Ramy El-Ganainy

Theory of coupled optical PT-symmetric structures

Starting from Lagrangian principles we develop a formalism suitable for describing coupled optical parity-time symmetric systems.

Analytical solutions to a class of nonlinear Schrödinger equations with {\cal PT} -like potentials

We present closed form solutions to a certain class of one- and two-dimensional nonlinear Schr¨ odinger equations involving potentials with broken and unbroken PT symmetry. In the one-dimensional case, these solutions are given in terms of Jacobi elliptic functions, hyperbolic and trigonometric functions. Some of these solutions are possible even when the corresponding PT -symmetric potentials have a zero threshold. In two-dimensions, hyperbolic secant type solutions are obtained for a nonlinear Schr¨ odinger equation with a nonseparable complex potential.

Enhanced third-order nonlinear effects in optical AlGaAs nanowires

We report the first observation of enhanced third-order nonlinear effects in AlGaAs nanowires. AlGaAs nanowaveguides with widths varying from 100 to 600nm were fabricated and characterized. Nonlinear phase shifts of approximately pi were experimentally observed at 1.55mum with peak powers of 30-40W in 600mum long, 550nm wide guides.

Soliton dynamics and self-induced transparency in nonlinear nanosuspensions

We study spatial soliton dynamics in nano-particle suspensions. Starting from the Nernst-Planck and Smoluchowski equations, we demonstrate that in these systems the underlying nonlinearities as well as the nonlinear Rayleigh losses depend exponentially on optical intensity. Two different nonlinear regimes are identified depending on the refractive index contrast of the nanoparticles involved and the interesting prospect of self-induced transparency is demonstrated. Soliton stability is systematically analyzed for both 1D and 2D configurations and their propagation dynamics in the presence of Rayleigh losses is examined. The possibility of synthesizing artificial nonlinearities using mixtures of nanosuspensions is also considered.

Nonlinear optical response of colloidal suspensions.

We experimentally probed the nonlinear optical response of aqueous nano-colloidal suspensions to provide a test of the theoretical approaches that have been proposed for the nonlinearity, namely an exponential model, an artificial Kerr medium, and a non-ideal gas model. The best agreement with experiment is found using the non-ideal gas model for the colloidal suspension which in turn can be used to infer values for the second virial coefficient of the medium and the nonlinear coefficients.

Optical beam instabilities in nonlinear nanosuspensions

We investigate the modulation instability of plane waves and the transverse instabilities of soliton stripe beams propagating in nonlinear nanosuspensions. We show that in these systems the process of modulational instability depends on the input beam conditions. On the other hand, the transverse instability of soliton stripes can exhibit new features as a result of 1D collapse caused by the exponential nonlinearity.

Power circulation via negative energy-flux wormholes in optical nanowaveguides.

We show that energy circulation within a pulse is possible when it propagates in a high-contrast dielectric nanowire. This process is accomplished through electromagnetic wormhole regions, in which the Poynting vector associated with the guided mode is negative with respect to the direction of propagation. For demonstration purposes this mechanism is elucidated in AlGaAs and silicon nanowaveguides. The effect of dispersion on the power circulation is also considered.

Shockwave based nonlinear optical manipulation in densely scattering opaque suspensions.

Optical manipulation of particulate-loaded, highly scattering (opaque) suspensions is considered impossible. Here we demonstrate theoretically and experimentally optical manipulation of the local properties of such opaque suspensions. We show that the optical forces exerted by multiply-scattered light give rise to dense shock fronts of particle concentration, propagating deep inside the opaque suspensions, where the optical field is completely diffuse. We exploit these waves to demonstrate a plethora of optofluidic manipulations, ranging from optical transport and concentration of large populations of nanoparticles, to light-induced writing of concentrated spots in the suspensions and light-induced phase-transition from suspension to gel in localized volumes inside the fluids.

Light-induced self-synchronizing flow patterns

In this paper, we present the observation of light-induced self- synchronizing flow patterns in a light-fluid system. A light beam induces local flow patterns in a fluid, which oscillate periodically or chaotically in time. The oscillations within different regions of the fluid interact with each other through heat- and surface-tension-induced fluid waves, and they become synchronized. We demonstrate optical control over the state of synchronization and over the temporal correlation between different parts of the flow field. Finally, we provide a model to elucidate these results and we suggest further ideas on light controlling flow and vice versa.

Nonlinear Photonics in AlGaAs Photonics Nanowires: Self Phase and Cross Phase Modulation

AlGaAs nanowires are ideal waveguides for the observation of nonlinear effects. The sub-micron geometry and tight field confinement, due to the high index contrast between core and cladding, yield very intense pulsed beams at low power levels. Observations of Kerr nonlinearities are thus possible at power levels of a few watts in just a few millimeters of propagation. By using pulses of 52 W of peak power we obtain a 0.9pi nonlinear phase shift from self-phase modulation. By way of cross-polarised co-propagating pulses we also obtain a cross-phase modulation to self-phase modulation ratio of 0.88 at 1557 nm. Nonlinear transmission arising from three photon absorption is also analysed. We find good agreement between our experimental results and those from our simulations based on a nonlinear Schrodinger model. These results may eventually lead to the development of nonlinear all-optical devices operating in sub-micrometer interaction lengths with but a few watts of power.