Long-wave infrared tunable thin-film perfect absorber utilizing highly doped silicon-on-sapphire.

We show that nearly perfect absorption can be achieved in a simple structure with highly doped silicon on a sapphire (SOS) substrate. An SOS structure with the n-Si film being 600 nm thick and having doping concentration of 2e19 cm(-3) has an absorption peak of 96% in the film at a wavelength of 12.1 μm. More generally, 95% absorption in the n-Si can be achieved and tailored to specific wavelengths in the range of 11.6-15.1 μm utilizing dopings of 1-2.4e19 cm(-3) and film thicknesses of 600-1000 nm. Regions of 90% absorption can be achievable down to 11 μm and up to as much as 22 μm with tailoring of doping and film thickness. It is also shown that choice of substrate with large k/n (imaginary over real part of refractive index) is imperative for high absorption in the thin-film and will play a role in tailoring possibilities. Shown here are results for n-Si, but in general these results also apply to p-Si and the methods may be used to investigate structures with alternative films or substrates. This investigated SOS structure has high potential since desired film thickness and doping investigated here for perfect absorption can be purchased commercially and easily tuned by etching the silicon film.