Generalized Inverse Design of a Compact Polarization Beam Splitter on X-cut TFLN

We develop a generalized inverse design method and demonstrate its effectiveness through the design of a compact polarization beam splitter (PBS) on an X-cut thin-film lithium niobate (TFLN) platform. This method combines dielectric tensor-based material modeling with topology optimization, enabling precise control of electromagnetic field distributions. Based on this approach, an inverse-designed PBS with a compact footprint of 20 μm × 8 μm is demonstrated, enabling efficient separation of TE0 and TM0 modes. For the TE₀ mode, the device achieves an insertion loss (IL) of −0.56 dB, with a 3 dB bandwidth of 25 nm (1539-1564 nm), and a polarization extinction ratio (PER) of greater than 10 dB across the wavelength range of 1536-1561 nm. For the TM₀ mode, the IL is −0.78 dB, with a 3 dB bandwidth of 42 nm (1526-1568 nm) and a PER greater than 10 dB across the wavelength range of 1528-1566 nm. These results demonstrate the feasibility of applying inverse design method to X-cut TFLN platform and establish a robust pathway toward high-density photonic integration.