Hybrid materials combining organic chromophores in inorganic matrices have been investigated over the past 20 years as extremely interesting systems for optical and non-linear optical applications, such as screens, filters, sensors, optical protections, modulators. In this perspective, the synthesis of monolithic siloxane-based hybrid materials highly doped by various type of organic or organometallic chromophores was achieved with a new protocol based on a fast gelation reaction. Besides the synthesis protocol, the optical properties of these hybrid materials (absorption, luminescence, nonlinear absorption) will be discussed. The impact of the structure and the chemical composition of the matrix on the optical properties will be emphasized during this presentation. Furthermore, the possibility of using the plasmon effect of metal nanoparticles to magnify the nonlinear optical properties of the chromophores will be discussed.
References :
R. Zieba, C. Desroche, F. Chaput, M. Carlsson, B. Eliasson, C. Lopes, M. Lindgren, S. Parola, Adv. Funct. Mater., 2009, 19, 235.
O. Maury, A. D&’Aleo, A. Picot, S. Brasselet, P.L. Baldeck, F. Chaput, S. Parola, C. Andraud, J. Lumin., 2013, doi:10.1016/j.jlumin.2011.09.039
D. Chateau, F. Chaput, C. Desroche, C. Lopes, S. Parola, 2010, EP 10305377.3-2111 (2010).
D. Chateau, F. Chaput, C. Lopes, C. Brännlund, J. Öhgren, M. Lindgren, N. Djourelov, P. Nedelec, C. Desroche, B. Eliasson, T. Kindhal, F. Lerouge, C. Andraud, S. Parola, ACS Appl. Mater. Interfaces, 2012, 4, 2369.

The highly sensitive and molecule specific technique of surface-enhanced Raman spectroscopy (SERS) generates high signal enhancements via localized optical fields on nano-scale metallic materials, which can be tuned by manipulation of the surface roughness and architecture at the sub-micron level. Carbon nanotubes (CNTs), on the other hand, have been, concurrently, a topic of extensive research and applications, in particular because of their extraordinary electrical, thermal and mechanical properties. However, the application of CNTs for plasmonics including SERS has been only recently beginning to gain interest. We generate low-cost gold functionalized vertically-aligned carbon nanotube forests (VACNTs), which are systematically optimized for their performance as straightforward SERS nano-platforms. Modeling of the VACNT-based SERS substrates reveals consistent dependence on structural parameters as observed experimentally. The created nanostructures span over large substrate areas, are readily configurable and yield uniform and reproducible SERS enhancement factors. These large surface area substrates demonstrate remarkable SERS enhancement factors, up to 10^7 due the high concentration of hot-spots provided by the VACNT forests. Gold-coated small diameter VACNTs forests patterned into pre-designed pillar structures are further utilized for duplex detection demonstrating that patterned VACNTs arrays can act as straightforward and cost-effective substrates for high-throughput multiplex SERS detection. Since vertically oriented CNTs exhibit functionalities such as electrical conductivity and unique adsorption properties, these can be further harnessed in their development as novel chemical and bio-sensing platforms. Based on optimization and developments undertaken, this work also paves the way towards construction and development of fully portable system suited for real time detection focused on areas with a point-of-care need. The unique properties of CNTs, which can be synergistically utilized in VACNT based substrates and patterned arrays, can thus provide new generation platforms for SERS detection.

Illumination of light into the plasmon bands of noble metal nanostructures creates large electric fields that decay away from the surface. Placement of molecules at defined distances from the surface can lead to enhancement or quenching of photophysical processes in molecules due to the electric field. In this talk we will summarize our recent work placing chromophores near the surface of gold nanorods, and we will explore molecular fate for on- and off-resonant plasmon excitation.

One focus of our research program involves the study of sensitized triplet-triplet annihilation (TTA) or triplet fusion (TF) in solution using highly photostable metal-organic chromophores in conjunction with energetically appropriate organic molecules with large singlet-triplet gaps. Selective excitation of the long-wavelength absorbing sensitizer efficiently generates long-lived triplet states that serve as energy transfer donors. In the presence of appropriate molecular acceptors, diffusion controlled triplet-triplet energy transfer takes place, producing the excited triplet state of the acceptor while regenerating the ground state of the sensitizer. When sufficient numbers of the sensitized triplets are produced, TTA takes place which results in either frequency upconverted light or the formation of desired chemical products. Various combinations of donor and acceptor have been explored and data will be presented on a number of these systems spanning light conversions ranging from the near-visible to the near-IR. This presentation will also describe many examples of upconversion phenomena realized in solid-state polymeric materials along with emerging classes of acceptor/annihilator chromophores. TF processes will be shown to operate at high efficiencies with concomitant linear incident power density response, demonstrated in both theory and experiment using non-coherent photons.

Photon upconversion based on sensitized triplet-triplet annihilation (TTA) continues to emerge as a promising low-power initiated wavelength-shifting technology. It is a nonlinear process involving multiple diffusion controlled energy transfer steps, ultimately resulting in the generation of delayed singlet fluorescence that is of higher energy than the incident light. In this presentation, recent work related to using the red-light absorbing platinum(II) tetraphenyltetranaphthylporphyrin (PtTPTNP) as a triplet sensitizer with yellow emitter rubrene as the acceptor in both solution and polymer films will be described. In deaerated toluene solution, stable and high quantum efficiency (Phi;_UC=0.0663±0.006) was observed. Stern-Volmer analysis of the photoluminescence quenching of PtTPTNP by rubrene yields a Stern-Volmer constant of 11819 M^-1. The quadratic-to-linear power dependence of the upconverted rubrene fluorescence with respect to 690 nm incident laser power density was observed to conform to two extreme kinetic limits (weak and strong annihilation regimes), which demonstrate that the upconverted emission intensity is directly proportional to incident light power in the strong annihilation limit whereas a quadratic dependence is predicted in the weak annihilation limit.

The polycrystalline and single crystalline semiconducting degenerated layered tellurides and selenides, pure and doped were prepared and studied. The plasma frequency and density of free carriers (le; 10 up 21/cm up 3) can be tuned by doping and non-stoichiometry in relatively broad spectral region. Real part of permittivity of studied materials can be negative in a part of the spectrum. The studied crystals and films can be applied as plasmonic materials in near and mid IR spectral region.

Near-infrared (NIR) organic light-emitting devices (OLEDs) could serve as a new class of NIR light sources offering advantages such as light weight, low power consumption, and compatibility with large area and flexible substrates and thus have attracted much attention due to their potential applications in telecommunications, displays and bio-imaging. However, NIR OLEDs typically require sophisticated multilayer structures and low-work-function cathodes to optimize device efficiencies, influencing their competitiveness with other solid-state NIR emitting technologies. Compared with conventional NIR OLEDs, solid-state NIR light-emitting electrochemical cells (LECs) possess several promising advantages. NIR LECs require only a single emissive layer, which can be easily processed from solutions, and can conveniently use inert electrodes, e.g., Au and Ag. Ohmic contacts near electrodes induced by electrochemical doping of the emissive layer facilitate carrier injection. Thus, a single-layered NIR LEC device can be operated at very low voltages (close to Eg/e, where Eg is the energy gap of the emissive material and e is elementary charge) with balanced carrier injection, giving high power efficiencies.
In this work, we report phosphor-sensitized fluorescent NIR LECs utilizing a phosphorescent cationic transition metal complex [Ir(ppy)₂(dasb)]⁺(PF₆^-) (where ppy is 2-phenylpyridine and dasb is 4,5-diaza-9,9&’-spirobifluorene) as the host and two fluorescent ionic NIR emitting dyes 3,3'-diethyl-2,2'-oxathiacarbocyanine iodide (DOTCI) and 3,3prime;-diethylthiatricarbocyanine iodide (DTTCI) as the guests. Photoluminescence measurements show that the host-guest films containing low guest concentrations effectively quench host emission due to efficient host-guest energy transfer. Electroluminescence (EL) measurements reveal that the EL spectra of the NIR LECs doped with DOTCI and DTTCI center at c.a. 730 and 810 nm, respectively. These NIR LECs can be operated under bias voltages as low as 2.4 V. Moreover, the DOTCI and DTTCI doped NIR LECs achieve peak external quantum efficiency (EQE) up to 0.80% and 1.24 %, respectively. The device efficiencies achieved are among the highest reported for NIR LECs and thus confirm that phosphor-sensitized fluorescence is useful for achieving efficient NIR LECs. This technique is useful for NIR devices in power-effective applications.

White organic light-emitting diodes (OLEDs) have attracted much attention because of their potential applications in flat-panel displays and solid-state lighting. Compared with conventional white OLEDs, solid-state white light-emitting electrochemical cells (LECs) possess several promising advantages. They generally require only a single emissive layer, which can be easily processed from solutions, and can conveniently use inert electrodes. Electrochemical doping regions of the emissive layer near electrodes induce ohmic contacts for carrier injection. As a result, a single-layered LEC device can be operated at very low voltages (close to Eg/e, where Eg is the energy gap of the emissive material and e is elementary charge) with balanced carrier injection, giving high power efficiencies. A host-guest strategy, i.e., utilizing an emissive layer composed of a blue-emitting host doped with a red-emitting guest was commonly used in LECs to generate white EL emission. However, carrier trapping effect due to offsets in energy levels between the host and the guest results in significant voltage-dependent EL spectra, precluding their potential applications in solid-state lighting.
In this work, we report efficient and color-stable white LECs by combining single-layered blue-emitting LECs with red-emitting color conversion layers (CCLs) on the inverse side of the glass substrate. By judicious choosing of the red-emitting dye doped in CCLs, good spectral overlap between the absorption spectrum of the red-emitting dye and the emission spectrum of the blue-emitting emissive material results in efficient energy transfer and thus sufficient down-converted red emission at low doping concentrations of the red-emitting dye in the CCLs. Low doping concentration is beneficial in reducing self-quenching of the red-emitting dye, rendering efficient red emission. EL measurements showed that peak external quantum efficiency and the peak power efficiency of the white LECs employing red CCLs reach 5.93% and 15.34 lm/W, respectively, which are among the highest reported for white LECs. These devices exhibit almost bias-independent white EL spectra, which are required for practical applications, due to nondoped emissive layers. These results reveal that single-layered blue-emitting LECs combined with red-emitting CCLs are one of the potential candidates for efficient and color-stable white light-emitting devices.

Dye-sensitized solar cell (DSSC) has been a hot research topic for last two decades. There were steep increase in energy conversion efficiency, and large advance in stability. However, many factors that impede the better performance of DSSCs still exist, and charge recombination is one of them.
Semiconductors with one-dimensional structure attracted many researchers, because random-walking electrons are confined to move directly to the outer circuit in 1-D semiconductors. Among many candidates, TiO2 nanotubes (NTs) were most frequently used in DSSCs, because they can be easily fabricated by a simple method, potentiostatic anodization. Charge recombination decreased in NT applied DSSCs, when compare to NP-used DSSCs.
However, TiO2 NTs had bad influence on energy conversion efficiency, because the surface area of nanotubes is much less than that of nanoparticles. Smaller amount of dyes were adsorbed on NTs, and the cell performance regressed. For this reason, there were many attempts to increase the roughness of the TiO2 NTs.
In this research, structural engineering of TiO2 NTs was conducted to increase the surface area, and the amount of loaded dyes. Voltage profiles with patterns at the anodization process led to the production of TiO2 with patterned structure. When these structurally engineered NTs were applied to the DSSCs, energy conversion efficiency increased, due to the rise in the amount of adsorbed sensitizers.

A chromonic liquid crystal (CLC) polarizer was fabricated by coating the self-assembly of lyotropic CLCs and the subsequent photo-polymerizing processes. Their molecular packing structures and optical behaviors were investigated from using the combined techniques of microscopy, scattering and spectroscopy. To stabilize the oriented Sunset Yellow FCF (H-SY) films and to minimize the possible defects generated during and after the coating, acrylic acid (AA) was added and photo-polymerized. The polymer-stabilized H-SY films showed good mechanical and chemical stabilities without sacrificing a high polarizability. Additionally, patterned polarizers were fabricated by applying a photomask during the photo-polymerization of AA, which may open new doors for the practical applications in electro-optic devices. This work was mainly supported by the Human Resource Training Project for Regional Innovation and the Converging Research Center Program (2012K001428) of Korean government.

Enaminones are chemical substances that present a conjugated system of the type N-C=C-C=O with different geometric forms. A particular class of enaminones are the azo-enaminones which are obtained by incorporation of an azo group to the N-C=C-C=O conjugated system. The presence of an azo group plays an important role on the electronic properties of the system. We have calculate the dipole moment, linear polarizability and second hyperpolarizability of an azo-enaminones crystal using a hybrid procedure that mixes a supermolecular approach and an interactive electrostatic polarization scheme. Within this approach, the polarization effects are taken into account by assuring the convergence of the properties of interest within the polarization field created by the surrounding molecules, treated as point charges. In the interactive procedure, the closest 249 asymmetric units were taken into account by replacing all atoms with point charges. The simulation was carried out using a cluster of 5x5x5 unit cells. Our results show that in the presence of the embedded charges, the value of dipole moment increases by 11% while the static values of linear polarizability decreases by 5%, compared to the isolated molecule values. All calculations were performed at MP2/6-31+G* level of calculation using the Gaussian 09 suite of programs.

For the purpose of development of highly energy-efficient light sources, one needs to design highly efficient green, red and yellow phosphors, which are able to absorb excitation energy and generate emissions. In this contribution, we present our results on producing some efficient phosphors with improved luminescence properties.The effects of zinc on the zinc-doped CaTiO3:Eu3+ phosphors have been investigated by varying the zinc concentrations. X-ray powder diffraction (XRD), SEM and Fourier transform infrared spectroscopy (FTIR) characterizations were studies for their structural, morphology and functional group analysis.The variation of zinc concentration in fluencies the crystallinity and morphology of the phosphors. The crystallinity and PL intensity increase as zinc content increases from 0 to 25mol%. (Zn,Ca)TiO3:Eu3+ at 25mol% Zn exhibits enhanced PL intensity compared to CaTiO3:Eu3+. Moreover, there demitting phosphor developed in this study can be very effectively excited at the wavelengths of 396nm. We think that (Zn,Ca)TiO3:Eu3+ can be used as a complementary phosphor in there red region for the white LEDs.

Conventional white-LEDs (WLEDs) use yellow phosphors to convert blue light source. Phosphors are mostly mounted on the LED chip via organic binders such as epoxy or silicone resin. However, due to weak thermal and photonic stability of organic binders, color coordination can be easily changed when they are used in high power LED restricting long-term stability. Phosphor-in-glass (PiG) uses glass matrix to embed phosphors and becomes a strong candidate for next generation color converter for high power WLEDs along with phosphor ceramics. PiG is highly promising due to its relatively low sintering temperature and versatility to various phosphors when compared to phosphor ceramics. However, it is essential to find proper glass frit material to embed phosphors to realize the PiG. The glass should show high transparency after the sintering and the sintering temperature should be low enough to avoid thermal degradation of phosphors during the sintering process. PbO-SiO2 based glasses mostly showed high transparency and reasonable viscosity at low temperature (< 600 oC) but cannot be used in practice due to its high lead contents. Thus, in this study, we investigated SiO2-B2O3-RO(R= Ba, Zn) glass system as a Pb-free glass frit material. While examining various compositions, we found a glass composition which show high visible transparency after the sintering at 750 °C for 30 min. When the glass was mixed with YAG:Ce3+ yellow phosphor, it showed good wetting behavior to phosphors without noticeable interaction. By simply changing the thickness of the PiG plate or mixing ratio of glass to phosphor, CIE chromaticity coordinates of the LED could be easily controlled. Thermal quenching behavior also proved its higher stability than conventional silicone resin. Structural modification of PiG plate has been also examined in order to improve color rendering index.

Near Infrared (NIR) technologies have rapidly emerged due to their numerous potential applications. In particular, NIR detection based on photo-electric conversion with extremely high sensitivity has become in great demand for light weight, miniaturized, portable sensing devices. A majority of the previous works have vertically stacked diode type architectures in which carriers photo-generated in NIR absorbing layer effectively moved to the corresponding electrodes, making the rectified dark current much enhanced.
Their inferior device performances to inorganic ones, however, make them hardly applicable for reliable devices. Alternatively, high performance NIR absorbing materials such as colloidal semiconducting quantum dots were embedded in pliable polymer matrix. To facilitate transfer of the carriers generated in the quantum dots by photo-excitation to the electrode, electrically conducting multi wall carbon nanotubes were employed with the quantum dots adhering to the tube surface.
Besides diode type devices, very sensitive NIR detectors with field effect transistor (FET) architecture have been recently proposed in which source-drain current of a semiconducting channel controlled by gate field was additionally affected by photocurrent arising from NIR excitation on the channel. FET-NIR detectors are beneficial because not only photocurrent can be further modulated with gate bias but also the channel dimensions are readily controlled in their planar type structure. These types of devices are particularly suitable for nanowire semiconductors efficiently absorbing NIR with large length-to-width ratio. Semiconducting single wall carbon nanotubes (SWNTs) are good candidates due to their characteristic NIR absorption near 900 nm. A FET-NIR device with a single wire channel of SWNT exhibited excellent photo-sensitivity greater than 4 under NIR illumination. In spite of great potential of FET-NIR detectors with SWNTs, they are in early stage of the development and further efforts should made to address several important issues such as high sensitivity, fast switching and more importantly easy fabrication of a device solution-processed as well as mechanically flexible.
We present a high performance FET-NIR detector with solution-processed SWNTs network channel consisting of the mixture of metallic and semiconducting nanotubes. The gate controlled p-type current modulation which arose from the unique Schottky barriers evolved between the individual metallic and semiconducting SWNTs in the network allowed for extremely sensitive NIR detection. The OFF state dark current of the device was significantly enhanced more than 10 times upon illumination of a 950 nm NIR laser for facile photo-carrier generation of SWNTs. The device operating at a power of a few mW exhibited excellent ON/OFF endurance cycles over 100 times with fast switching less than 5 seconds.

Applied magnetic field represents an effective tool to rapidly assemble micro- and nanoscale magnetic objects into defined structures. Ordered assembly is typically achieved by using magnetic micro-patterns, for which the downside is that they require advanced micro-fabrication techniques to produce. In addition, most conventional magnetic assembly strategies are restricted to target objects that possess magnetic properties. Herein we present a general strategy that allows convenient magnetically-driven assembly of nonmagnetic objects in defined locations with high spatial resolution. The process involves immersing a polymer relief pattern in a uniformly magnetized ferrofluid, which modulates the local magnetic fields around the pattern. Nonmagnetic target objects dispersed in the same ferrofluid can then be magnetically assembled at positions defined by the polymer pattern. As the nonmagnetic polymer patterns can be conveniently fabricated at low cost through photolithography and soft-lithography processes, our method provides a general yet very effective means to assemble a wide range of nonmagnetic objects with controlled spatial distribution, paving the way towards patterning functional photonic nanostructures.

Electroluminescent (EL) devices based on solution-processed printable materials that include fluorescent polymers and, more recently, colloidal semi-conducting quantum dots (QDs) and their hybrids are quite attractive for a variety of white lightning applications due to their low production costs and potential for fabrication into flexible, large area, lightweight devices.
In addition to conventional EL device architecture in which EL from either the polymer or QDs is in principle achieved by holes and electrons injected from their own ohmic electrodes followed by formation of excitons in the emissive layer that recombine radiatively, new EL devices with different mechanisms for emission have been proposed based on alternating current (AC) electric fields. Similar to well established inorganic electroluminescent devices, solution-processible devices are also mechanistically understood as being either solid-state cathode luminescence or field-induced luminescence in which bipolar charges injected in an AC field form excitons followed by recombination in the emissive layer.
To enhance brightness and to reduce the driving power of AC EL devices arising from low intrinsic density and mobility of charge carriers of the emitters as well as the high contact resistance between the electrode and emissive material, we have recently employed individually networked single wall carbon nanotubes dispersed in a fluorescent polymer layer. This nanocomposite resulted in significantly enhanced brightness at several tens of operating volts due to facilitated injection of both holes and electrons into the ambipolar carbon nanotubes from an electrode and subsequent transfer of the carriers to the fluorescent polymer. Great challenge still remains of fine control of emitting color by mixing as well as tuning with further improvement of brightness.
Here, we report the fabrication of a novel field-induced EL device containing a thin organic/inorganic hybrid nanocomposite film of QDs blended in a light emitting polymer matrix that functions under the influence of an AC electric field. The AC EL device featured an emissive layer composed of a blue fluorescent poly(9,9-di-n-octylfluorenyl-2,7-diyl) polymer and orange CdSe-ZnS core-shell type QDs that exhibited both color tuning and color mixing from different blending ratios together with homogeneous incorporation of a small amount of SWNTs to facilitate carrier injection from the electrode and transfer to the emissive layer. The single layer nanocomposite had a unique phase-separated microstructure at the optimized blend composition, and high performance field-induced white EL was realized with a brightness of approximately 460 cd/m2 at an applied voltage and AC frequency of 40 V and 300 kHz, respectively. Our simple device platform afforded facile preparation of the hybrid nanocomposite film by spin coating and enabled fabrication of a reliable FWEL device capable of color tuning by control of the blend composition.

In this presentation we will review recent advances in the design, synthesis, characterization and application of polymethine dyes with large real third-order optical nonlinearities, focusing on how variation in chemical structure relates to the observed nonlinearities. We will then describe recent studies to develop dyes that can be used for studies of their third-order nonlinear optical properties at high number density in the solid state.
Acknowledgements:
This work was supported in part by the NSF CMDITR Science and Technology Center (Grant DMR-0120967), the DARPA MORPH and ZOE programs, and a MURI through the Air Force Office of Scientific Research

Supermolecular chromophores and new conjugated organic materials can be prepared from ethyne-elaborated porphyrin synthons through the utilization of metal-mediated cross-coupling methodologies. Examples of such neoteric chromophores include species that combine porphyrin-derived pigments with strong (polypyridyl)metal-based oscillators; these structures manifest exceptional electronic interactions between the component pigments. These assemblies show the efficacy of coupled oscillator photophysics to delineate new classes of optoelectronic materials that exhibit unusual excited-state dynamics and manifest exceptional nonlinear responses. Modifying the nature of excitonic and electronic interactions between the component oscillators of these systems elaborates chromophores that manifest extraordinarily large hyperpolarizabilities, precisely tuned depolarization ratios, and exceptional, low energy electronically excited triplet states. The electronically excited triplet states of these species can be further manipulated to provide intense absorptivity over the 0.75 - 1.75 mu;m wavelength domain, and lifetimes that exceed several microseconds. The combination of these exceptional properties enables exploitation of these supermolecular chromophores in waveguides and optical limiters, as well as sensitizers for NIR-to-visible fluorescence upconversion via triplet-triplet annihilation (TTA) photochemistry.

Multi-photon absorption processes are currently of considerable interest for applications such as three-dimensional (3D) fluorescence microscopy, 3D microfabrication, and optical limiting (OL). Substantial efforts have been directed towards identification of structure-property relationships of organic chromophores in an effort to design molecules with large two-photon absorption (2PA) cross sections (δ). Such materials are very desirable as they offer the potential of greater sensitivity, thus allowing for lower laser intensities to be utilized in various two-photon activated processes. For OL via 2PA, we desire materials that are highly transparent in the relevant spectral region and also possess reasonably large 2PA and excited-state absorption (ESA) cross sections, to facilitate strong attenuation of high intensity pulses. Additionally, 2PA absorbers that have very high solubility allow for large macroscopic 2PA coefficients and ESA absorption, both of which favor large pulse suppression.
We have synthesized and performed linear and nonlinear optical characterization on two bis-donor-substituted distyrylthiophene (DST) dyes that show exceptional solubility, over 1 M in several organic solvents. The one-photon, excited-state, and 2PA spectra, as well as the excited-state dynamics show some sensitivity to the solvent polarity and polarizability. Particularly interesting changes were observed in the 2PA, where both an ~2-fold increase in peak 2PA cross section (δ_max) and a considerable broadening of the tail of the 2PA band were seen upon going from cyclohexane to acetonitrile solvent. DST shows substantial 2PA cross sections of ~2000 GM at 600 nm, 1300 GM at 730 nm, and >250 GM from 600 to 825 nm, in acetonitrile. The 2PA spectrum overlaps well with the singlet-singlet and triplet-triplet ESA bands. Consequently, DST exhibits effective optical limiting of nanosecond laser pulses through two-photon induced excited-state absorption over a broad spectral range of approximately 200 nm in the red and near-IR spectral region.

Silicon photonics has been considered a promising technology, mainly due to its intrinsic characteristic of allowing high integration of optical devices in small footprints and to its synergy with existing CMOS processes, promising to be useful in a wide range of applications, comprising: conventional long-distance down to intra-chip communications and optical sensors in general.
For the past years, several research groups have developed essential building blocks and proof-of-concept devices overcoming some of the challenges in Silicon Photonics platform, for example: efficient coupling systems from optical fibers to optical waveguides, low-loss optical waveguides, electro-optic modulators, all optical devices, tunable and reconfigurable devices.
In addition to those devices, tunable resonators is a class of device of great relevance, since they have played an important role as a versatile building block, enabling the demonstration of feasibility of several specific devices and functionalities in Silicon Photonics platform, such as: tunable lasers, tunable filters, modulators, switches, reconfigurable devices, and sensors.
Amongst the main resonators reported in the Silicon Photonics technical literature, the ring resonators and the photonic crystal nanobeam cavities are the most used structures, and have been widely used to demonstrate the feasibility of the devices previously mentioned. These structures have distinct and complementary characteristics regarding their functionalities, depending on the application.
In this work, a proof-of-concept for a new and entirely CMOS compatible tunable nanobeam cavity is demonstrated. Preliminary results show that a compact nanobeam cavity (~20 um2) with high Q-factor (~50,000) and integrated with a micro-heater atop, is able of tuning the resonant wavelength up to 15 nm with low power consumption (0.35nm/mW), and of attaining high modulation depth with only ~100 uW. Additionally, a tunable bi-stable behavior is reported.

Polymethines are being considered as candidate materials for AOSP recently due to their very large third order molecular polarizabilites (γ) and good 2PA molecular figure of merit. However, processing the materials made of polymethines with high number densities into thick, stable, optical-quality films for AOSP applications remains as a challenge. Here, we present our recent results on understanding the structure/ property relationships of rationally engineered polymethines to control their aggregation properties in solid state. Excellent off-resonant Kerr coefficient (n2 = -54.1±9.9 ×10-18 m2w-1) with a very low linear loss of 1.8 dB/cm at 1550 nm was obtained for a silicon waveguide device coated with film of the engineered polymethine salt.

Organic conjugated macromolecules have received great attention due to their
use in optical and electronic applications. There has been a great deal of effort in
the design and characterization of a number of different molecular architectures.
Our recent developments have centered around the newly discovered very
enhanced NLO properties of biradical (open shell) character organic NLO
materials. The great excitement around these materials stems from the
suggestion that third order NLO effects on the order of 10-5 esu may be possible.
This is extremely large in comparison to some of the presently best known NLO
(organic and inorganic) materials which only have coefficients up to 10-8 esu.
Recent theoretical studies have suggested that certain organic open shell
structures would provide such large enhanced NLO effects. In this talk the results
of NIR femtosecond pulsed excitation to drive three-photon absorption (3PA) in a
quinoidal oligothiophene (QOT) biradical system will be discussed. This results
in 3PA emission bright enough to be visible by eye. Through data from steady
state, multiphoton, four-wave mixing, ultra-fast fluorescence decay, and ultrafast
transient spectroscopy the details of the excited states and nonlinear optical
mechanisms of these and related systems will be discussed. In this presentation
a new approach of using a rigid, molecular wire-like biradical system as a strong
nonlinear optical responsive material will be provided.
Representative Publications
Raymond, J.E.; Soon, Y.K.; Casado, J.; Lopez Navarrete, J.T.; Takimiya, K.;
Goodson, T. J. Phys. Chem. Lett. 2011, 2, 2179 - 2183.
Donehue, J.E., Varnavski, O.P., Chemborski, R., Lyoda, M., Goodson, III,T.;
"Probing Coherence in Synthetic Cyclic light-Harvesting Pigments",J. Am. Chem.
Soc., 2011, 133, 4819-4828.
Daniel C. Flynn, Guda Ramakrishna, Hai-Bo Yang, Brian H. Northrop, Peter J.
Stang and Theodore Goodson, III J. Am. Chem. Soc., 2010, 132 (4), pp 1348-
1358
Varnavski, O.; Lee, D.I.; Ramakrishna, G.; Goodson, T.; J. Am. Chem. Soc.,
2010, 132 (1), pp 16-17.
Williams, M.; Bhaskar, A.; Guda, R.,Imamura, M.; Mawatari, A.; Nakao, K.;
Enozawa, H.; Niishinaga, T.; Iyoda, M.; Goodson, T., III, J. Amer. Chem. Soc.,
2008 , 130, 3252

A hybrid structure consisting of a silicon waveguide and an organic cladding containing nonlinear optical (NLO) dyes can enable cost-effective, low power, and ultrahigh bandwidth all-optical signal processing (AOSP) devices. The challenges associated with the implementation of this structure are sizable and involve improving the macroscopic third-order optical nonlinearities, chi;(3), while minimizing both linear and nonlinear absorption losses, and ensuring conformal deposition of the cladding since the optical modes lie in close proximity to the waveguide surface. Previously, chalcogenopyrylium-terminated polymethine dyes in solution have shown large microscopic third-order nonlinearities and low nonlinear absorption throughout the telecommunications spectral region. In a high number density environment, however, unmodified thiopyrylium-terminated polymethines suffer from aggregation resulting in a reduction in nonlinearity and an increase in linear absorption loss. To address this problem, rigid steric groups have been introduced onto the bridge and terminal positions of the molecule, resulting in drastic changes to the linear absorption spectra of high number density films of these modified dyes, suggestive of reduced aggregation. Consequently, this approach gave films with large values of chi;(3) of ~2.7×10-11 esu, very low nonlinear absorption and reasonably low linear losses (~4 dB/cm) at 1.55 mu;m. Furthermore, to translate effectively these optical properties to device performance, the surface of the waveguide substrate has been successfully modified by applying a monolayer-thick coupling agent, thereby promoting a stronger attractive interaction with the organic cladding and ensuring its conformal deposition. This material system with its large NLO response, low absorption loss, and ease of integration with existing silicon platforms could be enabling for AOSP.

Photon upconversion (UC), the emission of light at shorter wavelength than the excitation, has the potential for overcoming the thermodynamic limits of sunlight-powered devices and processes. However, conventional UC systems such as two-photon absorption and second-harmonic generation require high excitation intensities and as a result may be undesirable. An attractive route to lowering the incident power density for UC lies in harnessing energy transfer through triplet-triplet annihilation (TTA). The efficiencies of TTA-UC are determined by diffusion length of the triplet excitons; spatial overlap between the donor and acceptor wave functions is required for energy transfer. To maximize energy migration, molecular diffusivity within an inert medium is of paramount importance, especially in solid-state matrices for practical operation. Although many rubbery polymers with a low glass-transition temperature have been investigated as a matrix, their restricted molecular mobility resulted in much lower UC efficiency than liquid solutions. Here, we have developed an alternative UC system composed of uniform capsules fabricated via a microfluidic approach using a photocurable resin. The capsules comprise a core-shell structure consisting of a fluidic active core which allows for high efficient molecular interaction required for TTA-UC thus preventing the large decrease in the efficiency observed in solids, and an elastomeric shell for facile device integration with sufficient mechanical strength and air stability. We have investigated photochemical properties related to diffusive energy-transfer-driven photoluminescence in a bi-molecular UC system through controlled photo-polymerization at the surface of a microdroplet and concomitant tuning of the properties of the capsules.

Since the birth of nonlinear optics, researchers have continually focused on developing efficient nonlinear optical devices that require low optical powers. Silicon nanophotonics has emerged as a highly promising platform for such devices and for enabling massively parallel, integrated optical and electronic devices on a single chip. The key feature for nonlinear photonics in Silicon is the strong light confinement that enables both a high effective nonlinearity and tuning of the dispersion, which is essential for phase matching of parametric nonlinear optical processes such as four-wave-mixing (FWM). We demonstrate a wide range of devices based on FWM in Silicon chips that offer the potential for ultrahigh bandwidth all-optical processing, CMOS-compatible multiple-wavelength sources, and all-optical clocks.

Suitable design of periodic metal/polymer composite materials facilitates resonant tunneling of light at absorbing wavelengths and is shown to provide a means to significantly reduce optical absorption losses in polymer-based metallodielectric composite structures. The conditions for resonant tunneling are established based on the concept of ‘photonic band edge alignment&’ in 1D-periodic systems. For the particular case of a four-layer gold/polystyrene laminate structure we show that the matching of the lower band edge of the 1D periodic structure with the plasma frequency of the metal component facilitates increased optical transmission at the plasma frequency by about 500% as compared to monolithic metal-film structures of equal total thickness. The resonant metal/polymer composite materials retain the flexural stability of the polymer matrix and thus could find application not only as flexible transparent conductors in areas such as ‘plastic electronics&’ but also in flexible anti-reflecting coatings and optical filters.

Dual electroluminescence (EL) emissions have been realized from nanostructure of CdSe (core)/CdS (arm) tetrapod at room temperature. The dual emission light emitting diode (LED) was formed by a thin anode layer with indium tin oxide (ITO) coated with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS; a LiF/Al cathode, and CdSe/CdS tetrapod as active emitting layer. The device showed a main emission peak centered at ~660 nm, which is due to the electron-hole recombination at CdSe core. Significantly, another visible emission arisen from CdS arm (~ 500 nm) was detected, which is due to the competing hole supply from CdSe core. Furthermore, the device performance as a function of arm length has been studied, which shows that tetrapods with CdS arm length of ~28 nm have the strongest EL emission intensity from both peaks with a turn-on voltage of sim;2.5 V.

The efficiency limit of solar cells has been traditionally studied using detailed balance model based on the absorption and emission fluxes into and out of the device. The absorption flux is directly related to the photocurrent of the cell and the emission flux pertains to the photovoltage. Existing theories use a ray optical picture for the light entering and leaving the cell, which is valid for systems with dimensions larger than the wavelength. On the other hand, the demand for cost effective photovoltaics has prompted the consideration of thinner devices. The emergence of nanostrcutured and ultra-thin solar cells is accompanied by optical phenomena emanating from the near-field light matter interactions. Sub-wavelength photonic and plasmonic effects are introduced in such systems to guide and concentrate light, trap photons, and enhance the field, in order to maintain substantial absorption. While such effects have been considered for increasing the cell current, their effect on the emission, and thus voltage, has not been investigated due to lack of appropriate theory.
In the present work, we introduce a framework that allows calculation of the emission rate out of a solar cell based on a complete electromagnetic picture including near-field optics. We use the Fluctuation-Dissipation theorem to model the radiative recombination as dipole emission events inside the device, and estimate the photon flux ultimately leaving the cell based on the Green&’s function of the electromagnetic system. With the developed model, it is possible to calculate both absorption and emission, and thus current and voltage of the cell, for the specific optical design used for light trapping. It thus provides a powerful tool to optimize the photonic design of solar cells for maximum power (current-voltage product) rather than current only.
We apply the concept to a simple planar GaAs solar cell with gold back reflector to demonstrate the ability of the developed model in capturing the entirety of electromagnetic effects, in particular near-field optics.
Reference:
A. Niv, M. Gharghi, C. Gladden, O. Miller, X. Zhang, “Near-Field Electromagnetic Theory for Thin Solar Cells ” Phys. Rev. Lett. 109, 138701 (2012).

Plastic photonic devices provide a number of considerable advantages over those fabricated from inorganic materials, including the technologically attractive promise of allowing straight-forward fabrication and patterning procedures[1,2]. Such methods can readily be applied to create large area, highly reproducible sub-wavelength patterns exhibiting a range of optically interesting functional structures. We show that using both melt-processing and embossing techniques, we are able to create transparent one and two-dimensional patterns, the response of which are further controlled and enhanced through the introduction of microspheres (polymeric and metallic) at selected spatial positions on the structures[3-5]. This additional ‘nano-pinballing&’ step allows for further design of the optical response through the controlled formation of various close packed and sparse arrays of microspheres. Measurements performed, which include both structural and optical characterization, confirm the various stages of construction and, in the case of the optical scattering properties, faithfully reproduce the expected response. We also show that further opportunities exist by combining the facile photonic structure fabrication methods with the properties of metallic nanostructures - also through a nano-pinballing step. Such complex architectures may be expected to provide a number of novel opportunities, for example, emulating the behavior of metamaterials, plasmonic sinks for triplet quenching and light scattering or trapping layers [6-8].
1. Scherer, A., Vuckovic, J., Doll, T. & Loncar, M. Design and fabrication of silicon photonic crystal optical waveguides. Journal of Lightwave Technology 18, 1402-1411 (2000).
2. Stutzmann, N., Tervoort, T. A., Bastiaansen, C. W. M., Feldmann, K. & Smith, P. Solid-State Replication of Relief Structures in Semicrystalline Polymers. Advanced Materials 12, 557-562 (2000).
3. Yabu, H., Inoue, K. & Shimomura, M. Multiple-periodic structures of self-organized honeycomb-patterned films and polymer nanoparticles hybrids. Colloids and Surfaces A: Physicochemical and Engineering Aspects 284-285, 301-304 (2006).
4. Escalé, P., Rubatat, L., Billon, L. & Save, M. Recent advances in honeycomb-structured porous polymer films prepared via breath figures. European Polymer Journal 48, 1001-1025 (2012).
5. Zhang, Z., Wang, Z., Xing, R. & Han, Y. How to form regular polymer microstructures by surface-pattern-directed dewetting. Surface Science 539, 129-136 (2003).
6. Kena-Cohen, S., Wiener, A., Sivan, Y ., Stavrinou, PN., Bradley, DCC., Horsfield, A., Maier, SA. Plasmonic Sinks for the Selective Removal of Long-Lived States. ACS Nano 5, 9958-9965 (2012).
7. Atwater, H. & Polman, A. Plasmonics for improved photovoltaic devices. Nature materials 9, 205-214 (2010).
8. Rockstuhl, C. & Scharf, T. A metamaterial based on coupled metallic nanoparticles and its band-gap property. Journal of Microscopy 229, 281-286 (2008).

In this work, we present that metallic gratings consisting of narrow slits have been demonstrated to become transparent for extremely broad bandwidths. This phenomenon can be explained by a concrete picture in which the incident wave drives free electrons on the conducting surfaces and part of the slit walls to form surface plasmons (SPs). The SPs then propagate on the slit walls but are abruptly discontinued by the bottom edges to form oscillating charges that emit the transmitted wave. This picture explicitly demonstrates the conversion between light and SPs and indicates clear guidelines for enhancing SP excitation and propagation. Meanwhile, the broadband optical transmission is verified for the structured metals with significant thickness in the range of half a wavelength, and the high transmission efficiency is insensitive to the metal thickness. Furthermore, this approach can implement transparent metals nearly over the entire spectrum ranging from the radio frequency to the visible. The investigations provide a guideline to develop many novel devices, including transparent conducting panels, white-beam polarizers, broadband metamaterials, and antireflective solar cells. References: 1) X. R. Huang, R. W. Peng, R. H. Fan, Phys. Rev. Lett. 105, 243901 (2010); 2) R. H. Fan , R. W. Peng, X. R. Huang, J. Li, Yongmin Liu, Qing Hu, Mu Wang, and Xiang Zhang, Adv. Mater. 24, 1980 (2012).

Inspired by natural structures in insect compound eyes, nanocone structures form an effective medium with axial gradient in refractive index, which leads to elimination of impedance mismatch at optical interfaces and hence suppresses Fresnel reflection. In addition, such textured surfaces have controllable wetting properties, depending on the surface chemistry deposited on the nanocone structures. In the past we have shown that the functionality of these surfaces can be optimized by increasing the aspect ratio, since taller, narrow nanostructures contribute to a lower reflection coefficient and more robust superhydrophobicity or superhydrophilicity. However, it is inevitable that the high aspect ratio of the slender cones is also detrimental to the mechanical robustness of the nanostructured surface, which is a critical barrier when developing textured substrates for real world applications.
Here, we propose and then fabricate an inverted nanocone structure to create an improved biomimetic multifunctional film with greater mechanical robustness. Whilst retaining the high aspect ratio of tapered nanostructures for enhancing the optical and wetting performance, the inverted nanocone structure also provides high mechanical robustness regardless of its aspect ratio, since the nanostructures are structurally supported by the matrix material around the neighboring structures.
The inverted nanocone surface is fabricated using a simple UV replication method with high throughput. A negative master mold comprising of a wide area periodic array of nanoconical features was prepared using laser interference lithography in a fused silica substrate. Then it was replicated into UV curable polymer by a sequence of pressing, UV curing and demolding steps. The fabricated inverted nanocone arrays have a pitch of 200 nm and aspect ratio that varies between 4:1 and 5:1 depending on the uniformity of the mold and local distribution of the applied pressure.
The mechanical strength of the proposed inverted nanocone structure was quantitatively simulated and compared with that of conventional nanocone structures using finite element calculations. The inverted nanocone structure has lower stress concentration and smaller tip deflection compared to that of the corresponding nanocone structure under the same shear force, and the structure itself is free from bucking and collapse problems.
Measurements of the transmission spectra for the inverted nanocone texture were carried out in the visible to near infrared range (300nm < lambda; < 1300nm) at different incident angles from 0° to 70°, and were compared to calculated data evaluated using Finite Difference Time Domain (FDTD) calculations. The structured surface exhibits enhanced optical transmittance compared to the flat fused silica surface over a wide range of wavelengths and various incident angles due to the high aspect ratio (4:1) of the inverted nanocone arrays.

Oxide alloys such as Hafnium Titanium oxide (HfTiOx) are extensively studied as electronic and catalytic materials, their potential for photonic devices (e.g., optical waveguide couplers) has been overlooked perhaps because they tend to crystallize resulting in characteristic microstructures (e.g., crystalline columnar structures) with poor transparency. Amorphous HfTiOx can be an ideal base material for exotic photonic devices with optical paths having a refractive index that continuously and spatially varies. Low optical extinction coefficient is necessary for those devices that require long optical paths with low optical loss. We report HfTiOx alloy thin films deposited by pulsed DC reactive magnetron co-sputtering on 300mm diameter glass and silicon(100) substrates. Two sputtering targets, 3N Hafnium and 3N Titanium, were used to control the cation atomic percentage of HfTiOx films with a range of compositions with an emphasis on refractive index in the range of 2.2-2.6, matching the index of LEDs and laser diodes and providing with numerical aperture of .7 or higher, suitable for étendue matched optical coupling to semiconductor sources and receivers. The sputtering process was carried out with and without 13.56MHz AC substrate bias to control microscopic structures. HfTiOx alloy mixtures of end binary provide tenability of both optical and electrical properties. The resulting HfTiOx alloys uniquely demonstrate deviation in the dispersion of their refractive index from linear mixture, which is increased by AC bias. Our sputtering system provides continuous uniform deposition of layers of each oxide from much less than a monolayer to several monolayers offering complete mixing and flexibility in designing optical waveguide devices with graded refractive index. We used spectroscopic ellipsometry, transmission electron microscopy, and x-ray photoemission spectroscopy to correlate optical and structural/chemical properties and discuss our finding that HfTiOx alloys and their end binaries deposited with the AC substrate bias have much higher refractive index and significantly lower optical extinction coefficient (<1x10-8 for Hf rich alloys) than those of alloys deposited without the AC bias, suggesting that microstructures with less distinct columnar characteristics are responsible for the improved optical properties. In addition, the dependence of optical constants on the alloy composition indicates the presence of a large bowing effect. We also discuss current-voltage and capacitance-voltage measurement performed on the HfTiOx alloys to assess correlation between their dielectric properties and optical properties.

It is tempting to develop a self-assembly-based method for growth of plasmonic nanostructures that would allow for engineering and tuning of their optical properties via control of growth parameters. We shall demonstrate such a method. The wavelength of the plasmonic resonance could be tuned at will to cover the entire visible range in a highly controlled fashion.
We discuss a versatile novel atomic terrace low-angle shadowing (ATLAS) self-assembly method for producing ordered planar NW arrays with wire widths ranging between 15 nm and 1000 nm. This method utilizes the geometrical shadowing, caused by the step- and terrace- morphology of a step-bunched vicinal surface, of the deposition flux under ultrahigh vacuum (UHV) conditions. The beam is deposited at a grazing angle on a vicinal single crystalline substrate. The method is a bottom up based method and yet, it is not specific to a particular combination of the materials of the substrate and the nanostructures. Therefore, it is highly versatile.
For the plasmonic studies, one-dimensional Ag, Au and Cu nanoparticle (NP) arrays (nanowire-like arrays) have been grown on faceted vicinal c-plane Al2O3. The structures showed a strong optical anisotropy in the visible region, with a shift between longitudinal and transverse resonances. This phenomenon has been attributed to the combined effects of both shape anisotropy and strong inter-particle interaction. Due to the presence of “hot spot” areas in the inter-particle sites, the structure is appealing for surface enhanced Raman spectroscopy measurements and biosensing application.
Reflection Anisotropy Spectroscopy (RAS) has been used, for the first time on such structures, to monitor their optical anisotropy in-situ, during growth. The relevant optical properties were determined as a function of NP height, deposition angle and substrate morphology. The average NP diameter and inter-particle distance are below limitations of lithographic methods, with standard deviations of 20%. We could set the plasmonic resonance peak at will anywhere in the range from 1.1 eV to 2.6 eV by changing the material of the nanoparticles, their size and shape. Thus, engineering of the optical properties of the array is readily achievable by the versatile ATLAS method.

Complex fluids have attracted attention owing to their intricate response to optical fields. Despite the vivid interest, several relevant aspects remain unclear. A mysterious light-soft matter coupling is the unexpected material organization along a cw-visible laser beam, observed in homogeneous entangled solutions of 1,4-polydienes. When irradiated with mild (few mW) laser light, the non-absorbing solutions responded by a local increase of polymer concentration δc. In turn, the enhanced refractive index δn altered the light propagation and the self-focusing optical nonlinearity caused the formation of various micro-patterns, such as soliton-like single filaments, multi-filament arrays and gratings, depending on the irradiation conditions.
The opposite effect of a light-induced δn decrease was recently uncovered: Polymer chains were effectively either attracted or repelled by visible light depending on the solvent environment but irrespectively of the optical contrast. Its versatile nature was documented by the observation of all possible material responses to light; polymers in good solvents of higher or lower refractive indices could be accumulated in the irradiated volume or be dispersed outside it.
The writing efficiency exhibited strong wavelength dependence for the solutions with δn>0. Under similar conditions, δc increases fast in the red (633-671 nm), slows significantly down towards blue (532 and 488 nm) and is hardly discernible at 830 nm.
The effect is not limited to homopolymers. In transparent dispersions of polystyrene-b-polyisoprene in hexane, irradiation led to copolymer-rich fibers. Turbid dispersions of the same copolymer in ethyl acetate revealed a unique case of self-induced transparency. The polyisoprene-light coupling and the metastable character of the dispersion are believed to be at the origin of the effect.
In contrast to electrostriction, the sign of the refractive index difference Δn between the polymer and the solvent does not solely determine the system&’s response to light. The minute absorption in the visible and the positive Soret coefficients for two solutions with Δn>0 but opposite responses (δn>0 or δn<0) should exclude photo-thermal and thermophoretic driving forces. The qualitative difference between the responses of the solutions in different solvents, points towards a microscopic origin of the light-matter interaction. Shedding light on the specificity of these materials might unlock the quest for other complex fluids presenting similar phenomena. Elucidation of the underlying mechanism might facilitate macromolecular manipulation and the investigation of soft matter nonlinear optics, and offer several perspectives utilizing the richness of the polymer science.
1. Sigel et al., Science 297, 67 (2002)
2. Loppinet et al., J. Am. Chem. Soc. 127, 9678 (2005)
3. Anyfantakis et al., Opt. Lett. 33, 2839 (2008)
4. Anyfantakis et al., Soft Matter 8, 2382 (2012)
5. Anyfantakis et al., Opt. Lett. 37, 2487 (2012)

The talk will highlight recent work focused on the synthesis, photophysical characterization, and non-linear optical response of chromophores that feature platinum(II) centers. In one line of investigation we have developed a series of oligo(phenylene vinylene)s that are linked to platinum(II) acetylide centers. These chromophores display remarkable two-photon absorption throughout the visible and near-infrared regions. In a second line of work, we have developed a new family of platinum(II) acetylide chromophores that contain N-heterocyclic carbene (NHC) ligands. The linear and non-linear photophysics of the NHC complexes is compared to analogs that feature trialkylphosphine ligands. The two photon absorption and nanosecond non-linear response of many of these chromophores has been characterized in poly(methyl methacrylate) glassy monoliths.
For a recent review see:
Chen Liao, Abigail H. Shelton, Kye-Young Kim, and Kirk S. Schanze, ACS Appl. Mater. Interfaces, 2011, 3 (9), pp 3225-3238.

Suprasil quartz is widely employed as a substrate for thin-film optical components with different applications. In particular, it can be used in hostile conditions such as space missions and high energy experiments, where it is exposed to several kinds of ionizing radiations, such as gamma rays, electrons, neutrons and heavy particles.
It is well known that gamma irradiation on quartz material induces the formation of intrinsic defects which correspond to different absorption bands in the UV-VIS spectral range. Such defects can modify the optical response of the substrate itself and of the optical components deposited on it.
The knowledge of the effects of gamma irradiation on materials is extremely important to foresee their behavior in hostile environments. Moreover it is fundamental to separate the effects due to substrate modifications from those due to changes in the coating. In many cases, in fact, only the substrate is damaged, while the coating materials are not affected by irradiation.
In previous works several synthetic Suprasil samples exposed to gamma rays at low total doses (50-8400 Gy) were investigated and no evidence of defect formation was found. In this work, substrates of the same kind were irradiated by gamma rays at higher doses (between 300 and 1200 kGy). The irradiations were performed at the 60Co Calliope plant in ENEA Casaccia (Rome). Radiation induced defects formation was investigated by means of UV-VIS transmittance measurements and Electron Spin Resonance spectroscopy before and after irradiation. The time evolution of the defects was also monitored in samples kept in dark and light to study the different behaviors in such conditions. Radiation-induced damage was evident in all samples.

Nanowire waveguides have attracted significant attention in nanoscale photonic and optoelectronic devices because of their important role in linking various optical elements to perform complex functions in the devices.(1, 2) One of the major concerns in designing waveguides is unavoidable directional bending that causes additional energy loss due to the distortion of modal field.(3) Bending loss in nanowire waveguides has been studied while including substrate coupling loss(4), energy transfer to the underneath substrate.(1, 5) Pure bending loss, i.e. bending loss unaffected by substrate coupling, in nanowire waveguides still remains unclear. A challenging task in study of pure bending loss is to introduce bending on nanowire waveguides in the air and to tune the radius of bending.
We report the characterization of pure bending loss in nanowire waveguides unaffected by substrate coupling. Our idea is to directly bend a vertical freestanding nanowire that has been fabricated on a Si wafer in the air. Specifically, vertical freestanding active nanowire waveguides of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene](MEH-PPV) have been fabricated by the meniscus-guided method.(6) To characterize pure bending loss, the bending of each as-grown waveguide with different diameter was remotely controlled by applying an electrostatic force near the top end of the waveguide. Finite-difference-time-domain (FDTD) simulation was performed to confirm the experimental result and to validate the effect of the substrate. We show that the bending losses, previously reported by conventional experimental approaches of nanowires rested on substrates, were strongly overestimated. The significant overestimation is attributed to the coupling of the enhanced evanescent field around the bent nanowires to the substrate. We suggest that our freestanding nanowire system could be also utilized for studying various intrinsic properties of nanowire waveguides as an ideal model of air-clad nanowire waveguide.
1. M. Law et al., Nanoribbon waveguides for subwavelength photonics integration, Science 305, 1269-1273 (2004).
2. R. Yan, D. Gargas, P. Yang, Nanowire photonics, Nature Photonics 3, 569-576 (2009).
3. R. Olshansky, Propagation in glass optical waveguides, Rev Mod Phys 51, 341-369 (1979).
4. W. Wang, Q. Yang, F. Fan, H. Xu, Z. L. Wang, Light Propagation in Curved Silver Nanowire Plasmonic Waveguides, Nano Letters 11, 1603-1608 (2011).
5. F. Di Benedetto et al., Patterning of light-emitting conjugated polymer nanofibres, Nature Nanotechnology 3, 614-619 (2008).
6. J. T. Kim et al., Three-Dimensional Writing of Conducting Polymer Nanowire Arrays by Meniscus-Guided Polymerization, Advanced Materials 23, 1968-1970 (2011).

Organic materials with broadband nonlinear absorption in the visible to the near-IR region are desired for a variety of photonic device applications. Many platinum diimine complexes exhibit broadband excited-state absorption in the visible to the near-IR region. However, due to the very weak ground-state absorption of the Pt(II) diimine complexes in the near-IR region, it is not possible to populate the excited state via one-photon absorption in this region. To overcome this disadvantage, we designed and synthesized several series of Pt(II) diimine complexes containing fluorenyl component on the diimine ligand or/and on the acetylide ligands to increase the two-photon absorption of these complexes in the near-IR region. In such a case, we can access the excited state via two-photon absorption. Combination of two-photon absorption and excited-state absorption not only expands the nonlinear absorption spectral region but also enhances the nonlinear transmission performance in the near-IR region.

White organic light-emitting diodes (OLEDs) have attracted intense attention due to their potential applications in flat-panel displays and solid-state lighting. Compared with conventional white OLEDs, solid-state white light-emitting electrochemical cells (LECs) possess several promising advantages. They generally require only a single emissive layer, which can be easily processed from solutions, and can conveniently use air-stable electrodes. Electrochemical doping regions of the emissive layer near electrodes induce ohmic contacts for carrier injection. As a result, a single-layered LEC device can be operated at very low voltages (close to Eg/e, where Eg is the energy gap of the emissive material and e is elementary charge) with balanced carrier injection, giving high power efficiencies. White LECs can be fabricated by employing a host-guest strategy, i.e., utilizing an emissive layer composed of a blue-emitting host doped with a red-emitting guest. However, carrier trapping effect due to offsets in energy levels between the host and the guest results in significant voltage-dependent electroluminescence (EL) spectra. In addition, multiple components in the emissive layer complicate fabrication processes and affect reproducibility of EL characteristics.
In this work, we report white solid-state LECs based on a single-component emissive layer. A novel white-emitting ionic copolymer containing blue-emitting hole-transporting segments and yellow-emitting electron-transporting moieties was used as the emissive material of white LECs. These white LECs showed high peak external quantum efficiency (EQE) and power efficiency up to 0.69% and 1.56 lm/W, respectively. Such EQE is approaching the upper limit (~0.7%) that one would expect from the photoluminescence quantum yield of the thin film of the copolymer (0.14) when fluorescent spin statistics of ca. 25% and an optical out-coupling efficiency of ca. 20% are estimated. Thus, superior carrier balance in white LECs based on this copolymer could be speculated. It may result from the bipolar characteristic of the molecular structure of this copolymer, which is consisted of hole-transporting segments and electron-transporting moieties. Furthermore, white EL spectra with Commission Internationale de l'Eclairage (CIE) coordinates approaching (0.33, 0.33) and high color rendering indices (CRI) >90 have been reported for the first time in white LECs to the best of our knowledge.

We describe the fabrication of a polariton organic semiconductor LED utilizing a distributed Bragg reflector as one of the cavity mirrors. The device is characterised through angle-resolved photoluminescence and electroluminescence spectroscopy, with the efficiency of the polariton OLED being compared with that of an equivalent non-strongly coupled device. We find that the external emission efficiency of the polariton OLED is approximately 6 times smaller than that of the standard OLED; an effect that we believe results from the limited scattering of excitons towards states at the bottom of the polariton branches. We measure the temporal response of the polariton and standard OLED devices under pulsed voltage excitation and conclude that device dynamics are limited by relatively slow charge transport and recombination.