Energy-dependent temperature dynamics in femtosecond laser microprocessing clarified by Raman temperature measurement

Focused femtosecond laser pulses can be used for fabricating photonic devices inside transparent materials. However, the processing mechanisms are not fully clarified. Previously, we investigated the local and rapid temperature dynamics of fused silica during femtosecond laser microprocessing by Raman temperature measurement. In this paper, we report on the energy-dependent temperature dynamics and the spatiotemporal evolution of heat. In the experiment, a Ti:sapphire laser system generated 80-fs pulses and a frequency-doubled Nd: YAG laser system generated 10-ns pulses. These pulses were used for microprocessing and Raman excitation, respectively. They were focused into the sample by a microscope objective. The sample was transferred mechanically during the processing to prevent multiple irradiations. The temperature at the focus was calculated from the ratio of the intensity of Stokes and anti-Stokes Raman scattering components of the measured spectrum. The measured temperature near the focal point decreased with different delays depending on the pulse energy. The spatial distribution of the temperature showed heat diffusion and temperature decrease. The measured temperature fitted well with the thermal diffusion model. In this way, energy-dependence of temperature dynamics and spatiotemporal evolution of heat were successfully investigated by using the present system.