Microcavity Polaritons Research Articles

Polariton–polariton scattering and the nonequilibrium condensation of exciton polaritons in semiconductor microcavities

Polariton–polariton scattering and the nonequilibrium condensation of exciton polaritons in semiconductor microcavities

Resonant acousto-optics of microcavity polaritons

We propose and analyse theoretically a resonant acousto-optic Stark effect for microcavity (MC) polaritons parametrically driven by a surface acoustic wave. For GaAs-based microcavities our scheme ‘acoustic pumping–optical probing’ deals with surface acoustic waves of frequency νSAW ≃ 0.5–3 GHz and intensity ISAW ≃ 0.1–10 mW mm−1. The acoustically-induced stop gaps in the MC polariton spectrum drastically change the optical response of MC polaritons. Because an acoustically pumped intrinsic semiconductor microcavity remains in its ground electronic state, no many-body effects screen and weaken the resonant acousto-optic Stark effect. In the meantime, this allows us to work out an exactly-solvable model for resonant acousto-optics of MC polaritons which deals with giant acousto-optical nonlinearities. Finally, we discuss possible applications of the proposed resonant acoustic Stark effect for optical modulation and switching and describe an acousto-optic device based on a (GaAs) microcavity driven by a surface acoustic wave.

Polaritons in microcavities containing a two-dimensional electron gas

We review the spectroscopic and dynamic properties of cavity polaritons in a λ-wide GaAs/GaAlAs microcavity (MC) structure having a single GaAs/AlAs quantum well in which a variable density two-dimensional electron gas (2DEG) is photoexcited, with a density in the range 0 < ne < 5 × 1011 cm−2. The main observed effects that will be discussed here are: (a) photoexcitation of negatively charged polaritons. These polaritons result of the coupling of the MC photon and the negatively charged excitons. They appear at low temperatures (T <25 K), in the range of low 2DEG densities (ne < 5 × 1010 cm−2), and their coupling strength increases as √ne. (b) The effect of electron–polariton scattering on the polariton spectral linewidth, its complex dependence on the detuning energy, and the electron-induced transition from strong to weak coupling regimes, as ne increases. (c) The efficient polariton transfer across the relaxation bottleneck in the lower branch induced by electron–polariton scattering, and the resulting non-linear dependence of the polariton photoluminescence intensity on ne.

Exciton–polariton spin rotation in microcavities in zero magnetic field

We show theoretically that the longitudinal–transverse splitting of exciton–polaritons in semiconductor microcavities induces precession of polariton pseudospins. This precession manifests itself in pronounced oscillations of the polarization degree of time-resolved photoluminescence of microcavities. This effect combined with spin-dependent stimulated scattering of exciton–polaritons is responsible for the unusual polarization dynamics of emission of microcavities, recently reported [M. D. Martin et al., Phys. Rev. Lett 89, 077402 (2002)].

II–VI semiconductor microcavity angle‐resolved coherent dynamics

We used angle-resolved time-integrated four-wave mixing to study the pump dynamics in a II–VI semiconductor microcavity polariton parametric amplifier. The low-energy polariton dephasing time decreases at large angles when the excitonic reservoir becomes accessible to polariton–polariton scattering. Additionally, in the parametric amplification regime, with phase matching for the polariton–polariton scattering at the magic angle, we observe a strong reduction of the FWM decay time. Numerical analysis of the polariton coherent dynamics in this regime allows to interpret the observed features in terms of time dependent pump depletion.

Linewidth of a polariton laser: Theoretical analysis of self-interaction effects

Polaritons in semiconductor microcavities can experience a Bose-Einstein condensation experimentally detectable in the spectrum of the emitted light. Scattering with noncondensed particles as well as self-interaction in the condensate provoke phase-diffusion limiting the coherence of the polariton condensate. We present a theoretical analysis of self-interaction effects on the lineshape of the emission from a polariton laser. Our calculations, for CdTe microcavities, show that there is an optimum pump at which the linewidth of the emitted light is reduced down to 1 $\mu$ eV.

Photoluminescence emission and Raman scattering polarization in birefringent organic microcavities in the strong coupling regime

We have investigated the polarization properties of photoluminescence and Raman scattering in strongly coupled organic microcavities containing cyanine dye J aggregates. The birefringence of radially aligned J aggregates results in a large energy splitting of polariton modes when the electric field of the incoming laser light is perpendicular or parallel to the alignment direction. This splitting allows the degree of polarization of doubly resonant Raman processes involving the vibrational modes of J aggregates to be controlled, where the ingoing and outgoing channels are in resonance with the lower polariton branch. As well as providing insight into the properties of polaritons in organic microcavities, these experiments are a sensitive probe of alignment effects arising during the spin-coating deposition process.

Giant enhancement of polariton relaxation in semiconductor microcavities by polariton-free carrier interaction: Experimental evidence and theory

Very large changes in the population distributions of microcavity polaritons are reported under conditions of simultaneous resonant and nonresonant excitation. The strong relaxation bottleneck, which results in weak emission from the lower polariton branch for resonant excitation alone, is fully suppressed if additional nonresonant excitation is employed. By comparison with theory, the massive enhancement of the polariton relaxation is shown to arise from scattering of polaritons by free carriers, present in quantum wells due to residual doping, which are then heated by the nonresonant excitation.

Non-linear resonant Rayleigh scattering from microcavity

A strong superlinear dependence of the resonant Rayleigh scattering (RRS) from a semiconductor microcavity (MC) after cw excitation is observed. The effect is associated with transition from normal mode coupling (NMC) to weak coupling regime of the MC modes due to the exciton bleaching. Since the MC-RRS mainly arises from the scattering of the photonic part of the MC polariton and in NMC regime the RRS is quenched by the excitonic absorption, the loss of exciton oscillator strength produces the strong RRS nonlinearity.

Cavity polaritons in microcavities containing disordered organic semiconductors

Many organic materials (disordered and crystalline as well) have rather broad and dispersionless electronic resonances. The excitations in these materials are localized, and thus for these excitations the wave vector is not a ``good'' quantum number. We analyze the optical properties of exciton polaritons in a microcavity, which utilize such organic materials as the optically active semiconductor. We show that as a result of strong light-matter coupling two polariton branches appear in the microcavity, which are analogous to the cavity-polariton branches observed in inorganic semiconductor structures. However, in contrast to the case of polaritons in inorganic semiconductors, the lower polariton branch exists only in a certain restricted intervals of wave vector values. We also show that in such materials the majority of the electronic excited states do not strongly couple to the cavity photon, and these states can be regarded as essentially incoherent. Applying this physical picture for the microcavities containing disordered cyanine dye J aggregates, we examine the decay of the upper cavity polaritons accompanied by the emission of an intramolecular phonon. We show that the main contribution to the upper polariton nonradiative decay rate arises from the transition to incoherent states, and the transition is very fast (of the order of 50 fs). From the results of our calculations we discuss the dynamics of excitations in a microcavity containing organic materials

Semiconductor microcavities: towards polariton lasers

In this review paper we address one of the most rapidly developing new domains of semiconductor optics: light-matter coupling in semiconductor microcavities. Using the non-local dielectric response theory and transfer matrix technique, we show how two-dimensional confinement of a photonic mode coupled to an exciton resonance results in the appearance of two branches of exciton-polaritons, quasi-particles combining properties of photons and excitons. We obtain the dispersion relations of polaritons in microcavities and derive a condition for strong-weak coupling threshold. We show that being bosons, exciton-polaritons are subject to Bose-condensation which might result in emission of a coherent and monochromatic light in the strong coupling regime. A source of such coherent light is referred to as a polariton laser. We show that polariton lasers have theoretically no threshold and require essentially new basic physics as compared to conventional lasers described by Einstein theory. We give examples of model polariton laser structures expected to work at room temperature and overview the main difficulties on the way to producing these new opto-electronic devices.

Spontaneous Bose coherence of excitons and polaritons.

In the past decade, there has been an increasing number of experiments on spontaneous Bose coherence of excitons and polaritons. Four major areas of research are reviewed here: three-dimensional excitons in the bulk semiconductor Cu2O, two-dimensional excitons in coupled quantum wells, Coulomb drag experiments in coupled two-dimensional electron gases, and polaritons in semiconductor microcavities. The unifying theory of all these experiments is the effect of spontaneous symmetry breaking in the Bose-Einstein condensation phase transition.

Microcavity Polaritons in Materials with Weak Intermolecular Interaction

We analyze the optical properties of exciton-polaritons in a microcavity, which utilizes materials with broad and dispersionless electronic resonances as the optically active semiconductor. The excitations in such non-cavity materials are localized, and for these excitations the wave vector is not a “good” quantum number. We show that in a microcavity as a result of strong light–matter coupling two polariton branches appear, which are analogous to the cavity-polariton branches observed in inorganic semiconductor structures. However, the polariton branches only exist in certain restricted intervals of wave vectors. We then show that in such materials the majority of the electronic excited states do not strongly couple to the cavity photon, and these states can be regarded as essentially incoherent. Based on these results, we discuss some peculiarities of the dynamics of excitations in a microcavity containing such type of materials.

Threshold power and internal loss in the stimulated scattering of microcavity polaritons

Circularly polarized excitation $({\ensuremath{\sigma}}^{+})$ and detection $({\ensuremath{\sigma}}^{+},{\ensuremath{\sigma}}^{\ensuremath{-}})$ techniques are employed to demonstrate the role of internal losses in determining threshold powers and spectroscopic energies in the regime of stimulated polariton scattering. Very marked increases of linewidth with temperature and excitation intensity are found for the nonstimulated ${\ensuremath{\sigma}}^{\ensuremath{-}}$ minority spin orientation, and provide a direct probe of the internal losses. The losses thus determined provide a very good explanation for the increases of threshold power and the blueshifts in signal and idler energies with temperature found for the stimulated ${\ensuremath{\sigma}}^{+}$ emission. Good semiquantitative agreement with recent theories is found.

Polariton linewidths in a semiconductor microcavity

The linewidths of microcavity polariton modes are measured form reflectivity spectra at 10 K as a function of the detuning between exciton and cavity modes. The narrowing of the polariton features is described in the framework of the semiclassical approach of linear dispersion theory including exciton asymmetric inhomogenous broadening. We have also measured the polariton linewidth for different sample temperatures and observed the same polariton linewidth behaviour as that for homogenous broadening system.

Stimulated spin dynamics of polaritons in semiconductor microcavities

The strong coupling of light to electronic transitions in semiconductors has recently become of great interest because of the additional interactions allowed. These interactions originate from the complete mixing of photons with hydrogenically bound electron-hole pairs, which produces mixed quasiparticles called exciton polaritons with radically different properties. 1‐3 In particular, pair scattering events

Bose-Einstein Condensation of Polaritons in Organic Semiconducting Microcavities

Bose-Einstein Condensation of Polaritons in Organic Semiconducting Microcavities

Polariton lasing by exciton-electron scattering in semiconductor microcavities

The relaxation bottleneck present in the dispersion relation of exciton polaritons in semiconductor microcavities has prevented the realization of low threshold lasing based on exciton-polariton condensation. Here we show theoretically that the introduction of a cold electron gas into such structures induces efficient electron-polariton scattering. This process allows the condensation of the polaritons accumulated at the bottleneck to the final emitting state with a transition time of a few picoseconds, opening the way to a new generation of low-threshold light-emitting devices.

Bose-Einstein Condensation of Polaritons in Organic Semiconducting Microcavities

Anti-crossing of organic exciton and photon modes and a room temperature Rabi splitting that is an order of magnitude larger than the highest values reported for inorganic semiconductor quantum wells are observed in organic microcavity embedded quantum wells. Also, this larger splitting due to the large oscillator strength of organic excitons is easily achieved with many organic systems and gives reasonable hope for reaching the strong coupling regime. However, dynamic condensation clearly involving stimulated emission of the k ≈ 0 lower polaritons is expected at relatively low exciton density, due to the very small polariton mass. We report on the theoretical study of non-linear emission of organic microcavity polaritons. In the ‘Boser’ model at low density, we include the acoustic phonon-assisted relaxation and polariton–polariton scattering stimulated by the quasi-bosonic polariton final state population.

Elastic Scattering of Polaritons in High Quality Microcavities and Weak Localization

We theoretically study the elastic scattering of lower cavity polaritons on the static disorder in high quality semiconductor microcavities in the strong coupling regime. We consider the dominant contribution of the resonant scattering on localized exciton levels and calculate for a model density of states the corresponding elastic mean free path and inhomogeneous broadening. Our analytical results compare well with available linewidth data and large scale numerical simulations. They also provide a justification for a widely used phenomenological model assuming the polariton wavevector conservation. Finally, we discuss the possibility of weak localization in state of the art microcavities.