Influence of temperature on supercontinuum generation induced by femtosecond laser filamentation in NaCl solution

Abstract

Supercontinuum generation is an important nonlinear phenomenon that occurs during the femtosecond laser filamentation in transparent medium, and its potential and promising applications like remote sensing, biomedical imaging and generation of few-cycle femtosecond pulses, etc. have aroused a great deal of interest. With the extensive and thorough theoretical simulation and experimental research of the supercontinuum generation in air, the mechanism of the supercontinuum induced by femtosecond laser filament in gaseous medium has become clear. However, the femtosecond laser filament-induced supercontinuum in liquid is still an open question. In this work, by taking NaCl solution for example, we investigate the influence of solution temperature on the supercontinuum induced by the femtosecond laser filamentation in solution. It is found that when the laser pulse energy is relatively low (e.g. 20 and 50 J), the influence of solution temperature on supercontinuum generation can be neglected. In contrast, when the laser pulse energy is relatively high (e.g. 200 J), with the increase of solution temperature, the supercontinuum generation shows a suppression tendency. The water molecules in NaCl solution are photo-ionized due to the high intensity of femtosecond laser filament, generating a great deal of oxygen (O2), hydrogen (H2) and water vapor (H2O), and thus forming bubbles that float upwards. In the case of lower pulse energy, the multi-photon ionization rate is low, therefore, only a few bubbles are generated, and they are small in size, which hardly affects the supercontinuum generation. In the case of higher pulse energy, a large number of bubbles can be observed in the NaCl solution, and their sizes become increasingly large when the temperature of NaCl solution increases. The generation of bubbles leads to the reflection and refraction of light, which inevitably influences the spectral intensity. Furthermore, the components (e.g. O2, H2 and H2O) in the bubbles also absorb the supercontinuum, which further lowers the spectral intensity. This work reveals that the main factors leading to the supercontinuum suppression in solution can be attributed to the generation of bubbles during femtosecond laser filamentation and the scattering and absorption of light caused by water vapor in bubbles. When we detect the components in solution via the femtosecond laser filament-induced supercontiunum, the influence of tempera-ture can be effectively eliminated by adjusting the incident pulse energy. Moreover, in the case of high pulse energy, the supercontinuum generation can be controlled by adjusting the solution temperature. This study is conducible to the application of supercontinuum as well as its generation.