The interference of additional waves of forbidden polaritons excited by allowed polaritons in CuGaS2

Additional waves of exciton polaritons are studied in thin (1.5–1.8 μm) CuGaS₂ crystals at 9 K. The reflectivity spectra show a fine structure related to the interference of Fabry–Perot and additional waves which is a consequence of the polariton spatial dispersion. The main parameters of the exciton polaritons were determined from the spectra calculations. The Γ₄ excitons of big oscillator strength are shown to excite the additional polariton waves of the Γ₅ excitons of small oscillator strength, which interfere determining the fine structure in exciton resonance optical spectra.     

Combined exciton–electron excitation in quantum wells with a two-dimensional electron gas of low density

A combined exciton–cyclotron resonance is found in the photoluminescence excitation and reflectivity spectra of semiconductor quantum wells with an electron gas of low density. In external magnetic fields an incident photon creates an exciton in the ground state and simultaneously excites an electron between Landau levels. A theoretical model is developed and suggests the dominating contribution of the exchange exciton–electron interaction.     

Bleaching in the region of exciton resonance of layered GaSe crystals

Light absorption in the region of exciton resonance of GaSe crystal is studied experimentally at high levels of optical excitation. A picosecond YAG:Nd³⁺ laser emitting 30-ps light pulses and a dye laser with a pulse width of ～3 ns tunable within the range 594–643 nm were used as light sources. It was found that, at high levels of optical excitation, the exciton absorption line of the GaSe crystal disappeared, which was attributed to increasing exciton density with arising mechanisms of their decay: exciton-exciton interactions and screening of excitons by the free charge-carrier plasma. It is shown that these mechanisms are also responsible for the arising new emission band in the long-wavelength region of the photoluminescence spectrum.     

Sasers: resonant transitions in narrow-gap semiconductors and in exciton system in coupled quantum wells

Two schemes for steady stimulated phonon generation (saser, i.e., phonon laser) are proposed. The first scheme exploits a narrow-gap indirect semiconductor or analogous indirect gap semiconductor heterostructure where the tuning into resonance of one-phonon transition of electron–hole recombination can be carried out by external pressure, magnetic or electric fields. The second scheme uses one-phonon transition between direct and indirect exciton levels in coupled quantum wells. The tuning into the resonance of this transition can be accomplished by engineering of dispersion of indirect exciton by external in-plane magnetic and normal electric fields. In the second scheme the magnitude of phonon wave vector is determined by magnitude of in-plane magnetic field and, therefore, such a saser is tunable. Both schemes are analyzed and estimated numerically.     

Resonant Rayleigh scattering from excitons in CdxZn1−xTe:ZnTe quantum wells: measurement of homogeneous linewidths

The technique of resonant Rayleigh scattering is used to determine the homogeneous linewidth across the inhomogeneously broadened exciton resonance in a Cd_0.25Zn_0.75Te/ZnTe multiple quantum well structure. An order of magnitude increase of the Rayleigh scattering signal over background is observed on tuning a narrow-band laser through the exciton resonance at low temperatures. Spectral and temporal measurements show the effect to be a true scattering process rather than luminescence. The interface and alloy fluctuations in the quantum well give rise to spatial fluctuations in the dielectric response of the system while the large exciton resonance causes strong enhancement of scattering. The homogeneous linewidth was calculated across the exciton resonance. The technique is compared with the dephasing and hole-burning techniques more commonly used in homogeneous linewidth measurements.     

Energy Exchange Dynamics and Relaxation of Excitations upon Strong Exciton–Plasmon Interaction in a Planar Nanostructure of Molecular J-Aggregates on a Metal Substrate

The kinetics of energy exchange between the plasmonic and excitonic subsystems in a “metal–dielectric–molecular layer” is studied for a planar composite nanostructure with a strong exciton–plasmon interaction. It is shown that the time dependence of the energy transfer between the components of the system has the character of damped oscillations, which depend on the prepared initial state, a number of relaxation parameters, the Rabi frequency, and the detuning from resonance.     

Center-of-mass quantization of excitons and Fabry–Perot modes of the polariton in ZnSe epilayers

Reflectance properties of ZnSe epilayers grown on GaAs substrates are studied at 80 K. Oscillation features are observed in the region of exciton resonance which are significantly different depending on the epilayer thickness L. For optically thin layers with thickness in the range of several tens of nanometer, reflectance oscillations appear above the light-hole (lh) exciton, while for optically thick layers with thickness in the range of a micrometer, reflectance oscillations appear between 1s and 2s excitons. These oscillations are interpreted as the quantized levels of the exciton center-of-mass motion in the lower branch of the polariton for optically thin layers and Fabry–Perot modes in the upper branch of the polariton for optically thick layers, respectively. The reflectance data are analyzed in the frame work of a dielectric function with a polariton dispersion.     

Optical nutation in the exciton range of spectrum

Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level.     

Direct observation of exciton relaxation in AgBr by time-resolved secondary emission

The dynamical behaviour of a hot exciton system is studied by measuring the time evolution of secondary emission after picosecond laser excitation in resonance with the indirect exciton. From analyzing the experimental data absolute transition rates are determined for exciton trapping at impurities or defects and scattering by LA(Λ) and TA(X) phonons. The deformation potentials for long wavelength acoustic and intervalley phonon scattering derived are 0.84 eV and 4.2 eV, respectively.     

Exciton-contribution to the reflection spectrum at high excitation intensities in CdS

We report measurements of changes in reflection spectrum of CdS due to increasing the density of photoexcited carriers at temperatures above the critical temperature for electron-hole liquid formation. The contribution of the exciton resonance is seen to decrease and analysis of the lineshape indicates that this decrease is due to exciton-exciton collision and a change in exciton polarizability. These results are consistent with a transition from exciton to free electron-hole plasma (Mott transition) at a density of n ～ 2.5 x 10¹⁷ cm^-3.     

Exciton coherence and electron energy loss spectroscopy of conjugated molecules

Signatures of the exciton coherence size, which controls the nonlinear optical response and luminescence of conjugated systems, in the electronic dynamic structure factor S(q,omega) are calculated. We find that for small molecules the momentum dependence of the lowest exciton resonance is purely geometric, reflecting the molecular size rather than a universal exciton size, as suggested recently. For long chains the q dependence is determined by the interplay of the exciton size and the bond-alternation length scales.     

Propagation of quasi-two dimensional exciton polaritons in a quantum well waveguide

The propagation of exciton polaritons in an optical waveguide with a quantum well is studied. Spatial dispersion of the excitons causes the wave vector of the exciton polaritons to split between waveguide and exciton modes at resonance. The magnitude of this splitting is determined by the radiative decay parameter of excitons with corresponding polarization in the quantum well. The group velocity of the waveguide exciton polaritons in the resonance region can be three or four orders of magnitude lower than the speed of light in vacuum. Fiz. Tverd. Tela (St. Petersburg) 40, 362–365 (February 1998)     

Observation of bound exciton and distant pair resonances in amorphous phosphorus

For the first time in an amorphous semiconductor magnetic resonance of a bound exciton is reported. ODMR investigations of amorphous phosphorous show that the luminescence consists of two bands, a low energy emission which is consistent with triple exciton recombination at IVAPs and a high energy emission due to electron-hole recombination at distant IVAPs. The triplet exciton has g = 2.13 ± 0.05, and the distant pair resonance occurs at g = 2.0 ± 0.02.     

Control of excitons in a bent bunch of molecular aggregates by dipole–dipole interaction with quantum dots

The nonlocal dipole–dipole interaction is studied between excitations in chromophores forming a bunch or a tube of J-aggregates and closely spaced quantum dots (QDs). Equations describing the evolution of exciton pulses in a quasi-one-dimensional medium are derived taking into account the interaction with the transition resonant to nanoparticles. It is shown that the efficient controllable resonance energy transfer can occur in the system between QDs and an exciton pulse. The efficiency of this process significantly increases if the bunch of aggregates is deformed to bend nanoparticles round. It is shown that the interaction of permanent dipole moments of QDs and chromophores leads to the formation of a potential barrier or a well. It is found that the combined influence of these factors can be used to efficiently control the dynamics of pulses in aggregates.     

Direct observation of excitons and a continuum of one-dimensional Mott insulators: a reflection-type third-harmonic-generation study of Ni-halogen chain compounds

The third-harmonic-generation (THG) spectrum was measured for a NiBr-chain compound, which is a one-dimensional Mott insulator, in a reflection configuration. A sharp peak and a shoulder structure in the THG spectrum are attributed to three-photon resonance to an exciton and a continuum, respectively. The band-edge energy, the exciton binding energy, and the spectral weights for the exciton and the continuum were determined from comparative studies of linear absorption, THG, and electroreflectance spectra. The excitonic effect is more pronounced in the NiCl chain than in the NiBr chain.     

Resonant Raman Scattering by excitonic polaritons in semiconductors

We report results on Resonant Raman Scattering (RRS) mediated by excitonic polaritons in a high-purity semiconductor (CdTe) at low temperatures. For the first time ingoing and outgoing resonances at the n = 1, 2, 3 exciton states are detected on the one and two LO-phonon RRS. The transformation at resonance of sharp Raman peaks into well-thermalized exciton luminescence bands is observed for every ingoing, outgoing or intermediate state resonance.     

Special frequencies in the optical reflectance spectra of resonant Bragg structures

The optical reflectance spectra of resonant Bragg quantum-well structures are studied theoretically. The existence of two special frequencies in the spectra at which the reflectance depends only weakly on the number of quantum wells in the structure is explained analytically. The effect of nonradiative exciton damping on the reflectance spectra in the vicinity of the special frequencies is analyzed. It is shown that the inclusion of the dielectric contrast leads to the appearance of a third special frequency, at which the contributions to the reflectance due to the dielectric contrast and exciton resonance completely cancel each other.     

Backscattering of light by a finite two-dimensional crystal plate in the region of exciton resonance: Antispecular reflection

It is shown that scattering of a plane monochromatic wave by a finite two-dimensional crystal (quantum well or Langmuir-Blodgett film) in the region of exciton resonance shows, along with the peak of specular reflection, a peak of comparable magnitude corresponding to backward scattering propagating toward the incident beam (antispecular reflection). The effect is related to reflection of the exciton by the boundary of the two-dimensional crystal. The solution of the scattering problem for a simple model system with a Frenkel exciton, demonstrating this phenomenon, is given.     

Excitonic optical absorption in semiconductors under intense terahertz radiation

The excitonic optical absorption of GaAs bulk semiconductors under intense terahertz （THz） radiation is investigated numerically. The method of solving initial-value problems, combined with the perfect matched layer technique, is used to calculate the optical susceptibility. In the presence of a driving THz field, in addition to the usual exciton peaks, 2p replica of the dark 2p exciton and even-THz-photon-sidebands of the main exciton resonance emerge in the continuum above the band edge and below the main exciton resonance. Moreover, to understand the shift of the position of the main exciton peak under intense THz radiation, it is necessary to take into consideration both the dynamical Franz-Keldysh effect and ac Stark effect simultaneously. For moderate frequency fields, the main exciton peak decreases and broadens due to the field-induced ionization of the excitons with THz field increasing. However, for high frequency THz fields, the characteristics of the exciton recur even under very strong THz fields, which accords with the recent experimental results qualitatively.     

The symmetry of the relative motion of excitons in semiconductor heterostructures

A theoretical model of excitonic states in semiconductor heterostructures is presented. The approach employs the envelope function approximation, and involves a two parameter variational calculation in which the symmetry of the component of the wave function representing the relative motion is allowed to vary between the two- and three-dimensional limits. Detailed calculations are described for a variety of single quantum wells and superlattices. The results show that the excitons are neither 2D nor 3D like, but are intermediate in character. Furthermore, in the main, they assume the symmetry of a prolate spheroid. An exception to this occurs in the special case of an asymmetric double quantum well close to resonance, where two stable exciton states are found for the same one-particle states. One of these ‘twin’ exciton states is an oblate spheroid. The results illustrate the need for accurate determination of excitonic properties if the dynamical evaluation of exciton states, in for example, quantum well lasers, is to be readily determined.