Space-time focusing of phase-stabilized nanojoule-level 2.5-cycle pulses to peak intensities ≫3×1013 W/cm2 at 80 MHz

In this work, the authors demonstrate the efficient focusing of 2.5-cycle laser pulses from a phase-stabilized broadband laser oscillator with a repetition rate of 80 MHz (Femtolasers Rainbow) using a simple and compact setup based on octave-spanning double-chirped mirrors to pre-compensate the dispersion of microscope objectives. The beam diameter was kept at <2 mm at the entrance of the objectives in order to minimize undesirable radius-depend group-delay dispersion effects. Temporal characterization was performed with a modified SHG interferometric autocorrelator capable of measuring the pulse duration directly at the focus of the objectives, and the spot size was measured by imaging it with another microscope objective and CCD camera. For an input pulse energy of 2.5 nJ, a 63 x microscope objective (N.A.= 0.85) resulted in clean 6.8 fs (FWHM) pulses with spot sizes as small as 1.1 mum (FWHM).