Complex gratings

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The concept of complex or aperiodic grating refers to a grating whose surface profile is a superposition of two or more binary or sinusoidal grating profiles. For the application as TPV radiators and infrared detectors, complex gratings may improve simple 1D gratings by reducing the sharpness in the spectral peak and the directional sensitivity [1,2]. The method of forming a complex grating is from a combination of a short-period and a long-period grating. The period of the complex grating is the least common multiple of the periods of two simple gratings. Since the profile of the complex gratings comes from two binary gratings, the normal spectral emittance may have features in common with either of them, whereas the band-folding will form very different emittance features. The emittance/absorptance peaks of the complex grating are wider than those of simple gratings, especially at long wavelengths. This is particularly useful for TPV emitters by enhancing the absorption to frequencies higher than the bandgap of the TPV cell and by suppressing absorption at longer wavelengths [1]. Another example of the application of complex grating is for applications as wavelength-selective absorbers using heavily doped silicon [2]. By properly choosing the carrier concentration and geometry, silicon complex gratings have been shown to exhibit a broadband absorptance peak that is insensitive to the angle of incidence. Furthermore, the peak wavelength can be engineered either with the height of the ridges or the period. Such type of absorptance peak comes from the SPP excitation and is dominated by the first evanescent diffraction order.


References

[1] Chen, Y.-B., and Zhang, Z. M., 2007, “Design of Tungsten Complex Gratings for Thermophotovoltaic Radiators,” Optics Communications, 269, pp. 411-417.

[2] Chen, Y.-B., and Zhang, Z. M., 2008, “Heavily Doped Silicon Complex Gratings as Wavelength-Selective Absorbing Surfaces,” Journal of Physics D: Applied Physics, 41, p. 095406.