The selective absorption of sunlight plays a critical role in solar-thermophotovoltaic (STPV) energy conversion by tailoring both the absorption and emission spectrums for efficient solar-thermal-electrical energy conversion. [1][2][3][4][5][6][7] By selectively absorbing solar energy while suppressing long wavelength emission, optimal solar-thermal energy conversion can be achieved. In practical STPV systems, selective absorbers must simultaneously contain optical, manufacturing, and reliability properties. Previous efforts have typically focused only on a subset of these requirements. In this communication, we present our solution which contains all of the ideal properties of a selective absorber for large-scale and efficient solar energy conversion.The effective absorption of solar energy requires selective absorption across the solar spectrum, high temperature reliability, omnidirectional absorption, and wafer-scale fabrication 2 for mass scalability. Recent developments of metal based selective absorbers have demonstrated 1D, 2D, and 3D metallic photonic crystal structures capable of tailoring the absorption spectrum. [2,[8][9][10][11][12][13][14] One dimensional metal dielectric stacks have demonstrated promising solar absorbing properties but are unstable at temperatures greater than approximately 600°C. [13] In particular, two-dimensional metallic air photonic crystals (MAPhC) have been shown to selectively absorb light in the near-IR via cavity modes and withstand high temperatures greater than 1000°C; however, the acceptance angle is limited to ±30°, and the absorption in the visible spectrum is limited due to diffraction. [2,11,15] Metamaterial and plasmonic based absorbers have demonstrated wide angle absorption due to their subwavelength periodic structures; however high temperature stability and wafer-scalefabrication have yet to be shown. [1,3,[16][17][18][19] Here we present our 2D metallic dielectric photonic crystal (MDPhC) structure, which simultaneously demonstrates broadband (visible to near-IR) absorption, omnidirectional absorption, wafer-scale fabrication, and high temperature robustness.[20] The wafer-scale fabricated MDPhC has a measured absorption of 85% for photon energies and an absorption below 10% for . Angled measurements show existence of the cavity modes for angles up to 70° from normal. Furnace tests at 1000°C for 24 hours show a robust optical performance due to its fully encapsulated design which helps to retain the metal cavity shapes at high temperatures. [21] Finite-difference time-domain (FDTD) and rigorous coupled wave analysis (RCWA) based simulations indicate that the broadband absorption is due to a high density of hybrid cavity and surface plasmon modes overlaped with an anti-reflection coating (ARC).A schematic image of the MDPhC is shown in Figure 1(a) and (b). The MDPhC utilizes cut-off frequencies of cavity modes to tailor the absorption. Since the cut-off frequency is dependent on the geometry of the cavities, the absorption spectrum can be tuned by simply modif...

Abstract:We present a theoretical study of the optical angular momentum transfer from a circularly polarized plane wave to thin metal nanoparticles of different rotational symmetries. While absorption has been regarded as the predominant mechanism of torque generation on the nanoscale, we demonstrate numerically how the contribution from scattering can be enhanced by using multipolar plasmon resonance. The multipolar modes in non-circular particles can convert the angular momentum carried by the scattered field and thereby produce scattering-dominant optical torque, while a circularly symmetric particle cannot. Our results show that the optical torque induced by resonant scattering can contribute to 80% of the total optical torque in gold particles. This scattering-dominant torque generation is extremely mode-specific, and deserves to be distinguished from the absorption-dominant mechanism. Our findings might have applications in optical manipulation on the nanoscale as well as new designs in plasmonics and metamaterials.

Ultrathin lossy fi lms have attracted much attention due to their strong interference persisting inside the lossy dielectric fi lm on a refl ective substrate. Here, a plasmon-enhanced ultrathin fi lm broadband absorber is proposed by combining the ultrathin fi lm absorber with localized surface plasmon resonances. This concept can be realized by patterning nanoholes on an absorber comprised of an absorptive ultrathin Ge fi lm and a refl ective Au layer, where the localized surface plasmon mode is activated by metallic pore-shaped holes. The plasmonic enhancement is resulting from the pore-shape localized resonance mode, which increases the optical path length through scattering and concentrates the incident light fi eld near the interface of Ge/Au. The experimental characterization results of a nanoporous ultrathin fi lm absorber, which is fabricated with a scalable laser interference lithography approach, demonstrate its superior light absorption performance. Several materials, such as Ag, Al, and Cu, are proposed as an alternative to Au, and they can also provide plasmonic enhancement to ultrathin fi lms. Furthermore, through an effi cient way to optimize the structural dimensions of the nanoporous ultrathin fi lm absorber, a trilayer system of TiO 2 /Ge/Au achieves the total solar absorptance over 89.3% with a wavelength range of 400-1100 nm.

Herein, we report a method that uses free-base amino acids to mediate the controlled hydrothermal growth of amorphous zinc oxide (a-ZnO) or nanocrystalline zinc sulfide (c-ZnS) shells on gold nanoparticles. By screening through a set of 13 candidate amino acids, we have identified four as being capable of mediating inorganic shell growth using an aqueous, low-temperature, one-pot process. In particular, unaggregated and monodisperse sols of exceptional quality are produced using l-histidine, which preserves colloidal stability and mediates the growth of continuous and remarkably uniform a-ZnO shells with a tunable thickness between 2 and 25 nm while avoiding the nucleation of free particles. By coupling spectral extinction measurements with generalized Mie theory calculations, we estimated the complex refractive index of the a-ZnO shell to be 1.47 + i0.09. It is expected not only that our Au@a-ZnO core–shell particles are suitable for both energy and biological applications but also that our process for growing inorganic shells could be extended to other nanocomposite systems comprised of different materials and geometries.

A universal property of resonant subwavelength scatterers is that their optical cross-sections are proportional to a square wavelength, λ 2 , regardless of whether they are plasmonic nanoparticles, twolevel quantum systems, or RF antennas. The maximum cross-section is an intrinsic property of the incident field : plane waves, with infinite power, can be decomposed into multipolar orders with finite powers proportional to λ 2 . In this Article, we identify λ 2 /c and λ 3 /c as analogous force and torque constants, derived within a more general quadratic scattering-channel framework for upper bounds to optical force and torque for any illumination field. This framework also solves the reverse problem: computing globally optimal "holographic" incident beams, for a fixed collection of scatterers. We analyze structures and incident fields that approach the bounds, which for wavelength-scale bodies show a rich interplay between scattering channels, and we show that spherically symmetric structures are forbidden from reaching the plane-wave force/torque bounds. This framework should enable optimal mechanical control of nanoparticles with light.