Dmitry V. Dylov

Observation of All-Optical Bump-on-Tail Instability

We demonstrate an all-optical bump-on-tail instability by considering the nonlinear interaction of two partially coherent spatial beams. For weak wave coupling, we observe momentum transfer with no variation in intensity. For strong wave coupling, modulations appear in intensity and evidence appears for wave (Langmuir) collapse at large scales. Borrowing plasma language, these limits represent regimes of weak and strong spatial optical turbulence. In both limits, the internal spectral energy redistribution is observed by recording and reconstructing a hologram of the evolving dynamics. The results are universal and can appear in any wave-kinetic system with short-wave-long-wave coupling.

Diffraction from an edge in a self-focusing medium

We experimentally demonstrate diffraction from a straight edge in a medium with self-focusing nonlinearity. Diffraction into the shadow region is suppressed with increasing nonlinearity, but mode coupling leads to excitations and traveling waves on the high-intensity side. Theoretically, we interpret these modulations as spatially dispersive shock waves with negative pressure.

Nonlinear focusing and defocusing of partially coherent spatial beams

We consider the propagation of a partially coherent spatial beam in both self-focusing and self-defocusing nonlinear media. Using a Gaussian-Schell model, we derive an equation governing the width of highly incoherent beams as they propagate in both types of media and confirm its validity by using numerical simulations. Experiments performed in a biased photorefractive crystal match the predicted scaling.

Modulation instability of a coherent–incoherent mixture

We study, experimentally and theoretically, the modulation instability of a mixture of coherent and spatially-incoherent beams. In contrast with incoherent-MI, which requires a threshold nonlinearity, we show that any amount of coherent component triggers instability.

Nonlinear Restoration of Diffused Images via Seeded Instability

We demonstrate a nonlinear method for restoring the quality of diffused images. It works by introducing a self-focusing medium into the optical path and allowing the underlying correlations to grow as they propagate. More specifically, the method is a dynamical stochastic resonance in which a weak signal seeds an instability in the diffuse background. The resonance is determined by the transition point between competing instabilities, with optimal growth occurring when the spatial scale of the instability matches that of the object of interest. The results are presented within the general framework of nonlinear statistical optics and have implications for information theory, basic wave physics, and nonlinear system design.

Instability-driven recovery of diffused images.

We demonstrate the nonlinear recovery of diffused images in a self-focusing photorefractive medium. The method is based on the convolution property of nonlinearity, in which related modes reinforce each other as they propagate. The resulting mode coupling enables energy transfer from the scattered light to the underlying signal. The dynamics is well described by a model in which the signal seeds a modulation instability in the diffused background.

Plasma membrane temperature gradients and multiple cell permeabilization induced by low peak power density femtosecond lasers

Calculations indicate that selectively heating the extracellular media induces membrane temperature gradients that combine with electric fields and a temperature-induced reduction in the electropermeabilization threshold to potentially facilitate exogenous molecular delivery. Experiments by a wide-field, pulsed femtosecond laser with peak power density far below typical single cell optical delivery systems confirmed this hypothesis. Operating this laser in continuous wave mode at the same average power permeabilized many fewer cells, suggesting that bulk heating alone is insufficient and temperature gradients are crucial for permeabilization. This work suggests promising opportunities for a high throughput, low cost, contactless method for laser mediated exogenous molecule delivery without the complex optics of typical single cell optoinjection, for potential integration into microscope imaging and microfluidic systems.

Photonic Plasma Instabilities and Soliton Turbulence in Spatially Incoherent Light

We develop a plasma theory of nonlinear statistical optics. In this model, partially spatially incoherent light is treated as an ensemble of speckles which can interact through the nonlinearity. A photonic plasma frequency is defined, as is a photonic Debye length. This approach unifies previous observations using partially coherent light and predicts a new class of optical phenomena. Examples include the two-scale energy transfer common to modulation instability and the continuous excitation of modes from the gradient-driven bump-on-tail instability. The latter example, well-known from plasma physics, represents a new regime for optical experiments. We observe it here by considering the nonlinear coupling of two partially coherent beams in a self-focusing photorefractive crystal. For weak wave coupling, determined by small modal density within a Debye sphere, we observe momentum exchange with no variation in intensity. For strong wave coupling, modulations in intensity appear, as does evidence for wave (Langmuir) collapse at large scales. To achieve a broader range of wave coupling, we consider a double bump-on-tail geometry. This system can be modeled as a pair of coupled single-hump instabilities whose interaction involves general issues of nonlinear competition, synchronization, etc. For the case of strong wave coupling, the multiple humps merge into a single-peaked profile with an algebraic k-2 inertial range. This self-similar spectrum, representing an ensemble of dynamically-interacting solitons atop a sea of radiation modes, is a definitive observation of soliton (Langmuir) turbulence.

Spectral dynamics of spatially incoherent bump-on-tail instability

We examine an all-optical bump-on-tail instability by considering the nonlinear interaction of two partially incoherent spatial beams. Using a radiation transport approach, we develop plasmalike dispersion relations for perturbation modes and show that a positive gradient in the power spectrum can trigger instability. Theoretical considerations are confirmed by experiment and numerical simulation.

Nonlinear recovery of noise-hidden signals via modulation instability

We demonstrate a spatial, all-optical version of steganography in a nonlinear medium. After hiding a coherent image in spatially-incoherent noise, we recover the signal by seeding modulation instability in a self-focusing photorefractive crystal.