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.
PT-symmetric optical systems can be used for unconventional control and manipulation of light. We designed chirped circular Bragg lasers with radial PT symmetry and analyzed their modal properties. Compared with the conventional circular Bragg lasers, PT-symmetric circular Bragg lasers possess more versatile modal control with further enhanced modal discrimination between the radial modes and between the azimuthal modes. PT-symmetric circular Bragg lasers also have advantages in rotation sensing by exploiting the effects of rotation on modal threshold gain. Their rotation sensitivity can be two orders of magnitude higher than that of conventional circular Bragg lasers with similar geometry and threshold gain. Additionally, they are more robust against perturbation induced by environmental variations than sensors that must operate at the exceptional point.
The force exerted by electromagnetic fields is of fundamental importance in physics. Intense debates on the conventionally accepted Lorentz formulation and the recently suggested Einstein–Laub formulation still continue due to lack of experimental evidences. To distinguish these two formulations, we experimentally investigated the topological charge of optical force in a solid dielectric, and found that the force exerted by a Gaussian beam has components with topological charge of both 2 and 0, which agrees with neither the Lorentz nor Einstein–Laub formulation. Instead, we found a modified Helmholtz theory could explain our experimental results. This work not only contributes to the ultimate determination of the correct force formulation in classical electrodynamics, but also has broad and far-reaching impact on many subjects involving electromagnetic forces.
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.
We develop a unified theory to analyze the modal properties of surface emitting chirped circular grating lasers. Based on
solving the resonance conditions which involve two types of reflectivities of chirped circular gratings, this theory is both
easy to understand and convenient to apply to different configurations of circular grating lasers. Though in a more
concise format, this approach is shown to be in agreement with previous derivations which use the characteristic
equations. With this unified analysis, the modal properties of circular DFB, disk-, and ring- Bragg resonator lasers are
obtained, and the threshold gains, single mode ranges, quality factors, emission efficiencies, and modal areas of these
types of circular grating lasers are compared.
We derive a comprehensive coupled-mode theory, including resonant vertical radiation, for the analysis of non-periodic grating circular Bragg lasers. We analyze the threshold levels and modal properties of such lasers employing mixed-order Bragg gratings to achieve both strong confinement and efficient vertical emission. By reducing the threshold gain and maximizing the emission efficiency, we suggest an optimal design for the circular Bragg microdisk lasers which indicates low-threshold and high-efficiency operation is possible.