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.     

Ultrafast carrier dynamics of resonantly excited InAs/GaAs self-assembled quantum dots

We study theoretically the time development of electronic relaxation in quantum dots. We consider the process of relaxation of the state with an electron prepared at the beginning of relaxation in the electronic ground state. We obtain a fast (in picoseconds) increase of electronic population in the excited state. Also, we consider the process of relaxation of an electron from an excited state in the dot. Here we obtain an incomplete depopulation of the electron from the excited state. We compare these results to experiments in which a fast decrease of luminescence is reported during the first period of relaxation after resonant excitation of the ground state. We estimate numerically the role of electron–LO–phonon (Fröhlich's coupling) mechanism in these processes. We show that this effect may be attributed to the influence of multiple scattering of quantum dot electrons on LO phonons. A single-electron two-energy-level quantum dot model is used to demonstrate this effect in an isolated semiconductor quantum dot.     

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)     

Polariton-polariton kinematic interaction modulation and kinematic bipolaritons in organic microcavities

We study polariton-polariton kinematic interactions in organic microcavities. Using the Agranovich-Toshich transformation, to transform the Frenkel excitons from Paulions into Bosons, the exciton-exciton kinematic interaction is derived. In the strong coupling regime, the polariton excitonic part results in the polariton-polariton kinematic interaction. The scattering amplitude is calculated and the effective potential is obtained for a scattering between two free polaritons. The effective potential can be modulated by changing the exciton-cavity photon detuning, and we show the crossover of the effective potential from attractive into repulsive one. A pole in the two-particle Green's function is the signature of the formation of polariton bound state, i.e. bipolariton. Due to the smallness of the polariton effective mass, the obtained bound state is very shallow and appears below the minimum of the lower polariton branch, and falls inside the natural bandwidth of the polariton branch.     

Electron-polariton scattering in semiconductor microcavities

In semiconductor microcavities, electron-polariton scattering has been proposed as an efficient process that can drive polaritons from the bottleneck region to the ground state, achieving Bose amplification of the optical emission. We present clear experimental observation of this process in a structure that allows control of the electron density and we report substantial enhancement of photoluminescence. We show that this enhancement is more effective at higher temperatures due to the different way that electron scattering processes either broaden or relax polaritons.     

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.     

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.     

Relaxation kinetics for temperature and degeneracy of microcavity polaritons

We apply a semi-classical Boltzmann kinetics for a gas of laser-pulse excited microcavity polaritons taking into account their mutual interaction and their interaction with acoustic phonons. Fitting the temporally evolving polariton distribution above the ground state with a Bose–Einstein distribution, we find the evolution of the temperature and the degeneracy parameter, i.e. the ratio of the chemical potential to the thermal energy. Studying the relaxation in particular for GaAs microcavities we compare our results with recent measurements by Deng et al. In agreement with the experiment we find that the lattice temperature can be reached and that the degeneracy of the condensed gas holds up to 60–80 ps provided a detuning of the cavity mode is applied which increases the exciton component of the lower-branch polaritons and thus their scattering rates.     

Anomalous Doppler-effect and polariton-mediated cooling of two-level atoms

We consider an atom moving in a near resonant laser field with its dipole strongly coupled to a resonator field mode. As compared to the standard Doppler shift, we find a substantially different and counterintuitive linear velocity dependence of the light scattering properties. The mechanical force of the laser field exhibits strong velocity selectivity at a polariton resonance, which gives rise to an enhanced friction force and Doppler cooling even in the directions perpendicular to the resonator axis. This effect allows for sub-Doppler cooling of atoms even with a nondegenerate ground state.     

Direct observation of polariton velocity in CuCl

Dynamic behaviour of the polariton-polariton resonant scattering process in CuCl was observed in a forward scattering configuration using a picosecond tunable source. The propagation times of the polariton in the crystal was estimated at various energies near the transverse exciton. They are well fitted with the theoretical group velocity calculated from the dispersion curve of the polariton.     

Electron dynamics in modulation p-doped InGaAs/GaAs quantum dots

We investigate the electron dynamics of p-type modulation doped and undoped InGaAs/GaAs quantum dots using up-conversion photoluminescence at low temperature and room temperature. The rise time of the p-doped sample is significantly shorter than that of the undoped at low temperature. With increasing to room temperature the undoped sample exhibits a decreased rise time whilst that of the doped sample does not change. A relaxation mechanism of electron-hole scattering is proposed in which the doped quantum dots exhibit an enhanced and temperature independent relaxation due to excess built-in holes in the valence band of the quantum dots. In contrast, the rise time of the undoped quantum dots decreases significantly at room temperature due to the large availability of holes in the ground state of the valence band. Furthermore, modulation p-doping results in a shorter lifetime due to the presence of excess defects.     

Polarization control of the nonlinear emission of semiconductor microcavities

The degree of circular polarization ( Weierstrass p ) of the nonlinear emission in semiconductor microcavities is controlled by changing the exciton-cavity detuning. The polariton relaxation towards K approximately 0 cavitylike states is governed by final-state stimulated scattering. The helicity of the emission is selected due to the lifting of the degeneracy of the +/-1 spin levels at K approximately 0. At short times after a pulsed excitation Weierstrass p reaches very large values, either positive or negative, as a result of stimulated scattering to the spin level of lowest energy (+1/-1 spin for positive/negative detuning).     

Open-system density matrix description of an STM-driven atomic switch: H on Si(100)

Previous experiments indicate that an STM (scanning tunnelling microscope) can be used to switch a hydrogen atom at a partially hydrogen-covered Si(100)-2×1 surface, from one Si atom of a Si dimer to a neighbouring, empty Si site [U.J. Quaade et al., Surf. Sci. 415, L1037, 1998]. It has been suggested that the switching occurs via a transient positive ion resonance state. In an earlier paper, we have examined the switching process for the “above threshold” regime when the bias is large enough to directly populate the positive ion resonance. In the present paper we study the “below threshold” regime instead, where the switching is more appropriately modelled as a ladder climbing over the barrier, in the ground electronic state. For this purpose we solve the Liouville–von Neumann equation in Lindblad form, describing a switching H atom on a Si dimer. STM-induced transition rates between vibrational levels are estimated from cluster calculations, assuming contributions both from a dipole and a resonance scattering mechanism. Vibrational relaxation is also included, as well as finite temperature and field effects. The switching rate in a current regime of about 1 to 10 nA scales highly non-linearly with current, and it is found to be governed by vibrational “ladder climbing” and subsequent tunnelling through the top of the ground state barrier. Multi-phonon processes also play a role. As a result of tunnelling, pronounced isotope effects are observed when replacing H with D. It is further argued that resonance-mediated inelastic scattering dominates over dipole excitation, and that the STM switch is stable also at room temperature.     

Long-lived two-dimensional triplet magnetoexcitons in a Hall insulator

An experimental technique is developed to perform photoexcitation of an ensemble of translationinvariant triplet excitons, to manipulate this ensemble, and to detect the properties of its components. In particular, the influence of temperature on the radiationless decay during the relaxation of an exciton spin into the ground state of a Hall insulator at a filling factor ν = 2 is studied. The generation of photoexcited electrons and holes is controlled using photoinduced resonance reflection spectra, which makes it possible to estimate the density of light-generated electron–hole pairs and to independently control the self-consistent generation of electrons at the first Landau level and holes (vacancies) at the ground (zero) cyclotron electronic level. The existence of triplet excitons is established from inelastic light scattering spectra, which are used to determine the singlet–triplet exciton splitting. The lifetimes of triplet excitons, which are closely related to the relaxation time of an electron spin, are extremely long: they reach 100 μs in perfect GaAs/AlGaAs heterostructures with a high mobility of two-dimensional electrons at low temperatures. These long spin relaxation times are qualitatively explained, and the expected collective behavior of high-density triplet magnetoexcitons at sufficiently low temperatures, which is related to their Bose nature, is discussed.     

Direct spin–phonon coupling of spin-flip relaxation in quantum dots

Within the frame of the Pavlov–Firsov spin–phonon coupling model, we study the spin-flip assisted by the acoustical phonon scattering between the first-excited state and the ground state in quantum dots. We analyze the behaviors of the spin relaxation rates as a function of an external magnetic field and lateral radius of quantum dot. The different trends of the relaxation rates depending on the magnetic field and lateral radius are obtained, which may serve as a channel to distinguish the relaxation processes and thus control the spin state effectively.     

Spin-flip relaxation in a two-electron quantum dot with spin-phonon coupling

We study the spin-flip process from the first excited state to the ground state due to the spin-phonon coupling in a two-electron quantum dot in the presence of a magnetic field. We give several possible relaxation channels before and after the crossing of the Zeeman sublevels. Our results show that the Coulomb interactions between the electrons of different channels play quite different roles and thus inducing different spin relaxation behaviors.     

锐钛矿和金红石二氧化钛中光生载流子的超快动力学：电声相互作用研究

本文结合玻尔兹曼输运方程和电声散射速率计算研究锐钛矿和金红石二氧化钛中光生载流子的超快动力学过程. 其中,动力学模拟所需的结构参数均通过第一性原理计算获得. 结果表明,由于存在强Fr?hlich型电声耦合,纵光学声子模对两个晶相的能量弛豫过程均有十分显著的影响,但是两个晶相的弛豫机理却表现出明显的差异. 对于单条导带内的弛豫过程,锐钛矿和金红石的能量弛豫时间分别为24.0 fs和11.8 fs,前者约为后者的二倍. 这一差异来源于两个晶相中不同的电子扩散分布以及不同的声学模散射贡献. 对于涉及多条导带的弛豫过程,预测的锐钛矿和金红石的总体弛豫时间分别为47 fs和57 fs,其相对大小与单条导带的情况相反. 分析表明金红石相弛豫较慢是因为存在多个速率控制步骤. 这些发现为调控电子动力学以及设计高效的二氧化钛器件提供了有价值的信息.     

Carrier capture and relaxation through a continuum background in InAs quantum dots

Carrier capture and relaxation in self-assembled InAs/GaAs quantum dots (QDs) have been studied, using bleaching rise time measurements for both the ground state (GS) and the first excited state (ES) transition, as a function of temperature (5, 77 and 293 K) and excitation density. We surprisingly observe that the bleaching rise time is longer for the ES than for the GS, indicating that the ES does not act as an intermediate state. At intermediate excitation density where the carrier relaxation is usually explained by Auger scattering, we still observe a temperature dependence pointing towards a single phonon emission process. For high excitation density, we observe a temperature-dependent plateau in the initial bleaching rise time, contradicting an Auger scattering-based relaxation model. Both these experimental results point towards a relaxation through the continuum background, followed by a single LO-phonon emission towards the QD GS.     

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.