Mechanism of femtosecond-laser-induced periodic nanostructure formation on crystalline silicon surface immersed in water

Abstract

Focused on silicon surface in water, superimposed multiple shots of linearly polarized 800-nm, 100-fs, 10-Hz laser pulses at lower fluence than the single-pulse ablation threshold are shown to produce two kinds of periodic nanostructures with almost constant periods of 150 nm and 400 nm. Surface plasmon polaritons excited in the surface layer illustrates well the formation of nanostructures and its dynamic properties observed. Pump and probe measurements of the ultrafast change in surface reflectivity during the interaction have demonstrated that the multiple low-fluence fs pulses are crucial to the nanostructuring through the accumulation of non-thermal bonding structure change and the subsequent nanoscale ablation.

Nanograting formation through surface plasmon fields induced by femtosecond laser pulses

“…It is clear that the reduction of K is non-thermally developed with an increase in N, being accompanied by neither thermal melting nor other effects to destroy the initial interference pattern. These results strongly suggest that the ablation to reduce K is induced by the spatial standing SPP waves 23,30 excited in the fringe period, and a single standing SPP mode survives with increasing N to form the nanograting at q ¼ 5. The origin of integer q will be identified by a model calculation in Sec.…”

“…Those are concerned with self-organization of surface instability, 1,16 linear and/or nonlinear refractive index change, 3,7 Coulomb explosion, 4 second-harmonic generation, 5,17 nanoplasma formation, 18,19 optical near-fields, 20,21 and excitation of surface plasmon polaritons (SPPs) in the surface layer. 22,23 Some of these mechanisms should contribute to the observed periodic nanostructure formation in specific cases. However, there have been few experiments to successfully demonstrate or control the fundamental interaction process responsible for the nanostructure formation.…”

“…In our studies of fs-laser-induced nanostructure formation, [20][21][22][23][24] we have focused our attention on the experimental condition of nanostructuring that uses superimposed multiple shots of fs laser pulses at lower fluence than the single-pulse ablation threshold of the target. This condition has been employed in most common to form the periodic surface nanostructure for different kinds of materials.…”

“…[1][2][3][4][5][6][7][8][9][10][20][21][22][23][24][25][26] We expect that under the condition, the incident fs laser pulse hardly induces strong thermal effects such as thermal melting and erosion before the onset of ablation, and then the ablation trace should be the fingerprint of ultrafast interaction of fs laser pulses with the target surface. The experimental results obtained in our studies support the following scenario of nanostructuring in progress with an increase in the superimposed shot number N of fs laser pulses at a low fluence: 23,27 On a flat target surface, multiple shots of low-fluence fs pulses induce bonding structure change of the material through the linear or nonlinear light absorption. 20,25 An increase in the bonding structure change with increasing N leads to a decrease in the ablation threshold of the target, followed by the onset of nanoscale ablation through the optical near-fields enhanced around the high curvatures on the surface.…”

“…22 Development of the nanoscale surface corrugation allows the incident fs laser pulses to couple with SPPs, of which periodically enhanced near-fields induce further ablation to form the periodic nanostructure. 22,23 These processes of nanostructuring should non-thermally be developed so that the energy deposition periodically 114, 153108 (2013) modulated in the nanoscale surface layer is never destroyed prior to the onset of ablation. In the recent experiment study, 26 we have confirmed that the low fluence of fs pulses certainly ensures the non-thermal process for nanostructuring.…”