Orientation-Dependent Multiphoton Ionization in Wide Band Gap Crystals

Using different crystalline dielectrics and intense femtosecond laser pulses, we show that nonlinear absorption depends on sample orientation. This arises primarily because of the direction dependence of the effective mass of the electron. The multiphoton nature of the interaction creates a local probe that can be used anywhere in the material. We show that the structure of crystal quartz is not changed by repeated illumination of the focal region with 45 fs pulses.

Figure 1

Transmission measured in $\alpha$ quartz as a function of the alignment of the laser polarization to the crystal $x$ axis. A 10-points moving average has been applied to the raw data.

Figure 2

Transmission modulation spectra for (a) $\alpha$ quartz, (b) sapphire, and (c) lithium fluoride. The vertical scale is 0–0.75% for (a),(b) and 0–1.5% for (c). Corresponding crystal structures (the bottom panel) [18, 19] ($z$-cut view) are shown looking down the $z$ axis with dashed lines as eye guides for the symmetry.

Figure 3

The amplitude of the sixfold (▴) and twofold (◆) modulation in transmission as a function of focus position $z$ in an optically bonded fused silica and $\alpha$-quartz sample. The focus is in fused silica for negative $z$ and in $\alpha$ quartz for positive $z$. The pulse energy was 90 nJ.

Figure 4

Comparison of the energy dependence of the sixfold transmission modulation in $\alpha$ quartz with the expectation from an alignment dependence of multiphoton ionization. (▴) Sixfold transmission amplitude; (line) the quantity $-E_{\mathrm{IN}}\partial T/\partial E_{\mathrm{IN}}$, with $\partial T/\partial E_{\mathrm{IN}}$ obtained as the numerical derivative of the transmission measured as a function of $E_{\mathrm{IN}}$. The two series of data have been normalized for comparison purposes.