Harnessing scanning probe lithography for integrated photonics with anisotropic materials

The integration of high-index, wide-bandgap layered semiconductors into nanophotonic circuits is hindered by the lack of damage-free fabrication techniques compatible with their mechanical anisotropy. Here we report a deterministic, crystallography-guided approach to patterning photonic waveguides in the biaxial layered semiconductor GaInS₃ (n > 2.5, bandgap 2.73 eV) using mechanical scanning probe lithography in the frictional mode. A preliminary "compass-rose" lithography step maps the in-plane cleavage anisotropy, identifying low-energy armchair directions that yield trenches with sub-10 nm edge roughness. Aligning waveguide edges with these axes, we fabricate 70 nm-thick strip waveguides and demonstrate robust single-mode operation in the visible range (505-630 nm), with the operational bandwidth limited only by the intrinsic absorption edge. In stark contrast, identical structures produced by focused ion beam milling exhibit complete extinction of guided modes due to amorphization and ion-induced damage, underscoring the method's ability to preserve crystalline integrity. Numerical simulations incorporating measured sidewall roughness reproduce the transmittance spectra, confirming that the achieved edge quality enables on-chip signal transmission. The frictional SPL protocol provides a resist-free, etch-free route for rapid prototyping of nanophotonic components in mechanically anisotropic high-index materials and establishes a platform for exploring complex photonic circuits where lattice damage must be avoided.