Metamaterials of metal-insulator-metal structures represent effective ways in manipulating light absorbance for photodetection, sensing, and energy harvesting etc. Most of the time, specular reflection has been used in characterizing resonances of metamaterials without considering diffuse scattering from their periodic subwavelength units. In this paper, we investigate diffuse reflection in metasurfaces made of periodic metallic disks in the mid-infrared region. Integrating sphere-based spectral measurements indicate that diffuse reflection is dominated by grating diffractions, which cause diffuse scattering in a spectral region with wavelengths less than that of the first order Rayleigh anomaly. The diffuse reflection is greatly enhanced by the metasurface resonance and exhibits a general increase towards shorter wavelengths, which not only causes a significant difference in evaluating the metamaterial resonant absorption efficiency but also a small blue-shift of the resonance frequency. These findings are helpful for designing and analyzing metamaterial resonant properties when diffuse scattering is taken into account.

As a planar resonant structure, Salisbury screen offers a cost-effective way of manipulating electromagnetic waves for both fundamental studies and practical applications in optoelectronics. In this paper, we demonstrate Salisbury screen absorbers using epsilon-near-zero substrate, which reduces the spacer thickness below typical one quarter wavelength limit. Three-layered thin-film absorbers made of SiC substrate, ZnSe spacer layer and top NiCr film are designed and fabricated, which exhibit near-perfect absorption at 11.72 μm with spacer thickness of about half of a quarter-wavelength. For ideal zero-index material without optical loss, our proposed thin-film absorber simplifies to a two-layered structure even without the spacer layer in theory. These results suggest that epsilon-near-zero materials provide an alternative approach in developing compact planar absorbing structures without involving lithographic patterning.

The reaction between CO2 and electrochemically reduced bis(phthalocyaninato)neodymate(III) complex in a dimethylformamide solution was investigated by cyclic voltammetry and in situ visible spectroscopy. The introduction of gaseous CO2 into the phthalocyanine solution caused a considerable increase in the first reduction peak current of the complex and a shift in the second reduction peak to more positive potentials. In addition, new anodic current peaks appeared in the reverse sweep at potentials different from those of the complex. These observations were explained in terms of the formation of a CO2-adduct. The formation and decomposition of the CO2-adducts occurred reversibly with the applied potential, which was confirmed by in situ visible spectroscopy.

Structure-engineered silicon exhibits a wealth of unique optical properties below its bandgap, which holds promise for mid-infrared and terahertz applications such as photodetection, thermophotovoltaics, radiative cooling, and spectroscopy. In this paper, we investigate enhancement mechanisms of sub-bandgap absorption of black silicon fabricated into periodic pyramids. Our measurements indicate that the pyramid structure leads to an enhanced broadband absorption in the wavelength region from 1.5 to 13.07 μm with an efficiency of over 80%. The broadband absorption enhancement is shown to originate from the Rayleigh–Wood anomaly, localized magnetic plasmonic resonance, and graded-index effect, which together facilitate the interaction between light and free-carriers in silicon. These results are helpful for understanding the interaction between light and black silicon.