We show that a quasi-steady-state photorefractive spatial soliton forms a waveguide structure in the bulk of a photorefractive material. Although the optically induced waveguide is formed by a very low-power (microwatts) soliton beam, it can guide a powerful (watt) beam of a longer wavelength at which the medium is nonphotosensitive. Furthermore, the waveguide survives, either in the dark or when guiding the longerwavelength beam, for a long time after the soliton beam is turned off. We take advantage of the solitons' property of evolution from a relatively broad input beam into a narrow channel and show that the soliton induces a tapered waveguide (an optical funnel) that improves the coupling efficiency of light into the waveguiding structure.
The interaction between nanoparticles and ultrashort laser pulses holds great interest in laser nanomedicine, introducing such possibilities as selective cell targeting to create highly localized cell damage. Two models are studied to describe the laser pulse interaction with nanoparticles in the femtosecond, picosecond, and nanosecond regimes. The first is a two-temperature model using two coupled diffusion equations: one describing the heat conduction of electrons, and the other that of the lattice. The second model is a one-temperature model utilizing a heat diffusion equation for the phonon subsystem and applying a uniform heating approximation throughout the particle volume. A comparison of the two modeling strategies shows that the two-temperature model gives a good approximation for the femtosecond mode, but fails to accurately describe the laser heating for longer pulses. On the contrary, the simpler one-temperature model provides an adequate description of the laser heating of nanoparticles in the femtosecond, picosecond, and nanosecond modes.
We study experimentally self-trapping of optical beams in photorefractive media and show that the trapping is inherently asymmetric with respect to the two (transverse) trapping dimensions. We also present experimental results that show how the sizes of the resultant photorefractive spatial solitons are independent (within their range of existence) of the amplitude of the externally applied electric field used to generate them.Self-trapping of optical beams in photorefractive (PR) media occurs when diffraction is exactly balanced by self-scattering (two-wave mixing) of the spatial (plane-wave) components the soliton beam. 1 ' 2 Intuitively, since diffraction involves accumulation, by each plane-wave component of a beam, of a phase that is linear in the propagation distance, it is desirable to balance the diffraction by nonlinear phase coupling that leaves the complex amplitudes of the plane-wave components unchanged. PR materials, however, typically exhibit amplitude coupling (energy-exchange interaction) that is due to a dominant diffusion transport mechanism for the redistribution of the photogenerated charge carriers, which, inherently, cannot compensate for diffraction since it alters the amplitudes of the plane-wave components rather than balancing their phases. The presence of an external bias field, however, can cause strong phase coupling and is therefore required for the formation of PR solitons. We have predicted that PR solitons exist for a well-defined range of external fields, and, within this range, the soliton size (cross section) is independent of this external field. This property is a consequence of the optical nonlinear property of the medium: the perturbation in the refractive index is proportional to the light-induced space-charge fields, which depend primarily on the beam profile.Our recent observation of what is to our knowledge the first PR spatial solitons 3 ' 4 revealed, among a variety of features (such as independence of the absolute light intensity), that, unlike the Kerr-like solitons, the PR solitons may be trapped in two transverse dimensions and maintain their stability. We have also shown theoretically and experimentally 5 that the PR solitons are stable for perturbations in their waveforms that are much smaller (in size) than their transverse cross sections but break down for perturbations that are comparable with their cross sections. Our theoretical model, however, is at this point limited to a single transverse dimension and cannot fully explain the trapping in two transverse dimensions.In this Letter we present experimental results that address the two-transverse-dimensional problem and point out where a one-dimensional analysis is valid. Furthermore, we find experimentally, in agreement with our predictions," 2 that the size of the PR soliton is independent (within the range of its existence) of the externally applied voltage used to generate it.First, we address the trapping in two transverse dimensions. Assuming the anisotropy of the PR medium to be negligible, the only ...
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