The use of a saturable absorber as a passive mode locker in a solid-state laser can introduce a tendency for Q-switched mode-locked operation. We have investigated the transition between the regimes of cw mode locking and Q-switched mode locking. Experimental data from Nd:YLF lasers in the picosecond domain and soliton mode-locked Nd:glass lasers in the femtosecond domain, both passively mode locked with semiconductor saturable absorber mirrors, were compared with predictions from an analytical model. The observed stability limits for the picosecond lasers agree well with a previously described model, while for soliton mode-locked femtosecond lasers we have developed an extended theory that takes into account nonlinear soliton-shaping effects and gain filtering.

Coherent manipulation of spin ensembles is a key issue in the development of spintronics. In particular, multivalued spin switching may lead to new schemes of logic gating and memories. This phenomenon has been studied with atom vapours 30 years ago, but is still awaited in the solid state. Here, we demonstrate spin multistability with microcavity polaritons in a trap. Owing to the spinor nature of these light-matter quasiparticles and to the anisotropy of their interactions, we can optically control the spin state of a single confined level by tuning the excitation power, frequency and polarization. First, we realize high-efficiency power-dependent polarization switching. Then, at constant excitation power, we evidence polarization hysteresis and determine the conditions for realizing multivalued spin switching. Finally, we demonstrate an unexpected regime, where our system behaves as a high-contrast spin trigger. These results open new pathways to the development of advanced spintronics devices and to the realization of multivalued logic circuits. Spin manipulation is the object of an intense research activity in a great variety of solid-state systems [1][2][3] . Owing to significant advances in tunability and miniaturization, semiconductor nanostructures have turned into ideal laboratories to address spintronics challenges 4 . In this respect, microcavity polaritons hold great potential 5,6 . Arising from the normal-mode coupling between cavity photons and quantum-well excitons, polaritons behave as bosons and possess unique coherence properties that have led to the demonstration of Bose-Einstein condensation and superfluidity [7][8][9] . A great advantage of polaritons is the one-to-one correspondence between the polariton spin and the polarization of the emitted light. This allowed the observations of the optical spin Hall effect 10 , or of half-quantum vortices 11 , which have shown that polaritons exhibit remarkable spin carrier properties. Finally, recent realizations of optical bistability 12,13 and electrical injection in polariton diodes 14 allow the implementation of low-power polaritronic devices working at room temperature 15,16 . Spin multistability refers to the possibility for a system to present three or more stable spin states for a given excitation condition. It requires precise control of coherence and interactions and is therefore difficult to realize. The only successful studies of multistability with a spinor system were carried out with atomic vapours 30 years ago 17,18 . Its demonstration in the solid state would clearly lead to new schemes of spin-based logic devices 19,20 . Microcavity polaritons were recently predicted to be promising candidates to explore spin multistability 21 . This phenomenon rapidly emerged as an innovative solution for the design of spin memory elements 22 , and for the realization of logic gates based on the selective transport of spin-polarized polaritons 23,24 . Such developments first require an experimental demonstration of spin multistability in a pattern...

Quantized vortices appear in quantum gases at the breakdown of superfluidity. In liquid helium and cold atomic gases, they have been indentified as the quantum counterpart of turbulence in classical fluids. In the solid state, composite light-matter bosons known as exciton polaritons have enabled studies of non-equilibrium quantum gases and superfluidity. However, there has been no experimental evidence of hydrodynamic nucleation of polariton vortices so far. Here we report the experimental study of a polariton fluid flowing past an obstacle and the observation of nucleation of quantized vortex pairs in the wake of the obstacle. We image the nucleation mechanism and track the motion of the vortices along the flow. The nucleation conditions are established in terms of local fluid density and velocity measured on the obstacle perimeter. The experimental results are successfully reproduced by numerical simulations based on the resolution of the Gross-Pitaevskii equation.H ydrodynamic instabilities in classical fluids were studied in the pioneering experiments of Bénard in the 1910's. Convective Bénard-Rayleigh flows and Bénard-Von Kár-mán streets are now well known examples in nonlinear and chaos sciences 1 . In conventional fluids, the flow around an obstacle is characterized by the dimensionless Reynolds number Re = vR/ν, with v and ν the fluid velocity and dynamical viscosity, respectively, and R the diameter of the obstacle. When increasing the Reynolds number, laminar flow, stationary vortices, Bénard-Von Kármán streets of moving vortices and fully turbulent regimes are successively observed in the wake of the obstacle 1 .In quantum fluids, such as liquid helium or atomic BoseEinstein condensates, quantum turbulence has long been predicted to appear at the breakdown of superfluidity 2-8 . In superfluid systems, the Reynolds number cannot be defined owing to the absence of viscosity. However, quantum turbulence, in the form of quantized vortices, appears simultaneously with dissipation and drag on the obstacle once a critical velocity is exceeded. This critical velocity is predicted to be lower than the Landau criterion for superfluidity far from the obstacle, because of a local increase of the fluid velocity in the vicinity of the impenetrable obstacle 2,4,5 .Experimental evidence has been given for the appearance of a drag force or heat above some critical velocity in superfluid helium 5 and atomic Bose-Einstein condensates 9,10 . In stirred atomic gases, vortex lattices appear above a critical stirring frequency 11-13 , analogously to the rotating bucket experiments originally performed with superfluid helium 14 . Irregular vortex tangle patterns were also observed under an external oscillating perturbation, indicating the presence of turbulence in the atomic cloud 15 . Finally, vortex nucleation has been reported in the wake of a blue-detuned laser moving above a critical velocity through the condensate 16,17 . However, no experiment has yet allowed the imaging of the hydrodynamic nucleation mechanism with ...

Pulses of sub-6-fs duration have been obtained from a Kerr-lens mode-locked Ti:sapphire laser at a repetition rate of 100 MHz and an average power of 300 mW. Fitting an ideal sech(2) to the autocorrelation data yields a 4.8-fs pulse duration, whereas reconstruction of the pulse amplitude profile gives 5.8 fs. The pulse spectrum covers wavelengths from above 950 nm to below 630 nm, extending into the yellow beyond the gain bandwidth of Ti:sapphire. This improvement in bandwidth has been made possible by three key ingredients: carefully designed spectral shaping of the output coupling, better suppression of the dispersion oscillation of the double-chirped mirrors, and a novel broadband semiconductor saturable-absorber mirror.

In this paper we introduce a new paradigm for nanowire growth that explains the unwanted appearance of parasitic nonvertical nanowires. With a crystal structure polarization analysis of the initial stages of GaAs nanowire growth on Si substrates, we demonstrate that secondary seeds form due to a three-dimensional twinning phenomenon. We derive the geometrical rules that underlie the multiple growth directions observed experimentally. These rules help optimizing nanowire array devices such as solar or water splitting cells or of more complex hierarchical branched nanowire devices.

Exciton polaritons have been shown to be an optimal system in order to investigate the properties of bosonic quantum fluids. We report here on the observation of dark solitons in the wake of engineered circular obstacles and their decay into streets of quantized vortices. Our experiments provide a timeresolved access to the polariton phase and density, which allows for a quantitative study of instabilities of freely evolving polaritons. The decay of solitons is quantified and identified as an effect of disorderinduced transverse perturbations in the dissipative polariton gas. DOI: 10.1103/PhysRevLett.107.245301 PACS numbers: 67.10.Jn, 03.75.Lm, 71.36.+c, 78.67.Àn Perturbations of quantum fluids can lead to the creation of solitary waves called solitons resulting from the compensation between dispersion and particle interaction. In the particular case of repulsive interaction, dark solitons are created. These density depressions move in the fluid while keeping a constant shape and they are characterized by a phase jump across the density minimum. Since the first theoretical prediction [1], dark solitons have been studied and then observed in a variety of systems such as nonlinear lattices [2], thin magnetic films [3], and complex plasma [4]. They have attracted considerable interest especially in the field of nonlinear optics [5], because of their consequent use in communication devices (i.e., optical fibers [6]), and in atomic Bose-Einstein condensation (BEC) [7]. As quantized vortices [8][9][10], dark solitons are BEC excitations, which arise spontaneously upon the phase transition. As such, they are clear evidences for the onset of a quantum behavior and powerful tools to understand BEC instabilities. Controlling these instabilities is of crucial importance for the development of optoelectronic devices based on quantum fluids in which stable regimes and structures are required. Dark solitons are considered as the dispersive and nonlinear analog of shock waves of supersonic motion [11]. The creation of solitons by phase imprinting in BEC has been reported [12,13], triggering a growing interest in their hydrodynamic formation and stability of solitons.In this Letter we report on the observation of hydrodynamic oblique dark solitons in a 2D polariton fluid and the formation of quantized vortex streets. Polaritons are bosonic quasiparticles arising from the strong coupling between quantum well excitons and photons in semiconductor microcavities. Because of their small effective mass and their strong nonlinearity, they have turned out to be an optimal system in order to investigate the properties of a bosonic quantum fluid. Polaritons can undergo BEC [14] and, by virtue of their nonequilibrium nature, they are accompanied with the spontaneous appearance of quantized vortices [15]. With the demonstration of polariton superfluidity [16,17] much effort has been performed to better understand the nature of different turbulences arising from the breakdown of this fascinating state of matter. Recently, the hydrodynam...

We report on the realization of polariton quantum boxes in a semiconductor microcavity under strong coupling regime. The quantum boxes consist of mesas, etched on the top of the spacer of a microcavity, that confine the cavity photon. For mesas with sizes of the order of a few microns in width and nanometers in depth, we observe quantization of the polariton modes in several states, caused by the lateral confinement. We evidence the strong exciton-photon coupling regime through a typical anticrossing curve for each quantized level. Moreover, the growth technique permits one to obtain high-quality samples, and opens the way for the conception of new optoelectronic devices. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2172409͔Confining semiconductor structures allows the study of various fundamental effects, ranging from the Purcell effect to the full quantum confinement. Such confinement is also used for applications in many fields, from optoelectronics to quantum information. Previous works have focused on different aspects: on the one hand, on the matter part, with the confinement of the excitonic resonances in quantum wells, quantum wires, and quantum dots. On the other hand, environment for the electromagnetic field has been modified by optical confinement in different types of cavities. Additionally since the middle of the 90s, low dimensional devices have been realized in the strong coupling regime. 1 Confinement can enhance the interactions, modify the real and imaginary parts of the resonance's energy, or open access to new interaction processes. It is also often considered as a possible way to obtain a condensed phase of bosons in semiconductors, 2 but so far the fermionic nature of excitons has always become dominant upon increasing density. In this sense, polaritons are of great interest as, despite their excitonic content, they have a very small effective mass in comparison to the exciton ͑thanks to their photonic component͒, which theoretically increases their temperature of condensation ͑above 0.1 K͒. 3 The peculiar trap shape of the lower microcavity polariton dispersion curve has motivated several relaxation experiments towards the bottom of this "trap" 4,5 but no clear evidence for the formation of spontaneous coherence formation has been given yet.Zero-dimensional ͑0D͒ Polariton confinement can be achieved either through their excitonic or through their photonic component. Recently, evidence for 0D polaritons has been given with single quantum dots in micropillars, 6 photonic nanocavities, 7 or microdisks, 8 and for a large number of excitations in micropillar structures. 9-11 Here we consider a novel system under strong coupling regime, where 0D confinement is achieved through the photonic part of polaritons in high Q cavities. Our original structure contains polariton quantum boxes, constituted by mesas in the spacer layer of a semiconductor microcavity, allowing to keep the strong coupling regime. Each mesa, by acting on the two degrees of freedom of the photonic component of the two...

We demonstrate three-dimensional spatial confinement of exciton-polaritons in a semiconductor microcavity. Polaritons are confined within a micron-sized region of slightly larger cavity thickness, called mesa, through lateral trapping of their photon component. This results in a shallow potential well that allows the simultaneous existence of extended states above the barrier. Photoluminescence spectra were measured as a function of either the emission angle or the position on the sample. Striking signatures of confined states of lower and upper polaritons, together with the corresponding extended states at higher energy, were found. In particular, the confined states appear only within the mesa region, and are characterized by a discrete energy spectrum and a broad angular pattern. A theoretical model of polariton states, based on a realistic description of the confined photon modes, supports our observations.