10.1117/2.1201310.005129 A metamaterial to convert heat to light Yosuke Ueba and Junichi Takahara An array of split-ring resonators efficiently absorbs and re-emits thermal energy as IR radiation to power thermophotovoltaic systems. Thermophotovoltaic (TPV) power generation systems recover exhaust heat from various heat sources, such as heat engines or an ironworks rolling process. An example TPV system consists of an IR solar cell and a thermal emitter as a heat–light converter: see Figure 1. The thermal emitter modifies the emission spec- trum of the heat source to match the absorption spectrum of the solar cell. Ideally, the maximum emissivity will be at the wave- length at which the solar cell has its absorption maximum. In general, the shape of a thermal radiation spectrum depends on its temperature. As the temperature increases, the radiation power increases and the peak wavelength is shifted toward the blue end of the spectrum. Additionally, artificial structures on the source can produce resonant modes to enhance the spec- trum as was first proposed by Hesketh and coworkers in 1986. 1 Thermal radiation control by artificial structures has since been studied using open-end resonance, 1, 2 surface waves, 3–5 pho- tonic bandgaps, 6, 7 and cross antennas. 8 The recent development of metamaterials has realized many novel phenomena such as invisibility cloaking, negative refrac- tion, and novel optical filters. 9 In these studies, light interacts with a structure that is much smaller than its own wavelength. An artificial optical material with a 3D structure and an ultra- thin film (metasurface) can behave as a homogeneous medium. Recently, metamaterial techniques have been used to design lu- minescence spectra of thermal emitters and so improve TPV gen- erators’ energy conversion efficiency. Until now, TPV systems have been based on microcavity ar- rays and need deep (i.e., several microns) hole structures on a thick substrate. Advanced dry etching processes are required to achieve these deep-aspect-ratio microstructures and this is chal- lenging over a wide area. In addition, there is no flexibility in such a substrate. We have studied the control of thermal radiation spectra to develop more efficient converters and proposed a metasurface Figure 1. Schematic view of a thermophotovoltaic (TPV) system. : Wavelength. made up of split-ring resonators (SRRs) for efficient heat–light energy conversion with a remarkably low heat capacity. It is very thin (100nm), compatible with conventional planar processes, and easily expanded to make a very wide, thin and flexible sub- strate that can, for example, be used to patch heat engines. Our metasurface is made up of an array of gold SRRs on a plane substrate with glass, chromium, silver, chromium, and sil- ica layers: see Figure 2. 10 To fabricate it, we used electron-beam lithography, and then deposited thin layers of chromium (3nm) and gold (80nm) by vacuum deposition. Finally, we lifted off the redundant resist. The resonant wavelength is greater than the thickness of the SRRs. Note that the silver layer blocks thermal radiation from the underlying substrate. To measure the thermal emission, we used a Fourier-transform IR spectrometer and a ce- ramic heater with constant electric power. Figure 3(a) shows the measured thermal radiation spectra from the SRRs and the plane substrate. The metasurface emis- sion spectrum peaks at about 38THz and 60–80THz. The en- hanced peaks from the SRR emission spectrum were about three and six times larger than the emission by the plane substrate at the same frequencies and we could also modify the peak wave- lengths by changing the size of the SRRs. We also performed a numerical simulation to obtain electric field distributions and observed the nodes attributed to the var- ious resonant modes of an SRR: see Figure 3(b). We were able to Continued on next page