Phase Modulation of Intense Ultrashort Laser Pulses Reflected From Steep, Dense Plasmas

The phase modulation of intense ( I = 10¹⁸ W/cm²) ultrashort laser pulses ( τ_p = 70 fs) after reflection from steep, dense plasmas has been temporally resolved for the first time in particle-in-cell simulations. The position of the turning point from where the pulse reflects has been compared to the phase modulation, over a range of angles of incidence. At normal incidence or s polarization the phase modulation almost exactly represents the movement of the turning point due to the light pressure. As the angle of incidence is increased for p polarization, the simple Fresnel relationship between phase modulation and displacement of the reflection position, Δφ\(t\) = -2k₀Δx\(t\)cosθ₀, increasingly breaks down.