Exciton Saturation Research Articles

Two-Dimensional Exciton Oriented Diffusion via Periodic Potentials.

Excitonic devices operate based on excitons, which can be excited by photons as well as emitting photons and serve as a medium for photon-carrier conversion. Excitonic devices are expected to combine the advantages of both the high response rate of photonic devices and the high integration of electronic devices simultaneously. However, because of the neutral feature, exciton transport is generally achieved via diffusion rather than using electric fields, and the efficient control of exciton flux directionality has always been difficult. In this work, a precisely designed one-dimensional periodic nanostructure (1DPS) is used to introduce periodic strain field along with resonant mode to the WS2 monolayer, achieving exciton oriented diffusion with a 7.6-fold exciton diffusion coefficient enhancement relative to that of intrinsic, while enhancing the excitonic emission intensity by a factor of 10 and reducing exciton saturation threshold power by 2 orders of magnitude. Based on the analysis of the density functional theory (DFT) and the finite-element method (FEM), we attribute the anisotropy of exciton diffusion to exciton funneling induced by periodic potentials, which do not require excessive potential height difference for an efficient oriented diffusion. As a result of resonant emission, the exciton diffusion is dragged into the nonlinear regime owing to the high exciton density close to saturation, which improves the exciton diffusion coefficient and diffusion anisotropy more appreciably.

Exciton-Exciton Interaction beyond the Hydrogenic Picture in a MoSe_{2} Monolayer in the Strong Light-Matter Coupling Regime.

In transition metal dichalcogenides' layers of atomic-scale thickness, the electron-hole Coulomb interaction potential is strongly influenced by the sharp discontinuity of the dielectric function across the layer plane. This feature results in peculiar nonhydrogenic excitonic states in which exciton-mediated optical nonlinearities are predicted to be enhanced compared to their hydrogenic counterparts. To demonstrate this enhancement, we perform optical transmission spectroscopy of a MoSe_{2} monolayer placed in the strong coupling regime with the mode of an optical microcavity and analyze the results quantitatively with a nonlinear input-output theory. We find an enhancement of both the exciton-exciton interaction and of the excitonic fermionic saturation with respect to realistic values expected in the hydrogenic picture. Such results demonstrate that unconventional excitons in MoSe_{2} are highly favorable for the implementation of large exciton-mediated optical nonlinearities, potentially working up to room temperature.

Ultrafast Control of the Dimensionality of Exciton-Exciton Annihilation in Atomically Thin Black Phosphorus.

Using microtransient absorption spectroscopy, we show that the dynamical form of exciton-exciton annihilation in atomically thin black phosphorous can be made to switch between time varying 1D scattering and time-independent 2D scattering. At low carrier densities, anisotropy drives the 1D behavior, but as the photoexcitation density approaches the exciton saturation limit, the 2D nature of exciton-exciton scattering takes over. Furthermore, lowering the temperature provides a handle on the ultrafast timescale at which the 1D to 2D transition occurs. We understand our results quantitatively using a diffusion based model of exciton-exciton scattering.

Ultra-low threshold polariton lasing at room temperature in a GaN membrane microcavity with a zero-dimensional trap

Polariton lasers are coherent light sources based on the condensation of exciton-polaritons in semiconductor microcavities, which occurs either in the kinetic or thermodynamic (Bose-Einstein) regime. Besides their fundamental interest, polariton lasers have the potential of extremely low operating thresholds. Here, we demonstrate ultra-low threshold polariton lasing at room temperature, using an all-dielectric, GaN membrane-based microcavity, with a spontaneously-formed zero-dimensional trap. The microcavity is fabricated using an innovative method, which involves photo-electrochemical etching of an InGaN sacrificial layer and allows for the incorporation of optimally-grown GaN active quantum wells inside a cavity with atomically-smooth surfaces. The resulting structure presents near-theoretical Q-factors and pronounced strong-coupling effects, with a record-high Rabi splitting of 64 meV at room-temperature. Polariton lasing is observed at threshold carrier densities 2.5 orders of magnitude lower than the exciton saturation density. Above threshold, angle-resolved emission spectra reveal an ordered pattern in k-space, attributed to polariton condensation at discrete levels of a single confinement site. This confinement mechanism along with the high material and optical quality of the microcavity, accounts for the enhanced performance of our polariton laser, and pave the way for further developments in the area of robust room temperature polaritonic devices.

Overcoming power broadening of the quantum dot emission in a pure wurtzite nanowire

One of the key challenges in developing quantum networks is to generate single photons with high brightness, purity, and long temporal coherence. Semiconductor quantum dots potentially satisfy these requirements; however, due to imperfections in the surrounding material, the coherence generally degrades with increasing excitation power yielding a broader emission spectrum. Here we overcome this power broadening regime and demonstrate the longest coherence at exciton saturation where the detection count rates are highest. We detect single-photon count rates of 460,000 counts per second under pulsed laser excitation while maintaining a single-photon purity of greater than 99%. Importantly, the enhanced coherence is attained with quantum dots in ultraclean wurtzite InP nanowires, where the surrounding charge traps are filled by exciting above the wurtzite InP nanowire bandgap. By raising the excitation intensity, the number of possible charge configurations in the quantum dot environment is reduced, resulting in a narrower emission spectrum. Via Monte Carlo simulations we explain the observed narrowing of the emission spectrum with increasing power. Cooling down the sample to 300 mK, we further enhance the single-photon coherence two-fold as compared to operation at 4.5 K, resulting in a homogeneous coherence time, T2, of 1.2 ns.

Exciton-dominant electroluminescence from a diode of monolayer MoS2

In two-dimensional monolayer MoS2, excitons dominate the absorption and emission properties. However, the low electroluminescent efficiency and signal-to-noise ratio limit our understanding of the excitonic behavior of electroluminescence. Here, we study the microscopic origin of the electroluminescence from a diode of monolayer MoS2 fabricated on a heavily p-type doped silicon substrate. Direct and bound-exciton related recombination processes are identified from the electroluminescence. At a high electron-hole pair injection rate, Auger recombination of the exciton-exciton annihilation of the bound exciton emission is observed at room temperature. Moreover, the efficient electrical injection demonstrated here allows for the observation of a higher energy exciton peak of 2.255 eV in the monolayer MoS2 diode, attributed to the excited exciton state of a direct-exciton transition.

Impact of saturation on the polariton renormalization in III-nitride based planar microcavities

It has been widely observed that an increasing carrier density in a strongly coupled semiconductor microcavity (MC) alters the dispersion of cavity polaritons, below and above the condensation threshold. The interacting nature of cavity polaritons stems from their excitonic fraction being intrinsically subject to Coulomb interactions and the Pauli-blocking principle at high carrier densities. By means of injection-dependent photoluminescence studies performed nonresonantly on a GaN-based MC at various temperatures, it is shown that already below the condensation threshold saturation effects generally dominate over any energy variation in the excitonic resonance. This observation is in sharp contrast to the usually assumed picture in strongly coupled semiconductor MCs, where the impact of saturation is widely neglected. These experimental findings are confirmed by tracking the exciton emission properties of the bare MC active medium and those of a high-quality single GaN quantum well up to the Mott density. The systematic investigation of renormalization up to the polariton condensation threshold as a function of lattice temperature and exciton-cavity photon detuning is strongly hampered by photonic disorder. However, when overcoming the latter by averaging over a larger spot size, a behavior in agreement with a saturation-dominated polariton renormalization is revealed. Finally, a comparison with other inorganic material systems suggests that for correctly reproducing polariton renormalization, exciton saturation effects should be taken into account systematically.

Large vacuum Rabi splitting in a multiple quantum well GaN-based microcavity in the strong-coupling regime

An $\mathrm{Al}\mathrm{In}\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{N}$ microcavity (MC) containing a $\mathrm{Ga}\mathrm{N}∕{\mathrm{Al}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$ multiple quantum well (MQW) structure is investigated through room temperature (RT) photoluminescence and reflectivity experiments. A vacuum Rabi splitting as large as $50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at RT is reported for this MC structure, the highest value reported so far for a semiconductor MC containing QWs. This is shown to result from a geometry combining a state of the art nitride-based MC with a MQW system where the built-in electric field has a reduced impact on the oscillator strength of optical transitions. The contribution of Bragg modes seen in optical spectra is well accounted for by transfer matrix simulations. In addition, the sensitivity of the present system to the tuning between the various optical components of the microcavity (bottom and top Bragg reflectors and active cavity region) to maximize the strong-coupling regime is also shown through simulations. Prospects regarding the nonlinear polariton emission from such a structure indicate that this type of MCs could potentially sustain ultrafast polariton parametric amplification up to $440\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, thanks to an increased exciton binding energy. More generally, it is predicted that, owing to a large exciton saturation density in excess of $1\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ per QW, such a MC structure would be suitable for the observation of nonlinear effects associated with cavity polaritons (polariton lasing and polariton Bose-Einstein condensates) at RT and above.

Reduction in Escape Times of Photogenerated Charge Carriers with Asymmetric Intrastep Quantum Wells and Subsequent Improvement in Saturation Optical Intensity

Escape times of photogenerated charge carriers from an InGaAsP intrastep quantum well (IQW) are calculated and compared with those from a conventional quantum well (QW). Since the red shift of the IQW is initiated at a higher electric field, the escape times from the IQW are smaller than those from the conventional QW at given values in modulator transmission. From the reduction in escape times, improvements in the exciton saturation intensity are estimated to be a factor of ∼5 for both high and low modulator-transmission points.

Microcavity polariton kinetics for bosonic condensation and lasing in II-VI compound materials

We investigate with a Boltzmann approach the relaxation kinetics of II-VI compound microcavity polaritons after a nonresonant pump pulse for the example of CdTe. In these polar materials large vacuum Rabi microcavity splittings guarantee robust polariton modes in the strong-coupling regime with an extremely small effective mass at the bottom of the lower branch, but with very short cavity lifetimes. Our kinetic treatment for stationary and picosecond-pulsed pumping takes the polariton--acoustic-phonon as well as the polariton-polariton scattering into account. For cavities with a vacuum Rabi splitting around $\ensuremath{\Omega}=10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, we find for nonresonant picosecond-pulse excitation a relaxation scenario which leads to a transient bosonic condensation in the lowest polariton state and coherent laser action at densities smaller than the exciton saturation density. For microcavities with a large vacuum Rabi splitting of $\ensuremath{\Omega}=26\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ we find due to the steep increase of radiative losses and the simultaneous steep decrease of the scattering rates with decreasing energy a nonequilibrium phase transition with a macroscopically occupied state in the bottleneck region at a finite transverse momentum and a connected coherent laser emission under an angle of 17\ifmmode^\circ\else\textdegree\fi{}. All of our results for medium and large Rabi vacuum splitting are in good qualitative agreement with corresponding observations of Dang and co-workers

New Design of InGaAs–InGaAlAs MQW Electroabsorption Modulator for High-Speed All-Optical Wavelength Conversion

We have optimized the structure of InGaAlAs multiple quantum-well electroabsorption modulators for all-optical signal processing. As wavelength conversion is a key function for all-optical telecommunication network, we have based our optimization upon the main parameters of this function, namely, polarization insensitivity, exciton saturation, and photogenerated carrier sweep-out time. Four samples have been studied with different well tensile strain and barrier height. We find low polarization dependence over a range of applied bias. Less than 10-ps absorption-saturation recovery time at lower biases and smaller saturation power than in the literature are reported.

Spectral investigation for phosphorescent polymer light-emitting devices with doubly doped phosphorescent dyes

The photoluminescence and electroluminescence (EL) spectral characteristics of two phosphorescent dyes, green fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3] and red tris-(4,7-diphenyl-1,10-phenanthroline)rhenium dications [Ru(4,7-Ph2-phen)3]2+, doubly doped poly(N-vinylcarbazol) (PVK) films were investigated. The efficient energy transfers from PVK host to both Ir(ppy)3 and [Ru(4,7-Ph2-phen)3]2+ dopants were observed in photoexcitation and EL devices. The intensity ratio (Ri) of red emission versus green emission as function of doping ratio (Rc) of [Ru(4,7-Ph2-phen)3]2+ and Ir(ppy)3 in PVK host is found according to a power-law function with the slopes of 2.28±0.2 for PL which obey the power-law with Ri–Rc2 predicated by Förster energy transfer model. EL spectra show a driving voltage depentent power law of Ri–Rc1.53±0.14 at low voltage and Ri–Rc1.16±0.08 at high voltage, respectively. The shift in the power law of Förster energy transfer is likely due to additional emission mechanisms such as carrier trapping and triplet exciton saturation at high current density.

Coherent effects in spectrally resolved pump-probe differential reflectivity measurements at exciton resonance in GaAs quantum wells

We perform femtosecond spectrally resolved pump-probe differential reflectivity measurements on excitons in GaAs quantum wells in both coherent and incoherent regimes. The spectral signal is nonzero and oscillatory at $\ensuremath{-}\mathrm{ve}$ delay up to a few ps indicating persistent coherence between excitonic polarization induced by the probe pulse and the delayed pump pulse. The signal for small $+ve$ and $\ensuremath{-}\mathrm{ve}$ delay $(\ensuremath{\tau})$ shows modulation caused by quantum beats due to coexcitation of heavy and light hole excitons. The rise of the signal with $\ensuremath{\tau}$ for $\ensuremath{\tau}l0$ is rather nonexponential and is sensitive to detuning of the detection energy with respect to the exciton energy. The signal decay with $\ensuremath{\tau}$ for small $+ve$ $\ensuremath{\tau}$ shows dependence on pump-probe relative polarization. The excitonic reflectivity shows a large reduction caused by excitonic absorption saturation for both $+ve$ and $\ensuremath{-}\mathrm{ve}$ $\ensuremath{\tau}$ when the pump pulse intensity is increased to large values. To compare the experimental results with theory, we obtain an expression for the PPDR signal in terms of the third order excitonic polarization. This has an additional term not considered earlier to describe pump-probe experiments. The calculations are performed using optical Bloch equations, modified to incorporate exciton inhomogeneous broadening, quantum beats, and many body effects such as local field, excitation induced energy shift, and dephasing. These effects are found essential in explaining the observed PPDR spectral data. The delay dependence of the signal near the exciton energy is mainly controlled by inhomogeneous broadening for $\ensuremath{-}\mathrm{ve}$ $\ensuremath{\tau}.$ The signal decay for large $+ve$ $\ensuremath{\tau}$ has an exponential behavior determined by the lifetime of incoherent excitons. Experiments for collinear polarization reveal an additional exponential component for small $+ve$ delay. Theoretically, this is found to be related to exciton dephasing and is present only when many body effects are included.

Optical Cavity Effects in ZnO Nanowire Lasers and Waveguides

Wide band gap semiconductor nanostructures with near-cylindrical geometry and large dielectric constants exhibit two-dimensional ultraviolet and visible photonic confinement (i.e., waveguiding). Combined with optical gain and suitable resonant feedback, the waveguiding behavior facilitates highly directional lasing at room temperature in controlled-growth nanowires. We have characterized the nanowire emission in detail with high-resolution optical microscopy. The waveguiding behavior of individual zinc oxide (ZnO) nanowires depends on the wavelength of the emitted light and the directional coupling of the photoluminescence (PL) to the emission dipoles of the nanowire. Polarization studies reveal two distinct regimes of PL characterized by coupling to either guided (bound) or radiation modes of the waveguide, the extent of which depends on wire dimensions. Pumping with high pulse energy engenders the transition from spontaneous to stimulated emission, and analysis of the polarization, line width, and line spacing of the laser radiation facilitates identification of the transverse and longitudinal cavity modes and their gain properties. Interpretation of the lasing spectra as a function of pump fluence, with consideration of ZnO material properties and ultrafast excitation dynamics, demonstrates a transition from exciton (fluence 1 IJ/cm 2 ) and gain saturation behavior (fluence > 3 IJ/cm 2 ) modified by the constraints of the nanoscale cylindrical cavity.

Time-resolved differential reflectivity as a probe of on-resonance exciton dynamics in quantum wells

We investigate the use of time-resolved differential reflectivity for the study of the ultrafast and recombination dynamics of resonantly excited quantum-well excitons. We illustrate the technique with extensive time-resolved measurements using femtosecond laser pulses on a 4-nm ${\mathrm{In}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{As}|\mathrm{GaAs}$ quantum well. Simple considerations applicable to quantum-well systems allow us to relate a generic change in reflectivity to a change in transition oscillator strength, which in turn is proportional to the exciton density (population inversion) created by an intense pump beam. Having as a starting point the temporal evolution of a two-level system under resonant excitation, we interpret differential reflectivity measurements in terms of dynamical processes that range from polarization dephasing (hundreds of femtoseconds) to radiative and nonradiative recombination (nanoseconds). These various dynamical processes span over four orders of magnitude in the time domain and may be extracted from a single time-resolved differential reflectivity experiment. We also find that the observed temporal decays are quite sensitive to the initial (and cold) photogenerated population. We show that a power dependence of the differential reflectivity signal can be utilized to obtain estimates of the exciton saturation density as well as to study the role of exciton-exciton scattering and thermalization on the total recombination time. Under low-power excitation conditions, we relate the differential reflectivity decay to the total exciton recombination lifetime. Lifetime measurements as a function of temperature confirm this assignment as they show an initial and quasilinear rise at low temperatures $(T<70\mathrm{K})$ followed by a decrease at higher temperatures owing to the dominance of nonradiative recombination channels.

Numerical model of the dynamic absorption variation in QW-EAM for ultrafast all-optical signal processing

Noise reduction techniques, such as all-optical 3R regeneration, can be realised using ultrafast saturable absorbers, both for the clock recovery block and for the reshaping block. Electroabsorption modulators (EAM) can be used as saturable absorbers when they are traversed by a strong pump optical pulse, which can reduce the absorption experienced by a second probe signal that propagates along the device together with the pump: this feature is called cross-absorption modulation (XAM). The authors present a novel numerical model to evaluate the dynamic behaviour of a quantum well EAM absorption, that can be applied in all-optical signal processing schemes for ultrafast transmission systems. The effects of both electric field screening and exciton saturation are taken into account. Numerical results agree with the experimental device behaviour reported in the literature.

Mechanism of Polariton-Stimulation in a CdTe-Based Microcavity

At low temperature and for excitation densities well below the exciton saturation density in CdTe microcavities, the photoluminescence (PL) intensity of the lower polariton mode strongly increases with a marked threshold behaviour. We present results of pump–probe experiments which show that the stimulation of the lower polariton PL actually occurs in the strong exciton–photon coupling regime. We present also a detailed study of this stimulation effect as a function of the number of quantum wells in the microcavity and as a function of the detuning δ = EC — EX between exciton energy EX and the cavity mode energy EC. We show that: (i) our results cannot be interpreted in terms of the localised exciton model; (ii) phonon relaxation and exciton–exciton collisions or polariton–exciton collisions are involved in the stimulation.

Ultrafast nonlinear absorption of quantum well excitons periodically distributed in a Bragg reflector

Ultrafast absorptive nonlinearity of excitons is enhanced in a Bragg reflector in which quantum wells are periodically distributed. Absorptive excitonic nonlinearity in the exciton Bragg reflector is characterised by femtosecond transient transmission and reflectance measurements with e–h pair density far above the exciton saturation density. The absorptive excitonic nonlinearity is dominated by two-photon absorption enhanced under the optical confinement in this Bragg reflector. Optical frequency shift caused by ultrafast refractive index change due to the enhanced two-photon absorption, is evaluated from frequency-resolved absorption transients. Spectral modulation of optical pulses and the resultant pulse chirping are produced by this frequency shift and the optical pulses reflected from the exciton Bragg reflector are compressed by compensating the pulse chirping.

Optical Nonlinearities in the Band Edge Region of Highly Excited (In)GaN Thin Films Studied via Femtosecond and Nanosecond Optical Pump-Probe Spectroscopy

We present the results of a detailed study of the band edge absorption changes in GaN and InGaN thin films induced by the presence of excess photo-generated free carriers. The 1s A and B free excitonic resonances in GaN are shown to decrease with increasing above-gap nanosecond optical excitation due to screening by free carriers and exciton–exciton scattering. The decrease in excitonic absorption with increasing above-gap excitation is accompanied by a significant increase in the below-gap absorption coefficient. To further explore this behavior, femtosecond nondegenerate optical pump–probe experiments were also performed using an amplified Ti : sapphire laser. Exciton saturation due to screening by free carriers and excitonic phase space filling was again observed, along with a modest amount of below-gap induced absorption attributed to band gap renormalization. Similar experiments were performed on InGaN thin films. The band edge absorption changes observed in InGaN films were found to be significantly different than those observed in GaN.

Exciton saturation in GaAs multiple quantum wells at room temperature

Pump-probe experiments in GaAs/AlGaAs multiple quantum well samples are described using both picosecond and femtosecond laser pulses in different polarization configurations. The excitonic and free carrier components of exciton absorption saturation were analyzed as a function of well width. The relative importance of phase space filling, Coulomb screening and broadening contributions was determined from polarization and laser bandwidth dependencies via spin-dependent and spin-independent components. A five-level model was used to fit the data and nonlinear coefficients for the individual contributions were determined for each well width. The Coulomb contributions arising from screening and broadening of the excitons was found to dominate the absorption bleaching using picosecond pulses, whereas phase space filling was largest in the femtosecond regime. Exciton phase space filling was found to be four times larger than free carrier phase space filling at narrower well widths. A sublinear dependence of exciton broadening with carrier density was observed.