Excitonic Spectra Research Articles

Exciton spectra in disordered quantum wells

Exciton spectra in disordered quantum wells

Interaction-Induced Crystalline Topology of Excitons.

We apply the topological theory of symmetry indicators to interaction-induced exciton band structures in centrosymmetric semiconductors. Crucially, we distinguish between the topological invariants inherited from the underlying electron and hole bands and those that are intrinsic to the exciton wave function itself. Focusing on the latter, we show that there exists a class of exciton bands for which the maximally localized exciton Wannier states are shifted with respect to the electronic Wannier states by a quantized amount; we call these excitons shift excitons. Our analysis explains how the exciton spectrum can be topologically nontrivial and sustain exciton edge states in open boundary conditions even when the underlying noninteracting bands have a trivial atomic limit. We demonstrate the presence of shift excitons as the lowest energy neutral excitations of the Su-Schrieffer-Heeger model in its trivial phase when supplemented by local two-body interactions.

Effects of dynamical dielectric screening on the excitonic spectrum of monolayer semiconductors

Effects of dynamical dielectric screening on the excitonic spectrum of monolayer semiconductors

Quantum chaotic features of the spin-orbit coupled excitons in disordered two-dimensional insulators.

We study the synergy between disorder (phenomenologically modeled by the introduction of Riesz fractional derivative in the corresponding Schrödinger equation) and spin-orbit coupling (SOC) on the exciton spectra in two-dimensional (2D) semiconductor structures. We demonstrate that the joint impact of "fractionality" and SOC considerably modifies the spectrum of corresponding "ordinary" (i.e., without fractional derivatives) hydrogenic problem, leading to the non-Poissonian statistics of the adjacent level distance distribution. The latter fact is strong evidence of the possible emergence of quantum chaotic features in the system. Using analytical and numerical arguments, we discuss the possibilities to control the above chaotic features using the synergy of SOC, Coulomb interaction, and "fractionality," characterized by the Lévy index μ.

Exciton interacting with a moiré lattice: Polarons, strings, and optical probing of spin correlations

The ability to create and stack different atomically thin transition metal dichacogenide (TMD) layers on top of each other has opened up a rich playground for exploring new and interesting two-dimensional (2D) quantum phases. As a consequence of this remarkable development, there is presently a need for new sensors to probe these 2D layers, since conventional techniques for bulk materials such as x-ray and neutron scattering are inefficient. Here, we develop a general theory for how an exciton in a TMD monolayer couples to spin and charge correlations in an adjacent moiré lattice created by a TMD bilayer. Virtual tunneling of charge carriers, assumed for concreteness to be holes, between the moiré lattice and the monolayer combined with the presence of bound hole-exciton states, i.e., trions, give rise to an effective interaction between the moiré holes and the exciton. In addition to the Umklapp scattering, we show that this interaction is spin-dependent and therefore couples the exciton to the spin correlations of the moiré holes, which may be in- as well as out-of-plane. We then use our theory to examine two specific examples where the moiré holes form in-plane ferromagnetic or antiferromagnetic order. In both cases, the exciton creates spin waves in the moiré lattice, which we analyze by using a self-consistent Born approximation that includes such processes to infinite order. We show that the competition between magnetic order and exciton motion leads to the formation of a well-defined quasiparticle consisting of the exciton surrounded by a cloud of magnetic frustration in the moiré lattice sites below. For the antiferromagnet, we furthermore demonstrate the presence of the elusive geometric string excitations and discuss how they can be observed via their smoking gun energy dependence on the spin-spin coupling, which can be tuned by varying the twist angle of the moiré bilayer. All these phenomena have clear signatures in the exciton spectrum, and as such our results illustrate that excitons are promising quantum probes providing optical access to the spin correlations of new phases predicted to exist in TMD materials. Published by the American Physical Society 2024

Temperature dependence of the near infrared absorption spectrum of single-wall carbon nanotubes dispersed by sodium dodecyl sulfate in aqueous solution: experiments and molecular dynamics study.

Single-wall carbon nanotubes (SWCNT) dispersed in water with the help of sodium dodecyl sulfate (SDS) surfactants exhibit a temperature dependent near infrared (NIR) exciton spectrum. Due to their biocompatibility and NIR spectrum falling within the transparent window for biological tissue, SWCNTs hold potential for sensing temperature inside cells. Here, we seek to elucidate the mechanism responsible for this temperature dependence, focusing on changes in the water coverage of the SWCNT as the surfactant structure changes with temperature. We compare optical absorption spectra in the UV-Vis-IR range and fully atomistic molecular dynamics (MD) simulations. The observed temperature dependence of the spectra for various SWCNTs may be attributed to changes in the dielectric environment and its impact on excitons. MD simulations reveal that the adsorbed SDS molecules effectively shield the SWCNT, with ~ 70% of water molecules removed from the first two adlayers; this coverage shows a modest temperature dependence. Although we are not able to directly demonstrate how this influences the NIR spectrum, this represents a potential pathway given the strong influence of the water environment on the excitons in SWCNTs. Optical absorption measurements were carried out in the UV-Vis-NIR range using a Varian Cary 5000 spectrophotometer in a temperature-controlled environment. PeakFit™ v. 4.06 was used as peak-fitting program in the spectral range 900-1400nm (890-1380meV) as a function of the temperature. Fully atomistic molecular dynamics simulations were conducted using the NAMD2 package. The CHARMM force field comprising two-body bond stretching, three-body angle deformation, four-body dihedral angle deformation, and nonbonded interactions (electrostatic and Lennard-Jones 6-16 potentials) was employed.

Photoluminescence properties of two-dimensional semiconductor heterointerfaces.

Two-dimensional metal-sulfur compounds have attracted much attention due to their novel physical properties, such as layered structure, ultrathin physical dimensions, and continuously tunable bandgap. The vertical stacking of different 2D semiconductors enables the heterojunction to retain the excellent properties of its constituent materials and has physical properties such as interlayer energy transfer and interlayer carrier transfer. In this paper, we utilize the carrier interlayer transfer properties of p-n heterojunctions and form heterojunctions using p-type Te and PdSe2 prepared with n-type monolayer WS2 using the microzone transfer technique. We found that the PL spectrum of monolayer WS2 is purer after heterojunction formation. The photoluminescence peaks representing exciton recombination are sharper, while the peaks represented by trions almost disappear. These phenomena indicate that we can utilize p-n junctions to capture the PL spectra of excitons in WS2, which is important for the further study of the optical properties of 2D metal-sulfur compounds.

Fast relaxation of interlayer excitons from the free to self-trapped states in lead halide perovskite van der Waals heterostructures

We study the thermal relaxation of interlayer excitons from the free to momentary self-trapped states in lead halide perovskite van der Waals heterostructures based on the well-known Huang-Rhys model. We find that these relaxation processes (self-trapped processes) are very fast ranging from nanoseconds to picoseconds. Moreover, the self-trapped time displays different variational trends by regulating three key structural parameters of the heterostructure, which could be intrinsically attributed to the modulation of exciton–phonon coupling by these structural parameters, resulting in the variation of the self-trapping depth. The underlying physical pictures that the changing of intersection points of two adiabatic potential between the free and momentary self-trapped states in the configuration coordinates are proposed to explain these relaxation processes. These results not only provide the significant enlightenments for analyzing the abnormal features of excitonic spectra in experiments but also present the practical ways to modulate the dynamical processes of excitons in two-dimensional structures.

Exciton states in molecular superlattices

The structure of molecular superlattices consists of van der Waals heterostructures of two-dimensional atomic crystals (ultrathin films) connected to each other with regular and periodic patterns. Inspired by the increasing importance of excitons for optical spectroscopy and the engineering of photonic materials in recent years, we have calculated the energy spectra of excitons in superlattices in this theoretical study. The number of film layers and the ratio between the exciton energy transfer within each motif (film) and the exciton energy transfer between neighboring motifs (between films) are important parameters that determine the behavior in an external electromagnetic field.

Surface optical phonon replica in photoluminescence spectroscopy of nitride nanostructures: Crystal structure and surface effects

The surface optical (SO) phonon replica in photoluminescence (PL) spectroscopy of nitride nanowires (NWs) was theoretically investigated in this study. The dispersive relationships of SO phonon mode in anisotropic wurtzite (WZ) and isotropic zinc-blende (ZB) crystal structure NWs with circular and square cross sections (CSs) were derived within the framework of the dielectric continuum model. Based on the energy and momentum conservation laws, a constraint relationship between the frequency and wave-number was constructed for SO phonon-assisted excitonic PL spectra in the NW structure. By combining the dispersive and constraint relationships, the frequency and wave-number of the SO phonon replica in the PL spectra could be determined. The WZ and ZB crystal structures of nitride semiconductor were considered. The influences of surface factors including the CS shape, dielectric medium, and environment temperature on the frequency and photon wavelength of the band-edge emission of the SO phonon replica were studied in detail. Numerical results reveal that the crystal structure, surface factors, and environment temperature greatly affect the frequency and photon wavelength of the band-edge emission of the SO phonon replica. The calculated results for the photon wavelength agree well with the experimental values of the SO phonon replica in AlN NWs. The results of the dielectric effect obtained here are also supported by previous experimental and theoretical results for nitrides and other semiconductor NWs. The present theoretical scheme and numerical results can be used to analyze and design the SO phonon replica in PL spectra of nanostructures.

Gaussian-process-regression-based method for the localization of exceptional points in complex resonance spectra

Resonances in open quantum systems depending on at least two controllable parameters can show the phenomenon of exceptional points (EPs), where not only the eigenvalues but also the eigenvectors of two or more resonances coalesce. Their exact localization in the parameter space is challenging, in particular in systems, where the computation of the quantum spectra and resonances is numerically very expensive. We introduce an efficient machine learning algorithm to find EPs based on Gaussian process regression (GPR). The GPR-model is trained with an initial set of eigenvalue pairs belonging to an EP and used for a first estimation of the EP position via a numerically cheap root search. The estimate is then improved iteratively by adding selected exact eigenvalue pairs as training points to the GPR-model. The GPR-based method is developed and tested on a simple low-dimensional matrix model and then applied to a challenging real physical system, viz., the localization of EPs in the resonance spectra of excitons in cuprous oxide in external electric and magnetic fields. The precise computation of EPs, by taking into account the complete valence band structure and central-cell corrections of the crystal, can be the basis for the experimental observation of EPs in this system.

Exciton and biexciton transient absorption spectra of CdSe quantum dots with varying diameters.

Transient absorption (TA) spectroscopy of semiconductor nanocrystals (NCs) is often used for excited state population analysis, but recent results suggest that TA bleach signals associated with multiexcitons in NCs do not scale linearly with exciton multiplicity. In this manuscript, we probe the factors that determine the intensities and spectral positions of exciton and biexciton components in the TA spectra of CdSe quantum dots (QDs) of five diameters. We find that, in all cases, the peak intensity of the biexciton TA spectrum is less than 1.5 times that of the single exciton TA spectrum, in stark contrast to a commonly made assumption that this ratio is 2. The relative intensities of the biexciton and exciton TA signals at each wavelength are determined by at least two factors: the TA spectral intensity and the spectral offset between the two signals. We do not observe correlations between either of these factors and the particle diameter, but we find that both are strongly impacted by replacing the native organic surface-capping ligands with a hole-trapping ligand. These results suggest that surface trapping plays an important role in determining the absolute intensities of TA features for CdSe QDs and not just their decay kinetics. Our work highlights the role of spectral offsets and the importance of surface trapping in governing absolute TA intensities. It also conclusively demonstrates that the biexciton TA spectra of CdSe QDs at the band gap energy are less than twice as intense as those of the exciton.

Optical spectra and exciton radiative lifetimes in bulk transition metal dichalcogenides.

The optical response of layered transition metal dichalcogenides (TMDCs) exhibits remarkable excitonic properties which are important from both fundamental and device application viewpoints. One of these phenomena is the observation of intralayer/interlayer excitons. While much effort has been done to characterize excitons in monolayer TMDCs and their heterostructures, a quite limited number of works have addressed the exciton spectra of their bulk counterparts. In this work, we employ ab initio many-body perturbation calculations to investigate the exciton dynamics and spectra of bulk 2H-MX2 (M = Mo, W, and X = S, Se). For molybdenum-based systems, we find the presence of interlayer excitons at energies higher than the first bright exciton (XA), with non-negligible strength intensity. Our results also show that interlayer excitons in tungsten-based systems are almost degenerate in energy with XA and possess very small oscillator strengths when compared with molybdenum-based systems. At room temperature, and considering the thermal exciton fine-structure population for the XA-exciton, we estimate effective radiative lifetimes in the range of ∼4-14 ns. For higher energy excitons we predict longer effective lifetimes of tens of nanoseconds.

Pressure effects on both fluorescent emission and charge transport properties of organic semiconductors: a computational study.

External pressure can regulate the photophysical property and charge transport performance of organic semiconductors, however, the underlying mechanism at the microscopic level is still elusive. Using thermal vibrational correlation function coupled quantum mechanics/molecular mechanics and full quantum charge transfer rate theory, we systematically explore the influence of pressure on fluorescence emission and charge transport behaviours of representative cyclooctatetrathiophene (COTh). It is found that, upon pressurization, the intramolecular configurations of COTh became more twisted, leading to the blue-shifted emission. The fluorescence quantum efficiency (FQE) of COTh crystals decreases monotonically in a wide pressure range of 0-4.38 GPa, because the increase of intermolecular electronic energy transfer rate constant (keet) is larger than the decrease of internal conversion rate constant (kic), and the variation of keet is dominant. The decrease in kic is attributed to the decreasing reorganization energy, reflecting the suppression of the low-frequency flipping vibrations of four thiophene rings and the high-frequency stretching vibrations of central cyclooctatetraene, while the keet increase is due to the simultaneous increase in exciton coupling and spectra overlap. Moreover, we predicted that the hole mobility of COTh increases monotonically by nearly an order of magnitude from 0.39 to 3.00 cm2 V-1 s-1 upon compression, because of the increase in transfer integral and the decrease of charged reorganization energy. Furthermore, its hole mobility exhibits obvious anisotropy. Our work systematically builds the external pressure, molecular packing, luminescence and transport properties relationships of organic semiconductors and provides theoretical guidance for the rational design of pressure responsive organic semiconductors with excellent photoelectric performance.

Simulation of exciton spectra in disordered supramolecular polymers: exciton localization in cisoid indolenine squaraine hexamers.

In order to understand the effects of disorder and defects in oligomers and polymers on the localization of excitons, we investigated the spectral properties of the squaraine B hexamer using long range corrected tight-binding TDDFT (lc-TDDFTB) and Frenkel-exciton model based calculations. Employing classical molecular dynamics, the cisoid indolenine squaraine hexamers helix was propagated in DCM and acetone to obtain ensembles of realistic structures, which naturally exhibit considerable disorder. The trajectories together with several model squaraine systems were studied to show the profound effects of disorder in the superstructure and disorder of the local monomer geometry on optical properties like absorption and exciton localization. We further compared lc-TDDFTB and exciton theory derived spectral data to related experimental data on absorption, exciton transfer and localization in squaraine polymers and oligomers.

Versatile optical manipulation of trions, dark excitons and biexcitons through contrasting exciton-photon coupling

Various exciton species in transition metal dichalcogenides (TMDs), such as neutral excitons, trions (charged excitons), dark excitons, and biexcitons, have been individually discovered with distinct light-matter interactions. In terms of valley-spin locked band structures and electron-hole configurations, these exciton species demonstrate flexible control of emission light with degrees of freedom (DOFs) such as intensity, polarization, frequency, and dynamics. However, it remains elusive to fully manipulate different exciton species on demand for practical photonic applications. Here, we investigate the contrasting light-matter interactions to control multiple DOFs of emission light in a hybrid monolayer WSe2-Ag nanowire (NW) structure by taking advantage of various exciton species. These excitons, including trions, dark excitons, and biexcitons, are found to couple independently with propagating surface plasmon polaritons (SPPs) of Ag NW in quite different ways, thanks to the orientations of transition dipoles. Consistent with the simulations, the dark excitons and dark trions show extremely high coupling efficiency with SPPs, while the trions demonstrate directional chiral-coupling features. This study presents a crucial step towards the ultimate goal of exploiting the comprehensive spectrum of TMD excitons for optical information processing and quantum optics.

Efficient computation of optical excitations in two-dimensional materials with the Xatu code

Here we describe an efficient numerical implementation of the Bethe-Salpeter equation to obtain the excitonic spectrum of semiconductors. This is done on the electronic structure calculated either at the simplest tight-binding level or through density functional theory calculations based on local orbitals. We use a simplified model for the electron-electron interactions which considers atomic orbitals as point-like orbitals and a phenomenological screening. The optical conductivity can then be optionally computed within the Kubo formalism. Our results for paradigmatic two-dimensional materials such as hBN and MoS2, when compared with those of more sophisticated first-principles methods, are excellent and envision a practical use of our implementation beyond the computational limitations of such methods. Program summaryProgram Title: XatuCPC Library link to program files:https://doi.org/10.17632/kj4rt95pvc.1Developer's repository link:https://github.com/alejandrojuria/xatuLicensing provisions: GPLv3Programming language: C++, Fortran, PythonNature of problem: The exciton spectrum is obtained as the solution of the Bethe-Salpeter equation for insulators and semi-conductors. Constructing the equation involves determining the screening of the electrostatic interaction and then determining the matrix elements of the interaction kernel, which are computationally-intensive tasks, specially if one takes a purely ab-initio approach.Solution method: The Bethe-Salpeter equation can be efficiently set up and solved assuming that the basis of the reference electronic structure calculation, obtained either from tight-binding models or density functional theory with actual localized orbitals, corresponds to point-like localized orbitals. This, in addition to using an effective screening instead of computing the dielectric constant, allows to obtain the interaction kernel at very low computational cost and, thereof, the exciton spectrum as well as the light absorption of materials.Additional comments including restrictions and unusual features: The code requires using at least C++11, given that it uses version-specific features. All linear algebra routines have been delegated to the Armadillo library.

Exciton absorption spectrum of thin films of ternary compounds in the CsCl–CuCl system

The absorption spectra of thin films (CsCl)1–x(CuCl)x were studied at T = 90 K in the spectral range 2–6 eV. From the analysis of the spectra, the formation of CsCu2Cl3 and Cs2CuCl3 compounds in the CsCl–CuCl system was created. The Cs2CuCl3 compound is stable only in a vacuum; in air, it apparently decomposes into CsCu2Cl3 with a disordered crystal lattice and CsCl. The exciton spectrum of both compounds is interpreted on the basis of transitions in the Cu+ ion. The one-dimensional character of excitons in CsCu2Cl3 and the two-dimensional character in Cs2CuCl3 were established.

Iterative density matrix revisited: Excitonic phenomena on the InAs quantum well as a saturable absorber

By solving the Liouville equation, third-order nonlinear terms of the optical absorptive two quantum-level system are obtained through iterative density matrix calculations. In the improved model, the full frequency range is taken into account instead of just the weak absorptive limit. he process investigated in this study can be likened to saturation fitting for heavy-hole excitons in the InAs quantum well. Additionally, we demonstrate that the iterative method used for analyzing band-to-band excitonic spectra enables easy retrieval of the static dielectric constant.

One‐Particle and Excitonic Band Structure in Cubic Boron Arsenide

Cubic BAs has received recent attention for its large electron and hole mobilities and large thermal conductivity. This is a rare and much desired combination in semiconductor industry: commercial semiconductors typically have high electron mobilities, or hole mobilities, or large thermal conductivities, but not all of them together. Herein, predictions from an advanced self‐consistent many‐body perturbative theory are reported and it is shown that with respect to one‐particle properties, BAs is strikingly similar to Si. There are some important differences, notably there is an unusually small variation in the valence band masses. With respect to two‐particle properties, significant differences with Si appear. The excitonic spectrum for both q = 0 and finite q is reported, and it is shown that while the direct gap in cubic BAs is about 4 eV, dark excitons can be observed down to about ≈1.5 eV, which may play a crucial role in application of BAs in optoelectronics.