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.

One dimensional spatial localization of polychromatic stationary wave-packets in normally dispersive media.

In this paper we illustrate how the localization of the stationary two-dimensional solution of the propagation equation strongly depends on the features of its spatio-temporal spectral bandwidth. We especially investigate the role of the ultra-broad temporal support and of the spatial bandwidth of the spectrum on the high localization in one spatial dimension of "Bessel-like" or "blade-like" beams, quasi-stationarily propagating in normally dispersive materials, and potentially interesting for microfabrication applications.

Rogue-wave-like statistics in ultrafast white-light continuum generation in sapphire.

We experimentally study the statistics of the white-light continuum generated by focusing of 130 fs, 800 nm pulses in a sapphire plate and show that the statistical distributions of the spectral intensity of the blue-shifted continuum components obey the extreme-value statistics. This rogue-wave-like behavior is detected only within a narrow input-pulse energy interval. By the use of numerical simulations, we show that the observed rogue-wave-like behavior is associated with pulse splitting and build-up of intense trailing pulse. The extreme events are thereafter suppressed by the intensity clamping.

Axial emission and spectral broadening in self-focusing of femtosecond Bessel beams.

We report on the experimental observations of on-axis spectral broadening arising from self-focusing of the axicon-generated femtosecond Bessel beam in water. The observed spectral broadening is interpreted by a nonlinearly phase-matched four-wave mixing process involving the intense conical pump, the axial signal and a conical idler wave.

Highly efficient four-wave parametric amplification in transparent bulk Kerr medium.

We report on highly efficient four-wave optical parametric amplification in a water cell pumped by an elliptically shaped, ultrashort pulsed laser beam under non-collinear phase-matching configuration. Energy conversion from pump to parametric waves as high as 25 % is obtained owing to the achievement of 1-dimensional spatial-soliton regime, which guarantees high intensity over a large interaction length and ensures high beam quality.

Observation of quadratic optical vortex solitons

We report on the generation of stable dark-vortex solitons in large-phase-mismatched second-harmonic generation of self-defocusing type, sustained by a combined effect of transverse walk-off and finite beam size.

Noise-seeded spatiotemporal modulation instability in normal dispersion.

In optical second-harmonic generation with normal dispersion, the virtually infinite bandwidth of the unbounded, hyperbolic, modulational instability leads to quenching of spatial multisoliton formation and to the occurrence of a catastrophic spatiotemporal breakup when an extended beam is left to interact with an extremely weak external noise with a coherence time much shorter than that of the pump.

Nonlinear X waves in second-harmonic generation: experimental results.

X waves, spatiotemporal generalization of the monochromatic Bessel- (or Durnin-) type beams, are known in linear acoustic, microwave and optics for their unique property of defeating both spatial and temporal spreadings. Recently, we brought to the attention that X-type waves are also the key to understand the spatiotemporal dynamics observed in the nonlinear (high intensity) regime. Indeed, X waves represent the normal-propagation mode for a wide class of parametric interactions described by hyperbolic nonlinear models featuring spatial self-focusing and temporal self-broadening. Here, we provide a complete and detailed description of the experiment in which the spontaneous appearance of X waves has been observed. The experiment concerns frequency doubling of a 170-fs, 50-microm standard laser wave packet in a 22-mm lithium triborate crystal, tuned for second-harmonic generation with positive phase mismatch, positive group-velocity dispersion, and large group-velocity mismatch. Conventional beam-profile and autocorrelation measurements at the crystal output face show evidence of spatiotemporal self-trapping. The characterization of the free-space propagation reveals sub-Gaussian diffraction and pulse broadening, consistent with the presence of angular dispersion. Space-resolved autocorrelations indicate the generation of an X-type profile.

Spatiotemporal three-dimensional mapping of nonlinear X waves.

The spatiotemporal intensity profile of a 100-fs wave packet at the output of a X2 crystal, tuned for mismatched second-harmonic generation, is probed via sum-frequency generation with a compressed, 20-fs pulse, revealing the appearance of an X-type wave shape.

Parametric excitation of X-waves by downconversion of Bessel beams in nonlinear crystals.

We predict that in traveling-wave degenerate parametric downconversion the Bessel beam pump stimulates the appearance of a nondiffracting X-wave from quantum noise amplification. Numerical simulation results of downconversion in ADP crystal are presented, along with preliminary experimental data.

Fundamental-frequency pulse compression through cascaded second-order processes in a type II phase-matched second-harmonic generator.

We show that cascading of the second-order processes under large group-velocity mismatch can lead to the efficient compression of pulses at fundamental frequency in the type II phase-matched second-harmonicgeneration process. We demonstrate that any of three interacting pulses can be compressed if the proper ratio of intensities of the o- and e-polarized pump pulses is chosen on input. We experimentally achieved compression of o-polarized pulses from 1.3 ps down to 280 fs with energy conversion close to 50% when a conventional second-harmonic compressor with unbalanced intensities of the pump pulses was used.

Two-Photon Absorbing Properties of Ultraviolet Phase-Matchable Crystals at 264 and 211 nm.

We investigated the intensity-dependent loss properties of nonlinear crystals by using subpicosecond laser pulses at 264 and 211 nm. Two-photon absorption coefficients for potassium dihydrogen phosphate, beta-barium borate, and lithium triborate crystals were obtained from the intensity-dependent transmission measurements.

Generation of compressed 600-720-nm tunable femtosecond pulses by transient frequency mixing in a beta-barium borate crystal.

We demonstrate the feasibility of the compression of tunable pulses by means of sum-frequency generation under special group-velocity mismatch conditions. beta-Barium borate crystal has been shown to support the upconversion process, resulting in 160-fs pulses tunable within 600-720 nm when mixing 1.3-ps 1055-nm pulses from a Nd:glass laser with 0.95-ps pulses tunable within 1400-2200 nm from a traveling-wave parametric generator. A similar compression of ~150-fs Ti:sapphire pulses predicted for the sum-frequency tuning range of 520-590 nm.

Nonlinear second-harmonic pulse compression with tilted pulses.

We demonstrate that the group velocities of interacting pulses can be adjusted by pulse tilting to optimize second-harmonic pulse compression for a variety of nonlinear crystals and wavelengths. As an experimental proof we present results of ninefold compression of 1.3-ps Nd:glass laser pulses in beta -barium borate crystal.

Generation of femtosecond radiation at 211 nm by femtosecond pulse upconversion in the field of a picosecond pulse.

We show that, in the case of sum-frequency mixing, one can alleviate group-velocity mismatch between IR and UV pulses by choosing different pulse widths, thus extending the interaction length of ultrashort pulses within nonlinear crystals. By fifth-harmonic generation with a Nd:glass laser, we demonstrate efficient frequency upconversion of 195-fs 264-nm pulses under the envelope of 0.9-ps 1055-nm pulses in beta-barium borate crystal, yielding <270-fs pulses with energy of up to 110muJ at 211 nm.

Parametric generation of high-energy 14.5-fs light pulses at 1.5 mum.

High-energy light pulses that are tunable from 1.1 to 2.6 mum, with a duration as short as 14.5 fs were generated in a type II phase-matching beta-BaB(2)O(4) traveling-wave parametric converter pumped by 18-fs pulses obtained from a Ti:sapphire laser with chirped-pulse amplification, followed by a hollow-fiber compressor.

Highly efficient parametric conversion of femtosecond Ti:sapphire laser pulses at 1 kHz.

Pulses with energies as high as 150 microJ and durations as low as 60 f(s) were generated from 1.1 to 2.6 microm by a traveling-wave parametric converter pumped by femtosecond pulses of a Ti:sapphire laser with chirped-pulse amplification.

Nonlinear electromagnetic X waves.

Nonlinear optical media that are normally dispersive support a new type of localized (nondiffractive and nondispersive) wave packets that are X shaped in space and time and have slower than exponential decay. High-intensity X waves, unlike linear ones, can be formed spontaneously through a trigger mechanism of conical emission, thus playing an important role in experiments.

Spontaneously generated X-shaped light bullets.

We observe the formation of an intense optical wave packet fully localized in all dimensions, i.e., both longitudinally (in time) and in the transverse plane, with an extension of a few tens of fsec and microns, respectively. Our measurements show that the self-trapped wave is an X-shaped light bullet spontaneously generated from a standard laser wave packet via the nonlinear material response (i.e., second-harmonic generation), which extend the soliton concept to a new realm, where the main hump coexists with conical tails which reflect the symmetry of linear dispersion relationship.

Spatial versus temporal deterministic wave breakup of nonlinearly coupled light waves.

We investigate experimentally the competition between spatial and temporal breakup due to modulational instability in chi((2)) nonlinear mixing. The modulation of the wave packets caused by the energy exchange between fundamental and second-harmonic components is found to be the prevailing trigger mechanism which, according to the relative weight of diffraction and dispersion, leads to the appearance of a multisoliton pattern in the low-dimensional spatial or temporal domain.