Photonic Band Gap (PBG) materials are artificial periodic microstructures that enable microscopic trapping and guiding of light in a 3D optical chip. In addition to optical information processing, light trapping in photonic crystal thin films can be utilized for solar energy harvesting. Engineering of the electromagnetic density of states in metallic photonic crystal filaments can be used to control thermal spontaneous emission for incandescent lighting. I review these applications and discuss present challenges in the large scale, low cost fabrication of three-dimensional PBG materials.

The ability of photonic crystals to modify spontaneousemission and as a consequence, their ability to tailor thermalradiation has received significant attention in recent years.It was shown that photonic crystals offer unparalleledpossibilities for designing thermal radiation sources with propertiesthat are often non-intuitive and deviate significantly from those of typicalgrey-body sources.There are two general research areas being pursued with regard toshaping thermal radiation using photonic crystals. On one hand,photonic crystals are used to design highly selective narrow-bandthermal emitters, exhibiting wavelength, directional andpolarization selectivity.These structures show promise for applications in IR sensors and avariety of IR sources. On the other hand, photonic crystals areexplored to design wide-band thermal emitters exhibitingnear-blackbody thermal emission within a given wavelength range andlargely attenuated emission outside the givenrange. Applications such as thermophotovoltaic energy conversion,solar-thermophotovoltaic conversion, solar absorbers/reflectors areconsidered as main drivers behind advances in broad-band selective thermal emitters.In this paper we explore theory, design and fabrication of one- and two-dimensionalPhC's as either broad-band or narrow-band selective thermal emitters and filters.We introduce 1D Si/SiO₂ PhC as broadband selective filter for thermophotovoltaicapplications exhibiting pass-band from 1-1.8 μm and stop-band from 1.8-3.4 μm, and discuss design and fabricationtradeoffs. In addition, we explore design, simulation, and fabrication of 2D tungsten PhC as high-temperature broadband selectivethermal emitters with photonic bandgap of 1.8 μm. Both of these devices areexplored in the context of efficient TPV generation.Furthermore we introduce two narrow-band thermal radiation sources that exhibitboth temporal and spatial coherence, based on 1D and 2D PhC designs.

For many years, conventional wisdom has held that the lowest possible thermal conductivities are found in electrically-insulating glasses and certain classes of strongly disordered crystals. The thermal conductivity of these materials can be successfully predicted by a simple model, i.e., the minimum thermal conductivity, based on Einstein’s 1911 theory of heat conduction. We have discovered recently, however, that anisotropic solids that combine order and disorder in the random stacking of two-dimensional crystalline sheets, so-called “disordered layered crystals” have a thermal conductivity that is many times smaller than the predicted minimum. In fact, the conductivity of thin films of disordered layered WSe2 is only a factor of 2 larger than air. The cause of this ultralow thermal conductivity is not fully understood but may be explained by the large anisotropy in elastic constants that suppresses the density of phonon modes that propagate along the cross-plane direction. Using time-domain thermoreflectance (TDTR), we can rapidly and routinely measure the thermal conductivity of almost any material that has a relatively smooth surface. The spatial resolution of TDTR is a few microns and our development of a “two-tint” version of this pump-probe technique greatly increases the tolerance to surface roughness. I will describe how we are using TDTR to explore the physics of heat conduction in a wide variety of layered crystalline materials, e.g., graphites, silicates, and transition metal chalcogenides.

The room temperature nanomechanics of carbon nanotubes (CNTs) has been well established. However, the high temperature nanomechanics of CNTs remains almost unexplored. By using a transmission electron microscopy – scanning tunneling microscopy (TEM-STM) platform, rich high temperature nanomechanics of CNTs was revealed. In this talk, I will review our recent progress in using the TEM-SPM platform to probe the electrical and mechanical properties of CNTs and nanowires [1]. First, individual multiwall CNTs were peeled off layer-by-layer by electric breakdown inside the TEM. This provided new insights into the transport property of each individual wall within a multiwall nanotubes. Second, plastic deformation, such as superplasticity, kink motion, dislocation climb, and vacancy migration, was discovered in nanotubes. Third, shrink-wrap buckyballs, quasimelting of diamond, and close-cap growth of single-wall nanotubes were observed. Finally, a combination of TEM with microelectromechanical (MEMS) device to enable in-situ thermal and thermoelectric measurements were achieved.[1]J.Y. Huang et al., Nature 439, 281 (2006); J.Y. Huang et al., Phys. Rev. Lett. 94, 236802 (2005); 97, 075501 (2006); 98, 185501 (2007); 99, 175593 (2007); 100, 035503 (2008); J.Y. Huang et al., Nano Lett. 7, 1699 (2006); 7, 2335-2340 (2007); J.Y. Huang et al., Small 3, 1735-1739 (2007); J.Y. Huang et al., Phys. Rev. B 78, 155436 (2008).

We survey our recent results on processing and characterization of heteroepitaxial all-garnet magneto-optical (MO) photonic crystals (MOPCs). 1D MOPCs are composed of λ/4 garnet layers alternating highly gyrotropic Bi3Fe5O12 with transparent rare earth gallium garnets. Completely substituted bismuth iron garnet Bi3Fe5O12 films show a record value of Faraday rotation (FR) as high as θF = -8.4 deg/μm@633nm and a peak value of -28 deg/μm@537nm. As designed, MOPCs’ spectra exhibit optical stop band with the transmittance central peak caused by light localization in λ/2 thick MO-cavity and giant enhancement of nonreciprocal optical performance. At the resonance wavelength of 750 (980) nm, [Bi3Fe5O12/Sm3Ga5O12]^6/Bi3Fe5O12^2/[Sm3Ga5O12/Bi3Fe5O12]^6 MOPC demonstrates 50 dB peak-to-valley light rejection and the highest specific FR achieved so far: θF = – 20.5 (– 7.3) deg/μm that is 470 (810) % enhancement compared to a single layer Bi3Fe5O12 film. MO-remanence (optical memory latching capability) has been engineered in Bi3Fe5O12:Gd3Ga5O12(n:m) superlattices by atomic layer deposition. Regular alternating of lattice mismatched garnet layers impedes the nucleation of misfit dislocations, preserves a long range coherent compressive strain through the whole superlattice stack thus results in a strong uniaxial magnetic anisotropy. Nanostructured garnets were used to build MO-visualizer and current driven magneto-optical spatial light modulator. We provide an assessment of ferromagnetic/ferroelectric photonic crystals for optical data recognition, processing and storage, color filtering, bandwidth control as well as their integrability with gain materials and lasing crystals.

We consider the possibility of drastically reducing the thermal conductance below that of vacuum, with the use of multi-layer photonic crystals consisting of alternating dielectric and vacuum layers. We show that for a single crystal structure, there is a theoretical limit of the photonic thermal conductance at high temperature. Such limit depends only on the dielectric contrast, and is independent of the thickness of the layers. We also show that a photonic crystal heterostructure, formed by interfacing two crystals with different lattice structures, can achieve thermal conductance below the theoretical limit of that of a single crystal.

Metallic photonic crystals, metal-based structures with periodicities on the scale of the wavelength of light, have attracted considerable attention due to the potential for new properties, including the possibility of a complete photonic bandgap with reduced structural constraints compared to purely dielectric photonic crystals, unique optical absorption, potential for narrow band thermally stimulated emission, and interesting plasmonic physics. In order to realize many of these applications however, 3D photonic crystals with exceptionally thermal stability will be required. Over the last few years, we have demonstrated, through both holographic and self-assembly based routes, the fabrication of metallic three-dimensional photonic crystals which can withstand over 1200 K. Electrochemical strategies have been employed to fabricate structures from materials including nickel, copper, and tungsten, and further employed to control the degree of structural openness of the metallic photonic crystal. More conventional chemical vapor deposition and atomic layer deposition approaches have also been applied to fill colloidal and holographic templates with materials such as platinum and tungsten. The optical properties of these structures at both room and elevated temperatures have been measured.

Artificial control of thermal radiation from material surface is gathering much attention because of its wide variety of applications as well as physically interesting phenomena. This paper reviews our research on thermal radiation from metallic surface microstructures to develop selective radiators for thermophotovoltaic (TPV) generation. In this application, selective enhancement of emissivity in the near infrared region is one of the key issues to improve the power generation efficiency.To understand and estimate the spectral properties of metallic surface gratings, we have performed numerical simulations based on electromagnetic wave theory. The results show that several peaks having different features appear on the emissivity spectra of metallic gratings; One is originated from surface plasmon polaritons supported by surface gratings, which is dependent on the grating period and angle. Second is originated from local surface plasmon supported by each narrow slit or narrow microcavity located on a grating surface, which is independent on angle. Third one is explained by the microcavity effect which arises from each microcavity with a relatively large opening size on a grating surface. This effect is also not sensitive to angle. Among these mechanisms, we have chosen the microcavity effect that exhibits suitable properties for selective radiators in TPV generation: a high emissivity within the visible and near-infrared regions, and angle independence. We have developed two kinds of two-dimensional metallic gratings based on Si and W with the period of 1.0–2.0 μm. Si gratings with inverted pyramid structures were fabricated by electron beam (EB) lithography for patterning and successive wet anisotropic etching. W gratings were also patterned by EB lithography. For deep etching of W, we utilized accelerated neutral molecular mass beam in combination with thick resist mask. The W gratings composed of rectangular microcavities displays a high emissivity in the near-infrared region as expected from the calculation results. The cut-off wavelength, which corresponds to the threshold wavelength for enhanced emissivity, is reasonably changed by changing the microcavity size. In addition, it has been confirmed experimentally that the W selective radiators have advantages for high-power and high-efficiency TPV systems. The high thermal stability of the W gratings has been also demonstrated over 1000K.(1)H. Sai et al., J. Opt. Soc. Am. A, 22, 1805-1813 (2005).(2)H. Sai et al., J. Micromechanics and Microengineering, 15, S243-S249 (2005)(3)H. Sai et al., Appl. Phys. Lett., 82, 1685-1687 (2003)(4)H. Sai et al., J. Opt. Soc. Am. A, 18, 1471-1476 (2001).

Thermal barrier coatings (TBCs) are typically composed of translucent ceramic oxides that provide thermal protection for metallic components exposed to high-temperature environments, such as in jet turbine engines. Taking advantage of the translucent nature of TBCs, optical diagnostics have been developed that can provide an informed assessment of TBC health that will allow mitigating action to be taken before TBC degradation threatens performance or safety. In particular, rare-earth-doped luminescent sublayers have been integrated into the TBC structure to produce luminescence that monitors TBC erosion, delamination, and temperature gradients. Erosion monitoring of TBC-coated specimens is demonstrated by utilizing visible luminescence that is excited from a sublayer that is exposed by erosion. TBC delamination monitoring is achieved in TBCs with a base rare-earth-doped luminescent sublayer by the reflectance-enhanced increase in luminescence produced in regions containing buried delamination cracks. TBC temperature monitoring is demonstrated using the temperature-dependent decay time for luminescence originating from the specific coating depth associated with a rare-earth-doped luminescent sublayer. The design and implementation of these TBCs with integrated luminescent sublayers is discussed, including co-doping strategies to produce more penetrating near-infrared luminescence. It is demonstrated that integration of the rare-earth-doped sublayers is achieved with no reduction in TBC life. In addition, results for multilayer TBCs designed to also perform as radiation barriers are also presented.

Modern industry requires the development of thermal barrier coatings with service temperatures of about 1000-1500°C and higher. Nano- and micro-grained metal oxide materials are commonly used (for example stabilized zirconia, α-Al2O3). However, being good thermal insulators at intermediate temperatures (T<1000°C) these materials demonstrate an increase of thermal conductivity at higher temperatures. Typical temperature dependence of the thermal conductivity contains a rapid increase at high temperatures (T>1000°C) due to radiative effects. On the contrary, porous materials (for example plasma sprayed stabilized zirconia coatings) do not demonstrate any increase of the thermal conductivity at high temperatures.In this report we analyze the radiative component of the thermal conductivity. The problem of heat photon propagation in non-homogeneous media is considered. In particular, photon propagation in nano-grained and porous materials is investigated. Dependence of the radiation component on the pore size and total porosity is investigated. It is shown that materials having pores of size comparable with wavelength of the thermal photons are most efficient for insulating radiative heat. Heat transfer in porous photonic bandgap structures is investigated. We consider the photon propagation in the photonic crystals and its reflection at the photonic crystal boundary. This allows us to estimate the radiation component of thermal conductivity. Comparison of this component of the photonic crystal and random porous structure of the same porosity is discussed.

Atomic layer deposited iridium is coated onto three-dimensional tungsten woodpile photonic crystals to modify the optical properties of the structure. Because of the inherent material absorption present in real metals, band-edge scaling with feature size deviates from ideal behavior as the metal plasma frequency is approached and the metal’s optical properties begin to dominate those of the periodic structure. As feature sizes drop to the scale of the near infrared wavelengths, the band-edge becomes “pinned” and cannot be pushed to shorter wavelengths regardless of how small the lattice constant is made. For a tungsten photonic lattice, this pinning occurs at about λ ∼ 1.8 μm. Iridium is a metal which, like tungsten, has a very high melting point and thus is suitable for high temperature applications. Iridium however has a more ideal behavior in the near-infrared wavelengths, with a higher reflectance in the wavelength range 0.5–1.5 μm. We find that with a thin coating of iridium is sufficient to mimic a bulk-iridium woodpile structure, and correspondingly the band-edge is pushed below λ = 1.5 μm. This shift in the reflectance band-edge will be accompanied by a reduction in the absorptance of the structure in the 1.5 – 2.0 μm range, allowing for shift in the thermal emission spectrum of an iridium woodpile structure with the appropriate dimensions compared to a tungsten structure. This technique allows for precise control of the optical properties of a photonic lattice via the engineering of the constituent materials.

Recent progress in fabrication technology allows for the efficient control of electromagnetic waves by means of photonic devices. This is attractive and promising for high-temperature photonic structures, which minimize undesired electromagnetic heat transfer at temperatures above 1000 oC. We review the literature and present our own results on modeling fiber reinforced ceramics (or ceramic matrix composites, CMC), i.e., composite materials with thermo-mechanical properties, which could be superior to high-temperature metals or monolithic ceramics and can be designed for photonic applications. Possible applications of CMC include the protection of non-rotating components in high-temperature engines and turbines such as combustors and liners, coatings and parts for aerospace vehicles. The main types of fiber CMC are briefly discussed, starting from non-oxide/non-oxide CMC (CMC built of non-oxide fibers in non-oxide matrices). Our discussion includes fiber CMC temperature capabilities, material science aspects, and some relevant structure features such as accessible dimensions, thermal conductivity, high-temperature stability. We present a technique to study radiation heat transfer properties of CMC, which are currently not well understood. Known CMC have predominantly 1D and 2D fiber arrangements. CMC approaches such as weavable materials and interpenetrating microstructures, could be used to design new fiber CMC with both complete photonic band gaps and omnidirectional reflectivities.