Engineering the electromagnetic environment of a nanometre-scale light emitter by use of a photonic cavity can significantly enhance its spontaneous emission rate, through cavity quantum electrodynamics in the Purcell regime. This effect can greatly reduce the lasing threshold of the emitter, providing a low-threshold laser system with small footprint, low power consumption and ultrafast modulation. An ultralow-threshold nanoscale laser has been successfully developed by embedding quantum dots into a photonic crystal cavity (PCC). However, several challenges impede the practical application of this architecture, including the random positions and compositional fluctuations of the dots, extreme difficulty in current injection, and lack of compatibility with electronic circuits. Here we report a new lasing strategy: an atomically thin crystalline semiconductor--that is, a tungsten diselenide monolayer--is non-destructively and deterministically introduced as a gain medium at the surface of a pre-fabricated PCC. A continuous-wave nanolaser operating in the visible regime is thereby achieved with an optical pumping threshold as low as 27 nanowatts at 130 kelvin, similar to the value achieved in quantum-dot PCC lasers. The key to the lasing action lies in the monolayer nature of the gain medium, which confines direct-gap excitons to within one nanometre of the PCC surface. The surface-gain geometry gives unprecedented accessibility and hence the ability to tailor gain properties via external controls such as electrostatic gating and current injection, enabling electrically pumped operation. Our scheme is scalable and compatible with integrated photonics for on-chip optical communication technologies.

We design metalenses to capture colored images with white light by combining metasurfaces and computational imaging.

The miniaturization of current image sensors is largely limited by the volume of the optical elements. Using a subwavelength-patterned quasi-periodic structure, also known as a metasurface, one can build planar optical elements based on the principle of diffraction. Recent demonstrations of high-quality metasurface optical elements are mostly based on high-refractive-index materials. Here, we present a design of low-contrast metasurface-based optical elements. We fabricate and experimentally characterize several silicon nitride-based lenses and vortex beam generators. The fabricated lenses achieved beam spots of less than 1 μm with numerical apertures as high as ∼0.75. We observed a transmission efficiency of 90% and focusing efficiency of 40% in the visible regime. Our results pave the way toward building low-loss metasurface-based optical elements at visible frequencies using low-contrast materials and extend the range of prospective material systems for metasurface optics.

:Efficient conversion of electrical signal to optical signal in nano-photonics enables solid state integration of electronics and photonics. Combination of graphene with photonic crystals is promising for electro-optic modulation. In this paper, we demonstrate that by electrostatic gating a single layer of graphene on top of a photonic crystal cavity, the cavity resonance can be changed significantly. A ~2nm change in the cavity resonance linewidth and almost 400% (6 dB) change in resonance reflectivity is observed. In addition, our analysis shows that a graphenephotonic crystal device can potentially be useful for a high speed, and low power absorptive and refractive modulator, while maintaining a small physical footprint.