Formation of nanogratings in a porous glass immersed in water by femtosecond laser irradiation

Abstract

Irradiation of intense ultrafast laser pulses in glasses can lead to formation of nanogratings whose periods are significantly smaller than the incident irradiation wavelength. The mechanism of the exotic phenomenon is still under hot debate. Here, we access the snapshots of morphologies in the laser affected regions in a porous glass which reveal the evolution of the formation of nanogratings with increasing number of laser pulses. Combined with further theoretical analyses, our observation provides important clues which suggest that excitation of standing plasma waves at the interfaces between areas modified and unmodified by the femtosecond laser irradiation plays a crucial role for promoting the growth of periodic nanogratings. The finding indicates that the formation of volume nanogratings induced by irradiation of femtosecond laser pulses is initiated with a mechanism similar to the formation of surface nanoripples.

From random inhomogeneities to periodic nanostructures induced in bulk silica by ultrashort laser

“…They also enhanced and improved the nanoplasma incubation process [15,[59][60][61]. In previous studies, the dynamics of laser interaction with inhomogeneities of different nature was studied for voids [48], for nanospheres of densified fused silica [41], and for metallic nanoparticles [62]. In all these cases, multiphoton absorption led to the generation of free electron plasma in the near field of the inhomogeneity and to the reinforcement of scattering, which induced new plasma formation in the backward propagation direction.…”

“…In contrast to plasma wave [1] and nanoplasmonic models [2,4], this approach requires low electron densities, as at higher densities the exciton-polariton interaction is heavily screened by the electron plasma when it begins to efficiently absorb laser energy [44]. Finally, Liao et al suggested that excitation of standing plasma waves at the interfaces between modified and unmodified areas played a crucial role in promoting the growth of periodic nanogratings [7] and their self-organization mechanism had similarities with femtosecond-laser-induced surface ripples formation [41]. However, it was further underlined that the evidence of the defect-assisted local field rearrangement excluded the scenario that the nanograting was a result of interference between the writing beam and the surface plasma waves [27].…”

“…Some of them were based on the nonlinear Schrödinger equation (NLSE) [45][46][47], others used Maxwell's equations coupled with rate equation for free electron generation [34,35,41,[48][49][50][51]. However, the NLSE being an asymptotic parabolic approximation of Maxwell's equations, requires the unidirectionality of the light beam and cannot describe cases where dense electron plasma is generated causing light scattering to large angles [35,36].…”

“…None of the hypotheses presented above explained the whole physics of the phenomenon and very few attempts were made to prove or to invalidate any of them based on numerical calculations [35,36,41,48,49].…”

“…Several hypotheses were proposed to explain the organization of periodic structures as an interference between the incident wave and electron plasma waves [1,13], an interplay between nanoplasmonic and incubation effects [2,4,29,30], self-trapping of excitons [31,32], standing wave [33], ionization scattering instabilities [34][35][36], second harmonic generation [37], Boson condensation [38], Coulomb explosion [39], excitation of surface plasmon polaritons [40,41], and a spacecharge built from ponderomotive force [42]. The first model for nanograting formation was proposed by Shimotsuma et al [1] based on the interference of the laser field with the laserinduced plasma waves.…”