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.     

Theory of nonlinear excitation of surface polaritons in lossy media

The purpose of this paper is to study the nonlinear excitation of surface polaritons taking fully into account the damping of the active medium and the finite cross-section of the nonlinear polarization on the interface. This problem is solved using the guided wave calculation techniques where the EM field at the surface polariton frequency is expanded over a complete set of normal modes of the unperturbed interface. Using a “table method” we find that this set includes one guided mode, which is the surface polariton mode, and two classes of radiation modes. The expressions of all these modes are derived and interpreted physically. We then get the expression of the EM field excited at the surface polariton frequency inside and outside the pumped region and show that, in the general case, it is a mixture of all these normal modes. The end of the paper is mainly devoted to the study of the surface term occurring in the expression of the EM field at the surface polariton frequency: we point out the existence of a resonance phenomenon with two kinds of surface polariton modes: the “spatial” one and the “temporal” one. The corresponding dispersion curves, or resonance curves, are given and it is explained how each of them can be obtained experimentally.     

The dispersion curves of the excitonic polariton and the excitonic molecule in CdS and their interaction

By means of the two-photon Raman scattering (TPRS) process we investigate the dispersion relation of the excitonic polariton in the energetic regions around the A-exciton resonance and near half the bi-exciton energy in CdS. In a high resolution experiment an anomaly is observed due to two-polariton transitions to the excitonic molecule (biexciton) state. This anomaly is explained on the basis of a previously developed theory of the intensity dependent dielectric function. Excitation spectroscopy of the TPRS-lines yields information about the damping of the excitonic molecule. Luminescence assisted two polariton spectroscopy (LATS) experiments are performed to determine the eigenenergy of the biexciton as well as its dispersion curve.     

“Right-handed” and “left-handed” polaritons in material media

The dispersion law for polariton waves is deduced from analysis of Maxwell equations in a dielectric medium characterized by the presence of resonance in the frequency range of lattice vibrations or exciton transitions. The theory considers polariton waves with right- and left-oriented vectors $\vec E,\vec H,\vec k$ , corresponding to “right-handed” and “left-handed” polaritons. Dispersion dependences of group velocity of polariton waves and effective mass are established for “right-handed” and “left-handed” polaritons. Expressions are obtained for the effective refractive index and reflection coefficient in a wide spectral range including the resonance region. The specific features of lattice reflection spectra in alkali halide crystals are explained using the proposed theory.     

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.     

Anomalously slow group velocity of upper branch polariton in CuCl

Group velocity of the Z₃ excitonic polariton was determined by measuring the propagation time of a picosecond light pulse in CuCl. Anomalously slow group velocity of the upper branch polariton was observed and the characteristic feature of the group velocity was qualitatively explained by taking account of the dispersion effect of the Z₁₂ exciton. This technique is available for the precise determination of the dispersion curve of the polariton.     

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.     

Phase sensitive spectroscopy of the polariton modes in a ZnSe-ZnSxSe1−x heterostructure

Applying the method of spectral interferometry we investigate the phase of reflected light at a ZnSe-ZnS_xSe_1−x heterostructure. We find a series of polariton modes propagating through the ZnSe layer. They can be related to the different polariton branches split of at the heavy- and light-hole excitons. The phase shows pronounced changes around these modes. The strongest changes by 2π appear at the modes of lowest order located weakly above the exciton resonances, while they are smaller for higher modes. Our experimental findings can be explained considering spatial dispersion, Pekar's additional boundary conditions and a weak extension of the excitonic polarization into the ZnS_xSe_1−x cladding layers.     

Room-temperature polariton parametric scattering driven by a one-dimensional polariton condensate

We demonstrate a novel way to realize room-temperature polariton parametric scattering in a one-dimensional ZnO microcavity. The polariton parametric scattering is driven by a polariton condensate, with a balanced polariton pair generated at the adjacent polariton mode. This parametric scattering is experimentally investigated by the angle-resolved photoluminescence spectroscopy technique under different pump powers and it is well described by the rate equation of interacting bosons. The direct relation between the intensity of the scattered polariton signal and that of the polariton reservoir is acquired under nonresonant excitation, exhibiting the explicit nonlinear characteristic of this room-temperature polariton parametric process.     

Dynamics of a Spinor Exciton–Polariton System in Laterally Strained GaAs Microcavities under Resonant Photoexcitation

The evolution of the spatial coherence and the polarization has been studied in a freely decaying polariton condensate that is resonantly excited by linearly polarized picosecond laser pulses at the lower and upper sublevels of the lower polariton branch in a high-Q GaAs-based microcavity with a reduced lateral symmetry without excitation of the exciton reservoir. It is found that the condensate inherits the coherence of the exciting laser pulse at both sublevels in a wide range of excitation densities and retains it for several dozen picoseconds. The linear polarization of the photoexcited condensate is retained only in the condensate at the lower sublevel. The linearly polarized condensate excited at the upper sublevel loses its stability at the excitation densities higher a threshold value: it enters a regime of internal Josephson oscillations with strongly oscillating circular and diagonal linear degrees of polarization. The polariton–polariton interaction leads to the nonlinear Josephson effects at high condensate densities. All the effects are well described in terms of the spinor Gross–Pitaevskii equations. The cause of the polarization instability of the condensate is shown to be the spin anisotropy of the polariton–polariton interaction.     

Intrinsic decoherence mechanisms in the microcavity polariton condensate

The fundamental mechanisms which control the phase coherence of the polariton Bose-Einstein condensate (BEC) are determined. It is shown that the combination of number fluctuations and interactions leads to decoherence with a characteristic Gaussian decay of the first-order correlation function. This line shape, and the long decay times ( approximately 150 ps) of both first- and second-order correlation functions, are explained quantitatively by a quantum-optical model which takes into account interactions, fluctuations, and gain and loss in the system. Interaction limited coherence times of this type have been predicted for atomic BECs, but are yet to be observed experimentally.     

Light scattering by electrons in the excitonic absorption region of GaAs

A study has been made of the effect of the additional generation of photoexcited electrons on the excitonic absorption and luminescence spectra of ultrapure GaAs samples at T=2 K. The observed increase in the absorption coefficient for the ground (n=1) excitonic state is shown to originate from the polariton character of the energy spectrum of this state and to be due to an increase of polariton damping. The increased damping observed under electron generation is caused by polariton scattering from hot electrons as the latter undergo thermalization. As a result, the polaritons are heated. The changes observed in the luminescence spectra are produced by the reverse effect of electron heating and polariton cooling. Fiz. Tverd. Tela (St. Petersburg) 39, 1011–1016 (1997)     

Bragg polaritons: strong coupling and amplification in an unfolded microcavity

Periodic incorporation of quantum wells inside a one-dimensional Bragg structure is shown to enhance coherent coupling of excitons to the electromagnetic Bloch waves. We demonstrate strong coupling of quantum well excitons to photonic crystal Bragg modes at the edge of the photonic band gap, which gives rise to mixed Bragg polariton eigenstates. The resulting Bragg polariton branches are in good agreement with the theory and allow demonstration of Bragg polariton parametric amplification.     

Propagating wave packets and quantized currents in coherently driven polariton superfluids

We study the properties of propagating polariton wave packets and their connection to the stability of doubly charged vortices. Wave-packet propagation and related photoluminescence spectra exhibit a rich behavior dependent on the excitation regime. We show that, because of the nonquadratic polariton dispersion, doubly charged vortices are stable only when initiated in wave packets propagating at small velocities. Vortices propagating at larger velocities, or those imprinted directly into the polariton optical parametric oscillator signal and idler, are unstable to splitting.     

Dynamics of polaritons resonantly created at the upper polariton branch

We have studied the polariton relaxation dynamics in a CdTe microcavity at low temperatures after resonant excitation into the upper polariton branch (UPB). Initially, we have set a negative exciton–cavity detuning, such that the energy difference between the two polariton branches coincides with that of an LO phonon. Our experimental results reveal a sublinear dependence of the integrated emission from the lower polariton branch (LPB) with excitation power. This evidences not only an inefficient LO phonon mediated relaxation from the UPB to the LPB but also a substantial inhibition of polariton relaxation along the LPB. After that, we have progressively reduced the negative detuning, approaching the exciton–cavity resonance. Under these conditions it is possible to observe a nonlinear emission arising from K0 LPB-states similar to that observed after nonresonant excitation. Marked oscillations are present in the time evolution traces, with a period that does not depend on excitation power or detuning.     

Superradiant properties of a suspension of composite nanoparticles

The characteristics of gold nanoparticles coated with silicon dioxide containing embedded dye molecules are experimentally investigated at room and liquid-nitrogen temperatures. The origin of the observed narrowing of the luminescence line upon cooling is discussed. Model analysis of the nanosystem under study indicates that the observed behavior cannot be related to the temperature dependence of individual parameters of the components of the medium. The effect is explained by the occurrence of nonlinear feedback that leads to the enhancement of phase synchronization between the polarizations of the active transitions in the dye molecules and plasmons. This effect is an analog of plasmon polariton superradiance.     

极化激元(polaritons)与拉曼散射

在给出介质中的光（电磁）波与极化波混合作用而得出的极化激元方程。极化激元色散方程之后又讨论了相应的物理意义及相关的物理问题；获得极化激元的实验方法。接着还介绍和讨论了极化激元的拉曼散射。     

Dynamics of “cold polariton” near a bottleneck region in CuCl

A “cold polariton” which has very sharp distributions of enery and momentum was created in the large kinetic energy region of the lower polariton branch by using a tunable picosecond laser in CuCl. The time behavior of the induced absorption from the “cold polariton” to the excitonic molecule was studied. From these experiments, the lifetime of the polariton at various energies in the bottleneck region was determined. A rather small decrease of the lifetime with the increase of the kinetic energy of the polariton was observed. This decrease was attributed to the increase of the group velocity of the polariton. The main scattering process of the polariton in the bottleneck region was discussed.     

Four-wave polariton scattering of light in LiNbO3

Four-wave Stokes k-spectra for light scattering on polaritons in lithium niobate crystals with an Mg impurity are studied experimentally. The mechanisms for direct, cascade, coherent, and incoherent four-wave mixing of light are discussed in the course of interpreting the angular dependences of the scattered light intensity. It is shown that the dispersion of the real part of the polariton wave vector and the refractive index of the crystals at the polariton frequencies can be measured with an order of magnitude greater accuracy than by spontaneous three-wave polariton light scattering. A significant discrepancy is found between determinations of the polariton absorption coefficient from the angular spectra of three-wave scattering and four-wave scattering in terms of the model employed here. Zh. éksp. Teor. Fiz. 112, 441–452 (August 1997)     

Extremely long range surface polaritons in a thin corrugated metal film

The essential increase of propagation length of a long range surface plasmon polariton in a thin symmetrically corrugated plasmon-carrying film embedded in a dielectric medium is theoretically predicted. The calculations are based on the differential formalism for the system of Maxwells equations where the solution for electromagnetic fields is written as a superposition of partial plane waves in the presentation of a curvilinear non-orthogonal coordinates system for simplifying the boundary conditions. The spectral and angular dependencies of p-polarized light transmittance/reflectance demonstrate that the in-plane shift between both profiles of corrugated film drastically changes the surface plasmon polariton propagation length from minimum of the asymmetric profile to maximum of the symmetric one. The obtained results were qualitatively explained using the model of weakly coupled photonic wells.