Exciton-acoustic Phonon Coupling Research Articles

Monolayer indium selenide: an indirect bandgap material exhibits efficient brightening of dark excitons

Atomically thin indium selenide (InSe) exhibits a sombrero-like valence band, leading to distinctive excitonic behaviors. It is known that the indirect band gap of atomically thin InSe leads to a weak emission from the lowest-energy excitonic state (A peak). However, the A peak emission of monolayer (ML) InSe was observed to be either absent or very weak, rendering the nature of its excitonic states largely unknown. Intriguingly, we demonstrate that ML InSe exhibits pronounced PL emission because of the efficient brightening of the momentum-indirect dark excitons. The mechanism is attributed to acoustic phonon-assisted radiative recombination facilitated by strong exciton-acoustic phonon coupling and extended wavefunction in momentum space. Systematic analysis of layer-, power-, and temperature-dependent PL demonstrates that a carrier localization model can account for the asymmetric line shape of the lowest-energy excitonic emission for atomically thin InSe. Our work reveals that atomically thin InSe is a promising platform for manipulating the tightly bound dark excitons in two-dimensional semiconductor-based optoelectronic devices.

Room-temperature wavelike exciton transport in a van der Waals superatomic semiconductor.

The transport of energy and information in semiconductors is limited by scattering between electronic carriers and lattice phonons, resulting in diffusive and lossy transport that curtails all semiconductor technologies. Using Re6Se8Cl2, a van der Waals (vdW) superatomic semiconductor, we demonstrate the formation of acoustic exciton-polarons, an electronic quasiparticle shielded from phonon scattering. We directly imaged polaron transport in Re6Se8Cl2 at room temperature, revealing quasi-ballistic, wavelike propagation sustained for a nanosecond and several micrometers. Shielded polaron transport leads to electronic energy propagation lengths orders of magnitude greater than in other vdW semiconductors, exceeding even silicon over a nanosecond. We propose that, counterintuitively, quasi-flat electronic bands and strong exciton-acoustic phonon coupling are together responsible for the transport properties of Re6Se8Cl2, establishing a path to ballistic room-temperature semiconductors.

Study of the Electron-Phonon Coupling in PbS/MnTe Quantum Dots Based on Temperature-Dependent Photoluminescence.

The temperature dependence of photoluminescence (PL) emission is a valuable tool for investigating carrier localization, recombination, and carrier–phonon interactions. Herein, electron–phonon couplings in lead sulfide (PbS) quantum dots (QDs) and lead sulfide/manganese tellurite (PbS/MnTe) QDs is reported. The effect of temperature on the PL emission of PbS and PbS/MnTe was explored within a temperature range of 10 to 300 K. When temperature increased, PL emission was blue-shifted due to the confinement effect. The gradual broadening of the full width at half maximum (FWHM) with increasing temperature indicates electron–phonon interactions. An analysis based on the Boson model revealed that the values of the exciton acoustic phonon coupling coefficient, σ, and temperature-dependent linewidth, γ, for PbS/MnTe were larger than those for PbS, indicating stronger exciton longitudinal-optical–phonon coupling in the compound structure.

Exciton-acoustic phonon coupling revealed by resonant excitation of single perovskite nanocrystals

Single perovskite nanocrystals have attracted great research attention very recently due to their potential quantum-information applications, which critically depend on the development of powerful optical techniques to resolve delicate exciton photophysics. Here we have realized resonant and near-resonant excitations of single perovskite CsPbI3 nanocrystals, with the scattered laser light contributing to only ~10% of the total collected signals. This allows us to estimate an ultranarrow photoluminescence excitation linewidth of ~11.32 µeV for the emission state of a single CsPbI3 nanocrystal, corresponding to an exciton dephasing time of ~116.29 ps. Meanwhile, size-quantized acoustic phonons can be resolved from a single CsPbI3 nanocrystal, whose coupling with the exciton is proposed to arise from the piezoelectric potential. The ability to collect resonance fluorescence from single CsPbI3 nanocrystals, with the subsequent revelation of exciton-acoustic phonon coupling, has marked a critical step towards their steady advancement into superior quantum-light sources.

Reduced carrier trapping in CdSe/ZnS/CdSe heterostructure quantum dots inferred from temperature dependent spectral studies

We report temperature dependent photoluminescence (TDPL) characteristics of CdSe/ZnS core-shell and coupled 0D-2D CdSe/ZnS/CdSe heterostructure quantum dots (HQDs). Anomalous, behavior of temperature dependent PL linewidth seen in core-shell HQDs is ascribed to the relaxation of carriers into interfacial strain related local energy minimum. TDPL characteristics show largest exciton-acoustic phonon and exciton-LO phonon coupling in core-shell HQDs compared to other QDs. These are caused by enhanced deformation potential interaction and enhanced Fröhlich interaction due to the interfacial strain field. Further, relatively lower strength of exciton-acoustic phonon coupling in 0D-2D HQDs is due to relaxation of interfacial strain. Temperature dependence of spectrally integrated PL intensity shows nonradiative relaxation below 150 K due to thermal activation of carriers in surface tarp states while at temperatures above, it is dominated by carrier thermal escape mediated by exciton-LO phonon interaction involving lesser number of phonons in 0D-2D HQDs.

(Invited) Interplay of Spectral Diffusion and Phonon Broadening in Carbon Nanotubes: Implications for Quantum Optics

We performed a thorough and quantitative investigation of spectral diffusion at low temperature in carbon nanotubes wrapped in PFO and embedded in a polystyren matrix, a system of interest for applications for quantum light states generation. We show that an intrinsic line shape typical of exciton-acoustic phonon coupling can be traced back down to the µs time-scale, while spectral diffusion yields a feature-less spectral broadening of the zero-phonon line. By tuning physical parameters such as temperature and pump power we investigate the line-shape deformation at several integration time-scales. Finally, we discuss the implications of these optical properties as regards the efficiency of the static and dynamical coupling of carbon nanotube to optical micro-cavities for single photon generation.

Photoluminescence characteristics of CdSe quantum dots: role of exciton–phonon coupling and defect/trap states

In this paper, we report temperature dependent photoluminescence (PL) characteristics of CdSe colloidal QDs with average diameter ~2.8 nm. Temperature dependence of strongly confined exciton PL peak position, linewidth and intensity were investigated in 30 K to 300 K temperature range. Our studies reveal nearly four times weaker exciton-LO phonon coupling than bulk CdSe crystal. Theoretically, it should be vanishingly small due to near identical electron and hole charge distributions in strongly confined QDs. On the other hand, exciton-acoustic phonon coupling is an order of magnitude larger than its bulk counterpart. Observed finite value of exciton-LO phonon coupling and enhanced exciton-acoustic phonon coupling are due to piezoelectric strain fields. PL intensity exhibits anomalous behavior in the temperature range 100–230 K. This has been explained by thermally activated detrapping of the charge carriers trapped in the potential wells formed at the interface adjoining dislocations/stacking faults developed during the synthesis process. Above 230 K, PL is partially quenched by thermal escape of charge carriers from luminescing exciton state to higher lying nonluminescing states.

The implications of grading on the emission line width of core–shell nanocrystals

An excitonic coupling with phonons leads to broadening of the emission line. The narrow excitonic emission line width observed in CdZnS/ZnS alloy core graded shell nanocrystals (NCs) with varying compositions is studied using temperature-dependent photoluminescence measurements. Contrary to the consensus that a narrow emission line width is observable with a reduction in size (due to the increased exciton–acoustic phonon coupling coefficient σ), an increased value is noted with reduced size. Based on a theoretical approach to graded core–shell NCs, the relationship between the electron–hole wave function overlap and exciton lifetime is invoked to understand this anomaly. Smaller alloy core–shell NCs (CdZnS/ZnS-I) have a longer lifetime than larger NCs (CdZnS/ZnS-II), indicating reduced electron–hole wave function overlap for CdZnS/ZnS-I NCs and hence a larger ‘effective size’ of NCs, even though the actual size is smaller. The experimental findings demonstrate that graded core–shell NCs reveal an additional functionality, facilitating control of the emission line width of NCs via minimal interaction with the solid state environment.

Photoreflectance investigation of exciton-acoustic phonon scattering in GaN grown by MOVPE

In this paper, we report a systematic investigation of the near band edge (NBE) excitonic states in GaN using low temperature photoluminescence (PL) and photoreflectance (PR) measurements. For this purpose, GaN films of different thicknesses have been grown on silicon nitride (SiN) treated c-plane sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Low temperature PR spectra exhibit well-defined spectral features related to the A, B and C free excitons denoted by FXA FXB and FXC, respectively. In contrast, PL spectra are essentially dominated by the A free and donor bound excitons. By combining PR spectra and Hall measurements a strong correlation between residual electron concentration and exciton linewidths is observed. From the temperature dependence of the excitonic linewidths, the exciton-acoustic phonon coupling constant is determined for FXA, FXB and FXC. We show that this coupling constant is strongly related to the exciton kinetic energy and to the strain level.

Electrical Control of Interdot Electron Tunneling in a Double InGaAs Quantum-Dot Nanostructure

We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photogenerated charge carriers. By tuning the relative energy of the orbital states in the two dots and monitoring the temporal evolution of the pump-probe spectra the electron and hole tunneling times are separately measured and resonant tunneling between the two dots is shown to be mediated both by elastic and inelastic processes. Ultrafast (<5 ps) interdot tunneling is shown to occur over a surprisingly wide bandwidth, up to ∼8 meV, reflecting the spectrum of exciton-acoustic phonon coupling in the system.

Anomalous Stokes shift in CdSe nanocrystals

We investigate the temperature dependence of exciton fluorescence, absorption, and Stokes shift for five sizes of high quality $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ nanocrystals from the perspective of the fine structure model and exciton-acoustic phonon coupling. The Stokes shift is found to have a weak temperature dependence for all sizes. Within the fine structure model, we use the temperature dependence of the Stokes shift to infer the upper level energy and oscillator strength. We also interpret our ensemble measurements using an exciton-acoustic phonon scattering model. We find that neither the fine structure nor exciton-acoustic phonon scattering is able to adequately explain our data in isolation and conclude that a comprehensive theory which includes the physics of both models on an equal footing is required.

Reduction of exciton-phonon interaction due to stronger confinement in single quantum dots

We study optical emission from single CdTe quantum dots embedded in ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{Te}$ barriers. A progressive and simultaneous reduction of (i) exciton-acoustic phonon coupling and (ii) anti-Stokes scattering with acoustic and optical phonons with increasing $\mathrm{Mg}$ concentration in the barrier material is observed. A model is presented for quantitative analysis of exciton-phonon interactions in the presence of strong phonon sidebands, with a data processing method that extracts the phonon sideband contribution from the single quantum dot spectra.

Exciton relaxation and level repulsion inGaAs/AlxGa1−xAsquantum wires

V groove GaAs/AlGaAs quantum wires are investigated by spatially resolved photoluminescence spectroscopy using a low-temperature scanning near-field optical microscope. The spectra along the wires feature several sharp emission lines, which are understood as the emission from exciton states localized in inhomogeneities of the confining potential. It is expected that these exciton states spatially overlap and that their energies are correlated, which leads to level repulsion. The statistical analysis of the spectra in terms of autocorrelation functions clearly reveals this effect. In order to model photoluminescence rather than absorption spectra, we perform detailed simulations of exciton relaxation and luminescence kinetics in a disordered one-dimensional system, taking into account the realistic structure of the facets at the bottom of the V groove in our disorder model. Combining near-field measurements and numerical simulations we show that disorder prevents the full relaxation of excitons towards the local ground states in the dips of the disordered potential. As a consequence, luminescence from excited states is observed. We identify in the spatial and energy correlation between localized states the origin of the observed level repulsion and discuss the role played by the two facets at the bottom of the V groove in this mechanism. This analysis also highlights the relevance of the broad photoluminescence background observed in this and in analogous experiments. We propose as explanation for the origin of this broad background the strong exciton-acoustic phonon coupling that results in phonon sidebands in the spectrum of each localized state.

Exciton–Acoustic Phonon Coupling in Single CdTe Quantum Dots

Exciton–Acoustic Phonon Coupling in Single CdTe Quantum Dots

Exciton-Acoustic Phonon Coupling in Single CdTe Quantum Dots

The photoluminescence spectra of individual quantum dots have been measured with high spectral and spatial resolution spectroscopy. The temperature dependence of the line broadening is analyzed for different single-confined excitons. At low temperatures, the photoluminescence spectra can be very well fitted by a Lorentzian, as expected from the lifetime broadening and the pure dephasing described by a linear phase damping. However, surprisingly, when increasing the temperature, the line shape progressively deviates from a Lorentzian and an additional broad background is observed. This non-Lorentzian line shape and its temperature dependence are theoretically modeled on the basis of lattice relaxation due to the exciton–acoustic phonon coupling.

Coherent transient phenomena of confined excitons in CuCl nanocrsytals: non-Markovian dephasing and quantum beats

We have investigated coherent transient phenomena of confined excitons in CuCl nanocrystals with mean radii R in the range 2.7–23 nm embedded in glass by means of femtosecond degenerated four-wave mixing (DFWM). In the transient DFWM signal for R<5 nm, the non-Markovian behavior is observed, and the analysis based on the stochastic model yields the reservoir correlation time of 4.4–8.5 ps. The size dependence of non-Markovian behavior is explained by an increase of exciton–acoustic phonon coupling strength and a change in the acoustic phonon density of states due to quantum confinement of acoustic phonons. In the spectrally resolved DFWM signal, oscillating behaviors have been observed. In large nanocrystals with R>6.5 nm, quantum beats between the confined exciton sublevels have been observed, while for R<6.5 nm, we have observed quantum beats between the confined exciton and LO-phonon-assisted transitions due to the formation of exciton–LO-phonon complexes.

Triplet—triplet annihilation and exciton—exciton interactions

A theoretical discussion is presented on the nature of direct and indirect interactions between two triplet excitons. It is shown that the direct exciton interaction through the coulombic interaction of electrons is important when there is a large change in the polarizability of the molecule during its excitation. The indirect exciton—exciton interaction is mainly due to the exciton—phonon interaction arising from the variation in the D-shift term. It is independent of temperature and spin substate. It is attractive in nature for excitons of narrow band-width, and if the exciton—acoustic phonon coupling is inducing the indirect interaction, it falls as R −3. It is suggested that a kinematic theory of the triplet—triplet annihilation should incorporate the effects of such interactions.