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.
Optical resonances of nanoparticle have been studied for a long time in various optical devices. However, the difficulties in fabrication of uniform nanoparticles hinders the applications. Herein, we formed Si nanoparticles having a uniform size via laser irradiation on an amorphous-Si thin film and found the optical resonances of red, green, and blue (RGB) colors originated from the Si nanoparticles. Two-dimensional scanning of 355-nm wavelength of nanosecond laser with a Gaussian spot beam created Si nanoparticles of 100~200 nm at laser fluences of 150~200 mJ/cm2 . We demonstrated the color resonances could be tuned to red, green, and blue adjusting the laser fluence and scan pitch. The size and distribution are characterized by scanning electron microscopy (SEM), which revealed the Si nanoparticles are ellipsoidal shape, embedded in the residual Si layer. The optical properties are measured by dark field microscopy and fiber coupled spectroscopy. The RGB samples show peak wavelengths of 628 nm, 570 nm, and 495 nm, respectively, which are attributed to the dipole resonance as predicted by the Mie theory.
A 355-nm solid state laser was irradiated onto 55-nm thick amorphous Si on a glass substrate using a spot beam steering method. After irradiation with a single pulse of Gaussian beam of varying laser energies, the Si surfaces were examined using Nomarski optical images. The images were composed of several color regions including pink, orange, dark red, and yellow. The energy fluences were 30-40 mJ/cm2 (pink), 40-70 mJ/cm2 (orange), 70-110 mJ/cm2 (dark red), and over 110 mJ/cm2 (yellow). Within the pink to dark-red area, as the fluence increases, the surface roughness and Si crystallinity also increase. However, in the yellow region, the Si surface is partially ablated due to the excess laser intensity. The laser beam, not exceeding the peak fluence of 110 mJ/cm2 , was scanned in the horizontal direction with beam overlap number of 20-100. The result revealed that high stain was observed in 20 pulses (scan pitch of 8.0 μm) caused by the difference in energy density from pulse to pulse. However, in 100 pulses (scan pitch of 1.6 μm), the Si surface was smooth and uniform with a roughness of 22 nm.
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