Abstract
1 Introduction
White-light continuum generation is a universal process that occurs when intense ultrashort laser pulses interact with transparent gases[
Since the discovery of white-light continuum by Robert R. Alfano and Stanley Shapiro in 1969, extensive progress has been achieved on the experimental and theoretical understanding of ultrafast nonlinear and linear processes responsible for white-light continuum generation[
A white-light continuum is believed to be polarized in the direction of the incident pump laser polarization for an isotropic medium[
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In this paper, we focus a femtosecond laser into bulk fused silica to induce white-light continuum and also use two orthogonal polarizers to analyze the continuum. As expected, continuum signals can still be detected behind the second polarizer. We name such a phenomenon as self-induced birefringence of continuum. In contrast to previous work, a deeper study of depolarization is presented by exploring the time evolution of the transmitted continuum signal as well as imaging the induced structures. In the following sections, we will find more characteristics of self-induced birefringence of continuum and decode how self-induced birefringence of continuum proceeds in fused silica.
2 Experiment
The laser source is an amplified Ti:sapphire mode-locked laser system (Legend Elite Series, Coherent) with a pulse duration of 50 fs operating at 800 nm with a measured bandwidth of 10 nm and a frequency of 1 kHz. The single pulse energy is varied from 20 to 850 μJ. In this work, commercially available fused silica glass (JGS1) samples with the dimension of 10 mm × 10 mm × 2 mm are used, and their six surfaces are polished to optical grade. The apparatus used in the experiment is depicted in Figure
3 Filamentation without polarizers
When the two lenses shown in Figure
4 Time evolution and pulse energy dependence of white-light continuum
When white-light continuum is generated, we can detect the transmission signal behind the second polarizer, which indicates that the polarization of the continuum is changed. A laser beam profiler is used to record the beam profile. Figure
For a more accurate demonstration, we also recorded the power evolution of the probe signal in Figure
Fused silica is an isotropic material. Though depolarization of continuum has been demonstrated in isotropic medium such as BK-7 glass[
Laser irradiated bulk fused silica was then viewed under microscope. In Figure
Femtosecond laser induced birefringence in glass is not a new phenomenon. Previous work has indicated that when a femtosecond laser focuses into a bulk glass, optical anisotropy can be created due to anisotropic refractive-index change[
Figures
In Figure
Finally, the transmitted spectra behind the second polarizer are also measured in Figure
5 Conclusion
In conclusion, we have recorded the time evolution of self-induced birefringence of white-light continuum in fused silica. This indicates that white-light continuum is synchronously modulated anisotropically in the interaction process of a focused femtosecond laser with fused silica. The birefringence signal of the generated continuum has a growth–saturation property with time evolution, and it becomes more intense for higher laser pulse energies. Optical morphology analysis finally demonstrates that time-evolved anisotropic structures are responsible for self-induced birefringence of the continuum. These properties may be useful for fabrication of polarization-dependent devices.
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