Multiphoton Ionization in Dielectrics: Comparison of Circular and Linear Polarization

Ionization mechanisms in bulk dielectrics irradiated by single intense 50-fs-laser pulses are investigated by ultrafast time-resolved imaging interferometry. Polarization-sensitive 6-photon ionization is shown to be the dominant ionization mechanism in fused silica and sapphire at intensities around $10\mathrm{TW}/{\mathrm{cm}}^2$. For both materials the cross sections of 6-photon ionization are found to be significantly higher for linear polarization than for circular. Our experimental results corroborate an earlier theoretical prediction on the dominance of linear polarization in high-order multiphoton ionization.

Figure 1

Schematic of experimental setup for ultrafast imaging Mach-Zehnder interferometry.

Figure 2

Interferometrically measured phase shifts $\Delta \Phi$ and transmission $T$ of probe pulses versus pump-probe delay time (a) in sapphire, peak pump intensity $I=12\mathrm{TW}/{\mathrm{cm}}^2$ and (b) in fused silica, peak pump intensity $I=20\mathrm{TW}/{\mathrm{cm}}^2$.

Figure 3

Dependence of free-carrier density $\rho /m^{*}\sim |\Delta \Phi |$ in sapphire and fused silica on the intensity of linearly and circularly polarized pump pulses. The original dependencies of the phase shift $|\Delta \Phi |$ and the logarithm of transmission $|\ln T|$ on the intensity are given on the right-hand side. The dotted straight lines are proportional to $I^6$; the arrows in the upper right corner mark the threshold intensities of surface breakdown by linearly and circularly polarized pump pulses.