Exciton-polaritons by two-photon absorption in semiconductors

The creation of excitonic polariton states by two-photon absorption is theoretically investigated. A semi-classical approach is adopted to compute the two-photon transition probability to polariton states through an intermediate exciton state. The numerical results in CuCl show two peaks corresponding to the longitudinal exciton and transverse polariton states, respectively. These results are in good agreement with the experiment.     

Polariton laser using single micropillar GaAs-GaAlAs semiconductor cavities

Polariton lasing is demonstrated on the zero-dimensional states of single GaAs/GaAlAs micropillar cavities. Under nonresonant excitation, the measured polariton ground-state occupancy is found as large as 10(4). Changing the spatial excitation conditions, competition between several polariton lasing modes is observed, ruling out Bose-Einstein condensation. When the polariton state occupancy increases, the emission blueshift is the signature of self-interaction within the half-light half-matter polariton lasing mode.     

Optical polariton properties in ZnSe-based planar and pillar structured microcavities

Strong coupling is demonstrated in monolithic ZnSe-based microcavities. Under nonresonant excitation the polariton dispersion has been investigated in dependence on the photon-exciton detuning for different excitation densities at low temperatures. For zero detuning indications of a polariton lasing threshold are observed like a k-space and energy dispersion narrowing of the lower polariton branch with increasing excitation density. Furthermore, it is observed that this effect is hampered for measurements at negative detunings as a result of the less effective polariton relaxation to the ground state. Latter results in the formation of a discrete polariton distribution at finite k values as known for the polariton bottleneck. In order to investigate the influence of a three-dimensional optical confinement on the polariton relaxation, pillar structured microcavities were fabricated. The formation of discrete polariton states in the k-space distribution is observed. Furthermore, indications for a softening of the k-conservation arising from the structural confinement are found leading to a more effective polariton relaxation. This process would be beneficial for the realization of efficient polariton lasing processes.     

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.     

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.     

The influence of critical points on polariton dispersion in the band of two-particle states

The presence of critical points in the two-phonon band in conditions of the Fermi resonance on a polariton may result in a considerable polariton dispersion inside the band of two-particle states. Moreover, the polariton linewidth, conditioned by the decay into two particles, is shown to become zero near critical points; this fact holds certain promise for the determination of the positions of critical points in side the band of two-particle states.     

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.     

Phonon-polariton density of states in semiconductor superlattices

An interesting property of modulated semiconductor materials is that their reflectance and absorption spectra can nearly be chosen at will by adjusting the layer geometry. Introducing the concept of phonon-polariton density of states, this paper is aimed at investigating spectral properties of multilayered materials in the infra-red frequency range. Using powerful analytical methods, we will successively consider the cases of infinite and semi-infinite superlattices. The local density of states of polariton modes is obtained using a Green's function technique. Complete information is then available on allowed radiative and non-radiative electromagnetic excitations, (as a function of frequency and wavelength), at any depth in the stratified material. This approach will depict the essential role played by the surface, which changes significantly the polariton density of states as compared to ideal unbounded materials. In multilayered materials, in addition to the effect induced by the surface, one can similarly investigate the influence of the internal interfaces on the polariton local density of states and, from these, on the optical properties of those systems. Electromagnetic eigenmodes arising from the accumulation of interfaces are crucial to assess the spectral properties involving TM-polarized radiations. Effects related to the TE-polarized radiations are explained from the macroscopic anisotropy due to the alternate growth of different semiconductors. These results will be used to discuss reflectance experiments and simulated ATR spectra.     

Superradiance of polaritons in finite one-dimensional crystals of increasing length: Passage from a dimer to an infinite crystal

Solutions of the dispersion equations for polariton states in finite one-dimensional crystals of arbitrary length are obtained. The appearance and evolution of the radiative and nonradiative polariton branches are traced as the length varies from two monomers to limiting values, above which the spectrum no longer undergoes significant changes. The dependences of the frequencies and radiative widths on the polariton wave vector are found for various orientations of the dipole moment of the quantum transition. The evolution of superradiance as the length of the crystal increases is traced. Some previously unknown significant features of the polariton spectrum are noted particularly the damping of the branch traditionally termed nonradiative as a consequence of emission from the end faces. Fiz. Tverd. Tela (St. Petersburg) 40, 2136–2140 (November 1998)     

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.     

Quantum theory of polaritons with spatial dispersion: Exact solutions

Summary We give the exact solution of the quantum polariton problem with spatial dispersion. The exact polariton eigenstates are obtained in terms of photon and polariton states. For all values ofk a nonlinear contribution of the photon and polarization states is present in the polariton wave function. Two- and three-photon components are explicitly singled out.

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Riassunto Si presentano le soluzioni esatte del problema quantistico del polaritone con dispersione spaziale. Gli autostati polaritonici esatti sono ottenuti come combinazioni di stati fotonici e di stati di polarizzazione. Contributi non lineari di fotoni e di quanti di polarizzazione sono presenti a tutti i valori dik. Componenti a due fotoni e a tre fotoni sono date in forma esplicita.

     

Electrically injected cavity polaritons

We have realized an electroluminescent device operating in the light-matter strong-coupling regime based on a GaAs/AlGaAs quantum cascade structure embedded in a planar microcavity. At zero bias, reflectivity measurements show a polariton anticrossing between the intersubband transition and the cavity mode. Under electrical injection the spectral features of the emitted light change drastically, as electrons are resonantly injected in a reduced part of the polariton branches. Our experiments demonstrate that electrons can be selectively injected into polariton states up to room temperature.     

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.     

Coherent effects under suppressed spontaneous emission

An ensemble of resonance atoms is considered, which are doped into a medium with well developed polariton effect, when in the spectrum of polariton states there is a band gap. If an atom with a resonance frequency inside the polariton gap is placed into the medium, the atomic spontaneous emission is suppressed. However, a system of resonance atoms inside the polariton gap can radiate when their coherent interaction is sufficiently strong. Thus the suppression of spontaneous emission for a single atom can be overcome by a collective of atoms radiating coherently. Conditions when such collective effects can appear and their dynamics are analysed. Received 7 June 2000     

Bose glass and superfluid phase transitions of exciton-polaritons in GaN microcavities

Microcavity exciton-polaritons within GaN-based structures are the object of the present work. The impact of the structural imperfections on the properties of the two-dimensional polariton gas is investigated through the calculation of its phase diagram. We demonstrate that the presence of disorder first induces a quasi-phase transition of the polariton system towards a Bose-glass phase before it becomes superfluid as its density increases. Calculations of the density of states as well as the condensate wavefunction and the related spectrum of elementary excitations in the framework of the Gross-Pitaevskii theory provide further insight into the properties of exciton-polaritons in GaN-based microcavities.     

Strong light‐matter coupling between a molecular vibrational mode in a PMMA film and a low‐loss mid‐IR microcavity

Microcavity devices exhibiting strong light‐matter coupling in the mid‐infrared spectral range offer the potential to explore exciting open physical questions pertaining to energy transfer between heat and light and can lead to a new generation of efficient wavelength tunable mid‐infrared sources of coherent light based on polariton Bose‐Einstein Condensation. Vibrational transitions of organic molecules, which often have strong absorption peaks in the infrared and considerably narrower linewidths than organic excitonic resonances, can generate polaritonic states in the mid‐infrared spectral range using microcavity devices. Here, narrow linewidth polaritonic resonances are exhibited in the mid‐infrared by coupling the carbonyl stretch vibrational transition of a polymethyl methacrylate film to the photonic resonance of a low optical‐loss mid‐infrared microcavity, which consisted of two Ge/ZnS dielectric Bragg reflectors. Rabi‐splitting of 14.3 meV is observed, with a 4.4 meV polariton linewidth at anti‐crossing. The large Rabi‐splitting relative to linewidth indicates efficient impedance‐matching between the bare vibrational and photonic states, and suggests molecular‐vibration polaritons incorporated in dielectric microcavities can be an enabling step towards realizing polariton optical switching and polariton condensation in the mid‐infrared spectral range.     

Removal of surface plasmon polariton eigenmodes degeneracy

The effect of the tilt angle of a metal film on the transmissivity of subwavelength holes in optically thick metal film is investigated. We found that the transmission efficiency can be highly dependent on the tilt angle. It is also found that when the input photons are not polarized along the eigenmode directions of surface plasmon polariton, a birefringent phenomenon is observed when a periodic array of subwavelength holes is tilted. Linear polarization states can be changed to elliptical polarization states, and a phase can be added between two eigenmodes. The phase is changed with the tilt angle. A model based on surface plasmon polariton eigenmodes degeneracy is presented to explain these experimental results. PACS 81.07.-b; 71.36.+c; 78.66.Bz     

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.     

Continuous wave observation of massive polariton redistribution by stimulated scattering in semiconductor microcavities

A massive redistribution of the polariton occupancy to two specific wave vectors, zero and approximately 3.9x10(4) cm(-1), is observed under conditions of continuous wave excitation of a semiconductor microcavity. The "condensation" of the polaritons to the two specific states arises from stimulated scattering at final state occupancies of order unity. The stimulation phenomena, arising due to the bosonic character of the polariton quasiparticles, occur for conditions of resonant excitation of the lower polariton branch. High energy nonresonant excitation, as in most previous work, instead leads to conventional lasing in the vertical cavity structure.     

A new type of local polariton at the interface of gyrotropic enantiomorphic crystals

It is shown that a new type of local polariton can propagate at the interface of enantiomorphic twins—gyrotropic crystals with left-handed and right-handed rotation of the polarization plane. The wave function of these local polaritons oscillates strongly, with changes of sign, over lengths of the order of the lattice constant near the interface, and the period of the spatial oscillations grows with increasing distance from the interface. The local polariton term is detached from the band of delocalized states toward higher frequencies. Calculations of the radiation broadening of this term show that, for a local polariton, the effect of a giant increase of the decay (“superradiance”) is possible. The magnitude of the polarization rotation due to a local polariton has been found. Fiz. Tverd. Tela (St. Petersburg) 41, 337–340 (February 1999)