Handling beam propagation in air for nearly 10-fs laser damage experiments

Predictable surface ablation of dielectrics with few-cycle laser pulse even beyond air ionization

“…For every beam that we measured for a given energy E, the free parameters of the numerical fit are: the order n of each participating SG function (note that n ¼ 1 corresponds to a Gaussian function), the radius w n at 1/e 2 , and the weighting coefficient a n ensuring that the 2D integration over r of the fluence function FðrÞ equals the energy carried by the beam. Note that we neglect the small loss of energy due to plasma creation in air, since we measured in previous work 12 that it is limited to $1%, in agreement with recent numerical predictions 20,21 in such sharp focusing conditions. In fact, the numerical fitting of the beam enables to evaluate the peak fluence value (used to calibrate Fig.…”

“…Figure 2 shows that the experimental data are fully in accordance with this equation only for low incident energies, until E air NL ¼ 7:5 lJ (corresponding intensity I air NL ¼ 1:65 Â 10 14 Wcm À2 ). As expected, this value is slightly above the intensity we previously determined, 12 the small difference being related to the observable used for defining it. In our former experiment, we indeed measured the energy above which the beam focal plane begins to move out of the linear focus while here we observe the consequences of such displacement on an outcome of the interaction, e.g., the ablation diameter.…”

“…2). In the regime E air NL < E < E air ioniz: , the beam profile is still Gaussianshaped but the focus is shifted towards the incoming laser beam direction (essentially due to self-focusing 12 ) and its waist dimension w air 0 is decreased compared to the one measured at linear focus. After this nonlinear focus, the beam diffracts and spreads transversely, further leading to larger dimension w air sample on the sample surface (see Fig.…”

“…Very few experimental studies have examined femtosecond pulse propagation in air with tightly focused beams. [8][9][10][11] In previous works done in such conditions, 12 we determined an intensity, equal to I NL ¼ 1.4 Â 10 14 W/cm 2 , above which the beam undergoes distortions due to the nonlinear effects taking place in air. Nevertheless, the consequences on ablation outcomes (as for instance the diameter of the ablated crater) using few-cycle pulses have not been investigated so far.…”

“…The experimental setup using ASUR platform has been described in detail elsewhere. 12 It relies on a Ti:Sa laser system delivering linearly polarized 25 fs, 1 mJ pulses at 100 Hz, and 800 nm central wavelength. The spectrum is further enlarged up to 160 nm at 1/e 2 (720-880 nm) through crosspolarized wave generation in two cascaded BaF 2 crystals.…”