Time-resolved backside optical probing of picosecond-laser-pulse-produced plasma in solid materials

We report on pump-probe measurements of reflectivity and transmissivity of a plasma produced in transparent solid materials. The plasma is created by irradiating uncoated transparent planar glass targets with 1.0-ps FWHM laser pulses at a peak intensity of 2.0×${10}^{13}$ ${\mathrm{W}/\mathrm{c}\mathrm{m}}^2$. Time-resolved measurements using a probe light pulse incident from the backside are presented, revealing two competing mechanisms: one is highly absorptive due to a bulk underdense plasma formed behind the target surface, and the other is highly reflective due to an overdense plasma layer at the surface. A simple self-consistent and analytical model, similar to the avalanche model, is proposed, leading to both time-dependent and time-integrated solutions to the evolution of plasma density profiles and characteristics of high-intensity-laser-pulse propagation and absorption in the transparent material. Calculations of the probe light interacting with this plasma show that excellent agreement with experimental measurements can only be obtained by including a contribution from the bulk plasma formed behind the surface. Experimental measurements with Au-coated targets are also shown to illustrate elimination of the bulk plasma.