40th International Workshop on High Energy Density Physics with Intense Ion and Laser Beams

It was recently shown [1] that plasma mirror can be an applicable pulse cleaning method even for UV lasers, as up to 70~\% efficiency can be obtained for intensities of $10^{15}$~W/cm$^2$. High acceleration of KrF laser produced plasmas was also observed [2]. Even recent results show that absorption and reflection of intense ultrashort laser pulses from laser plasmas depend strongly on the temporal contrast of the laser beam [3]. In our lab a new non-linear Fourier-filter method [4] was demonstrated for the contrast improvement of short-pulse KrF lasers and this was applied the first time here for high-intensity laser plasma experiments. It was found that increasing the intensity of the 248~nm, 600~fs laser pulse from $10^{15}$~W/cm$^2$ to $10^{18}$~W/cm$^2$ the plasma reflectivity not only saturates but decreases below 20~\% for different target materials and different polarizations. The spectral shift of the reflected beam depends strongly on the contrast of the beam. Using the improved contrast of $5\cdot10^{11}$ with the Fourier filtering spectral blue shift up to 0.6~nm was observed, corresponding to the plasma acceleration of $4\cdot10^{18}$~ms$^{-2}$. This is approximately four times higher than the previous result [2] and it does not depend strongly on the incoming beam polarization. Thus the acceleration is probably caused by the ponderomotive force. \vspace{10mm} \noindent\textbf{References} \vspace{5mm} \noindent[1] B. Gilicze et al.; Rev. Sci. Instrum. \textbf{87}, 083101 (2016) \noindent[2] R. Sauerbrey; Physics of Plasmas \textbf{3}, 4712 (1996) \noindent[3] P.K. Singh et. al.; Scientific Reports \textbf{5}, 17870 (2015) \noindent[4] B. Gilicze et. al.; Optics Express \textbf{27}, 17377 (2019)