Study of thermal lensing effect induced in C2H4Cl2 and C2H4Br2 by femtosecond laser pulses
Yu-Ting Kuo1*, Yi-Ci Li1, Po-Yuan Huang1, Cheng-I Lee2, Tai-Huei Wei1
1Department of Physics, National Chung Cheng University, ChiaYi, Taiwan
2Department of Life Science, National Chung Cheng University, ChiaYi, Taiwan
* presenting author:Yu-Ting Kuo,
Using the Z-scan technique with 82 MHz 20 femtosecond laser pulses at 820 nm, we investigate the thermal lensing effect induced in C2H4Br2 and C2H4Cl2, respectively. Because the peak-to-valley separation for each curve pertaining to either sample is 1.7 times of the diffraction length, we attribute the heat generation to non-radiative relaxation subsequent to one-photon excitation rather than two-photon excitation or stimulated light scattering. When C2H4Br2 shows stronger thermal lensing effect than C2H4Cl2, we compare the following thermal-effect-related properties of both samples: excitation of molecular motions by individual 20 fs-laser pulses, heat generation due to relaxation of the excited molecular motions, sample density change induced by temperature gradient driven thermal acoustic waves and thermal diffusivity.

Keywords: Z-scan, thermal lensing effect, femtosecond laser, C2H4Cl2, C2H4Br2