Exciton Ground State Research Articles

The dark exciton ground state promotes photon-pair emission in individual perovskite nanocrystals

Cesium lead halide perovskites exhibit outstanding optical and electronic properties for a wide range of applications in optoelectronics and for light-emitting devices. Yet, the physics of the band-edge exciton, whose recombination is at the origin of the photoluminescence, is not elucidated. Here, we unveil the exciton fine structure of individual cesium lead iodide perovskite nanocrystals and demonstrate that it is governed by the electron-hole exchange interaction and nanocrystal shape anisotropy. The lowest-energy exciton state is a long-lived dark singlet state, which promotes the creation of biexcitons at low temperatures and thus correlated photon pairs. These bright quantum emitters in the near-infrared have a photon statistics that can readily be tuned from bunching to antibunching, using magnetic or thermal coupling between dark and bright exciton sublevels.

Band structure properties, phonons, and exciton fine structure in 4H -SiC measured by wavelength-modulated absorption and low-temperature photoluminescence

Owing to its hexagonal symmetry, indirect band gap, and relatively large unit cell, the electronic band structure of $4H$-SiC is comprised of a complicated series of anisotropic valence and conduction band extrema even very near to the uppermost valence band maximum and lowest conduction band minimum. This has presented a difficult challenge to those experiments which have attempted to resolve the small energy separations between these band extrema. To overcome this challenge, we have measured the wavelength-modulated absorption (WMA) spectrum of $4H$-SiC over a broader wavelength range (3500--3800 \AA{}) and at a higher resolution ($l0.1$ \AA{}) than in previous work. By comparing these measurements with the low-temperature photoluminescence spectrum in ultrapure $4H$-SiC, we have identified several features, which we attribute to a $56\ifmmode\pm\else\textpm\fi{}3$ meV crystal-field splitting of the valence band maximum or a $136\ifmmode\pm\else\textpm\fi{}3$ meV separation between the two lowest conduction band minima. We also show that the spin-orbit split-off valence band, which has been observed in previous measurements of $4H$-SiC, contributes to nonparabolic dispersion near the valence band maximum, and this is responsible for several previously misidentified features in the WMA spectrum. Finally, we report the first experimental measurement of fine structure splittings in the free exciton ground state, which manifests as four small ($0.7\ifmmode\pm\else\textpm\fi{}0.1$ meV) splittings in the WMA spectrum due to mass anisotropy and electron-hole exchange interaction.

Quantum dynamical simulations of intra-chain exciton diffusion in an oligo (para-phenylene vinylene) chain at finite temperature.

We report on quantum dynamical simulations of exciton diffusion in an oligo(para-phenylene vinylene) chain segment with 20 repeat units (OPV-20) at finite temperature, complementary to our recent study of the same system at T = 0 K [R. Binder and I. Burghardt, J. Chem. Phys. 152, 204120 (2020)]. Accurate quantum dynamical simulations are performed using the multi-layer multi-configuration time-dependent Hartree method as applied to a site-based Hamiltonian comprising 20 electronic states of Frenkel type and 460 vibrational modes, including site-local quinoid-distortion modes along with site-correlated bond-length alternation (BLA) modes, ring torsional modes, and an explicit harmonic-oscillator bath. A first-principles parameterized Frenkel-Holstein type Hamiltonian is employed, which accounts for correlations between the ring torsional modes and the anharmonically coupled BLA coordinates located at the same junction. Thermally induced fluctuations of the torsional modes are described by a stochastic mean-field approach, and their impact on the excitonic motion is characterized in terms of the exciton mean-squared displacement. A normal diffusion regime is observed under periodic boundary conditions, apart from transient localization features. Even though the polaronic exciton species are comparatively weakly bound, exciton diffusion is found to be a coherent-rather than hopping type-process, driven by the fluctuations of the soft torsional modes. Similar to the previous observations for oligothiophenes, the evolution for the most part exhibits a near-adiabatic dynamics of local exciton ground states (LEGSs) that adjust to the local conformational dynamics. However, a second mechanism, involving resonant transitions between neighboring LEGSs, gains importance at higher temperatures.

Excitonic terahertz emission from Si at intense interband photoexcitation

Terahertz photoluminescence from silicon crystals due to radiative transitions between the energy levels of free excitons has been studied up to intensities of interband photoexcitation significantly exceeding the threshold for the formation of electron-hole liquid droplets. At temperatures above 20 K and a photoexcitation density in excess of 7 W/cm2, the appearance of stimulated intraexciton terahertz radiation was clearly demonstrated. Terahertz gain spectrum was obtained. The gain spectrum indicates that population inversion arises between the highly excited states of excitons and sublevels of the ground state of free excitons, as well as possibly between two-exciton and biexciton states. The performed calculations of the times of energy relaxation of exciton states show the fundamental possibility of the appearance of population inversion between the states of excitons in the process of their energy relaxation.

Nonlinear Refraction in Colloidal Silver Sulfide Quantum Dots

We analyze the features of nonlinear refraction in colloidal solutions containing Ag2S quantum dots capped with thioglycolic acid (Ag2S/TGA) or L-cysteine (Ag2S/L-Cys), with the quantum-dot (QD) sizes ranging from 1.7 to 5.5 nm and their concentration in the solution being 0.1%. The Z-scans of the samples were obtained using 10 ns pulses of 532 nm radiation. For samples with various average sizes and positions of the characteristic feature in the optical density spectrum, both self-focusing and self-defocusing of radiation were observed. The contribution of thermal refraction to the recorded Z-scans was estimated, and the impact of thermal effects on the observed nonlinear refraction was shown to be negligible. The solutions of the Ag2S/TGA QDs with the ground state exciton absorption peaked at 590 nm exhibit self-focusing. The samples with recombination luminescence and the ground state exciton absorption peaked at 750 nm reveal self-defocusing. We show that nonlinear refraction in the case of nanosecond pulses is predominantly nonthermal in nature and is due to absorption by free charge carriers, as well as due to the band filling effect in the Ag2S QDs, which involves localized states of impurities.

Exciton oscillator strength in two-dimensional Dirac materials

Exciton problem is solved in the two-dimensional Dirac model with allowance for strong electron-hole attraction. The exciton binding energy is assumed smaller than but comparable to the band gap. The exciton wavefunction is found in the momentum space as a superposition of all four two-particle states including electron and hole states with both positive and negative energies. The matrix element of exciton generation is shown to depend on the additional components of the exciton wavefunction. Both the Coulomb and the Rytova-Keldysh potentials are considered. The dependence of the binding energy on the coupling constant is analyzed for the ground and first excited exciton states. The binding energy and the oscillator strength are studied as functions of the environmental-dependent dielectric constant for real transition metal dichalcogenide monolayers. We demonstrate that the multicomponent nature of the exciton wavefunction is crucial for description of resonant optical properties of two-dimensional Dirac systems.

The ultrafast onset of exciton formation in 2D semiconductors

The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process.

Quantum interference between the optical Stark effect and resonant harmonic generation in WS2

An applied field can modulate optical signals by resonance shifting via the Stark effect. The optical Stark effect (OSE) uses ultrafast light in the transparency region of a material to shift resonances with speeds limited by the pulse duration or system coherence. In this Letter we investigate the OSE in resonant optical harmonic generation (OHG) using the ground state exciton transition of WS2 with a variety of morphologies. Multidimensional pump-harmonic-probe measurements, in which the probe is second- or third-harmonic emission, reveal not only large Stark shifts that are commensurate with the large optical susceptibilities common to WS2 excitons, but also behaviors more complex than simple OSE treatments predict. We show how a new manifestation of the Stark Effect, brought forth by coherent photon exchange between the pump and OHG fundamental fields, can strongly enhance or suppress OHG.

Efficient MoWO3/VO2/MoS2/Si UV Schottky photodetectors; MoS2 optimization and monoclinic VO2 surface modifications

The distinctive properties of strongly correlated oxides provide a variety of possibilities for modulating the properties of 2D transition metal dichalcogenides semiconductors; which represent a new class of superior optical and optoelectronic interfacing semiconductors. We report a novel approach to scaling-up molybdenum disulfide (MoS2) by combining the techniques of chemical and physical vapor deposition (CVD and PVD) and interfacing with a thin layer of monoclinic VO2. MoWO3/VO2/MoS2 photodetectors were manufactured at different sputtering times by depositing molybdenum oxide layers using a PVD technique on p-type silicon substrates followed by a sulphurization process in the CVD chamber. The high quality and the excellent structural and absorption properties of MoWO3/VO2/MoS2/Si with MoS2 deposited for 60 s enables its use as an efficient UV photodetector. The electronically coupled monoclinic VO2 layer on MoS2/Si causes a redshift and intensive MoS2 Raman peaks. Interestingly, the incorporation of VO2 dramatically changes the ratio between A-exciton (ground state exciton) and trion photoluminescence intensities of VO2/(30 s)MoS2/Si from < 1 to > 1. By increasing the deposition time of MoS2 from 60 to 180 s, the relative intensity of the B-exciton/A-exciton increases, whereas the lowest ratio at deposition time of 60 s refers to the high quality and low defect densities of the VO2/(60 s)MoS2/Si structure. Both the VO2/(60 s)MoS2/Si trion and A-exciton peaks have higher intensities compared with (60 s) MoS2/Si structure. The MoWO3/VO2/(60 s)MoS2/Si photodetector displays the highest photocurrent gain of 1.6, 4.32 × 108 Jones detectivity, and ~ 1.0 × 1010 quantum efficiency at 365 nm. Moreover, the surface roughness and grains mapping are studied and a low semiconducting-metallic phase transition is observed at ~ 40 °C.

Twistronics: A Recent Avenue in van der Waals Heterostructures

Two-dimensional (2D) materials, the thinnest form of materials to ever occur in nature, have the potential to dramatically alter and revolutionize our material world. After the discovery of graphene, the most prominent representative of this class of materials, many other 2D crystals have been identified. Even if individual 2D materials own various interesting and unexpected properties, the stacking of such layers leads to ‘artificial vdW solids’ called van der Waals heterostructures (vdW HSs) that result in the emergence of new states of matter with novel functionalities. The vdW HSs not only depend on the combination of different 2D crystal but also on their rotational alignment opening the avenue for a new field called twistronics. Coupling between the two layers depends on the stacking angle, which can be used as an external degree of freedom to tune their optical and electronic properties. Apart from excitonic ground states, 2D transition metal dichalcogenides (TMDs) and their heterostructures offer an excellent platform to explore fascinating higher-order excitations such as trion, biexciton, interlayer exciton, hybrid exciton, moiré exciton, and so on. The emergence of these higher-order excitations mostly depends on the symmetry, temperature, and the band alignment of the heterobilayer systems.

Intense Dark Exciton Emission from Strongly Quantum-Confined CsPbBr3 Nanocrystals

Dark exciton as the lowest-energy (ground) exciton state in metal halide perovskite nanocrystals is a subject of much interest. This is because the superior performance of perovskites as the photon source combined with long lifetime of dark exciton can be attractive for many applications of exciton. However, the direct observation of the intense and long-lived dark exciton emission, indicating facile access to dark ground exciton state, has remained elusive. Here, we report the intense photoluminescence from dark exciton with microsecond lifetime in strongly confined CsPbBr3 nanocrystals and reveal the crucial role of confinement in accessing the dark ground exciton state. This study establishes the potential of strongly quantum-confined perovskite nanostructures as the excellent platform to harvest the benefits of extremely long-lived dark exciton.

Ground and excited state exciton polarons in monolayer MoSe2.

Monolayer transition metal dichalcogenide semiconductors, with versatile experimentally accessible exciton species, offer an interesting platform for investigating the interaction between excitons and a Fermi sea of charges. Using hexagonal boron nitride encapsulated monolayer MoSe2, we study the impact of charge density tuning on the A and B series of exciton Rydberg states, including A:1s, A:2s, B:1s, and B:2s. The doping dependence of the A:2s state provides an opportunity to examine such interactions with greatly reduced exciton binding energy and more spatially diffuse structures, and we found that the impact of the Fermi sea becomes much more dramatic compared to the A:1s state. Using photoluminescence upconversion, we verify that the B:2s exciton state displays similar behavior when interacting with the Fermi sea despite being well above the bare bandgap in energy. Photoluminescence and reflection spectra of the A:1s state show clear evidence that the interaction of the exciton with a Fermi sea is best described by the exciton-polaron model, rather than a trion model. Our experimental results demonstrate that overall features of charge interaction are quite generic and highly robust, offering key insights into the dressed many body states in a Fermi sea.

Accurate modeling of electron-hole binding in CuCl. I. Exciton states

We present an accurate variational approach for calculating the ground and excited exciton states in CuCl, and provide detailed comparison with measured polariton energies of excited exciton states. Computed exciton energies and oscillator strengths allow us to reproduce the polariton dispersion of up to four exciton states with unprecedented accuracy. A reinterpretation of the observed $1s$-exciton binding energy shows that the actual Coulomb energy in the exciton ground state is more than 50% larger than the observed binding energy, with important consequences for calculation of exciton complexes such as the biexciton.

Extraction of temperature-dependent exciton-polariton damping in InP bulk crystal

The temperature dependence of exciton-polariton damping in InP bulk crystal was extracted by the method of integrated absorption. The extraction procedure excluding the contribution of inhomogeneous broadening into the exciton ground state absorption linewidth is graphically illustrated. The extracted temperature-dependent damping is analyzed regarding the primary dissipative mechanism in order to determine the material parameters of exciton-polariton scattering by acoustic and optical phonons.

IR luminescence mechanism in colloidal Ag[formula omitted]S quantum dots

In the present paper, we discuss general features of luminescent properties for various samples of colloidal Ag2S quantum dots (QDs), passivated with thioglycolic (Ag2S/TGA) and 2-mercaptopropionic acids (Ag2S/2-MPA), L-cysteine (Ag2S/L-cys), and also Ag2S QDs stabilized in gelatin (Ag2S/Gel) and Poly(N-vinylpyrrolidone) (Ag2S/PVP). It was found that for all samples, the Stokes shift of the luminescence peak relative to ground state exciton absorption is greater than the exciton binding energy in Ag2S (0.096-0.104 eV). Moreover, the Stokes shift turned out to be caused by not only the quantum dot size but also by the type of the organic ligand. With decreasing the quantum dot size, the Stokes shift increases significantly. The estimates of the energy level shifts for electrons, holes, as well as the position of the luminescence peak, led us to a conclusion that trap state luminescence occurs. The specific luminescence mechanism is determined by the recombination of an electron trapped at the luminescence center with a free hole. It was found that decay of the trap state luminescence has non-exponential behavior. A model is proposed that explains the non-exponential luminescence decay.

Even-Parity Self-Trapped Excitons Lead to Magnetic Dipole Radiation in Two-Dimensional Lead Halide Perovskites.

Recently, unconventional bright magnetic dipole (MD) radiation was observed from two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs). According to commonly accepted HOIP band structure calculations, such MD light emission from the ground-state exciton should be strictly symmetry forbidden. These results suggest that MD emission arises in conjunction with an as-yet unidentified symmetry-breaking mechanism. In this paper, we show that MD light emission originates from a self-trapped p-like exciton stabilized at energies below the primary electric dipole (ED)-emitting 1s exciton. Using suitable combinations of sample and collection geometries, we isolate the distinct temperature-dependent properties of the ED and MD photoluminescence (PL). We show that the ED emission wavelength is nearly constant with temperature, whereas the MD emission wavelength exhibits substantial red shifts with heating. To explain these results, we derive a microscopic model comprising two distinct parity exciton states coupled to lattice distortions. The model explains many experimental observations, including the thermal red shift, the difference in emission wavelengths, and the relative intensities of the ED and MD emission. Thermodynamic analysis of temperature-dependent PL reveals that the MD emission originates from a locally distorted structure. Finally, we demonstrate unusual hysteresis effects of the MD-emitting state near structural phase transitions. We hypothesize that this is another manifestation of the local distortions, indicating that they are insensitive to phase changes in the equilibrium lattice structure.

Crossover from weakly indirect to direct excitons in atomically thin films of InSe

We perform a $\mathbf{k \cdot p}$ theory analysis of the spectra of the lowest energy and excited states of the excitons in few-layer atomically thin films of InSe taking into account in-plane electric polarizability of the film and the influence of the encapsulation environment. For the thinner films, the lowest-energy state of the exciton is weakly indirect in momentum space, with its dispersion showing minima at a layer-number-dependent wave number, due to an inverted edge of a relatively flat topmost valence band branch of the InSe film spectrum and we compute the activation energy from the momentum dark exciton ground state into the bright state. For the films with more than seven In$_2$Se$_2$ layers, the exciton dispersion minimum shifts to $\Gamma$-point.

Electro-Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling.

Ultrafast interlayer charge transfer is one of the most distinct features of van der Waals (vdW) heterostructures. Its dynamics competes with carrier thermalization such that the energy of nonthermalized photocarriers may be harnessed by band engineering. In this study, nonthermalized photocarrier energy is harnessed to achieve near-infrared (NIR) to visible light upconversion in a metal-insulator-semiconductor (MIS) vdW heterostructure tunnel diode consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS2 ). Photoexcitation of the electrically biased heterostructure with 1.58 eV NIR laser in the linear absorption regime generates emission from the ground exciton state of WS2 , which corresponds to upconversion by ≈370 meV. The upconversion is realized by electrically assisted interlayer transfer of nonthermalized photoexcited holes from FLG to WS2 , followed by formation and radiative recombination of excitons in WS2 . The photocarrier transfer rate can be described by Fowler-Nordheim tunneling mechanism and is electrically tunable by two orders of magnitude by tuning voltage bias applied to the device. This study highlights the prospects for realizing novel electro-optic upconversion devices by exploiting electrically tunable nonthermalized photocarrier relaxation dynamics in vdW heterostructures.

Manipulation of linear and nonlinear optical properties of GaSb quantum ring in AlGaAs/GaAs/AlGaAs quantum well and AlAs/GaAs/InGaAs/AlAs double quantum well

Linear and nonlinear optical properties of GaSb quantum ring inside AlxGa1−xAs/GaAs/AlxGa1−xAs quantum well (QW) and AlAs/GaAs/InGaAs/AlAs double QW have been theoretically investigated. The effects of the Al concentration in the barrier, the widths of the layers, the position of the quantum ring inside the wells and the incident optical intensity on linear and nonlinear absorption and refractive index, associated to the ground excitonic state, have been explored. We found that by varying the different parameters of the studied structures and the incident optical intensity, one can manipulate the linear and nonlinear absorption coefficient and refractive index. These results make the studied systems a promising candidate for application in tunable nano-optoelectronic devices.

Strongly localized states in a single carbon nanotube with polymer molecules

A single-walled carbon nanotube (SWCNT) with conjugated polymer molecules is analyzed via optical spectroscopy. The presence of strongly localized excitonic states in the SWCNT is confirmed using time-integrated photoluminescence (PL). The PL spectrum exhibits extremely narrow width (~0.8 meV) which is attributed to the strong confinement of the states by polymer molecules. In addition, I observed that the excited states are gradually filled as a function of the excitation power, which supports the localized excitonic behavior. Only the ground excitonic state is observed at low excitation powers, but three additional PL peaks appear as the excitation power is increased. Especially, the power-dependent PL spectrum shows a blueshift and increased width, which can be elucidated in terms of quantum confined stark effect and the screening of induced electric fields. Overall, I demonstrate that the presence of polymer molecules induces several localized states in a single SWCNT.