Excitonic Interband Transitions Research Articles

Optical force exerted on the two dimensional transition-metal dichalcogenide coated dielectric particle by Gaussian beam

Two-dimensional transition-metal dichalcogenide (TMDC) exhibits a series of distinctive optical and electrical characteristics, which make it has a good application prospect in the field of optical manipulation. Based on the Mie theory, we investigate the radiation force exerted on the TMDC wrapped dielectric particle by Gaussian wave. Theoretical calculations show that the optical force spectra exhibit two resonant peaks in the visible region, which are generated by the interband exciton transitions in TMDC. Magnitude and morphology of the excitonic peaks could be modulated effectively by tuning the number of coated TMDC layers. Furthermore, the excitonic peaks transform significantly with particle size due to the variation of coupling strength between the dielectric particle and TMDC coating. The investigation provides potential applications in optical manipulations and light-matter interactions.

A large size single crystal growth, scientific evaluation, and giant Faraday effect of cadmium manganese telluride

The growth of large-sized Cd0.9Mn0.1T: V ingot was performed using the Te solution vertical Bridgman method. The ingot was cut into wafers and marked for scientific evaluation and property analysis. Through infrared microscopic analysis, we observed the planar growth interface for the first time, and the X-ray diffraction analyses indicated that the crystal possessed excellent crystallinity. Electron probe microanalysis revealed the homogeneous distribution of Mn, Cd, and Te not only in axial, but in radial direction. Inductively Coupled Plasma Mass Spectrometer investigated the distribution rules of various impurities in the axial direction. Te inclusions investigations showed the smaller sizes and the lower densities than previous studies, and the Infrared transmittance was almost up to the theoretical value 63%. These tests have confirmed that the grown crystals are perfect and can be used for device fabrication. The giant Faraday rotation of monochromatic polarized light was generated in the band edge because of the great contributions of the interband exciton transitions. Based on the Faraday effect, the 780 nm optical isolator was fabricated.

Quasiparticle energies and excitonic effects of chromium trichloride: From two dimensions to bulk

Layered van der Waals (vdW) magnetic materials have attracted significant research interest to date. In this work, we employ the first-principles many-body perturbation theory to calculate excited-state properties of a prototype vdW magnet, chromium trichloride (CrCl3), covering monolayer, bilayer, and bulk structures. Unlike usual non-magnetic vdW semiconductors, in which many-electron interactions and excited states are sensitive to dimensionality, many-electron interactions are always enhanced and dominate quasiparticle energies and optical responses of both two-dimensional and bulk CrCl3. The electron-hole (e-h) binding energy can reach 3 eV in monolayer and remains as high as 2 eV in bulk. Because of the cancellation effect between self-energy corrections and e-h binding energies, the lowest-energy exciton (optical gap) is almost not affected by the change of dimensionality. Besides, for the excitons with similar e-h binding energies, their dipole oscillator strength can differ by a few orders of magnitude.Our analysis shows that such a big difference is from a unique interference effect between complex exciton wavefunctions and interband transitions. Finally, we find that the interlayer stacking sequence and magnetic coupling barely change quasiparticle band gaps and optical absorption spectra of CrCl3. Our calculated low-energy exciton peak positions agree with available measurements. These findings give insight into the understanding of many-electron interactions and the interplay between magnetic orders and optical excitations in vdW magnetic materials.

Giant Efficiency of Visible Second-Harmonic Light by an All-Dielectric Multiple-Quantum-Well Metasurface

A high-efficiency nonlinear source of visible light in a multiple-quantum-well (MQW) metasurface is in high demand for photonic quantum technology, but existing systems suffer from the limited conduction-band offset of MQWs and high dissipative losses of metal, which severely limit applicability in the visible range. This study presents a simple, reliable system without metal losses, by utilizing the interband excitonic transitions of MQWs and the Mie resonances of the metasurface's structure. This shows a viable path toward a coherent, nonlinear light source for use in the visible region and beyond with high conversion efficiency, to enable nanophotonic quantum information processing.

Symmetry regimes for circular photocurrents in monolayer MoSe2

In monolayer transition metal dichalcogenides helicity-dependent charge and spin photocurrents can emerge, even without applying any electrical bias, due to circular photogalvanic and photon drag effects. Exploiting such circular photocurrents (CPCs) in devices, however, requires better understanding of their behavior and physical origin. Here, we present symmetry, spectral, and electrical characteristics of CPC from excitonic interband transitions in a MoSe2 monolayer. The dependence on bias and gate voltages reveals two different CPC contributions, dominant at different voltages and with different dependence on illumination wavelength and incidence angles. We theoretically analyze symmetry requirements for effects that can yield CPC and compare these with the observed angular dependence and symmetries that occur for our device geometry. This reveals that the observed CPC effects require a reduced device symmetry, and that effects due to Berry curvature of the electronic states do not give a significant contribution.

Influence of Polar Pressure Transmission Medium on the Pressure Coefficient of Excitonic Interband Transitions in Monolayer WSe2**Supported by the National Key Research and Development Program of China under Grant No 2016YFA0301202, the National Natural Science Foundation of China under Grant Nos 11474275, 61674135 and 91536101, the Strategic Priority Research Program of the Chinese Academy of Sciences under Grant No XDPB0603,

The influence of the pressure transmission medium (PTM) on the excitonic interband transitions in monolayer tungsten diselenide (WSe2) is investigated using photoluminescence (PL) spectra under hydrostatic pressure up to 5GPa. Three kinds of PTMs, condensed argon (Ar), 1:1 n-pentane and isopentane mixture (PM), and 4:1 methanol and ethanol mixture (MEM, a PTM with polarity), are used. It is found that when either Ar or PM is used as the PTM, the PL peak of exciton related to the direct K–K interband transition shows a pressure-induced blue-shift at a rate of 32±4 or 32±1 meV/GPa, while it turns to be 50±9 meV/GPa when MEM is used as the PTM. The indirect Λ–K interband transition presents almost no shift with increasing pressure up to approximately 5GPa when Ar and PM are used as the PTM, while it shows a red-shift at the rate of −17±7 meV/GPa by using MEM as the PTM. These results reveal that the optical interband transitions of monolayer WSe2 are very sensitive to the polarity of the PTM. The anomalous pressure coefficient obtained using the polar PTM of MEM is ascribed to the existence of hydrogen-like bonds between hydroxyl in MEM and Se atoms under hydrostatic pressure.

Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers.

We report that the refractive index of transition metal dichacolgenide (TMDC) monolayers, such as MoS2, WS2, and WSe2, can be substantially tuned by >60% in the imaginary part and >20% in the real part around exciton resonances using complementary metal-oxide-semiconductor (CMOS) compatible electrical gating. This giant tunablility is rooted in the dominance of excitonic effects in the refractive index of the monolayers and the strong susceptibility of the excitons to the influence of injected charge carriers. The tunability mainly results from the effects of injected charge carriers to broaden the spectral width of excitonic interband transitions and to facilitate the interconversion of neutral and charged excitons. The other effects of the injected charge carriers, such as renormalizing bandgap and changing exciton binding energy, only play negligible roles. We also demonstrate that the atomically thin monolayers, when combined with photonic structures, can enable the efficiencies of optical absorption (reflection) tuned from 40% (60%) to 80% (20%) due to the giant tunability of the refractive index. This work may pave the way toward the development of field-effect photonics in which the optical functionality can be controlled with CMOS circuits.

Temperature dependent dielectric function and the E critical points of hexagonal GaN from 30 to 690 K

The complex dielectric function ɛ and the E0 excitonic and band-edge critical-point structures of hexagonal GaN are reported for temperatures from 30 to 690 K and energies from 0.74 to 6.42 eV, obtained by rotating-compensator spectroscopic ellipsometry on a 1.9 μm thick GaN film deposited on a c-plane (0001) sapphire substrate by molecular beam epitaxy. Direct inversion and B-splines in a multilayer-structure calculation were used to extract the optical properties of the film from the measured pseudodielectric function ⟨ɛ⟩. At low temperature sharp E0 excitonic and critical-point interband transitions are separately observed. Their temperature dependences were determined by fitting the data to the empirical Varshni relation and the phenomenological expression that contains the Bose-Einstein statistical factor.

Exciton optical transitions in quantum wells with spin-orbit coupling

The energy spectrum of light-hole and heavy-hole excitons and optical absorption in a quantum well have been analyzed taking into account Rashba spin-orbit coupling. Interband and intraband exciton transitions have been considered. It has been shown that, in the presence of spin-orbit coupling, the probabilities of the interband and intraband photoelectric effects diverge in the vicinity of the threshold if the electron-hole interaction is neglected. The threshold probabilities of the interband and intraband photoelectric effects become finite when Coulomb interaction is taken into account.

Electroluminescence from chirality-sorted (9,7)-semiconducting carbon nanotube devices

We have measured the electroluminescence and photoluminescence of (9,7) semiconducting carbon nanotube devices and demonstrate that the electroluminescence wavelength is determined by the nanotube's chiral index (n,m). The devices were fabricated on Si3N4 membranes by dielectrophoretic assembly of tubes from monochiral dispersion. Electrically driven (9,7) devices exhibit a single Lorentzian shaped emission peak at 825 nm in the visible part of the spectrum. The emission could be assigned to the excitonic E22 interband transition by comparison of the electroluminescence spectra with corresponding photoluminescence excitation maps. We show a linear dependence of the EL peak width on the electrical current, and provide evidence for the inertness of Si3N4 surfaces with respect to the nanotubes optical properties.

Exploring optimal UV emission windows for AlGaN and AlInN alloys grown on different templates

The interband excitonic transition energies and their corresponding polarization properties of AlGaN and AlInN alloys pseudomorphically grown on c-plane GaN and AlN templates, have been systematically investigated under the framework of k · p perturbation theory. It is found that by changing alloy composition which is accompanied with misfit strain modulation, the valence band structure of both AlGaN and AlInN can be switched from AlN-like to GaN-like, thus leading to enhancement of the surface ultraviolet emission efficiency. Optimal alloy composition windows for efficient ultraviolet emissions have been established where AlGaN and AlInN alloys grown on AlN template show wider optimal windows than that grown on GaN template.

Near band edge anisotropic optical transitions in wide band gap semiconductor Cu2ZnSiS4

In this study, anisotropic near band edge transitions of Cu2ZnSiS4 single crystals grown by chemical vapor transport were characterized by using polarization-dependent absorption, piezoreflectance (PzR) and surface photovoltage (SPV) spectroscopy techniques at room temperature. The measurements were carried out on the as grown basal plane with the normal along [2 1 0] and the axis c parallel to the long edge of the crystal platelet. Analysis of absorption and SPV spectra reveal indirect allowed transitions for the absorption edge of Cu2ZnSiS4. The estimated values of indirect band gap are 2.97 eV and 3.07 eV, respectively, for E⊥c and E∥c polarization configurations. The polarization-dependent PzR and SPV spectra in the vicinity of the direct band gap of Cu2ZnSiS4 reveal features E⊥ex and E∥ex at around 3.32 eV and 3.41 eV for E⊥c and E∥c polarizations, respectively. Both features E⊥ex and E∥ex are associated with the interband excitonic transitions at point Γ and can be explained by crystal-field splitting of valence band. Based on the experimental observations, a plausible band structure near band edge of Cu2ZnSiS4 is proposed.

Temperature-dependent study of the band-edge excitonic transitions of Cu 2ZnSiS 4 single crystals by polarization-dependent piezoreflectance

The temperature dependence of the band-edge excitonic transitions of Cu 2ZnSiS 4 single crystals were characterized by using polarization-dependent piezoreflectance (PzR) in the temperature range of 10–300 K. The PzR measurements were carried out on the as-grown basal plane with the normal along [2 1 0] and the c axis parallel to the long edge of the crystal platelet. The PzR spectra revealed polarization-dependent E ⊥ ex and E | | ex features for E⊥ c and Е||c polarization, respectively. Both E ⊥ ex and E | | ex features are associated with the interband excitonic transitions at Γ point and can be explained by crystal-field splitting of valence band. From a detailed lineshape fit to the PzR spectra, the temperature dependence of the transition energies and broadening parameters of the band-edge excitons were determined accurately. The temperature dependence of near band-edge excitonic transition energies were analyzed using Varshni and Bose–Einstein expressions. The temperature dependence of the broadening parameter of excitonic features also has been studied in terms of a Bose–Einstein equation that contains the electron (exciton)–longitudinal optical phonon-coupling constant. The parameters that describe the temperature variation of the excitonic transition energies and broadening parameters were evaluated and discussed.

Growth Strategy and Physical Properties of the High Mobility P-Type CuI Crystal

Acquiring stable binary wide band-gap semiconductor (WBS) materials with high p-type mobility is essential for the development of WBS optoelectronic devices. CuI is a p-type WBS material with a large band gap (3.1 eV) and high exciton binding energy (62 meV). However, the semiconductor characteristics of the CuI single crystal are unknown due to the lack of a large sized and high quality crystal. Our approach focuses on the design of the mineralizer for the hydrothermal method to effectively control the growth habit and the impurity concentration in the crystal. A large size (15 mm × 10 mm × 1 mm) and high quality CuI single crystal is obtained by using a new mineralizer (NH4I + KI). The crystal shows high p-type mobility (43.9 cm2·V−1·S1−). The strong and sharp band-edge emission at 410 nm indicates that the interband excitonic transition dominates the optical response in the spectrum. Such a binary crystalline material may open the way to new applications in optoelectronic devices.

Resonant enhancement of second order sideband generation for intraexcitonic transitions in GaAs/AlGaAs multiple quantum wells

We present an experimental study on efficient second order sideband generation in symmetric undoped GaAs/AlGaAs multiple quantum wells. A near-infrared laser tuned to excitonic interband transitions is mixed with an in-plane polarized terahertz beam from a free-electron laser. The terahertz beam is tuned either to the intraexcitonic heavy-hole 1s-2p transition or to the interexcitonic heavy-hole light-hole transition. We find strong evidence that the intraexcitonic transition is of paramount influence on n=±2 sideband generation, leading to an order-of-magnitude resonant enhancement of the conversion efficiency up to 0.1% at low temperature. At room temperature, the efficiency drops only by a factor of 7 for low terahertz powers.

Electro‐modulation enhancement in the InGaNAs/GaAs quantum well structures

AbstractElectrooptical properties of InGaNAs/GaAs quantum well structures were investigated by photoreflectance spectroscopy. The MBE grown structures consist of a central InGaNAs well, GaAs barriers, and GaAsN strain compensating buffer layers of different thickness. More quantum states and extended wave functions in the system with strain relief GaAsN barriers due to broader and lower confinement profile. Particular electro‐modulated spectral features corresponding to excitonic interband transitions are enhanced. Compared with the numerically calculated quantum state energies and wave functions, the electromodulation enhanced transitions were those involving quantum states with wave functions distribute in wider regions and extend more from the InGaNAs well into the space charge region near interfaces. It was the internal electric field near the heterointerfaces strongly enhanced the electro‐modulation efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of InGaNAs/GaAs quantum well structures with GaNAs strain relief buffer layers

The modulated optical responses of InGaNAs/GaAs quantum well structures were characterized by photoreflectance spectroscopy. The quantum well excitonic interband transitions were observed in the spectral range above hν= E g (InGaNAs). When the InGaNAs well is sandwiched in between GaNAs strain relief buffer layers, the quantum confinement potential becomes one step wider and increases the number of quantum states in the system. The quantum well subband energies and wave functions were calculated numerically to compare with the major optical features. The modulated optical responses are enhanced, especially for those quantum states with wave function extending into GaNAs and GaAs barriers where interface space charges induce strong internal field and enhance the electro-modulation efficiency.

Optical characterization of a Cd0.85Mg0.15Se mixed crystal

We present an optical characterization of a Bridgman-grown wurtzite-typeCd0.85Mg0.15Se mixed crystal in the near-band-edge interband transitions using temperature-dependentcontactless electroreflectance (CER) and photoreflectance (PR) in the temperature range15–400 K. The interband excitonic transitions A and C originating from the band edge andspin–orbital splitting critical points of the sample, respectively, have been observed inthe CER/PR spectra. The transition energies and broadening function of theexcitonic features are determined via a lineshape fit to the CER/PR spectra. Theparameters that describe the temperature dependence of the transition energies ofexcitons A and C, and the broadening function of exciton A, are evaluated anddiscussed.

Determination of deformation potentials in strained InSb quantum wells

We use interband exciton transitions in InSb∕AlxIn1−xSb multi-quantum-well samples to determine the heavy-hole and light-hole energy gaps as the strain is varied using Al concentrations up to 25%. The gaps are compared to deformation-dependent calculations of the energy gaps in the presence of biaxial strain to obtain a measure of the deformation potentials α and β.

Study of Be δ-doped GaAs/AlAs multiple quantum wells by the surface photovoltage spectroscopy

Abstract We report a surface photovoltage and differential surface photovoltage (DSPV) study of Be δ-doped GaAs/AlAs multiple quantum wells (QWs) with widths ranging from 3 to 20 nm and sheet doping densities from 2 × 10 10 to 2.5 × 10 12 cm −2 per well aiming to characterize their electronic properties and structural quality. From a line shape analysis of room temperature DSPV spectra the interband excitonic transition energies and broadening parameters for a large number of QW-related subbands have been established. A study of well-width and quantum number dependencies of the excitonic linewidths allowed us to evaluate the various broadening contributions to the spectral line shapes in QWs of different design. It was found that an average half monolayer well-width fluctuations are the dominant broadening mechanism of the excitonic line for QWs thinner than 10 nm. In QWs thicker than 10 nm, the spectral line broadening originates mainly from thermal broadening as well as Stark broadening due to random electric fields of ionized impurities and exciton scattering by free holes.