Phonon-assisted Exciton Research Articles

Recombination dynamics and pronounced re-absorption effect in ZnO single crystals under two-photon excitation

Time-resolved photoluminescence measurements were conducted on ZnO single crystals using two-photon (2P) excitation at low temperatures ranging from 25 to 260 K. The decay constants of various emission peaks are determined and compared between one-photon and two-photon excitations using a recently modified localized-state ensemble model. The analysis is focused on the redshift of the phonon-assisted free exciton transition energy with increasing temperature. A significantly longer radiative recombination lifetime of approximately 840 ps is observed under 2P excitation. The extended lifetime of excitons during 2P excitation is achieved through the interplay of multiple mechanisms. Photon-recycling, enabled by high absorption coefficients and total internal reflection, facilitates re-absorption and new carrier generation. Phonon scattering lowers photon energy, allowing their escape from the crystal, while exciton–polariton interactions delay photon travel, collectively contributing to the extension of exciton lifetimes. The re-absorption effect and elongated carrier lifetime observed from our work could be beneficial for applications of ZnO materials in 2P imaging, particularly of biological samples.

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.

Excitation-dependent photoluminescence intensity of monolayer MoS2: Role of heat-dissipating area and phonon-assisted exciton scattering

We studied the excitation-dependent photoluminescence (PL) quantum yield (QY) of monolayer MoS2 (1L-MoS2) with various flake areas grown on SiO2/Si substrates. The PL measurements were carried out by 532, 488, and 325 nm excitations which fulfill the conditions of quasi-resonant excitation of A-exciton, above bandgap, and far above the bandgap excitations, respectively. The PL QY was found to be reduced by decreasing the excitation wavelength, and it is attributed to variation in the thermal energy dissipated to the lattice. PL emission from 1L-MoS2 was observed with 325 nm excitation in large-area flakes (≥532 μm2) because of efficient heat dissipation. In the literature, PL emission of 1L-MoS2 is hardly reported with 325 nm laser excitations. Under 325 nm laser irradiation, 50% of excitation energy is converted to heat, which substantially increases the local temperature. From the temperature-dependent Raman analysis, the rise in the local temperature is approximated to be ∼382 K in the case of a small-area flake, whereas such an effect is alleviated in large-area flakes. Moreover, inter-valley exciton scattering dominates as the excitation wavelength decreases because of a substantial rise in the phonon population for small-area flakes. As a consequence of inter-valley exciton scattering, dark excitons (K-Σ) dominate over the bright excitons (K-K) under the 325 nm excitation. Hence, total suppression of PL emission was observed for small-area flakes because of dark exciton recombination. The noticeable PL emission of large-area flakes is attributed to the improved bright exciton recombination.

Hot exciton transport in WSe2 monolayers

We experimentally demonstrate hot exciton transport in h-BN encapsulated WSe2 monolayers via spatially and temporally resolved photoluminescence measurements at room temperature. We show that the nonlinear evolution of the mean squared displacement of the non-resonantly excited hot exciton gas is primarily due to the relaxation of its excess kinetic energy and is characterized by a density-dependent fast expansion that converges to a slower, constant rate expansion. We also observe saturation of the hot exciton gas' expansion rate at high excitation densities due to the balance between Auger-assisted hot exciton generation and the phonon-assisted hot exciton relaxation processes.

Temperature-dependent inhomogeneous optical behavior of AlInGaN quaternary alloys

The temperature-dependent anomalous optical properties of AlInGaN quaternary alloys have been studied using Monte Carlo simulation of phonon-assisted exciton hopping. The simulation results agreed well with the experimental photoluminescence linewidths and the peak energy positions while considering the additional inhomogeneous broadening with band potential fluctuation and the phonon-induced radiative lifetime of the exciton. The incorporation of band-gap shrinkage into the conventional Monte Carlo simulation shows a good fit for the photoluminescence peak energy positions in AlInGaN quaternary alloys with temperature-induced experimental unusual behavior (redshift-blueshift-redshift). The W-shaped temperature-dependent inhomogeneities are observed for the PL linewidths. The temperature-induced S-shaped photoluminescence band peaks and W-shaped linewidth have been attributed to the change of exciton dynamics due to the indium aggregated potentials and the exciton localization effects. These results could be important to understanding the realistic optical properties of AlInGaN quaternary alloy-based electronic and optoelectronic devices.

Phonon-assisted oscillatory exciton dynamics in monolayer MoSe2

In monolayer semiconductor transition metal dichalcogenides, the exciton–phonon interaction strongly affects the photocarrier dynamics. Here, we report on an unusual oscillatory enhancement of the neutral exciton photoluminescence with the excitation laser frequency in monolayer MoSe2. The frequency of oscillation matches that of the M-point longitudinal acoustic phonon, LA(M), suggesting the significance of zone-edge acoustic phonons and hence the deformation potential in exciton-phonon coupling in MoSe2. Moreover, oscillatory behavior is observed in the steady-state emission linewidth and in time-resolved PLE data, which reveals variation with excitation energy in the exciton lifetime. These results clearly expose the key role played by phonons in the exciton formation and relaxation dynamics of two-dimensional van der Waals semiconductors.

Unexpectedly Fast Phonon-Assisted Exciton Hopping between Carbon Nanotubes

Carbon-nanotube (CNT) aggregates are promising light-absorbing materials for photovoltaics. The hopping rate of excitons between CNTs directly affects the efficiency of these devices. We theoretically investigate phonon-assisted exciton hopping, where excitons scatter with phonons into a same-tube transition state, followed by intertube Coulomb scattering into the final state. Second-order hopping between bright excitonic states is as fast as the first-order process (∼1 ps). For perpendicular CNTs, the high rate stems from the high density of phononic states; for parallel CNTs, the reason lies in relaxed selection rules. Moreover, second-order exciton transfer between dark and bright states, facilitated by phonons with large angular momentum, has rates comparable to bright-to-bright transfer, so dark excitons provide an additional pathway for energy transfer in CNT composites. As dark excitons are difficult to probe in experiment, predictive theory is critical for understanding exciton dynamics in CNT com...

Phonon-assisted dark exciton preparation in a quantum dot

In semiconductor quantum dots the coupling to the environment, i.e. to phonons, plays a crucial role for optical state preparation. We analyze the phonon impact on two methods for a direct optical preparation of the dark exciton, which is enabled by a tilted magnetic field: the excitation with a chirped laser pulse and the excitation with a detuned pulse. Our study reveals that for both methods phonons either do not impede the proposed mechanism or are even useful by widening the parameter range where a dark state preparation is possible due to phonon-assisted dark exciton preparation. In view of the positive impact of phonons on the optical preparation the use of dark excitons in quantum dots becomes even more attractive.

High Conversion Efficiency Carbon Nanotube-Based Barrier-Free Bipolar-Diode Photodetector.

Conversion efficiency (CE) is the most important figure of merit for photodetectors. For carbon nanotubes (CNT) based photodetectors, the CE is mainly determined by excitons dissociation and transport of free carriers toward contacts. While phonon-assisted exciton dissociation mechanism is effective in split-gate CNT p-n diodes, the CE is typically low in these devices, approximately 1-5%. Here, we evaluate the performance of a barrier-free bipolar diode (BFBD), which is basically a semiconducting CNT asymmetrically contacted by perfect n-type ohmic contact (Sc) and p-type ohmic contact (Pd) at the two ends of the diode. We show that the CE in short channel BFBD devices (e.g., 60 nm) is over 60%, and it reduces rapidly with increasing channel length. We find that the electric-field-assisted mechanism dominates the dissociation rate of excitons in BFBD devices at zero bias and thus the photocurrent generation process. By performing a time-resolved and spatial-resolved Monte Carlo simulation, we find that there exists an effective electron (hole)-rich region near the n-type (p-type) electrode in the asymmetrically contacted BFBD device, where the electric-field strength is larger than 17 V/μm and exciton dissociation is extremely fast (<0.1 ps), leading to very high CE in the BFBD devices.

Effect of seed layer on optical properties and visible photoresponse of ZnO/Cu2O composite thin films

ZnO/Cu2O composite thin films were deposited on Cu foil using a simple solution method with a variety of ZnO seed layers inserted between the Cu2O and ZnO layers. Effects of the ZnO seed layers on the microstructure, optical properties and photoresponse of the thin films were investigated. The intensity of the ZnO (002) diffraction peak increases with increasing thickness of the seed layer. Dissolution of ZnO nanoparticles also occurs during growth of the ZnO thin films. The optical band gap of the Cu2O layer decreases gradually from 1.86 to 1.66eV as the thickness of the ZnO seed layer increases. Change of the optical band gap could be attributed to Cu2+ impurities in the Cu2O and (or) ZnO layers. A visible emission from sample with a seed layer of four times can be attributed to phonon-assisted exciton recombination in Cu2O. An ultraviolet emission centered at 380nm and a broad visible emission band is observed in the two samples with a seed layer of six and eight times. The best photoresponse performance has been observed in the sample with a seed layer of eight times. This is likely a result of the stronger visible absorption and better band alignment in samples, which facilitates electron transfer from the conduction band of Cu2O to the conduction band of ZnO.

Observation of Efficient Upconversion Photoluminescence of Single-Walled Carbon Nanotubes

Because of their potential applications in optoelectronics and bioimaging, photoluminescence (PL) properties of single-walled carbon nanotubes (SWNTs) have attracted much attention over the past decade [1-3]. Here, we report a novel aspect of the PL properties of SWNTs: availability of near-infrared PL (~950-1000 nm) by excitation with longer wavelength light (>1100-1200 nm), namely, upconversion PL. On the basis of the observations of temperature and excitation photon energy dependence, the observed phenomenon is attributed to the efficient phonon-assisted exciton upconversion processes. Furthermore, we demonstrate that strong enhancement of the upconversion PL occurs with additional chemical treatment on SWNTs. Materials exhibiting efficient upconversion PL in the near infrared range have been strongly desired for the observation of deep inside of highly scattering biological media. Our findings may thus open the door to the application of SWNTs in upconversion imaging of unprecedentedly deep inside of biological tissues. [1] Y. Miyauchi, J. Mater. Chem. C 1, 6499 (2013). [2] Y. Miyauchi, M. Iwamura, S. Mouri, T. Kawazoe, M. Ohtsu, and K. Matsuda, Nature Photon. 7, 715 (2013). [3] M. Iwamura, N. Akizuki, Y. Miyauchi, S. Mouri, J. Shaver, Z. Gao, L. Cognet, B. Lounis and K. Matsuda, ACS Nano, asap (2014).

Dissipative preparation of the exciton and biexciton in self-assembled quantum dots on picosecond time scales

Pulsed resonant fluorescence is used to probe ultrafast phonon-assisted exciton and biexciton preparation in individual self-assembled InGaAs quantum dots. By driving the system using large area $(\ensuremath{\ge}10\ensuremath{\pi})$ near resonant optical pulses, we experimentally demonstrate how phonon-mediated dissipation within the manifold of dressed excitonic states can be used to prepare the neutral exciton with a fidelity $\ensuremath{\ge}70%$. By comparing the phonon-assisted preparation with resonant Rabi oscillations we show that the phonon-mediated process provides the higher-fidelity preparation for large pulse areas and is less sensitive to pulse area variations. Moreover, by detuning the laser with respect to the exciton transition, we map out the spectral density for exciton coupling to the bulk LA-phonon continuum. Similar phonon-mediated processes are shown to facilitate direct biexciton preparation via two-photon biexciton absorption, with fidelities $&gt;80%$. Our results are found to be in very good quantitative agreement with simulations that model the quantum dot--phonon bath interactions with Bloch-Redfield theory.

Slow exciton spin relaxation in single self-assembledIn1−xGaxAs/GaAsquantum dots

We calculate the acoustic phonon-assisted exciton spin relaxation via spin-orbit coupling in single self-assembled ${\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}\mathrm{As/GaAs}$ quantum dots using an atomistic empirical pseudopotential method. We show that the transition rate from bright to dark exciton states is zero under Hartree-Fock approximation. The exciton spin relaxation time obtained from sophisticated configuration interaction calculations is approximately 15--55 $\ensuremath{\mu}$s in pure InAs/GaAs QDs and even longer in alloy dots. These results are more than three orders of magnitude longer than previous theoretical and experimental results (a few ns), but agree with more recent experiments which suggest that excitons have long spin-relaxation times ($&gt;$1 $\ensuremath{\mu}$s).

Phonon-Assisted Exciton Transfer into Silicon Using Nanoemitters: The Role of Phonons and Temperature Effects in Förster Resonance Energy Transfer

We study phonon-assisted Förster resonance energy transfer (FRET) into an indirect band-gap semiconductor using nanoemitters. The unusual temperature dependence of this energy transfer, which is measured using the donor nanoemitters of quantum dot (QD) layers integrated on the acceptor monocrystalline bulk silicon as a model system, is predicted by a phonon-assisted exciton transfer model proposed here. The model includes the phonon-mediated optical properties of silicon, while considering the contribution from the multimonolayer-equivalent QD film to the nonradiative energy transfer, which is derived with a d(-3) distance dependence. The FRET efficiencies are experimentally observed to decrease at cryogenic temperatures, which are well explained by the model considering the phonon depopulation in the indirect band-gap acceptor together with the changes in the quantum yield of the donor. These understandings will be crucial for designing FRET-enabled sensitization of silicon based high-efficiency excitonic systems using nanoemitters.

Phonon-assisted exciton formation in ZnO/(Zn, Mg)O single quantum wells grown on C-plane oriented substrates

We report on absorption phenomena in ZnO/(Zn, Mg)O quantum wells grown along the c-axis by molecular beam epitaxy. The optical properties of such quantum wells are affected by a huge internal electric field. For wide quantum wells the absorption is driven by Quantum Confined Stark Effect. Phonon-assisted formation of excitons is observed in the case of thin quantum wells. The physical origin of these hot excitons is determined by using both low temperature (T=10K) photoluminescence excitation spectroscopy and reflectivity measurements.

Dynamics of elastic and inelastic energy transfer between quantum dots in a microcavity

We develop a theory of exciton energy transfer dynamics between two quantum dots in a planar microcavity. We study the dynamics of quantum dot excitons in three different cases: (i) a single dot coupled only to cavity modes, (ii) two dots coupled to cavity modes, and (iii) two dots coupled to cavity modes and to acoustic phonons. In the latter case, we focus on the process of phonon-assisted inelastic exciton energy transfer between the quantum dots. We take into account phonon effects by introducing a light-matter Hamiltonian with operators describing the exciton-photon-phonon coupling and we truncate the Hilbert space by assuming that at most one excitation is present in the system. Using this approach we simulate the exciton dynamics with realistic parameters in the zero temperature limit. From the dynamics we extract the dependence of the characteristic energy transfer rate as a function of the interdot separation. This theoretical approach can be used to optimize exciton energy transfer by designing structures with engineered photon and phonon density of states.

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.

Radiative phonon-assisted and Auger recombination in Si nanocrystals

Recent analysis of the literature shows that the photoluminescence (PL) of Si nanocrystals and porous silicon is caused by phonon-assisted exciton radiative recombination, as well as by direct radiative electron transfer from the second to the first conduction sub-band, which is related to the Auger recombination. The PL decay curve for porous silicon after excitation with ultraviolet laser pulse has been established experimentally. We have constructed continuity equations for the first and the second conduction sub-bands, including radiative phonon-assisted exciton recombination, Auger recombination and direct radiative transition from the second to the first conduction sub-band. The solution of these equations yields the theoretical PL decay curve. These equations contain adjustable parameters. We have estimated their values by using the least-squares fit of the theoretical curve to the experimental data. We have shown that in our case both radiative components are approximately equal at the beginning, but the relative part of the phonon-assisted recombination increases with time and finally tends to 100%. Using the obtained values of the fit parameters, we have constructed the decay curves at different excitation intensities. We conclude that, at a small excitation power, the decay curve is exponential, the phonon-assisted radiative exciton recombination is dominating, and quantum efficiency is close to one. At a high excitation power, the decay curve is approximately stretched-exponential, the direct radiative transitions from the second to the first conduction sub-band are dominating, and quantum efficiency is about 1/3 for porous silicon sample.

Two-Qubit Conditional Phase Gate in Laser-Excited Semiconductor Quantum Dots Using the Quantum Zeno Effect

We propose a scheme for a two-qubit conditional quantum Zeno phase gate for semiconductor quantum dots. The proposed system consists of two charged dots and one ancillary neutral dot driven by a laser pulse tuned to the exciton resonance. The primary decoherence mechanism is phonon-assisted exciton relaxation, which can be viewed as continuous monitoring by the environment. Because of the Zeno effect, a strong possibility of emission is sufficient to strongly modify the coherent dynamics, with negligible probability of actual emission. We solve analytically the master equation and simulate the dynamics of the system using a realistic set of parameters. In contrast to standard schemes, larger phonon relaxation rates increase the fidelity of the operations.

Influence of surface structure on the phonon-assisted emission process in the ZnO nanowires grown on homoepitaxial films

We carried out temperature-dependent photoluminescence measurements to investigate the influence of surface roughness on the phonon-assisted emission of the surface-tailored ZnO nanowires (NWs). For the rough ZnO NWs, the observation of strong defect emission with vibration peaks by the exciton-phonon coupling reflects the presence of a high density of surface defects, resulting in a rapid shift to lower energy region of free exciton emission and a strong contribution of the first order phonon-assisted free exciton in ultraviolet emission. This investigation indicates that the surface defects associated with roughness have a significant influence on the phonon-assisted exciton emission.