Symposium S.EL05—Scalable Photonic Material Platforms—Applications and Manufacturing Advances

Effectively transparent contacts (ETCs) constitute the only front contact technology to date overcoming the inherent trade-off between grid conductivity and transparency[1]. When applied to solar cells relative efficiency improvements of 4-10% can be achieved. ETCs consist of triangular cross-section microscale silver grids that redirect incoming light towards the absorber material of the photovoltaic (PV) device instead of reflecting it back to the light source. Key features of this technology are the mesoscale structure size leading to ray-optical near-field interaction and low sheet resistivity [2] in combination with a novel fabrication method allowing for mirror-like optical properties. Here, the design of ETCs, their physical and visual [3] properties, application in different solar cell technologies and the principle of residue-free imprint lithography will be presented. Commercialization of the ETC technology is underway and executed by the award-winning Caltech and UTwente spin-off ETC Solar.
[1] R. Saive, A. M. Borsuk, H. S. Emmer, C. R. Bukowsky, S. Yalamanchili, and H. A. Atwater: Effectively Transparent Front Contacts for Optoelectronic Devices. Advanced Optical Materials, 4 (10), 1470-1474, 2016
[2] R. Saive, and H. A. Atwater: Mesoscale Trumps Nanoscale: Metallic Mesoscale Contact Morphology for Improved Light Trapping, Optical Absorption and Grid Conductance in Si Solar Cells. Optics Express 26 (6), A275-A282, 2018
[3] L. J. Myers, H. A. Atwater, R. Saive: Visual appearance of micro contacts for solar windows. Journal of Photonics for Energy 9 (2), 027001, 2019

Radiative heat management is a promising approach for thermoregulation because it is ubiquitous, rapid, lightweight, and energy-efficient. The sensitivity and spectral tunability also make thermal radiation particularly powerful in many energy applications, such as rooftop radiative cooling, solar thermal heating, wearable thermal management, and so on. As the radiative heat management concept becomes more and more prevalent and influential, it also calls for more functions integrated into one single device. In this talk, I will introduce a triple-mode mid-infrared modulator that can switch among transmission, reflection, and emission states. The device is composed of an elastomer/metal thin film with a rationally designed coupling relationship between mechanical and optical properties. The fabrication is highly scalable and does not need any photolithography. By controlling the biaxial strain of the elastomer/metal film, the device can switch among the three modes with unprecedented performance of emittance contrast |Δε|= 0.44, transmittance contrast |Δτ| =0.48, and reflectance contrast |Δρ|=0.38, and these modulation remains stable after 100 cycles. Being able to switch among three modes means the device can perform heating and cooling on objects with all kinds of surface emissivities, no matter it is perfect thermal emitter such as the human body or poor emitters such as metals. We also performed semi-empirical structure optimization to serve as the future pattern design guideline. It provides a new paradigm in the cross-disciplining area of soft and shape-changing photonic materials.

The warm white light emission phemena from a single solid state incandescent light emitting device (SSI-LED) was reported by Kuo's group (1,2). This device is composed of a very large number of nano-resistors formed from the breakdown of a MOS capacitor with a nm-thick high-k gate dielectric on a p-type Si wafer. Light is emitted during the passage of the current thorugh the nano-resistors, i.e., following the Planck's law on balckbody emission. The light emission phenomena can last for more than 20,000 hours under the continuous operation condition in air without a passivation layer (3). The emitted light can be narrowed down to a narrow wavelength width near 850 nm (4), which is a potential light source in an on-chip optical interconnect system.

In this paper, the light emission phonomena over a SSI-LED has been simulated. The emission spectra from the thermal excitation of the nano-resistor at different temperatures with and without inclusion of an ITO or a-Si thin film filter are simulated and compared with that measured from the actual device. The device size effect on the distribution of nano-resistors and therefore the distribution of light dots is estimated and compared with that of the actual device, which will be correlated to the measured currrent density of the device.

(1) Y. Kuo and C.-C. Lin, Appl. Phys. Lett., 102, 031117 (2013).
(2) Y. Kuo, invited, “Solid State Incandescent Light Emitting Devices Made of IC Compatible Material and Fabrication Process,” IEEE Elec. Dev. Soc. News Letters, 22-2, 1-5 (2015).
(3) Y. Kuo and S. Zhang, AVS 63rd meeting, Abst. 1844, Nashville, TN, November 6, 2016.
(4) S. Zhang and Y. Kuo, ECS J. Solid State Sci. Technol., 6(4) Q39-Q41 (2017).

Solar thermal energy has been one of the major energy sources to realize zero energy buildings for a sustainable future. The high efficiency and low cost make it particularly attractive to reduce the energy consumption for hot water and space heating. On the other hand, the radiative cooling uses the rest of the sky, i.e. the outer space, as the cold source to generate cooling that can offset the usage of refrigeration and space cooling. Both technologies show rapid and promising improvement in the past decade, which calls for new engineering and design principles to further maximize the energy saving for buildings. In this talk, I will introduce a smart building envelope that can switch between solar heating and radiative cooling, so it can be applied to buildings at various climate zones (spatial variation) under different weather and forms of energy demand (temporal variation). The dual-function material which possesses both heating and cooling property is switched mechanically between ~600 W/m² of solar heating and ~100 W/m² of radiative cooling in less than one minute. This high performance is the outcome of heat transport analysis and engineering. We demonstrate this smart heat managing building envelope can be adopted in all 16 climate zones in the US and produce in more energy-saving on a yearly basis.

While selective emitters can lead to significant radiative cooling by reflecting sunlight and emitting heat to the surroundings, current materials solutions such as multilayer structures or metallic reflectors are inadequate for large-scale radiative cooling systems that require lightweight, flexible and low-cost materials. Commonly used coatings for scalable applications, including commercial pigment-embedded white paints, suffer from strong ultraviolet absorption from embedded high-refractive-index (n~2.5) TiO₂ particles and a low near-infrared reflectivity. The strong ultraviolet absorption induces severe photocatalytic degradation for the organic binder components in TiO₂-based paints. Integrating low refractive index particles such as SiO₂ microspheres can be used to avoid high ultraviolet absorption because the extinction coefficient of SiO₂ remains approximately zero throughout the ultraviolet/visible/near-infrared spectrums. Here we create a lightweight, flexible and scalable polymer coating by integrating controlled volume concentrations of hollow glass microspheres within a polydimethylsiloxane (PDMS) matrix since PDMS is a commonly used low-cost polymer with little solar absorption and easy accessibility. We utilize hollow SiO₂ microspheres instead of solid microspheres as they provide a higher interfaces-to-volume ratio and a much smaller density. Our size analysis shows that the diameter of the hollow glass microspheres ranges from 2 µm to over 40 µm and the shell thickness are roughly 0.05 times the diameter (0.1 µm to 2 µm). The prepared polymer coating offers a maximum mass density of 681 kg/m³ and the corresponding areal density of 0.885 kg/m². The polymer coating also shows excellent flexibility and scalability which is beneficial for large-scale applications. As the volume concentration of hollow glass microspheres increases from 0 to 70%, the average reflectivity in the visible spectrum accordingly increases from 0.06 to 0.92 while the emissivity in the mid-infrared remains above 0.85. We use computations based on rigorous coupled-wave analysis to explain the role of hollow glass microspheres in increasing the solar reflectivity. The high solar reflectivity is attributed to the strong backscattering of sunlight enabled by refractive index difference between PDMS matrix (n~1.4) and SiO₂ shell (n~1.45), as well as between SiO₂ shell and air void (n~1) inside the microsphere. Our computation based on the Finite-difference time-domain method shows that hollow glass microspheres with a hierarchical arrangement of diameters are more efficient in scattering the sunlight and thus the polymer coating composed of large numbers of such hierarchical systems randomly distributed in a PDMS matrix can provide high solar reflectivity. The high mid-infrared emissivity is attributed to multiple extinction peaks of PDMS in the mid-infrared wavelength caused by different vibrational modes of its molecular structures. Our outdoor temperature measurement shows that the polymer coating on a concrete surface can lead to a temperature reduction of 25°C during the day, which is also 9°C below ambient temperature when conduction and convection are limited. Our building energy analysis shows the potential impact of the polymer coating on common buildings in Phoenix and estimates annual cooling energy savings of 6-38 MJ/m². The demonstrated ultraviolet-resistant property shows that the polymer coating exhibits great reliability and thus outperforms commercially available white paints. The selectively reflective polymer coating material based on hollow glass microspheres presents optimal material solutions for large-scale radiative cooling applications in buildings.

Controlling the propagation, absorption and emission of light has been a topic of fundamental interest in contemporary photonics research. In the context of controlling the emission of light from thermal sources, a central challenge is that of angular selectivity. Over the past two decades, a range of photonic strategies, including using surface phonon-polariton modes, have enabled anomalous angular ‘beaming’ of thermal radiation, albeit only at narrow ranges of wavelengths. However, thermal radiation is inherently a broadband phenomenon. We currently lack the ability to constrain broad-spectrum thermal emission over a narrow range of angles of incidence, but consistently over a broad range of wavelengths. This form of broadband thermal beaming would be a fundamentally enabling capability for a range of applications, including infrared imaging and camouflage, thermophotovoltaics and radiative cooling.

Here, we employ multiple epsilon-near-zero (ENZ) materials in a layered configuration to realize a broadband angularly selective thermal emitter. ENZ materials can fulfill high absorption and emission of incident light at wavelengths where the epsilon is near zero. Aluminum oxide (Al₂O₃), Silicon monoxide (SiO) and Silica (SiO₂) are all polariton materials with different ENZ wavelength. When combining them on top of each other into a multilayer thin film structure, we can obtain broadband near-blackbody emission from 7 to 11µm at a certain range of angles of incidence in the p polarization. We demonstrate this broadband angularly selective thermal emitter through theory, simulations, and an experimentally fabricated structure.

By employing transfer matrix method, we analytically solve the dispersion relation of this multilayer thin film structure and able to observe the EM field using the simulation tool. Following that, experimental results are compared with the analytical models with excellent agreement. The ENZ multilayer stack was coated on a Φ=100mm wafer via electrical beam evaporation method on CHA MARK 40. First, a 100nm Aluminum layer was deposited on the silicon wafer and then with 200nm Al₂O₃, 300nm SiO and 100nm SiO₂ on top of each other. Then the sample was characterized by a Fourier-transform infrared spectroscopy (FTIR)-Jacob 6100. The strongest emission occurs at an AOI around θ = 70° and can be tuned from 65°to 80° by just varying the thickness of the layers. Outside of this AOI range the multilayer film is highly reflective which allows a switch between emission and reflection mode by changing the AOI. The materials used and structure designed in our research are cheap and easy to fabricate and fully compatible with wafer-scale manufacturing. Overall, this thin film based ENZ approach opens a path towards broadband thermal beaming and angular selectivity at low cost, at large scale and with simple structures.

Radiative cooling is an attractive method to save energy usage for terrestrial entitiies, such as buildings and vehicles. To achieve efficient radiative cooling, high solar reflectance and high thermal emittance are needed. In this talk I will present out recent results on developing porous polymer paint for achieving high-performance radiative cooling. Strong sunlight reflectance at the polymer/air interface leads to solar reflectance of 96-99.6% and thermal emittance of 0.97. Sub-ambient cooling of 6 oC is achieved under strong sunlight. Such paint can also be prepared in scalable fashion and applied by wide range of surface with irregular shapes. Moreover, we have achieved dynamic switching inside by wetting/dewetting the porous film by liquid. A bilayer design is also developed for colored cooling paint, which shows 10-30% higher reflectance in the inrafred part of solar spectrum.

References:
Jyotirmoy Mandal et al., Science, 362, 315 (2018).
Jyotirmoy Mandal et al., Joule, online.

In the field of tunable optics, transparent conducting oxides (TCOs) have heralded a new generation of all-optical switches. The extraordinary enhancement of reflection modulation by epsilon-near-zero materials has paved the way for lithography-free all-optical switches in the telecommunication regime. We have demonstrated reflection modulation of up to 40% with optical pump-powers in the millijoules/cm² range, utilizing planar films of aluminum-doped zinc oxide [1]. Furthermore, we have demonstrated the addition and subtraction of the optically induced nonlinearities utilizing simultaneous intraband and interband pumping, performing operations in the sub-picosecond timescale [2]. We have also demonstrated low power optical switching with cadmium oxide in the mid-infrared regime, where the free-carrier-induced changes in the refractive index can achieve reflection modulation up to 135% with pump-powers as low as 1.3 mJ/cm². Finally, we have brought together low-loss dielectrics such as undoped ZnO and metals such as titanium nitride in hybrid metal-dielectric resonators to demonstrate broadband optical switching in the near-infrared wavelengths, achieving reflection modulation over 50% with relaxation times in the order of 2 picoseconds [3].
Transdimensional materials (TDMs) are yet another material platform gaining traction for dynamically tunable nanophotonic devices. The optical responses of atomically thin plasmonic films can be engineered by precise control of their structural and compositional parameters. This unique tailorability establishes TDMs as an attractive material for the design of tailorable and dynamically switchable metasurfaces [4]. Transition metal nitrides (TMNs) are ideal materials to investigate the tailorable properties of plasmonic films with thicknesses of just a few monolayers. The optical and electrical properties of smooth, epitaxial, TiN ranging from 2 to 10 nm have been characterized by ellipsometry and Hall measurements [5]. Although the metallicity decreases in smaller thicknesses, the thinnest film (2 nm) still retains a carrier density of 10²²cm^-3and is plasmonic at visible wavelengths. We have also explored the role of oxidation, and strain through first principles density functional (DFT) calculations, providing a way to tailor the optical response [6]. Gate dependent Hall measurements on a 1 nm film also reveals extreme tunability of the carrier concentration (up to 8.5%).
TCOs and TMNs have been reported as promising material platforms for actively tunable nanophotonic devices. While our results have long established TCOs as a major player for ultrafast, switchable metasurfaces, our progress in developing tunable atomically thin TiN demonstrates the potential for tunable plasmonics based on TDMs.

[1] N. Kinsey, C. DeVault, J. Kim, M. Ferrera, V. M. Shalaev, and A. Boltasseva, “Epsilon-near-zero Al-doped ZnO for ultrafast switching at telecom wavelengths,” Optica, vol. 2, no. 7, pp. 616–622, Jul. 2015.
[2] M. Clerici et al., “Controlling hybrid nonlinearities in transparent conducting oxides via two-colour excitation,” Nat. Commun., vol. 8, no. 1, pp. 1–7, Jun. 2017.
[3] S. Saha et al., “Ultrafast Tunable Metasurface with Transparent Conducting Oxide Antenna Array,” in Conference on Lasers and Electro-Optics (2018), paper FTh4M.4, 2018, p. FTh4M.4.
[4] A. Boltasseva and V. M. Shalaev, “Transdimensional Photonics,” ACS Photonics, vol. 6, no. 1, pp. 1–3, Jan. 2019.
[5] D. Shah, H. Reddy, N. Kinsey, V. M. Shalaev, and A. Boltasseva, “Optical Properties of Plasmonic Ultrathin TiN Films,” Adv. Opt. Mater., vol. 5, no. 13, p. 1700065, Jul. 2017.
[6] D. Shah et al., “Controlling the Plasmonic Properties of Ultrathin TiN Films at the Atomic Level,” ACS Photonics, vol. 5, no. 7, pp. 2816–2824, Jul. 2018.

Planar optical elements with efficient light control at the nanoscale can be designed based on transdimensional photonic lattices that operate in the translational regime between two and three dimensions. Such transdimensional lattices include 3D-engineered nanoantennas supporting multipole Mie resonances and arranged in the 2D arrays to harness collective effects in the nanostructure [1]. Optical antennas made out of van der Waals material with naturally-occurring hyperbolic dispersion is a promising alternative to plasmonic and high-refractive-index dielectric structures in the practical realization of nanoscale photonic elements. The antenna made out of hexagonal boron nitride (hBN) possesses different multipole resonances enabled by the supporting high-k modes and their reflection from the antenna boundaries. The full range of the resonances is demonstrated for the hBN cuboid antenna, a decrease of reflection from the array, and highly directional resonant scattering from antennas pairs. We show that transdimensional lattices consisting of resonant hBN antennas in the engineered periodic arrays have great potential to serve as functional elements in ultra-thin optical components and photonic devices.
We show that a periodic array of particles with a large imaginary part of their permittivity (LIPP) can support well-defined modes localized within the particle, with the spectral position of the modes mainly defined by the array period. It has been known for a long time that a non-zero imaginary part of the permittivity enables propagating surface modes at the interface of materials with positive permittivities, known as Zenneck modes. Recently, it has been demonstrated that transition metal dichalcogenides (TMDC) layers support different kinds of propagating Zenneck waves defined by the layer thickness and interfaces with air and substrate [2]. Here, we show that localized Zenneck modes are possible on subwavelength particles and that the particles can serve as antennas for direction light scattering, reflection suppression, and Kerker-effect applications. To illustrate a possibility to control light, we show that simultaneous excitation of the several particle multipole resonances (electric dipole and quadrupole) allows for achieving significant reflection suppression and observation of the generalized lattice Kerker effect [3]. Materials with LIPP are common in nature, including TMDCs and lossy metals, titanium, chromium, and others.
Acknowledgment. This material is based upon work supported by the Air Force Office of Scientific Research under Grant No. FA9550-19-1-0032.

[1] V. E. Babicheva, "Multipole Resonances in Transdimensional Lattices of Plasmonic and Silicon Nanoparticles," MRS Advances 4, 713-722 (2019).
[2] V. E. Babicheva, S. Gamage, L. Zhen, S. B. Cronin, V. S. Yakovlev, Y. Abate, "Near-field Surface Waves in Few-Layer MoS2," ACS Photonics 2018, 5, 2106.
[3] V. E. Babicheva and J. Moloney, "Lattice Zenneck modes on subwavelength antennas," Laser & Photonics Reviews 13, 1800267 (2019).

Metasurfaces enable arbitrary control of the wavefront of light by locally manipulating amplitude, phase and polarization.^1,2 I will present advances in components (metalenses and structurally birefringent phase plates) that have enabled the realization of high-performance polarization sensitive cameras and depth cameras. Recent developments have enabled the practical realization of optical elements in which the polarization of light may vary spatially.³ We recently presented an extension of Fourier optics—matrix Fourier optics—for understanding these devices and apply it to the design and realization of metasurface gratings implementing arbitrary, parallel polarization analysis.⁴ We show how these gratings enable a compact, full-Stokes polarization camera without standard polarization optics. Our single-shot polarization camera requires no moving parts, specially patterned pixels, or conventional polarization optics and may enable the widespread adoption of polarization imaging in machine vision, remote sensing, and other areas.⁴ We also have demonstrated a compact depth sensor that is inspired by the jumping spider.⁵ It combines novel metalens optics, which modifies the phase of incident light at a subwavelength scale, with efficient computations to measure depth from image defocus. The sensor uses a metalens to split the light that passes through an aperture, and concurrently form two differently-defocused images at distinct regions of a single planar photosensor. We have demonstrated a system that deploys a 3mm-diameter metalens to measure depth over a 10cm distance range, using fewer than 700 floating point operations per output pixel., at video rates.⁵ Compared with previous passive depth sensors, our metalens depth sensor is compact, single-shot and requires a small amount of computation. This integration of nano-photonics and efficient computation brings artificial depth sensing closer to being feasible on millimeter-scale, micro-Watts platforms such as microrobots and micro-sensor networks.
1. N. Yu and F. Capasso Nature Materials 13, 139 (2014)
2. M. Khorasaninejad and F. Capasso Science 358,1146 (2017)
3. J. P. Balthasar Mueller et al. Physical Review Letters 118, 113901 (2017)
4. N. Rubin et al. Science 365, 6448 (2019)
5. Qi Guo et al. PNAS https://doi.org/10.1073/pnas.1912154116 (2019)

Metamaterials consist of subwavelength structured units have demonstrated many promising applications in optics and photonics. However, practical applications of metamaterials have been prohibited by low productivity of conventional fabrication methods. Electron beam lithography has been widely used to realize metamaterials which operate in the visible because necessary critical dimension of the unit structure is smaller than the diffraction limit of typical photolithography. Most metamaterials have been demonstrated by electron beam lithography whereas it is not suitable for practical uses due to high cost and ridiculously low throughput. Here, we demonstrate large scale metamaterials operating in the visible based on nanoimprint lithography (NIL) which allows rapid replication of nanostructures.
An efficient and convenient new imaging device made of a wafer-scale array of spherical hyperlenses are demonstrated based on NIL. Hyperlens is a metamaterial based imaging device whose super-resolving power comes from the hyperbolic dispersion relation in anisotropic media, which enables the sub-diffraction-limited objects to be resolved in the far-field [1]. However, hyperlens suffered from the extreme difficulties in fabrication process and objects placement which restrict the practical uses of the hyperlens devices. To overcome this limitations, direct pattern transfer techniques using NIL is applied as implementing large-scale hyperlens arrays is desirable [2]. We demonstrate 4-inch wafer-scale array of hyperlenses using NIL and it is integrated into a conventional microscopy system for imaging hippocampal neurons with resolution down to 151 nm, much smaller than the diffraction limit of conventional imaging systems [3].
We also demonstrate one-step fabrication method based on NIL to realize dielectric metasurfaces [4,5]. Despite many advantages of conventional NIL, it is still inefficient to fabricate dielectric metasurfaces because conventional NIL requires secondary operations such as thin film deposition and etching, which reduce productivity, substrate compatibility and cost competitiveness of NIL. To overcome the limitations, we develop nanoparticle composite (NPC) that consists of high-index nanoparticle inclusions in UV-curable resin. The refractive index of the NPC can be controlled by the volume fraction of the inclusion. Since the transferred pattern by the NPC can itself work as metasurfaces due to sufficiently high refractive index of the NPC, dielectric metasurfaces can be realized by a single step of NIL without any secondary operations. Experimental demonstration of metahologram and metalens verify the feasibility of our method, thereby we believe our approach provides a promising opportunity to manufacture metamaterials for practical uses.

References
[1] J. Rho*, Z. Ye*, Y. Xiong, X. Yin, Z. Liu, H. Choi, G. Bartal, X. Zhang, Nat. Commun. 1, 143 (2010).
[2] M. Byun*, D. Lee*, M. Kim, Y. Kim, K. Kim, J. G. Ok, J. Rho, H. Lee, Sci. Rep. 7, 46314 (2017).
[3] D. Lee*, Y. D. Kim*, M. Kim, S. So, H.-J. Choi, J. Mun, D. M. Nguyen, T. Badloe, J. G. Ok, K. Kim, H. Lee, J. Rho, ACS Photonics 5, 2549–2554 (2018).
[4] K. Kim*, G. Yoon*, S. Baek, J. Rho, H. Lee, ACS Appl. Mater. Interfaces 11, 26109–26115 (2019).
[5] G. Yoon et al., Nature Commun. (in revision)

Bacterial infections such as pneumonia, staphylococcus, and streptococcus do not often make headlines, yet they are responsible for more deaths than AIDS and many cancers and rank among the most expensive medical conditions to treat. Bacterial identification and antibiotic susceptibility testing can span days, even in state-of-the-art laboratories using the most advanced technologies, delaying the use of targeted antibiotics and accelerating the spread of infectious disease. Here, we present development of scalable, clinically-compatible nanophotonic imaging tools for bacterial pathogen identification, their antibiotic susceptibility, and their interactions with immune cells. First, we combine Raman spectroscopy and deep learning to accurately classify bacteria by both species and antibiotic resistance in a single step. We design a convolutional neural network (CNN) for spectral data and train it to identify 30 of the most common bacterial strains from single-cell Raman spectra, achieving antibiotic treatment identification accuracies exceeding 99% and species identification accuracies similar to leading mass spectrometry identification techniques. Our combined Raman-CNN system represents a proof-of-concept for rapid, culture-free identification of bacterial isolates and antibiotic resistance. Then, we introduce a new class of in vivo optical probes to monitor the forces between pathogens and immune cells with high spatial and temporal resolution. Our design is based on upconverting nanoparticles that, when excited in the near-infrared, emit light of a different color and intensity in response to nano-to-microNewton forces. The nanoparticles are sub-30nm in size, do not bleach or photoblink, and can enable deep tissue imaging with minimal tissue autofluorescence. We present the design, synthesis, and characterization of these nanoparticles both in vitro and in vivo; both chronic and acute cytotoxicity assays are used to confirm biocompatibility. Long-term, our Raman-CNN platform and optical-force sensors could enable several advances for personalized medicine and mitigation of infectious disease, allowing patients to receive the appropriate drug at the appropriate dosage and for the appropriate duration.

During the last decade, III-V semiconductor nanowires (NWs) have received great research interest due to their promising potential for next-generation electronic, optoelectronic, and photonic devices. One of the main challenges for epitaxially-grown III-V NWs, however, is their high manufacturing cost, which can be divided to two individual categories: (1) the cost of starting III-V substrates, and (2) the cost of pre-epitaxy fabrication associated with substrate patterning. Large-scale integration of III-V NW-based technologies requires novel nanofabrication strategies that mitigate both cost streams. Wafer costs can be substantially reduced simply through growth of III-V NWs on foreign substrates such as silicon or graphene, instead of on bulk III-V wafers. One strategy for simultaneously reducing fabrication complexity and pre-growth processing costs is to reuse Si substrates with existing predefined growth masks for selective-area epitaxy (SAE) of III-V NW arrays. Here, we present wafer-scale, bottom-up SAE growth of InAs NW arrays and fabrication of flexible NW-based infrared (IR) photodetectors realized through delamination of embedded NW arrays and reuse of Si substrates with predefined growth masks. Silicon (111) substrates are patterned by conventional photolithography to obtain pores with 0.5 µm diameters and 1 µm pitch in SiO₂ growth masks of 55 nm thickness. Next, InAs NW arrays are grown via the SAE approach using metalorganic chemical vapor deposition (MOCVD). Trimethyl-indium (TMIn) and arsine (AsH₃) were used as precursor gases for the supply of In and As growth species, respectively, at a set-point growth temperature of 700 °C. A two-step growth technique is introduced, whereby a separate nucleation step using a TMIn flow rate of 16 μmol/min precedes a NW growth step at a TMIn flow rate of 0.77 μmol/min. This procedure is necessary for optimization of pore occupation toward a global NW yield of >85%, with NW aspect ratios of ~15. The InAs NW arrays are embedded in various polymer membranes and mechanically delaminated from their growth substrates, then bonded to carrier substrates for subsequent device fabrication. The polymer-embedded NW array exfoliation process leaves NW bases with heights of ~50 nm below the fracture plane attached to the growth surface. Consequently, five different substrates restoration processes are investigated prior to substrate-reuse. It is observed that the best results for re-growth of daughter generation InAs NW arrays on reused Si substrates can be realized when no additional wet etching, chemical treatment, or mechanical polishing steps are introduced between subsequent re-growth runs, as the remaining NW bases serve as SAE growth sites. Direct re-growth on reused Si substrates results in vertical extension of existing NW bases and growth of InAs NWs with aspect ratios >80 under the same growth conditions as the primary run with comparable global NW yield. For fabrication of flexible IR photodetectors using both primary and subsequent generation InAs NW arrays, a unique process flow is reported, which enables NW transfer yields of ~100% and preserves the original position and orientation of as-grown NWs. This work demonstrates that multiple generations of InAs NW-based devices can be fabricated through reuse of a single, templated Si substrate, without introduction of intermediate substrate restoration processes such as wet etching or chemical mechanical polishing. This approach can serve as an effective strategy toward growth and fabrication of large-area, high-efficiency, low-cost, flexible, and wearable device applications in III-V nanoelectronics, optoelectronics, and photovoltaics.

Low-cost, large-area thermal management is desirable for the operation of many modern technologies including smart clothing, electronic circuits, building environments, and outdoor equipment to control heat flow. Inspired by the space blanket and the dynamic skin of cephalopods, we have demonstrated a large-area, highly uniform, low-cost nanostructured material with tunable thermoregulatory and infrared properties. We have implemented scalable nanofabrication processes to achieve a material with an area > 500 cm², a > 40 % change in infrared transmittance and reflectance, and a dynamic environmental setpoint temperature window of ~ 8 °C. Due to characteristics of scalability and associated figures of merit, our material affords new scientific and technological opportunities not only for adaptive optics and thermoregulation but also for any platform that would benefit from dynamic control of infrared radiation and heat.

The use of nonthermal plasmas has emerged as an effective method for the synthesis of high-quality nanoparticles. Extensive study of nonthermal plasma reactors under low pressure conditions has been conducted which has yielded systems capable of producing nanoparticles with size tunable properties. Miniaturization to a microscale reactor allows for effective operation of a RF nonthermal plasma reactor at atmospheric pressure. The reduced scale of the reactor also provides the opportunity for combining nanoparticle synthesis and deposition into a single system for manufacturing with nanoparticles.
Presented here is our work on the additive manufacturing of silicon nanoparticles synthesized at atmospheric pressure with a nonthermal plasma. The plasma was generated with a 13.56 MHz RF power supply inside a glass capillary tube between two external ring electrodes. A silane precursor gas together with an argon background gas were flown to the reactor for the synthesis of silicon nanoparticles. The reactor was attached to a motorized system responsible for controlled deposition of the nanoparticles in predetermined geometries. TEM and XRD has shown that control between crystalline and amorphous particles can be achieved by controlling the applied power. Control over the nanoparticle size has also been demonstrated by varying the overall gas flow rate to the reactor. In efforts to increase the capability of this system for manufacturing processes we have investigated methods to control the deposition resolution. Spreading of the particles after exiting the reactor leads to wide linewidths which can limit the performance of this tool for manufacturing. Manipulation of the gas flow with aerodynamic lenses allows for reduced expansion of the particles and improved resolution. With this method we have been able to reduce the spot size to 1.3 times the size of the reactor tube outlet diameter.

Here we present Subsurface Controllable Refractive Index via laser Beam Exposure (SCRIBE), a novel lithographic approach to fabricate polymer optical elements with arbitrary 3D index profiles at precise locations within a mesoporous silicon scaffold. We achieve a broad index range at 633 nm of 1.55 to 2.01 by adjusting the laser exposure. We demonstrate a microscale doublet objective that focuses visible light to 750 ± 20 μm and a smallest-on-record 15-μm diameter spherical Luneburg (gradient index) lens. Finally, we realize integrated photonic devices that use 3D waveguides to couple light vertically from the surface to a horizontally oriented subsurface all-pass microring (Q = 10,000). These demonstrations present a new paradigm for lithographically-aligned 3D fabrication of polymer micro-optics and photonic systems.

Material properties are governed by the chemical composition and spatial arrangement of constituent elements at multiple length-scales. This fundamentally limits material properties with respect to each other creating trade-offs when selecting materials for specific applications. For example, strength and density are inherently linked so that, in general, the more dense the material, the stronger it is in bulk form. We are combining inverse design methods such as topology optimization, with advanced additive micro- and nanomanufacturing techniques to create new material systems with previously unachievable property combinations – architected mechanical metamaterials. The performance of these materials is fundamentally controlled by geometry at multiple length-scales, from the nano- to the macroscale, rather than chemical composition alone. We have demonstrated designer properties of these mechanical metamaterials in polymers, metals, ceramics and combinations thereof. Properties include ultra-stiff lightweight materials, negative stiffness, and negative thermal expansion to name a few, as well as functional properties such electrical, optical, and even chemical responses. Recently, we have also demonstrated metamaterials which respond to external stimuli; namely magnetic fields. We have primarily utilized our custom developed additive micro- and nano- additive manufacturing techniques to create these structures and materials. These include projection microstereolithography (PuSL), direct ink writing (DIW), and electrophoretic deposition (EPD). I will also touch on new advanced concepts such as volumetric additive manufacturing (VAM), computed axial lithography (CAL), parallel two-photon polymerization, and diode-based additive manufacturing (DiAM) as well as new materials including graphene aerogel and nanoporous gold.

Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344.

We report the relationship between measures of the crystal quality of self-assembled colloidal crystal films and the intensity of their structural color. Structural color arises from geometric diffraction; it has potential applications in optical materials because it is more resistant to environmental degradation than coloration mechanisms that are of chemical origin. Structural color can be produced from self-assembled films of colloidal particles. To study the connection between crystal film quality and structural color intensity, we investigate the peak intensity and peak width of structural color reflection spectra as a function of defect density, film thickness and impurity concentration using a combined experimental and computational approach. Polystyrene microspheres are self-assembled into defect-laden colloidal crystals via solvent evaporation. Colloidal crystal growth via sedimentation is simulated with molecular dynamics and the reflection spectrum of simulated crystals is calculated using the finite-difference time-domain method. We examine the impact of several commonly observed defect types (vacancies, stacking fault tetrahedra, planar faults, and microcracks) on structural color peak intensity. We find that the reduction in peak intensity scales with increased defect density; stacking fault tetrahedra have a particularly detrimental effect. The thickness profiles are characterized by cross-sectional scanning electron microscopy and profilometry. The reflective spectral response of the colloidal crystals is measured with a white light spectrophotometer. Our results show that reflectance of structural color increases as a function of the crystal thickness, until a plateau is reached at thicknesses greater than about 10 μm. The plateau reflection is 83.2% ± 2.8%; this value is significantly less than the 100% reflectivity predicted for a fully crystalline material; the difference is caused by defects present in the experimental crystal structures. Finally, we manipulate the defect structures of colloidal crystals by introducing differently sized particles that function as impurities. As the concentration of the impurity is increased, the measured intensity of structural color reflection decreases by amounts that we correlate with the observed microstructure. These findings can guide the design of optical materials which tune the intensity of structural color.

This talk will overview our work toward two general-purpose nanomanufacturing processes for the hyper-scaling of semiconductor nanowire-based electronic and photonic devices: SCALES and Geode. The SCALES process (Selective CoAxial Lithography via Etching of Surfaces) leverages chemical differences between compositionally distinct nanowire segments to yield polymer surface masks with nanoscale precision. The Geode process utilizes an unconventional substrate – the interior surface of hollow silica microcapsules lined with the metal nanoparticles that seed nanowire growth – to increase the throughput of nanowire production by orders-of-magnitude. These processes are part of an emerging platform for the massively scalable fabrication and deployment of high-performance semiconductor materials and devices for applications in electronics and photonics.

Sprayable coatings for frequency-selective reflectors or absorbers in the visible and infrared are enabled by heterogeneous and hierarchal microspheres, such as high index contrast core-shell structures or spherical Bragg stacks. However, such nano- to micron-scale architecture can be difficult to achieve for a wide array of visible and infrared transparent materials. In this talk, we describe a fast, scalable, and material-agnostic process to make heterogeneous structures for omnidirectional IR reflection and scattering. We apply our self-assembly method to multiple nanoparticle compositions (SiO₂, TiO₂, LiYF₄, InP, PbSe) and nanoparticle shapes (spheres, pyramids, hexagonal prisms), yielding smooth and uniform microspheres 10-100 micrometers in diameter. The microspheres have a high nanoparticle packing density and do not contain any IR-active organic ligands or binders. Next, we show control of the microsphere internal structure by co-assembling two materials, yielding structures ranging from core-shell to a periodic mixture. Diffuse reflectance measurements of the assembled microspheres show that MWIR scattering is enhanced for materials with differing refractive indexes (Δn > 1). Our process is capable of making scattering microspheres at a rate of >0.1 grams / minute, thereby leveraging existing nanoparticle capabilities and unique properties and scaling them into macroscale coatings (m² surfaces) for optical and photonics applications.
This material is based upon work supported by the United States Air Force under Contract No. FA8650-15-7549. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessary reflect the views of the United States Air Force.

With the promise of novel optoelectronic devices using black phosphorus (BP), there is a need for a large-scale synthesis method for BP compatible with on-chip integration. Herein, we attain large scale mechanochemical conversion of red phosphorus (RP) to black phosphorus (BP) via high energy ball milling. During the milling process, the collisions of hardened steel media rapidly compress amorphous RP into crystalline, orthorhombic BP flakes, with a conversion yield >95% for up to 5 g of bulk BP powder. Commercial scale-up of BP production can be extrapolated by empirically derived conversion kinetics from lab scale, high energy ball mills. RP’s milling conversion kinetics, as monitored via ex situ XRD, shows a sigmoidal behavior best described by the Avrami rate model. For <60% conversion, BP is rapidly formed at the expense of RP following an autocatalytic behavior with induction times proportional to the milling intensity. At ≥60% conversion, milling conversion rate suddenly decreases, indicative of a growth mechanism change with presumably a higher activation energy, thereby requiring higher impact energies for complete conversion. Further, by comparing the specific milling dose, namely, the energy input required for the phase transformation to reach completion, versus the conversion progress, we can determine the optimum milling conditions. We therefore ascertain the BP specific milling dose (50 kJ/g) and minimum impact energy (25 mJ/impact), both necessary to scale up production and minimize the power consumed in a tumbler milling vessel. Finally, the as-synthesized BP powder exfoliates more readily than single crystal BP sources in solution, as revealed by concentrations obtained from UV-Vis absorption under the same exfoliation conditions. The milled, converted BP solution is roughly 6-fold the concentration of the single crystal solution. High energy ball milling is demonstrated as a large scale, viable synthesis route for BP.

Nano-gap structure has attracted great scientific interests. For example, plasmonic nanostructures with nano-gaps concentrate light to a small volume, which can lead to many potential applications. While it is theoretically predicted that the optimal plasmonic hot spot is a gap of less than 1 nanometer between two metallic particles, there is still no manufacturing technology to reliably fabricate it with high-precision and controllability at practical cost. In this work, we present a novel fabrication technique that combines both top-down and bottom-up process to produce large-area nanogap structures with atomic-precision gap size control. First, two layers of photoresist and one liftoff layer were spin-coated on substrates, then Nanoimprint (NIL)^[1], Reactive Ion Etching (RIE) and Metal Deposition were utilized to achieve desired metal patterns. Afterwards, uncovered areas were etched by RIE to form high aspect ratio nanofingers and Atomic Layer Deposition was used to grow selected thin dielectric layer outside nanofingers. After dripping ethanol onto samples, high aspect ratio nanofingers would be driven to collapse by capillary force during ethanol evaporation. By manipulating nanofinger heights and dielectric thicknesses, gap size down to 0.8 nm can be precisely defined after finger collapsing and such structure can be utilized as an ideal platform to study the rich sciences of plasmon and quantum effect. For example, we experimentally demonstrated that tunneling barrier height across the gap can be adjusted by using appropriate gap spacer materials and gap sizes.^[2] As a result, plasmonic enhancement can be strongly affected since the electron tunneling mechanism is the most essential part in determining the plasmonic enhancement factor. It was found that as the tunneling barrier height is decreased, the enhancement of electron tunneling manifests itself in a wider optimal gap, a redshift of the plasmon frequency with increasing gap size was also observed when the gaps narrowed to sub-5 nm range^[2]. Moreover, many photonic applications have been studied and implemented intensively based on our controllable nanogap structures. Those include plasmonic enhanced fluorescence, single-molecule label-free sensing by SERS^[3] and plasmonic enhanced photocatalysis.
Reference:
[1] S. Y. Chou, P. R. Krauss, P. J. Renstrom, Appl. Phys. Lett. 1995, 67, 3114.
[2] B. Song, Y. Yao, R. E. Groenewald, Y. Wang, H. Liu, Y. Wang, Y. Li, F. Liu, S. B. Cronin, A. M. Schwartzberg, S. Cabrini, S. Haas, W. Wu, Acs Nano 2017, 11, 5836.
[3] F. Liu, B. Song, G. Su, O. Liang, P. Zhan, H. Wang, W. Wu, Y. Xie, Z. Wang, Small 2018, 14, 1801146.

The next generation of functional coatings will likely be composed of nanomaterials. At the laboratory scale nanomaterial-based coatings have shown the potential to improve performance and enable completely new functionality in applications like thermal barrier coatings on high-temperature parts, active layers in electronic devices, anti-reflective coatings for PV modules, or biocompatible coatings for medical applications. In all cases, these coatings should be able to be formed over large areas at commercial production rates and have a uniform and controllable thickness, and their constituent particles should retain their nano-properties.
Present nanomaterial films developed in laboratories are often unable to be reproduced on a commercial scale; the manufacturing methods are inherently nanomaterial, substrate, and application specific. This specificity results in manufacturing processes that are expensive or offer limited utility. We propose an advanced nanomaterial film manufacturing technology based on Aerosol Impact Driven Assembly (AIDA) that overcomes these deficiencies.
AIDA begins by aerosolizing a nanomaterial using any desired technique, from atomizing a nanoparticle-laden solution to feeding nanoparticle-precursor gas into a plasma. The aerosolized material is fed into the AIDA system, which consists of two chambers separated by a slit-shaped nozzle, with the bottom chamber held under vacuum (< 1 Torr). The nanomaterial is accelerated to a velocity of several hundred meters/second as it and the aerosol background gas are forced through the nozzle, resulting in the formation of a curtain of nanoparticles directed into the downstream chamber. A substrate is passed through the curtain, and the nanomaterial collides with and adheres to the substrate, forming a thin coating.
AIDA’s unique approach to film formation enables independent control over nearly every aspect of the coating’s morphology. As a dry spray technique, there are no inherent limitations on the thickness. Coatings ranging in thickness from a sub-monolayer to >1mm have been produced with dynamic deposition rates as high as 6 mm-m/min. By adjusting the particle impact velocity, the porosity of the coating can be controlled between 5% and 95%. This enables control over film properties like the refractive index, heat transfer coefficient, and dielectric constant. By changing the angle of impaction, the surface roughness of the film can be tuned between 0 nm and >100 nm allowing engineers to tune the effective surface energy of the coating. Combining this intimate control of film morphology with the wide range of nanomaterials compatible with AIDA allows engineers to produce complex, multi-layered, multi-component, and multi-functional films using a single platform.
As a case study, we will showcase an AIDA system capable of depositing on 14”x24” substrates that uses an in-situ non-thermal plasma reactor to synthesize metal-oxide nanoparticles (SiO₂, TiO₂, and ZnO) and produce a multi-layered coating that is simultaneously highly photocatalytic (self-cleaning) and anti-reflective for use in PV module glass. The composition of each layer will be tuned to maximize the photocatalytic self-cleaning effect. The thickness and refractive index of each layer will be precisely tuned to optimize its anti-reflective properties and provide a 3% transmittance gain compared to uncoated glass. The surface roughness of each layer will be tuned to promote adhesion and provide the desired water contact angle on the top surface to enhance self-cleaning.

Single molecule (SM) detection represents the ultimate limit of chemical detection. Over the years, many experimental techniques have emerged with this capacity. Yet, SM detection and imaging methods require large, reproducible spectral datasets in order to benefit from chemometric methods. We will present a scalable chemical assembly platform for plasmonic and metamaterial architectures that yield reproducible optical response. For example, surface enhanced Raman scattering spectroscopy (SERS), with extensive applications in biosensing, is demonstrated to be particularly promising because Raman active molecules can be identified without recognition elements and is capable of SM detection. Yet quantification at ultralow analyte concentrations requiring detection of SM events remains an ongoing challenge, with the few existing methods requiring carefully developed calibration curves that must be redeveloped for each analyte molecule. We will present work where we demonstrate that using 2-dimensional physically activated chemical (2PAC) self-assembly, we can achieve reproducible enhancement factors of 10⁹ over 1 cm². 2PAC crosslinks nanoparticles into discrete assemblies with a small molecule linker, where the crosslinking reaction is driven by electrohydrodynamic flow at on a substrate surface. The hotspots observed in a 2PAC fabricated assembly have gap spacing is 0.9 nm, corresponding to the length of the chemical crosslinker. Large area SERS maps are acquired from these 2PAC assembled surfaces from analyte dissolved in water and even complex biological media.

One of the most important applications of contemporary machine learning, i.e., image analysis, has remained relatively untouched by the SERS community. We will show that a combination of large area and reproducible SERS substrates with a CNN model is used to address the long standing challenge in SERS: quantification of sub-nanomolar analyte concentrations. A convolutional neural network (CNN) model when applied to bundles of reliable SERS spectra yields a robust, facile method for concentration quantification down to 10 fM using SM detection events. The demonstrated limit of blank (LOB) is 1 fM for Rhodamine 800, with a limit of quantification (LOQ) of 10 fM. Furthermore, the model’s predicted concentrations have an average r² value of 0.958 over 6 orders of magnitude as determined by k-folds cross validation. A key advantage of the analysis is its compatibility with transfer learning, the use of large datasets to train a model and then generalize this pretrained model to new molecules quickly with significantly smaller datasets. The uniformity of enhancement factor is essential for our approach, as large variance in the dataset not only increases the variance in predictions, but also requires more data for convergence and prevents a transfer learning approach. Generalization of the CNN model to other analytes using transfer learning is demonstrated with methylene blue. Transfer learning achieves good results with as few as 50 8x8 pixel maps. Thus entire datasets are acquired quickly, requiring just 5.3 minutes of total laser exposure time per concentration to train a robust model that is much faster and more scalable than required for building SM concentration regression models. This demonstrates a proof of concept for hyperspectral CNN image analysis of SERS, which could be broadly applicable within SERS, especially in the biological setting where classification of images could improve analysis of bacteria that are already imaged with SERS. We will also present results showing this approach is able to differentiate susceptible and resistant bacterial strains for rapid antimicrobial susceptibility tests.

Bottom up self-assembly of block copolymers (BCPs) offers a scalable, economical alternative to lithography for producing large area, highly uniform nanopatterns required for engineering materials with tailored light-matter interactions. At their typical scales (~20-50 nm feature spacing), thin film BCP nanopatterns may be used to generate deep subwavelength like nanotextures that reduce visible reflectance to < 1% [1,2], or nanoholes that improve light trapping in photovoltaics at carrier selective rear contacts [3]. Nevertheless a greater latitude for optical property engineering would be possible using self-assembled BCP patterns at length scales commensurate with visible wavelengths of light in order to exploit optical diffraction and resonance effects. This is prohibited by exponentially slower ordering kinetics in the ultrahigh higher molecular weight BCPs (> 1000 kg/mol) required for this scaling. Here we report dramatically accelerated ordering afforded by blending in very low molecular weight (~3 kg/mol) homopolymer plasticizers. These formulations enable a 10× reduction in the time required to achieve ordered nanopatterns with periods > 200 nm. Subsequent reactive ion etching demonstrates that these nanopatterns are readily transferrable to other materials. Exemplar applications such as structural colors made possible using these self-assembled patterns will be described.

References:
[1] A. Rahman, et al., Nat. Commun. 6, 5963 (2015).
[2] A.C. Liapis, et al., Appl. Phys. Lett. 111, 183901 (2017).
[3] M.J. Hossain, et al., Proc. SPIE 11089, 110890P (2019).

Research performed at the CFN and NSLS-II, U.S. DOE Office of Science Facilities at Brookhaven National Laboratory under Contract No. DE-SC0012704.

Recently, R2R-UV nanoimprinting has proven to be very useful and unrivalled for the large-area fabrication of high-resolution, hierarchical structures at reasonable throughput deployed for mechanically flexible and lightweight functional surfaces in bionic applications like drag-reducing films or transparent conductive electrodes [1], [2].
Here we report on the deployment of our UV-Nanoimprinting pilot line for the manufacturing and upscaling of optical components in lighting and microfluidic applications. This pilot line covers all steps needed for upscaling, namely design & simulation, origination via maskless laser lithography, tooling via Step & Repeat UV-NIL and large area replication via R2R UV-nanoimprinting.
One key prerequisite for the versatile deployment of UV-nanoimprinting is the adjustability of the imprint resin towards the targeted application scenario. It should be tuneable in terms of elasticity and surface tension to account for easy demoulding and for low (water-repellent) as well as high (water-wicking) energy surfaces and it should have low enough viscosity to allow for large-area coating. Special requirements for the channel forming resin in microfluidics are a sufficiently high surface energy to allow for a fast and long distance filling of analytes, the ability to be bio-functionalized and low cytotoxicity. For optical polymer components, the tuneability of the refractive index is important as well. Another key aspect is the seamless tiling of master structures via Step &Repeat UV nanoimprinting on PET sheets to form large-area flexible imprint stamps for readily mounting around the imprint roller.
On this base, we R2R-fabricated complex freeform microoptical element (FFMOE) designs with small height on polymer films that enable a tailored redirection of light from discrete LED sources. These films enabled the generation of ultra-thin direct-lit luminaires with very homogeneous irradiance over a defined area shape [3]. FFMOE-covered films were also applied for light management in wall-wash luminaires. Finally, we used FFMOEs to improve the outcoupling of the chemiluminescence signal in in-vitro diagnostic chips for the detection of diverse pathogens. The biosensor signal was increased by a factor of ~1.7 as tested in a portable read-out device of a commercial chip testing platform.
[1] M. Leitgeb et al., ACS Nano 10 (5), 4926 (2016).
[2] D. Nees et al., Proc. of SPIE Vol. 9777, 97770D, doi: 10.1117/12.22181 (2016).
[3] P. Hartmann et al., LpR 67, p. 50 (2018)

Acknowledgement: Parts of this work have been performed within the H2020-funded projects R2RBiofluidics and Phabulous, and the FFG-funded project Green Photonics.

Millions of years of evolution in the biological world has developed a plethora of micro- and nanoscopic photonic structures that are frequently superior to synthetic analogs. These biophotonic materials show interesting novel and tunable unforeseen properties with deliberately introduced disorder in their respective geometries and compositions¹. Over the last decade, photonic materials with tailored -i.e. with deliberately introduced- structural disorder have also attracted considerable interest in various optical applications due to their extended spectral and angular range of effectiveness². Most bio-inspired nanostructured devices or optical metamaterials designed to date have been demonstrated at small-scales using expensive top-down techniques. However, biological structure formation in nature utilizes several bottom-up self-assembly approaches for manufacturing hierarchical mesoscopic nanostructures (100 – 550 nm) with immense diversity. Despite recent efforts aimed at increasing top-down fabrication writing speed, alternative routes based on self-assemblies still possess major advantages for industrial implementation of disordered structures as they allow rapid processing over large areas (>>cm²).
In this communication, we show that up-scalable polymer blend lithography technique can be used as a versatile platform for fabricating 2D planar, disordered nanostructures that can be exploited in both top-down and bottom-up strategies. Polymer blend lithography utilizes the polymer-phase separation process following nature’s way of forming nanostructures. The tailored disorder is achieved here by adjusting the process parameters (polymer blend composition and deposition conditions), enabling us to tune the morphology and the spatial distribution of the nanostructures produced, and in turn their light management properties.
First, we use our approach to pattern a resist etching mask, employed for transferring disordered nanopillars by dry etching (top-down route) onto a Fabry-Perot-resonator-based intraocular pressure (IOP) sensor for glaucoma management³. The nanostructure integration onto the IOP sensor led to a 2.5-fold improvement in readout angle allowing easy handheld monitoring and in a one-month in vivo study conducted in rabbits, showed a 3-fold reduction in IOP error and 12-fold reduction in tissue encapsulation and inflammation, compared to an IOP sensor without nanostructures. Second, we demonstrate that similar structures can serve as a template in a bottom-up configuration, whereby aluminum thin film is directly deposited into the disordered nanoholes to form scalable plasmonic metasurfaces⁴. These metasurfaces generate hybrid multipolar lossless plasmonic modes resulting in a broadband fluorescence-enhancement factor above 1000 for visible wavelengths with respect to glass chips commonly used in bioassays. Using the metasurface and a multiplexing technique involving three visible wavelengths, we successfully detected three biomarkers, insulin, vascular endothelial growth factor, and thrombin relevant to diabetes, ocular and cardiovascular diseases, respectively, in a single 10-μL droplet containing only 1 femtomole of each biomarker.

References
1. Kolle, M. and Lee, S., Progress and opportunities in soft photonics and biologically inspired optics. Advanced Materials 30(2), 2018.
2. Wiersma, D.S., Disordered photonics. Nature Photonics 7(3), 2013.
3. Narasimhan, V., Siddique, R.H., Lee, J.O., Kumar, S., Ndjamen, B., Du, J., Hong, N., Sretavan, D. and Choo, H., Multifunctional biophotonic nanostructures inspired by the longtail glasswing butterfly for medical devices. Nature Nanotechnology 13(6), 2018.
4. Siddique, R.H., Kumar, S., Narasimhan, V., Kwon, H., and Choo, H., Aluminum Metasurface With Hybrid Multipolar Plasmons for 1000-Fold Broadband Visible Fluorescence Enhancement and Multiplexed Biosensing. ACS Nano, 2019.

Colloidal nanoparticles have been established as a diverse class of material with many interesting and useful properties. However, their practical application often requires some form of patterning procedure such as photolithography, ink-jet printing and nanoimprint lithography. Direct optical lithography of functional inorganic nanomaterials (DOLFIN) is a solution-processed additive patterning technique that utilizes a photosensitive colloidal nanomaterial ink. Here we engineer ligand-colloid interactions to be optically sensitive, allowing us to achieve micron-resolution patterning of metal oxide nanomaterials. Using this approach, we demonstrate the facile lithography of thick (>500 nm) metal oxide structures, which have a high-refractive index (n > 1.8) and are highly transparent. As a proof of concept, we manufacture diffractive optical elements (e.g. gratings) to show that significant phase modulation of light is attainable. Unlike polymer-based optical elements, our optical devices are more robust and can tolerate high temperatures due to its primarily inorganic content. We also show the patterning of conductive colloidal oxide nanomaterials (e.g. ITO), which facilitates its implementation as a transparent conductive electrode. These advances aim to facilitate the integration of colloidal nanomaterials in real-world applications.

We investigate nanoparticle film structures as components for daytime radiative cooling, using a generalized effective medium theory which is capable of recovering the Maxwell-Garnett, Bruggeman, and Coherent Potential mixing formulas. We show that, in all cases, nanoparticle fill fraction is a degree of freedom which can be used to improve free space coupling and spectrally tune the emissivity and absorptivity curves so as to optimize radiative emission within the atmospheric transmission window (8 – 14 μm). Based on this theory, we design different radiative cooling structures from composites of SiO₂ and Si₃N₄ nanoparticle films, which were chosen as the example emissive materials because of their strong absorption peaks within the atmospheric transmission window. Our results show that two-layer nanoparticle films of these materials outperform all thin-film analogs and are sufficient to achieve cooling performances comparable to leading reports in literature. Further research will explore how the low thermal conductivity of nanoparticle films could be used to further enhance daytime radiative cooling performance. This research supports the idea that simple nanoparticle films couple provide a viable method for designing radiative cooling structures, provided scalable methods are identified for nanoparticle fabrication.

The history of coloration has been proceeded by trials and development of many technologies for selectively splitting the light. Structural coloration, among them, has grown up as a potential candidate alternating traditional pigments/dyes coloration since their great durability, sustainable production, and fine color tuning by controlling materials and dimension. As this property, structural color facilitates the creation of security features for anti-counterfeiting. However, conventionally, most of them have used metal/dielectric materials to make a resonating system at a specific wavelength and polarization condition which demands complex fabrication processes such as e-beam lithography or ion beam lithography technology. Even though this system can be suitable to an ultra-high definition structural color display, it is limited to only a few millimeter scales or less. To apply into a realistic application such as banknote, luxury products, or customer goods, it is necessary to make it over centimeter scale with a flexible medium.

In this work, to break through these significant flexibility/scalability limitations, we developed polarization distinguishable covert display with ultrathin porous nanocolumns (PNCs) on a metal film by glancing angle deposition (GLAD) method. Particularly, the unique property of this covert display showed powerful functions which provide a hurdle to the inadvertent viewing of stored optical information without compromising their aesthetic. This structure with lossy PNCs on a metal film makes a non-trivial phase shift at the interfaces causing short reflection path and finally shows strong resonance [1]. Furthermore, the PNCs fabricated with GLAD produce anisotropic media and has different effective complex refractive index causing different resonating condition depending on polarization direction [2]. Using this concept, we designed polarization distinguishable color filters with a variety of materials and dimensions with rigorous coupling wave analysis (RCWA) method. In this process, we obtained an enlarged color range occupying over 80% of standard RGB area. In this color response, some showed a small color difference (ΔE), the other showed a large color difference following polarization angle. Additionally, we confirmed this result is originated from human eye’s spectral responses according to between tristimulus value and reflectance spectra having a single minimum. Using these results, we designed optical data label (i.e., QR code) which shows inadvertent viewing in the naked eye (under unpolarized light), on the other hand, this label can be selectively detected by polarized light. For practical application, we designed this label into daily consumer goods with the color matching between labels and products to avoid compromising aesthetics of products and successfully demonstrated onto wine bottleneck, food packaging with confirming the authentication process. As a multi-functional color display, we demonstrated a colorimetric detection. Due to the low complex refractive index of porous medium, PNCs are sensitive to subtle external environment change. Consequently, PNCs reveal a sensitive color change according to background refractive index or additional layer thickness change. Using this unique property, we presented colorimetric detection with external change for volatile organic compounds (VOCs) and humidity.

[1] M. A. Kats, R. Blanchard, P. Genevet and F. Capasso, “Nanometre optical coatings based on strong interference effects in highly absorbing media,” Nat. Mater. 12 (2012): 20–24.
[2] Y. J. Yoo, J. H. Lim, G. J. Lee, K. I. Jang, and Y. M. Song, "Ultra-thin films with highly absorbent porous media fine-tunable for coloration and enhanced color purity," Nanoscale 9 (2017): 2986-2991.

Controlling the propagation, absorption and emission of light has been a topic of fundamental interest in contemporary photonics research. In the context of controlling the emission of light from thermal sources, a central challenge is that of angular selectivity. Over the past two decades, a range of photonic strategies, including using surface phonon-polariton modes, have enabled anomalous angular ‘beaming’ of thermal radiation, albeit only at narrow ranges of wavelengths. However, thermal radiation is inherently a broadband phenomenon. We currently lack the ability to constrain broad-spectrum thermal emission over a narrow range of angles of incidence, but consistently over a broad range of wavelengths. This form of broadband thermal beaming would be a fundamentally enabling capability for a range of applications, including infrared imaging and camouflage, thermophotovoltaics and radiative cooling.

Here, we employ multiple epsilon-near-zero (ENZ) materials in a layered configuration to realize a broadband angularly selective thermal emitter. ENZ materials can fulfill high absorption and emission of incident light at wavelengths where the epsilon is near zero. Aluminum oxide (Al₂O₃), Silicon monoxide (SiO) and Silica (SiO₂) are all polariton materials with different ENZ wavelength. When combining them on top of each other into a multilayer thin film structure, we can obtain broadband near-blackbody emission from 7 to 11µm at a certain range of angles of incidence in the p polarization. We demonstrate this broadband angularly selective thermal emitter through theory, simulations, and an experimentally fabricated structure.

By employing transfer matrix method, we analytically solve the dispersion relation of this multilayer thin film structure and able to observe the EM field using the simulation tool. Following that, experimental results are compared with the analytical models with excellent agreement. The ENZ multilayer stack was coated on a Φ=100mm wafer via electrical beam evaporation method on CHA MARK 40. First, a 100nm Aluminum layer was deposited on the silicon wafer and then with 200nm Al₂O₃, 300nm SiO and 100nm SiO₂ on top of each other. Then the sample was characterized by a Fourier-transform infrared spectroscopy (FTIR)-Jacob 6100. The strongest emission occurs at an AOI around θ = 70° and can be tuned from 65°to 80° by just varying the thickness of the layers. Outside of this AOI range the multilayer film is highly reflective which allows a switch between emission and reflection mode by changing the AOI. The materials used and structure designed in our research are cheap and easy to fabricate and fully compatible with wafer-scale manufacturing. Overall, this thin film based ENZ approach opens a path towards broadband thermal beaming and angular selectivity at low cost, at large scale and with simple structures.

While selective emitters can lead to significant radiative cooling by reflecting sunlight and emitting heat to the surroundings, current materials solutions such as multilayer structures or metallic reflectors are inadequate for large-scale radiative cooling systems that require lightweight, flexible and low-cost materials. Commonly used coatings for scalable applications, including commercial pigment-embedded white paints, suffer from strong ultraviolet absorption from embedded high-refractive-index (n~2.5) TiO₂ particles and a low near-infrared reflectivity. The strong ultraviolet absorption induces severe photocatalytic degradation for the organic binder components in TiO₂-based paints. Integrating low refractive index particles such as SiO₂ microspheres can be used to avoid high ultraviolet absorption because the extinction coefficient of SiO₂ remains approximately zero throughout the ultraviolet/visible/near-infrared spectrums. Here we create a lightweight, flexible and scalable polymer coating by integrating controlled volume concentrations of hollow glass microspheres within a polydimethylsiloxane (PDMS) matrix since PDMS is a commonly used low-cost polymer with little solar absorption and easy accessibility. We utilize hollow SiO₂ microspheres instead of solid microspheres as they provide a higher interfaces-to-volume ratio and a much smaller density. Our size analysis shows that the diameter of the hollow glass microspheres ranges from 2 µm to over 40 µm and the shell thickness are roughly 0.05 times the diameter (0.1 µm to 2 µm). The prepared polymer coating offers a maximum mass density of 681 kg/m³ and the corresponding areal density of 0.885 kg/m². The polymer coating also shows excellent flexibility and scalability which is beneficial for large-scale applications. As the volume concentration of hollow glass microspheres increases from 0 to 70%, the average reflectivity in the visible spectrum accordingly increases from 0.06 to 0.92 while the emissivity in the mid-infrared remains above 0.85. We use computations based on rigorous coupled-wave analysis to explain the role of hollow glass microspheres in increasing the solar reflectivity. The high solar reflectivity is attributed to the strong backscattering of sunlight enabled by refractive index difference between PDMS matrix (n~1.4) and SiO₂ shell (n~1.45), as well as between SiO₂ shell and air void (n~1) inside the microsphere. Our computation based on the Finite-difference time-domain method shows that hollow glass microspheres with a hierarchical arrangement of diameters are more efficient in scattering the sunlight and thus the polymer coating composed of large numbers of such hierarchical systems randomly distributed in a PDMS matrix can provide high solar reflectivity. The high mid-infrared emissivity is attributed to multiple extinction peaks of PDMS in the mid-infrared wavelength caused by different vibrational modes of its molecular structures. Our outdoor temperature measurement shows that the polymer coating on a concrete surface can lead to a temperature reduction of 25°C during the day, which is also 9°C below ambient temperature when conduction and convection are limited. Our building energy analysis shows the potential impact of the polymer coating on common buildings in Phoenix and estimates annual cooling energy savings of 6-38 MJ/m². The demonstrated ultraviolet-resistant property shows that the polymer coating exhibits great reliability and thus outperforms commercially available white paints. The selectively reflective polymer coating material based on hollow glass microspheres presents optimal material solutions for large-scale radiative cooling applications in buildings.

The warm white light emission phemena from a single solid state incandescent light emitting device (SSI-LED) was reported by Kuo's group (1,2). This device is composed of a very large number of nano-resistors formed from the breakdown of a MOS capacitor with a nm-thick high-k gate dielectric on a p-type Si wafer. Light is emitted during the passage of the current thorugh the nano-resistors, i.e., following the Planck's law on balckbody emission. The light emission phenomena can last for more than 20,000 hours under the continuous operation condition in air without a passivation layer (3). The emitted light can be narrowed down to a narrow wavelength width near 850 nm (4), which is a potential light source in an on-chip optical interconnect system.

In this paper, the light emission phonomena over a SSI-LED has been simulated. The emission spectra from the thermal excitation of the nano-resistor at different temperatures with and without inclusion of an ITO or a-Si thin film filter are simulated and compared with that measured from the actual device. The device size effect on the distribution of nano-resistors and therefore the distribution of light dots is estimated and compared with that of the actual device, which will be correlated to the measured currrent density of the device.

(1) Y. Kuo and C.-C. Lin, Appl. Phys. Lett., 102, 031117 (2013).
(2) Y. Kuo, invited, “Solid State Incandescent Light Emitting Devices Made of IC Compatible Material and Fabrication Process,” IEEE Elec. Dev. Soc. News Letters, 22-2, 1-5 (2015).
(3) Y. Kuo and S. Zhang, AVS 63rd meeting, Abst. 1844, Nashville, TN, November 6, 2016.
(4) S. Zhang and Y. Kuo, ECS J. Solid State Sci. Technol., 6(4) Q39-Q41 (2017).

The use of nonthermal plasmas has emerged as an effective method for the synthesis of high-quality nanoparticles. Extensive study of nonthermal plasma reactors under low pressure conditions has been conducted which has yielded systems capable of producing nanoparticles with size tunable properties. Miniaturization to a microscale reactor allows for effective operation of a RF nonthermal plasma reactor at atmospheric pressure. The reduced scale of the reactor also provides the opportunity for combining nanoparticle synthesis and deposition into a single system for manufacturing with nanoparticles.
Presented here is our work on the additive manufacturing of silicon nanoparticles synthesized at atmospheric pressure with a nonthermal plasma. The plasma was generated with a 13.56 MHz RF power supply inside a glass capillary tube between two external ring electrodes. A silane precursor gas together with an argon background gas were flown to the reactor for the synthesis of silicon nanoparticles. The reactor was attached to a motorized system responsible for controlled deposition of the nanoparticles in predetermined geometries. TEM and XRD has shown that control between crystalline and amorphous particles can be achieved by controlling the applied power. Control over the nanoparticle size has also been demonstrated by varying the overall gas flow rate to the reactor. In efforts to increase the capability of this system for manufacturing processes we have investigated methods to control the deposition resolution. Spreading of the particles after exiting the reactor leads to wide linewidths which can limit the performance of this tool for manufacturing. Manipulation of the gas flow with aerodynamic lenses allows for reduced expansion of the particles and improved resolution. With this method we have been able to reduce the spot size to 1.3 times the size of the reactor tube outlet diameter.

Colloidal nanoparticles have been established as a diverse class of material with many interesting and useful properties. However, their practical application often requires some form of patterning procedure such as photolithography, ink-jet printing and nanoimprint lithography. Direct optical lithography of functional inorganic nanomaterials (DOLFIN) is a solution-processed additive patterning technique that utilizes a photosensitive colloidal nanomaterial ink. Here we engineer ligand-colloid interactions to be optically sensitive, allowing us to achieve micron-resolution patterning of metal oxide nanomaterials. Using this approach, we demonstrate the facile lithography of thick (>500 nm) metal oxide structures, which have a high-refractive index (n > 1.8) and are highly transparent. As a proof of concept, we manufacture diffractive optical elements (e.g. gratings) to show that significant phase modulation of light is attainable. Unlike polymer-based optical elements, our optical devices are more robust and can tolerate high temperatures due to its primarily inorganic content. We also show the patterning of conductive colloidal oxide nanomaterials (e.g. ITO), which facilitates its implementation as a transparent conductive electrode. These advances aim to facilitate the integration of colloidal nanomaterials in real-world applications.

Millions of years of evolution in the biological world has developed a plethora of micro- and nanoscopic photonic structures that are frequently superior to synthetic analogs. These biophotonic materials show interesting novel and tunable unforeseen properties with deliberately introduced disorder in their respective geometries and compositions¹. Over the last decade, photonic materials with tailored -i.e. with deliberately introduced- structural disorder have also attracted considerable interest in various optical applications due to their extended spectral and angular range of effectiveness². Most bio-inspired nanostructured devices or optical metamaterials designed to date have been demonstrated at small-scales using expensive top-down techniques. However, biological structure formation in nature utilizes several bottom-up self-assembly approaches for manufacturing hierarchical mesoscopic nanostructures (100 – 550 nm) with immense diversity. Despite recent efforts aimed at increasing top-down fabrication writing speed, alternative routes based on self-assemblies still possess major advantages for industrial implementation of disordered structures as they allow rapid processing over large areas (>>cm²).
In this communication, we show that up-scalable polymer blend lithography technique can be used as a versatile platform for fabricating 2D planar, disordered nanostructures that can be exploited in both top-down and bottom-up strategies. Polymer blend lithography utilizes the polymer-phase separation process following nature’s way of forming nanostructures. The tailored disorder is achieved here by adjusting the process parameters (polymer blend composition and deposition conditions), enabling us to tune the morphology and the spatial distribution of the nanostructures produced, and in turn their light management properties.
First, we use our approach to pattern a resist etching mask, employed for transferring disordered nanopillars by dry etching (top-down route) onto a Fabry-Perot-resonator-based intraocular pressure (IOP) sensor for glaucoma management³. The nanostructure integration onto the IOP sensor led to a 2.5-fold improvement in readout angle allowing easy handheld monitoring and in a one-month in vivo study conducted in rabbits, showed a 3-fold reduction in IOP error and 12-fold reduction in tissue encapsulation and inflammation, compared to an IOP sensor without nanostructures. Second, we demonstrate that similar structures can serve as a template in a bottom-up configuration, whereby aluminum thin film is directly deposited into the disordered nanoholes to form scalable plasmonic metasurfaces⁴. These metasurfaces generate hybrid multipolar lossless plasmonic modes resulting in a broadband fluorescence-enhancement factor above 1000 for visible wavelengths with respect to glass chips commonly used in bioassays. Using the metasurface and a multiplexing technique involving three visible wavelengths, we successfully detected three biomarkers, insulin, vascular endothelial growth factor, and thrombin relevant to diabetes, ocular and cardiovascular diseases, respectively, in a single 10-μL droplet containing only 1 femtomole of each biomarker.

References
1. Kolle, M. and Lee, S., Progress and opportunities in soft photonics and biologically inspired optics. Advanced Materials 30(2), 2018.
2. Wiersma, D.S., Disordered photonics. Nature Photonics 7(3), 2013.
3. Narasimhan, V., Siddique, R.H., Lee, J.O., Kumar, S., Ndjamen, B., Du, J., Hong, N., Sretavan, D. and Choo, H., Multifunctional biophotonic nanostructures inspired by the longtail glasswing butterfly for medical devices. Nature Nanotechnology 13(6), 2018.
4. Siddique, R.H., Kumar, S., Narasimhan, V., Kwon, H., and Choo, H., Aluminum Metasurface With Hybrid Multipolar Plasmons for 1000-Fold Broadband Visible Fluorescence Enhancement and Multiplexed Biosensing. ACS Nano, 2019.

During the last decade, III-V semiconductor nanowires (NWs) have received great research interest due to their promising potential for next-generation electronic, optoelectronic, and photonic devices. One of the main challenges for epitaxially-grown III-V NWs, however, is their high manufacturing cost, which can be divided to two individual categories: (1) the cost of starting III-V substrates, and (2) the cost of pre-epitaxy fabrication associated with substrate patterning. Large-scale integration of III-V NW-based technologies requires novel nanofabrication strategies that mitigate both cost streams. Wafer costs can be substantially reduced simply through growth of III-V NWs on foreign substrates such as silicon or graphene, instead of on bulk III-V wafers. One strategy for simultaneously reducing fabrication complexity and pre-growth processing costs is to reuse Si substrates with existing predefined growth masks for selective-area epitaxy (SAE) of III-V NW arrays. Here, we present wafer-scale, bottom-up SAE growth of InAs NW arrays and fabrication of flexible NW-based infrared (IR) photodetectors realized through delamination of embedded NW arrays and reuse of Si substrates with predefined growth masks. Silicon (111) substrates are patterned by conventional photolithography to obtain pores with 0.5 µm diameters and 1 µm pitch in SiO₂ growth masks of 55 nm thickness. Next, InAs NW arrays are grown via the SAE approach using metalorganic chemical vapor deposition (MOCVD). Trimethyl-indium (TMIn) and arsine (AsH₃) were used as precursor gases for the supply of In and As growth species, respectively, at a set-point growth temperature of 700 °C. A two-step growth technique is introduced, whereby a separate nucleation step using a TMIn flow rate of 16 μmol/min precedes a NW growth step at a TMIn flow rate of 0.77 μmol/min. This procedure is necessary for optimization of pore occupation toward a global NW yield of >85%, with NW aspect ratios of ~15. The InAs NW arrays are embedded in various polymer membranes and mechanically delaminated from their growth substrates, then bonded to carrier substrates for subsequent device fabrication. The polymer-embedded NW array exfoliation process leaves NW bases with heights of ~50 nm below the fracture plane attached to the growth surface. Consequently, five different substrates restoration processes are investigated prior to substrate-reuse. It is observed that the best results for re-growth of daughter generation InAs NW arrays on reused Si substrates can be realized when no additional wet etching, chemical treatment, or mechanical polishing steps are introduced between subsequent re-growth runs, as the remaining NW bases serve as SAE growth sites. Direct re-growth on reused Si substrates results in vertical extension of existing NW bases and growth of InAs NWs with aspect ratios >80 under the same growth conditions as the primary run with comparable global NW yield. For fabrication of flexible IR photodetectors using both primary and subsequent generation InAs NW arrays, a unique process flow is reported, which enables NW transfer yields of ~100% and preserves the original position and orientation of as-grown NWs. This work demonstrates that multiple generations of InAs NW-based devices can be fabricated through reuse of a single, templated Si substrate, without introduction of intermediate substrate restoration processes such as wet etching or chemical mechanical polishing. This approach can serve as an effective strategy toward growth and fabrication of large-area, high-efficiency, low-cost, flexible, and wearable device applications in III-V nanoelectronics, optoelectronics, and photovoltaics.

Planar optical elements with efficient light control at the nanoscale can be designed based on transdimensional photonic lattices that operate in the translational regime between two and three dimensions. Such transdimensional lattices include 3D-engineered nanoantennas supporting multipole Mie resonances and arranged in the 2D arrays to harness collective effects in the nanostructure [1]. Optical antennas made out of van der Waals material with naturally-occurring hyperbolic dispersion is a promising alternative to plasmonic and high-refractive-index dielectric structures in the practical realization of nanoscale photonic elements. The antenna made out of hexagonal boron nitride (hBN) possesses different multipole resonances enabled by the supporting high-k modes and their reflection from the antenna boundaries. The full range of the resonances is demonstrated for the hBN cuboid antenna, a decrease of reflection from the array, and highly directional resonant scattering from antennas pairs. We show that transdimensional lattices consisting of resonant hBN antennas in the engineered periodic arrays have great potential to serve as functional elements in ultra-thin optical components and photonic devices.
We show that a periodic array of particles with a large imaginary part of their permittivity (LIPP) can support well-defined modes localized within the particle, with the spectral position of the modes mainly defined by the array period. It has been known for a long time that a non-zero imaginary part of the permittivity enables propagating surface modes at the interface of materials with positive permittivities, known as Zenneck modes. Recently, it has been demonstrated that transition metal dichalcogenides (TMDC) layers support different kinds of propagating Zenneck waves defined by the layer thickness and interfaces with air and substrate [2]. Here, we show that localized Zenneck modes are possible on subwavelength particles and that the particles can serve as antennas for direction light scattering, reflection suppression, and Kerker-effect applications. To illustrate a possibility to control light, we show that simultaneous excitation of the several particle multipole resonances (electric dipole and quadrupole) allows for achieving significant reflection suppression and observation of the generalized lattice Kerker effect [3]. Materials with LIPP are common in nature, including TMDCs and lossy metals, titanium, chromium, and others.
Acknowledgment. This material is based upon work supported by the Air Force Office of Scientific Research under Grant No. FA9550-19-1-0032.

[1] V. E. Babicheva, "Multipole Resonances in Transdimensional Lattices of Plasmonic and Silicon Nanoparticles," MRS Advances 4, 713-722 (2019).
[2] V. E. Babicheva, S. Gamage, L. Zhen, S. B. Cronin, V. S. Yakovlev, Y. Abate, "Near-field Surface Waves in Few-Layer MoS2," ACS Photonics 2018, 5, 2106.
[3] V. E. Babicheva and J. Moloney, "Lattice Zenneck modes on subwavelength antennas," Laser & Photonics Reviews 13, 1800267 (2019).

Low-cost, large-area thermal management is desirable for the operation of many modern technologies including smart clothing, electronic circuits, building environments, and outdoor equipment to control heat flow. Inspired by the space blanket and the dynamic skin of cephalopods, we have demonstrated a large-area, highly uniform, low-cost nanostructured material with tunable thermoregulatory and infrared properties. We have implemented scalable nanofabrication processes to achieve a material with an area > 500 cm², a > 40 % change in infrared transmittance and reflectance, and a dynamic environmental setpoint temperature window of ~ 8 °C. Due to characteristics of scalability and associated figures of merit, our material affords new scientific and technological opportunities not only for adaptive optics and thermoregulation but also for any platform that would benefit from dynamic control of infrared radiation and heat.

We report the relationship between measures of the crystal quality of self-assembled colloidal crystal films and the intensity of their structural color. Structural color arises from geometric diffraction; it has potential applications in optical materials because it is more resistant to environmental degradation than coloration mechanisms that are of chemical origin. Structural color can be produced from self-assembled films of colloidal particles. To study the connection between crystal film quality and structural color intensity, we investigate the peak intensity and peak width of structural color reflection spectra as a function of defect density, film thickness and impurity concentration using a combined experimental and computational approach. Polystyrene microspheres are self-assembled into defect-laden colloidal crystals via solvent evaporation. Colloidal crystal growth via sedimentation is simulated with molecular dynamics and the reflection spectrum of simulated crystals is calculated using the finite-difference time-domain method. We examine the impact of several commonly observed defect types (vacancies, stacking fault tetrahedra, planar faults, and microcracks) on structural color peak intensity. We find that the reduction in peak intensity scales with increased defect density; stacking fault tetrahedra have a particularly detrimental effect. The thickness profiles are characterized by cross-sectional scanning electron microscopy and profilometry. The reflective spectral response of the colloidal crystals is measured with a white light spectrophotometer. Our results show that reflectance of structural color increases as a function of the crystal thickness, until a plateau is reached at thicknesses greater than about 10 μm. The plateau reflection is 83.2% ± 2.8%; this value is significantly less than the 100% reflectivity predicted for a fully crystalline material; the difference is caused by defects present in the experimental crystal structures. Finally, we manipulate the defect structures of colloidal crystals by introducing differently sized particles that function as impurities. As the concentration of the impurity is increased, the measured intensity of structural color reflection decreases by amounts that we correlate with the observed microstructure. These findings can guide the design of optical materials which tune the intensity of structural color.

With the promise of novel optoelectronic devices using black phosphorus (BP), there is a need for a large-scale synthesis method for BP compatible with on-chip integration. Herein, we attain large scale mechanochemical conversion of red phosphorus (RP) to black phosphorus (BP) via high energy ball milling. During the milling process, the collisions of hardened steel media rapidly compress amorphous RP into crystalline, orthorhombic BP flakes, with a conversion yield >95% for up to 5 g of bulk BP powder. Commercial scale-up of BP production can be extrapolated by empirically derived conversion kinetics from lab scale, high energy ball mills. RP’s milling conversion kinetics, as monitored via ex situ XRD, shows a sigmoidal behavior best described by the Avrami rate model. For <60% conversion, BP is rapidly formed at the expense of RP following an autocatalytic behavior with induction times proportional to the milling intensity. At ≥60% conversion, milling conversion rate suddenly decreases, indicative of a growth mechanism change with presumably a higher activation energy, thereby requiring higher impact energies for complete conversion. Further, by comparing the specific milling dose, namely, the energy input required for the phase transformation to reach completion, versus the conversion progress, we can determine the optimum milling conditions. We therefore ascertain the BP specific milling dose (50 kJ/g) and minimum impact energy (25 mJ/impact), both necessary to scale up production and minimize the power consumed in a tumbler milling vessel. Finally, the as-synthesized BP powder exfoliates more readily than single crystal BP sources in solution, as revealed by concentrations obtained from UV-Vis absorption under the same exfoliation conditions. The milled, converted BP solution is roughly 6-fold the concentration of the single crystal solution. High energy ball milling is demonstrated as a large scale, viable synthesis route for BP.

Bottom up self-assembly of block copolymers (BCPs) offers a scalable, economical alternative to lithography for producing large area, highly uniform nanopatterns required for engineering materials with tailored light-matter interactions. At their typical scales (~20-50 nm feature spacing), thin film BCP nanopatterns may be used to generate deep subwavelength like nanotextures that reduce visible reflectance to < 1% [1,2], or nanoholes that improve light trapping in photovoltaics at carrier selective rear contacts [3]. Nevertheless a greater latitude for optical property engineering would be possible using self-assembled BCP patterns at length scales commensurate with visible wavelengths of light in order to exploit optical diffraction and resonance effects. This is prohibited by exponentially slower ordering kinetics in the ultrahigh higher molecular weight BCPs (> 1000 kg/mol) required for this scaling. Here we report dramatically accelerated ordering afforded by blending in very low molecular weight (~3 kg/mol) homopolymer plasticizers. These formulations enable a 10× reduction in the time required to achieve ordered nanopatterns with periods > 200 nm. Subsequent reactive ion etching demonstrates that these nanopatterns are readily transferrable to other materials. Exemplar applications such as structural colors made possible using these self-assembled patterns will be described.

References:
[1] A. Rahman, et al., Nat. Commun. 6, 5963 (2015).
[2] A.C. Liapis, et al., Appl. Phys. Lett. 111, 183901 (2017).
[3] M.J. Hossain, et al., Proc. SPIE 11089, 110890P (2019).

Research performed at the CFN and NSLS-II, U.S. DOE Office of Science Facilities at Brookhaven National Laboratory under Contract No. DE-SC0012704.

Single molecule (SM) detection represents the ultimate limit of chemical detection. Over the years, many experimental techniques have emerged with this capacity. Yet, SM detection and imaging methods require large, reproducible spectral datasets in order to benefit from chemometric methods. We will present a scalable chemical assembly platform for plasmonic and metamaterial architectures that yield reproducible optical response. For example, surface enhanced Raman scattering spectroscopy (SERS), with extensive applications in biosensing, is demonstrated to be particularly promising because Raman active molecules can be identified without recognition elements and is capable of SM detection. Yet quantification at ultralow analyte concentrations requiring detection of SM events remains an ongoing challenge, with the few existing methods requiring carefully developed calibration curves that must be redeveloped for each analyte molecule. We will present work where we demonstrate that using 2-dimensional physically activated chemical (2PAC) self-assembly, we can achieve reproducible enhancement factors of 10⁹ over 1 cm². 2PAC crosslinks nanoparticles into discrete assemblies with a small molecule linker, where the crosslinking reaction is driven by electrohydrodynamic flow at on a substrate surface. The hotspots observed in a 2PAC fabricated assembly have gap spacing is 0.9 nm, corresponding to the length of the chemical crosslinker. Large area SERS maps are acquired from these 2PAC assembled surfaces from analyte dissolved in water and even complex biological media.

One of the most important applications of contemporary machine learning, i.e., image analysis, has remained relatively untouched by the SERS community. We will show that a combination of large area and reproducible SERS substrates with a CNN model is used to address the long standing challenge in SERS: quantification of sub-nanomolar analyte concentrations. A convolutional neural network (CNN) model when applied to bundles of reliable SERS spectra yields a robust, facile method for concentration quantification down to 10 fM using SM detection events. The demonstrated limit of blank (LOB) is 1 fM for Rhodamine 800, with a limit of quantification (LOQ) of 10 fM. Furthermore, the model’s predicted concentrations have an average r² value of 0.958 over 6 orders of magnitude as determined by k-folds cross validation. A key advantage of the analysis is its compatibility with transfer learning, the use of large datasets to train a model and then generalize this pretrained model to new molecules quickly with significantly smaller datasets. The uniformity of enhancement factor is essential for our approach, as large variance in the dataset not only increases the variance in predictions, but also requires more data for convergence and prevents a transfer learning approach. Generalization of the CNN model to other analytes using transfer learning is demonstrated with methylene blue. Transfer learning achieves good results with as few as 50 8x8 pixel maps. Thus entire datasets are acquired quickly, requiring just 5.3 minutes of total laser exposure time per concentration to train a robust model that is much faster and more scalable than required for building SM concentration regression models. This demonstrates a proof of concept for hyperspectral CNN image analysis of SERS, which could be broadly applicable within SERS, especially in the biological setting where classification of images could improve analysis of bacteria that are already imaged with SERS. We will also present results showing this approach is able to differentiate susceptible and resistant bacterial strains for rapid antimicrobial susceptibility tests.

The next generation of functional coatings will likely be composed of nanomaterials. At the laboratory scale nanomaterial-based coatings have shown the potential to improve performance and enable completely new functionality in applications like thermal barrier coatings on high-temperature parts, active layers in electronic devices, anti-reflective coatings for PV modules, or biocompatible coatings for medical applications. In all cases, these coatings should be able to be formed over large areas at commercial production rates and have a uniform and controllable thickness, and their constituent particles should retain their nano-properties.
Present nanomaterial films developed in laboratories are often unable to be reproduced on a commercial scale; the manufacturing methods are inherently nanomaterial, substrate, and application specific. This specificity results in manufacturing processes that are expensive or offer limited utility. We propose an advanced nanomaterial film manufacturing technology based on Aerosol Impact Driven Assembly (AIDA) that overcomes these deficiencies.
AIDA begins by aerosolizing a nanomaterial using any desired technique, from atomizing a nanoparticle-laden solution to feeding nanoparticle-precursor gas into a plasma. The aerosolized material is fed into the AIDA system, which consists of two chambers separated by a slit-shaped nozzle, with the bottom chamber held under vacuum (< 1 Torr). The nanomaterial is accelerated to a velocity of several hundred meters/second as it and the aerosol background gas are forced through the nozzle, resulting in the formation of a curtain of nanoparticles directed into the downstream chamber. A substrate is passed through the curtain, and the nanomaterial collides with and adheres to the substrate, forming a thin coating.
AIDA’s unique approach to film formation enables independent control over nearly every aspect of the coating’s morphology. As a dry spray technique, there are no inherent limitations on the thickness. Coatings ranging in thickness from a sub-monolayer to >1mm have been produced with dynamic deposition rates as high as 6 mm-m/min. By adjusting the particle impact velocity, the porosity of the coating can be controlled between 5% and 95%. This enables control over film properties like the refractive index, heat transfer coefficient, and dielectric constant. By changing the angle of impaction, the surface roughness of the film can be tuned between 0 nm and >100 nm allowing engineers to tune the effective surface energy of the coating. Combining this intimate control of film morphology with the wide range of nanomaterials compatible with AIDA allows engineers to produce complex, multi-layered, multi-component, and multi-functional films using a single platform.
As a case study, we will showcase an AIDA system capable of depositing on 14”x24” substrates that uses an in-situ non-thermal plasma reactor to synthesize metal-oxide nanoparticles (SiO₂, TiO₂, and ZnO) and produce a multi-layered coating that is simultaneously highly photocatalytic (self-cleaning) and anti-reflective for use in PV module glass. The composition of each layer will be tuned to maximize the photocatalytic self-cleaning effect. The thickness and refractive index of each layer will be precisely tuned to optimize its anti-reflective properties and provide a 3% transmittance gain compared to uncoated glass. The surface roughness of each layer will be tuned to promote adhesion and provide the desired water contact angle on the top surface to enhance self-cleaning.