Transformation of liquid water to ice VII during propagation of picosecond laser pulses: effects of wavelength and polarization

The dependence on the input wavelength and polarization of the formation of the ice VII phase of liquid water during the propagation of 30 ps laser pulses using stimulated Raman scattering (SRS) is studied. Raman (Stokes and anti-Stokes) shifts corresponding to both liquid water and ice VII peaks are observed with 532 and 1064 nm excitation, whereas with 355 nm Raman shifts due to liquid water alone are observed. An interesting energy transfer between anti-Stokes modes of ice VII and that of water is observed at 1064 nm excitation. A higher Stokes percentage of conversion efficiency with circular polarization for 532 nm and 355 nm and higher anti-Stokes with linear polarization for 532 nm and 1064 nm are observed. SRS signals in the forward and backward directions with 532 nm excitation have shown that the Raman shifts confirm the generation of GPa pressures in liquid water around the focal volume. The evolution of Raman shifts of ice VII phase with input polarization studied at 532 nm excitation wavelengths indicated the presence of pressures in the range of 8.7–10.7 GPa in the forward direction and 8.7–11.5 GPa in the backward direction. The longitudinal imaging of the self-emission due to filamentation of ps pulses propagating through liquid water indicated the localization of energy around the focal volume leading to cavitation supporting the presence of higher plasma pressures.