Magnetopolaritons in ZnTe

From an invariant expansion, we construct the exciton Hamiltonian for the Γ₆×Γ₈ excitons in theT _d -type material ZnTe represented by an 8×8 matrix including the influences of a finite wave vector and an external magnetic field. We diagonalize the Hamiltonian matrix to obtain the exciton states. Then the excitons are coupled to the electromagnetic radiation field thus giving the polariton states. The theoretical dispersion curves are fitted to the results of two-photon Raman scattering and reflection experiments in magnetic fields up to 22 T. From this fit we deduce precise values for the eigenergies, exciton masses,g-factors, and diamagnetic shifts.     

Anomalous wave interference at Z3-exciton resonance of CuCl

The transmission and the reflection spectra of a thin CuCl single crystal of 0.15μ thickness have been measured in the Z₃-exciton resonance region at 1.6K by using a weak dye-laser light as a light source. Well resolved interference fringes have been obtained over the exciton resonance. In the higher energy region than the longitudinal exciton's energy, the separation of adjacent fringes cannot be explained by interference of the lower branch polariton waves (LBP) or the upper branch polariton waves (UBP). These structures have been explained by the mutual interference effect between the UBP and LBP waves, anomalous waves. This has been confirmed by the measurements of two-photon absorption due to the excitonic molecule via respective polariton states.     

Optical properties of microcavity polaritons

This work contains a theoretical analysis of the optical properties of semiconductor quantum wells embedded in planar Fabry-Perot microcavities. In particular, the properties of the system in correspondence to the excitonic transition are studied by means of the polariton formalism. The polariton states in microcavities are derived and the polar-iton dispersion is presented. Particular emphasis is put on the existence of two well distinct regimes depending on the exciton and cavity parameters: strong coupling and weak coupling regime. The main experimental results are reviewed and compared with the prediction of the theory. After the polariton states have been characterized, the optical response of the system is discussed, with particular attention to the photoluminescence measurements. The polariton formation and relaxation through phonon scattering and the effect of the exciton inhomogeneous broadening are considered and, finally, a phe-nomenological model for the polariton photoluminescence spectra is presented.     

Exciton polaritons in monoclinic zinc diphosphide

A classical theory based on excitons is insufficient to explain the reflectivity spectra of βZnP₂. Instead an exciton polariton picture is invoked and the polariton parameters are determined. Photoluminescence spectra are consistent with the polariton interpretation. Higher energy states of the exciton have been observed and from these the band gap and exciton binding energy in βZnP₂ have been deduced.     

Theorie der nichtstationären Anregung von Exziton-Polaritonen durch Zweiphotonenabsorption

Theory of nonstationary Exciton-Polariton Generation by Two-photon Absorption A theory of nonstationary two-photon absorption for nonlinear non-centrosymmetric crystals is presented. The time-dependent radiation flux density of the generated polariton beam is calculated under the condition of an ultrashort laser pulse duration T_L ? ΓT^?1 (ΓT is the exciton line width) and a stationary incoherent test field. The “mechanical” response of the excitons and different group velocities of the polariton and the laser and test fields are taken into account. the spatial dispersion of the exciton motion being neglected. With a rectangular laser pulse shape the analytically nonstationary solutions are discussed for identical and for different group velocities of laser and test fields. Using the conservation laws for energy and quasimomentum we illustrate, by considering the polariton dispersion branches, the various possible cases of two-photon absorption in uniaxial piezocrystals.     

Polariton spin beats in semiconductor quantum well microcavities

Time-resolved Kerr (Faraday) rotation experiments allow for the observation of polariton spin beats in both InGaAs and CdMnTe quantum well (QW) microcavities. The existence of these beats is an unambiguous manifestation of the coherent energy exchange between exciton and photon components of polariton states created by a circularly polarized and spectrally wide femtosecond laser pulse. The polariton states are also shown to be split into a linearly polarized doublet. This splitting is responsible for the polarization transfer between linearly and circularly polarized states. In a highest-quality sample, the resulting spin dynamics could be detected.     

Exciton warming in III–V semiconductors and microcavities

We present a time resolved experiment in which we dynamically tailor the occupation and temperature of a photogenerated exciton distribution in QWs by excitation with two delayed picosecond pulses. The modification of the excitonic distribution results in ultrafast changes in the PL dynamics. Our experimental results are well accounted by a quasiequilibrium thermodynamical model, which includes the occupation and momentum distribution of the excitons. We use this model and the two-pulse experimental technique to study the polariton dynamics in InGaAs-based microcavities in the strong coupling regime. In particular, we demonstrate that resonantly injected upper polaritons mainly relax to the lower polariton branch via scattering to large momentum polariton states, producing the warming of the polariton reservoir.     

On a new mechanism of polariton–polariton scattering

Condensate states of a two-dimensional exciton–polariton system have been considered under the conditions of direct resonant photoexcitation. It has been theoretically predicted that splitting of eigenmodes with orthogonal polarizations leads to the emergence of a new channel of parametric scattering. A polariton condensate spontaneously decays into a set of states in a finite region of the momentum space, thus leading to a strong inhomogeneity in the distributions of the intensity and polarization even in the case of a strictly constant amplitude and zero in-plane momentum of the external field. The new scattering mechanism makes possible self-oscillating and chaotic states of polariton systems.     

Resonant three-dimensional photonic crystals

The theory of the exciton-polariton band structure of a resonant three-dimensional photonic crystal is developed for an arbitrary dielectric contrast and an arbitrary effective mass of an exciton excited in a composite material. The calculation is performed for a periodic array of semiconductor balls embedded in a dielectric matrix. The position of the lower polariton dispersion branches is shown to depend monotonically on the exciton effective mass and to be governed by the interaction of light with the first several states of a mechanical exciton quantum-confined within each ball. The effect of excitonic states on the band gap of a photonic crystal in the [001] direction is considered analytically in terms of a two-wave approximation.     

Microcavity polariton scattering probed by coherent control experiments

We present coherent control experiments which simultaneously probe both the coherence and the population dynamics of the exciton–photon polariton states in a semiconductor microcavity. The coherent manipulation of either the spin orientation or the density of polaritons is demonstrated leading to the measurement of the optical dephasing time. The polariton scattering by acoustical phonons or by mutual collision processes are investigated by a simultaneous measurement of both the optical dephasing time T₂and the decay time T₁of the radiant states. These results clearly evidence a quenching of the different scattering processes at resonance.     

Quantum optical effects in semiconductor microcavities

Investigations of quantum effects in semiconductor quantum-well microcavities interacting with laser light in the strong-coupling regime are presented. Modifications of quantum fluctuations of the outgoing light are expected due to the non-linearity originating from coherent exciton–exciton scattering. In the strong-coupling regime, this scattering translates into a four-wave mixing interaction between the mixed exciton–photon states, the polaritons. Squeezing and giant amplification of the polariton field and of the outgoing light field fluctuations are predicted. However, polariton–phonon scattering is shown to yield excess noise in the output field, which may destroy the non-classical effects. Experiments demonstrate evidence for giant amplification due to coherent four-wave mixing of polaritons. Noise reduction below the thermal noise level was also observed. To cite this article: E. Giacobino et al., C. R. Physique 3 (2002) 41–52     

Entangled photon pairs produced by a quantum dot strongly coupled to a microcavity

We show theoretically that entangled photon pairs can be produced on demand through the biexciton decay of a quantum dot strongly coupled to the modes of a photonic crystal. The strong coupling allows us to tune the energy of the mixed exciton-photon (polariton) eigenmodes and to overcome the natural splitting existing between the exciton states coupled with different linear polarizations of light. Polariton states are moreover well protected against dephasing due to their lifetime of ten to a hundred times shorter than that of a bare exciton. Our analysis shows that the scheme proposed is achievable with the present technology.     

Exciton-photon interaction in low-dimensional semiconductor microcavities

The structure of the photon states and dispersion of cavity polaritons in semiconductor microcavities with two-dimensional optical confinement (photon wires), fabricated from planar Bragg structures with a quantum well in the active layer, are investigated by measuring the angular dependence of the photoluminescence spectra. The size quantization of light due to the wavelength-commensurate lateral dimension of the cavity causes additional photon modes to appear. The dispersion of polaritons in photon wires is found to agree qualitatively with the prediction for wires having an ideal quantum well, for which the spectrum is formed by pairwise interaction between exciton and photon modes of like spatial symmetry. The weak influence of the exciton symmetry-breaking random potential in the quantum well indicates a mechanism of polariton production through light-induced collective exciton states. This phenomenon is possible because the light wavelength is large in comparison with the exciton radius and the dephasing time of the collective exciton state is long. Zh. éksp. Teor. Fiz. 114, 1329–1345 (October 1998)     

Energy relaxation of excitonlike polaritons in semiconductor microcavities: Effect on the parametric scattering of polaritons

Polariton emission in GaAs-based microcavities has been studied under variable conditions, which made it possible to excite (a) polaritons from the upper polariton branch and hot free polaritons and electrons, (b) polaritons from the lower polariton branch (LPB) and localized excitons, and (c) the mixed system. Variation of the excitation conditions leads to substantial differences in the energy distributions of polaritons and in the temperature dependences of polariton emission. It is established that the energy relaxation of resonantly excited LPB polaritons via polariton and localized exciton states at liquid helium temperatures is ineffective. Instead, the relaxation bottleneck effect is suppressed with increasing temperature by means of exciton delocalization (due to thermal excitation by phonons). The most effective mechanism of relaxation to the LPB bottom is via scattering of delocalized excitons on hot free carriers. It is found that the slow energy relaxation of polaritons excited below the free exciton energy can be significantly accelerated at low temperatures by means of additional weak generation of hot excitons and, especially, hot electrons. This acceleration of the energy relaxation of polaritons by means of additional overbarrier photoexcitation sharply decreases the barrier for stimulated parametric scattering of polaritons excited at an LPB inflection point. Therefore, additional illumination can be used to control the polariton-polariton scattering.     

Intensity of two-photon excitonic absorption in two-band and three-band models

The intensity of two-photon absorption to Wannier excitons is estimated in simplified models. Both two-band and three-band models simultaneously contribute to the optical two-photon spectrum, but it is shown that the two-band model is favoured in crystals with relevant electron-hole interaction. As an example, it is explained why two-photon absorption spectrum in CuCl exhibits 1s, 2p, 3p exciton states.     

Motional narrowing in semiconductor microcavities

We describe experiments on a semiconductor microcavity which provide the first demonstration of motional narrowing in semiconductor inter-subband optical transitions. Significant narrowing occurs because of the small mass of the polaritons in a microcavity. The demonstration is made possible by the control provided in a microcavity of the mixing between photon and exciton states, and hence the dispersion of the polariton.     

Two-photon absorption in the vicinity of the exciton absorption in CdS

The two-photon absorption spectrum of CdS in the wavelength region of the free and bound exciton lines has been measured using a pulsed dye laser and a differential transmittance technique. An intense absorption band is observed which is possibly due to the interference of the contributions of different intermediate states to the interband two-photon absorption. No absorption is observed which can be attributed to the formation of free excitonic molecules.     

Electromagnetically induced transparency in bulk and microcavity semiconductors

We discuss our recent results on electromagnetically induced transparency (EIT) effects based on intrinsic free exciton and biexciton states in semiconductors. The Λ configuration obtained from the 1S and 2P yellow exciton levels of Cu₂O leads to a well-developed EIT regime, akin to the atomic case. The coherent driving of the exciton–biexciton transition in CuCl induces a tunable transparency window within the polaritonic stop-band, due to the presence of a third polariton branch in the dressed system. In a microcavity configuration, this gives rise to three reflectivity dips in the strong coupling regime.     

Resonant Brillouin scattering via free and bound excitons in CdSe

We report resonant Brillouin scattering results in CdSe. Enhancements in Brillouin scattering have been observed at both the I₂ bound exciton and the free exciton. As a result of the spatial dispersion of the exciton-polariton, the Brillouin frequencies vary with the polariton energy. From this variation of the Brillouin frequencies, we deduced the following parameters in CdSe: transverse exciton frequency =14713 cm^?1, splitting between longitudinal and transverse exciton frequencies = 4cm^?1 and exciton effective mass (perpendicular to the c-axis) =0.40 times free electron mass. The Brillouin linewidths were found to vary with polariton energies in qualitative agreement with the theory of Brenig, Zeyher and Birman.     

Excitons in monoclinic zinc diphosphide: Orthoexciton and polariton effects at n=1 resonance

Quantitative investigations of the hydrogen-like exciton B series in the absorption spectra of the β-ZnP₂ crystal for various wave vector directions and polarization states of radiation are conducted. It is shown that the B spectrum constitutes a single orthoexciton series with S-type envelope functions, and low-energy components in doublet lines belong to the S-type for lines in the series with n≥3. Polariton effects are clearly manifested at the B _n=1 exciton resonance, and Bouguer’s law is violated. The oscillator strength tensor components are determined for transitions to the exciton states of the B series, and the polariton parameters at the B _n=1 exciton resonance are calculated.