Electron-photon Interaction Research Articles

Oscillations in electron transport caused by multiple resonances in a quantum dot-QED system in the steady-state regime

We model the electron transport current as the photon energy is swept through several resonances of a multi-level quantum dot, embedded in a short quantum wire, coupled to photon cavity. We use a Markovian quantum master equation appropriate for the long-time evolution and include the electron-electron and both the para- and the diamagnetic electron-photon interactions via diagonalization in a truncated many-body Fock space. Tuning the photon energy, several anti-crossings caused by Rabi-splitting in the energy spectrum of the quantum dot system are found. The strength of the Rabi-splittings and the photon-exchange between the resonant states depend on the polarization of the cavity photon field. We observe oscillations of the charge in the system and several resonant transport current peaks for the photon energies corresponding to the resonances in the steady-state regime.

High-efficiency photon–electron coupling resonant emission in GaN-based microdisks on Si**Project supported by the National Key R&D Program of China (Grant Nos. 2016YFB0400102 and 2016YFB0400602), the National Natural Science Foundation of China (Grant Nos. 61674076, 61422401 and 51461135002), the Collaborative Innovation Center of Solid State Lighting and Energy-Saving Electronics, Open Fund of the State Key Laboratory on Integrated Optoelectronics (Grant No.

Resonance effects caused by the photon–electron interaction are a focus of attention in semiconductor optoelectronics, as they are able to increase the efficiency of emission. GaN-on-silicon microdisks can provide a perfect cavity structure for such resonance to occur. Here we report GaN-based microdisks with different diameters, based on a standard blue LED wafer on a Si substrate. A confocal photoluminescence spectroscopy is performed to analyze the properties of all microdisks. Then, we systematically study the effects of radial modes and axial modes of these microdisks on photon–electron coupling efficiency by using three-dimensional finite-difference time-domain simulations. For thick microdisks, photon–electron coupling efficiency is found to greatly depend on the distributions of both the radial modes and the axial modes, and the inclined sidewalls make significant influences on the axial mode distributions. These results are important for realization of high-efficiency resonant emission in GaN-based microcavity devices.

One-photon absorption by inorganic perovskite nanocrystals: A theoretical study

The one-photon absorption cross section of nanocrystals (NCs) of the inorganic perovskite CsPbBr$_{3}$ is studied theoretically using a multiband $\mathbf{k}\cdot\mathbf{p}$ envelope-function model combined with a treatment of intercarrier correlation by many-body perturbation theory. A confined exciton is described first within the Hartree-Fock (HF) approximation, and correlation between the electron and hole is then included in leading order by computing the first-order vertex correction to the electron-photon interaction. The vertex correction is found to give an enhancement of the near-threshold absorption cross section by a factor of up to 4 relative to the HF (mean-field) value of the cross section, for NCs with an edge length $L=9$-12 nm (regime of intermediate confinement). The vertex-correction enhancement factors are found to decrease with increasing exciton energy; the absorption cross section for photons of energy $\omega=3.1$ eV (about 0.7 eV above threshold) is enhanced by a factor of only 1.4-1.5 relative to the HF value. The $\mathbf{k}\cdot\mathbf{p}$ corrections to the absorption cross section are also significant; they are found to increase the cross section at an energy $\omega=3.1$ eV by about 30% relative to the value found in the effective-mass approximation. The theoretical absorption cross section at $\omega=3.1$ eV, assuming a Kane parameter $E_{P}=20$ eV, is found to be intermediate among the set of measured values (which vary among themselves by nearly an order of magnitude) and to obey a power-law dependence $\sigma^{(1)}(\omega)\propto L^{2.9}$ on the NC edge length $L$, in good agreement with experiment. The dominant contribution to the theoretical exponent 2.9 is shown to be the density of final-state excitons. The main theoretical uncertainty in these calculations is in the value of the Kane parameter $E_{P}$.

Pronounced non-Markovian features in multiply excited, multiple emitter waveguide QED: Retardation induced anomalous population trapping

The Markovian approximation is widely applied in the field of quantum optics due to the weak frequency dependence of the vacuum field amplitude, and in consequence non-Markovian effects are typically regarded to play a minor role in the optical electron-photon interaction. Here, we give an example where non-Markovianity changes the qualitative behavior of a quantum optical system, rendering the Markovian approximation quantitatively and qualitatively insufficient. Namely, we study a multiple-emitter, multiple-excitation waveguide quantum-electrodynamic (waveguide-QED) system and include propagation time delay. In particular, we demonstrate anomalous population trapping as a result of the retardation in the excitation exchange between the waveguide and three initially excited emitters. Allowing for local phases in the emitter-waveguide coupling, this population trapping cannot be recovered using a Markovian treatment, proving the essential role of non-Markovian dynamics in the process. Furthermore, this time-delayed excitation exchange allows for a novel steady state, in which one emitter decays entirely to its ground state while the other two remain partially excited.

Breakdown of the electric dipole approximation at Cooper minima in direct two-photon ionisation

We predict breakdown of the electric dipole approximation at nonlinear Cooper minimum in direct two-photon K–shell atomic ionisation by circularly polarised light. According to predictions based on the electric dipole approximation, we expect that tuning the incident photon energy to the Cooper minimum in two-photon ionisation results in pure depletion of one spin projection of the initially bound 1s electrons, and hence, leaves the ionised atom in a fully oriented state. We show that by inclusion of electric quadrupole interaction, dramatic drop of orientation purity is obtained. The low degree of the remaining ion orientation provides a direct access to contributions of the electron-photon interaction beyond the electric dipole approximation in the two-photon ionisation of atoms and molecules. The orientation of the photoions can be experimentally detected either directly by a Stern-Gerlach analyzer, or by means of subsequent Kα fluorescence emission, which has the information about the ion orientation imprinted in the polarisation of the emitted photons.

Surface‐Emitting Lasers: Multiwavelength GaN‐Based Surface‐Emitting Lasers and Their Design Principles (Ann. Phys. 1/2020)

In article number 1900308, Shaoqiang Chen and co-workers propose a controllable multiwavelength GaN-based vertical-cavity surface-emitting laser using asymmetric quantum well or multistacked size-varied quantum dot active regions. The components and intensity of the multi-laser outputs are demonstrated to be continuously adjustable. The analysis reveals that the electron–photon interaction, carrier tunneling, and photon reabsorption play a crucial role in the multiwavelength lasing processes. The design principles of the proposed active regions are also elaborated upon to provide guidelines for high performance multiwavelength emissions in GaN-based surface-emitting lasers.

Генерация когерентного рентгеновского излучения гармоник в однопроходном лазере на свободных электронах со сдвигом фаз электронов относительно фотонов

Theoretical study of the radiated FEL power and bunching is conducted in single-pass free electron laser (FEL) with repetitive shift of the photon pulse with respect to the electron bunch in between the sections of the undulator with field harmonics. Using the analytical calculations, the asymmetric elliptic and planar undulator fields are identified for the undulators to be able to reduce the fundamental tone and enhance the harmonic radiation. This effect is amplified by the phase shift kπ/n, k=2,4,6… of the electrons and photons between the undulator sections. This allows n-th FEL harmonic reach its peak power beyond the fundamental saturation FEL length. It is not possible to achieve in a common FEL due to fast increase of the energy spread, induced by the fundamental tone along FEL and its early saturation. The phenomenological model of the FEL is used for the analysis of the bunching coefficients and harmonic powers evolution in the FEL. The model describes the debunching between the undulator sections, where the phase shift is imposed, and accounts for the higher sensitivity to losses of the electron-photon interaction at high harmonic wavelengths. The losses are due to the beam energy spread and emittance, deviation off the axis, diffraction etc. Explicit expressions for the Bessel coefficients of the planar and elliptic undulators with field harmonics are obtained and analyzed. The combination of the undulator field harmonics is identified, which reduces the fundamental tone and enhances the 3rd and 5th UR harmonics as compared with the radiation from a planar undulator. We propose the use of these undulators in a FEL with the electron-photon phase shift, which increases high harmonic powers in X-ray band by ~100 times with respect to common FELs.

Theoretical study of the effects of electron-phonon and electron-photon interaction in optoelectronic properties of armchair graphene nano-flakes –a renormalization method

Abstract In this work, we study the effects of electron-phonon and electron-photon interaction on the electronic and optoelectronic properties of nano systems consisting of graphene nano-flakes (small length armchair graphene nanoribbons) connected to two semi-infinite metal electrodes. Our calculations are based on the use of non-equilibrium Green's function formalism. The non-interacting Hamiltonian is written within the nearest neighbor tight binding approximation. The full interacting Hamiltonian is then obtained by addition the electron-phonon and electron –photon interaction to the above non-interacting Hamiltonian. Using unitary transformations the interacting Hamiltonian is renormalized into a non-interacting tight-binding form with effective onsite energy and hopping parameter which contain the interacting effects. The Landuer-Buttiker formalism can now be used for the system with renormalized non-interacting Hamiltonian for calculating the electronic current. Within the above framework, we calculate the electron current, density of states (DOS) and photocurrent in the presence and absence of electron-phonon interaction in a nano-system consisting of graphene nano-flake with different width as central molecule. Our results show that electron-phonon interaction leads to decreasing the number of peaks in the DOS. Also the photocurrent has oscillatory behavior versus electron-photon coupling for different widths of the nano-flake. Finally, incident photon energy dependence of threshold electron-photon coupling (i.e photocurrent becomes larger than Ballistic case) and its increasing trend versus photon energy are studied.

Multiwavelength GaN‐Based Surface‐Emitting Lasers and Their Design Principles

AbstractDual‐wavelength lasing operations are demonstrated in GaN‐based vertical‐cavity surface‐emitting lasers (VCSELs) comprising ingeniously designed asymmetric InGaN quantum wells (AS‐QWs). The dual laser modes show exact positive‐correlated polarization dependences with a high degree of polarization of up to 98%. By simply tuning the pump energy, the components and intensity of the laser outputs can be continuously changed, making wavelength selection and switching available for the GaN‐based VCSELs. Detailed theoretical analysis and experimental measurements show that the intensity of optical gain and the coupling between the active layer and optical field, namely the electron–photon interaction, as well as carrier tunneling and photon reabsorption play a crucial role in the multiwavelength lasing processes. Moreover, the design principles of the proposed AS‐QWs and multistacked size‐varied quantum dot (MS‐QD) active regions are elaborated to provide guidelines for controllable multiwavelength emissions in GaN‐based surface‐emitting lasers. These results not only provide better understanding of lasing in nitride‐based microcavity systems but also shed insight into the more fundamental issues of electron–photon coupling in such systems. Importantly, such controllable multiwavelength laser operations may extend nitride‐based VCSELs to previously inaccessible areas, for example, flip‐flop, ultrafast switches, and other functional devices such as Raman lasers and sensors.

Modeling the electron-photon interaction in monolayers of graphene and transition metal dichalcogenides in a tight binding approximation

In the presence of a strong electromagnetic field, the spectrum of charge carriers in a Dirac material is changed. The interaction of a linearly polarized field with electrons in the Dirac material leads to anisotropy. The anisotropy axis coincides with the direction of the electric field in the linearly polarized wave. We show that the major contribution to the interaction of electrons with an electromagnetic field comes from elastic scattering processes, whereas taking into account inelastic processes leads to an error of about 2%.

Entanglements of Electrons and Cavity Photons in the Strong-Coupling Regime.

This Letter sets a road map towards an experimental realization of strong coupling between free electrons and photons and analytically explores entanglement phenomena that emerge in this regime. The proposed model unifies the strong-coupling predictions with known electron-photon interactions. Additionally, this Letter predicts a non-Columbic entanglement between freely propagating electrons. Since strong coupling can map entanglements between photon pairs onto photon-electron pairs, it may harness electron beams for quantum communication, thus far exclusive to photons.

Cavity‐Photon‐Induced High‐Order Transitions between Ground States of Quantum Dots

AbstractIt is shown that quantum electromagnetic transitions to high orders are essential to describe the time‐dependent path of a nanoscale electron system in a Coulomb blockade regime when coupled to external leads and placed in a 3D rectangular photon cavity. The electronic system consists of two quantum dots embedded asymmetrically in a short quantum wire. The two lowest in energy spin degenerate electron states are mostly localized in each dot with only a tiny probability in the other dot. In the presence of the leads, a slow high‐order transition between the ground states of the two quantum dots is identified. The Fourier power spectrum for photon–photon correlations in the steady state shows a Fano type of resonance for the frequency of the slow transition. Full account is taken of the geometry of the multilevel electronic system, and the electron–electron Coulomb interactions together with the para‐ and diamagnetic electron–photon interactions are treated with step‐wise exact numerical diagonalization and truncation of appropriate many‐body Fock spaces. The matrix elements for all interactions are computed analytically or numerically exactly.

Time: a Constructal viewpoint & its consequences

In the environment, there exists a continuous interaction between electromagnetic radiation and matter. So, atoms continuously interact with the photons of the environmental electromagnetic fields. This electromagnetic interaction is the consequence of the continuous and universal thermal non-equilibrium, that introduces an element of randomness to atomic and molecular motion. Consequently, a decreasing of path probability required for microscopic reversibility of evolution occurs. Recently, an energy footprint has been theoretically proven in the atomic electron-photon interaction, related to the well known spectroscopic phase shift effect, and the results on the irreversibility of the electromagnetic interaction with atoms and molecules, experimentally obtained in the late sixties. Here, we want to show how this quantum footprint is the “origin of time”. Last, the result obtained represents also a response to the question introduced by Einstein on the analysis of the interaction between radiation and molecules when thermal radiation is considered; he highlighted that in general one restricts oneself to a discussion of the energy exchange, without taking the momentum exchange into account. Our result has been obtained just introducing the momentum into the quantum analysis.

Stability of Ferromagnetism in Many-Electron Systems

We construct a model-independent framework describing stabilities of ferromagnetism in strongly correlated electron systems; Our description relies on the operator theoretic correlation inequalities. Within the new framework, we reinterpret the Marshall–Lieb–Mattis theorem and Lieb’s theorem; in addition, from the new perspective, we prove that Lieb’s theorem still holds true even if the electron–phonon and electron–photon interactions are taken into account. We also examine the Nagaoka–Thouless theorem and its stabilities. These examples verify the effectiveness of our new viewpoint.

Negative Differential Conductance Assisted by Optical Fields in a Single Quantum Dot with Ferromagnetic Electrodes.

In a single quantum dot (QD) system connected with ferromagnetic electrodes, the electron transport properties, assisted by the thermal and Fock state optical fields, are theoretically studied by the Keldysh nonequilibrium Green’s function approach. The results show that the evolution properties of the density of state and tunneling current assisted by the Fock state optical field, are quite different from those of the thermal state. The photon sideband shift decreases monotonously with the increase in the electron–photon coupling strength for the case of the thermal state, while the shift is oscillatory for the case of the Fock state. Negative differential conductance (NDC) appears obviously in a QD system contacted with parallel (P) and antiparallel (AP) magnetization alignment of the ferromagnetic electrode leads, assisted by the Fock state optical field in a wide range of electron–photon interaction parameters. Evident NDC usually only arises in an AP configuration QD system assisted by the thermal state optical field. The results have the potential to introduce a new way to actively manipulate and control the single-electron tunneling transport on a QD system by the quantum states of the optical field.

Energy Characteristics of the Anomalous Diffusion Process

We consider an anomalous diffusion model in which space-time nonlocalities are generated by singular zones forming sub- and superdiffusion transfer regimes. The dynamical equation for these regimes appears in the form of the quasiparticle interaction law and is an analogue of the dynamical equation for the photon-electron interaction.

Time: a footprint of irreversibility

In the environment there exists a continuous interaction between electromagnetic radiation and matter. So, atoms continuously interact with the photons of the environmental electromagnetic fields. This electromagnetic interaction is a consequence of thermal non-equilibrium. It introduces an element of randomness into atomic and molecular motion, which leads to the decrease of the path probability required for the microscopic reversibility of evolution. Recently, an energy footprint has been theoretically proven in the atomic electron-photon interaction, as well as the well known spectroscopic phase shift effect and the results on the irreversibility of the electromagnetic interaction with atoms and molecules, experimentally obtained by R. O. Doyle in 1968. Here, we want to show that such a quantum footprint is nothing more than the generation of time .

Tracking Sub-Nanometer Shift in the Scattering Centroid of Single Gold Nanorods during Electrochemical Charging.

While conventional wisdom suggests the scattering centroid of a plasmonic nanoparticle reflects its geometric center, here we uncover the dependence of a scattering centroid of a single gold nanorod (AuNR) on its electron density when the geometric features (position and morphology) do not change at all. When periodically altering the electron density of a single AuNR during nonfaradaic charging and discharging processes, the optical centroid of the scattering dot in a series of dark-field images was found to reversibly shift back and forth by ∼0.4 nm, in pace with the sweeping potential. A Fourier-transform-based demodulation method was proposed to determine the centroid displacement as small as 0.1 nm, allowing for validating the generality of the observed phenomenon. The dependence of an optical centroid on the potential was attributed to the displacement of the electron density center as a result of inhomogeneous accumulation of injected electrons on the surface of a single AuNR. Not only does the present work shed light on studying the photon-electron interactions at sub-nanoparticle level, Fourier transform-based demodulation also provides a superior strategy for other fast and reversible processes such as electrochromic and photothermal conversions.

Quantum Effects In Imaging Nano-Structures Using Photon-Induced Near-Field Electron Microscopy

In this paper, we introduce the quantum mechanical approach as a more physically-realistic model to accurately quantify the electron-photon interaction in Photon-induced near-field electron microscopy (PINEM). Further, we compare the maximum coupling speed between the electrons and the photons in the quantum and classical regime. For a nanosphere of radius 2.13 nm, full quantum calculations show that the maximum coupling between photon and electron occurs at a slower speed than classical calculations report. In addition, a significant reduction in PINEM field intensity is observed for the full quantum model. Furthermore, we discuss the size limitation for particles imaged using the PIMEN technique and the role of the background material in improving the PINEM intensity. We further report a significant reduction in PINEM intensity in nearly touching plasmonic particles (0.3 nm gap) due to tunneling effect.

Interlayer hybridization and moiré superlattice minibands for electrons and excitons in heterobilayers of transition-metal dichalcogenides

Geometrical moir\'e patterns, generic for almost aligned bilayers of two-dimensional (2D) crystals with similar lattice structure but slightly different lattice constants, lead to zone folding and miniband formation for electronic states. Here, we show that moir\'e superlattice (mSL) effects in $\mathrm{MoSe}_2/\mathrm{WS}_2$ and $\mathrm{MoTe}_2/\mathrm{MoSe}_2$ heterobilayers that feature alignment of the band edges are enhanced by resonant interlayer hybridization, and anticipate similar features in twisted homobilayers of TMDs, including examples of narrow minibands close to the actual band edges. Such hybridization determines the optical activity of interlayer excitons in transition-metal dichalcogenide (TMD) heterostructures, as well as energy shifts in the exciton spectrum. We show that the resonantly hybridized exciton (hX) energy should display a sharp modulation as a function of the interlayer twist angle, accompanied by additional spectral features caused by umklapp electron-photon interactions with the mSL. We analyze the appearance of resonantly enhanced mSL features in absorption and emission of light by the interlayer exciton hybridization with both intralayer A and B excitons in $\mathrm{MoSe}_2/\mathrm{WS}_2$, $\mathrm{MoTe}_2/\mathrm{MoSe}_2$, $\mathrm{MoSe}_2/\mathrm{MoS}_2$, $\mathrm{WS}_2/\mathrm{MoS}_2$, and $\mathrm{WSe}_2/\mathrm{MoSe}_2$.