Franco Prati

Cavity solitons in a driven VCSEL above threshold

We experimentally demonstrate the existence and the control of cavity solitons in externally driven vertical-cavity semiconductor lasers above threshold. A model including material polarization dynamics is used to predict and confirm the experimental findings.

Dissipative Phase Solitons in Semiconductor Lasers.

We experimentally demonstrate the existence of nondispersive solitary waves associated with a 2π phase rotation in a strongly multimode ring semiconductor laser with coherent forcing. Similarly to Bloch domain walls, such structures host a chiral charge. The numerical simulations based on a set of effective Maxwell-Bloch equations support the experimental evidence that only one sign of chiral charge is stable, which strongly affects the motion of the phase solitons. Furthermore, the reduction of the model to a modified Ginzburg-Landau equation with forcing demonstrates the generality of these phenomena and exposes the impact of the lack of parity symmetry in propagative optical systems.

Terahertz optically tunable dielectric metamaterials without microfabrication

We theoretically investigate the terahertz (THz) dielectric response of a semiconductor slab hosting a tunable grating photogenerated by the interference of two tilted infrared (IR) plane waves. In the case where the grating period is much smaller than the THz wavelength, we numerically evaluate the ordinary and extraordinary component of the effective permittivity tensor by resorting to electromagnetic full-wave simulation coupled to the dynamics of charge carriers excited by IR radiation. We show that the photo-induced metamaterial optical response can be tailored by varying the grating and it ranges from birefringent to hyperbolic to anisotropic negative dielectric without resorting to microfabrication.

Cavity Solitons in Semiconductor Devices

Cavity solitons represent a class of dissipative solitons which are generated inside an optical resonator. They have attracted considerable interest in the recent years due to their possible application to optical information processing. First of all, this review chapter illustrates the physics of cavity solitons in semiconductor devices. We discuss the experiments which demonstrated cavity solitons in vertical-cavity surface-emitting lasers, both below and above threshold, and all the theory which accompanied such experiments. Those features of the experimental results, which relate to prospective applications, are highlighted. The final part of the chapter deals with the theory of the cavity soliton laser.

Spatiotemporal pattern formation and chaos in passive optical systems

Abstract The interaction and competition of transverse cavity modes in passive optical systems can lead to the formation of spatial field structures whose symmetry differs from that of the empty cavity, while the time behaviour can become irregular. After having introduced dynamical equations in which the spatial dependence of the electric field is described by frequency-degenerate Gauss-Laguerre (GL) modes, the stationary solutions corresponding to a single or many modes are calculated and their stability is discussed. A scan of the parameter space allows for the identification of several interesting dynamical regimes. An extensive study of the chaotic states is performed in order to understand the relation between GL mode excitation and the relevant active degrees of freedom. Based on this analysis, the conditions for the onset of spatiotemporal chaos, for its characterisation and experimental observation are finally discussed.

Spatiotemporal extreme events in a laser with a saturable absorber

Extreme events in optics have been recently attracting a great attention due to the well-known analogy between optics and hydrodynamics, where rogue wave formation and prediction is a priority field of investigations. In this contribution, we show numerical results about extreme events occurring in the transverse section of the field emitted by a broad-area semiconductor laser with saturable absorber. We believe that our system, being intrinsically two-dimensional, may give some precious insights on the focusing mechanisms giving rise to rogue wave formation in oceans, mechanisms that could be absent in one-dimensional systems.

Reconfigurable photoinduced metamaterials in the microwave regime

We investigate optically reconfigurable dielectric metamaterials at gigahertz frequencies. More precisely, we study the microwave response of a subwavelength grating optically imprinted into a semiconductor slab. In the homogenized regime, we analytically evaluate the ordinary and extraordinary component of the effective permittivity tensor by taking into account the photo-carrier dynamics described by the ambipolar diffusion equation. We analyze the impact of semiconductor parameters on the gigahertz metamaterial response which turns out to be highly reconfigurable by varying the photogenerated grating and which can show a marked anisotropic behavior.

Refractory period of an excitable semiconductor laser with optical injection

Injection-locked semiconductor lasers can be brought to a neuronlike excitable regime when parameters are set close to the unlocking transition. Here we study experimentally the response of this system to repeated optical perturbations and observe the existence of a refractory period during which perturbations are not able to elicit an excitable response. The results are analyzed via simulations of a set of dynamical equations which reproduced adequately the experimental results.

Controlling cavity solitons by means of photorefractive soliton electro-activation

We consider a hybrid system consisting of a centrosymmetric photorefractive crystal in contact with a vertical-cavity surface-emitting laser. We numerically investigate the generation and control of cavity solitons (CSs) by propagating a plane wave through electro-activated solitonic waveguides in the crystal. In such a compound scheme, which couples a propagative/conservative field dynamics to a bistable/dissipative one, we show that by changing the electro-activation voltage of the crystal, the CSs can be turned on and shifted with controlled velocity across the device section, on the scale of tens of nanoseconds. The configuration can be exploited for applications to optical information encoding and processing.

Extrinsic electromagnetic chirality in all-photodesigned one-dimensional terahertz metamaterials

We suggest that all-photodesigned metamaterials, sub-wavelength custom patterns of photo-excited carriers on a semiconductor, can display an exotic extrinsic electromagnetic chirality in terahertz (THz) frequency range. We consider a photo-induced pattern exhibiting 1D geometrical chirality, i.e. its mirror image can not be superposed onto itself by translations without rotations and, in the long wavelength limit, we evaluate its bianisotropic response. The photo-induced extrinsic chirality turns out to be fully reconfigurable by recasting the optical illumination which supports the photo-excited carriers. The all-photodesigning technique represents a feasible, easy and powerful method for achieving effective matter functionalization and, combined with the chiral asymmetry, it could be the platform for a new generation of reconfigurable devices for THz wave polarization manipulation.