Sotiris Droulias

X - waves in nonlinear normally dispersive waveguide arrays

We theoretically demonstrate that optical discrete X-waves are possible in normally dispersive nonlinear waveguide arrays. We show that such X-waves can be effectively excited for a wide range of initial conditions and in certain occasions can be generated in cascade. The possibility of observing this family of waves in AlGaAs array systems is investigated in terms of pertinent examples.

Waveguide array-grating compressors

We show that efficient optical pulse compression can be achieved when normally dispersive nonlinear waveguide arrays are used in conjunction with dispersive elements such as gratings or other programmable phase filters. Our computations indicate that the compression resulting from such discrete arrays can be of better quality as compared to that obtained traditionally from a single nonlinear waveguide element. The performance of the array-grating compression method is assessed in normally dispersive highly nonlinear AlGaAs array systems.

Propagation of chirped solitary pulses in optical transmission lines: perturbed variational approach

Abstract The evolution of a dressed solitary pulse subjected to filtered amplification is examined. The model equation used is complex cubic Ginzburg–Landau equation (CCGLE). A system of ordinary differential equations is derived on the basis of an extended-perturbed variational method. These equations are solved numerically for a set of initial conditions in the vicinity of the fixed point (corresponding to the exact solution of CCGLE) of the dissipative system these equations model. The stability and degree of stationarity (in propagation distance) of pulses with initial (launching) parameters falling in the vicinity of the fixed point are examined in the context of this method. A fully numerical simulation of the CCGLE finally tests the results of this investigation. Detailed comparisons reveal a wide class of initial pulse profiles, which are characterized by adequate stationarity and long propagation, distances before they disintegrate. In the anomalous dispersion regime there is an adequate quantitative agreement while in the normal dispersion regime the predictability of the method is impressive. Limitations of the proposed method are also discussed.

Mechanism of the metallic metamaterials coupled to the gain material

We present evidence of strong coupling between the gain material and the metallic metamaterials. It is of vital importance to understand the mechanism of the coupling of metamaterials with the gain medium. Using a four-level gain system, the numerical pump-probe experiments are performed in several configurations (split-ring resonators (SRRs), inverse SRRs and fishnets) of metamaterials, demonstrating reduction of the resonator damping in all cases and hence the possibility for loss compensation. We find that the differential transmittance ΔT/T can be negative in different SRR configurations, such as SRRs on the top of the gain substrate, gain in the SRR gap and gain covering the SRR structure, while in the fishnet metamaterial with gain ΔT/T is positive.

Dissipative soliton acceleration in nonlinear optical lattices.

An effective mechanism for dissipative soliton acceleration in nonlinear optical lattices under the presence of linear gain and nonlinear loss is presented. The key idea for soliton acceleration consists of the dynamical reduction of the amplitude of the effective potential experienced by the soliton so that its kinetic energy eventually increases. This is possible through the dependence of the effective potential amplitude on the soliton mass, which can be varied due to the presence of gain and loss mechanisms. In contrast to the case where either the linear or the nonlinear refractive index is spatially modulated, we show that when both indices are modulated with the same period we can have soliton acceleration and mass increasing as well as stable soliton propagation with constant non-oscillating velocity. The acceleration mechanism is shown to be very robust for a wide range of configurations.

LOCALIZED MODES IN A CIRCULAR ARRAY OF COUPLED NONLINEAR OPTICAL WAVEGUIDES

A circular array of optical waveguides collectively coupled with a central core is investigated. Nonlinear losses, both linear and nonlinear coupling as well as energy transfer between neighboring array elements and between the array and the core are allowed. The concept is ideal for the design of high power stable amplifiers as well as of all-optical data processing devices in optical communications. The existence of stable steady-state continuous wave modes as well as of localized solitary and breathing type modes is demonstrated. These properties render the proposed system functionally rich, far more controllable than a planar one and easier to stabilize.

Lasing threshold control in two-dimensional photonic crystals with gain

We demonstrate how the lasing threshold of a two dimensional photonic crystal containing a four-level gain medium is modified, as a result of the interplay between the group velocity and the modal reflectivity at the interface between the cavity and the exterior. Depending on their relative strength and the optical density of states, we show how the lasing threshold may be dramatically altered inside a band or, most importantly, close to the band edge. The idea is realized via self-consistent calculations based on a finite-difference time-domain method. The simulations are in good agreement with theoretical predictions.

Novel Lasers Based on Resonant Dark States

The route to miniaturization of laser systems has so far led to the utilization of diverse materials and techniques for reaching the desired laser oscillation at small scales. Unfortunately, at some point all approaches encounter a trade-off between the system dimensions and the Q factor, especially when going subwavelength, mostly because the radiation damping is inherent to the oscillating mode and can thus not be controlled separately. Here, we propose a metamaterial laser system that overcomes this trade-off and offers radiation damping tunability, along with many other features, such as directionality, subwavelength integration, and simple layer-by-layer fabrication.

Finite-Size Effects in Metasurface Lasers Based on Resonant Dark States

The quest for subwavelength coherent light sources has recently led to the exploration of dark-mode based surface lasers, which allow for independent adjustment of the lasing state and its coherent radiation output. To understand how this unique design performs in real experiments, we need to consider systems of finite size and quantify finite-size effects not present in the infinite dark-mode surface laser model. Here we find that, depending on the size of the system, distinct and even counterintuitive behavior of the lasing state is possible, determined by a balanced competition between multiple loss channels, including dissipation, intentional out-coupling of coherent radiation, and leakage from the edges of the finite system. The conclusions are crucial for the design of future experiments that will enable the realization of ultrathin coherent light sources.

Gain-controlled Soliton Routing in Dissipative Optical Lattices

We investigate dynamical soliton trapping in optical lattices under the presence of gain and loss mechanisms. It is shown that depending on soliton initial power and velocity dynamical gain-controlled routing can take place.