Self-Organized Nanogratings in Glass Irradiated by Ultrashort Light Pulses

Periodic nanostructures are observed inside silica glass after irradiation by a focused beam of a femtosecond Ti:sapphire laser. Backscattering electron images of the irradiated spot reveal a periodic structure of stripelike regions of $\sim 20\mathrm{n}\mathrm{m}$ width with a low oxygen concentration, which are aligned perpendicular to the laser polarization direction. These are the smallest embedded structures ever created by light. The period of self-organized grating structures can be controlled from $\sim 140$ to 320 nm by the pulse energy and the number of irradiated pulses. The phenomenon is interpreted in terms of interference between the incident light field and the electric field of the bulk electron plasma wave, resulting in the periodic modulation of electron plasma concentration and the structural changes in glass.

Figure 1

(a) Secondary electron images of silica glass surface polished close to the depth of focal spot. (b) Light “fingerprints”: Backscattering electron images of the same surface. The magnification of the upper and lower images is $10000\times$ and $30000\times$, respectively.

Figure 2

Auger spectra maps and corresponding line scans of oxygen (a) and silicon (b) on same silica glass surface polished close to the depth of focal spot.

Figure 3

Theoretical dependence of self-organized grating period on electron temperature for $n_e=1.7\times {10}^{21}{\mathrm{c}\mathrm{m}}^{-3}$. Note that this is not a fit of experimental data. Experimental grating periods versus pulse energy are also shown. Inset is a wave vector matching diagram.