Exciton dynamics in GaAs quantum wells under resonant excitation

Room-Temperature Exciton-Polariton-Driven Self-Phase Modulation in Planar Perovskite Waveguides

Optical nonlinearities are crucial for advanced photonic technologies since they allow photons to be managed by photons. Exciton-polaritons resulting from strong light-matter coupling are hybrid in nature: they combine the small mass and high coherence of photons with strong nonlinearity enabled by excitons, making them ideal for ultrafast all-optical manipulations. Among the most prospective polaritonic materials are halide perovskites since they require neither cryogenic temperatures nor expensive fabrication techniques. Here, we study strikingly nonlinear self-action of ultrashort polaritonic pulses propagating in planar MAPbBr3 perovskite slab waveguides. Tuning the input pulse energy and central frequency, we experimentally observe various scenarios of its nonlinear evolution in the spectral domain, which include peak shifts, narrowing, or splitting driven by self-phase modulation, group velocity dispersion, and self-steepening. The theoretical model provides complementary temporal traces of pulse propagation and reveals the transition from the birth of a doublet of optical solitons to the formation of a shock wave, both supported by the system. Our results presented here represent an important step in ultrafast nonlinear on-chip polaritonics in perovskite-based systems.

Carrier dynamics in (Ga,In)(Sb,Bi)/GaSb quantum wells for laser applications in the mid-infrared spectral range

We present experimental studies on low-temperature ([Formula: see text]) carrier dynamics in (Ga,In)(Sb,Bi)/GaSb quantum wells (QWs) with the nominal In content of 3.7% and the Bi ranging from 6 to 8%. The photoreflectance experiment revealed the QW bandgap evolution with [Formula: see text] % Bi, which resulted in the bandgap tunability roughly between 629 and [Formula: see text], setting up the photon emission wavelength between 1.97 and [Formula: see text]. The photoluminescence experiment showed a relatively small 3-10[Formula: see text] Stokes shift regarding the fundamental QW absorption edge, indicating the exciton localisation beneath the QW mobility edge. The localised state's distribution, being the origin of the PL, determined carrier dynamics in the QWs probed directly by the time-resolved photoluminescence and transient reflectivity. The intraband carrier relaxation time to the QW ground state, following the non-resonant excitation, occurred within 3-25[Formula: see text] and was nearly independent of the Bi content. However, the interband relaxation showed a strong time dispersion across the PL emission band and ranging nearly between 150 and [Formula: see text], indicating the carrier transfer among the localised state's distribution. Furthermore, the estimated linear dispersion variation parameter significantly decreased from [Formula: see text] to [Formula: see text] with increasing the Bi content, manifested the increasing role of the non-radiative recombination processes with Bi in the QWs.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS₂, WS₂, MoSe₂, and WSe₂, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.

Lifetime-resolved photon-correlation Fourier spectroscopy

The excited state population of single solid-state emitters is subjected to energy fluctuations around the equilibrium driven by the bath and relaxation through the emission of phonons or photons. Simultaneous measurement of the associated spectral dynamics requires a technique with a high spectral and temporal resolution with an additionally high temporal dynamic range. We propose a pulsed excitation-laser analog of photon-correlation Fourier spectroscopy (PCFS), which extracts the linewidth and spectral diffusion dynamics along the emission lifetime trajectory of the emitter, effectively discriminating spectral dynamics from relaxation and bath fluctuations. This lifetime-resolved PCFS correlates photon-pairs at the output arm of a Michelson interferometer in both their time-delay between laser-excitation and photon-detection T and the time-delay between two photons τ. We propose the utility of the technique for systems with changing relative contributions to the emission from multiple states, for example, quantum emitters exhibiting phonon-mediated exchange between different fine-structure states.

Transient circular dichroism and exciton spin dynamics in all-inorganic halide perovskites

All-inorganic metal halides perovskites (CsPbX₃, X = Br or Cl) show strong excitonic and spin-orbital coupling effects, underpinning spin-selective excitonic transitions and therefore exhibiting great promise for spintronics and quantum-optics applications. Here we report spin-dependent optical nonlinearities in CsPbX₃ single crystals by using ultrafast pump-probe spectroscopy. Many-body interactions between spin-polarized excitons act like a pseudo-magnetic field and thus lift the degeneracy of spin states resulting in a photoinduced circular dichroism. Such spontaneous spin splitting between “spin-up” and “spin-down” excitons can be several tens of milli-electron volts under intense excitations. The exciton spin relaxation time is ~20 picoseconds at very low pump fluence, the longest reported in the metal halides perovskites family at room temperature. The dominant spin-flip mechanism is attributed to the electron-hole exchange interactions. Our results provide essential understandings towards realizing practical spintronics applications of perovskite semiconductors.

Strong excitonic effects and spin-orbit coupling in all-inorganic halide perovskite is promising for spintronic application, yet the spin-dependent phenomenon is not well understood. Here, the authors reveal that many-body interactions between spin-polarized excitons act like pseudo-magnetic field, lifting the degeneracy and resulting in circular dichroism.

Measurement of the spin-forbidden dark excitons in MoS2 and MoSe2 monolayers

Excitons with binding energies of a few hundreds of meV control the optical properties of transition metal dichalcogenide monolayers. Knowledge of the fine structure of these excitons is therefore essential to understand the optoelectronic properties of these 2D materials. Here we measure the exciton fine structure of MoS2 and MoSe2 monolayers encapsulated in boron nitride by magneto-photoluminescence spectroscopy in magnetic fields up to 30 T. The experiments performed in transverse magnetic field reveal a brightening of the spin-forbidden dark excitons in MoS2 monolayer: we find that the dark excitons appear at 14 meV below the bright ones. Measurements performed in tilted magnetic field provide a conceivable description of the neutral exciton fine structure. The experimental results are in agreement with a model taking into account the effect of the exchange interaction on both the bright and dark exciton states as well as the interaction with the magnetic field.

Observation of chiral surface excitons in a topological insulator Bi2Se3

The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin-orbit interaction in solids composed of heavy elements. Here, we study the composite particles-chiral excitons-formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical 3D TI, [Formula: see text] Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate from the (apparent) optical band gap by hundreds of millielectronvolts, and that the orientation remains preserved even at room temperature. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin-orbit coupling. A theoretical model based on this proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin-orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics.

How pump-probe differential reflectivity at negative delay yields the perturbed-free-induction-decay: theory of the experiment and its verification

We present a simple but mathematically complete first-principles theory for the pump-probe differential reflectivity experiment at negative delay (probe preceding the pump) to show how it gives information about the perturbed-free-induction-decay of coherent polarization. The calculation, involving the optical Bloch equations to describe the induced polarization and the Ewald-Oseen idea to calculate the reflected signal as a consequence of the free oscillations of perturbed dipoles, also explicitly includes the process of lock-in detection of a double-chopped signal after it has passed through a monochromator. The theory giving a closed form expression for the measured signal in both time and spectal domains is compared with experiments on high quality GaAs quantum well sample. The dephasing time inferred experimentally at 4 K compares remarkably well with the inverse of the absorption linewidth of the continuous-wave photoluminescence excitation spectrum. Spectrally-resolved signal at negative delay calculated from our theoretical expression nicely reproduces the coherent spectral oscillations observed in our experiments, although exact fitting of the experimental spectra with the theoretical expression is difficult on account of multiple resonances.

Anti-Stokes photoluminescence probing k-conservation and thermalization of minority carriers in degenerately doped semiconductors

It has recently been found that anti-Stokes photoluminescence can be observed in degenerately n-doped indium phosphide nanowires, when exciting directly into the electron gas. This anti-Stokes mechanism has not been observed before and allows the study of carrier relaxation and recombination using standard photoluminescence techniques. It is important to know if this anti-Stokes photoluminescence also occurs in bulk semiconductors as well as its relation to carrier recombination and relaxation. Here we show that similar anti-Stokes photoluminescence can indeed be observed in degenerately doped bulk indium phosphide and gallium arsenide and is caused by minority carriers scattering to high momenta by phonons. We find in addition that the radiative electron-hole recombination is highly momentum-conserving and that photogenerated minority carriers recombine before relaxing to the band edge at low temperatures. These observations challenge the use of models assuming thermalization of minority carriers in the analysis of highly doped devices.

Anti-Stokes luminescence - the emission of photons with higher energy than those absorbed – in nanomaterials is widely used for optoelectronic applications. Here the authors observe it in degenerately doped bulk InP and GaAs, indicating it as a more general property of direct bandgap semiconductors.

Dark Solitons in High Velocity Waveguide Polariton Fluids

We study exciton-polariton nonlinear optical fluids in the high momentum waveguide regime for the first time. We demonstrate the formation of dark solitons with the expected dependence of width on fluid density for both main classes of soliton-forming fluid defects. The results are well described by numerical modeling of the fluid propagation. We deduce a continuous wave nonlinearity more than ten times that on picosecond time scales, arising due to interaction with the exciton reservoir.

Bose-Einstein condensation and indirect excitons: a review

We review recent progress on Bose-Einstein condensation (BEC) of semiconductor excitons. The first part deals with theory, the second part with experiments. This Review is written at a time where the problem of exciton Bose-Einstein condensation has just been revived by the understanding that the exciton condensate must be dark because the exciton ground state is not coupled to light. Here, we theoretically discuss this missed understanding before providing its experimental support through experiments that scrutinize indirect excitons made of spatially separated electrons and holes. The theoretical part first discusses condensation of elementary bosons. In particular, the necessary inhibition of condensate fragmentation by exchange interaction is stressed, before extending the discussion to interacting bosons with spin degrees of freedom. The theoretical part then considers composite bosons made of two fermions like semiconductor excitons. The spin structure of the excitons is detailed, with emphasis on the crucial fact that ground-state excitons are dark: indeed, this imposes the exciton Bose-Einstein condensate to be not coupled to light in the dilute regime. Condensate fragmentations are then reconsidered. In particular, it is shown that while at low density, the exciton condensate is fully dark, it acquires a bright component, coherent with the dark one, beyond a density threshold: in this regime, the exciton condensate is 'gray'. The experimental part first discusses optical creation of indirect excitons in quantum wells, and the detection of their photoluminescence. Exciton thermalisation is also addressed, as well as available approaches to estimate the exciton density. We then switch to specific experiments where indirect excitons form a macroscopic fragmented ring. We show that such ring provides efficient electrostatic trapping in the region of the fragments where an essentially-dark exciton Bose-Einstein condensate is formed at sub-Kelvin bath temperatures. The macroscopic spatial coherence of the photoluminescence observed in this essentially dark region confirms this conclusion.

Interfacially Al-doped ZnO nanowires: greatly enhanced near band edge emission through suppressed electron-phonon coupling and confined optical field

Aluminium (Al)-doped zinc oxide (ZnO) nanowires (NWs) with a unique core-shell structure and a Δ-doping profile at the interface were successfully grown using a combination of chemical vapor deposition re-growth and few-layer Al_xO_y atomic layer deposition. Unlike the conventional heavy doping which degrades the near-band-edge (NBE) luminescence and increases the electron-phonon coupling (EPC), it was found that there was an over 20-fold enhanced NBE emission and a notably-weakened EPC in this type of interfacially Al-doped ZnO NWs. Further experiments revealed a greatly suppressed nonradiative decay process and a much enhanced radiative recombination rate. By comparing the finite-difference time-domain simulation with the experimental results from intentionally designed different NWs, this enhanced radiative decay rate was attributed to the Purcell effect induced by the confined and intensified optical field within the interfacial layer. The ability to manipulate the confinement, transport and relaxation dynamics of ZnO excitons can be naturally guaranteed with this unique interfacial Δ-doping strategy, which is certainly desirable for the applications using ZnO-based nano-photonic and nano-optoelectronic devices.

Exciton Dynamics in Monolayer Transition Metal Dichalcogenides

Since the discovery of semiconducting monolayer transition metal dichalcogenides, a variety of experimental and theoretical studies have been carried out seeking to understand the intrinsic exciton population recombination and valley relaxation dynamics. Reports of the exciton decay time range from hundreds of femtoseconds to ten nanoseconds, while the valley depolarization time can exceed one nanosecond. At present, however, a consensus on the microscopic mechanisms governing exciton radiative and non-radiative recombination is lacking. The strong exciton oscillator strength resulting in up to ~ 20% absorption for a single monolayer points to ultrafast radiative recombination. However, the low quantum yield and large variance in the reported lifetimes suggest that non-radiative Auger-type processes obscure the intrinsic exciton radiative lifetime. In either case, the electron-hole exchange interaction plays an important role in the exciton spin and valley dynamics. In this article, we review the experiments and theory that have led to these conclusions and comment on future experiments that could complement our current understanding.

Photon upconversion in degenerately sulfur doped InP nanowires

Radiative recombination in degenerately n-doped InP nanowires is studied for excitation above and below the Fermi energy of the electron gas, using photoluminescence. Laser-induced electron heating is observed, which allows absorption below the Fermi energy. We observe photon upconversion where photo-excited holes recombine with high |k| electrons. This can be attributed to hole scattering to high |k|-values, and the temperature dependence of this process is measured. We show that hole relaxation via phonon scattering can be observed in continuous wave excitation luminescence measurements.

Exciton dynamics in semiconductor nanocrystals

This review article provides an overview of recent advances in the study and understanding of dynamics of excitons in semiconductor nanocrystals (NCs) or quantum dots (QDs). Emphasis is placed on the relationship between exciton dynamics and optical properties, both linear and nonlinear. We also focus on the unique aspects of exciton dynamics in semiconductor NCs as compared to those in bulk crystals. Various experimental techniques for probing exciton dynamics, particularly time-resolved laser methods, are reviewed. Relevant models and computational studies are also briefly presented. By comparing different materials systems, a unifying picture is proposed to account for the major dynamic features of excitons in semiconductor QDs. While the specific dynamic processes involved are material-dependent, key processes can be identified for all the materials that include electronic dephasing, intraband relaxation, trapping, and interband recombination of free and trapped charge carriers (electron and hole). Exciton dynamics play a critical role in the fundamental properties and functionalities of nanomaterials of interest for a variety of applications including optical detectors, solar energy conversion, lasers, and sensors. A better understanding of exciton dynamics in nanomaterials is thus important both fundamentally and technologically.

EXCITON MIGRATION IN DENDRITIC AGGREGATE SYSTEMS USING THE QUANTUM MASTER EQUATION APPROACH INVOLVING WEAK EXCITON-PHONON COUPLING

The quantum master equation approach involving a weak exciton-phonon coupling is applied to the exciton migration dynamics of dendritic molecular aggregates modeled after a phenylacetylene dendrimer, D25, which exhibits an efficient light-harvesting property. The mechanism of efficient exciton migration from the periphery to the core is studied by analyzing relaxation terms among the exciton states originating in weak exciton-phonon coupling. Partial overlaps of exciton distributions between neighboring exciton states are found to be important for realizing the unique migration behavior by step-by-step transfer from the periphery to the core via multi-step exciton states. The same calculation method is applied to the exciton dynamics of a larger dendritic aggregate model, D127, modeled after the largest synthesized phenylacetylene dendrimer D127 in order to examine the dependencies of the exciton migration on the strength of the intermolecular interaction and the temperature of phonon bath. In the case of the relatively weak dipole-dipole coupling, exciton is not observed to migrate efficiently from the periphery to the core, while the largest exciton population is found to remain in the intermediate generations. This is ascribed to the fact that the thermal excitation to the higher exciton states significantly contributes to the exciton distribution in the equilibrium state when the weak intermolecular interaction reduces the energy difference between exciton states. This indicates that the intermolecular interaction is important not only for the overlaps of the exciton distributions between the exciton states, but also the relation between the exciton energy differences and thermal excitations, which spoil the distinct concentration of exciton distribution in the core generation. In the case of a low temperature, even if the intermolecular interaction is weak, the exciton population in the core region is found to be the largest in all the generations. This suggests that exciton tends to efficiently migrate from the periphery to the core when the temperature is sufficiently low. Implications of these theoretical results are discussed in relation to design of magneto-optical materials and other technological applications.

Polarization bistability and resultant spin rings in semiconductor microcavities

The transmission of a pump laser resonant with the lower polariton branch of a semiconductor microcavity is shown to be highly dependent on the degree of circular polarization of the pump. Spin dependent anisotropy of polariton-polariton interactions allows the internal polarization to be controlled by varying the pump power. The formation of spatial patterns, spin rings with a high degree of circular polarization, arising as a result of polarization bistability, is observed. A phenomenological model based on effective semiclassical equations of motion provides a good description of the experimental results. Inclusion of interactions with the incoherent exciton reservoir, which provides spin-independent blueshifts of the polariton modes, is found to be essential.

Electron-hole overlap dictates the hole spin relaxation rate in nanocrystal heterostructures

Hole spin relaxation dynamics in CdTe/CdSe core-shell nanocrystals are measured by an ultrafast polarization transient grating technique. Photoexcited charge separation in type II structures suppresses the electron-hole exchange interaction and the hole spin relaxation time constant is found to increase from ∼0.3  ps to ∼10  ps at 293 K as the CdSe shell thickness increases from ∼0.2  nm to ∼2.4  nm. Analysis of these data suggests that spin relaxation in semiconductor nanostructures is tunable between type I and type II localization according to an electron-hole overlap function.

Spin transport of excitons

We report on observation of the spin transport of spatially indirect excitons in GaAs/AlGaAs coupled quantum wells (CQW). Exciton spin transport over substantial distances, up to several micrometers in the present work, is achieved due to orders of magnitude enhancement of the exciton spin relaxation time in CQW with respect to conventional quantum wells.

Exciton fine structure and spin relaxation in semiconductor colloidal quantum dots

Quantum dots (QDs) have discrete quantum states isolated from the environment, making QDs well suited for quantum information processing. In semiconductor QDs, the electron spins can be coherently oriented by photoexcitation using circularly polarized light, creating optical orientation. The optically induced spin orientation could serve as a unit for data storage and processing. Carrier spin orientation is also envisioned to be a key component in a related, though parallel, field of semiconductor spintronics. However, the oriented spin population rapidly loses its coherence by interaction with the environment, thereby erasing the prepared information. Since long-lasting spin orientation is desirable in both areas of investigation, spin relaxation is the central focus of investigation for optimization of device performance. In this Account, we discuss a topic peripherally related to these emerging areas of investigation: exciton fine structure relaxation (EFSR). The radiationless transition occurring in the exciton fine structure not only highlights a novel aspect of QD exciton relaxation but also has implications for carrier spin relaxation in QDs. We focus on examining the EFSR in connection with optical spin orientation and subsequent ultrafast relaxation of electron and hole spin densities in the framework of the exciton fine structure basis. Despite its significance, the study of exciton fine structure in colloidal QDs has been hampered by the experimental challenge arising from inhomogeneous line broadening that obscures the details of closely spaced fine structure states in the frequency domain. In this Account, we show that spin relaxation occurring in the fine structure of CdSe QDs can be probed by a time-domain nonlinear polarization spectroscopy, circumventing the obstacles confronted in the frequency-domain spectroscopy. In particular, by combining polarization sequences of multiple optical pulses with the unique optical selection rules of semiconductors, fast energy relaxation among the QD exciton fine structure states is selectively measured. The measured exciton fine structure relaxation, which is a nanoscale analogue of molecular radiationless transitions, contains direct information on the relaxation of spin densities of electron and hole carriers, that is, spin relaxation in QDs. From the exciton fine structure relaxation rates measured for CdSe nanorods and complex-shaped nanocrystals using nonlinear polarization spectroscopy, we elucidated the implications of QD size and shape on the QD exciton properties as well, for example, size- and shape-scaling laws governing exciton spin flips and how an exciton is delocalized in a QD. We envision that the experimental development and the discoveries of QD exciton properties presented in this Account will inspire further studies toward revealing the characteristics of QD excitons and spin relaxation therein, for example, spin relaxation in QDs made of various materials with different electronic structures, spin relaxation under an external perturbation of QD electronic states using magnetic fields, and spin relaxation of separated electrons and holes in type-II QD heterostructures.

Mechanism and Origin of Exciton Spin Relaxation in CdSe Nanorods

The dynamics of exciton spin relaxation in CdSe nanorods of various sizes and shapes are measured by an ultrafast transient polarization grating technique. The measurement of the third-order transient grating (3-TG) signal utilizing linear cross-polarized pump pulses enables us to monitor the history of spin relaxation among the bright exciton states with a total angular momentum of F = +/-1. From the measured exciton spin relaxation dynamics, it is found that the effective mechanism of exciton spin relaxation is sensitive to the size of the nanorod. Most of the measured cross-polarized 3-TG signals show single-exponential spin relaxation dynamics, while biexponential spin relaxation dynamics are observed in the nanorod of the largest diameter. This analysis suggests that a direct exciton spin flip process between the bright exciton states with F = +/-1 is the dominant spin relaxation mechanism in small nanocrystals, and an indirect spin flip via the dark states with F = +/-2 contributes as the size of the nanocrystal increases. This idea is examined by simulations of 3-TG signals with a kinetic model for exciton spin relaxation considering the states in the exciton fine structure. Also, it is revealed that the rate of exciton spin relaxation has a strong correlation with the diameter, d, of the nanorod, scaled by the power law of 1/d4, rather than other shape parameters such as length, volume, or aspect ratio.

Nanocrystal shape and the mechanism of exciton spin relaxation

The rate of exciton spin relaxation (flips) between the bright exciton states (F = +/-1) of CdSe nanocrystals is reported as a function of shape, for dots and nanorods. The spin relaxation is measured using an ultrafast transient grating method with a crossed linearly polarization sequence. It is found that the spin relaxation rate depends on the radius, not length, of the nanocrystals. That observation is explained by deriving an expression for the electronic coupling matrix element that mixes the bright exciton states.

Dynamics within the Exciton Fine Structure of Colloidal CdSe Quantum Dots

Evidence for an interaction between the quantum dot exciton fine structure states F = +/-1 is obtained by measuring the dynamics of transitions among those states, exciton spin relaxation or flipping. An ultrafast transient grating experiment based on a crossed-linear polarization grating is reported. By using the quantum dot selection rules for absorption of circularly polarized light, it is demonstrated that it is possible to detect transitions between nominally degenerate fine structure states, even in a rotationally isotropic system. The results for colloidal CdSe quantum dots reveal a strong size dependence for the exciton spin relaxation rate from one bright exciton state (F = +/-1) to the other in CdSe colloidal quantum dots at 293 K, on a time scale ranging from femtoseconds to picoseconds, depending on the quantum dot size. The results are consistent with an interaction between those states attributed to a long-range contribution to the electron-hole exchange interaction.

Vanishing and emerging of absorption quantum beats from electron spin coherence in GaAs quantum wells

We report experimental studies of absorption quantum beats induced by electron spin coherence in GaAs quantum wells. Absorption quantum beats occur for strongly localized excitons, but nearly vanish for mobile excitons in the third order nonlinear optical response. Pronounced quantum beats for mobile excitons emerge in an unusual fifth order process. These results, along with a qualitative analysis based on the use of N-exciton eigenstates, elucidate how the manifestation of electron spin coherence in the excitonic nonlinear optical response can differ fundamentally from that in an atomic system.

Spin relaxation quenching in semiconductor quantum dots

We have studied the spin dynamics in self-organized InAs/GaAs quantum dots by time-resolved photoluminescence performed under strictly resonant excitation. At low temperature, we observe strictly no decay of both the linear and the circular luminescence polarization. This demonstrates that the carrier spins are totally frozen on the exciton lifetime scale.

Homogeneous Linewidths in the Optical Spectrum of a Single Gallium Arsenide Quantum Dot

The homogeneous linewidths in the photoluminescence excitation spectrum of a single, naturally formed gallium arsenide (GaAs) quantum dot have been measured with high spatial and spectral resolution. The energies and linewidths of the homogeneous spectrum provide a new perspective on the dephasing dynamics of the exciton in a quantum-confined, solid-state system. The origins of the linewidths are discussed in terms of the dynamics of the exciton in zero dimensions, in particular, in terms of lifetime broadening through the emission or absorption of phonons and photons.