Periodic nanostructures are observed in silica glass after irradiation with a femtosecond Ti:sapphire laser. Backscattering electron images reveal stripelike regions of ~20 nm width with low oxygen concentration, aligned perpendicular to the laser polarization. These are the smallest embedded structures ever created by light. The period of self-organized grating structures can be controlled from ~140 to 320 nm by pulse energy and number of pulses. The phenomenon is interpreted as interference between incident light and bulk electron plasma wave, leading to periodic modulation of electron plasma concentration and structural changes in glass. The study used silica glass samples and a femtosecond laser focused into the material. After irradiation, the sample was polished and analyzed using scanning electron microscopy and Auger electron spectroscopy. The results showed periodic structures of oxygen-deficient regions (~SiO₂₋ₓ) aligned perpendicular to the laser polarization. The grating period decreased with exposure time and increased with pulse energy. Theoretical analysis suggests that the grating period depends on electron temperature and density, with the period increasing as electron concentration and temperature rise. The observed phenomenon is explained by interference between light and electron density waves, leading to periodic structural changes in glass. The formation of stripelike regions with low oxygen concentration is attributed to multiphoton absorption, bond breaking, and formation of nonbridging oxygen-hole centers and interstitial oxygen atoms. These oxygen atoms can diffuse and are repelled by high electron concentration regions. The observed light "fingerprints" are the smallest embedded structures ever created by light, with potential applications in optical recording and photonic crystal fabrication. The study provides new insights into light-matter interactions and the formation of periodic nanostructures in glass.Periodic nanostructures are observed in silica glass after irradiation with a femtosecond Ti:sapphire laser. Backscattering electron images reveal stripelike regions of ~20 nm width with low oxygen concentration, aligned perpendicular to the laser polarization. These are the smallest embedded structures ever created by light. The period of self-organized grating structures can be controlled from ~140 to 320 nm by pulse energy and number of pulses. The phenomenon is interpreted as interference between incident light and bulk electron plasma wave, leading to periodic modulation of electron plasma concentration and structural changes in glass. The study used silica glass samples and a femtosecond laser focused into the material. After irradiation, the sample was polished and analyzed using scanning electron microscopy and Auger electron spectroscopy. The results showed periodic structures of oxygen-deficient regions (~SiO₂₋ₓ) aligned perpendicular to the laser polarization. The grating period decreased with exposure time and increased with pulse energy. Theoretical analysis suggests that the grating period depends on electron temperature and density, with the period increasing as electron concentration and temperature rise. The observed phenomenon is explained by interference between light and electron density waves, leading to periodic structural changes in glass. The formation of stripelike regions with low oxygen concentration is attributed to multiphoton absorption, bond breaking, and formation of nonbridging oxygen-hole centers and interstitial oxygen atoms. These oxygen atoms can diffuse and are repelled by high electron concentration regions. The observed light "fingerprints" are the smallest embedded structures ever created by light, with potential applications in optical recording and photonic crystal fabrication. The study provides new insights into light-matter interactions and the formation of periodic nanostructures in glass.