Zur Quantentheorie der stimulierten RAMAN-Streuung in Molekülkristallen. I. Einige Bemerkungen zur Theorie nach K. GROB

Stimulated RAMAN scattering in molecular crystals is quantum theoretically described as stimulated polariton scattering. The quantum theoretical treatment of molecular crystals is given within the framework of a second quantization described in an earlier work. In the HEISENBERG picture the equations of motion for the operators of the polaritons are derived and specialized for RAMAN scattering. By supposing quasi-stationary behaviour the operators of the polaritons that are mixtures of phonons and photons are eliminated. The resulting equations are shown to have the same structure but a generalized physical meaning as those derived by GROB on other way.     

Zur Quantentheorie der stimulierten RAMAN-Streuung in Molekülkristallen. II. Spezielle Fälle eines stationären RAMAN-Oszillators

Two kinds of stationary RAMAN oscillators are investigated theoretically for molecular crystals. The calculations are done firstly for the generation of one first order anti-STOKES mode and secondly for the generation of one second order STOKES mode. By using a quantum theoretical model described in an earlier paper for treatment of molecular crystals RAMAN scattering is assumed to be polariton scattering. Within this framework coupled nonlinear equations for the polariton operators of the excited modes are derived, stationary occupation numbers for the different modes and threshold conditions are calculated. The influence of phase fluctuations of the pump wave on the line widths of the RAMAN modes are investigated.     

Theorie des stimulierten Raman-Effekts

We consider stimulated Raman emission in solids, placed in a plane laser beam external to the cavity. The Hamiltonian of the system of phonons, electrons and electromagnetic fields is derived within the framework of a generalized adiabatic approximation for electrons and nuclei. It contains terms due to nonlinear interactions between electrons and phonons. Because the usual time-dependent perturbation theory cannot describe coherence effects properly we turn toHeisenberg's equations of motion for the operators of photons, phonons and electron excitations. In order to solve these equations in the steady state we apply an iteration procedure. We start with the light waves which give rise to electron transitions. The electrons such excited create phonons which then react on the electrons. Finally the electrons are coupled again to the lightfield. This procedure yields besides the usual wellknown Raman process two main processes occurring in stimulated Raman emission: a coupled two step Raman process and a parametric process. In the first one two phonons are involved. If the linewidth of phonons is comparable to the phonon frequencies the non-resonant parts of the above processes also become important. In solving the set of coupled equations for the light amplitudes, obtained from the iteration procedure, we only consider terms due to the first Stokes, the first anti-Stokes and the laser line. We then find frequency shifts of these lines due to the stimulated emission which are of the order of the linewidth of photons if this linewidth is very much smaller than that of phonons as it is the case in solids. This means that the coupled two step Raman process is dominant, in good agreement with measurements ofChiao andStoicheff in calcite.     

Bose-Einstein condensation quantum kinetics for a gas of interacting excitons

A quantum kinetics of the Bose-Einstein condensation in the self-consistent (s.c.) Hartree-Fock-Bogoliubov (HFB) model of the interacting Bose gas is formulated and numerically solved for the example of excitons scattering with a thermal bath of acoustic phonons. The theory describes the condensation in real time starting from a nonequilibrium initial state towards the equilibrium HFB solution. The s.c. changes of the spectrum are automatically incorporated in the scattering terms.     

Tomographic Description of Stimulated Brillouin Scattering of Light

The process of stimulated Brillouin scattering is described by the twodimensional oscillator model. Photons of the Stokes mode are described by one mode of the oscillator, and acoustic phonons are described by the other mode. The interaction of photons and acoustic phonons is assumed to be quadratic in the creation and annihilation operators of photons and phonons. The laser is considered as a classical light source and its depletion is neglected. New timedependent integrals of motion and the photon–phonon probability distribution function are found. The mean Stokes photon number and the mean acoustic phonon number are expressed as functions of the medium parameters (initial dispersions) and the interaction parameter (coupling constant). The classical propagator for stimulated Brillouin scattering and tomograms of the photon and phonon states are investigated within the framework of the symplectictomography scheme.     

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.     

Electromagnetic Field Theory Without Divergence Problems 1. The Born Legacy

Born's quest for the elusive divergence problem-free quantum theory of electromagnetism led to the important discovery of the nonlinear Maxwell–Born–Infeld equations for the classical electromagnetic fields, the sources of which are classical point charges in motion. The law of motion for these point charges has however been missing, because the Lorentz self-force in the relativistic Newtonian (formal) law of motion is ill-defined in magnitude and direction. In the present paper it is shown that a relativistic Hamilton–Jacobi type law of point charge motion can be consistently coupled with the nonlinear Maxwell–Born–Infeld field equations to obtain a well-defined relativistic classical electrodynamics with point charges. Curiously, while the point charges are spinless, the Pauli principle for bosons can be incorporated. Born's reasoning for calculating the value of his aether constant is re-assessed and found to be inconclusive.     

Intensities of resonant brillouin scatterings from multicomponent exciton-polaritons

To reduce the arbitrariness in the current analysis of resonant Brillouin scattering (RBS) from multicomponent polaritons, the intensities of scattering peaks are theoretically studied. The interaction among the multicomponent excitons may contain linear and quadratic terms in translational wave vector, electron-hole exchange interaction, and any other terms that retain translational symmetry. As the scattering mechanisms due to TA and LA phonons, we consider various deformation and piezoelectric potentials. In certain cases, this theory leads to a “selection rule”, which can solve the controversy between the two different dispersion curves for CuBr obtained from RBS and two photon resonant Raman scattering, in favor of the latter. The theory also provides a basis to discuss the problem of additional boundary conditions for multicomponent polaritons in terms of the relative intensities of scattering peaks.     

Two-polaron energy diffusion in a 1D lattice of hydrogen bonded peptide units

A generalized Pauli master equation is established for describing the vibrational energy flow in a 1D lattice of hydrogen bounded peptide units. A Lang-Firsov transformation is applied so that the relevant excitations are small polarons corresponding to vibrational excitons dressed by virtual phonons. A special attention is thus paid to characterize the energy transfer mediated by two polarons. At biological temperature, it is shown that the polaron-phonon coupling is sufficiently strong to prevent any coherent motion. The polaron-polaron interaction occurring in such a nonlinear lattice does not affect the long time behavior of the energy flow which results from the diffusion of two independent polarons. This diffusive motion originates from the competition between two contributions related to phonon mediated transitions (incoherent contribution) and to dephasing limited coherent motion (coherent contribution).     

Resonant Raman study of LO + acoustic phonon modes in CdSe

In studying resonant Raman scattering in the vicinity of the A and B excitons of CdSe, we have observed three new Raman peaks. Two of the peaks have been identified as two-phonon modes consisting of a longitudinal optical (LO) phonon plus respectively a transverse acoustic (TA) and a longitudinal acoustic (LA) phonon. A theory which involves the scattering of photoexcited B excitons to the A exciton by acoustic phonons via the piezoelectric exciton-phonon interaction was found to explain quantitatively the peak positions, lineshape and resonance enhancements of the observed peaks.     

Zur Theorie der nichtstationären stimulierten Raman-Streuung

The theory of transient stimulated Raman scattering is developed taking into account the deplation of the pump pulse. Two coupled differential equations describing the conversion of a laser pulse in a Stokes wave are derived with the aid of a semiclassical nonstationary perturbation calculation without the phonon concept. The equations are solved both analytically by a successive approximation and by computer calculations. Estimations are given of the delay of the Stokes pulse, the gain and a characteristic length concerning the conversion.     

Stimulated second-order processes in the interaction of a spinless quantum particle beam with a self-consistent electromagnetic field

A nonlinear theory of interaction of two electromagnetic waves with a beam or a gas of spinless charged particles is developed. The effects of stimulated Compton scattering of electromagnetic waves and stimulated particle pair production (annihilation) during a collision of two electromagnetic quanta are investigated. Other stimulated processes that can occur only in a medium and decelerate electromagnetic waves are also considered. The relation between stimulated processes and various types of instabilities considered in classical electrodynamics of plasmas and plasmalike media is demonstrated.     

Signatures of stimulated bosonic exciton-scattering in semiconductor luminescence

We observe signatures of stimulated bosonic scattering of excitons, a precursor of Bose-Einstein-Condensation (BEC), in the photoluminescence of semiconductor quantum wells. The optical decay of a spinless molecule of two excitons (biexciton) into an exciton and a photon with opposite angular momenta is subject to bosonic enhancement in the presence of other excitons. In a gas of biexcitons and spin polarized excitons the bosonic enhancement breaks the symmetry of two equivalent biexciton decay channels leading to circularly polarized luminescence of the biexciton with the sign opposite to the circularly polarized exciton luminescence. Comparison of experiment and many body theory clearly indicates the existence of stimulated exciton-scattering, but excludes the presence of a fully condensed BEC-like state.     

Theoretical treatment of the second-order Raman scattering

The theoretical analysis for the Raman scattering by 2LO phonons is carried out. The scattering process is considered, taking into account the Wannier excitons and the free electron-hole pairs as intermediate states.     

Zone-folding effects on phonons and excitons in polytypic BiI3 single crystals

Resonant Raman scattering (RRS) of new mode phonons in BiI₃ crystals containing stacking faults is reported. Under excitation with laser light tuned to a characteristic sharp absorption line, new mode Raman lines show the resonance behavior. The new phonon modes and the origin of the sharp absorption line can be interpreted in terms of folding back effects of the Brillouin zone for phonons and excitons in a polytypic structure     

Theory on the optical-phonon Raman scattering in GaAs/AlAs heterostructures

A microscopic theory of Raman scattering by optical phonons in GaAs/GaAlAs heterostructures is worked out systematically, on the basis of recent advances in our knowledge of the electronic structure and the optical-phonon modes in superlattices and quantum wells. Theories have shown that specific features of the intermediate states are of special importance for a quantitative theory. Thus, the heavy and light hole mixing effect, and the angular momentum state of the four-component excitons, can play a decisive role in determining the predominant scattering channels. Special attention has been paid to the Frolich-interaction induced scattering, which is dipole forbidden in bulk materials but allowed in multiple quantum wells owing to the barrier penetration and the hole mixing. Based on the microscopic theory, explanations are provided for such experimental facts as the asymmetry between the incoming and the outgoing resonance, the line shape of Raman spectra and the features of two-phonon Raman scattering in quantum well systems.     

Superfluidity of coherent light in self-focusing nonlinear waveguides

We establish the superfluidity theory of coherent light in waveguides made of nonlinear polar crystals.It is found that the pairing state of photons in a nonlinear polar crystal is the photonic superfluid state.The photon-photon interaction potential is an attractive effective interaction by exchange of virtual optical phonons.In the traveling-wave pairing state of photons,the photon number is conserved,which is similar to the Bose-Einstein condensation(BEC) state of photons.In analogy to the BCS-BEC crossover theory of superconductivity,we derive a set of coupled order parameter and number equations,which determine the solution of the traveling-wave superfluid state of photons.This solution gives the critical velocity of light in a self-focusing nonlinear waveguide.The most important property of the photonic superfluid state is that the system of photon pairs evolves without scattering attenuations.