Lead halide perovskites are emerging as an excellent material platform for optoelectronic processes. There have been extensive discussions on lasing, polariton formation, and nonlinear processes in this material system, but the underlying mechanism remains unknown. Here we probe lasing from CsPbBr3 perovskite nanowires with picosecond (ps) time resolution and show that lasing originates from stimulated emission of an electron-hole plasma. We observe an anomalous blue-shifting of the lasing gain profile with time up to 25 ps, and assign this as a signature for lasing involving plasmon emission. The time domain view provides an ultra-sensitive probe of many-body physics which was obscured in previous time-integrated measurements of lasing from lead halide perovskite nanowires.

These authors contributed equally to this work.Two dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) are promising for optical modulation, detection, and light emission since their material properties can be tuned on-demand via electrostatic doping 1-18 . The optical properties of TMDs have been shown to change drastically with doping in the wavelength range near the excitonic resonances [19][20][21][22] . However, little is known about the effect of doping on the optical properties of TMDs away from these resonances, where the material is transparent and therefore could be leveraged in photonic circuits. Here, we probe the electro-optic response of monolayer TMDs at near infrared (NIR) wavelengths (i.e. deep in the transparency regime), by integrating them on silicon nitride (SiN) photonic structures to induce strong light -matter interaction with the monolayer. We dope the monolayer to carrier densities of (7.2 ± 0.8) × 10 13 cm -2 , by electrically gating the TMD using an ionic liquid [P14 + ] [FAP -]. We show strong electro-refractive response in monolayer tungsten disulphide (WS2) at NIR wavelengths by measuring a large change in the real part of refractive index ∆n = 0.53, with only a minimal change in the imaginary part ∆k = 0.004. The doping induced phase change (∆n), compared to the induced absorption (∆k) measured for WS2 (∆n/∆k ∼ 125), a key metric for photonics, is an order of magnitude higher than the ∆n/∆k for bulk materials like silicon (∆n/∆k ∼ 10) 23 , making it ideal for various photonic applications 24-28 . We further utilize this strong tunable effect to demonstrate an electrostatically gated SiN-WS2 phase modulator using a WS2-HfO2 (Hafnia)-ITO (Indium Tin Oxide) capacitive configuration, that achieves a phase modulation efficiency (V π L) of 0.8 V · cm with a RC limited bandwidth of 0.3 GHz.

Energy transferred via thermal radiation between two surfaces separated by nanometer distances can be much larger than the blackbody limit. However, realizing a scalable platform that utilizes this near-field energy exchange mechanism to generate electricity remains a challenge. Here, we present a fully integrated, reconfigurable and scalable platform operating in the near-field regime that performs controlled heat extraction and energy recycling. Our platform relies on an integrated nano-electromechanical system that enables precise positioning of a thermal emitter within nanometer distances from a room-temperature germanium photodetector to form a thermo-photovoltaic cell. We demonstrate over an order of magnitude enhancement of power generation (P gen~1 .25 μWcm −2) in our thermophotovoltaic cell by actively tuning the gap between a hot-emitter (T E~8 80 K) and the cold photodetector (T D~3 00 K) from~500 nm down to~100 nm. Our nanoelectromechanical system consumes negligible tuning power (P gen /P NEMS~1 0 4) and relies on scalable silicon-based process technologies.

Highly doped graphene holds promise for next-generation electronic and photonic devices.However, chemical doping cannot be precisely controlled, and introduces external disorder that significantly diminishes the carrier mobility and therefore the graphene conductivity. Here, we show that monolayer tungsten oxyselenide (TOS) created by oxidation of WSe2 acts as an efficient and low-disorder hole-dopant for graphene. When the TOS is directly in contact with graphene, the induced hole density is 3 × 10 13 cm -2 , and the room-temperature mobility is 2,000 cm 2 /V•s, far exceeding that of chemically-doped graphene. Inserting WSe2 layers between the TOS and graphene tunes the induced hole density as well as reduces charge disorder such that the mobility exceeds 20,000 cm 2 /V•s and reaches the limit set by acoustic phonon scattering, resulting in sheet resistance below 50 /□. An electrostatic model based on work-function mismatch accurately describes the tuning of the carrier density with WSe2 interlayer thickness. These films show unparalleled performance as transparent conductors at telecommunication wavelengths, as shown by measurements of transmittance in thin films and insertion loss in photonic ring resonators. This work opens up new avenues in optoelectronics incorporating two-dimensional heterostructures including infrared transparent conductors, electro-phase modulators, and various junction devices.

Compact beam steering in the visible spectral range is required for a wide range of emerging applications, such as augmented and virtual reality displays, optical traps for quantum information processing, biological sensing, and stimulation. Optical phased arrays (OPAs) can shape and steer light to enable these applications with no moving parts on a compact chip. However, OPA demonstrations have been mainly limited to the near-infrared spectral range due to the fabrication and material challenges imposed by the shorter wavelengths. Here, we demonstrate the first chip-scale phased array operating at blue wavelengths (488 nm) using a high-confinement silicon nitride platform. We use a sparse aperiodic emitter layout to mitigate fabrication constraints at this short wavelength and achieve wide-angle beam steering over a 50° field of view with a full width at half-maximum beam size of 0.17°. Large-scale integration of this platform paves the way for fully reconfigurable chip-scale three-dimensional volumetric light projection across the entire visible range.

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The inability to efficiently tune the optical properties of waveguiding structures has been one of the major hurdles for the future scalability of integrated photonic systems. In silicon photonics, although dynamic tuning has been achieved with various mechanisms 1-10 , even the most effective thermo-optic effect 5 offers a refractive index change of only 1.86 × 10 -4 K -1 . To enhance this small change, light recycling based on resonators has been employed in order to realize efficient modulators, phase shifters, and optical switches 1,7-11 . However, the resonant enhancement comes at a great cost of optical bandwidth, fabrication tolerance and system scalability. Here we demonstrate a scalable light recycling approach based on spatialmode multiplexing. Our approach offers a fabrication tolerance of ± 15 nm, in stark contrast to the non-scalable subnanometer tolerance in typical silicon resonators. We experimentally demonstrate light recycling up to 7 passes with an optical bandwidth greater than 100 nm.We realize power-efficient thermo-optic phase shifters that require only 1.7 mW per π, representing more than an 8-fold reduction in the power consumption.The scalability of active silicon photonic circuits depends on two major tuning mechanisms of silicon, the thermo-optic effect 2-5 and the plasma dispersion effect 1,6-10 , which offer only ∆ =1.86 × 10 -4 K -1 and -[8.8×10 -22 × + 8.5×10 -18 × ℎ 0.8 ], respectively ( and ℎ are the