No abstract

Spectrally selective absorbers have received considerable interest due to their applications in thermophotovoltaic devices and as solar absorbers. Due to extreme operating conditions in these applications, such as high temperatures, thermo-mechanically stable and broadband spectrally selective absorbers are of interest. This paper demonstrates anisotropic random rough surfaces that provide broadband spectrally selective absorption for the thermo-mechanically stable Tungsten surfaces. Anisotropic random rough surface has different correlation lengths in the x- and y-directions, which means their topography parameters have directional dependence. In particular, we demonstrate that spectral absorptance of Tungsten random rough surfaces at visible (VIS) and near-infrared (NIR) spectral regions are sensitive to correlation length and RMS height variations. Our results indicate that by optimizing random rough surface parameters, absorption values exceeding 95% can be obtained. Moreover, our results indicate that anisotropic random rough surfaces broaden the bandwidth of the high absorption region. It is shown that in VIS and NIR regions, the absorption enhancements of up to 47% and 52% are achieved for the isotropic and anisotropic rough surfaces, respectively.

Although black silicon is utilized in a wide range of applications due to its broadband spectral emission and absorption, the underlying electromagnetic mechanisms are not well explored. In this study, the underlying phenomena that are responsible for these enhanced spectral features are investigated. The absorption spectra of the black silicon with random textures are analyzed, and the electromagnetic mechanisms that drive elevated absorption are explored. Our findings reveal that two separate electromagnetic phenomena occur in the textures, effective wavelength matching and waveguide modes. Detailed analysis reveals that the occurrence condition of those phenomena is highly dependent on the dimensions of the textures in the transverse direction. The effect of the texture dimensions and doping concentration both on absorption characteristics and physical phenomena is analyzed in detail. The findings of this study explain the absorption mechanisms of black silicon observed in experimental studies, which can lead to designer materials with rough surfaces for the desired spectral emissivity.

Passive radiative cooling has garnered significant attention in recent years due to its potential in addressing the energy consumption of conventional cooling systems. Plasmonic and metamaterial structures have been found to be effective broadband absorbers due to their selective emissive spectra, thin thickness, design flexibility, and the ability to excite plasmonic or photonic resonances. This study explores the use of bowtie shape plasmonic metamaterials for the development of novel, structurally simple radiative cooling devices. We show that by designing and optimizing a periodic high index-low index alternating layers (SiO2-TiO2), broadband reflection in visible and near-infrared spectrums is achievable. While to achieve broadband absorption in the transparency window (8-13 um), metamaterial is utilized.