An important challenge in the field of three-dimensional ultrafast laser processing is to achieve in the bulk structuring of silicon and narrow gap materials. Recent attempts by increasing the energy of infrared ultrashort pulses have simply failed. Here, we establish that it is because focusing with a maximum numerical aperture of about 1.5 with conventional schemes does not allow overcoming strong nonlinear and plasma effects before the focus. We circumvent this limitation by exploiting solid-immersion focusing, in analogy to techniques applied in advanced microscopy and lithography. By creating in a perfect spherical sample the conditions for an interaction with an extreme numerical aperture near 3, repeatable femtosecond optical breakdown and controllable refractive index modifications are achieved for the first time inside silicon. Our findings re-open the horizon towards the direct writing of 3D monolithic devices for silicon photonics. It also provides perspectives for new strong-field physics and warm-dense-matter plasma experiments.

More information can be found in: Chanal, M, Fedorov, VY, Chambonneau, M, Clady, R, Tzortzakis, S, Grojo, D. “Crossing the threshold of ultrafast laser writing in bulk silicon“, NATURE COMMUNICATIONS 8, 773 (2017).

[Highlighted in Phys.org]