Versatile generation and manipulation of phase-structured light beams using on-chip subwavelength holographic surface gratings

Optical vortex arrays in a multimode silicon waveguide

The immense potential of optical vortex arrays (OVAs) has been validated in applications such as particle manipulation, optical imaging, metrology, quantum, and optical communication. Photonic integrated technology has provided a powerful platform for the manipulation of vortex beams, which has the advantages of compact footprint, high controllability, and flexibility. However, since vortex beams are not eigenmodes of rectangular optical waveguides, the on-chip generation of in-plane OVAs has not been reported yet. Here, we propose a controlled method to generate in-plane OVAs by superimposing polarization-multiplexed modes in multimode silicon waveguides. Based on the proposed method, two distinct types of OVAs can be flexibly generated over an ultra-wide optical bandwidth. One exhibits opposite topological charge directions between adjacent vortices (vortex-antivortex), which are achieved by superimposing the fundamental transverse magnetic (TM0) mode with the i-order transverse electric (TEi) mode. The other shows consistent topological charge directions between neighboring vortices, which is realized by superimposing the i-order transverse magnetic (TMi) mode with the TEi mode. By controlling the phase of modes and waveguide width, the topological charge, transmission state, and quantity of vortex beams can be flexibly adjusted.

Integrated spatial light receivers based on inverse design

Photonic integrated spatial light receivers play a crucial role in free space optical (FSO) communication systems. In this paper, we propose a 4-channel and 6-channel spatial light receiver based on a silicon-on-insulator (SOI) using an inverse design method, respectively. The 4-channel receiver has a square receiving area of 4.4 µm × 4.4 µm, which enables receiving four Hermite-Gaussian modes (HG00, HG01, HG10, and HG02) and converting them into fundamental transverse electric (TE00) modes with insertion losses (ILs) within 1.6∼2.1 dB and mean cross talks (MCTs) less than -16 dB, at a wavelength of 1550 nm. The 3 dB bandwidths of the four HG modes range from 28 nm to 46 nm. Moreover, we explore the impact of fabrication errors, including under/over etching and oxide thickness errors, on the performance of the designed device. Simulation results show that the 4-channel receiver is robust against fabrication errors. The designed 6-channel receiver, featuring a regular hexagon receiving area, is capable of receiving six modes (HG00, HG01, HG10, HG02, HG20, and HG11) with ILs within 2.3∼4.1 dB and MCTs less than -15 dB, at a wavelength of 1550 nm. Additionally, the receiver offers a minimum optical bandwidth of 26 nm.

Intersecting of circular apertures to measure integer and fractional topological charge of vortex beams

Diffraction patterns of optical vortex beams (VBs) by differently shaped apertures are used to determine their topological charge (TC). In this paper, we show by simulations and experiments that diffraction of a Laguerre-Gaussian (LG) beam by intersecting circular apertures can be used to reveal the TC. The presented aperture structure has the advantage of the measurement of fractional TC in addition to the integer, sensitivity to the sign of TC, and low sensitivity to adjusting apertures. Accordingly, in addition to the integer TC up to 8, the fractional TC is measured with a step of 0.1 by two intersecting circular apertures (TICA). By examining a wide range of similarity criteria between the diffraction pattern of the fractional TC and the pattern of the lower integer TC, three metrics for measuring the fractional TC are found. Furthermore, the determination of integer TC up to 6 for three intersecting circular apertures (THICA) is demonstrated.

Integrated circuits based on broadband pixel-array metasurfaces for generating data-carrying optical and THz orbital angular momentum beams

There is growing interest in using multiple multiplexed orthogonal orbital angular momentum (OAM) beams to increase the data capacity of communication systems in different frequency ranges. To help enable future deployment of OAM-based communications, an ecosystem of compact and cost-effective OAM generators and detectors is likely to play an important role. Desired features of such integrated circuits include generating and detecting multiple coaxial OAM beams, tunability of OAM orders, and operation over a wide bandwidth. In this article, we discuss the use of pixel-array-based metasurfaces as OAM transmitters and receivers for mode division multiplexing (MDM) communications in near-infrared (NIR) and terahertz (THz) regimes.

Metasurfaces on silicon photonic waveguides for simultaneous emission phase and amplitude control

Chip-scale photonic systems that manipulate free-space emission have recently attracted attention for applications such as free-space optical communications and solid-state LiDAR. Silicon photonics, as a leading platform for chip-scale integration, needs to offer more versatile control of free-space emission. Here we integrate metasurfaces on silicon photonic waveguides to generate free-space emission with controlled phase and amplitude profiles. We demonstrate experimentally structured beams, including a focused Gaussian beam and a Hermite-Gaussian TEM₁₀ beam, as well as holographic image projections. Our approach is monolithic and CMOS-compatible. The simultaneous phase and amplitude control enable more faithful generation of structured beams and speckle-reduced projection of holographic images.