Nanostructured chromium-based broadband absorbers and emitters to realize thermally stable solar thermophotovoltaic systems.

The efficiency of traditional solar cells is constrained due to the Shockley-Queisser limit, to circumvent this theoretical limit, the concept of solar thermophotovoltaics (STPVs) has been introduced. The typical design of an STPV system consists of a wideband absorber with its front side facing the sun. The back of this absorber is physically attached to the back of a selective emitter facing a low-bandgap photovoltaic (PV) cell. We demonstrate an STPV system consisting of a wideband absorber and emitter pair achieving a high absorptance of solar radiation within the range of 400-1500 nm (covering the visible and infrared regions), whereas the emitter achieves an emittance of >95% at a wavelength of 2.3 µm. This wavelength corresponds to the bandgap energy of InGaAsSb (0.54 eV), which is the targeted PV cell technology for our STPV system design. The material used for both the absorber and the emitter is chromium due to its high melting temperature of 2200 K. An absorber and emitter pair is also fabricated and the measured results are in agreement with the simulated results. The design achieves an overall solar-to-electrical simulated efficiency of 21% at a moderate temperature of 1573 K with a solar concentration of 3000 suns. Furthermore, an efficiency of 15% can be achieved at a low temperature of 873 K with a solar concentration of 500 suns. The designs are also insensitive to polarization and show negligible degradation in solar absorptance and thermal emittance with a change in the angle of incidence.

Engineering multimodal dielectric resonance of TiO2 based nanostructures for high-performance refractive index sensing applications.

Optical metasurface based refractive index (RI) sensors find applications in chemical, environmental, biomedical, and food processing industries. The existing RI sensors based on metals suffer from the plasmonic loss in the optical regime; in contrast, those based on Fano-type resonances generated by dielectric materials are either polarization-sensitive or are based on complex geometrical structures prone to fabrication imperfections that can lead to severe performance degradation. Here, we demonstrate that careful engineering of resonance modes in dielectric metasurfaces based on simple symmetric meta-atoms can overcome these limitations. More specifically, we have designed low-loss high-performance RI sensors using all-dielectric metasurfaces composed of TiO2 based nanostructures of three different shapes (i.e., cylindrical, square and elliptical) operating at near-infrared (NIR) wavelengths, which are robust against the perturbations of geometric parameters. In terms of physics, this work reports sensor structures achieving sharp resonant dips of high Q-factor in the transmission spectra corresponding to multiple dielectric resonance modes (i.e., electric quadrupole, magnetic dipole, and electric dipole) with superior performance as compared to the state-of-the-art. Four absolute liquids (water, ethanol, pentanol, and carbon tetrachloride) with a refractive index ranging from 1.333 to 1.453 are used to numerically validate the performance, and a maximum sensitivity of 798 nm/RIU with FOM up to 732 has been achieved.