A Single-Step Bottom-up Approach for Synthesis of Highly Uniform Mie-Resonant Crystalline Semiconductor Particles at Visible Wavelengths.

Optically Mie-resonant crystalline silicon nanoparticles have long attracted interest for their unique scattering behaviors. Here, we report a bottom-up nonthermal plasma process that produces highly monodisperse particles, with diameters controllable between 60 and 214 nm, by temporarily electrostatically trapping nanoparticles inside a continuous-flow plasma reactor. The particle size is tuned by adjusting the gas residence time in the reactor. By dispersing the nanoparticles in water, optical extinction measurements indicate colloidal solutions of a particle-based metafluid in which particles support both strong magnetic and electric dipole resonances at visible wavelengths. The spectral overlap of the electric and magnetic resonances gives rise to directional Kerker scattering. The extinction measurements show excellent agreement with Mie theory, supporting the idea that the fabrication process enables particles with narrow distributions in size, shape, and composition. This single-step gas-phase process can also produce Mie-resonant nanoparticles of dielectric materials other than silicon and directly deposit them on the desired substrates.

Thermal metasurface with tunable narrowband absorption from a hybrid graphene/silicon photonic crystal resonance.

We report the design of a tunable, narrowband, thermal metasurface that employs a hybrid resonance generated by coupling a tunable permittivity graphene ribbon to a silicon photonic crystal. The gated graphene ribbon array, proximitized to a high quality factor Si photonic crystal supporting a guided mode resonance, exhibits tunable narrowband absorbance lineshapes (Q > 10,000). Actively tuned Fermi level modulation in graphene with applied gate voltage between high absorptivity and low absorptivity states gives rise to absorbance on/off ratios exceeding 60. We employ coupled-mode theory as a computationally efficient approach to elements of the metasurface design, demonstrating an orders of magnitude speedup over typical finite element computational methods.

Publisher Correction: Materials challenges for the Starshot lightsail.

In the version of this Perspective originally published, Fig. 1 was missing the following credit line from the caption: 'Background image from ESA/Hubble (A. Fujii).' This has now been corrected in the online versions of the Perspective.

Publisher Correction: Materials challenges for the Starshot lightsail.

In the version of this Perspective originally published, the titles of the references were missing; all versions have now been amended to include them.

Nanophotonic Heterostructures for Efficient Propulsion and Radiative Cooling of Relativistic Light Sails.

Light sails propelled by radiation pressure from high-power lasers have the potential to achieve relativistic spaceflight. In order to propel a spacecraft to relativistic speeds, an ultrathin, gram-sized light sail will need to be stably accelerated by lasers with ∼MW/cm2 intensities operating in the near-infrared spectral range. Such a laser-driven sail requires multiband electromagnetic functionality: it must simultaneously exhibit very low absorptivity in the (Doppler-broadened) laser beam spectrum in the near-infrared and high emissivity in the mid-infrared for efficient radiative cooling. These engineering challenges present an opportunity for nanophotonic design. Here, we show that designed thin-film heterostructures could become multifunctional building-block elements of the light sail, due to their ability to achieve substantial reflectivity while maintaining low absorption in the near-infrared, significant emissivity in the mid-infrared, and a very low mass. For a light sail carrying a payload, we propose a relevant figure of merit-the reflectivity adjusted area density-that can capture the trade-off between sail mass and reflectivity, independent of other quantities such as the incident beam power, phased array size, or the payload mass. Furthermore, we present designs for effective thermal management via radiative cooling and compare propulsion efficiencies for several candidate materials, using a general approach that could apply to a broad range of high-power laser propulsion problems.

Limits to the Optical Response of Graphene and Two-Dimensional Materials.

Two-dimensional (2D) materials provide a platform for strong light-matter interactions, creating wide-ranging design opportunities via new-material discoveries and new methods for geometrical structuring. We derive general upper bounds to the strength of such light-matter interactions, given only the optical conductivity of the material, including spatial nonlocality, and otherwise independent of shape and configuration. Our material figure-of-merit shows that highly doped graphene is an optimal material at infrared frequencies, whereas single-atomic-layer silver is optimal in the visible. For quantities ranging from absorption and scattering to near-field spontaneous-emission enhancements and radiative heat transfer, we consider canonical geometrical structures and show that in certain cases the bounds can be approached, while in others there may be significant opportunity for design improvement. The bounds can encourage systematic improvements in the design of ultrathin broadband absorbers, 2D antennas, and near-field energy harvesters.

Overcoming the black body limit in plasmonic and graphene near-field thermophotovoltaic systems.

Near-field thermophotovoltaic (TPV) systems with carefully tailored emitter-PV properties show large promise for a new temperature range (600 ­ 1200K) solid state energy conversion, where conventional thermoelectric (TE) devices cannot operate due to high temperatures and far-field TPV schemes suffer from low efficiency and power density. We present a detailed theoretical study of several different implementations of thermal emitters using plasmonic materials and graphene. We find that optimal improvements over the black body limit are achieved for low bandgap semiconductors and properly matched plasmonic frequencies. For a pure plasmonic emitter, theoretically predicted generated power density of 14 W/cm2 and efficiency of 36% can be achieved at 600K (hot-side), for 0.17eV bandgap (InSb). Developing insightful approximations, we argue that large plasmonic losses can, contrary to intuition, be helpful in enhancing the overall near-field transfer. We discuss and quantify the properties of an optimal near-field photovoltaic (PV) diode. In addition, we study plasmons in graphene and show that doping can be used to tune the plasmonic dispersion relation to match the PV cell bangap. In case of graphene, theoretically predicted generated power density of 6(120) W/cm2 and efficiency of 35(40)% can be achieved at 600(1200)K, for 0.17eV bandgap. With the ability to operate in intermediate temperature range, as well as high efficiency and power density, near-field TPV systems have the potential to complement conventional TE and TPV solid state heat-to-electricity conversion devices.

Shaping contactless radiation forces through anomalous acoustic scattering.

Waves impart momentum and exert force on obstacles in their path. The transfer of wave momentum is a fundamental mechanism for contactless manipulation, yet the rules of conventional scattering intrinsically limit the radiation force based on the shape and the size of the manipulated object. Here, we show that this intrinsic limit can be broken for acoustic waves with subwavelength-structured surfaces (metasurfaces), where the force becomes controllable by the arrangement of surface features, independent of the object's overall shape and size. Harnessing such anomalous metasurface scattering, we demonstrate complex actuation phenomena: self-guidance, where a metasurface object is autonomously guided by an acoustic wave, and tractor beaming, where a metasurface object is pulled by the wave. Our results show that bringing the metasurface physics of acoustic waves, and its full arsenal of tools, to the domain of mechanical manipulation opens new frontiers in contactless actuation and enables diverse actuation mechanisms that are beyond the limits of traditional wave-matter interactions.

Broadband, Angle- and Polarization-Invariant Antireflective and Absorbing Films by a Scalable Synthesis of Monodisperse Silicon Nanoparticles.

Optically induced magnetic resonances (OMRs) are highly tunable scattering states that cannot be reproduced in systems that only support electric resonances, such as in metals, lossy, or low-index materials. Despite offering unique scattering and coupling behavior, the study of OMRs in thin films has been limited by synthesis and simulation constraints. We report on the absorption and scattering response of OMR-based thin films composed of monodisperse crystalline silicon nanoparticles synthesized using a scalable nonthermal plasma growth technique and tractable simulation framework. The synthesis is solvent and ligand free, ensuring minimal contamination, and crystalline particles form with high yield and a narrow size distribution at close to room temperature. Using a scalable high-throughput deposition method, we deposit random particle films, without the need of a solid host matrix, showing near complete blackbody absorption at the collective OMR. This is achieved using 70% less material than an optimized antireflective-coated crystalline silicon thin film. The film exhibits strongly directional forward scattering with very low reflectivity, thus giving rise to angle- and polarization-insensitive antireflection properties across the visible spectrum. We find that, while commonly used effective medium models cannot capture the optical response, a modified effective medium accounting for multipole resonances and interparticle coupling shows excellent agreement with experiment. The effective permittivity and permeability are written in a mode and cluster resolved form, providing useful insight into how individual resonances and nanoparticle clusters affect the overall film response. Electric and magnetic-mode coupling show dramatically different behavior, resulting in uniquely different spectral broadening.

Frequency-selective near-field radiative heat transfer between photonic crystal slabs: a computational approach for arbitrary geometries and materials.

We demonstrate the possibility of achieving enhanced frequency-selective near-field radiative heat transfer between patterned (photonic-crystal) slabs at designable frequencies and separations, exploiting a general numerical approach for computing heat transfer in arbitrary geometries and materials based on the finite-difference time-domain method. Our simulations reveal a tradeoff between selectivity and near-field enhancement as the slab-slab separation decreases, with the patterned heat transfer eventually reducing to the unpatterned result multiplied by a fill factor (described by a standard proximity approximation). We also find that heat transfer can be further enhanced at selective frequencies when the slabs are brought into a glide-symmetric configuration, a consequence of the degeneracies associated with the nonsymmorphic symmetry group.

All-day fresh water harvesting by microstructured hydrogel membranes.

Solar steam water purification and fog collection are two independent processes that could enable abundant fresh water generation. We developed a hydrogel membrane that contains hierarchical three-dimensional microstructures with high surface area that combines both functions and serves as an all-day fresh water harvester. At night, the hydrogel membrane efficiently captures fog droplets and directionally transports them to a storage vessel. During the daytime, it acts as an interfacial solar steam generator and achieves a high evaporation rate of 3.64 kg m-2 h-1 under 1 sun enabled by improved thermal/vapor flow management. With a homemade rooftop water harvesting system, this hydrogel membrane can produce fresh water with a daily yield of ~34 L m-2 in an outdoor test, which demonstrates its potential for global water scarcity relief.

Active Radiative Thermal Switching with Graphene Plasmon Resonators.

We theoretically demonstrate a near-field radiative thermal switch based on thermally excited surface plasmons in graphene resonators. The high tunability of graphene enables substantial modulation of near-field radiative heat transfer, which, when combined with the use of resonant structures, overcomes the intrinsically broadband nature of thermal radiation. In canonical geometries, we use nonlinear optimization to show that stacked graphene sheets offer improved heat conductance contrast between "ON" and "OFF" switching states and that a >10× higher modulation is achieved between isolated graphene resonators than for parallel graphene sheets. In all cases, we find that carrier mobility is a crucial parameter for the performance of a radiative thermal switch. Furthermore, we derive shape-agnostic analytical approximations for the resonant heat transfer that provide general scaling laws and allow for direct comparison between different resonator geometries dominated by a single mode. The presented scheme is relevant for active thermal management and energy harvesting as well as probing excited-state dynamics at the nanoscale.

Topologically enabled optical nanomotors.

Shaping the topology of light, by way of spin or orbital angular momentum engineering, is a powerful tool to manipulate matter on the nanoscale. Conventionally, such methods focus on shaping the incident beam of light and not the full interaction between the light and the object to be manipulated. We theoretically show that tailoring the topology of the phase space of the light particle interaction is a fundamentally more versatile approach, enabling dynamics that may not be achievable by shaping of the light alone. In this manner, we find that optically asymmetric (Janus) particles can become stable nanoscale motors even in a light field with zero angular momentum. These precessing steady states arise from topologically protected anticrossing behavior of the vortices of the optical torque vector field. Furthermore, by varying the wavelength of the incident light, we can control the number, orientations, and the stability of the spinning states. These results show that the combination of phase-space topology and particle asymmetry can provide a powerful degree of freedom in designing nanoparticles for optimal external manipulation in a range of nano-optomechanical applications.

Tailoring high-temperature radiation and the resurrection of the incandescent source.

In solar cells, the mismatch between the Sun's emission spectrum and the cells' absorption profile limits the efficiency of such devices, while in incandescent light bulbs, most of the energy is lost as heat. One way to avoid the waste of a large fraction of the radiation emitted from hot objects is to tailor the thermal emission spectrum according to the desired application. This strategy has been successfully applied to photonic-crystal emitters at moderate temperatures, but is exceedingly difficult for hot emitters (>1,000 K). Here, we show that a plain incandescent tungsten filament (3,000 K) surrounded by a cold-side nanophotonic interference system optimized to reflect infrared light and transmit visible light for a wide range of angles could become a light source that reaches luminous efficiencies (∼40%) surpassing existing lighting technologies, and nearing a limit for lighting applications. We experimentally demonstrate a proof-of-principle incandescent emitter with efficiency approaching that of commercial fluorescent or light-emitting diode bulbs, but with exceptional reproduction of colours and scalable power. The ability to tailor the emission spectrum of high-temperature sources may find applications in thermophotovoltaic energy conversion and lighting.

Efficient plasmonic emission by the quantum Cerenkov effect from hot carriers in graphene.

Graphene plasmons have been found to be an exciting plasmonic platform, thanks to their high field confinement and low phase velocity, motivating contemporary research to revisit established concepts in light-matter interaction. In a conceptual breakthrough over 80 years old, Cerenkov showed how charged particles emit shockwaves of light when moving faster than the phase velocity of light in a medium. To modern eyes, the Cerenkov effect offers a direct and ultrafast energy conversion scheme from charge particles to photons. The requirement for relativistic particles, however, makes Cerenkov emission inaccessible to most nanoscale electronic and photonic devices. Here we show that graphene plasmons provide the means to overcome this limitation through their low phase velocity and high field confinement. The interaction between the charge carriers flowing inside graphene and the plasmons enables a highly efficient two-dimensional Cerenkov emission, giving a versatile, tunable and ultrafast conversion mechanism from electrical signal to plasmonic excitation.