Toluene fragmentation by intense femtosecond laser pulses is experimentally investigated. A strong increase of the toluene fragmentation appears to correlate with an increase of the focal area due to changes in the focal geometry and nonlinear small-scale self-focusing. The scenario of Raman modes excitation proposed in an earlier publication ͓A. M. Müller et al., Phys. Rev. Lett. 88, 023001 ͑2002͔͒ is ruled out as the dominant effect for the enhancement.

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In this paper, we theoretically studied the propagation of the first-order spatiotemporal coupled ultrafast pulse in fused silica glass by using the Cartesian nonlinear Schrödinger equation coupled with the electron density rate equation. We found that wavefront rotation, one parameter of the shaped pulse, may play an integral role to adjust the asymmetrical modification under different initial coupling conditions. During the focusing process, the pulse wavefront generates or rotates the pulse front through angular dispersion, and this process directly causes the change of inclined direction of the pulse front tilt at the focal plane. The spatiotemporal coupling change could make a difference in the asymmetric distributions of intensity, fluence, and electron density at the plane of interaction, and further affect the photoionization process. Therefore, the research on the influence of wavefront rotation on the pulse front tilt under the initial pulse incidence condition helps us to understand the temporal and spatial evolution of ultrafast laser pulses. Based on our numerical simulation, the possible mechanism of nonreciprocal direct-writing phenomena is revisited by taking into account the effect of wavefront rotation.

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