Efficient silicon polarization rotator based on mode-hybridization in a double-stair waveguide

On-Chip Polarization Rotators Using Metasurface

Metasurfaces, with their ability to finely manipulate light properties (e.g., polarization), represent a frontier in optical technology. Here, we report innovative gold-based metasurfaces that are adept at changing the light polarization across a broad range of incident angles. Their expansive acceptance angle facilitates seamless integration with silicon waveguides, culminating in the realization of a novel, compact, broadband, and low-loss on-chip polarization rotator. The demonstrated metasurface devices show polarization conversion efficiencies as high as 98.5% at 1550 nm in the free space measurements. Unlike conventional plasmonic metasurfaces, which lack broadband capabilities, our metasurface-based waveplates show extinction ratios of >15 dB for a 120 nm bandwidth. These metasurfaces are positioned on top of up-reflecting mirrors fabricated in a micrometer-scale silicon-on-insulator platform to demonstrate on-chip polarization rotation. The demonstrated polarization rotator shows an extinction ratio of more than 10 dB for a 100 nm bandwidth. This study marks the first successful demonstration of on-chip polarization manipulation utilizing metasurface integration, heralding significant potential impacts for enhancing the functionality and miniaturization of photonic integrated circuits.

Roughness Suppression in Electrochemical Nanoimprinting of Si for Applications in Silicon Photonics

Metal-assisted electrochemical nanoimprinting (Mac-Imprint) scales the fabrication of micro- and nanoscale 3D freeform geometries in silicon and holds the promise to enable novel chip-scale optics operating at the near-infrared spectrum. However, Mac-Imprint of silicon concomitantly generates mesoscale roughness (e.g., protrusion size ≈45 nm) creating prohibitive levels of light scattering. This arises from the requirement to coat stamps with nanoporous gold catalyst that, while sustaining etchant diffusion, imprints its pores (e.g., average diameter ≈42 nm) onto silicon. In this work, roughness is reduced to sub-10 nm levels, which is in par with plasma etching, by decreasing pore size of the catalyst via dealloying in far-from equilibrium conditions. At this level, single-digit nanometric details such as grain-boundary grooves of the catalyst are imprinted and attributed to the resolution limit of Mac-Imprint, which is argued to be twice the Debye length (i.e., 1.7 nm)-a finding that broadly applies to metal-assisted chemical etching. Last, Mac-Imprint is employed to produce single-mode rib-waveguides on pre-patterned silicon-on-insulator wafers with root-mean-square line-edge roughness less than 10 nm while providing depth uniformity (i.e., 42.9 ± 5.5 nm), and limited levels of silicon defect formation (e.g., Raman peak shift < 0.1 cm^-1 ) and sidewall scattering.

Design of Waveguide Polarization Convertor Based on Asymmetric 1D Photonic Crystals

Photonic crystals possess metastructures with a unique dispersion relation. An integrated optical circuit plays a crucial role in quantum computing, for which miniaturized optical components can be designed according to the characteristics of photonic crystals. Because the stable light transmission mode for a square waveguide is transverse electric or transverse magnetic polarization, we designed a half-waveplate element with a photonic crystal that can rotate the polarization direction of the light incident on a waveguide by 90°. Using the dispersion relation of photonic crystals, the polarization rotation length and the optical axis’s angle of deviation from the electric field in the eigenmode can be effectively calculated. Polarization rotators designed on the basis of photonic crystal structures can effectively reduce the insertion loss of components and exhibit favorable polarization rotation performance.

Inverse design of a single-step-etched ultracompact silicon polarization rotator

We propose and experimentally demonstrate a novel ultracompact silicon polarization rotator based on equivalent asymmetric waveguide cross section in only single-step etching procedure for densely integrated on-chip mode-division multiplexing system. In the conventional mode hybridization scheme, the asymmetric waveguide cross section is employed to excite the hybridized modes to realize high performance polarization rotator with compact footprint and high polarization extinction ratio. However, the fabrication complexity severely restricts the potential application of asymmetric waveguide cross section. We use inverse-designed photonic-crystal-like subwavelength structure to realize an equivalent asymmetric waveguide cross section, which can be fabricated in only single-step etching process. Besides, a theory-assisted inverse design method based on a manually-set initial pattern is employed to optimize the device to improve design efficiency and device perform. The fabricated device exhibited high performance with a compact footprint of only 1.2 × 7.2 µm², high extinction ratio (> 19 dB) and low insertion loss (< 0.7 dB) from 1530 to 1590 nm.

Fundamental mode hybridization in a thin film lithium niobate ridge waveguide

The high performance of thin film lithium niobate on insulator (LNOI) platform shows potential for electro-optical signal processing and nonlinear optics systems. To realize precise polarization management for sub-wavelength devices, we theoretically and experimentally investigate fundamental transverse electric (TE) and transverse magnetic (TM) mode hybridization in an x-cut LNOI ridge waveguide. Sudden jumps in the free-spectrum-range (FSR) of these modes in a fabricated microring resonator demonstrate the mode hybridization. The measured Q-factor of the lithium niobate (LN) microring is 1.78 million near the critical coupling condition. The hybridization wavelength was designed at 1562 nm and observed at 1537 nm. Potential applications include fundamental mode conversion, polarization rotation, polarization splitter, and polarization insensitive waveguides in optical receiver module.

Broadband and compact polarization beam splitter based on an asymmetrical directional coupler with extra optimizing designs

In this paper, a novel, to the best of our knowledge, polarization beam splitter (PBS) based on an asymmetrical directional coupler (DC) was proposed, which consists of a strip waveguide (WG) and a ${{\rm Si}_3}{{\rm N}_4}$Si₃N₄ loaded horizontal slot WG. By carefully adjusting the geometric parameters of the DC, the phase match condition between these two WGs can be satisfied for the transverse magnetic (TM) polarization, while the coupling efficiency of the transverse electric (TE) polarization is frustrated due to the large phase mismatch. The extra optimizing designs include adding filters to the output ports as well as introducing the tapered structure into the DC, which is settled by the particle swarm optimizing (PSO) algorithm so that the performance of the proposed PBS is improved over a broadband range. Numerical simulations show that the bandwidths for the extinction ratio (ER) $ \gt {20}\;{\rm dB}$>20dB, 30 dB, and 40 dB are 160 nm, 95 nm, and 50 nm, respectively, with insertion loss (IL) $ \lt {1}\;{\rm dB}$<1dB for the wavelength of 1.49-1.58 µm. The analysis of the deviations demonstrates that the proposed PBS allows high fabrication tolerances.

Design and numerical study of a compact, broadband and low-loss TE-pass polarizer using transparent conducting oxides

A compact, broadband, and low-loss TE-pass polarizer using transparent conducting oxides (TCOs) embedded in the center of the strip waveguide and deposited on its top is proposed and analyzed in detail. With the tunable permittivity of TCO, epsilon-near-zero (ENZ) of its real part and significant increase of its imaginary part can be achieved around the wavelength of 1.55 μm under a certain electron concentration. By introducing this ENZ material into the strip waveguide, huge polarization dependence can be realized, that is, the TE mode is almost not affected due to its quite weak interaction with TCOs, while the TM mode is extremely confined in the accumulation layers of TCO with high absorption loss, leading to a great reduction in length for the present polarizer. Moreover, the top TCO layer is applied to further enhance the polarizer performance. Results show that a polarizer of only 4.5 μm in length with an extinction ratio (ER) of 25.26 dB and an insertion loss of 0.21 dB is achieved at 1.55 μm, and its bandwidth can be extended to ~140 nm for an ER>20 dB. In addition, the ER can also be increased only by enlarging the length of the TCO-based polarizer.

High-extinction-ratio silicon polarization beam splitter with tolerance to waveguide width and coupling length variations

We demonstrate a compact silicon polarization beam splitter (PBS) based on grating-assisted contradirectional couplers (GACCs). Over 30-dB extinction ratios and less than 1-dB insertion losses are achieved for both polarizations. The proposed PBS exhibits tolerance in width variation, and the polarization extinction ratios remain higher than 20 dB for both polarizations when the width variation is adjusted from + 10 to -10 nm. Benefiting from the enhanced coupling by the GACCs, the polarization extinction ratio can be kept higher than 15 dB and the insertion loss is lower than 2 dB for both polarizations when the coupling length varies from 30.96 to 13.76 μm.

Highly efficient silicon optical polarization rotators based on mode order conversions

We design and demonstrate the novel silicon optical polarization rotators (PRs) based on the TM(0)-TE(n)-TE(0) mode conversions inside the waveguide. The TM(0)-TE(n) mode converters are realized by the mode hybridization of the tapered rib waveguides. The TE(n)-TE(0) mode converters based on the beam shaping method are followed to complete the PRs function. By using the TE(1), TE(2), and TE(3) mode as the transitional mode, the fabricated PRs show the insert losses of less than 0.4, 0.5, and 1 dB, respectively. The corresponding polarization extinction ratios of larger than 21, 18, and 23 dB, over a wavelength range of 100 nm.

Ultracompact polarization rotator in an asymmetric single dielectric loaded rib waveguide

A compact polarization rotator (PR) with an asymmetric single dielectric loaded rib waveguide is proposed. The core of the waveguide is designed to have a specific rectangular configuration. The waveguide requires only a single asymmetrical dielectric loading on the core to complete the polarization conversion. The optical field is confined to the vicinity of the core center, which matches the optical field of the input/output waveguides. The transition loss of the PR is as low as 0.03-0.21 dB/facet without the taper or offset schemes. Such results can facilitate the fabrication of a PR with an operating length of 10 μm. In a comprehensively designed PR with a length of 7.92 μm, a -1  dB bandwidth for polarization conversion efficiency (PCE) is greater than 100 nm at the communicating wavelength of 1550 nm. The loading width and thickness with ±20  nm tolerance exhibit -0.87 and -0.49  dB changes in PCE, respectively.

Design of a compact and integrated TM-rotated/TE-through polarization beam splitter for silicon-based slot waveguides

A compact and integrated TM-rotated/TE-through polarization beam splitter for silicon-based slot waveguides is proposed and characterized. For the input TM mode, it is first transferred into the cross strip waveguide using a tapered directional coupler (DC), and then efficiently rotated to the corresponding TE mode using an L-shaped bending polarization rotator (PR). Finally, the TE mode for slot waveguide at the output end is obtained with the help of a strip-to-slot mode converter. By contrast, for the input TE mode, it almost passes through the slot waveguide directly and outputs at the bar end with nearly neglected coupling due to a large mode mismatch. Moreover, an additional S-bend connecting the tapered DC and bending PR is used to enhance the performance. Results show that a total device length of 19.6 μm is achieved, where the crosstalk (CT) and polarization conversion loss are, respectively -26.09 and 0.54 dB, for the TM mode, and the CT and insertion loss are, respectively, -22.21 and 0.41 dB, for the TE mode, both at 1.55 μm. The optical bandwidth is approximately 50 nm with a CT<-20  dB. In addition, fabrication tolerances and field evolution are also presented.