Electronic Modulation of Near-Field Radiative Transfer in Graphene Field Effect Heterostructures.

Manipulating heat flow in a controllable and reversible manner is a topic of fundamental and practical interest. Numerous approaches to perform thermal switching have been reported, but they typically suffer from various limitations, for instance requiring mechanical modulation of a submicron gap spacing or only operating in a narrow temperature window. Here, we report the experimental modulation of radiative heat flow by electronic gating of a graphene field effect heterostructure without any moving elements. We measure a maximum heat flux modulation of 4 ± 3% and an absolute modulation depth of 24 ± 7 mW m-2 V-1 in samples with vacuum gap distances ranging from 1 to 3 µm. The active area in the samples through which heat is transferred is ∼1 cm2, indicating the scalable nature of these structures. A clear experimental path exists to realize switching ratios as large as 100%, laying the foundation for electronic control of near-field thermal radiation using 2D materials.

Anisotropic Quantum Well Electro-Optics in Few-Layer Black Phosphorus.

The incorporation of electrically tunable materials into photonic structures such as waveguides and metasurfaces enables dynamic, electrical control of light propagation at the nanoscale. Few-layer black phosphorus is a promising material for these applications due to its in-plane anisotropic, quantum well band structure, with a direct band gap that can be tuned from 0.3 to 2 eV with a number of layers and subbands that manifest as additional optical transitions across a wide range of energies. In this Letter, we report an experimental investigation of three different, anisotropic electro-optic mechanisms that allow electrical control of the complex refractive index in few-layer black phosphorus from the mid-infrared to the visible: Pauli-blocking of intersubband optical transitions (the Burstein-Moss effect); the quantum-confined Stark effect; and the modification of quantum well selection rules by a symmetry-breaking, applied electric field. These effects generate near-unity tuning of the BP oscillator strength for some material thicknesses and photon energies, along a single in-plane crystal axis, transforming absorption from highly anisotropic to nearly isotropic. Lastly, the anisotropy of these electro-optical phenomena results in dynamic control of linear dichroism and birefringence, a promising concept for active control of the complex polarization state of light, or propagation direction of surface waves.

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.

Emerging photonic architectures in two-dimensional opto-electronics.

The isolation of thermodynamically stable, free standing materials with single to few atom thicknesses has brought about a revolution in materials science, condensed matter physics and device engineering for opto-electronic applications. These two dimensional (2D) materials cover a broad range of electronic properties ranging from zero-band gap, semi-metallic graphene to wide band gap semiconductors in sulfides and selenides of Mo and W to metallic behavior in Ti, Nb and Ta sulfides and selenides. This permits their potential application in opto-electronic devices from terahertz frequencies up to the ultraviolet portion of the spectrum. However, their atomically thin nature poses fundamental challenges in driving efficient light-matter interactions. A range of strategies have been explored from the area of photonics and resonant optics that enhance the coupling and interaction of light with atomically thin layers to overcome this challenge. By comparing and contrasting critical advantages of integrating nanophotonic elements with 2D materials, this review highlights the challenges and advantages of such opto-electronic devices.

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.

High Photovoltaic Quantum Efficiency in Ultrathin van der Waals Heterostructures.

We report experimental measurements for ultrathin (<15 nm) van der Waals heterostructures exhibiting external quantum efficiencies exceeding 50% and show that these structures can achieve experimental absorbance >90%. By coupling electromagnetic simulations and experimental measurements, we show that pn WSe2/MoS2 heterojunctions with vertical carrier collection can have internal photocarrier collection efficiencies exceeding 70%.

Experimental Demonstration of >230° Phase Modulation in Gate-Tunable Graphene-Gold Reconfigurable Mid-Infrared Metasurfaces.

Metasurfaces offer significant potential to control far-field light propagation through the engineering of the amplitude, polarization, and phase at an interface. We report here the phase modulation of an electronically reconfigurable metasurface and demonstrate its utility for mid-infrared beam steering. Using a gate-tunable graphene-gold resonator geometry, we demonstrate highly tunable reflected phase at multiple wavelengths and show up to 237° phase modulation range at an operating wavelength of 8.50 µm. We observe a smooth monotonic modulation of phase with applied voltage from 0° to 206° at a wavelength of 8.70 µm. Based on these experimental data, we demonstrate with antenna array calculations an average beam steering efficiency of 23% for reflected light for angles up to 30° for this range of phases, confirming the suitability of this geometry for reconfigurable mid-infrared beam steering devices. By incorporating all nonidealities of the device into the antenna array calculations including absorption losses which could be mitigated, 1% absolute efficiency is achievable up to 30°.