Ultrafast nanoporous silica formation driven by femtosecond laser irradiation

Abstract

A type of glass modifications occurring after femto‐second laser irradiation gives rise to strong (10−2) from birefringence. This form birefringence is thought to be related to index nanostructure (called nanogratings). Analyzing induced tracks in fused silica using scanning electron microscopy (SEM) with nm resolution shows that nanostructures are porous nanoplanes with an average index lower than typical silica (Δn ∼ –0.20). Their origin is explained as arising from fast decomposition of the glass under localized, high‐intensity femtosecond laser radiation where strong nonlinear, multiphoton‐induced photoionization leads to plasma generation. Mechanistic details include Coulombic explosions characteristic of strong photoionization and the production of self‐trapped exciton (STE). Rapid relaxation of these STE prevents recombination and dissociated atomic oxygen instead recombines with each other to form molecular oxygen pointed out using Raman microscopy. Some of it is dissolved in the condensed glass whilst the rest is trapped within nanovoids. A chemical recombination can only occur at 1200 °C for many hours. This explains the thermal stability of such a nanostructure. Precise laser translation and control of these birefringent nanoporous structures allo arbitrarily tuning and positioning within the glass, an important tool for controlling optical properties for photonic applications, catalysts, molecular sieves, composites and more.

Compact Birefringent Waveplates Photo-Induced in Silica by Femtosecond Laser

“…Whatever the repetition rate, the retardance shows a trend to saturation with decreasing scanning speed (i.e., for 1000 pulses/μm). This can be understood taking into account the mechanism (O2 migration and nanoporous silica formation arising from condensation under confinement stresses inside nanolayers [5]). Quantitatively, the major part of the birefringence is due to form birefringence [B] even if a significant part (up to 20%) is related to stress birefringence.…”

“…For a 0.6 NA aspheric lens the scattering increases by a factor 2 from 0.5 μJ up to 1 μJ, then it exhibits a trend to saturation up to 1.5 μJ and then it increases dramatically for higher pulse energies that correspond to disrupted nanogratings microstructure. We speculate that the scattering losses are introduced by nanoporous glass observed inside the nanoplanes with characteristic dimensions of few tens of nanometers [5]. The scattering losses also tend to be smaller for the structures written with lower translation speed (more pulses per micron).…”

“…The main part of the laser-induced losses observed for Type II modification is likely due to the light scattering of the inhomogeneous nanostructure (porous nanoplanes) [5]. Additionally, two intense absorption bands can be distinguished at short wavelengths, which are attributed to SiE' centers at 210 nm (≡Si, an unpaired electron in a silicon atom bound to three oxygen atoms), and oxygen deficiency center (ODC)(II) at 245 nm, (-O-Si-O-, a divalent silicon atom) [27,28].…”

“…This was the first time that quasi-periodic sub-wavelength structures made up by the light were fabricated in volume, in contrast to 2D nanogratings produced by lasers on surfaces [8]. Recently, we showed that these nanogratings consist of thin regions with a low refractive index (typically Δn = −0.15) due to volume expansion and the formation of nanoporous silica [5,6], surrounded by homogeneous regions with a positive index change. These porous nanolayers exhibit strong uniaxial negative form birefringence (typically ~10 −2 ) with a fast axis parallel to the polarization of the writing laser [9] and essentially arise as a result of condensation of vaporized glass within a confined but inflated volume after excitation.…”

“…Recently, further unique properties of laser induced glass modifications have been discovered including orientational dependent writing [2][3][4], glass decomposition [5,6] and elemental distribution with sub-wavelength spatial resolution [7]. To our knowledge, no other technique holds the same potential for realizing 3D multi-functional photonic devices fabricated in a single step in a wide variety of transparent materials, particularly high temperature silica.…”