A polariton condensate transistor switch is realized through optical excitation of a microcavity ridge with two beams. The ballistically ejected polaritons from a condensate formed at the source are gated using the 20 times weaker second beam to switch on and off the flux of polaritons. In the absence of the gate beam the small builtin detuning creates potential landscape in which ejected polaritons are channelled toward the end of the ridge where they condense. The low loss photon-like propagation combined with strong nonlinearities associated with their excitonic component makes polariton based transistors particularly attractive for the implementation of all-optical integrated circuits. 71.36.+c Contemporary electronics face ever increasing obstacles in achieving higher speeds of operation. Down-scaling which has served Moore's law for decades is approaching the inherent limits of semiconductor materials [1][2][3][4]. Even though a number of novel approaches [5][6][7] have managed to improve operating frequency and power consumption, it is commonly acknowledged that in the future, charged carriers will have to be replaced by information carriers that do not suffer from scattering, capacitance and resistivity effects. Although photonic circuits have been proposed, a viable optical analogue to an electronic transistor has yet to be identified as switching and operating powers of these devices are typically high [8].Polaritons which are hybrid states of light and electronic excitations offer an attractive solution as they are a natural bridge between these two systems. Their excitonic component allows them to interact strongly giving rise to the nonlinear functionality enjoyed by electrons. On the other hand, their photonic component restricts their dephasing allowing them to carry information with minimal data loss. Notably from the view of solid state physics polaritons are bosonic particles with a particularly light effective mass. These properties allow for the condensation of polaritons into a massively occupied single low-energy state, which shows many similarities to atomic Bose Einstein condensates [9][10][11][12]. The macroscopic quantum properties of polariton condensates combined with their photonic nature make them ideal candidates for use in quantum information devices and all optical circuits [13][14][15][16][17]. Several recent works address the possibility of optical manipulation of polariton condensate flow however these stop short of demonstrating actual gating of polariton condensate flow a prerequisite for implementation of integrated optical circuits [17][18][19][20].In this paper, a high finesse microcavity sample fabricated into a ridge is utilized to develop an exciton-polariton condensate transistor switch. A polariton condensate formed by optical excitation serves as a source of polaritons which are ballis-tically ejected along the channel as shown in Figure 1(a). Polariton propagation can be controlled using a second weaker beam that gates the polariton flux by modifying the energy ...

We perform an all-optical spin-dynamic measurement of the Rashba spin-orbit interaction in ͑110͒-oriented GaAs/ AlGaAs quantum wells under applied electric field. This crystallographic orientation allows us to isolate the Rashba from other contributions, giving precise values of the Rashba coefficient. At low temperature, we find good agreement between our measurements and the k · p theory. Unexpectedly, we observe a temperature dependence of the Rashba coefficient that may signify the importance of higher-order terms of the Rashba coupling.

In a polariton, laser coherent monochromatic light is produced by a low-energy state of the system at the bottom of a polariton 'trap', where a condensate of polaritons is formed, requiring no conventional population inversion. Following the recent realization of polariton light-emitting diodes (LEDs) based on GaAs microcavities (MCs) operating up to room temperature, efforts have been directed towards the demonstration of an electrically injected polariton laser. However, until now, low-threshold polariton lasing in GaAs MCs under optical pumping has been reported only at low temperatures. Here, we investigate the temperature dependence of lasing threshold across the border of the strong-to-weak coupling regime transition in high-finesse GaAs MCs under non-resonant optical pumping. Remarkably, we find that although lasing in the strong coupling regime is lost when the temperature is raised from 25 to 70 K, the threshold only doubles, in stark contrast with the expected difference of two orders of magnitude. Our results can be explained by considering temperatureinduced thermalization of carriers to high wavevector states, increasing the reservoir's overall carrier lifetime, resulting in an order of magnitude higher steady-state carrier density at 70 K under similar pumping conditions.

Optically pumping high quality semiconductor microcavities allows for the spontaneous formation of polariton condensates, which can propagate over distances of many microns. Tightly focussed pump spots here are found to produce expanding incoherent bottleneck polaritons which coherently amplify the ballistic polaritons and lead to the formation of unusual ring condensates. This quantum liquid is found to form a remarkable sunflower-like spatial ripple pattern which arises from self interference with Rayleigh-scattered coherent polariton waves in theČerenkov regime. PACS numbers: 71.36.+c, 42.65.Yj, 73.63.Hs, 78.67.De Condensing solid state bosonic quasiparticles brings prospects for macroscopic quantum-coherent integrated devices. Although still cryogenic, advances in coherent atom matter-wave optics [1,2] shows what might be possible with room temperature condensation, as sought for years with superconductors. In the last decade it has become apparent that polaritons formed from coupling semiconductor excitons with cavity photons produce bosonic quasiparticles that can condense [3,4], even at room temperature [6], with superfluid transport and a number of unusual vortex states now discussed [7][8][9].Polariton condensation requires high-Q optical cavities, large light-matter Rabi coupling exceeding thermal energies (hΩ > k B T ), and efficient relaxation into bosonic quasiparticles from the high-energy injected fermionic carriers. Successful condensation in microcavities made of III-V arsenides [3], II-VI tellurides [4], III-V nitrides [6, 10] and anthracene [11] have relied on natural spatial traps produced by the disordered energy landscape to confine polaritons sufficiently to initiate condensation, as well as artificially created ones. This is largely due to the existence of a 'bottleneck' [12] in the relaxation pathway where polaritons collect, thus forming a high-energy reservoir from which they cannot relax further. Condensation requires an additional localized concentration of the bosons. However such disorder-induced localization severely hampers development of chip-based condensate circuits.Here we demonstrate an efficient scheme to pump such non-equilibrium quantum liquids, that produces unusual coherent spatial ring states. These macroscopic condensate rings experience spatially-localized stimulated scattering as the bosons expand ballistically. Despite their outwards acceleration the quantum liquid remains phase coherent. We show that spatial ripples appearing as interferometric sunflowers arise from elastic scattering in the superfluid.The microcavities consist of a 5λ/2 AlGaAs cavity containing four sets of three GaAs quantum wells placed at the antinodes of the cavity electric field, surrounded by two AlGaAs/AlAs Bragg mirrors of 35 (bottom) and 32 (top) pairs. The cavity quality factor is measured to exceed Q>16000, with cavity photon lifetime τ c =9ps. Strong coupling is obtained with a Rabi splitting between lower and upper polariton energies of 9 meV. All data presented here use a negative...

We measure simultaneously the in-plane electron g factor and spin-relaxation rate in a series of undoped inversion-asymmetric (001)-oriented GaAs/AlGaAs quantum wells by spin-quantum beat spectroscopy. In combination the two quantities reveal the absolute values of both the Rashba and the Dresselhaus coefficients and prove that the Rashba coefficient can be negligibly small despite huge conduction-band potential gradients which break the inversion symmetry. The negligible Rashba coefficient is a consequence of the "isomorphism" of conduction-and valence-band potentials in quantum systems where the asymmetry is solely produced by alloy variations. Symmetry is a thread which runs through all of physics, and symmetry reduction discloses basic physical principles. We employ crystallographically engineered symmetry reduction to study the intricate effects of spin-orbit interaction on the electron spin in semiconductor nanostructures. Symmetry reduction is an especially powerful tool in semiconductor physics because the variety of crystallographic directions combined with band-gap engineering allows enormous freedom.The interplay between structure, symmetry, and electron spin in semiconductors directly affects the spin-relaxation rate s and the effective electron Landé factor g. Early studies of s and g focused on bulk zinc-blende material where both entities are isotropic. 1 Subsequently, the reduction in symmetry from T d to D 2d symmetry in symmetrical (001)-oriented quantum wells (QWs) was shown to give rise to anisotropy between the in-plane (x,y) and the out-of-plane (z) directions. 2,3 Further reduction in symmetry to C 2v is achieved in (001) quantum wells by removing the mirror symmetry of the quantum well potential and allows an in-plane, twofold symmetric anisotropy of both s (Ref. 4) and g. 5 Fundamentally, s and g are both determined by spin-orbit interaction but the basic mechanisms for their anisotropies are quite different. Theoretically the in-plane anisotropy of g is proportional to the asymmetry of the electron wave function in the z direction with the proportionality constant given by the Dresselhaus or bulk inversion asymmetry (BIA) spin-splitting coefficient γ . 5,6 In contrast, s is in many cases dominated by the Dyakonov-Perel (DP) spin-relaxation mechanism and the related in-plane anisotropy depends on the ratio (α/β) of the Rashba structural inversion asymmetry (SIA) to the BIA spin splitting. 4 The SIA component is determined in a rather subtle way by the asymmetry of the structure along the z direction. 7,8 In this work, we determine the absolute value of both the Rashba and Dresselhaus coefficients for a series of quantum well structures by simultaneously measuring the in-plane anisotropy of s and g by spin quantum beat spectroscopy. 9 The specially designed undoped (001) quantum well samples, with reduced C 2v symmetry but without external electric fields, illustrate clearly the different origins of the two anisotropies as they possess a strong anisotropy of g and nearly negligible anisot...