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The operation of THz quantum cascade laser in the region of negative differential resistance
摘要： We investigate the light-current-voltage characteristics and emission spectra of 2.3 THz quantum cascade laser operating in the negative differential resistance (NDR) region. It was shown that the formation of electric ?eld domains (EFDs) leads to a large number of discontinuities on the current-voltage and the total optical power on current characteristics. Measurements of emission spectra at different current (before the NDR region and in the NDR region) shows that the formation of EFDs results in decrease of the output intensity, but does not in?uence on lasing frequencies. The performed calculations qualitatively explain the experimental results.
关键词： THz region,Resonant-phonon design,Quantum-cascade laser,Negative differential resistivity
更新于2025-09-11 14:15:04
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Fast Electron and Slow Hole Relaxation in InP-Based Colloidal Quantum Dots
摘要： Colloidal InP-based quantum dots are a promising material for light-emitting applications as an environment friendly alternative to their Cd-containing counterparts. Especially for their use in optoelectronic devices, it is essential to understand how charge carriers relax to the emitting state after injection with excess energy and if all of them arrive at this desired state. Herein, we report on time-resolved differential transmission measurements on colloidal InP/ZnS and InP/ZnSe core/shell quantum dots. By optically exciting and probing individual transitions, we are able to distinguish between electron and hole relaxation. This in turn allows us to determine how the initial excess energy of the charge carriers affects the relaxation processes. According to the electronic level scheme, one expects a strong phonon bottleneck for electrons, while holes should relax easier as their energy levels are more closely spaced. On the contrary, we find that electrons relax faster than holes. The fast electron relaxation occurs via an efficient Auger-like electron-hole scattering mechanism. On the other hand, a small wave function overlap between core and shell states slows the hole relaxation down. Additionally, holes can be trapped at the core/shell interface leading to either slow detrapping or nonradiative recombination. Overall, these results demonstrate that it is crucial to construct devices enabling the injection of charge carriers energetically close to their emitting states in order to maximize the radiative efficiency of the system.
关键词： InP,differential transmission spectroscopy,phonon bottleneck,charge carrier relaxation,trap states,electron-hole scattering,colloidal quantum dots
更新于2025-09-11 14:15:04
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Exciton Peierls mechanism and universal many-body gaps in carbon nanotubes
摘要： “Metallic” carbon nanotubes exhibit quasiparticle gaps when isolated from a screening environment. The gap-opening mechanism is expected to be of electronic origin, but the precise nature is debated. In this work, we show that hybrid density functional theory predicts a set of excitonic instabilities capable of opening gaps of the size found in experiment. The excitonic instabilities are coupled to vibrational modes and, in particular, the modes associated with the (cid:2) ? E2g and K ? A(cid:2) 1 Kohn anomalies of graphene, inducing Peierls lattice distortions with a strong electron-phonon coupling. In the larger tubes, the longitudinal optical phonon mode becomes a purely electronic dimerization that is fully symmetry conserving in the zigzag and chiral tubes, but breaks the symmetry in the armchair tubes. The resulting gaps are universal (i.e., independent of chirality) and scale as 1/R with tube radius.
关键词： exciton Peierls mechanism,electron-phonon coupling,many-body gaps,carbon nanotubes,hybrid density functional theory
更新于2025-09-11 14:15:04
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Solid-state emitter embedded in a microcavity under intense excitation: A variational master-equation approach
摘要： In this work, dissipative effects from a phonon bath on the resonance ?uorescence of a solid-state two-level system embedded in a high-quality semiconductor microcavity and driven by an intense laser are investigated. Within the density operator formalism, we derive a variational master equation valid for broader ranges of temperatures, pumping rates, and radiation-matter couplings than previous studies. From the obtained master equation, ?uorescence spectra for various thermal and exciting conditions are numerically calculated and compared to those computed from weak coupling and polaronic master equations, respectively. Our results evidence the breakdown of those rougher approaches under increased temperature and strong pumping.
关键词： solid-state emitter,variational master equation,microcavity,phonon bath,resonance ?uorescence
更新于2025-09-11 14:15:04
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Chiral phonons in two-dimensional materials
摘要： There has been growing interest in investigating chiral phonons since they are theoretically found and experimentally verified recently. In a magnetic system with time inversion symmetry breaking, phonon can have nonzero angular momentum, which makes a correction to the gyromagnetic ratio measured in the Einstein-de Haas Effect. Though total phonon angular momentum is zero in a nonmagnetic two-dimensional (2D) hexagonal system with space inversion symmetry breaking, phonons at high symmetry points of the Brillouin zone can have nonzero phonon angular momentum, which means they are chiral phonons. Chiral phonons decide selection rules in the electronic intervalley scattering, which has been experimentally verified in tungsten-diselenide monolayers very recently (Science 359, 579 (2018)). In this review, after a brief introduction of related background and some basic concepts, we mainly report recent progress of phonon angular momentum in magnetic systems and chiral phonon in nonmagnetic systems. We also review known experiments in verifications of the phonon chirality and finally conclude with an outlook of future developments.
关键词： two-dimensional materials,chiral phonons,phonon Hall effect,valleytronics,Einstein-de Haas Effect,phonon angular momentum
更新于2025-09-10 09:29:36
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Infrared reflectance, transmittance, and emittance spectra of MgO from first principles
摘要： By using density functional theory calculations we determined the influence of anharmonic effects on the infrared reflectance, transmittance, and emittance of MgO. The goal is to determine the limit of validity of a perturbative (multiphonon) approach. MgO is chosen as a test material because of the availability of different kinds of radiative properties measured experimentally. Nonanalytic terms of the three-phonon scattering coefficients are explicitly calculated and do not provide measurable effects. The agreement is overall very good to such an extent that, already at room temperature, one can clearly identify regions in which four-phonon scattering processes are dominant with respect to the three-phonon ones. The influence of isotopic disorder at cryogenic temperatures is also settled.
关键词： emittance,transmittance,density functional theory,MgO,infrared reflectance,anharmonic effects,phonon-phonon interactions
更新于2025-09-10 09:29:36
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Cavity quantum-electrodynamical polaritonically enhanced electron-phonon coupling and its influence on superconductivity
摘要： So far, laser control of solids has been mainly discussed in the context of strong classical nonlinear light-matter coupling in a pump-probe framework. Here, we propose a quantum-electrodynamical setting to address the coupling of a low-dimensional quantum material to quantized electromagnetic fields in quantum cavities. Using a prototypical model system describing FeSe/SrTiO3 with electron-phonon long-range forward scattering, we study how the formation of phonon polaritons at the two-dimensional interface of the material modifies effective couplings and superconducting properties in a Migdal-Eliashberg simulation. We find that through highly polarizable dipolar phonons, large cavity-enhanced electron-phonon couplings are possible, but superconductivity is not enhanced for the forward-scattering pairing mechanism due to the interplay between coupling enhancement and mode softening. Our results demonstrate that quantum cavities enable the engineering of fundamental couplings in solids, paving the way for unprecedented control of material properties.
关键词： phonon polaritons,electron-phonon coupling,superconductivity,cavity quantum electrodynamics,quantum materials
更新于2025-09-10 09:29:36
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Insight into phonon scattering in Si nanowires through high-field hole transport: Impacts of boundary condition and comparison with bulk phonon approximation
摘要： The impact of how to model phonon scattering on hole transport in Si nanowires was studied based on Boltzmann’s transport equation. Boundary conditions for atomistic description of phonons in nanowires and approximation by bulk acoustic and optical phonons were analyzed in terms of their impacts on high-field hole transport. The boundary conditions for phonons influence the drift velocity and momentum relaxation time, especially at low electric field, but the energy relaxation time hardly depends on the boundary conditions. The impacts by the change of boundary conditions can be approximated by the change of the strength of acoustic phonon scattering in bulk phonon picture, though the behavior of energy relaxation and distribution function of holes can not be reproduced by bulk phonon approximation.
关键词： Si nanowires,boundary condition,bulk phonon approximation,phonon scattering,high-field hole transport
更新于2025-09-10 09:29:36
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Effect of stacking faults and surface roughness on the thermal conductivity of InAs nanowires
摘要： Low thermal conductivity and high power factor are desirable for thermoelectric materials. These properties can be achieved by patterning devices into nano-structures such as nanowires (NWs). The thermal conductivity can be further reduced by altering the NW geometry through the introduction of surface roughness (SR) or stacking faults (SFs). In this paper, relaxation times for scattering of phonons at SFs and SR are developed to accurately compute the impact of both effects on the thermal conductivity of InAs NWs with different diameters. It is found that similar reductions of the thermal conductivity can be obtained with SFs instead of SR. For the shortest possible distance between SFs along a NW, the room temperature thermal conductivity can be reduced to 25% compared to an ideal NW. For a NW with rough surface, a more than 80% decrease of the thermal conductivity is possible for specific roughness profiles. All available experimental data on the lattice thermal conductivity of InAs NWs confirm the theoretical models and simulation results.
关键词： surface roughness,thermal conductivity,phonon scattering,stacking faults,InAs nanowires
更新于2025-09-10 09:29:36
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A study of 8 MeV e-beam on localized defect states in ZnO nanostructures and its role on Photoluminescence and third harmonic generation
摘要： In this article we have explored an effect of electron beam irradiation (EBI) on physical and nonlinear optical properties ZnO thin nano films. Nanostructured ZnO thin films were grown by low cost spray pyrolysis technique. The irradiation dosage has been fixed at 5kGy, 10kGY, 15kGy and 20kGy. The structural investigation by Glancing angle X-Ray Diffractometer (GAXRD) confirms a polycrystalline phase of ZnO with wurtzite structure. The variation in the surface morphology upon EBI has been demonstrated using 2D and 3D Atomic force microscopy (AFM) images. Nanoscope software analysis quantifies the variation in surface roughness and average particle height upon EBI. The defect states created in the films upon irradiation experiments were investigated using UV- visible spectrophotometer, Room temperature Photoluminescence (RTPL), Raman and X-ray photoelectron spectroscopy (XPS). The increase in urbach tail validates the creation of localized defect states in the films The Gaussian fitting on RTPL spectra shows the quenching in the luminescent centers upon irradiation arised as result of recombination of vacancy defects. Phonon confinement model fitting on Raman spectra endorses that shift in the phonon modes observed on irradiation is due to spatial confinement of phonons. The elemental composition and impurity states of the EBI ZnO thin films were studied using XPS spectra. The shift in the binding energy of Zn and O elements infers the electron beam induced changes in the films. The electron beam irradiation has resulted in the increment of third order optical susceptibility χ(3) from 3.5×10-4esu to 8.13×10-3esu due to the enhancement of electronic transition to different defect levels formed in the films and through local heating effects arising due to continuous wave (CW) laser illumination. The enhanced THG signal investigated using Nd:YAG laser at 1064nm and 8 ns pulse width shows the promising features of EBI ZnO films for frequency tripling applications.
关键词： Electron beam irradiation,Phonon confinement model,third harmonic generation,ZnO nanostructures,Localized defect states
更新于2025-09-10 09:29:36