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.
Developing a nanoscale, integrable, and electrically pumped single mode light source is an essential step toward on-chip optical information technologies and sensors. Here, we demonstrate nanocavity enhanced electroluminescence in van der Waals heterostructures (vdWhs) at room temperature. The vertically assembled light-emitting device uses graphene/boron nitride as top and bottom tunneling contacts and monolayer WSe as an active light emitter. By integrating a photonic crystal cavity on top of the vdWh, we observe the electroluminescence is locally enhanced (>4 times) by the nanocavity. The emission at the cavity resonance is single mode and highly linearly polarized (84%) along the cavity mode. By applying voltage pulses, we demonstrate direct modulation of this single mode electroluminescence at a speed of ∼1 MHz, which is faster than most of the planar optoelectronics based on transition metal chalcogenides (TMDCs). Our work shows that cavity integrated vdWhs present a promising nanoscale optoelectronic platform.
Nano-resonator integrated with two-dimensional materials (e.g. transition metal dichalcogenides) have recently emerged as a promising nano-optoelectronic platform. Here we demonstrate resonatorenhanced second-harmonic generation (SHG) in tungsten diselenide using a silicon photonic crystal cavity. By pumping the device with the ultrafast laser pulses near the cavity mode at the telecommunication wavelength, we observe a near visible SHG with a narrow linewidth and near unity linear polarization, originated from the coupling of the pump photon to the cavity mode. The observed SHG is enhanced by factor of ~200 compared to a bare monolayer on silicon. Our results imply the efficacy of cavity integrated monolayer materials for nonlinear optics and the potential of building a silicon-compatible second-order nonlinear integrated photonic platform. IntroductionNonlinear integrated photonics plays a crucial role in building all-optical information processors [1,2] and novel on-chip light-sources [3]. However, the weak optical nonlinearity of existing material systems results in large optical switching power, rendering optical information processing unattractive. The key to lower the required optical power is to incorporate nonlinear materials onto a nano-scale high quality factor resonator, where light can be stored in a small volume ( $ ) and for an extended period of time [4]. It can be shown that for a nonlinear optical switch, the switching power scales as $ / ' for the third order and $ / ( for the second order nonlinearity [5]. This stronger dependence on cavity , along with a larger value of second-order (') nonlinear coefficients compared to (() coefficients, make (') nonlinear processes more suitable to realize low-power nonlinear optical devices. Unfortunately, silicon lacks the desired (') nonlinearity due to its centrosymmetric crystal structure; thus devices based on (() processes dominate current efforts in nonlinear integrated photonics [3,6,7]. While materials with large (') nonlinearities, such as III-V materials [8] are well-studied, their incompatibility with current CMOS foundries [9] hinders the scalability sought by the integrated photonics community. This is further exacerbated by the fact that deposition of high refractive index III-V materials on silicon changes the optical mode profile significantly, making the phase matching condition more difficult to satisfy. Researchers have also studied aluminum nitride for nonlinear optics [10] and
Engineering an array of precisely located cavity-coupled active media poses a major experimental challenge in the field of hybrid integrated photonics. We deterministically position solution-processed colloidal quantum dots (QDs) on high quality (Q)-factor silicon nitride nanobeam cavities and demonstrate light-matter coupling. By lithographically defining a window on top of an encapsulated cavity that is cladded in a polymer resist, and spin coating the QD solution, we can precisely control the placement of the QDs, which subsequently couple to the cavity. We show rudimentary control of the number of QDs coupled to the cavity by modifying the size of the window. Furthermore, we demonstrate Purcell enhancement and saturable photoluminescence in this QD-cavity platform. Finally, we deterministically position QDs on a photonic molecule and observe QD-coupled cavity supermodes. Our results pave the way for precisely controlling the number of QDs coupled to a cavity by engineering the window size, the QD dimension, and the solution chemistry and will allow advanced studies in cavity enhanced single photon emission, ultralow power nonlinear optics, and quantum many-body simulations with interacting photons.
Abstract:Unprecedented material compatibility and ease of integration, in addition to the unique and diverse optoelectronic properties of layered materials, have generated significant interest in their utilization in nanophotonic devices. While initial nanophotonic experiments with layered materials primarily focused on light sources, modulators, and detectors, recent efforts have included nonlinear optical devices. In this paper, we review the current state of cavity-enhanced nonlinear optics with layered materials. Along with conventional non linear optics related to harmonic generation, we report on emerging directions of nonlinear optics, where layered materials can potentially play a significant role.
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