Thermalization Of Excitons Research Articles

Phonon‐Bottleneck Enhanced Exciton Emission in 2D Perovskites

AbstractLayered halide perovskites exhibit remarkable optoelectronic properties and technological promise, driven by strongly bound excitons. The interplay of spin‐orbit and exchange coupling creates a rich excitonic landscape, determining their optical signatures and exciton dynamics. Despite the dark excitonic ground state, surprisingly efficient emission from higher‐energy bright states has puzzled the scientific community, sparking debates on relaxation mechanisms. Combining low‐temperature magneto‐optical measurements with sophisticated many‐particle theory, the origin of the bright exciton emission in perovskites is elucidated by tracking the thermalization of dark and bright excitons under a magnetic field. The unexpectedly high emission is clearly attributed to a pronounced phonon‐bottleneck effect, considerably slowing down the relaxation toward the energetically lowest dark states. It is demonstrated that this bottleneck can be tuned by manipulating the bright‐dark energy splitting and optical phonon energies, offering valuable insights and strategies for controlling exciton emission in layered perovskite materials that is crucial for optoelectronics applications.

Time-domain observation of interlayer exciton formation and thermalization in a MoSe2/WSe2 heterostructure

Vertical heterostructures of transition metal dichalcogenides (TMDs) host interlayer excitons with electrons and holes residing in different layers. With respect to their intralayer counterparts, interlayer excitons feature longer lifetimes and diffusion lengths, paving the way for room temperature excitonic optoelectronic devices. The interlayer exciton formation process and its underlying physical mechanisms are largely unexplored. Here we use ultrafast transient absorption spectroscopy with a broadband white-light probe to simultaneously resolve interlayer charge transfer and interlayer exciton formation dynamics in a MoSe2/WSe2 heterostructure. We observe an interlayer exciton formation timescale nearly an order of magnitude (~1 ps) longer than the interlayer charge transfer time (~100 fs). Microscopic calculations attribute this relative delay to an interplay of a phonon-assisted interlayer exciton cascade and thermalization, and excitonic wave-function overlap. Our results may explain the efficient photocurrent generation observed in optoelectronic devices based on TMD heterostructures, as the interlayer excitons are able to dissociate during thermalization.

Exciton optics, dynamics, and transport in atomically thin semiconductors

Atomically thin semiconductors such as transition metal dichalcogenide (TMD) monolayers exhibit a very strong Coulomb interaction, giving rise to a rich exciton landscape. This makes these materials highly attractive for efficient and tunable optoelectronic devices. In this Research Update, we review the recent progress in the understanding of exciton optics, dynamics, and transport, which crucially govern the operation of TMD-based devices. We highlight the impact of hexagonal boron nitride-encapsulation, which reveals a plethora of many-particle states in optical spectra, and we outline the most novel breakthroughs in the field of exciton-polaritonics. Moreover, we underline the direct observation of exciton formation and thermalization in TMD monolayers and heterostructures in recent time-resolved, angle-resolved photoemission spectroscopy studies. We also show the impact of exciton density, strain, and dielectric environment on exciton diffusion and funneling. Finally, we put forward relevant research directions in the field of atomically thin semiconductors for the near future.

Temporal Evolution of Low-Temperature Phonon Sidebands in Transition Metal Dichalcogenides

Low-temperature photoluminescence (PL) of hBN-encapsulated monolayer tungsten diselenide (WSe$_2$) shows a multitude of sharp emission peaks below the bright exciton. Some of them have been recently identified as phonon sidebands of momentum-dark states. However, the exciton dynamics behind the emergence of these sidebands has not been revealed yet. In this joint theory-experiment study, we theoretically predict and experimentally observe time-resolved PL providing microscopic insights into thermalization of hot excitons formed after optical excitation. In good agreement between theory and experiment, we demonstrate a spectral red-shift of phonon sidebands on a timescale of tens of picoseconds reflecting the phonon-driven thermalization of hot excitons in momentum-dark states. Furthermore, we predict the emergence of a transient phonon sideband that vanishes in the stationary PL. The obtained microscopic insights are applicable to a broad class of 2D materials with multiple exciton valleys.

Excitonic emission in heavily Ga-doped zinc oxide films grown on GaN

There has been a great interest in development of materials technologies of transparent conducive electrodes for efficient solar cells, light-emitting diodes, radiation scintillators, displays and smart windows. Due to several advantages over other oxide and nitride materials, ZnO doped by gallium is a top contender in the considered field. Understanding the fundamental physics behind Ga doping effect on the optical and electronic properties of ZnO is thus a cornerstone of conceptualization of more efficient optoelectronic devices. Here, we use atmospheric pressure MOCVD approach for the preparation of well-aligned columnar Ga-doped ZnO films on GaN substrates. By performing complementary X-ray photoelectron spectroscopy and Raman spectroscopy studies, we evidence Ga substitutional doping leading to an upshift of the Fermi level and significant increase in the carrier density up to ~4·1020 cm−3. Temperature-dependent photoluminescence studies enabled unraveling the nature of the excitonic emission in heavily Ga-doped ZnO nanostructured films. Unlike well-known gradual thermalization of the donor bound excitons and prevailing of free excitonic emission in the high temperature regime for undoped ZnO films, we observe faster thermalization of D0X excitons and dominating FX-LO phonon replica for ZnO:Ga. A significant band gap renormalization of ~755 meV was found to be a major effect in heavily Ga-doped ZnO films. The obtained knowledge could be useful for smart design of ZnO-based transparent conductive electrodes with enhanced conductivity, improved quantum yield of emission and controlled optical transparency window.

Interlayer exciton dynamics in van der Waals heterostructures

Atomically thin transition metal dichalcogenides can be stacked to van der Waals heterostructures enabling the design of new materials with tailored properties. The strong Coulomb interaction gives rise to interlayer excitons, where electrons and holes are spatially separated in different layers. In this work, we reveal the time- and momentum-dependent elementary processes behind the formation, thermalization and photoemission of interlayer excitons for the exemplary MoSe2–WSe2 heterostructure. We identify tunneling of holes from MoSe2 to WSe2 on a ps timescale as the crucial process for interlayer exciton formation. We also predict a drastic reduction of the formation time as a function of the interlayer energy offset suggesting that interlayer excitons can be externally tuned. Finally, we explain the experimental observation of a dominant photoluminescence from interlayer excitons despite the vanishingly small oscillator strength as a consequence of huge interlayer exciton occupations at low temperatures.

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.

Mapping of the dark exciton landscape in transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) exhibit a remarkable exciton physics including optically accessible (bright) as well as spin- and momentum-forbidden (dark) excitonic states. So far the dark exciton landscape has not been revealed leaving in particular the spectral position of momentum-forbidden dark states completely unclear. This has a significant impact on the technological application potential of TMDs, since the nature of the energetically lowest state determines, if the material is a direct-gap semiconductor. Here, we show how dark states can be experimentally revealed by probing the intra-excitonic 1s-2p transition. Distinguishing the optical response shortly after the excitation (< 100$\,$fs) and after the exciton thermalization (> 1$\,$ps) allows us to demonstrate the relative position of bright and dark excitons. We find both in theory and experiment a clear blue-shift in the optical response demonstrating for the first time the transition of bright exciton populations into lower lying momentum- and spin-forbidden dark excitonic states in monolayer WSe$_2$.

Exciton Condensation in a Two-Dimensional System with Disorder

The Bose–Einstein condensation of excitons in two-dimensional (2D) systems has been studied theoretically, taking into account both the random potential associated with imperfections of the structure and the finite exciton lifetime. It is shown that the disorder existing in the system makes condensation possible. The finite exciton lifetime limits the thermalization of excitons in the disordered system and sets an additional limit on the critical temperature of the transition. The effects of interparticle interaction and pump fluctuations have been analyzed. The phase correlator has been calculated and the failure of the condensate due to the effects of interaction and fluctuations has been analyzed. The propagation of perturbations in the condensate has been investigated.

Dark and bright exciton formation, thermalization, and photoluminescence in monolayer transition metal dichalcogenides

The remarkably strong Coulomb interaction in atomically thin transition metal dichalcogenides (TMDs) results in an extraordinarily rich many-particle physics including the formation of tightly bound excitons. Besides optically accessible bright excitonic states, these materials also exhibit a variety of dark excitons. Since they can even lie below the bright states, they have a strong influence on the exciton dynamics, lifetimes, and photoluminescence. While very recently, the presence of dark excitonic states has been experimentally demonstrated, the origin of these states, their formation, and dynamics have not been revealed yet. Here, we present a microscopic study shedding light on time- and energy-resolved formation and thermalization of bright and dark intra- and intervalley excitons as well as their impact on the photoluminescence in different TMD materials. We demonstrate that intervalley dark excitons, so far widely overlooked in current literature, play a crucial role in tungsten-based TMDs giving rise to an enhanced photoluminescence and reduced exciton lifetimes at elevated temperatures.

Theory of Exciton Energy Transfer in Carbon Nanotube Composites

We compute the exciton transfer (ET) rate between semiconducting single-wall carbon nanotubes (SWNTs). We show that the main reasons for the wide range of measured ET rates reported in the literature are 1) exciton confinement in local quantum wells stemming from disorder in the environment and 2) exciton thermalization between dark and bright states due to intratube scattering. The SWNT excitonic states are calculated by solving the Bethe-Salpeter equation using tight-binding basis functions. The ET rates due to intertube Coulomb interaction are computed via Fermi's golden rule. In pristine samples, the ET rate between parallel (bundled) SWNTs of similar chirality is very high ($\sim 10^{14}\;\text{s}^{-1}$) while the ET rate for dissimilar or nonparallel tubes is considerably lower ($\sim 10^{12}\;\text{s}^{-1}$). Exciton confinement reduces the ET rate between same-chirality parallel SWNTs by two orders of magnitude, but has little effect otherwise. Consequently, the ET rate in most measurements will be on the order of $ 10^{12}\;\text{s}^{-1}$, regardless of the tube relative orientation or chirality. Exciton thermalization between bright and dark states further reduces the ET rate to about $10^{11}\;\text{s}^{-1}$. The ET rate also increases with increasing temperature and decreases with increasing dielectric constant of the surrounding medium.

Electronic Structure and Transition Energies in Polymer–Fullerene Bulk Heterojunctions

Photocurrent spectroscopy is used to measure both the charge transfer and exciton optical absorption spectra of various bulk heterojunction organic solar cells. The energy difference between the polymer HOMO energy and the fullerene LUMO energy is obtained from the spectra, along with the disorder energy. Combining information from cells with several different polymers and fullerenes allows measurements of the energy differences between HOMO or LUMO energies for about 10 different polymers and fullerenes, with an estimated uncertainty of 50 meV. Heterojunction band offsets are obtained for the various cells, distinguishing between the excitonic and the single-carrier band offsets. The cell open-circuit voltage is shown to be closely correlated with the interface band gap. The exciton disorder energy is directly correlated to the band-tail disorder and we also consider the effects of exciton thermalization on the charge generation mechanism. The data indicate that an energy offset between the polymer excit...

Novel Spectral Decay Dynamics of Hot Excitons in PbSe Nanocrystals: A Tunable Femtosecond Pump–Hyperspectral Probe Study

Ultrafast exciton cooling in highly monodisperse PbSe nanocrystals is followed with tunable pump–hyperspectral near-IR probe spectroscopy. Unexpected kinetic and spectral correlations between induced bleach and absorption features are revealed, which are incompatible with standard models for excited nanocrystal absorption. Interband optical excitation immediately generates a sharp bleach feature near the 1Sh1Se transition which is unchanged during exciton thermalization, while pumping well above the band edge induces an intense absorption at frequencies just below the band edge which decays concurrently with a buildup of renewed absorbance at the 1Ph1Pe peak during exciton cooling. Transient spectra of hot single and double excitons are nearly indistinguishable, arguing against the controversial involvement of Auger cooling in the rapid dissipation of excess energy in excited PbSe QDs. Finally, quantitative signal analysis shows no signs of multiexciton generation up to photon energies four times the sample band gap.

Resonant creation of indirect excitons in diamond at the phonon-assisted absorption edge

We have developed a new scheme for generating cold and high-density excitons in diamond by resonant excitation at the phonon-assisted absorption edge. Both the thermalization of excitons between fine-structure states and the effective temperature are examined with the tuning of the photon energy. The down-conversion rate for the excitons in the fine-structure states is found to be 3.5 ns independent of the photon energy, which means that the lowest-energy dark exciton can be created within a time scale comparable to its lifetime via the generation of a higher-energy bright exciton. With careful choice of excitation photon energy, we have achieved an electron-hole pair density of while maintaining the effective excitonic temperature below 15 K.

Pattern formation in the exciton inner ring

We report on the spatially separated pump-probe study of indirect excitons in the inner ring in the exciton emission pattern. A pump laser beam generates the inner ring and a weaker probe laser beam is positioned in the inner ring. The probe beam is found to suppress the exciton emission intensity in the ring. We also report on the inner ring fragmentation and formation of multiple rings in the inner ring region. These features are found to originate from a weak spatial modulation of the excitation beam intensity in the inner ring region. The modulation of exciton emission intensity anti-correlates with the modulation of the laser excitation intensity. The three phenomena - inner ring fragmentation, formation of multiple rings in the inner ring region, and emission suppression by a weak probe laser beam - have a common feature: a reduction of exciton emission intensity in the region of enhanced laser excitation. This effect is explained in terms of exciton transport and thermalization.

Engineering the Spin–Flip Limited Exciton Dephasing in Colloidal CdSe/CdS Quantum Dots

We have measured the intrinsic exciton dephasing in high-quality zinc blende CdSe/CdS colloidal quantum dots in the temperature range from 5 to 170 K using a sensitive three-beam photon echo technique in heterodyne detection, which is not affected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing. From the temperature dependence of the zero-phonon line dephasing, the exciton lifetime, and the exciton thermalization within its fine structure, we show that the zero-phonon line dephasing of the lowest bright state originates from the phonon-assisted spin–flip to dark exciton states. Importantly, we can control the dephasing by tailoring the exciton fine structure through its dependence on the dot core size and shell thickness, as expected from the spin–flip mechanism. By reducing the electron–hole exchange interaction with increasing core size and delocalization of the electron wave function in the quasi-type-II core/shell band alignment, we find the longest zero-phonon line dephasing time of ∼110 ps at 5 K in dots with the largest core diameter (5.7 nm) and the thickest CdSe shell (9 monolayers) in the series studied.

Effect of the doping level on the radiative life time in ZnO nanowires

The emission properties of ZnO nanowires grown by metal organic vapor phase epitaxy on sapphire and p-type GaN are compared using temperature dependent time resolved photoluminescence. The temperature dependence of the radiative decay time of nanowires grown on sapphire is well understood considering an exciton thermalization in a 3D density of states. In the case of growth on GaN, a great increase of the radiative decay time is observed compared to nanowires grown on sapphire. This increase of the radiative decay time could be due to a band bending effect that separates the electron and the hole. This band bending effect depends on the residual doping level and is not seen in the case of nanowires grown on sapphire probably because of a very high residual doping level that can be estimated from the radiative decay time temperature dependence.

Ultrafast relaxation and absorption saturation at near exciton resonance in a thin ZnO epilayer

We observed ultrafast free exciton thermalization time of 700–900 fs and obtained the magnitude of maximal differential absorption to be 1.8×104 cm−1 with the pumping fluence of 10 μJ/cm2 by measuring transient differential transmission in a thin ZnO epitaxial layer at room temperature. The largest induced transparency occurs near exciton resonance associated with absorption saturation by comparing the excitation from the above band-gap to band-tail states. The pumping dependent transient absorption reveals transition of excitonic relaxation from exciton-phonon scattering to exciton-exciton scattering or to an electron-hole plasma.

Exciton thermalization and state broadening contributions to the photoluminescence of colloidal PbSe quantum dot films from 295 to 4.5 K

The temperature-dependent photoluminescence (PL) from thick drop-cast films of $\ensuremath{\sim}4--5\text{ }\text{nm}$ diameter PbSe colloidal quantum dots was investigated in the temperature range of 4.5--295 K. Several samples all exhibit single-peak emission with a simple Arrhenius-type decrease in PL yield as temperature increases. The temperature dependence of the Stokes shift and PL line shape are reproducible and can be quantitatively described over the entire temperature range using a relatively simple, yet accurate thermal model, including state broadening. The Stokes shift is completely described by thermalization of the exciton population and the onset of degeneracy below $\ensuremath{\sim}100\text{ }\text{K}$. The variation in linewidth is dominated by the same degeneracy effects below $\ensuremath{\sim}100\text{ }\text{K}$ while above $\ensuremath{\sim}125\text{ }\text{K}$, the variation is controlled by thermal (Lorentzian) dephasing. There is also clear evidence for a substantial and temperature-independent Voigt-type contribution to the state scattering.

Four-wave-mixing imaging and carrier dynamics of PbS colloidal quantum dots

We demonstrate coherent multiphoton imaging of PbS colloidal quantum dots based on the detection of their transient resonant four-wave-mixing (FWM) emission under 150 fs pulsed laser excitation. Background-free imaging of PbS quantum dots in resonance with their ground-state excitonic absorption at $1.2\text{ }\ensuremath{\mu}\text{m}$ wavelength is shown, with intrinsic optical sectioning and a spatial resolution better than the one-photon diffraction limit, promising for deep tissue microscopy. The four-wave-mixing efficiency is found to be resonant to the ground-state absorption maximum, which is a prerequisite for spectral multiplexing. The measured FWM dynamics is consistent with the presence of charged exciton and biexiton Auger recombination in the 10--100 ps time scale, phonon-assisted exciton thermalization in the nanosecond time scale, and excitonic recombination in the microsecond time scale.