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The dependences of the exciton-polariton photoluminescence spectra on the intensity of optical excitation and external magnetic fields are investigated in cylindrical mesas based on high-quality microcavities. It was found that with an increase of the intensity of optical excitation above a certain threshold at sufficiently large magnetic field or with an increase of magnetic fields at constant optical excitation, the distribution of exciton-polaritons over Zeeman sublevels becomes substantially non-Boltzmannian.

The photoluminescence spectra of exciton polaritons in microcavities under conditions of three-dimensional quantization are studied as a factor of the density of the optical excitation and magnetic field. The behavior of the degree of circular polarization of the exciton luminescence in a magnetic field shows that, when the concentration of excitons increases, they condense at the lowest Zeeman sublevel.

The spin-switching of optically induced polariton condensates can be externally controlled with an electric field, with switching energies below 0.5 fJ. Practical challenges to extrapolating Moore’s law favour alternatives to electrons as information carriers. Two promising candidates are spin-based and all-optical architectures, the former offering lower energy consumption1, the latter superior signal transfer down to the level of chip-interconnects2. Polaritons—spinor quasi-particles composed of semiconductor excitons and microcavity photons—directly couple exciton spins and photon polarizations, combining the advantages of both approaches. However, their implementation for spintronics has been hindered because polariton spins can be manipulated only optically3,4 or by strong magnetic fields5,6. Here we use an external electric field to directly control the spin of a polariton condensate, bias-tuning the emission polarization. The nonlinear spin dynamics offers an alternative route to switching, allowing us to realize an electrical spin-switch exhibiting ultralow switching energies below 0.5 fJ. Our results lay the foundation for development of devices based on the electro-optical control of coherent spin ensembles on a chip.

Spatially confined, trapped polariton condensates have been shown to exhibit strong stochastic on-site spin polarization and in longer polariton condensate chains, distance controlled ferromagnetic and antiferromagnetic spin couplings. Until now, little is known, on how such polariton condensates spatially separated from their exciton reservoirs are trapped and formed. Here, we investigate the properties and formation dynamics of two main families of polariton condensates, those overlapping with the pump reservoir and those in confined geometries, under pulsed nonresonant excitation. The observed reduction in polariton condensation threshold and energy blueshift in trapped case is attributed to exciton reservoir-condensate spatial separation, whereas time-resolved photoluminescence measurements, reveal distinct relaxation and condensate formation dynamics with pair parametric scattering process being the dominant relaxation mechanism in trapped geometry.

S. T. acknowledges the financial support of the Stavros Niarchos Foundation within the framework of the project ARCHERS, P. S. acknowledges support form the bilateral Greece-Russia Polisimulator project cofinanced by Greece and the EU Regional Development Fund, A. T. acknowledges the Russian-Greek support from the project supported by the Ministry of Education and Science of The Russian Federation (Project No. RFMEFI61617X0085). A primary limitation of the intensively researched polaritonic systems compared to their atomic counterparts for the study of strongly correlated phenomena and many-body physics is their relatively weak two-particle interactions compared to disorder. Here, we show how new opportunities to enhance such on-site interactions and nonlinearities arise by tuning the exciton-polariton dipole moment in electrically biased semiconductor microcavities incorporating wide quantum wells. The applied field results in a twofold enhancement of exciton-exciton interactions as well as more efficiently driving relaxation towards low energy polariton states, thus, reducing condensation threshold. Postprint

Organic materials exhibit exceptional room temperature light emitting characteristics and enormous exciton oscillator strength, however, their low charge carrier mobility prevent their use in high-performance applications such as electrically pumped lasers. In this context, ultralow threshold polariton lasers, whose operation relies on Bose-Einstein condensation of polaritons - part-light part-matter quasiparticles, are highly advantageous since the requirement for high carrier injection no longer holds. Polariton lasers have been successfully implemented using inorganic materials owing to their excellent electrical properties, however, in most cases their relatively small exciton binding energies limit their operation temperature. It has been suggested that combining organic and inorganic semiconductors in a hybrid microcavity, exploiting resonant interactions between these materials would permit to dramatically enhance optical nonlinearities and operation temperature. Here, we obtain cavity mediated hybridization of GaAs and J-aggregate excitons in the strong coupling regime under electrical injection of carriers as well as polariton lasing up to 200 K under non-resonant optical pumping. Our demonstration paves the way towards realization of hybrid organic-inorganic microcavities which utilise the organic component for sustaining high temperature polariton condensation and efficient electrical injection through inorganic structure.

Optically controlled switching between modes of a polariton laser having different symmetries has been demonstrated experimentally. The microscopic shift of the optical excitation spot dramatically changes the shape of the polariton condensate formed in a cylindrical micropillar on the basis of the planar semiconductor microcavity. Switching between the ring and lobed condensate is achieved owing to the violation of the cylindrical symmetry of the effective potential formed by the lateral surface of the pillar and by the cloud of incoherent excitons created by optical pumping.

Organic materials exhibit exceptional room temperature light emitting characteristics and enormous exciton oscillator strength, however, their low charge carrier mobility prevent their use in high-performance applications such as electrically pumped lasers. In this context, ultralow threshold polariton lasers, whose operation relies on Bose-Einstein condensation of polaritons – part-light part-matter quasiparticles, are highly advantageous since the requirement for high carrier injection no longer holds. Polariton lasers have been successfully implemented using inorganic materials owing to their excellent electrical properties, however, in most cases their relatively small exciton binding energies limit their operation temperature. It has been suggested that combining organic and inorganic semiconductors in a hybrid microcavity, exploiting resonant interactions between these materials would permit to dramatically enhance optical nonlinearities and operation temperature. Here, we obtain cavity mediated hybridization of GaAs and J-aggregate excitons in the strong coupling regime under electrical injection of carriers as well as polariton lasing up to 200 K under non-resonant optical pumping. Our demonstration paves the way towards realization of hybrid organic-inorganic microcavities which utilise the organic component for sustaining high temperature polariton condensation and efficient electrical injection through inorganic structure.

We demonstrate that multiply coupled spinor polariton condensates can be optically tuned through a sequence of spin-ordered phases by changing the coupling strength between nearest neighbors. For closed four-condensate chains these phases span from ferromagnetic (FM) to antiferromagnetic (AFM), separated by an unexpected crossover phase. This crossover phase is composed of alternating FM-AFM bonds. For larger eight-condensate chains, we show the critical role of spatial inhomogeneities and demonstrate a scheme to overcome them and prepare any desired spin state. Our observations thus demonstrate a fully controllable nonequilibrium spin lattice., We acknowledge Grants No. EPSRC EP/L027151/1, No. EU INDEX 289968, No. ERC “POLAFLOW” Starting Grant, ERC LINASS 320503, Spanish MEC (MAT2008- 01555), Mexican CONACYT 251808, Leverhulme Trust Grant No. VP1-2013-011 and Fundación La Caixa. H. S. and I. S. acknowledge support by the Research Fund of the University of Iceland, The Icelandic Research Fund, Grant No. 163082-051. T. L. was supported by the MOE AcRF Tier 1 Grant No. 2016-T1-001-084. P. S. acknowledges financial support from the Stavros Niarchos Foundation, “ARCHERS” project.

We report spin and intensity coupling of an exciton-polariton condensate to the mechanical vibrations of a circular membrane microcavity. We optically drive the microcavity resonator at the lowest mechanical resonance frequency while creating an optically-trapped spin-polarized polariton condensate in different locations on the microcavity, and observe spin and intensity oscillations of the condensate at the vibration frequency of the resonator. Spin oscillations are induced by vibrational strain driving, whilst the modulation of the optical trap due to the displacement of the membrane causes intensity oscillations in the condensate emission. Our results demonstrate spin-phonon coupling in a macroscopically coherent condensate., 5 pages

Efficient single-photon emitters (SPEs) are key to quantum communications and information processing, and frequency tunability is an especially appealing feature. The authors find that at 100 K---above liquid-nitrogen temperature---piezoelectric (PZ) InAs quantum dots exhibit much greater Stark-effect tunability of their excitonic emissions than do non-PZ dots. The observed redshifts cannot be explained unless $n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}r$ PZ effects are taken into account. Beyond SPEs, these results are also important for understanding the behavior of strained nanostructures grown along polar crystal directions.

We study experimentally the oscillations of a non stationary transient signal of a semiconductor microcavity with embedded InGaAs quantum wells. The oscillations occur as a result of quantum beats between the upper and lower polariton modes due to the strong exciton-photon coupling in the microcavity sample (Rabi oscillations). The registration of spectrally resolved signal has allowed for separate observation of oscillations at the eigenfrequencies of two polariton modes. Surprisingly, the observed oscillations measured at the lower and upper polariton modes have opposite phases. We demonstrate theoretically that the opposite-phase oscillations are caused by the pump-induced modification of polariton Hopfield coefficients, which govern the ratio of exciton and photon components in each of the polartion modes. Such a behaviour is a fundamental feature of the quantum beats of coupled light-matter states. In contrast, the reference pump-probe experiment performed for the pure excitonic states in a quantum well heterostructure with no microcavity revealed the in-phase oscillations of the pump-probe signals measured at different excitonic levels., Comment: 6 pages, 5 figures

Tunable spin correlations are found to arise between two neighboring trapped exciton-polariton condensates which spin-polarize spontaneously. We observe a crossover from an antiferromagnetic- to a ferromagnetic pair state by reducing the coupling barrier in real-time using control of the imprinted pattern of pump light. Fast optical switching of both condensates is then achieved by resonantly but weakly triggering only a single condensate. These effects can be explained as the competition between spin bifurcations and spin-preserving Josephson coupling between the two condensates, and open the way to polariton Bose-Hubbard ladders., Comment: 5 pages, 4 figures

We explore two parabolic quantum well (PQW) samples, with and without Bragg mirrors, in order to optimise the building blocks of a Bosonic Cascade Laser. The photoluminescence spectra of a PQW microcavity sample is compared against that of a conventional microcavity with embedded quantum wells (QWs) to demonstrate that the weak coupling lasing in a PQW sample can be achieved. The relaxation dynamics in a conventional QW microcavity and in the PQW microcavity was studied by a non-resonant pump-pump excitation method. Strong difference in the relaxation characteristics between the two samples was found. The semi-classical Boltzmann equations were adapted to reproduce the evolution of excitonic populations within the PQW as a function of the pump power and the output intensity evolution as a function of the pump-pump pulse delay. Fitting the PQW data confirms the anticipated cascade relaxation, paving the way for such a system to produce terahertz radiation., Comment: 8 pages, 6 figures

An exciton polariton (a photon, electron, and a hole in a bound state, behaving as a boson) is a building block for a condensate that emits coherent light. On-chip manipulation of such condensates is an essential step toward polariton-based quantum information devices and lasers. In this work the energy of a polariton condensate in a high-finesse GaAs microcavity is tuned, bringing into reach polariton condensate devices that can be controlled by applied electrical bias.

We present preliminary experimental results on a doped-DBR microcavity diode sample, designed for electrically-pumped operation at the elevated temperatures around 200 K. Low temperature reflectivity measurements as a function of incidence angle have shown clear evidence of polaritons in the strong coupling regime in this sample. Moreover, reflectivity spectra recorded at various temperatures between 185 and 217 K have demonstrated that the strong coupling regime persists at these elevated temperatures. Our results are very promising for the realization of polaritonic devices under electrical injection. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on single dot spectroscopy of InAs/GaAs quantum dots grown by molecular beam epitaxy along the polar (211)B direction. Unlike the (001) dots, in the (211)B dots we have systematically observed large blue shifts of the biexciton lines with respect to the excitonic ones. This is attributed to the strong piezoelectric field present inside the polar dots. In order to better interpret our results, we performed preliminary photoluminesence studies under the influence of an external electric field, which showed large Stark shifts of the quantum dot ensemble. From the analysis of these results, we deduced that the piezoelectric field in the (211)B InAs/GaAs quantum dots is as large as 800kV/cm, which is consistent with the large anti-binding energies of bi-excitons observed in this system.

We report on single dot spectroscopy of polar (2 1 1)B InAs/GaAs quantum dots grown by molecular beam epitaxy. Exciton and biexciton peaks have been identified, revealing a negative biexciton binding energy attributed to the presence of strong piezoelectric field.

We observe condensation of exciton polaritons in quantum states composed of concentric rings when exciting cylindrical pillar GaAs/AlGaAs microcavities non-resonantly by a focused laser beam normally incident at the center of the pillar. The number of rings depends on the pumping intensity and the pillar size, and may achieve 5 in the pillar of 40 mkm diameter. Breaking the axial symmetry when moving the excitation spot away from the pillar center leads to transformation of the rings into a number of bright lobes corresponding to quantum states with nonzero angular momenta. The number of lobes, their shape and location are dependent on the spot position. We describe the out-of-equilibrium condensation of polaritons in the states with different principal quantum numbers and angular momenta with a formalism based on Boltzmann-Gross-Pitaevskii equations accounting for repulsion of polaritons from the exciton reservoir formed at the excitation spot and their spatial confinement by the pillar boundary., Comment: 9 figures, Submitted to the Physical Review B

Optically generated exciton-polaritons in cylindric semiconductor pillar microcavity with embedded GaAs/AlGaAs quantum wells demonstrate a clear polariton lasing regime. When exciting in the center of the pillar we detect a ring-shaped emission, where the peak of intensity can be separated from the excitation spot by more than 10 micrometers. The spatial coherence of the ring emission is verified by interferometry measurements. These observations are interpreted by drift of the exciton polariton condensate away from the excitation spot due to its repulsion from the exciton reservoir and by its spatial confinement by the pillar boundary., 9 pages, 4 figures. Submitted to the Applied Physics Letters

Direct electrical detection of the dispersion of electrically tuned light-matter strong coupling and consequent mappingofthepolaritondispersionisachievedbytunnelingphotocurrentoutofreverse-biased p-i-nmicrocavity structures.Thecoherentphoton-assistedtunnelingisbasedonmixingcavityphotonsintotheelectronicstates.Associated with this double-quantum-well tunneling we demonstrate the optical bistability of polaritons maintained within the strong-coupling regime. An intrinsic hole population gates bistability through the local modulation of the potential profile produced by an optically controlled buildup of free carriers within the quantum wells.

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 built-in detuning creates a potential landscape in which ejected polaritons are channelled toward the end of the ridge where they condense. The low-loss photonlike propagation combined with strong nonlinearities associated with their excitonic component makes polariton-based transistors particularly attractive for the implementation of all-optical integrated circuits. Published version

Tunneling Through with a Light Touch Quantum tunneling underpins a host of essential techniques, such as scanning tunneling microscopy and quantum cascade lasers, as well as chemical reactions. The tunneling particles are normally electrons, and control of the tunneling process has generally been by electric fields. By coupling tunneling electrons with cavity photons trapped inside a semiconductor microcavity, Cristofolini et al. (p. 704 , published online 5 April; see the Perspective by Szymańska ) produced mixed states that then allowed direct optical control of the tunneling process. Such an optical-based approach to manipulating and controlling the tunneling process may find applications in quantum information science.

The aim of this chapter is to highlight the recent progress in the rapidly developing field of electrically driven polariton devices. The unprecedented potential of polariton-based devices owes its origin mainly to the bosonic property of polaritons to condense in the same final state, thus requiring no population inversion to achieve lasing. Consequently, it is widely believed that the threshold of a polariton laser is at least two orders of magnitude lower than that of a conventional semiconductor photon laser operating in the weak-coupling regime [C. Weisbuch, M. Nishioka, A. Ishikawa, Y. Arakawa, Phys. Rev. Lett. 69, 3314–3317 (1992)]. This makes polariton lasers extremely promising as ultralow threshold lasers or as low-power sources of coherent and nonclassical light. Electrical injection of these entangled light-matter states is a key step toward the realisation of practical, compact devices. Recent demonstrations of electrically pumped polariton LEDs in organic as well as inorganic material systems operating up to room temperature have highlighted the potential of such devices for real-world polariton-based applications in optoelectronics.

Replacing independent single quantum wells inside a strongly-coupled semiconductor microcavity with double quantum wells produces a special type of polariton. Using asymmetric double quantum wells in devices processed into mesas allows the alignment of the electron levels to be voltage-tuned. At the resonant electronic tunnelling condition, we demonstrate that `oriented polaritons' are formed, which possess greatly enhanced dipole moments. Since the polariton-polariton scattering rate depends on this dipole moment, such devices could reach polariton lasing, condensation and optical nonlinearities at much lower threshold powers., 3 figures

We report high-speed electronic control of ultrafast polariton amplification in a semiconductor microcavity. A >90% reduction of the parametric scattering gain is obtained by tuning the intracavity electric field to turn on inter-well resonant tunneling.

Исследованы спектры фотолюминесценции экситонных поляритонов в микрорезонаторах в условиях 3-мерного квантования в зависимости от плотности оптического возбуждения и магнитного поля. По поведению степени круговой поляризации экситонной люминесценции в магнитном поле установлено, что при повышении концентрации экситонов они конденсируются на самом нижнем по энергии зеемановском подуровне.

The increasing ability to control light-matter interactions at the nanometre scale has improved the performance of semiconductor lasers in the past decade. The ultimate optimization is realized in semiconductor microcavities, in which strong coupling between quantum-well excitons and cavity photons gives rise to hybrid half-light/half-matter polariton quasiparticles. The unique properties of polaritons-such as stimulated scattering, parametric amplification, lasing, condensation and superfluidity-are believed to provide the basis for a new generation of polariton emitters and semiconductor lasers. Until now, polariton lasing and nonlinearities have only been demonstrated in optical experiments, which have shown the potential to reduce lasing thresholds by two orders of magnitude compared to conventional semiconductor lasers. Here we report an experimental realization of an electrically pumped semiconductor polariton light-emitting device, which emits directly from polariton states at a temperature of 235 K. Polariton electroluminescence data reveal characteristic anticrossing between exciton and cavity modes, a clear signature of the strong coupling regime. These findings represent a substantial step towards the realization of ultra-efficient polaritonic devices with unprecedented characteristics.

We report observation of strong light-matter coupling in an AlGaAs microcavity (MC) with an embedded single parabolic quantum well. The parabolic potential is achieved by varying aluminum concentration along the growth direction providing equally spaced energy levels, as confirmed by Brewster angle reflectivity from a reference sample without MC. It acts as an active region of the structure which potentially allows cascaded emission of terahertz (THz) light. Spectrally and time resolved pump-probe spectroscopy reveals characteristic quantum beats whose frequencies range from 0.9 to 4.5 THz, corresponding to energy separation between relevant excitonic levels. The structure exhibits strong stimulated nonlinear emission with simultaneous transition to weak coupling regime. The present study highlights the potential of such devices for creating cascaded relaxation of bosons, which could be utilized for THz emission.

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 temperature-induced 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.

InAs nanostructures were grown by molecular beam epitaxy on GaAs (111)B and (211)B substrates at various growth temperatures between 450°C and 530°C. Their structural properties were studied by atomic force microscopy. For the (111)B grown nanostructures, the formation of self-aligned InAs quantum dashes was revealed, which in some cases took the form of nanowires. In the (211)B case, the shape and size of nanostructures is drastically affected by growth temperature. At lower growth temperatures, fabrication of quantum dots has been achieved by deposition of 2MLs of InAs. However, when the same amount of InAs was deposited at higher growth temperatures, a drastic change of nanostructure from dots to dashes occurred. Towards a better understanding of nanostructure properties, systematic photoluminescence (PL) measurements were performed on the (211) sample series. No PL emission related to quantum dash formation was observed, while a PL peak position at 1.27 eV was attributed to the (211)B InAs dots grown at 500°C. The PL peak blueshifted upon increasing the excitation intensity, an effect suggested to be related with the existence of strong piezoelectric field in the [211]B direction. The latter was also confirmed in micro-PL experiments through metallic apertures, where a negative biexciton binding energy was observed.