Elastic and Thermal Dynamics in Femtosecond Laser-Induced Structural Change Inside Glasses Studied by the Transient Lens Method

Masaaki Sakakura,Kazuyuki Hirao,Yasuhiko Shimotsuma,Kiyotaka Miura,Masahide Terazima

doi:10.1155/2010/148268

Abstract

A technique to study femtosecond laser induced structural change inside glasses, the transient lens (TrL) method, is described. Because the TrL method is sensitive to the refractive index change around the photoexcited region, the time dependence of the density, pressure, and temperature changes, which accompany refractive index change, can be monitored over a broad range of timescales. In the picosecond-nanosecond time range, the pressure wave generation was observed as an oscillating TrL signal. By comparing the TrL signal with that calculated based on thermoelastic simulation, the density, pressure, and temperature changes in the photoexcited region can be estimated. In the longer time range (nanoseconds—milliseconds), the thermal diffusion process was observed. By fitting the TrL signal with that simulated based on thermal diffusion, the temporal evolution of the temperature distribution was obtained. Based on these observations, the features of femtosecond laser-induced structural change inside glasses are revealed. The advantages of the TrL method are described.

Highlights

Local structural change induced inside transparent materials using tightly focused femtosecond laser pulses enables us to fabricate various kinds of three-dimensional structures inside glasses, polymers, and inorganic crystals; this microfabrication technique with an fs laser has been used to produce small optical devices such as optical waveguides, photonic crystals, and 3D optical memory [1,2,3,4,5,6,7]
When an fs laser pulse is tightly focused inside a transparent material, only the material in the focal volume is photoexcited through nonlinear interaction with a strong laser field, resulting in localized permanent structural change inside the bulk of the material
We presented the investigation of refractive index dynamics, which reflects density, pressure, and temperature changes, after the photoexcitation by an fs laser pulse inside a glass by the transient lens (TrL) method

Summary

Introduction

Local structural change induced inside transparent materials using tightly focused femtosecond (fs) laser pulses enables us to fabricate various kinds of three-dimensional structures inside glasses, polymers, and inorganic crystals; this microfabrication technique with an fs laser has been used to produce small optical devices such as optical waveguides, photonic crystals, and 3D optical memory [1,2,3,4,5,6,7]. A structure with high refractive index has been observed around the laser focal region, when the photoexcitation occurs inside the material and the excitation laser energy is not high [1, 4,5,6,7, 12]. Chan et al observed an increase in the Raman band intensity from the structure inside a silica glass created by a tightly focused femtosecond laser They suggested that the irradiated region experiences high. The acoustic wave monitored by Kudryashov et al suggests that the photoexcited region is under high pressure after focusing fs laser pulse inside a glass [16] These studies suggest that the pressure and temperature changes should play an important role in the fs laserinduced structural change inside glasses. We summarize the dynamics that have been elucidated by the TrL method

Method

Observation of Thermoelastic Relaxation

Objective lens

Observation of Thermal Diffusion

Findings

Summary