Measurements of blue shifts due to collisionless absorption in harmonic generation from subpicosecond laser-produced plasmas
Аннотация
Harmonic generation from solid surface plasmas is studied using a subpicosecond Nd:glass laser system. For a $45\ifmmode^\circ\else\textdegree\fi{}$ angle of incidence, the speculars up to the fifth harmonics are blue shifted when the laser intensity exceeds $2\ifmmode\times\else\texttimes\fi{}{10}^{16} {\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2}.$ The second harmonic is blueshifted by $\ensuremath{\sim}16 \mathrm{\AA{}},$ and the fifth harmonic is blue shifted by $\ensuremath{\sim}51 \mathrm{\AA{}}$ for p polarization at the intensity of $1\ifmmode\times\else\texttimes\fi{}{10}^{17} {\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2}.$ We observed the blue shift of the fifth harmonics and found that the magnitude of blue shift is higher compared with that for the second harmonics. The blue shift is interpreted as a collisionless absorption due to the anomalous skin effect. It is also found that the divergence of harmonics preserves a smaller divergence when using a shorter pulse length for the driving laser.