We describe a new class of plasmonic photonic crystal emitters integrated into a MEMS platform for high temperature-intensity, high speed, and high efficiency tuned emitting and sensing applications in the infrared. We exploit 2D organized metallo-dielectric surface structures for angular and spectral control of reflection, absorption and emission from surfaces in the infrared. We have built a FDTD model that incorporates complex frequency dependent properties and provides quantitative agreement with measured spectral data. High temperature materials and special fabrication techniques allow high temperature operation. This technology offers new solutions for spectral control with application in thermophotovoltaic (TPV) energy conversion. Built on a MEMS platform, for thermal isolation from the environment, these devices also modulate at high speed, opening new applications in spectroscopy, infrared imaging, and signaling. Demonstrated wafer-level vacuum sealing improves the wall plug efficiency dramatically. We describe device architecture and fabrication considerations for plasmonic photonic crystal structures which simultaneously act as emitters and sensors in a defined narrow waveband radiation. In particular, this combined capability opens new avenues for research for vital commercial applications such as environmental protection, household safety, bio-hazardous material identification, meteorology and industrial environments.

Recently, wavelength selective emitting materials have attracted extensive interest due to their potential of high optical-to-electricity conversion efficiency for thermal photovoltaic (TPV) cells and realizing high efficient incandescent light sources. A substantial increase in spectral control over thermal radiation and photon recycling can accomplish this objective by the development of high-temperature photonic structures (HTPS) that simultaneously suppress unwanted radiation and enhance emission in a desirable wavelength range. In this paper, we shall review the properties of HTPS as a wavelength selective emitter, the radiative energy transfer relation in real devices, and photon recycling scheme using wavelength selective filters.

In this paper we explore theory, design, and fabrication of photonic crystal (PhC) based selective thermal emitters. In particular, we focus on tailoring spectral and spatial properties by means of resonant enhancement in PhC's. Firstly, we explore narrow-band resonant thermal emission in photonic crystals exhibiting strong spectral and directional selectivity. We demonstrate two interesting designs based on resonant Q-matching: a vertical cavity enhanced resonant thermal emitter and 2D silicon PhC slab Fano-resonance based thermal emitter. Secondly, we examine the design of 2D tungsten PhC as a broad-band selective emitter. Indeed, based on the resonant cavity coupled resonant modes we demonstrate a highly selective, highly-spectrally efficient thermal emitter. We show that an emitter with a photonic cut-off anywhere from 1.8 μm to 2.5 μm can be designed.

Spectral control is an enabling technology for greater than 10% efficient thermophotovoltaic (TPV) energy conversion [1-3]. Without control of the spectral distribution of photons reaching a TPV cell, the efficiency of a TPV energy conversion system suffers dramatically. Spectral control is the selection, design, and development of materials and structures used in TPV energy conversion systems to maximize the net flux of convertible (above band gap) photons, minimize the net flux of unconvertible (below band gap) photons, and recuperate (or reflect) any net flux of unconvertible (below band gap) photons that reach the TPV cell back to the radiating surface. Front surface, tandem filters consisting of an edge pass filter (short pass) in series with a highly doped, epitaxially grown layer have achieved the highest energy weighted spectral efficiency to date.

The performance, however, of these tandem filters is limited by at least two fundamental physical phenomena: the variation in performance with the incident angle of the photons and absorption in the filter materials. Three dimensional (3D), photonic crystal structures offer a potential solution to these fundamental limitations of tandem filters as well as the potential of spectral control for the radiating surfaces of TPV energy conversion system. First, 3D photonic crystal structures, in theory, perform equally for all incident angles. Second, the fabrication techniques for 3D photonic crystal structures may allow the use of materials (such as crystalline Si) with less absorption in the spectral range of interest (1-15μm) than the materials currently used in tandem filters. The fabrication techniques for 3D photonic crystal structures may also allow the use of high temperature (1000°C) materials for use as radiating surface spectral control. If so, 3D photonic crystal structures may be used to mitigate several of the inefficiencies associated with TPV systems.

The conclusion of this assessment is that 3D, photonic crystal structures can, in theory, provide near perfect TPV spectral control. If so, these structures may allow a 20% improvement over the performance achieved by tandem filters. Realizing this potential for TPV spectral control, however, will likely require significant development. At least two major challenges exist:

• Design techniques will likely need to be developed that will allow the analysis and design of 3D photonic crystal structures.

• Fabrication techniques will need to be developed for these structures that provide sufficient fidelity to a design to achieve the required performance and can be scaled to provide the area required for a given TPV application.

The effect of porosity on the radiation component of the thermal conductivity of the thermal barrier coatings is studied. Heat transfer in the disordered porous structures as well as the porous photonic band-gap structures is investigated. The pores, which size is comparable with the characteristic radiation wavelength λmax=2897.8/T μm, were found to be most efficient obstacles for the heat radiation.

The microcavity effect of two-dimensional W surface-relief gratings has been investigated by means of the finite-difference time-domain simulation. The peak structure of the spectral emissivity of W gratings with a number of microcavities is in good agreement with the spectral features of a single microcavity. This result shows that the emissivity enhancement by W gratings with microcavities is mainly attributable to the microcavity effect that arises from each microcavity. It is that the spectral emissivity can be controlled by a combination of several microcavities with different parameters, and that not only a rectangular but a cylindrical microcavity also shows the microcavity effect according to its cavity modes.

Photonic thermal conductance of a multi-layer photonic crystal, when normalized with the corresponding thermal conductance of vacuum at each temperature, can be significantly below unity. The minimum of this normalized thermal conductance occurs at the high-temperature limit, and the conductance value at this limit is independent of the layer thicknesses. We give an analytic theory to explain such independence, and show that it is related to the ergodic nature of the distribution of photonic bands in frequency space.

We have simulated the angle-dependent absorption and thermal emittance of two dimensional metallic and metallodielectric photonic crystals (PCs) with rigorous scattering matrix methods- where Maxwell's equations are solved in Fourier space. These metallic photonic crystals exhibit strong thermal emittance and absorption peaks in the normal direction. This peak splits into multiple peaks at larger and shorter wavelengths away from the normal direction. The thermal emission at different wavelengths is redistributed into different emission angles. There is partial suppression of photon emission at long wavelengths and enhancement at the shorter wavelength spectral range where the thermal emittance has a maximum. Angle-dependent measurements of the emission in metallo-dielectric photonic crystals are performed. Simulations compare well with these measurements and are consistent with the surface plasmon model. The strong dependence of the absorption with angle is very important for thermo-photovoltaic devices.

We design and fabricate metallic photonic crystals with sharp absorption peaks in the infrared regime. We have fabricated a metallic photonic crystal consisting of a triangular lattice of holes in a silicon layer conformally coated with gold at a lattice pitch of 3.8 microns. Conventional lithographic and deep reactive ion etching was used. The photonic crystal exhibits a deep reflection minimum and sharp thermal emission peak near the lattice spacing. Measurements agree well with rigorous scattering matrix simulations. This simple single-layer structure with a single patterned exposure has no emission sidebands and can be scaled to other lattice spacings.

Recent progress in fabrication technology allows for the efficient control of electromagnetic waves by means of photonic devices. This could be attractive and promising also for high-temperature photonic structures to control electromagnetic heat transfer at temperatures above 1000 oC. We discuss the literature and present our own results on Fiber Matrix Composites (FMC), which could be superior to high-temperature metals or monolithic ceramics and can be designed for photonic applications. Possible applications include the protection of non-rotating components in high-temperature engines and turbines such as combustors and liners, coatings and parts for aerospace vehicles. Our discussion includes the material aspect and some relevant structure features. The use of woven fabrics to design new photonic band gap structures is discussed. An example of the use of the plane-wave expansion method for FMC design is given.