High-speed asynchronous optical sampling ͑ASOPS͒ is a novel technique for ultrafast time-domain spectroscopy ͑TDS͒. It employs two mode-locked femtosecond oscillators operating at a fixed repetition frequency difference as sources of pump and probe pulses. We present a system where the 1 GHz pulse repetition frequencies of two Ti:sapphire oscillators are linked at an offset of ⌬f R = 10 kHz. As a result, their relative time delay is repetitively ramped from zero to 1 ns within a scan time of 100 s. Mechanical delay scanners common to conventional TDS systems are eliminated, thus systematic errors due to beam pointing instabilities and spot size variations are avoided when long time delays are scanned. Owing to the multikilohertz scan-rate, high-speed ASOPS permits data acquisition speeds impossible with conventional schemes. Within only 1 s of data acquisition time, a signal resolution of 6 ϫ 10 −7 is achieved for optical pump-probe spectroscopy over a time-delay window of 1 ns. When applied to terahertz TDS, the same acquisition time yields high-resolution terahertz spectra with 37 dB signal-to-noise ratio under nitrogen purging of the spectrometer. Spectra with 57 dB are obtained within 2 min. A new approach to perform the offset lock between the two femtosecond oscillators in a master-slave configuration using a frequency shifter at the third harmonic of the pulse repetition frequency is employed. This approach permits an unprecedented time-delay resolution of better than 160 fs. High-speed ASOPS provides the functionality of an all-optical oscilloscope with a bandwidth in excess of 3000 GHz and with 1 GHz frequency resolution.

Non-linear interactions in coherent gases are not only at the origin of bright and dark solitons and superfluids; they also give rise to phenomena such as multistability, which hold great promise for the development of advanced photonic and spintronic devices. In particular, spinor multistability in strongly coupled semiconductor microcavities shows that the spin of hundreds of exciton-polaritons can be coherently controlled, opening the route to spin-optronic devices such as ultrafast spin memories, gates or even neuronal communication schemes. Here we demonstrate that switching between the stable spin states of a driven polariton gas can be controlled by ultrafast optical pulses. Although such a long-lived spin memory necessarily relies on strong and anisotropic spinor interactions within the coherent polariton gas, we also highlight the crucial role of non-linear losses and formation of a non-radiative particle reservoir for ultrafast spin switching.

A thermal phase transition has been resolved in gold nanoparticles supported on a surface. By use of asynchronous optical sampling with coupled femtosecond oscillators, the Lamb vibrational modes could be resolved as a function of annealing temperature. At a temperature of 104°C the damping rate and phase changes abruptly, indicating a structural transition in the particle, which is explained as the onset of surface melting.Metallic nanoparticles are in the focus of fundamental and applied research. They show an optical response in the visible region that depends strongly on size, shape, and local dielectric environment, 1 making them appealing for nanoscale sensor applications. The interaction with light shows interesting peculiarities, such as ultrafast relaxation dynamics, coupling to nonradiative processes, or near field enhancement. 2 In particular nanoscale gold particles have received strong attention due to their versatile synthesis protocols, 3 shape transformations, or binding to biological molecules. 4 They served also to understand the generic structural properties that scale with size or shape. The melting point depression with decreasing particle size represents one of the prominent features, arising from the change in ratio between surface and bulk atoms and the reduction in binding energy. Since the seminal publication of Buffat and Borel 5 several improvements of the description have been made, while the general relation of the melting point depression ∆T mp ∝ 1/D as a function of size D remains valid. One of these points is the existence of surface melting, i.e., a coexistence of a liquid-like layer of atoms at the surface with the solid core material at temperatures below the respective melting point. 6,7 While this effect seems to be very basic, there is still a lack of unambiguous experimental proof regarding the details, in particular for gold nanoparticles.Inasawa et al. 8 have tried to investigate the shape transformation of elongated particles on a surface induced by static heating (ex situ) and concluded that a transformation into spheres takes places at temperatures as low as 400°C. A similar, very recent study has shown that nanorods with a large aspect ratio of the axes can be transformed at 200°C into spheres and partial relaxation occurs much earlier (100°C). 9 Surface melting is taken as the origin for these relaxations.It should also be borne in mind that the relaxation of rods could also be caused by other forces, as has been shown by laser-induced melting, 10 where the onset correlates with dislocations in the interior, as the rods are subjected to a large uniaxial strain due to surface tension.Hartland et al. have also reported on several experiments, which aimed at the structural transformations on a very short time scale, initiated by femtosecond excitation. 11,12 The vibrational response of the particles has been recorded by optical pump-probe techniques, reflecting the mechanical properties of the particles. While the change of the elastic constants of the spheres ...

We experimentally demonstrate a technique for the generation of optical beams carrying orbital angular momentum using a planar semiconductor microcavity. Despite being isotropic systems with no structural gyrotropy, semiconductor microcavities, because of the transverse-electric-transverse-magnetic polarization splitting that they feature, allow for the conversion of the circular polarization of an incoming laser beam into the orbital angular momentum of the transmitted light field. The process implies the formation of topological entities, a pair of optical vortices, in the intracavity field.

In this work, we study the influence of the excitation conditions on power-dependent spin switching and spin multistability of exciton polaritons in planar semiconductor microcavities. We obtain experimental evidence for the influence of a reservoir of nonradiative states which make a determining contribution to the dynamics of polaritons. While the spinor Gross-Pitaevskii equation (SGPE) fails in reproducing some critical experimental trends, an extended set of equations including a nonradiative reservoir allows us to reproduce the experiments quantitatively. We find that the energy renormalization of the exciton field due to the reservoir is crucial to describe power-dependent spin switching. The reservoir is also responsible for the effective repulsive interactions between polaritons of opposite spin obtained in the framework of the SGPE. Two important parameters, the coupling of the spinor polariton fields to the reservoir and the decay of the reservoir, are determined experimentally. We present indications that the reservoir originates from the formation of biexcitons and is constituted of localized exciton states.

Control of the wave function of confined microcavity polaritons is demonstrated experimentally and theoretically by means of tailored resonant optical excitation. Three dimensional confinement is achieved by etching mesas on top of the microcavity spacer layer. Resonant excitation with a continuous-wave laser locks the phase of the discrete polariton states to the phase of the laser. By tuning the energy and momentum of the laser, we achieve precise control of the momentum pattern of the polariton wave function. This is an efficient and direct way for quantum control of electronic excitations in a solid.Polaritons in semiconductor microcavities are hybrid quasiparticles consisting of a superposition of photons and excitons. Due to their photon component, polaritons are characterized by a quantum coherence length in the several micron range. Owing to their exciton content, they display sizeable interactions, both mutual and with other electronic degrees of freedom. These unique features have produced striking matter wave phenomena, such as Bose-Einstein condensation, 1,2 or parametric processes 3,4 able to generate entangled polariton states. 5,6 The key feature of polaritons with respect to confinement is their very small effective mass, which is about 10 5 times smaller than the free-electron mass. Hence, confinement within micrometer sized traps is sufficient to produce an atomlike spectrum with discrete energy levels. Recently, several paradigms for spatial confinement of polaritons in semiconductor devices have been established. 7-10 This opens the way to quantum devices in which polaritons can be used as a vector of quantum information. 11 Their electronic component can be accessed and controlled optically, through their photonic component. This holds promise for preparation of quantum states, which might then be transferred to longer lived elements of quantum storage ͑e.g., localized spins͒, or as a mechanism for mediating interactions between such elements over long distances, as proposed in Ref. 12. Precise control of the polariton wave function is then an essential requirement.In this Rapid Communication we demonstrate the manipulation of the wave function of confined zero-dimensional ͑0D͒ exciton polaritons under resonant optical excitation. The excited wave functions are monitored by collecting the coherent emission from the polariton traps. Control of the spatial and momentum probability distribution of the confined polaritons is achieved by tuning either the incidence angle or the energy of the excitation beam. Our results are supported by numerical simulations based on the coupled Gross-Pitaevskii equations for excitons and photons.The sample we used to carry out our studies consists of a single quantum well embedded in a planar microcavity with a pattern of differently sized round mesas on the spacer layer. 13 Patterning the cavity thickness results in a modulation of the photon resonance energy, which corresponds to a potential trap, with finite-energy barriers, able to confine polaritons. The con...

We experimentally investigate the relaxation of spatially confined microcavity polaritons. We measure the time-and energy-resolved photoluminescence under resonant excitation and in the low-density regime. In this way, we have access to the time evolution of the energy distribution of the polariton population. We show that, when one confined level is resonantly excited, after an initial transient, the population of the confined levels is thermally distributed. The reported efficiency of the relaxation process strongly depends on the confinement size. These experimental findings are well reproduced by a theoretical model accounting for the coupling between the confined states and a bath of acoustic phonons. Our results thus suggest that the phonon-mediated relaxation mechanisms are enhanced in the presence of spatial confinement.

We present the optical tomography of the probability density of quasiparticles, the microcavity polaritons, confined in three dimensions by cylindrical traps. Collecting the photoluminescence emitted by the quasimodes under continuous nonresonant laser excitation, we reconstruct a three-dimensional mapping of the photoluminescence, from which we can extract the spatial distribution of the confined states at any energy. We discuss the impact of the confinement geometry on the wave function patterns and give an intuitive understanding in terms of a light-matter quasiparticle confined in a box. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3126022͔Probing wave functions or probability densities ͑PDs͒ of confined carriers in semiconductor nanostructures is a very elegant way of retrieving textbook solutions of quantum confinement. It could also provide key information on the coupling between different nanostructures. To confine electrons and holes in semiconductor materials, traps in the nanometer range must be engineered due to the small de Broglie wavelength of the carriers. This reduced size generally prevents the optical imaging of the PDs of charge carriers. Sophisticated techniques such as insertion of probe layers, 1,2 magnetotransport measurements, 3 or magnetotunneling 4 allowed however to reconstruct the spatial variation of confined carrier PDs in the growth ͑vertical͒ direction of a quantum well ͑QW͒. Nevertheless, such techniques are dedicated to the study of PDs along a single confinement axis. They cannot be applied in the case of two-dimensional ͑2D͒ or threedimensional ͑3D͒ confining potentials. The study of the inplane ͑lateral͒ spatial extension of electronic wave functions is generally restricted to metallic surfaces and films, using scanning tunneling microscopy 5,6 or synchrotron radiation. 7The only measurement of fully confined carriers PDs has been achieved using near-field scanning optical microscopy and does not provide a significant resolution to access the spatial variation of the PDs inside the traps. In our work, we take advantage of the coupling between carriers and light in semiconductor microcavities. In the strong coupling regime, the interaction between excitons ͑Coulomb-correlated electron-hole pairs͒ and photons gives rise to the formation of quasiparticles called excitonpolaritons, split in two branches, the upper and lower polaritons.9 Their dispersion, being dominated by the photonic component, gives them an effective mass 10 4 smaller than the free electron mass. Consequently, they can be confined in micrometer-scale traps, above the resolution of optical microscopes. Moreover, as intracavity polaritons are directly coupled to extracavity photons, with energy and momentum conservation, 10 polaritonic states can be directly imaged through optical detection of the photoluminescence ͑PL͒ at the surface of the sample. We previously engineered a GaAs/AlAs microcavity featuring traps for the photonic modes and a single embedded InGaAs quantum well. The traps for the photon...