Wide-angle perfect absorber/thermal emitter in the terahertz regime

TLDR: In this article, the authors show that a perfect absorber/thermal emitter exhibiting an absorption peak of 99.9% can be achieved in metallic nanostructures that can be easily fabricated.

Abstract: We show that a perfect absorber/thermal emitter exhibiting an absorption peak of 99.9% can be achieved in metallic nanostructures that can be easily fabricated. The very high absorption is maintained for large angles with a minimal shift in the center frequency and can be tuned throughout the visible and near-infrared regime by scaling the nanostructure dimensions. The stability of the spectral features at high temperatures is tested by simulations using a range of material parameters. Since the beginning of the last century it is known that a perfect thermal emitter follows Planck’s law of blackbody radiation. 1 Realistic structures, however, generally do not follow Planck’s law but exhibit a smaller emission. The properties of these emitters strongly depend on the materials and their shapes. From the absorption spectra of a structure the emission properties can be deduced since Kirchhoff’s law directly relates the absorption with the emissivity. The emission is then determined by multiplying the emissivity with the blackbody radiation spectrum. Using photonic crystals, 2,3 it has been shown that this approach is also valid for periodically structured materials. For a number of applications such as thermophotovoltaic converters, it is necessary to control the spectral properties to achieve, e.g., selective emitters in a narrow frequency band corresponding to the band gap of solar cells. 4 In the case of structured metallic surfaces, the changes in the emission spectra are based on surface waves coupled to the external radiation through the periodic surface. 5,6 Alternatively, microcavity resonances can also be used to create narrow-band thermal radiation. 7 Unfortunately, most of the recent designs 6,8 for perfect absorbers/ emitters only work for one incident angle and one polarization. So, there is a need for wide-angle perfect absorber/ emitter nanostructures. In this Brief Report, we suggest a structure which exhibits a large absorption in the terahertz regime for a wide range of angles with respect to the surface. We show that the absorption characteristics are maintained even if the uncertainties in the estimated changes in the material parameters, due to high temperatures, are considered. The proposed structure can be easily manufactured with today’s planar microfabrication techniques. We also comment on the impact of deviations in the geometrical parameters caused by fabricational tolerances. The small size of the structure, in comparison to the wavelength together with the relatively straightforward fabrication, allows for easy integration into various devices, such as perfect thermal emitters, perfect absorbers, bolometers, and very effective light extraction light-emitting diodes LEDs. The suggested structure is shown in Fig. 1. It consists of a metal back plate black with a thickness larger than 200 nm. This is much larger than the typical skin depth in the terahertz regime and avoids transmission through the structure. In this case the reflection is the only factor limiting the absorption. The thickness of the back plate can be adjusted to the specific needs of the final application, e.g., to obtain good heat transport to sensors or to obtain a better stability. On top of the metal plate a spacer layer of silicon nitride SiN is deposited with a thickness Dt. The structure is terminated by an array of metallic stripes with a rectangular cross section. Their arrangement is described by a lattice constant a and their shape is given by a width Ww and a thickness Wt. In this setup a strong resonance with a large field enhancement in the dielectric spacer layer and in between the stripes can be obtained, as will be shown later. Adjusting the size of the metal stripes on the top, the coupling to this resonance can be tuned and the reflection can be minimized. Due to the scalability of Maxwell’s equations, in principle, the structure can be simulated using dimensionless units by dividing all sizes by the lattice constant and using =a / as frequency. However, the Drude model used to describe the metal requires frequencies in terahertz and therefore the lattice constant must be assigned in the simulation. If a shift in the frequencies of the spectral features by adjusting the lattice constant is intended, a different simulation must be done since changes in the dielectric constant would not be considered. In the simulation frequency-dependent material parameters are required. We calculate those using standard methods and adjust their values to take into consideration the high temperatures. The tungsten parts plate and stripes are described by a Drude model