On-chip Arbitrary Mode Spot Conversion

On-chip photonic devices show great potential in many applications, including communications, microwave photonic signal processing, quantum information, sensing and computing. Recently, researchers have proposed a novel design concept about operating a chip to control a free-space beam. Compared with free space devices, such as mirrors, on-chip components exhibit great advantages in trajectory and wavefront control. For example, fundamental mode spot converters (SSCs) use focusing lenses. Eaton lenses have been proven able to bend the light trajectory on the chip, and the aberration-free imaging characteristics of Maxwell's fisheye lenses have been used to construct crossed multimode waveguides (MWG). However, the reported scheme has only been proven to be used for fundamental modes, and higher-order modes or non-canonical modes are the preferred modes in specific applications. In Fourier optics, 4-f systems are widely used in optical information processing.

Recently, Wei Qi and others at Huazhong University of Science and Technology, inspired by the 4-f system and the beam expander in bulk optics, designed an SSC, with two metamaterial-based graded index waveguides havine been proposed and experimentally demonstrated. The proposed device can expand its arbitrary mode while maintaining its contour shape. Compared with the traditional adiabatic cone SSC, it can reduce the area usage by 91.5% under the same inter-mode crosstalk. Experiments have proved that the expansion ratio of the regular mode is 5. In addition, for the irregular pattern, the researchers numerically verified its function without changing the structure. This work provides a general solution for on-chip mode spot conversion, which can further expand the application prospects of on-chip Fourier optics.