Haiyang Huang

Proposal for fabrication-tolerant SOI polarization splitter-rotator based on cascaded MMI couplers and an assisted bi-level taper

A novel silicon-on-insulator (SOI) polarization splitter-rotator (PSR) with a large fabrication tolerance is proposed based on cascaded multimode interference (MMI) couplers and an assisted mode-evolution taper. The tapers are designed to adiabatically convert the input TM(0) mode into the TE(1) mode, which will output as the TE(0) mode after processed by the subsequent MMI mode converter, 90-degree phase shifter (PS) and MMI 3 dB coupler. The numerical simulation results show that the proposed device has a < 0.5 dB insertion loss with < -17 dB crosstalk in C optical communication band. Fabrication tolerance analysis is also performed with respect to the deviations of MMI coupler width, PS width, slab height and upper-cladding refractive index, showing that this device could work well even when affected by considerable fabrication errors. With such a robust performance with a large bandwidth, this device offers potential applications for CMOS-compatible polarization diversity, especially in the booming 100 Gb/s coherent optical communications based on silicon photonics technology.

A silicon-on-insulator polarization diversity scheme in the mid-infrared

We propose a silicon-on-insulator (SOI) polarization diversity scheme in the mid-infrared wavelength range. In consideration of absorption loss in silicon dioxide (SiO2), the polarization splitter-rotator (PSR) is designed and optimized with silicon nitride (SiN) upper-cladding and SiO2 lower-cladding. This asymmetry allows the PSR, which consists of mode-conversion tapers and subsequent mode-sorting asymmetric Y-junctions, to be fabricated with a simple one-step etching process. Simulation shows that our PSR has good performance with low mode conversion loss (< 0.25 dB) and low crosstalk (< -18 dB) in a very large wavelength range from 4.0 μm to 4.4 μm. The PSR also exhibits large fabrication tolerance with respect to the size deviations in waveguide width, height and refractive index of the upper-cladding. Additionally, PSR devices based on Y-junctions with SiO2 upper-cladding, and SiN upper- and lower-claddings are designed for potential applications at shorter and longer wavelengths, respectively. These PSR devices could facilitate the development of silicon photonic devices in the mid-infrared.

Ultrabroadband Silicon-on-Insulator Polarization Beam Splitter Based on Cascaded Mode-Sorting Asymmetric Y-Junctions

A very novel silicon-on-insulator polarization beam splitter is proposed based on cascaded mode-sorting asymmetric Y-junctions. The width and length of each Y-junction are optimized to achieve correct mode sorting with high conversion efficiency. The numerical simulation results show that the mode conversion efficiency increases with the length of the Y-junction for the waveguide widths varying in a large range. This proposed device has <; 0.7 dB insertion loss with > 22 dB polarization extinction ratio in an ultrabroad wavelength range from 1450 to 1750 nm. Fabrication tolerance analysis is also performed with respect to the deviation in device width and height. With such a broad operating bandwidth and robust fabrication tolerance, this device offers potential applications for complementary metal oxide semiconductor (CMOS)-compatible polarization diversity operating across the S, C, L, and U optical communication bands, particularly in the booming 100-Gb/s coherent optical communications based on silicon photonics technology.

Compressing and routing light through a silicon nanorod array

Over the past two decades, great efforts have been made in the study of routing and manipulating light waves at the subwavelength scale with open nanostructures such as photonic crystal waveguides (PCWs), surface plasmon waveguides (SPWs), and coupled resonator optical waveguides (CROWs), for their great potential in many photonic technologies such as highly integrated photonic signal-processing systems and sensors. To date, however, there have been some significant challenges for the open nanostructures in application, i.e., PCWs have relatively large sizes and complex structures, SPWs have large unavoidable metallic losses, and CROWs are disorder-sensitive and frequency-sensitive. Recently, we propose a new platform for the design of open nanostructures. Based on the flat form, we design an open nanostructure composed of a periodic subwavelength-nanoparticles chain, which is simple, ultrathin, compact, and can be compatible with CMOS. Our ultrathin open nanostructure provides a strong function not only in routing light at the subwavelength scale, but also in sharply bending and/or splitting light beams below the diffraction limit, exhibiting low-loss, broadband, incident-angle-insensitive, and robust against disorder. Experimental and numerical observations validate our findings.

Routing light with ultrathin nanostructures beyond the diffraction limit

An open nanostructure consisting of a periodic chain of subwavelength-nanoparticles for compressing and routing light beyond the diffraction limit is proposed. The open nanostructure is ultrathin and compact, with a size much smaller than the wavelength of light. We demonstrate that our ultrathin open nanostructure provides functions that can route and manipulate light at the subwavelength scale and can also sharply bend and split light beams below the diffraction limit while exhibiting broadband, incident-angle-tolerant, and robust against disorder. A physical picture based on all-angle self-collimation is presented to understand the manipulation of light using the ultrathin open nanostructure. Experimental and numerical observations validate our findings. This approach provides great flexibility in the design of nanophotonic devices for routing and manipulating light beyond the diffraction limit.

Broadband In-Plane Light Bending With a Doublet Silicon Nanopost Array

The in-plane tailoring of light propagation is significant in on-chip optical interconnections. Recently, an in-plane negative-angle refraction was realized with a thin line of silicon nanoposts, which are at resonance in the first angular momentum channel. This advancement is different from metasurface research, which mostly focuses on out-of-plane operations. In this paper, we experimentally demonstrate that a thin array of doublet silicon nanoposts, in which each unit comprises two tangent nanoposts functioning as an upright interface, remarkably improve efficiency for molding light. The designed upright interface exhibits a broadband response featuring high efficiency in a negative-angle light bending in the wavelength of 1480-1600 nm. The broadband, compactness, low loss, and complementary metal-oxide semiconductor (CMOS) compatibility enable the use of the subwavelength array as an alternative component for on-chip optical control.

All-Angle Quasi-Self-Collimation Effect in a Rod-Type Silicon Photonic Crystal

By changing the symmetry of a photonic crystal (PC) with a rectangular lattice to straighten one of the isofrequency contours, the PC shows an all-angle quasi-self-collimation (quasi-SC) effect. To investigate the straightness of the isofrequency contour and the quasi-SC effect, we propose a straightness factor L based on the method of least squares. With L ≤ L0 (L0 = 0.01 is the critical value), the isofrequency contour is sufficiently straight to induce the quasi-SC effect with the beam quasi-collimating in the structure. Furthermore, the efficiency of light coupling to the quasi-SC PC is studied and can be greatly improved by applying a carefully designed antireflection structure. This quasi-SC effect of the PC and the coupling structure may see applications in novel optical devices and photonic circuits.