L. Spinelli

Cavity solitons as pixels in semiconductor microcavities

Cavity solitons are localized intensity peaks that can form in a homogeneous background of radiation. They are generated by shining laser pulses into optical cavities that contain a nonlinear medium driven by a coherent field (holding beam). The ability to switch cavity solitons on and off and to control their location and motion by applying laser pulses makes them interesting as potential ‘pixels’ for reconfigurable arrays or all-optical processing units. Theoretical work on cavity solitons has stimulated a variety of experiments in macroscopic cavities and in systems with optical feedback. But for practical devices, it is desirable to generate cavity solitons in semiconductor structures, which would allow fast response and miniaturization. The existence of cavity solitons in semiconductor microcavities has been predicted theoretically, and precursors of cavity solitons have been observed, but clear experimental realization has been hindered by boundary-dependence of the resulting optical patterns—cavity solitons should be self-confined. Here we demonstrate the generation of cavity solitons in vertical cavity semiconductor microresonators that are electrically pumped above transparency but slightly below lasing threshold. We show that the generated optical spots can be written, erased and manipulated as objects independent of each other and of the boundary. Numerical simulations allow for a clearer interpretation of experimental results.

Cavity solitons in passive bulk semiconductor microcavities. I. Microscopic model and modulational instabilities

We consider a broad-area vertical microresonator with an active layer constituted by bulk GaAs driven by an external coherent homogeneous electromagnetic field, and we adopt a microscopic model that describes the field and carrier dynamics in the quasi-equilibrium regime. The theory is developed within the free-carrier approximation, with some relevant effects, such as the Urbach tail and the bandgap renormalization, which are taken into account in a phenomenological way. We include in the model the description of paraxial diffraction and carrier diffusion. A detailed study of the instabilities, both modulational and plane wave, affecting the homogeneous stationary state of the output field is performed. In this way we address the numerical research of cavity solitons, which appear as self-organized light peaks embedded in a homogeneous background, as discussed in a companion paper [J. Opt. Soc. Am. B16, 2095 (1999)]. Optimal conditions for cavity solitons’ existence are found in extended regions of the parameter space.

Cavity solitons in passive bulk semiconductor microcavities. II. Dynamical properties and control

We analyze numerically the microscopic model formulated in a companion paper [J. Opt. Soc. Am. B16, 2083 (1999)] to describe an externally driven broad-area bulk GaAs microcavity. We numerically predict the formation of patterns in the transverse profile of the output field. In particular, we find the existence of stable cavity solitons, which appear as self-organized light peaks embedded in a homogeneous background. We study the characteristics of such structures and specifically the possibility of switching them on and off at desired locations in the transverse field profile. Moreover, we analyze the interaction properties of cavity solitons with the purpose of applying them, in the future, to optical information treatment. Finally, we study the dynamical properties of cavity solitons, quantitatively evaluating the motion across the transverse plane induced by spatial gradients in the input field profile.

Thermal and electronic nonlinearities in semiconductor cavities

The dead-space carrier multiplication theory properly predicts the reduction in the excess noise factor in a number of APDs. The theory is applied to measurements, obtained from J. C. Campbell and collaborators at the University of Texas, for InP, InAlAs, GaAs, and AlGaAs APDs with multiplication-region widths ranging from 80 nm to 1600 nm. A refined model for the ionization coefficients is reported that is independent of the width of the device multiplication region of each device. In addition, in comparison to predictions from the conventional multiplication theory, the dead-space multiplication theory predicts a reduction in the mean bandwidth as well as a reduction in the power spectral density of the impulse response. In particular, it is shown that the avalanching noise at high-frequencies is reduced as a result of the reduction of the multiplication region width.

Modulational instabilities and cavity solitons in semiconductor microcavities

We consider a model recently proposed, describing a multiple quantum well semiconductor microcavity driven by a coherent holding beam. We discuss here some general features of stationary modulational instabilities affecting the system. By generalizing a method introduced in a previous paper on two-level atoms in optical cavities, we give a detailed description of the stationary instabilities in the parameter space. The modulational instability can be related to the presence of stable cavity solitons, which have been predicted in this system. Therefore we have a helpful tool to research the best parametric conditions for stable cavity solitons, a very demanding task due to the large number of parameters. Finally, we show that our analysis can be extended to more general models, describing a nonlinear diffusive material inside an optical cavity with a general form of the interaction terms.

Cavity solitons in semiconductor microcavities

This work demonstrates unequivocally the existence of cavity solitons (CSs) in semiconductor optical cavity. This study shows that CSs can be written, erased and manipulated as objects independent of each other and of the boundary.

Controlling cavity solitons in semiconductor microcavities for optical information treatment

We study the formation of self organized light peaks, in GaAs microcavities. By means of analytical and numerical techniques, experimentally accessible parametric domains can be found, where stable and robust CS can be addressed, shifted and brought to interaction ranges, thus realizing some basic schemes for optical information treatment. A Fourier-Newton approach is applied to gain quantitative information on CSs dynamical response to external control fields or on CS pair interaction.

Thermally-induced and guided motion of semiconductor cavity solitons

We describe here the spatio-temporal dynamics of a semiconductor microresonator driven by an external coherent field, including the diffusive dynamics of the lattice thermal field. This allows us to describe the thermal effects with particular regard to pattern formation and cavity solitons (CS). We consider both the case of a bulk medium in a passive configuration, and the case of a multiple quantum well (MQW) medium in an active configuration (that is, the device is electrically pumped, behaving as an amplifier). Thermal effects are taken into account through a temperature dependence of the band-gap energy of the semiconductor, a linear shift of the cavity resonance, and a rate equation for the temperature field dominated by lattice-carrier heat exchange. We formulate a set of dynamical equations which are characterised by three very different time scales. The system can eventually be affected by a thermally-induced dynamical instability of the Hopf type, giving rise to different dynamical regimes. In the passive case, it turns out that the Hopf instability, when present, has often a global character: it leads the system to oscillations of the whole beam, called regenerative oscillations, where the system displays an oscillatory homogeneous output intensity, for a constant homogeneous input intensity. There are also conditions in which the system is dominated by a stationary modulational instability (Turing); in this regime pattern formation seems to be unaffected by the presence of thermal effects, and stable CS can be obtained in wide parametric ranges. In other regimes, both in the active and in the passive case, we found that the Hopf instability is a dynamical modulational instability, and travelling spatial patterns develop. Under the same conditions, bright CS are possible in the active case, but after their excitation, they start travelling on the thermal time-scale (microseconds), but they can be guided or trapped using gradients in the holding beam.

First principle theory for cavity solitons in semiconductor microresonators

Summary form only given. Cavity solitons are similar to spatial solitons but arise in dissipative systems, which bestows them special properties. They are generated by shining short and narrow laser pulses into resonant cavities filled with nonlinear samples of large section, and driven by a cw coherent holding beam. The cavity soliton, which appears as a bright spot in the transverse intensity profile, persists after the passage of the pulse, until it is switched off by another pulse. This talk will present the recent progress in the theoretical numerical studies of cavity solitons in semiconductor microresonators, following the development of more refined models to adequately describe the complex physics of broad-area semiconductor microresonators. In particular, a microscopic model is discussed to describe the nonlinear response (via the complex susceptibility) of a multiple quantum well sample.

Theoretical aspects and potential applications of cavity solitons in semiconductor microresonators

Cavity solitons (CS) appear as self-confined light peaks embedded in the transverse profile of a homogeneous coherent field propagating in a nonlinear cavity. They have recently been predicted for GaAs semiconductor micro cavities for which we have developed a microscopic model that describes the field and the carrier dynamics inside the active region. Here we improve our previous model by adding the temperature dynamics. A detailed study of the instabilities affecting the homogeneous stationary state of the output field is performed. In this way we can address the numerical research of patterns and CS. We then show how it is possible to study intrinsic stability properties of CS by means of semi- analytical techniques that allow to describe the destabilizing mechanisms for solitons, mutual interaction properties and their response to perturbations; possible conceptual schemes for optical information treatment and logic gates are investigated.