Refractorymetal-based broadband absorber/narrowband emitters is a flourishing field in energy harvesting where the physical and chemical stability of the metals at high temperatures provide efficient absorption/emission of solar/ heat energy. [1][2][3] Advancements in solar/ thermophotovoltaics (S/TPV) must be accompanied by thermally stable devices in order to withstand extreme operating conditions. The fundamental limiting factor [Shockley Queisser (SQ) efficiency limit] in traditional single-junction solar cells is its inability in converting the broad solar spectrum into a narrow range of wavelengths defined by the PV cells. [4] Solar photons with energies below the bandgap of the PV cell are not converted, and the photons with energies higher than the bandgap lose their additional energy through a process known as thermalization. In solar thermophotovoltaics (STPV), an intermediate system composed of absorber and emitter is used to overcome the SQ limit by harvesting the solar energy followed by the emission of narrowband radiation. [5] The absorber should provide unitary absorption in the entire solar spectral range over a range of incidence angles with polarization insensitive nature, and minimum thermal reradiation to avoid near-infrared heat radiation at elevated temperatures. Through thermal conduction, the absorbed heat energy is transferred to the emitter, which is tailored to emit a narrowband radiation defined by the bandgap energy of a PV cell. Thus, the absorber needs to withstand high temperatures to transfer a large amount of heat energy to the emitter. The low-loss noble metals are successfully used in unitary absorbers so far, particularly Au and Ag. [6][7][8] However, noble metals are not compatible with hightemperature photovoltaic applications and standard silicon manufacturing processes (complementary metal oxide semiconductor, CMOS, technology), owing to low melting point and diffusion of noble metals into silicon. Annealing the substrates at high temperatures induce oxidation, surface diffusion, corrosion, cracking, and delamination of thin films from the substrate. [7] The situation is even worse in the case of the Broadband absorbers, with the simultaneous advantages of thermal stability, insensitivity to light polarization and angle, robustness against harsh environmental conditions, and large area fabrication by scalable methods, are essential elements in (solar) thermophotovoltaics. Compared to the noble metal and multilayered broadband absorbers, high-temperature refractory metal-based nanostructures with low-Q resonators are reported less. In this work, 3D titanium nitride (TiN) nanopillars are investigated for ultrabroadband absorption in the visible and near-infrared spectral regions with average absorptivities of 0.94, over a wide range of oblique angles between 0° and 75°. The effect of geometrical parameters of the TiN nanopillars on broadband absorption is investigated. By combining the flexibility of nanopillar design and lossy TiN films, ultrabroadband absorption in the vi...

Optical metasurfaces are judicously engineered electromagnetic interfaces that can control and manipulate many of light’s quintessential properties, such as amplitude, phase, and polarization. These artificial surfaces are composed of subwavelength arrays of optical antennas that experience resonant light-matter interaction with incoming electromagnetic radiation. Their ability to arbitrarily engineer optical interactions has generated considerable excitement and interest in recent years and is a promising methodology for miniaturizing optical components for applications in optical communication systems, imaging, sensing, and optical manipulation. However, development of optical metasurfaces requires progress and solutions to inherent challenges, namely large losses often associated with the resonant structures; large-scale, complementary metal-oxide-semiconductor-compatible nanofabrication techniques; and incorporation of active control elements. Furthermore, practical metasurface devices require robust operation in high-temperature environments, caustic chemicals, and intense electromagnetic fields. Although these challenges are substantial, optical metasurfaces remain in their infancy, and novel material platforms that offer resilient, low-loss, and tunable metasurface designs are driving new and promising routes for overcoming these hurdles. In this review, we discuss the different material platforms in the literature for various applications of metasurfaces, including refractory plasmonic materials, epitaxial noble metal, silicon, graphene, phase change materials, and metal oxides. We identify the key advantages of each material platform and review the breakthrough devices that were made possible with each material. Finally, we provide an outlook for emerging metasurface devices and the new material platforms that are enabling such devices.

Silver holds a unique place in plasmonics compared to other noble metals owing to its low losses in the visible and near-IR wavelength ranges. With a growing interest in local heating and high temperature applications of plasmonics, it is becoming critical to characterize the dielectric function of nanometer-scale thin silver films at higher temperatures, especially near the breakdown temperature, which depends on the film thickness and crystallinity. So far, such a comprehensive study has been missing. Here we report the in situ high temperature ellipsometry measurements of ultrasmooth and epitaxial quality crystalline silver films, along with electron beam evaporated polycrystalline silver films at temperatures up to 700 °C, in the wavelength range of 330−2000 nm. Our findings show that the dielectric function of all the films changes remarkably at elevated temperatures with larger relative changes observed in polycrystalline films. In addition, low-loss epitaxial films were found to be thermally more stable at elevated temperatures. We demonstrate the importance of our findings for high temperature applications with a numerical simulation of field enhancement in a bow-tie nanoantenna, a near field transducer commonly used for heat-assisted magnetic recording. The simulated field profiles at elevated temperatures showed significant deviations compared to those at room temperature, clearly suggesting that the use of room temperature optical properties in modeling elevated temperature applications can be misleading due to the thermal deviations in the Ag dielectric function. We also provide causal analytical models describing the elevated temperature Ag dielectric functions.

The strong electric and magnetic resonances in dielectric subwavelength structures have enabled unique opportunities for efficient manipulation of light–matter interactions. Besides, the dramatic enhancement of nonlinear light–matter interactions near so‐called bound states in the continuum (BICs) has recently attracted enormous attention due to potential advancements. However, the experimental realizations and the applications of high‐Q factor resonances in dielectric resonances in the visible have thus far been considerably limited. In this work, the interplay of electric and magnetic dipoles in arrays of dielectric nanoresonators is explored. The experimental realization of high‐Q factor resonances in the visible through the collective diffractive coupling of electric and magnetic dipoles is reported. It is also shown that coupling the Rayleigh anomaly of the array with the dipoles of the individual nanoresonators can result in the formation of different types of BICs. The resonances in the visible regime is utilized to achieve lasing action at room temperature with high spatial directionality and low threshold. Finally, multi‐mode, directional lasing is experimentally demonstrated and the BIC‐assisted lasing mode engineering in arrays of dielectric nanoresonators is studied. It is believed that the results enable a new range of applications in flat photonics through realizing on‐chip controllable single and multi‐wavelength micro‐lasers.

Germanium Oxynitride (GeON) gate interlayer (IL) dielectric formed using decoupled plasma nitridation (DPN) technique is compared with GeO2 and thermally nitrided GeON ILs for Ge gate stack applications using n-channel capacitors and transistors. Lower nitrogen concentration and roughness at the GeON/Ge interface lead to lower midgap interface trap density (Dit) and 1.5× higher electron mobility for the DPN versus thermally nitrided GeON IL. DPN GeON IL also exhibits enhanced thermal stability till 575 °C at the expense of a small degradation in Dit versus GeO2 IL, making it a more viable gate IL dielectric on Ge channels.