Shrinking the footprints of polarization splitter-rotators with highly-birefringent molybdenum disulfide waveguides

A polarization splitter-rotator (PSR) based on an asymmetric directional coupler (ADC) with molybdenum disulfide (MoS
2
) waveguides is proposed. Owing to the high in-plane refractive index and the extreme birefringence of MoS
2
, an ultra-small cross-polarization coupling length of 6.0 $\mu$m can be achieved. At the working wavelength of 1.55 $\mu$m, this ADC-type PSR exhibits a high TM
0
-to-TE
0
polarization conversion efficiency of 96.97% and a low conversion loss of 0.38 dB. Additionally, the TE
0
-to-TE
0
insertion loss and cross-talk are remarkably low, measuring at 0.006 dB and -31.53dB, respectively. We verify through simulations that the extremely high birefringence of MoS
2
is a necessity for achieving simple and ultra-compact ADC-PSR with straight waveguide geometries. This exceptional impact of extreme birefringence of MoS
2
indicates a unique and promising prospect for developing high-performance nanophotonic components based on the van der Waals on insulator (vdW-OI) configuration.