Longitudinal-transverse Splitting Research Articles

Phonon-Driven Femtosecond Dynamics of Excitons in Crystalline Pentacene from First Principles.

Nonradiative exciton relaxation processes are critical for energy transduction and transport in optoelectronic materials, but how these processes are connected to the underlying crystal structure and the associated electron, exciton, and phonon band structures, as well as the interactions of all these particles, is challenging to understand. Here, we present a first-principles study of exciton-phonon relaxation pathways in pentacene, a paradigmatic molecular crystal and optoelectronic semiconductor. We compute the momentum- and band-resolved exciton-phonon interactions, and use them to analyze key scattering channels. We find that both exciton intraband scattering and interband scattering to parity-forbidden dark states occur on the same ∼100 fs timescale as a direct consequence of the longitudinal-transverse splitting of the bright exciton band. Consequently, exciton-phonon scattering exists as a dominant nonradiative relaxation channel in pentacene. We further show how the propagation of an exciton wave packet is connected with crystal anisotropy, which gives rise to the longitudinal-transverse exciton splitting and concomitant anisotropic exciton and phonon dispersions. Our results provide a framework for understanding the role of exciton-phonon interactions in exciton nonradiative lifetimes in molecular crystals and beyond.

Exciton–Polaritons in CsPbBr3 Crystals Revealed by Optical Reflectivity in High Magnetic Fields and Two‐Photon Spectroscopy

Cesium lead bromide (CsPbBr3) is a representative material of the emerging class of lead halide perovskite semiconductors that possess remarkable optoelectronic properties. Its optical properties in the vicinity of the bandgap energy are greatly contributed by excitons, which form exciton polaritons due to strong light–matter interactions. Exciton–polaritons in solution‐grown CsPbBr3 crystals are examined by means of circularly polarized reflection spectroscopy measured in high magnetic fields up to 60 T. The excited 2P exciton state is measured by two‐photon absorption. Comprehensive modeling and analysis provides detailed quantitative information about the exciton–polariton parameters: exciton binding energy of 32.5 meV, oscillator strength characterized by longitudinal–transverse splitting of 5.3 meV, damping of 6.7 meV, reduced exciton mass of 0.18m0, exciton diamagnetic shift of 1.6 μeV T−2, and exciton Landé factor . It is shown that the exciton states can be described within a hydrogen‐like model with an effective dielectric constant of 8.7. From the measured exciton longitudinal–transverse splitting, Kane energy of Ep = 15 eV is evaluated, which is in reasonable agreement with values of 11.8–12.5 eV derived from the carrier effective masses.

Vibrational and dielectric properties of InN in orthorhombic Pnma phase

We have determined the lattice vibrational and dielectric properties of the newly predicted orthorhombic Pnma-InN, using the density functional perturbation theory. The origin of the observed phonon band gap separating optical modes is discussed. The full set zone center phonon modes and the longitudinal-transverse (LO-TO) splitting are presented and discussed. The LO-TO splitting is found to be largest for B2u mode at 471.33 cm−1 which is dominated by in-phase displacements of nitrogen atoms. The structural and dielectric properties of the orthorhombic Pnma-InN are compared to those of wurtzite and zinc blende phases of InN.

Photophysics of β-Ga2O3: Phonon polaritons, exciton polaritons, free-carrier absorption, and band-edge absorption

Monoclinic gallium oxide (β-Ga2O3) has attracted much attention from the fields of optoelectronic and electronic devices owing to the properties of wide bandgap, great breakdown field strength, as well as the economic advantages of low-cost growth of large-size single crystals. Here, the basic photophysical properties including absorption (free-carrier absorption and band-edge absorption) and reflection (phonon polaritons and exciton polaritons) of differently doped β-Ga2O3 with diverse carrier concentrations are studied in detail. The unpolarized reflection spectra of differently doped β-Ga2O3 crystals are well fitted based on the non-polarized reflection model. Besides, according to analysis, the longitudinal–transverse splitting energy of β-Ga2O3 direct excitons is estimated to be as high as 100 meV, reflecting the strong interaction between light and excitons. It is hoped that this work can provide beneficial reference for a comprehensive understanding on the spectral physical characteristics of β-Ga2O3, so as to deepen and expand the basic recognition of this material in the aspect of photophysical properties.

Observation of 3D Biexcitons in Pristine-Quality CH3 NH3 PbBr3 Single Crystals.

Halide perovskites (HPs) are fascinating materials whose optoelectronic properties are arguably excitonic. In the HP family, biexcitons are known to exist only in low dimensions where exciton-exciton binding is strongly enhanced by quantum and dielectric confinements. In this paper, however, it is shown that they indeed do exist in 3D bulk CH3 NH3 PbBr3 (MAPbBr3 ) single crystals if the pristine crystal quality is ensured for subtle binding of two excitons. The existence of biexcitons is clearly evidenced below 30 K with a binding energy of ≈3.9 ± 0.3 meV according to i) exciton-biexciton population dynamics, ii) giant resonant two-photon excitation of biexcitons, iii) inverted Boltzmann-type spectral feature, and iv) zero degree of circular polarization in the biexciton photoluminescence. Because of the polariton effect, the two-photon resonance occurs at the excited biexciton state from which longitudinal-transverse splitting is calculated to be 3.7 meV. The discovery of the 3D biexcitons underscores the very quality of HP crystals for generating various many-body excitonic phases in MAPbBr3 and its analogues toward the improved understanding of their fundamental properties and highly efficient optoelectronic applications.

Flat Bands and Giant Light-Matter Interaction in Hexagonal Boron Nitride.

Dispersionless energy bands in k space are a peculiar property gathering increasing attention for the emergence of novel electronic, magnetic, and photonic properties. Here, we explore the impact of electronic flat bands on the light-matter interaction. The van der Waals interaction between the atomic layers of hexagonal boron nitride induces flat bands along specific lines of the Brillouin zone. The macroscopic degeneracy along these lines leads to van Hove singularities with divergent joint density of states, resulting in outstanding optical properties of the excitonic states. For the direct exciton, we report a giant oscillator strength with a longitudinal-transverse splitting of 420meV, a record value, confirmed by our abinitio calculations. For the fundamental indirect exciton, flat bands result in phonon-assisted processes of exceptional efficiency, that compete with direct absorption in reflectivity, and that make the internal quantum efficiency close to values typical of direct band gap semiconductors.

Interference of exciton-polariton waves in GaSe nanocrystals

Absorption, reflection, photoluminescence and wavelength modulation transmission spectra of GaSe single crystals were investigated in temperature rage 300 – 10 K. From fitting of contours of excitonic reflection spectra parameters of excitons as energy of transversal exciton ω0 = 2.1212 eV, longitudinal transversal splitting ωLT = 2.3 meV, effective mass M = 2.5m0, background permittivity εb= 5.2 and damping constant γ = 1.4 were determined. Effective masses of electrons (mC1 = 0.37m0) and holes (mV1 = 2.13m0) in the Brillouin zone center were estimated. An interference of waves of upper and lower branches of exciton-polaritons was found out in wavelength modulation transmission spectra of GaSe nanocrystals.

Topological spin textures of exciton-polaritons manipulating by spin-orbit coupling

The topological spin textures of exciton-polariton in microcavities are governed by the interplay of the spin-orbit coupling and longitudinal-transverse splitting. We demonstrate the generation of exciton-polariton spin vortices by a linearly polarized optical pump and analyze the arising exciton-polariton topological spin textures in microcavities. By manipulating the spin-orbit coupling and polarization states, one can control the density, phase, and polarization distributions of exciton-polaritons. Moreover, we obtain a vortex of the pure phase winding number and spin-vortices characterized by quantized polarization winding, which holds a potential application for spinoptronic devices and quantum information.

Control of the exciton valley dynamics in atomically thin semiconductors by tailoring the environment

The exciton valley dynamics in van der Waals heterostructures with transition metal dichalcogenides monolayers is driven by the long-range exchange interaction between the electron and the hole in the exciton. It couples the states active in the opposite circular polarizations resulting in the longitudinal-transverse splitting of excitons propagating in the monolayer plane. Here we study theoretically the effect of the dielectric environment on the long-range exchange interaction and demonstrate how the encapsulation in the hexagonal boron nitride modifies the exciton longitudinal-transverse splitting. We calculate the exciton spin/valley polarization relaxation due to the long-range exchange interaction and demonstrate that the variation of the monolayer environment results in significant, up to five-fold, enhancement of the exciton valley polarization lifetime.

Зависимость светоэкситонного взаимодействия от ширины квантовой ямы во внешнем однородном электрическом поле

The paper is devoted to analyse of the dependance of exciton-light coupling on a quantum well thickness at presence of an uniform electric field. It is shown that the increasing of quantum well thickness at an enough strong field results in decreasing of the exciton-light coupling in compare with narrower wells.The numerical calculations of longitudinal-transverse splitting are performed for a wide ranges of both electric field and values of quantum well thickness.The analyse of obtained data allows to get analytical expressions which desсribe the exciton-light coupling dependence like on the electric field as quantum well thickness.

Up-conversion luminescence in GaSe nanocrystals

Abstract Optical properties of GaSe crystals have been investigated at wide temperature range from 10 to 300 K by help of reflection, absorption, and wavelength modulated reflection, and transmission spectra. Received data were compared with measured photoluminescence spectra. Three excitonic series were found out. The parameters of excitonic serie for С1-V1 bnads were determined. The longitudinal-transversal splitting of excitonic polariton ground states were determined. The doublet character of the transversal excitonic polariton mode (ωТ) was found out. The ground (n = 1) and excited (n = 2,3 … Eg) states of two high-energy excitonic series caused by C2–V1 and С3-V1 bands had been recognized, and its parameters were determined. The up-conversion of luminescence from high-energy excitonic levels (Е > 3 eV) was studied. The electrons were excited from V1 (Г1) to C1 (Г6) and C2 (Г5) bands with further transitions to higher energy levels С3-С6. So the luminescence from the excitonic levels of C3-С6 bands towards valence bands takes place. The band model in the Brillouin zone centerwas constructed based on the obtained data.

Optical phonons with a negative oscillator strength

We discuss the nature of additional (redundant from the point of view of selection rules) optical phonons observed in most crystals with ion–covalent bonds between atoms, including in their solid solutions. These ‘redundant’ phonons are located in the frequency range of the longitudinal–transverse splitting of fundamental phonons, where the real part of the crystal permittivity is negative. They are also split by the crystal field into transverse and longitudinal phonons such that the frequencies of redundant longitudinal phonons are lower than those of redundant transverse phonons and the oscillator strength of these phonons is negative.

Long-range electron-hole exchange interaction in aluminum nitride

To resolve the discrepancies in the exciton fine structure of aluminum nitride (AlN), polarization- and angle-resolved photoluminescence (PL) spectroscopies are performed. The excitonic PL spectra strongly depend on the optical polarization and detection angle. We propose that both the long-range and short-range electron-hole exchange interaction should be used to interpret the luminescence spectra. The theoretical framework fully explains the present and previous experimental results. The large longitudinal-transverse splitting energy obtained in this study suggests that AlN has strong light-matter coupling without quantum-confined structures.

Temperature-Dependent Total Absorption of Exciton Polaritons in Bulk Semiconductors

This paper highlights the temperature factor in the experimental study of total absorption as a method for experimental validation and the study of the exciton–polariton light transfer mechanism near the fundamental absorption edge in semiconductor crystals with spatial dispersion. The results of experimental studies of temperature-dependent total exciton absorption are generalized. The experimentally determined critical temperatures above which total absorption becomes constant, the corresponding critical damping parameter, and longitudinal–transverse splittings for the studied semiconductors CdTe, GaAs, InP, ZnSe, and ZnTe are presented.

Dependence of the Longitudinal-Transverse Exciton Splitting in a Quantum Well on an External Homogeneous Electric Field

A decrease in the longitudinal-transverse exciton splitting in a wide GaAs/AlGaAs quantum well (QW) in a homogeneous electric field has been theoretically considered. The dependences of the splitting on the applied field have been determined. The exciton reflection spectra of the wide QW have been calculated for the case where the field is oriented across the QW layer. The amplitude of exciton-related spectral oscillations is shown to decrease due to a decrease of the photo-excitonic interaction.

Температурно-зависимое интегральное поглощение экситонных поляритонов в полупроводниковых кристаллах

Abstract This paper highlights the temperature factor in the experimental study of total absorption as a method for experimental validation and the study of the exciton–polariton light transfer mechanism near the fundamental absorption edge in semiconductor crystals with spatial dispersion. The results of experimental studies of temperature-dependent total exciton absorption are generalized. The experimentally determined critical temperatures above which total absorption becomes constant, the corresponding critical damping parameter, and longitudinal–transverse splittings for the studied semiconductors CdTe, GaAs, InP, ZnSe, and ZnTe are presented.

Superradiance-to-Polariton Crossover of Wannier Excitons with Multiple Resonances.

We demonstrated the superradiance-to-polariton crossover of the blue excitons in Cu_{2}O by varying the sample thicknesses instead of controlling the cavity quality factor. The crossover behavior was compared with unprecedented calculations based on the nonlocal optical response theory with the inclusion of three exciton resonances. The crossover thickness, found to be 177±2 nm, was smaller than the predicted value for a single resonance by a factor of 5. The fact that there was much larger longitudinal-transverse splitting (40±5 meV) than in the bulk implies a surprisingly fast radiative recombination even without a cavity structure.

Nonlinear ring-shaped waves in spinor polariton condenates

Systems of interacting polaritons are known to support a rich variety of propagating states whose properties are essentially conditioned by the geometry of the potential and by the nonlinear nature of polariton-polariton interactions. We consider theoretically one-dimensional polariton ring accounting for both longitudinal-transverse (TE-TM) and Zeeman splitting of spinor polariton states and spin-dependent polariton-polariton interactions. We present the class of solutions which feature oscillations of the density in both spinor components and rotate with constant angular velocity. We show that the effects of the geometric phase arising from the interplay between external magnetic field and TE-TM splitting introduce chirality in the system and make solitons propagating in clockwise and anticlockwise directions non equivalent. This can be interpreted as solitonic analog of Aharonov-Bohm effect.

Chiral solitons in spinor polariton rings

We consider theoretically one-dimensional polariton ring accounting for both longitudinal-transverse (TE-TM) and Zeeman splitting of spinor polariton states and spin dependent polariton-polariton interactions. We present the novel class of solutions in the form of the localized defects rotating with constant angular velocity and analyze their properties for realistic values of the parameters of the system. We show that the effects of the geometric phase arising from the interplay between external magnetic field and TE-TM splitting introduce chirality in the system and make solitons propagating in clockwise and anticlockwise directions non equivalent. This can be interpreted as solitonic analog of Aharonov-Bohm effect.

Phonon states of B12P2 crystals: Ab initio calculation and experiment

Using methods of density functional theory (DFT) and Raman spectroscopy (RS) we investigate the phonon structure of the B12P2 crystal. The theory is consistent with the experiment for all twelve Raman-active modes. We have calculated Born charges, oscillator strengths, longitudinal-transverse splitting, and dielectric functions of dipole modes. The results are used to interpret the known IR spectra of B12P2. Analysis of phonon dispersion shows that the crystal is dynamically stable. The energy gap separating acoustic and optical phonons is absent due to the overlap of the phonon states at the Brillouin zone boundary. Vibrations of phosphorus and boron atoms contribute to the low- and high-frequency regions of the phonon density of states, respectively.