Femtosecond filamentation and supercontinuum generation in bulk silicon.

We experimentally study filamentation and supercontinuum generation in bulk silicon crystal using femtosecond mid-infrared pulses with carrier wavelengths in the range of 3.25-4.7 µm, in the presence of three-, four-, and five-photon absorption. Spectral measurements show a fairly stable blueshifted cutoff in the 2.5-2.7 µm range and gradual increase of the long-wave extent with increasing wavelength of the incident pulses, eventually yielding an octave-spanning supercontinuum, covering the wavelength range from 2.5 to 5.8 µm with the input pulses at 4.7 µm. The recorded spatiotemporal intensity distributions of a single filament revealed pulse splitting after the nonlinear focus, in line with the pulse-splitting-based filamentation scenario inherent to normally dispersive dielectric nonlinear media.

Odd harmonics-enhanced supercontinuum in bulk solid-state dielectric medium.

We report on generation of ultrabroadband, more than 4 octave spanning supercontinuum in thin CaF2 crystal, as pumped by intense mid-infrared laser pulses with central wavelength of 2.4 µm. The supercontinuum spectrum covers wavelength range from the ultraviolet to the mid-infrared and its short wavelength side is strongly enhanced by cascaded generation of third, fifth and seventh harmonics. Our results capture the transition from Kerr-dominated to plasma-dominated filamentation regime and uncover that in the latter the spectral superbroadening originates from dramatic plasma-induced compression of the driving pulse, which in turn induces broadening of the harmonics spectra due to cross-phase modulation effects. The experimental measurements are backed up by the numerical simulations based on a nonparaxial unidirectional propagation equation for the electric field of the pulse, which accounts for the cubic nonlinearity-induced effects, and which reproduce the experimental data in great detail.

Spatiotemporal light bullets and supercontinuum generation in ß-BBO crystal with competing quadratic and cubic nonlinearities.

We experimentally study filamentation and supercontinuum generation in a birefringent medium [beta-barium borate (ß-BBO) crystal] pumped by intense 90 fs, 1.8 µm laser pulses whose carrier wavelength falls in the range of anomalous group velocity dispersion of the crystal. We demonstrate that the competition between the intrinsic cubic and cascaded-quadratic nonlinearities may serve as a useful tool for controlling the self-action effects via phase matching condition. In particular, we found that spectral superbroadening of the ordinary polarization is linked to three-dimensional self-focusing and formation of self-compressed spatiotemporal light bullets that could be accessed within a certain range of either positive or negative phase mismatch. In the extraordinary polarization, we detect giant spectral shifts of the second harmonic radiation, which are attributed to a light bullet-induced self-phase matching.

Carrier-envelope phase-stable spatiotemporal light bullets.

We present an extensive experimental investigation of the self-focusing and filamentation of intense 90 fs, 1.8 µm, carrier-envelope phase-stable laser pulses in fused silica in the anomalous group velocity dispersion region. Spectral measurements in a wedge-shaped sample uncover dynamics of spectral broadening, which captures the evolution of third-harmonic, resonant radiation, and supercontinuum spectra as a function of the propagation distance with unprecedented detail. The relevant events of spectral broadening are linked to the formation and propagation dynamics of spatiotemporal light bullets as measured by a three-dimensional imaging technique. We also show that at a higher input power, the light bullet splits into two bullets, which retain characteristic O-shaped spatiotemporal intensity distributions and propagate with different group velocities. Finally, we demonstrate that the light bullets have a stable carrier-envelope phase that is preserved even after the bullet splitting event, as verified by f-2f interferometric measurements.