Compared with electronic integrated circuits, photonic integrated circuits have many advantages and arise as a promising candidate for our next-generation computation and communication systems. However, the feature size of photonic integrated circuits is still large for on-chip large-scale and high-density integration. Inverse design method, powered by advanced algorithms, has been adopted to greatly reduce the footprint of photonic components because it searches for the optimal photonic structures in the full structural parameter space. This talk will cover our recent efforts on inverse-designed photonic components with reduced feature size, including polarization rotators, reflectors, photonic welding points, waveguide crossings, and photonic jumpers. These components will contribute to construction and development of advanced photonic chips with significantly enhanced integration density, scale, and functionality.
We present our recent results on the use of two quite different approaches for photonic integration. First we shall describe how we used the concept of bound states in the continuum (BiC) to make channel guided devices without the need for any dry etching. The BiC channel waveguide employs a substrate that is completely flat. The completely flat structure is attractive for hybrid integration of 2D materials because it does not introduce sharp corners which can reduce the electrical mobility of the 2D material. Channel guiding of light can nonetheless be achieved by spin coating a lower-refractive-index polymer/photoresist on the 2D material and developing it to form a channel. This approach for integrating 2D materials also increases the optical overlap with the 2D material. We used this approach for the hybrid integration of graphene on lithium niobate for making 40-GHz-bandwidth channel-guided photodetectors and electroabsorption modulators on lithium niobate. The BiC concept facilitates the hybrid integration of 2D materials on different substrates and may also be used to increase the effective optical nonlinearity of the underlying substrate by hybrid integration of the appropriate 2D material. Second we shall discuss the InP membrane waveguide platform for nonlinear applications. InP has a third order nonlinearity that is over an order of magnitude larger than silicon, and is therefore of potential interest for spontaneous four wave mixing to produce entangled photons. The use of InP membranes can potentially facilitate the integration of active III-V lasers, and Geiger mode avalanche photodiodes for single photon detection and nonlinear devices on large scale silicon wafers which can integrate the large delay interferometers and filters needed for quantum information processing. We discuss the advantages and disadvantages of InP for SFWM and present recent results on the use of InP membranes for generating heralded single photons.
Optomechanical crystals (also referred to as photonic–phononic crystals or phoxonic crystals) exploit the simultaneous photonic and phononic bandgaps in periodic nanostructures. They have been utilized to colocalize, couple, and transduce optical and mechanical (acoustic) waves for nonlinear interactions and precision measurements. Devices that involve standing or traveling acoustic waves of high frequencies usually have advantages in many applications. Here, we review recent progress in nano-optomechanical devices where the acoustic wave oscillates at microwave frequencies. We focus on our development of an optomechanical crystal cavity and a phoxonic crystal waveguide with special features. The development of near-infrared optomechanical crystal cavities has reached a bottleneck in reducing the mechanical modal mass. This is because the reduction of the spatial overlap between the optical and mechanical modes results in a reduced optomechanical coupling rate. With a novel optimization strategy, we have successfully designed an optomechanical crystal cavity in gallium nitride with the optical mode at the wavelength of 393.03 nm, the mechanical mode at 14.97 GHz, the mechanical modal mass of 22.83 fg, and the optomechanical coupling rate of 1.26 MHz. Stimulated Brillouin scattering (SBS) has been widely exploited for applications of optical communication, sensing, and signal processing. A recent challenge of its implementation in silicon waveguides is the weak per-unit-length SBS gain. Taking advantage of the strong optomechanical interaction, we have successfully engineered a phoxonic crystal waveguide structure, where the SBS gain coefficient is greater than 3×104 W−1 m−1 in the entire C band with the highest value beyond 106W−1 m−1, which is at least an order of magnitude higher than the existing demonstrations.