Biomedical subjects
Publications and source records attributed to G Valiulis.
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.
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.
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.
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.
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.
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.
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.
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.