Charged Exciton States Research Articles

Electrical Control of Valley Polarized Charged Exciton Species in Monolayer WS2.

Excitons are key to the optoelectronic applications of van der Waals semiconductors, with the potential for versatile on-demand tuning of properties. Yet, their electrical manipulation remains challenging due to inherent charge neutrality and the additional loss channels induced by electrical doping. We demonstrate the dynamic electrical control of valley polarization in charged excitonic states of monolayer tungsten disulfide, achieving up to a 6-fold increase in the degree of circular polarization under off-resonant excitation. In contrast to the weak direct tuning of excitons typically observed using electrical gating, the charged exciton photoluminescence remains stable, even with increased scattering from electron doping. By exciting at the exciton resonances, we observed the reproducible nonmonotonic switching of the charged state population as the electron doping is varied under gate bias, indicating a resonant interplay between neutral and charged exciton states.

Efficient two-dimensional Fraunhofer diffraction pattern via electron spin coherence

In this letter, we have discussed the two-dimensional diffraction pattern via electron spin coherence in a GaAs quantum dot. Impulsive stimulated Raman excitation utilizing coherent optical fields is employed for the purpose of regulating the electron spin coherence within a charged ensemble of GaAs quantum dots, by means of an intermediate charged exciton (trion) state. We show that for the coupling two-dimensional standing wave (SW) field in the x and y directions, the two-dimensional Fraunhofer pattern can be formed for a weak probe light. By using the experimental parameters and controlling the Rabi frequency of the SW field and relative phase between applied lights, the symmetry and asymmetry diffraction pattern are obtained for the weak probe light due to the four-wave mixing mechanism. Our proposed model may have potential applications in high-capacity optical communications and quantum information technologies.

Theory of magneto-optical properties of neutral and charged excitons in GaAs/AlGaAs quantum dots

Detailed theoretical study of the magneto-optical properties of weakly confining GaAs/AlGaAs quantum dots is provided. We focus on the diamagnetic coefficient and the $g$-factor of the neutral and the charged excitonic states, respectively, and their evolution with various dot sizes for the magnetic fields applied along $[001]$ direction. For the calculations we utilize the combination of ${\bf k}\cdot{\bf p}$ and the configuration interaction methods. We decompose the theory into four levels of precision,~i.e., (i) single-particle electron and hole states, (ii) non-interacting electron-hole pair, (iii) electron-hole pair constructed from the ground state of both quasiparticles and interacting via the Coulomb interaction (i.e. with minimal amount of correlation), and (iv) that including the effect of correlation. The aforementioned approach allows us to pinpoint the dominant influence of various single- and multi-particle effects on the studied magneto-optical properties, allowing the characterization of experiments using models which are as simple as possible, yet retaining the detailed physical picture.

Robust room temperature emissions of trion in darkish WSe2 monolayers: effects of dark neutral and charged excitonic states

Owing to nonzero charge and spin degrees of freedom, trions offer unprecedented tunability and open new paths for applications in devices based on 2D semiconductors. However, in monolayer WSe2, the trion photoluminescence is commonly detected only at low temperatures and vanishes at room temperature, which undermines practical applications. To unveil how to overcome this obstacle, we have developed a comprehensive theory to probe the impact of different excitonic channels on the trion emission in WSe2 monolayers, which combines ab initio tight-binding formalism, Bethe–Salpeter equation and a set of coupled rate equations to describe valley dynamics of excitonic particles. Through a systematic study in which new scattering channels are progressively included, we found that, besides the low electron density, strong many-body correlations between bright and dark excitonic states quenches the trion emission in WSe2. Therefore, the reduction of scatterings from bright to dark states is required to achieve trion emission at room temperature for experimentally accessible carrier concentrations.

Electrical detection of excitonic states by time-resolved conductance measurements

We present time-resolved conductance measurements and charge spectra for the conduction-band states of InAs quantum dots after creating metastable holes by illumination. We demonstrate an electrical way of measuring the conduction-band energy offset and inverse tunnel rates of electrons from a two-dimensional electron gas into neutral (${X}^{0}$), single positively (${X}^{1+}$), and double positively (${X}^{2+}$) charged exciton states. The experiment also gives information about the metastable hole storage time and discharge dynamics.

Temperature dependence of optical properties of monolayer WS2 by spectroscopic ellipsometry

The complex dielectric function ɛ = ɛ1 + iɛ2 of monolayer tungsten disulfide (WS2) is investigated for the energy range from 1.5 to 6.0 eV and temperatures from 41 to 300 K. Measurements were performed under ultra-high vacuum conditions to avoid degradation and overlayer contamination. Fourteen critical-point (CP) energies were observed and their origins in Brillouin-zone were identified by band structure calculations. At low temperature the A and B excitonic peaks split into four CPs, which is understood as the neutral and charged exciton states of monolayer WS2. At low temperatures the CP energies show blue shifts with enhanced structure due to the reduction of electron-phonon interaction. The temperature dependence of these data was obtained by a phenomenological expression with Bose-Einstein statistical factor.

Single-particle-picture breakdown in laterally weakly confining GaAs quantum dots

We present a detailed investigation of different excitonic states weakly confined in single GaAs/AlGaAs quantum dots obtained by the Al droplet-etching method. For our analysis we make use of temperature-, polarization- and magnetic field-dependent $\mu$-photoluminescence measurements, which allow us to identify different excited states of the quantum dot system. Besides that, we present a comprehensive analysis of g-factors and diamagnetic coefficients of charged and neutral excitonic states in Voigt and Faraday configuration. Supported by theoretical calculations by the Configuration interaction method, we show that the widely used single-particle Zeeman Hamiltonian cannot be used to extract reliable values of the g-factors of the constituent particles from excitonic transition measurements.

(Invited) Correlations in Photon Emission from Neutral and Charged Exciton States in Functionalized Carbon Nanotubes

Covalently functionalized carbon nanotubes support localized excitons with a discrete photoluminescence spectrum and single photon emission statistics [1,2]. The spectrum of hexyl-modified nanotubes features two distinct photoluminescence bands attributed to neutral and charged configurations of exciton-localizing sp3-defects [3]. Here we report correlations in photon emission events by the respective states of neutral and charged defect-trapped excitons, and explain the observations in the framework of different exciton manifolds that compete for the photogenerated population. [1] X. Ma, N. F. Hartmann, J. K. S. Baldwin, S. K. Doorn, and H. Htoon, Nat. Nanotechnol.10, 671 (2015).[2] X. He, N. F. Hartmann, X. Ma, Y. Kim, R. Ihly, J. L. Blackburn, W. Gao, J. Kono, Y. Yomogida, A. Hirano, T. Tanaka, H. Kataura, H. Htoon, and S. K. Doorn, Nat. Photon. 11, 577 (2017).[3] H. Kwon, M. Kim, M. Nutz, N. F. Hartmann, V. Perrin, B. Meany, M. S. Hofmann, C. W. Clark, S. K. Doorn, A. Högele, and Y. Wang, submitted.

Size-Dependent Asymmetric Auger Interactions in Plasma-Produced n- and p-Type-Doped Silicon Nanocrystals

Nonradiative Auger recombination (AR) tends to dominate carrier dynamics in charged, quantum-confined structures. Consequently, it complicates the practical realization of many semiconductor nanocrystal (NC)-based devices such as light-emitting diodes, photovoltaic cells, and single-photon emitters, in which charged exciton states often occur. To this end, extensive experimental studies on direct band gap NCs have investigated the trion components (both positive and negative) that construct the total AR rate. However, such an analysis has remained elusive for indirect band gap Si NCs. In this study, we investigate AR decay of non-thermal plasma-produced n- and p-type-doped Si NCs. We expand the study over a large NC size range (DNC ≈ 3–8 nm), in which n- and p-type doping is achieved by either a substitutional or surface doping effect, respectively. First, we monitor the AR of charge-neutral multiexcitons by measuring the biexciton lifetime (τXX) as a function of the NC size and doping configuration. We s...

Stark shift of excitons and trions in two-dimensional materials

The effect of an external in-plane electric field on neutral and charged exciton states in two-dimensional (2D) materials is theoretically investigated. These states are argued to be strongly bound, so that electron-hole dissociation is not observed up to high electric field intensities. Trions in the anisotropic case of monolayer phosphorene are demonstrated to be especially robust under electric fields, so that fields as high as 100 kV/cm yield no significant effect on the trion binding energy or probability density distribution. Polarizabilities of excitons are obtained from the parabolicity of numerically calculated Stark shifts. For trions, a fourth order Stark shift is observed, which enables the experimental verification of hyperpolarizability in 2D materials, as observed in the highly excited states of the Rydberg series of atoms and ions.

Controlling dipole transparency with magnetic fields

We describe how magnetic fields can be exploited to control dipole-induced transparency in quantum dot cavity systems. Coupling a linearly-polarized microcavity mode to two spin charged exciton states of a single quantum dot, we demonstrate how cavity-mediated interference and magnetic-field resonance shifts can be utilized to control the transmission of light and on-chip photons, in both magnitude and phase. In particular, we show a triple resonance feature, which also survives with weakly coupled cavities, as long as one operates in the good cooperativity regime. The central peak, which is mediated by the applied magnetic field, is shown to exhibit spectral squeezing. We also demonstrate how the magnetic field allows five regions in which the phase changes by 2π over a small frequency window, where a possible phase gate could be implemented.

Dielectric environment and/or random disorder effects on free, charged and localized excitonic states in monolayer WS2

Excitonic effects play an important role on the optoelectronic behavior of atomically thin two-dimensional (2D) crystals of the WS2 transition metal dichalcogenide. In this paper, neutral and charged exciton behaviors in monolayer WS2 are handled within effective-mass approximation for which the critical parameters are ensured from our ab initio calculations. Firstly, we reveal an exciton series with a novel energy dependence on the orbital angular momentum. Considerable control of the dielectric environment on neutral and charged excitons binding energies is elucidated. We demonstrate that for accepted values of effective masses, the negative and positive trion binding energies should be identical. Secondly, localization of neutral exciton center of mass motion by random potential arising from monolayer defects is also studied. The results obtained are in agreement with available experimental work.

Quantum-Dot Single-Photon Sources for Entanglement Enhanced Interferometry.

Multiphoton entangled states such as "N00N states" have attracted a lot of attention because of their possible application in high-precision, quantum enhanced phase determination. So far, N00N states have been generated in spontaneous parametric down-conversion processes and by mixing quantum and classical light on a beam splitter. Here, in contrast, we demonstrate superresolving phase measurements based on two-photon N00N states generated by quantum dot single-photon sources making use of the Hong-Ou-Mandel effect on a beam splitter. By means of pulsed resonance fluorescence of a charged exciton state, we achieve, in postselection, a quantum enhanced improvement of the precision in phase uncertainty, higher than prescribed by the standard quantum limit. An analytical description of the measurement scheme is provided, reflecting requirements, capability, and restraints of single-photon emitters in optical quantum metrology. Our results point toward the realization of a real-world quantum sensor in the near future.

(Invited) Excited-State Dynamical Studies of Positively and Negatively Charged Excitons in Polymer-Wrapped Single-Walled Carbon Nanotube Superstructures

Conjugated, highly charged aryleneethynylene polymers exfoliate, individualize, and disperse single-walled carbon nanotubes (SWNTs) via a single-chain helical wrapping mechanism. These semiconducting polymer-SWNT superstructures are robust in a wide range of aqueous and organic solvents, maintaining a fixed polymer helical pitch length on the SWNT surface; importantly, this single chain polymer wrapping of the SWNT surface solubilizes the nanotube at a minimal polymer:SWNT molar ratio, and provides a facile means to organize functional organic moieties at predefined intervals along the SWNT surface. We have investigated the transient absorptive and dynamical properties of positively and negatively charged excitons (i.e., hole- and electron-trions) in these semiconducting polymer-wrapped SWNT superstructures. This work exploits redox titration experiments that fix SWNT electron or hole polaron concentrations, and ultrafast pump-probe transient spectroscopic measurements that determine trion transient absorptive signatures and dynamics as functions of absolute electron- or hole-doping levels. Global analysis of these data over the entire vis-NIR spectral domain, where these systems display characteristic transient absorptive spectral signatures, provide fundamental new understanding of trion formation and decay mechanisms in semiconducting SWNTs essential for developing electro-optic and photonic materials. Figure. (A) Structural schematic of a chiral aryleneethynylene polymer-wrapped SWNT. (B) Molecular structure of the binaphthalene-based conjugated, polyanionic semiconducting polymer, S-PBN(b)-Ph5 . (C) Schematic illustration highlighting semiconducting SWNT hole polaron, exciton, positively charged exciton (hole-trion) states. Figure 1

Long–lived femtosecond photon echo in thin films on localized exciton states at room temperature

Long–lived femtosecond photon echo was excited at room temperature in a two–photon regime on the charged exciton states (trion type) localized on the surface defects of a three–layer textured ZnO/Si(B)/Si(P) film with the thickness of each layer at 100 nm in a longitudinal uniform magnetic field. The irreversible longitudinal relaxation time T 1 for the long–lived photon echo regime was of the order of 40 ps, whereas when measured with the help of the stimulated photon echo generated in the single–photon mode, the value was found not to exceed 600 fs.

P-shell carrier assisted dynamic nuclear spin polarization in single quantum dots at zero external magnetic field

Repeated injection of spin polarized carriers in a quantum dot leads to the polarization of nuclear spins, a process known as dynamic nuclear spin polarization (DNP). Here, we report the first observation of p-shell carrier assisted DNP in single QDs at zero external magnetic field. The nuclear field - measured by using the Overhauser shift of the singly charged exciton state of the QDs - continues to increase, even after the carrier population in the s-shell saturates. This is also accompanied by an abrupt increase in nuclear spin buildup time as p-shell emission overtakes that of the s-shell. We attribute the observation to p-shell electrons strongly altering the nuclear spin dynamics in the QD, supported by numerical simulation results based on a rate equation model of coupling between electron and nuclear spin system. DNP with p-shell carriers could open up avenues for further control to increase the degree of nuclear spin polarization in QDs.

Epitaxial growth and photoluminescence excitation spectroscopy of CdSe quantum dots in (Zn,Cd)Se barrier

Design, epitaxial growth, and resonant spectroscopy of CdSe Quantum Dots (QDs) embedded in an innovative (Zn,Cd)Se barrier are presented. The (Zn,Cd)Se barrier enables shifting of QDs energy emission down to 1.87eV, that is below the energy of Mn2+ ions internal transition (2.1eV). This opens a perspective for implementation of epitaxial CdSe QDs doped with several Mn ions as, e.g., the light sources in high quantum yield magnetooptical devices. Polarization resolved Photoluminescence Excitation measurements of individual QDs reveal sharp (Γ<150μeV) maxima and transfer of optical polarization to QD confining charged exciton state with efficiency attaining 26%. The QD doping with single Mn2+ ions is achieved.

Dynamics of the Lowest-Energy Excitons in Single-Walled Carbon Nanotubes under Resonant and Nonresonant Optical Excitation

The dynamics of lowest-energy E11 excitons in undoped and hole-doped single-walled carbon nanotubes (SWCNTs) are investigated using transient absorption spectroscopy and theoretical calculations. In undoped SWCNT samples, E11 excitons under resonant E11 photoexcitation show faster decay than those under high-energy nonresonant photoexcitation. Doping SWCNTs with holes accelerates the E11 exciton decay through exciton-hole interactions; moreover, both resonant and nonresonant photoexcitation in hole-doped SWCNTs lead to nearly identical decay profiles. Theoretical calculations, which are based on a simple model that accounts for bright, dark, and charged exciton states, agree well with the experimental results. We ascribe the fast E11 exciton decay under the resonant E11 photoexcitation to the redistribution of exciton populations in the E11 bright exciton band, which includes optically accessible and inaccessible states; the rapid exciton redistribution is caused by exciton-hole scattering. Our study prov...

Multistate Blinking and Scaling of Recombination Rates in Individual Silica-Coated CdSe/CdS Nanocrystals

Nonradiative Auger recombination is the primary exciton loss mechanism in colloidal nanocrystals and an impediment for prospective optoelectronic applications. Recent development of new core/shell nanocrystals with suppressed Auger recombination rates has opened the possibility for studying multicarrier states using time-resolved photoluminescence (PL) spectroscopy. An important aspect not addressed in previous works is the scaling of radiative and nonradiative decay rates with the increasing number and type of excitons in individual nanocrystals. Here we conduct extensive single-dot PL spectroscopy of emissive states in PL blinking trajectories of giant silica-coated CdSe/CdS nanocrystals. At low fluences, we observe the appearance of neutral and charged exciton (trion) states. Both negative and positive trions show strongly suppressed Auger recombination rates resulting in PL quantum yields close to 50%. At higher excitation powers, we observe consecutive emergence of lower efficiency states, indicative...

Optical control of charged exciton states in tungsten disulfide

A method is presented for optically preparing WS2 monolayers to luminescence from only the charged exciton (trion) state–completely suppressing the neutral exciton. When isolating the trion state, we observed changes in the Raman A1g intensity and an enhanced feature on the low energy side of the E12g peak. Photoluminescence and optical reflectivity measurements confirm the existence of the prepared trion state. This technique also prepares intermediate regimes with controlled luminescence amplitudes of the neutral and charged exciton. This effect is reversible by exposing the sample to air, indicating the change is mitigated by surface interactions with the ambient environment. This method provides a tool to modify optical emission energy and to isolate physical processes in this and other two-dimensional materials.