Broadband silicon polarization beam splitter based on Floquet engineering

A broadband silicon polarization beam splitter (PBS) is proposed and experimentally demonstrated based on Floquet-engineered directional couplers. The total length of the coupling structure is 20 um . By periodically modulating the waveguide width of the directional couplers, the power exchange between the two waveguides for the transverse-electric (TE) mode is suppressed, whereas the power coupling for the transverse-magnetic (TM) mode is enhanced. The fabricated PBS exhibits polarization extinction ratios (PERs) > 20 dB for both polarizations over a broad wavelength range of 1483 nm-1620 nm. Additionally, the measured insertion losses (ILs) are 0.15 dB and 1.2 dB at 1550 nm for TE and TM polarizations, respectively.

💡 Research Summary

The paper presents a broadband silicon polarization beam splitter (PBS) that leverages Floquet engineering to achieve polarization‑dependent coupling in an ultra‑compact directional coupler (DC) architecture. Conventional silicon PBSs based on multimode interferometers, sub‑wavelength gratings, or standard DCs suffer from strong wavelength dependence, limiting their usable bandwidth. By periodically modulating the width of the two parallel strip waveguides with a sinusoidal profile (amplitude = 50 nm, period = 1.2 µm), the authors introduce a spatially periodic “drive” that can be described by a Floquet‑type Hamiltonian. This periodic perturbation yields a polarization‑specific effective coupling coefficient: for the transverse‑electric (TE) mode the modulation parameters satisfy the zero‑coupling condition (the first‑order Bessel function J₀(ξ)=0), suppressing power exchange; for the transverse‑magnetic (TM) mode the same parameters enhance coupling, allowing efficient power transfer to the cross port.

Numerical simulations first examine a single DC with various waveguide gaps. With a gap of 215 nm, the TE mode remains essentially uncoupled across the whole gap range, while the TM mode exhibits strong coupling, giving a ~35 nm bandwidth with PER > 20 dB for each polarization. To broaden the operational bandwidth, two identical DCs are cascaded: the through‑port of the first DC feeds the second DC, which further filters residual TM power. This cascaded configuration expands the TM bandwidth from ~35 nm to ~155 nm (1480 nm–1635 nm) while maintaining TE performance over 1450 nm–1650 nm. Simulated insertion losses are <0.1 dB for TE and <1 dB for TM.

Fabrication is carried out on a 220 nm‑thick silicon device layer on a 3 µm buried oxide SOI wafer. Electron‑beam lithography and inductively coupled plasma etching create the sinusoidally modulated waveguides with high fidelity. Tolerance analysis shows that ±20 nm variations in waveguide width or height still preserve PER > 20 dB over >100 nm bandwidth, demonstrating robust manufacturability. Grating couplers optimized for TE (630 nm period, 50 % duty) and TM (980 nm period, 60 % duty) provide coupling losses of ~7 dB per facet.

Experimental characterization uses a tunable laser (1483 nm–1637 nm) and power meter. Normalized transmission spectra reveal PER > 20 dB for both polarizations across the entire measured range (1483 nm–1620 nm). Insertion losses at 1550 nm are 0.15 dB for TE and 1.2 dB for TM; TE loss stays below 1 dB over the full band, while TM loss remains under 2 dB. The total device length is only 20 µm, an order of magnitude smaller than typical MMI‑based PBSs.

In summary, the work demonstrates that Floquet‑engineered width modulation provides a powerful degree of freedom to independently tailor coupling for orthogonal polarizations. The resulting PBS achieves ultra‑compact size, broadband operation (~137 nm 20 dB PER bandwidth), low insertion loss, and strong fabrication tolerance, positioning it as a highly attractive component for polarization‑division multiplexing in silicon photonic integrated circuits. The approach can be extended to more complex photonic lattices, dense waveguide arrays, and multi‑wavelength devices, opening new pathways for high‑density, low‑crosstalk silicon photonics.