Triple mechanisms were employed to trap mid-infrared (mid-IR) rays within a semi-transparent SiO(2) film sandwiched between gold gratings and a gold substrate. Dimensions of four absorbers were explicitly determined using an LC (inductor-capacitor) circuit model considering the role transition of SiO(2) film. The film behaves as a capacitance and an inductance when the real part of relative electric permittivity for SiO(2) is positive and negative, respectively. At the normal incidence of transverse magnetic waves, every absorptance spectrum of absorbers showed a peak at wavelength λ = 10 μm due to the first mode excitation of magnetic polaritons (MP). At oblique incidence, the Berreman mode led to another peak at λ = 8 μm while its bandwidth was expanded with epsilon near zero mode excited by diffracted waves. The full-width-at-half-maximum of both peaks exceeded 0.6 μm thanks to the SiO(2) loss. Other minor absorptance peaks in the mid-IR were caused by variants of the same MP mode.

The direct utilization of solar radiation has been considered a promising energy source because of its abundance, sustainability, and cleanness. The conversion of solar radiation into usable heat largely depends on the absorption characteristics of a solar thermal collector. In the present study, we conducted design analysis of a wavelength-selective absorber composed of a two-dimensional Nickel grating, a thin SÍO2 film, and a Nickel substrate. Dimensions of the two-dimensional grating were determined with the Taguchi method, which optimized the spectral absorptance for both polarizations. The spectral absorptance demonstrated a broad-band plateau within the visible and the near-infrared spectral region, but it was significantly suppressed at longer wavelengths. Moreover, the absorptance plateau was nearly insensitive to the incident orientation of solar radiation. Physical mechanisms of the absorption enhancement were elucidated with the local magnetic field distribution.Eigure I(í3) depicts the baseline structure of proposed solar thermal absorber. The scheme only shows a unit cell, which is one period of a 2D Ni grating on a SÍO2 film and a Ni substrate. Journal of Heat Transfer

The in-plane scattering patterns from a complex dielectric grating were both numerically and experimentally studied in contrast to those from well-known metallic gratings. The incidence was the transverse electric or transverse magnetic wave of wavelength λ=660  nm. The grating profile was complex with a period Λ=7.0  μm, while the material was lightly doped crystalline silicon. Patterns of the electric field, magnetic field, and spatial intensity distribution were demonstrated at the normal (θ(i)=0°) and oblique (θ(i)=+30°) incidence. Electric and magnetic fields were presented in the near field as well as the far field. The measured power ratio within -90°≤θ(r)≤+90° was plotted. Their major peaks and the numerically obtained diffraction efficiency of 21 orders (-10≤m≤+10 or -15≤m≤+5) of diffracted waves occurred at the same θ(r). Other peaks and stair-like shoulders of major peaks also exhibited in spectra.

A stochastic method and its variants, differential evolution (DE) and micro-DE, were employed to optimize profiles of omnidirectional gratings for desired emittance spectra. Both narrow-band and broad-band infrared emitters were developed successfully from assigned profile types with different complexity and dimension constraints. The efficiency in determining profiles from each method was compared to demonstrate that the superiority of each method is dependent on the number of parameters (dimensions). The performance of the proposed emitters was further discussed considering the emission orientation and fabrication tolerance.

The sunlight has been considered a promising alternative energy source because of its abundance and sustainability. A solar thermal collector can turn the solar irradiation into the usable heat, and thus, its performance highly depends on the efficiency of absorber. An ideal absorber should trap most incoming solar radiation in the visible and near-infrared spectral region, and minimize its emitted thermal energy at long wavelengths. One of the promising solutions for satisfying the aforementioned requirements is to employ periodic structured surfaces, whose tunable radiative properties were used in thermophotovoltaic devices and chemical sensors. Two-dimensional subwavelength gratings are thus proposed for the absorber surface profile in the present study. Design objectives are a broad-band peak in the absorption spectrum and a quasi-isotropic angular lobe at the incidence of both linear polarizations. Nickel is selected for its fabrication easiness and low cost. A SiO2 film sandwiched between gratings and a substrate is considered as extra design flexibility to possibly enhance performance without much difficulty. Radiative properties and electromagnetic fields will be obtained from programs based on the rigorous coupled-wave analysis (RCWA). The optimization is then realized with the Taguchi method.