Anomalous birefringence in periodic ripples induced on Si thin layer by ultraviolet laser scanning

When a laser beam is scanned over a material multiple times, periodic surface structures, known as laser-induced periodic surface structures (LIPSSs) are formed. In this study, a 355-nm ultraviolet laser with a Gaussian beam was scanned over a 45-nm-thick amorphous-Si layer at the maximum fluence of 115mJ/cm², resulting in the creation of periodic Si nanoparticles of approximately 50nm size. The measured transmittance of incident light with a polarization angle between transverse magnetic (TM) and transverse electric (TE) showed bumps at wavelength of 400-500nm and 690nm. The periodic surface was modeled as a 1D grating and analyzed using numerical simulation. Cross-sectional plots of electric and magnetic field distributions at peak wavelengths revealed strong localization in the grating region of the Si layer, suggesting that the bumps originate from guiding modes localized at the surface Si layer. Additionally, birefringence measured in ultraviolet-visible wavelength range using a standard polarizer-analyzer setup was enhanced at the corresponding wavelengths. This birefringence can be controlled by adjusting laser processing parameters, offering additional applications to polarization-sensitive printing, flat optics, optical data storage, and sensing devices.