The speed of the laser-supported hydrothermal wave is experimentally measured in porous polystyrene with overcritical average density. The results of the experiments are in agreement with the simulations performed with the MULTI-FM code, modeling the state of partly homogenized plasma. The measured velocity is 2 times smaller than the shock wave velocity calculated in simulations under the same conditions of laser irradiation in a homogeneous substance of the same density. The obtained results allow us to better investigate the possibility of using porous matter of overcritical density as an effective absorber-ablator in laser thermonuclear fusion targets.
Microscale filamentation of 0.25 NA-focused, linearly and circularly polarized 1030 nm and 515 nm ultrashort laser pulses of variable pulse widths in fused silica, fluorite, and natural and synthetic diamonds demonstrates the Raman–Kerr effect in the form of critical pulse power magnitudes, proportional to squared wavelength and inversely proportional to laser pulse width of 0.3–10 ps. The first trend represents the common spectral relationship between the quantities, while the second indicates its time-integrated inertial contribution of Raman-active lattice polarization, appearing in transmission spectra via ultrafast optical-phonon Raman scattering. The optical-phonon contribution to the nonlinear polarization could come from laser field-induced spontaneous/stimulated Raman scattering and coherent optical phonons generated by electron–hole plasma with its clamped density in the nonlinear focus. Almost constant product value of the (sub)picosecond laser pulse widths and corresponding critical pulse powers for self-focusing and filamentation in the dielectrics (“critical pulse energy”) apparently implies constant magnitude of the nonlinear polarization and other “clamped” filamentation parameters at the given wavelength.
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