Sunlight is one of the Earth's clean and sustainable natural energy resources, and extensive studies have been conducted on the conversion of solar energy into electricity using photovoltaic (PV) devices. However, single-junction PV devices cannot break the theoretical efficiency limit known as the Shockley-Queisser limit that is caused by the sub-bandgap transmission and heat dissipation losses in semiconductors. Solar thermal conversion approaches may provide an alternative way to exceed this limit and enable more efficient use of solar light than that in PV devices. Recently, spectrally or thermally engineered metamaterials have attracted considerable attention for solar energy applications because of their excellent physical properties. This review describes the recent research progress in the development of these photothermal and thermoplasmonic metamaterials, along with their promising applications in solar thermophotovoltaics, radiative cooling, and solar desalination.

We present a large-area and single-step fabrication approach based on phase spatial light modulator (SLM)-assisted interference lithography for the realization of submicrometer photonic structures on photoresist. A multimirror beam steering unit is used to reflect the SLM-generated phase-engineered beams leading to a large angle between interfering beams while also preserving the large area of the interfering plane beams. Both translational and rotational periodic submicrometer structures are experimentally realized. This approach increases the flexibility of interference lithography to fabricate more complex submicrometer photonic structures and photonic metamaterial structures for future applications.

Solar steam generation is a promising solar energy harvesting technology to address the global clean energy and water deficiency cost-effectively. In this work, we present a compact plasmonic nanostructure–nanoparticle composite based on haze/GO-rGO/Au enabling multiple purposes such as broadband solar absorption, solar steam generation, and solar desalination. The graphene oxide–reduced GO (GO-rGO) combination allows broadband optical absorption, and the presence of 5 nm Au nanoparticles creates high-density localized hotspots for enhanced photothermal effect as evidenced through Raman signal enhancement studies. Anodized aluminum oxide-based haze nanostructures provide maximum light interaction volume through multiple nanogaps and porosity through vertically aligned nanowires of 20–26 nm width and 5–10 μm depth for water channelization. The haze sample coated with GO-rGO/Au shows high solar absorbance of 92.5% over the 300–2500 nm wavelength range covering the whole solar spectrum (ultraviolet–visible–near-infrared). We have achieved 50 and 71.1% of solar-to-thermal conversion efficiencies in a single-layer microscale sample for saline and freshwater, respectively, with a maximum surface temperature of 95.7 °C. The efficiencies increase to 64 and 77% for two layers of the sample at 5 suns (5 kW m–2) illumination.

This Letter demonstrates a single-step optical realization method for hexagonal and square lattice-based dual periodic motheye and gradient-index-array photonic structures over large areas. Computed phase mask of gradient interference patterns are used as inputs to a phase-only spatial light modulator (SLM), and the first-order diffracting beams are coherently superposed with the help of a 2f-2f Fourier filtering setup to avoid complex optical geometry for generation and control of individual beams. The simulated interference patterns are verified experimentally through a CMOS camera. The fabricated micro-structures on a positive photoresist are shown to have a major periodicity of 638 μm and minor periodicity of 25.2 μm, with the air hole diameter varying from 22.7 to 6.9 μm along the X and Y axes. The depth of the fabricated structure gradually varies from 4.203 μm at the center to 1.818 μm at the corner. These structures may be scaled down to submicron features that can show improved anti-reflection properties for solar energy harvesting and GRIN lens for optical wavelength region.

In this Letter we report for the first time, to the best of our knowledge, a phase spatial light modulator (SLM)-based interference lithography (IL) approach for the realization of hexagonally packed helical photonic structures with a submicrometer scale spatial, as well as axial, periodicity over a large area. A phase-only SLM is used to electronically generate six phase-controlled plane beams. These six beams from the front side and a direct central backside beam are used together in an "inverted umbrella" geometry setup to realize the desired submicrometer axial periodic chiral photonic structures through IL. The realized structures with 650 nm spatial and 353 nm axial periodicities on negative photoresist can be used as an optical filter and refractive index sensor, as evidenced from the FDTD-based simulation study on its optical properties. Further, the fabricated templates can be transferred to metals such as silver or aluminum for the realization of a metamaterial-based broadband circular polarizer ranging from 1 to 3.5 μm of near-infrared spectra.