Salvador Balle

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.

Alternate oscillations in semiconductor ring lasers

We report on fabrication and characterization of single-longitudinal- and transverse-mode semiconductor ring lasers. A bifurcation from bidirectional stable operation to a regime with alternate oscillations of the counterpropagating modes was observed experimentally and is theoretically explained through a two-mode model. Analytical expressions for the onset and the frequency of the oscillations are derived, and L-I curves numerically evaluated. Good quantitative agreement between theory and measurements made over a large number of tested devices is obtained.

Mechanisms of polarization switching in single-transverse-mode vertical-cavity surface-emitting lasers: thermal shift and nonlinear semiconductor dynamics.

We analyze polarization switching in vertical-cavity surface-emitting lasers, taking into account a proper semiconductor frequency-dependent complex susceptibility and spin-flip processes. Thermal effects are included as a varying detuning, and gain differences arise from birefringence splitting. We find that, for large birefringence, gain differences between the two linearly polarized modes are preponderant, and switching occurs owing to thermal shift. For small birefringence polarization switching from the high- to the low-gain mode occurs owing to the combined effect of birefringence and semiconductor phase-amplitude-coupled dynamics for a finite value of the carrier spin-flip rate.

Emission Directionality of Semiconductor Ring Lasers: A Traveling-Wave Description

We use a traveling-wave model for explaining the experimentally observed changes in the directionality of the emission of semiconductor ring lasers and its different behavior when current is increased or decreased. The modulation of the cavity losses imposed by the light extraction sections together with the thermal shift of the gain spectrum and spatial hole burning in the carrier density play a crucial role in the directionality of the emission and its changes with operation current. The differences as the current is increased or decreased correspond to the different role played by spatial hole burning.

Mode-Locking in Semiconductor Fabry-Pérot Lasers

We theoretically study the dynamics and the mode-locking properties of semiconductor Fabry-Perot lasers with intracavity saturable absorber by using a travelling-wave model and a time-domain description of the optical response of the semiconductor materials. Our description enables us to incorporate important features as for instance the abrupt spectral variations of the absorption in the saturable absorber. We analyze the influence of several key parameters that affect the stability of the mode-locking regime and show that this modelling approach can be used, upon proper fitting of the material parameters, for optimization of the design of semiconductor mode-locked lasers.

Polarization and transverse-mode dynamics of gain-guided vertical-cavity surface-emitting lasers

Transverse-mode competition and polarization selection in gain-guided vertical-cavity surface-emitting lasers are studied by use of a transverse continuous model that incorporates basic physical mechanisms of polarization dynamics. Polarization stability and polarization switching within the fundamental Gaussian mode are described. The first-order transverse mode always starts lasing orthogonally polarized to the fundamental one. At larger currents polarization coexists with several active transverse modes. These results are shown to be sensitive to the carrier spin-flip relaxation rate.

Low frequency fluctuations and multimode operation of a semiconductor laser with optical feedback

Abstract We experimentally investigate low frequency fluctuations (LFF) in a Fabry-Perot semiconductor laser with optical feedback from an external mirror. During LFF, the time resolved optical spectrum shows that many longitudinal modes of the solitary laser enter into the transients. After each LFF event, the excited solitary-laser modes recover similarly. However, the recovery for the power in each mode is much slower than the recovery of the total power. The intermode exchange of energy during the recovery indicates that a single-longitudinal mode description of such LFF behavior will not capture important underlying dynamics. The relevance of multimode dynamics is confirmed in a feedback experiment where the external mirror is substituted by a diffraction grating.

Excitability and optical pulse generation in semiconductor lasers driven by resonant tunneling diode photo-detectors

We demonstrate, experimentally and theoretically, excitable nanosecond optical pulses in optoelectronic integrated circuits operating at telecommunication wavelengths (1550 nm) comprising a nanoscale double barrier quantum well resonant tunneling diode (RTD) photo-detector driving a laser diode (LD). When perturbed either electrically or optically by an input signal above a certain threshold, the optoelectronic circuit generates short electrical and optical excitable pulses mimicking the spiking behavior of biological neurons. Interestingly, the asymmetric nonlinear characteristic of the RTD-LD allows for two different regimes where one obtain either single pulses or a burst of multiple pulses. The high-speed excitable response capabilities are promising for neurally inspired information applications in photonics.

Dynamical mechanism of anticipating synchronization in excitable systems

We analyze the phenomenon of anticipating synchronization of two excitable systems with unidirectional delayed coupling which are subject to the same external forcing. We demonstrate for different paradigms of excitable system that, due to the coupling, the excitability threshold for the slave system is always lower than that for the master. As a consequence the two systems respond to a common external forcing with different response times. This allows us to explain in a simple way the mechanism behind the phenomenon of anticipating synchronization in excitable systems.

Polarization and transverse-mode selection in quantum-well vertical-cavity surface-emitting lasers: index- and gain-guided devices

We study polarization switching and transverse-mode competition in vertical-cavity surface-emitting lasers (VCSELs) in the absence of temperature effects. We use a model that incorporates the vector nature of the laser field, saturable dispersion, different carrier populations associated with different magnetic sublevels of the conduction and heavy hole valence bands in quantum-well media, spin-flip relaxation processes and cavity birefringence and dichroism. We consider both index- and gain-guided VCSELs and we find that spin-flip dynamics and the linewidth enhancement factor are crucial for the selection of the polarization state corresponding to a given injection current. For index-guided VCSELs the effect of spatial hole burning on the polarization behaviour within the fundamental mode regime is discussed. For gain-guided VCSELs, transverse-mode and polarization selection are studied within a Maxwell - Bloch approximation which includes field diffraction and carrier diffusion. Polarization switching is found in the fundamental mode regime. The first-order transverse mode starts lasing orthogonally polarized to the fundamental mode. At larger currents polarization coexistence with several active transverse modes occurs.