Effect of numerical aperture on the ultrashort laser pulses focusing in bulk of synthetic diamond

The effect of the focusing optics numerical aperture $(NA)$ on the plasma channels formation induced by 1030 nm ultrashort laser pulses in bulk of synthetic diamond is investigated. It is shown that in the studied peak power range $P_0$ = 0.45–0.9 MW at $NA <$ 0.2 the nonlinear focusing regime takes place, in which the significant influence of Kerr self-focusing leads to the extended plasma channels formation, and with tight focusing ($NA >$ 0.3) it happens in the linear regime, where due to the predominance of geometric focusing the formation of more compact structures is possible. The transition from a nonlinear to a linear focusing depends on the power of the laser pulses and at lower powers occurs at higher values of the numerical aperture. The results obtained can be used to improve the accuracy of in-bulk laser micro/nanomodification and to control the spatial parameters of the modified regions.