Exciton Spin Relaxation Research Articles

Time-resolved exciton photoluminescence and its linear polarization of an ensemble of CdSe/CdS colloidal nanoplatelets

We present experimental and theoretical studies of the time-resolved photoluminescence and its linear polarization of an ensemble of CdSe/CdS colloidal nanoplatelets in the Faraday magnetic fields up to 6 T. The linearly polarized photoluminescence kinetics stems from the excitons resonantly excited with the linearly polarized light pulse and comprises both the structural contribution and the exciton optical alignment effect. The emission from the bright and dark exciton states is distinguished by their different intensity decay timescales with lifetime at cryogenic temperature of about 1–2ns and 100–200ns, respectively. The longitudinal spin relaxation time of the bright exciton was determined to be from 4 to 6ns depending on the magnetic field. The dark exciton spin relaxation time was estimated to be in the order of 20 to 100ns. The developed theory for the bright exciton polarization kinetics and the modeling of the data allowed us to conclude that spin dephasing times of the bright as well as of the dark exciton are shorter than 0.3ns. The origin of such a strong spin relaxation time anisotropy in the ensemble of nanoplatelets is discussed.

Ultrafast Spin Relaxation of Charge Carriers in Strongly Quantum Confined Methylammonium Lead Bromide Perovskite Magic-Sized Clusters.

Spin relaxation of charge carriers in strongly quantum confined perovskite magic-sized clusters has been probed, for the first time, by using polarization-controlled femtosecond transient absorption (fs-TA) spectroscopy. Fs-TA measurements with a circularly polarized pump and probe allowed for the determination of the exciton spin relaxation lifetime (∼1.5 ps) at room temperature based on the dynamics of a photoinduced absorption (PIA) feature peaked at 458 nm. This spin lifetime is shorter than that of perovskite quantum dots (PQDs) with a larger size, and the results suggest that exciton confinement and defects likely play a more important role in these strongly quantum confined magic-sized clusters with a larger surface-to-volume ratio.

Challenges in Developing Perovskite Nanocrystals for Commercial Applications.

Zero-dimensional lead halide perovskite nanocrystals (NCs) exhibit size-dependent bandgap and carrier confinement compared to bulk counterparts due to the quantum confinement effect, making them essential for achieving wide-color-gamut displays, studying excitonic spin relaxation, and constructing superlattices. Despite their promising potential, they face a variety of technical bottlenecks, such as insufficient color reproducibility, limited large-scale production, low stability, and toxicity. An outline of a research roadmap is provided in the review, which highlights key challenges in developing perovskite NCs for commercial applications.

Orbital angular momentum swapping of light via biexciton coherence

In this paper we study orbital angular momentum (OAM) transfer between applied lights via biexciton coherence and exciton spin coherence. The quantum dot derives by two control fields that couple to a biexciton state which leads to the destructive interference, while a weak probe light interacts by one of the ground-level exciton transitions. In this scenario, a new weak signal light can be generated via the four-wave mixing (FWM) mechanism in a second ground-level exciton transition. We will study the role of exciton spin relaxation as well as intensity of coupling lights on the exchange efficiency of FWM mechanism. We will also discuss the spatially dependent optical effects via OAM state and azimuthal angle of vortex light.

Energy and spin relaxation of excitons in CdMnTe/CdMgTe quantum wells

The energy and spin relaxation of excitons and trions is studied in the (Cd, Mn)Te/(Cd,Mg)Te quantum wells containing hole magnetic polarons by means of polarized photoluminescence technique. It was found that under circularly polarized photoexcitation with an energy higher than the ground state energy of the exciton, photoluminescence is also polarized. This fact points to the presence of the spin memory of the excitons in these systems. It is suggested that the long spin memory is due to the magnetic polaron formation stabilizing the spin of the exciton. Under quasiresonant excitation of the trion states an inverted sign of circular polarization degree is observed in the radiation spectrum. Such inversion of the polarization degree requires the special mechanism of the spin relaxation, associated with the spin flip of an electron in the trion magnetic polaron state. Application of the external magnetic field in the Voigt geometry leads to the depolarization of the excitonic radiation. The cause of this depolarization is the destabilization of the magnetic polaron and the shortening of the spin relaxation time of the exciton.

Rapidly expanding spin-polarized exciton halo in a two-dimensional halide perovskite at room temperature.

Monitoring of the spatially resolved exciton spin dynamics in two-dimensional semiconductors has revealed the formation of a spatial pattern and long-range transport of the spin-polarized excitons, which holds promise for exciton-based spin-optoelectronic applications. However, the spatial evolution has been restricted to cryogenic temperatures because of the short exciton spin relaxation times at room temperature. Here, we report that two-dimensional halide perovskites can overcome this limitation owing to their relatively long exciton spin relaxation times and substantial exciton-exciton interactions. We demonstrate the emergence of a halo-like spatial profile in spin-polarized exciton population and its ultrafast expansion at room temperature by performing time-resolved Faraday rotation imaging of spin-polarized excitons in two-dimensional perovskite (C4H9NH3)2(CH3NH3)3Pb4I13. Exciton-exciton exchange interactions induce density-dependent nonlinear relaxation and ultrafast transport of exciton spins and give rise to a rapidly expanding halo-like spatial pattern. The density-dependent spatial control suggests the potential of using two-dimensional halide perovskites for spin-optoelectronic applications.

Optical control of exciton spin dynamics in layered metal halide perovskites via polaronic state formation

One of the open challenges of spintronics is to control the spin relaxation mechanisms. Layered metal-halide perovskites are an emerging class of semiconductors which possess a soft crystal lattice that strongly couples electronic and vibrational states and show promise for spintronic applications. Here, we investigate the impact of such strong coupling on the spin relaxation of excitons in the layered perovskite BA2FAPbI7 using a combination of cryogenic Faraday rotation and transient absorption spectroscopy. We report an unexpected increase of the spin lifetime by two orders of magnitude at 77 K under photoexcitation with photon energy in excess of the exciton absorption peak, and thus demonstrate optical control over the dominant spin relaxation mechanism. We attribute this control to strong coupling between excitons and optically excited phonons, which form polaronic states with reduced electron-hole wave function overlap that protect the exciton spin memory. Our insights highlight the special role of exciton-lattice interactions on the spin physics in the layered perovskites and provide a novel opportunity for optical spin control.

Plasmonic Nanocavity Induced Coupling and Boost of Dark Excitons in Monolayer WSe2 at Room Temperature.

Spin-forbidden excitons in monolayer transition metal dichalcogenides are optically inactive at room temperature. Probing and manipulating these dark excitons are essential for understanding exciton spin relaxation and valley coherence of these 2D materials. Here, we show that the coupling of dark excitons to a metal nanoparticle-on-mirror cavity leads to plasmon-induced resonant emission with the intensity comparable to that of the spin-allowed bright excitons. A three-state quantum model combined with full-wave electrodynamic calculations reveals that the radiative decay rate of the dark excitons can be enhanced by nearly 6 orders of magnitude through the Purcell effect, therefore compensating its intrinsic nature of weak radiation. Our nanocavity approach provides a useful paradigm for understanding the room-temperature dynamics of dark excitons, potentially paving the road for employing dark exciton in quantum computing and nanoscale optoelectronics.

Tuning Spin-Polarized Lifetime in Two-Dimensional Metal-Halide Perovskite through Exciton Binding Energy.

Metal-halide perovskite semiconductors have attracted attention for opto-spintronic applications where the manipulation of charge and spin degrees of freedom have the potential to lower power consumption and achieve faster switching times for electronic devices. Lower-dimensional perovskites are of particular interest since the lower degree of symmetry of the metal-halide connected octahedra and the large spin-orbit coupling can potentially lift the spin degeneracy. To achieve their full application potential, long spin-polarized lifetimes and an understanding of spin-relaxation in these systems are needed. Here, we report an intriguing spin-selective excitation of excitons in a series of 2D lead iodide perovskite (n = 1) single crystals by using time- and polarization-resolved transient reflection spectroscopy. Exciton spin relaxation times as long as ∼26 ps at low excitation densities and at room temperature were achieved for a system with small binding energy, 2D EOA2PbI4 (EOA = ethanolamine). By tuning the excitation density and the exciton binding energy, we identify the dominant mechanism as the D'yakonov-Perel (DP) mechanism at low exciton densities and the Bir-Aronov-Pikus (BAP) mechanism at high excitation densities. Together, these results provide new design principles to achieve long spin lifetimes in metal-halide perovskite semiconductors.

Exciton recombination and spin relaxation in strong magnetic fields in ultrathin (In,Al)As/AlAs quantum wells with indirect band gap and type-I band alignment

The exciton recombination and spin dynamics are studied in monolayer-thick (In,Al)As/AlAs quantum wells characterized by an indirect band gap and a type-I band alignment. The exciton recombination time and the photoluminescence intensity are strongly dependent on strength and orientation of an applied magnetic field. In contrast to no effect of an in-plane field, at a temperature of 1.8 K a magnetic field applied parallel to the growth axis drastically slows down the recombination and reduces the intensity of photoluminescence. The magnetic-field-induced circular polarization of photoluminescence is studied as a function of the magnetic field strength and direction, as well as sample temperature. The observed nonmonotonic behavior of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. Taking into account the magnetic-field-induced redistribution of the indirect excitons between their bright and dark states, we evaluate the heavy-hole longitudinal $g$ factor of 3.6, the radiative recombination time for the bright excitons of 0.13 ms, and the nonradiative recombination time of the bright and dark excitons of 0.43 ms, as well as the spin relaxation times of electron of $25\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$ and heavy hole of $16\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$, bound in the exciton.

Dynamic polaronic screening for anomalous exciton spin relaxation in two-dimensional lead halide perovskites.

Two-dimensional lead halide perovskites with confined excitons have shown exciting potentials in optoelectronic applications. It is intriguing but unclear how the soft and polar lattice redefines excitons in layered perovskites. Here, we reveal the intrinsic exciton properties by investigating exciton spin dynamics, which provides a sensitive probe to exciton coulomb interactions. Compared to transition metal dichalcogenides with comparable exciton binding energy, we observe orders of magnitude smaller exciton-exciton interaction and, counterintuitively, longer exciton spin lifetime at higher temperature. The anomalous spin dynamics implies that excitons exist as exciton polarons with substantially weakened inter- and intra-excitonic interactions by dynamic polaronic screening. The combination of strong light matter interaction from reduced dielectric screening and weakened inter-/intra-exciton interaction from dynamic polaronic screening explains their exceptional performance and provides new rules for quantum-confined optoelectronic and spintronic systems.

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.

Recombination and spin dynamics of excitons in thin (Ga,Al)(Sb,As)/AlAs quantum wells with an indirect band gap and type-I band alignment

The dynamics of exciton recombination and spin relaxation in thin (Ga,Al)(Sb,As)/AlAs quantum wells (QWs) with indirect band gap are studied. The band alignment in these QWs is identified as type I. The exciton recombination time exceeds hundreds of microseconds, while the spin relaxation times of the electron and the heavy hole in an exciton do not exceed hundreds of nanoseconds. The heavy-hole longitudinal $g$ factor is determined to be $+2.5$. Despite the long exciton lifetimes, the photoluminescence circular polarization degree induced by a magnetic field is unexpectedly small and does not exceed $25%$.

Spin relaxation of indirect excitons in asymmetric coupled quantum wells

Density and spin dynamics of indirect excitons in asymmetric GaAs/AlGaAs coupled quantum wells is studied by time-resolved photoluminescence. Under electric bias applied in the growth direction the lifetime of indirect excitons formed by an electron in a wide and hole in a narrow quantum well reaches 12 ns, while the circular polarisation lifetime is about 5 ns. The structure is suited for further studies of dark indirect excitons by time-resolved pump-probe spectroscopy.

Influence of spin relaxation induced by molecular vibration on thermally activated delayed fluorescence

Abstract Thermally activated delayed fluorescence (TADF), an effective mechanism to break the 25% statistic limit of organic light-emitting diodes (OLEDs) internal quantum efficiency, has become an active topic recently. The key to germinate TADF is the achievement of efficient reverse intersystem crossing from triplet spin state to singlet state by thermal activation, which is obviously a temperature dependence process. The direct way of thermal activation is the absorption of phonon energy, in which the transition rate from triplet state to singlet state has the Boltzmann distribution function dependence of the temperature. Nevertheless, the molecular vibration could engender spin relaxation of excitons, giving rise to different temperature dependence. This could be regarded as an indirect way of thermal activation. Here, we investigate the effect of spin relaxation caused by molecular vibration on TADF and analyze the change principles of the efficiency of TADF processes versus temperature. It is found that the experimental dependence could be well explained when the spin relaxation induced by molecular vibration is considered. Therefore, the consideration of this process helps us to understand TADF more comprehensively.

Optical Spectroscopy of Dark and Bright Excitons in CdSe Nanocrystals in High Magnetic Fields

Low-temperature polarized and time-resolved photoluminescence spectroscopy in high magnetic fields (up to 30 T) has been used to study the spin-polarization, spin-relaxation, and radiative lifetimes of excitons in wurtzite semiconductor (e.g., CdSe) colloidal nanocrystals. The applied magnetic field leads to a significant degree of circular polarization of the exciton photoluminescence, accompanied by a reduction in the photoluminescence decay time. The circular polarization arises from the Zeeman splitting of exciton levels, whereas lifetime reduction results from a polarization-preserving field-induced mixing of exciton levels. We analyze these experimental findings in terms of a simple model that combines both Zeeman effect and exciton-level mixing, as a function of the relative orientation of the nanocrystal c-axis and the magnetic field. This model is able to simultaneously describe the degree of circular polarization and lifetime reduction of the exciton photoluminescence, permitting us to quantify ...

Investigation of dual electromagnetically induced grating based on spatial modulation in quantum well nanostructures via biexciton coherence

A new scheme for obtaining an electromagnetically induced grating (EIG) via biexciton coherence in quantum well nanostructures is developed. It is theoretically shown that exciton spin relaxation and biexciton binding energy have important roles in producing efficient dual electromagnetically induced phase grating. In this structure, due to biexciton coherence, the higher order diffraction intensities of the grating can be observed. Furthermore, it is shown that the efficiency of different orders in the grating patterns could be controlled by biexciton energy renormalization (ESR) and relative phase between the applied laser fields.

Long-lived magnetoexcitons in 2D-fermion system

The paper addresses the experimental technique that, when applied to a 2D-electron system in the integer quantum Hall regime with filling factor ν = 2 (the Hall insulating state), allows resonant excitation of magnetoexcitons, their detection, control of an ensemble of long-lived triplet excitons and investigation of their radiationless decay related to exciton spin relaxation into the ground state. The technique proposed enables independent control of photoexcited electrons and Fermi-holes using photoinduced resonance reflection spectra as well as estimate with a reasonable degree of accuracy the resulting density of photoinduced electron-hole pairs bound into magnetoexcitons. The mere existence of triplet excitons was directly established by inelastic light scattering spectra which were analyzed to determine the value of singlet-triplet exciton splitting. It was found that the lifetimes of triplet excitons conditioned by electron spin relaxation in highly perfect GaAs/AlGaAs heterostructures with highly mobile 2D electrons are extremely long exceeding 100 μs at T < 1 K. The paper presents a qualitative explanation of the long-spin relaxation lifetimes which are unprecedented for translation-invariant 2D systems. This enabled us to create sufficiently high concentrations of triplet magnetoexcitons, electrically neutral excitations following Bose–Einstein statistics, in a Fermi electron system and investigate their collective properties. At sufficiently high densities of triplet magnetoexcitons and low temperatures, T < 1 K, the degenerate magnetofermionic system exhibits condensation of the triplet magnetoexcitons into a qualitatively new collective state with unusual properties which occurs in the space of generalized moments (magnetic translation vectors). The occurrence of a condensed phase is accompanied with a significant decrease in the viscosity of the photoexcited system, which is responsible for electron spin transport at macroscopic distances, as well as with the effects of threshold enhancement of the system response to the external action of the electromagnetic field and emergence of a new intensive radiative recombination channel.

Exciton spin relaxation in InAs/InGaAlAs/InP(001) quantum dashes emitting near 1.55μm

Exciton spin and related optical polarization in self-assembled InAs/In0.53Ga0.23Al0.24As/InP(001) quantum dashes emitting at 1.55 μm are investigated by means of polarization- and time-resolved photoluminescence, as well as photoluminescence excitation spectroscopy, at cryogenic temperature. We investigate the influence of highly non-resonant and quasi-resonant optical spin pumping conditions on spin polarization and spin memory of the quantum dash ground state. We show that a spin pumping scheme, utilizing the longitudinal-optical-phonon-mediated coherent scattering process, can lead to the polarization degree above 50%. We discuss the role of intrinsic asymmetries in the quantum dash that influence values of the degree of polarization and its time evolution.

Effects of thickness layer on the photoluminescence properties of InAlAs/GaAlAs quantum dots

We investigated the effect of InAlAs layer thickness on exciton-spin relaxation and optical properties of In0.62Al0.38As/Al0.67Ga0.33As QDs. The luminescence properties and carrier dynamics of QDs were studied by the temperature-dependent photoluminescence (PL) and pump-probe measurements. As the total amount of deposited In0.62Al0.38As alloy increased, the central position of the low-energy PL signal decreases, while its full width at half maximum (FWHM) increases. A monotonous redshift of the PL peak was observed with increasing temperature due to the electron–phonon scattering. From the pump-probe measurement, the spin relaxation time decreases with the monolayers at higher temperatures, in agreement with the phonon energy determinate by PL measurements.