Showing papers on "Free electron model published in 2019"

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

Abstract: 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.

Maximal Spontaneous Photon Emission and Energy Loss from Free Electrons

Abstract: Free electron radiation such as Cerenkov, Smith--Purcell, and transition radiation can be greatly affected by structured optical environments, as has been demonstrated in a variety of polaritonic, photonic-crystal, and metamaterial systems. However, the amount of radiation that can ultimately be extracted from free electrons near an arbitrary material structure has remained elusive. Here we derive a fundamental upper limit to the spontaneous photon emission and energy loss of free electrons, regardless of geometry, which illuminates the effects of material properties and electron velocities. We obtain experimental evidence for our theory with quantitative measurements of Smith--Purcell radiation. Our framework allows us to make two predictions. One is a new regime of radiation operation---at subwavelength separations, slower (nonrelativistic) electrons can achieve stronger radiation than fast (relativistic) electrons. The second is a divergence of the emission probability in the limit of lossless materials. We further reveal that such divergences can be approached by coupling free electrons to photonic bound states in the continuum (BICs). Our findings suggest that compact and efficient free-electron radiation sources from microwaves to the soft X-ray regime may be achievable without requiring ultrahigh accelerating voltages.

Spontaneous and stimulated emissions of a preformed quantum free-electron wave function

Abstract: Do the prior history and the wave-packet size and form of a free electron have a physical effect in its interaction with light? Here we answer these fundamental questions on the interpretation of the electron quantum wave function by analyzing spontaneous and stimulated emissions of a quantum electron wave packet, interacting with a general, quantized radiation field. For coherent radiation (Glauber state), we confirm that stimulated emission and absorption of photons depends on the preinteraction-history-dependent size, exhibiting spectral cutoff when it exceeds the interacting radiation wavelength. Furthermore, stimulated emission of an optically modulated electron wave packet has a characteristic harmonic emission spectrum beyond the cutoff, which depends on the modulation features. In either case, there is no wave-packet-dependent radiation of the Fock state, and particularly the vacuum state spontaneous emission is wave-packet independent. The classical-to-quantum transition of radiation from the point-particle to the plane-wave limits, and the effects of wave-packet modulation indicate a way of measuring the wave-packet size of a single electron wave function, and suggest an alternative direction for exploring light-matter interaction.

Observation of dissociative quasi-free electron attachment to nucleoside via excited anion radical in solution.

Abstract: Damage to DNA via dissociative electron attachment has been well-studied in both the gas and condensed phases; however, understanding this process in bulk solution at a fundamental level is still a challenge. Here, we use a picosecond pulse of a high energy electron beam to generate electrons in liquid diethylene glycol and observe the electron attachment dynamics to ribothymidine at different stages of electron relaxation. Our transient spectroscopic results reveal that the quasi-free electron with energy near the conduction band effectively attaches to ribothymidine leading to a new absorbing species that is characterized in the UV-visible region. This species exhibits a nearly concentration-independent decay with a time constant of ~350 ps. From time-resolved studies under different conditions, combined with data analysis and theoretical calculations, we assign this intermediate to an excited anion radical that undergoes N1-C1′ glycosidic bond dissociation rather than relaxation to its ground state. Radiation-induced low-energy electrons in solution are implicated in DNA damage, but their relaxation dynamics are not well understood. Here the authors observe how quasi-free electrons dissociate glycosidic bonds via an excited nucleoside anion radical, whereas solvated electrons reside on the nucleoside as a relatively stable anion radical.

Coherent interaction between free electrons and a photonic cavity

Abstract: Since its inception, research of cavity quantum electrodynamics (CQED) has extended our understanding of light-matter interactions and our ability to utilize them. Thus far, all the work in this field has been focused on light interacting with bound electron systems - such as atoms, molecules, quantum dots, and quantum circuits. In contrast, markedly different physical phenomena are found in free-electron systems, the energy distribution of which is continuous and not discrete, implying tunable transitions and selection rules. In addition to their uses for electron microscopy, the interaction of free electrons with light enables important phenomena such as Cherenkov radiation, Compton scattering, and free-electron lasing. However, no experiment has shown the integration of free electrons into the framework of CQED, because the fundamental electron-light interaction is limited in strength and lifetime. This limit explains why many phenomena have remained out of reach for experiments with free electrons. In this work, we developed the platform for studying CQED at the nanoscale with free electrons and demonstrated it by observing their coherent interaction with cavity photons for the first time. We also directly measure the cavity photon lifetime via a free electron probe and show more than an order of magnitude enhancement in the electron-photon interaction strength. These capabilities may open new paths toward using free electrons as carriers of quantum information, even more so after strong coupling between free electrons and cavity photons will have been demonstrated. Efficient electron-cavity photon coupling could also allow new nonlinear phenomena of cavity opto-electro-mechanics and the ultrafast exploration of soft matter or other beam-sensitive materials using low electron current and low laser exposure.

Observing the Quantum Wave Nature of Free Electrons through Spontaneous Emission.

Abstract: We investigate, both experimentally and theoretically, the interpretation of the free-electron wave function using spontaneous emission. We use a transversely wide single-electron wave function to describe the spatial extent of transverse coherence of an electron beam in a standard transmission electron microscope. When the electron beam passes next to a metallic grating, spontaneous Smith-Purcell radiation is emitted. We then examine the effect of the electron wave function transversal size on the emitted radiation. Two interpretations widely used in the literature are considered: (1) radiation by a continuous current density attributed to the quantum probability current, equivalent to the spreading of the electron charge continuously over space; and (2) interpreting the square modulus of the wave function as a probability distribution of finding a point particle at a certain location, wherein the electron charge is always localized in space. We discuss how these two interpretations give contradictory predictions for the radiation pattern in our experiment, comparing the emission from narrow and wide wave functions with respect to the emitted radiation's wavelength. Matching our experiment with a new quantum-electrodynamics derivation, we conclude that the measurements can be explained by the probability distribution approach wherein the electron interacts with the grating as a classical point charge. Our findings clarify the transition between the classical and quantum regimes and shed light on the mechanisms that take part in general light-matter interactions.

The electronic structure of ε-Ga2O3

Abstract: The electronic structure of e-Ga2O3 thin films has been investigated by ab initio calculations and photoemission spectroscopy with UV, soft, and hard X-rays to probe the surface and bulk properties. The latter measurements reveal a peculiar satellite structure in the Ga 2p core level spectrum, absent at the surface, and a core-level broadening that can be attributed to photoelectron recoil. The photoemission experiments indicate that the energy separation between the valence band and the Fermi level is about 4.4 eV, a valence band maximum at the Γ point and an effective mass of the highest lying bands of – 4.2 free electron masses. The value of the bandgap compares well with that obtained by optical experiments and with that obtained by calculations performed using a hybrid density-functional, which also reproduce well the dispersion and density of states.

Revealing the role of the Rh valence state, La doping level and Ru cocatalyst in determining the H2 evolution efficiency in doped SrTiO3 photocatalysts

Abstract: SrTiO3 (STO) has favorable opto-electronic properties for overall water splitting. Nevertheless, realizing a higher efficiency is impeded by its band gap which can only harvest UV light. In order to extend the spectral response towards visible light, STO is (co)doped with lanthanum (La) and rhodium (Rh). However, notwithstanding the amount of visible light absorbed, the H2 evolution rates are remarkably governed by the valence state of Rh, La doping level and ruthenium (Ru) cocatalyst loading. Hence, it is essential to unravel the underlying effect of doping on the photophysical processes to gain insight into material design. To this end, charge carrier dynamics was probed over a wide time (sub-picosecond to microsecond) and spectral (visible to IR) region using transient absorption spectroscopy. Depending on the dopant composition, an interplay between the electron trapping and the kinetics of the electron transfer to the Ru cocatalyst was rationalized. For Rh4+:STO, free electrons probed at 3435 nm decayed virtually completely by 20 ps resulting in a kinetic competition between the electron trapping and the electron transfer to Ru cocatalyst. In the case of Rh3+:STO, free electrons decayed by a factor of three by 100 ps, thus demonstrating the effect of Rh valence state on the electron lifetime. The time constant and quantum yield of electron transfer from Rh3+:STO to the Ru cocatalyst were found to be 1.6 ps and 14.7%, respectively. In addition to a longer electron lifetime, enhanced electron transfer to the Ru cocatalyst makes Rh3+:STO one of the promising photocatalysts for H2 generation. Engineering the energetic position of the dopant within the band gap to avoid undesirable carrier trapping is crucial to enhance the efficiency of photocatalytic reactions.

Odd Electron Wave Packets From Cycloidal Ultrashort Laser Fields

Abstract: Polarization-tailored bichromatic femtosecond laser fields with cycloidal polarization profiles have emerged as a powerful tool for coherent control of quantum processes. We present an optical scheme to create and manipulate three-dimensional free electron wave packets with arbitrary rotational symmetry by combining advanced supercontinuum pulse shaping with high resolution photoelectron tomography. Here we use carrier-envelope phase-stable polarization-tailored bichromatic (3ω:4ω) counter- and corotating femtosecond laser pulses to generate 7-fold rotational symmetric and asymmetric photoelectron momentum distributions by multiphoton ionization of sodium atoms. To elucidate the physical mechanisms, we investigate the interplay between the symmetry properties of the driving field and the resulting electron wave packets by varying the optical field parameters. Our results show that the symmetry properties of electron wave packets are not fully determined by the field symmetry, but completely described by multipath quantum interference of states with different angular momenta. Laser fields can be tuned to probe electronic motion in atoms and molecules. Here the authors ionize Na atoms using bichromatic pulses to generate electron wave packets of crescent-shaped and 7-fold rotational symmetry which do not follow the field symmetry but are determined by multiphoton interference.

Controlling free electrons with optical whispering-gallery modes.

Abstract: Free-electron beams serve as uniquely versatile probes of microscopic structure and composition, and have repeatedly revolutionized atomic-scale imaging, from solid-state physics to structural biology. Over the past decade, the manipulation and interaction of electrons with optical fields has seen significant progress, enabling novel imaging methods, schemes of near-field electron acceleration, and culminating in 4D microscopy techniques with both high temporal and spatial resolution. However, weak coupling strengths of electron beams to optical excitations are a standing issue for existing and emerging applications of optical free-electron control. Here, we demonstrate phase matched near-field coupling of a free-electron beam to optical whispering gallery modes of dielectric microresonators. The cavity-enhanced interaction with these optically excited modes imprints a strong phase modulation on co-propagating electrons, which leads to electron-energy sidebands up to hundreds of photon orders and a spectral broadening of 700 eV. Mapping the near-field interaction with ultrashort electron pulses in space and time, we trace the temporal ring-down of the microresonator following a femtosecond excitation and observe the cavity's resonant spectral response. Resonantly enhancing the coupling of electrons and light via optical cavities, with efficient injection and extraction, can open up novel applications such as continuous-wave acceleration, attosecond structuring, and real-time all-optical electron detection.

Anomalous Photon-Induced Near-Field Electron Microscopy

Abstract: We reveal the classical and quantum regimes of free electron interaction with radiation, common to the general variety of radiation sources (e.g., a Smith-Purcell radiation), the dielectric laser accelerator, and photo-induced near-field electron microscopy (PINEM). Modeling the electron with initial conditions of a coherent quantum electron wave packet, its topology in phase space uniquely defines a universal distinction of three interaction regimes: point-particle-like acceleration, a quantum wave function (PINEM), and a newly reported regime of anomalous PINEM (APINEM). The quantum interference beat of APINEM is capable of improving the spectral resolution of postselective electron microscopy. The particle-wave duality transition between regimes reveals the history-dependent nature of quantum electron interaction with light.

Designer photonic dynamics by using non-uniform electron temperature distribution for on-demand all-optical switching times

Abstract: While free electrons in metals respond to ultrafast excitation with refractive index changes on femtosecond time scales, typical relaxation mechanisms occur over several picoseconds, governed by electron-phonon energy exchange rates. Here, we propose tailoring these intrinsic rates by engineering a non-uniform electron temperature distribution through nanostructuring, thus, introducing an additional electron temperature relaxation channel. We experimentally demonstrate a sub-300 fs switching time due to the wavelength dependence of the induced hot electron distribution in the nanostructure. The speed of switching is determined by the rate of redistribution of the inhomogeneous electron temperature and not just the rate of heat exchange between electrons and phonons. This effect depends on both the spatial overlap between control and signal fields in the metamaterial and hot-electron diffusion effects. Thus, switching rates can be controlled in nanostructured systems by designing geometrical parameters and selecting wavelengths, which determine the control and signal mode distributions. Here, the authors engineer a non-uniform electron temperature distribution through nanostructuring and demonstrate a sub-300 fs switching time. This can assist in the design of nanostructures for nonlinear optics, hot carrier extraction and photocatalysis

Sub-cycle time resolution of multi-photon momentum transfer in strong-field ionization

Abstract: During multi-photon ionization of an atom it is well understood how the involved photons transfer their energy to the ion and the photoelectron. However, the transfer of the photon linear momentum is still not fully understood. Here, we present a time-resolved measurement of linear momentum transfer along the laser pulse propagation direction. Beyond the limit of the electric dipole approximation we observe a time-dependent momentum transfer. We can show that the time-averaged photon radiation pressure picture is not generally applicable and the linear momentum transfer to the photoelectron depends on the ionization time within the electromagnetic wave cycle using the attoclock technique. We can mostly explain the measured linear momentum transfer within a classical model for a free electron in a laser field. However, corrections are required due to the interaction of the outgoing photoelectron with the parent ion and due to the initial momentum when the electron appears in the continuum. The parent ion interaction induces a measurable negative attosecond time delay between the appearance in the continuum of the electron with minimal linear momentum transfer and the point in time with maximum ionization rate.

Longitudinal phonon plasmon mode coupling in β-Ga2O3

Abstract: In this letter, we investigate a set of n-type single crystals of monoclinic symmetry β-Ga2O3 with different free electron concentration values by generalized far infrared and infrared spectroscopic ellipsometry. In excellent agreement with our previous model prediction, we find here by experiment that longitudinal-phonon-plasmon coupled modes are polarized either within the monoclinic plane or perpendicular to the monoclinic plane. As predicted, all modes change the amplitude and frequency with the free electron concentration. The most important observation is that all longitudinal-phonon-plasmon coupled modes polarized within the monoclinic plane continuously change their direction as a function of free electron concentration.

Quantized plasmon excitations of electron gas in potential well

Abstract: Using the Schrodinger-Poisson system in this paper, the basic quantum features of plasmon excitations in a free noninteracting electron gas with arbitrary degeneracy are investigated. The standing wave solution of the free electron gas is derived from the corresponding linearized pseudo-force system with appropriate boundary conditions. It is shown that the plasmon excitation energies for electron gas confined in an infinite potential well are quantized eigenvalues of which are obtained. It is found that any arbitrary degenerate quantum electron gas possesses two different characteristic length scales, unlike the classical dilute electron gas, with the smaller length scale corresponding to the single particle oscillation and the larger one due to the collective Langmuir excitations. The probability density of the free electron gas in a box contains fine structures which are modulated over a larger pattern. The envelope probability density profile for the electron Fermi gas confined in an impenetrable well in different energy states is found to be quite similar to that of the free electron confined to an infinite potential well. However, the illustrative features of the plasmon theory presented in this research can be further elaborated in order to illuminate a wide range of interesting physical phenomena involving both the single particle and the collective features.Using the Schrodinger-Poisson system in this paper, the basic quantum features of plasmon excitations in a free noninteracting electron gas with arbitrary degeneracy are investigated. The standing wave solution of the free electron gas is derived from the corresponding linearized pseudo-force system with appropriate boundary conditions. It is shown that the plasmon excitation energies for electron gas confined in an infinite potential well are quantized eigenvalues of which are obtained. It is found that any arbitrary degenerate quantum electron gas possesses two different characteristic length scales, unlike the classical dilute electron gas, with the smaller length scale corresponding to the single particle oscillation and the larger one due to the collective Langmuir excitations. The probability density of the free electron gas in a box contains fine structures which are modulated over a larger pattern. The envelope probability density profile for the electron Fermi gas confined in an impenetrable well...

Electrically Tuneable Exciton‐Polaritons through Free Electron Doping in Monolayer WS2 Microcavities

Abstract: We demonstrate control over light-matter coupling at room temperature combining a field effect transistor (FET) with a tuneable optical microcavity. Our microcavity FET comprises a monolayer tungsten disulfide WS$_2$ semiconductor which was transferred onto a hexagonal boron nitride flake that acts as a dielectric spacer in the microcavity, and as an electric insulator in the FET. In our tuneable system, strong coupling between excitons in the monolayer WS$_2$ and cavity photons can be tuned by controlling the cavity length, which we achieved with excellent stability, allowing us to choose from the second to the fifth order of the cavity modes. Once we achieve the strong coupling regime, we then modify the oscillator strength of excitons in the semiconductor material by modifying the free electron carrier density in the conduction band of the WS$_2$. This enables strong Coulomb repulsion between free electrons, which reduces the oscillator strength of excitons until the Rabi splitting completely disappears. We controlled the charge carrier density from 0 up to 3.2 $\times$ 10$^{12}$ cm$^{-2}$, and over this range the Rabi splitting varies from a maximum value that depends on the cavity mode chosen, down to zero, so the system spans the strong to weak coupling regimes.

Experimental and theoretical investigation of ultrashort pulsed laser ablation of diamond

Abstract: Diamond is a wide bandgap material, which exhibits an abrupt increase of its free-electron density, when excited by an ultrashort laser pulse. The generation of free electrons transforms the insulator diamond to a conducting material with metallic optical behavior. This transformation process can be described by the multiple rate equation (MRE) model. The introduced MRE model considers strong-field excitation in the Keldysh picture as well as collisional excitation. The light attenuation results from the strong-field absorption and free-carrier absorption described in the Drude picture. Thus, the electron density and intensity distribution as functions of time, penetration depth, and laser beam radius are calculated. Furthermore, the model predicts the evolution of optical properties and estimates the ablation threshold value by the diameter and depth regression method. The calculated ablation threshold is compared to experimental results on a single crystalline chemical vapor deposited diamond by applying the diameter and depth regression method. Experimental and theoretical results are discussed with regard to the pulse duration. The discussion focuses on single pulse ablation but also addresses the multishot domain, which is essential for laser machining. At 1030 nm, the experimental single pulse ablation threshold fluence is determined to be 8.2 and 12.9 J/cm2 for pulse durations of 400 and 700 fs, respectively. This is in compliance with the simulation results.Diamond is a wide bandgap material, which exhibits an abrupt increase of its free-electron density, when excited by an ultrashort laser pulse. The generation of free electrons transforms the insulator diamond to a conducting material with metallic optical behavior. This transformation process can be described by the multiple rate equation (MRE) model. The introduced MRE model considers strong-field excitation in the Keldysh picture as well as collisional excitation. The light attenuation results from the strong-field absorption and free-carrier absorption described in the Drude picture. Thus, the electron density and intensity distribution as functions of time, penetration depth, and laser beam radius are calculated. Furthermore, the model predicts the evolution of optical properties and estimates the ablation threshold value by the diameter and depth regression method. The calculated ablation threshold is compared to experimental results on a single crystalline chemical vapor deposited diamond by applyin...

Unusual properties of the RY3 center in GaN

Abstract: The investigation and identification of point defects in GaN is crucial for improving the reliability of light-emitting and high-power electronic devices. The RY3 defect with a characteristic emission band at about 1.8 eV is often observed in photoluminescence (PL) spectra of $n$-type GaN grown by hydride vapor phase epitaxy, and it exhibits unusual properties. Its emission band consists of two components: a fast (10-ns lifetime) RL3 with a maximum at 1.8 eV and a slow (100--300 \ensuremath{\mu}s lifetime) YL3 with a maximum at 2.1 eV and zero-phonon line at 2.36 eV. In steady-state PL measurements, the YL3 component emerges with increasing temperature from 90 to 180 K, concurrently with a decrease in the RL3 intensity. The activation energy of both processes is about 0.06 eV. In time-resolved PL, the YL3 intensity abruptly rises when the RL3 intensity begins to saturate. These and other phenomena can be explained using a model of an acceptor with two excited states. A delocalized, effective-mass state at about 0.2 eV above the valence band captures photogenerated holes. These holes transition to the ground state, which produces the RL3 component with a lifetime of \ensuremath{\sim}10 ns. Alternatively, they may nonradiatively transition over a 0.06 eV-high barrier to a localized excited state with a level at 1.13 eV above the valence band. Recombination of free electrons or electrons at shallow donors with the holes at this localized excited state is responsible for the YL3 component. The relative intensities of the RL3 and YL3 components are dictated by the probabilities of holes at the shallow excited state to transition to the ground or to the localized excited states. Transition metals and complex defects are considered as the main candidates for the RY3 center.

Real-Time Dynamics of the Formation of Hydrated Electrons upon Irradiation of Water Clusters with Extreme Ultraviolet Light

Abstract: Free electrons in a polar liquid can form a bound state via interaction with the molecular environment. This so-called hydrated electron state in water is of fundamental importance, e.g., in cellular biology or radiation chemistry. Hydrated electrons are highly reactive radicals that can either directly interact with DNA or enzymes, or form highly excited hydrogen (H^{*}) after being captured by protons. Here, we investigate the formation of the hydrated electron in real-time employing extreme ultraviolet femtosecond pulses from a free electron laser, in this way observing the initial steps of the hydration process. Using time-resolved photoelectron spectroscopy we find formation timescales in the low picosecond range and resolve the prominent dynamics of forming excited hydrogen states.

Terahertz Driven Amplification of Coherent Optical Phonons in GaAs Coupled to a Metasurface.

Abstract: We demonstrate amplification of longitudinal optical (LO) phonons by polar-optical interaction with an electron plasma in a GaAs structure coupled to a metallic metasurface using two-color two-dimensional spectroscopy. In a novel scheme, the metamaterial resonator enhances broadband terahertz fields, which generate coherent LO phonons and drive free electrons in the conduction band of GaAs. The time evolution of the LO phonon amplitude is monitored with midinfrared pulses via the LO-phonon-induced Kerr nonlinearity of the sample, showing an amplification of the LO phonon amplitude by up to a factor of 10, in agreement with a theoretical estimate.

Kinetic energy density of nearly free electrons. II. Response functionals of the electron density

Abstract: We present electron-density-based response functionals yielding the non-negative kinetic energy density (KED) of nearly free electron systems. In a previous paper, for a canonical free-electron system perturbed by an external potential, we derived the first- and second-order corrections to the KED as functionals of the potential, providing the response functions in reciprocal space. Here, we formulate the KED response in terms of the electron density by converting the potential-based functionals into density functionals. We also determine the related response of the Pauli KED, which is the KED in excess of the von Weizs\"acker KED. We anticipate that the structure of these density functionals will help guide the design of the more sophisticated kinetic energy functionals required for orbital-free density functional theory simulations. We conclude by examining the performance of the first- and second-order density functionals for the KED when applied to electron densities generated from local pseudopotential calculations for Li, Al, and Si crystals.

On the Z 1 -dependence of electronic stopping in TiN

Abstract: We present a thorough experimental study of electronic stopping of H, He, B, N, Ne and Al ions in TiN with the aim to learn about the energy loss mechanisms of slow ions. The energy loss was measured by means of time-of-flight medium-energy ion scattering. Thin films of TiN on silicon with a δ-layer of W at the TiN/Si interface were used as targets. We compare our results to non-linear density functional theory calculations, examining electron-hole pair excitations by screened ions in a free electron gas in the static limit, with a density equivalent to the expected value for TiN. These calculations predict oscillations in the electronic stopping power for increasing atomic number Z1 of the projectile. An increasing discrepancy between our experimental results and predictions by theory for increasing Z1 was observed. This observation can be attributed to contributions from energy loss channels different from electron-hole pair excitation in binary Coulomb collisions.

A Plasmon-Mediated Electron Emission Process

Abstract: Light-driven electron emission plays an important role in modern optoelectronic devices. However, such a process usually requires a light field with either a high intensity or a high frequency, which is not favorable for its implementations and difficult for its integrations. To solve these issues, we propose to combine plasmonic nanostructures with nanoelectron emitters of low work function. In such a heterostructure, hot electrons generated by plasmon resonances upon light excitation can be directly injected into the adjacent emitter, which can subsequently be emitted into the vacuum. Electron emission of high efficiency can be obtained with light fields of moderate intensities and visible wavelengths, which is a plasmon-mediated electron emission (PMEE) process. We have demonstrated our proposed design using a gold-on-graphene (Au-on-Gr) nanostructure, which can have electron emission with light intensity down to 73 mW·cm-2. It should be noted that the field electron emission is not involved in such a PMEE process. This proposal is of interest for applications including cold-cathode electron sources, advanced photocathodes, and micro- and nanoelectronic devices relying on free electrons.

Coherent interaction between free electrons and cavity photons

Abstract: Since its inception, research of cavity quantum electrodynamics (CQED) has extended our understanding of light-matter interactions and our ability to utilize them. Thus far, all the work in this field has been focused on light interacting with bound electron systems - such as atoms, molecules, quantum dots, and quantum circuits. In contrast, markedly different physical phenomena are found in free-electron systems, the energy distribution of which is continuous and not discrete, implying tunable transitions and selection rules. In addition to their uses for electron microscopy, the interaction of free electrons with light enables important phenomena such as Cherenkov radiation, Compton scattering, and free-electron lasing. However, no experiment has shown the integration of free electrons into the framework of CQED, because the fundamental electron-light interaction is limited in strength and lifetime. This limit explains why many phenomena have remained out of reach for experiments with free electrons. In this work, we developed the platform for studying CQED at the nanoscale with free electrons and demonstrated it by observing their coherent interaction with cavity photons for the first time. We also directly measure the cavity photon lifetime via a free electron probe and show more than an order of magnitude enhancement in the electron-photon interaction strength. These capabilities may open new paths toward using free electrons as carriers of quantum information, even more so after strong coupling between free electrons and cavity photons will have been demonstrated. Efficient electron-cavity photon coupling could also allow new nonlinear phenomena of cavity opto-electro-mechanics and the ultrafast exploration of soft matter or other beam-sensitive materials using low electron current and low laser exposure.

Steady-state superconductivity in electronic materials with repulsive interactions

Abstract: We study the effect of laser driving on a minimal model for a hexagonal two-dimensional material with broken inversion symmetry. Through the application of circularly polarized light and coupling to a thermal free electron bath, the system is driven into a nonequilibrium steady state with asymmetric, nonthermal carrier populations in the two valleys. We show that, in this steady state, interband superconducting correlations between electrons can develop independent of the sign of the electronic interactions. We discuss how our results apply, for example, to transition metal dichalcogenides. This work opens the door to technological applications of superconductivity in a range of materials that were hitherto precluded from it.

Free electron laser polarization control with interfering crossed polarized fields

Abstract: Free electron lasers emit powerful and coherent radiation in a wide wavelength range extending to hard x-rays. This radiation is also characterized by a high degree of polarization that is generally linear and depends on the undulator properties. The possibility of controlling the polarization state of the radiation is an important option for free electron lasers that is critical for a large class of experiments. Such control can be achieved using variable polarization undulators or alternatively via the crossed polarized undulator scheme. We report the results of an extensive study for the characterization of the crossed-polarized undulator scheme in a number of different configurations. A simple model, based on Gaussian mode beam propagation, is presented and used to reproduce the experimental results obtained at the seeded free electron laser FERMI. A good agreement is found between the model and the experiment allowing us to understand the impact of the wavefront properties of the radiation coming from the consecutive undulators on the output radiation. The model is used not only for characterizing the control of the polarization but also for the control of the transverse mode.

1D Modeling of the Microwave Discharge in Liquid n -Heptane Including Production of Carbonaceous Particles

Abstract: Microwave plasma in the liquid is initiated inside a gas bubble formed at the end of the electrode-antenna, through which microwave energy is introduced into the liquid. A 1D model a set of gas phase kinetic reactions describes the evolution processes of ionization, heat transfer and formation of gas and solid products inside the plasma bubble. The code is based on joint solution of the Boltzmann equation for free electrons of the plasma, a simplified equation for the microwave field, the heat conduction equation, the balance equation for the electron density and the balance equations for the weight fraction for all gas and solid products of n-heptane pyrolysis. The Joule heat released in the plasma is expended on the evaporation of liquid n-heptane into the bubble and the decomposition of the n-heptane molecules. The model includes both the description of gas phase processes and formation of solid carbon-containing particles. The growth mechanism for generation of solid particles describes simultaneous processes of the initial nucleation, surface growth and coagulation of soot particles. The results of calculations are compared with known experimental results.

Pseudo-Resonance and Energy Band Gap in Plasmonic Crystals

Abstract: In this paper, using the generalized coupled pseudoforce model with driving elements, we develop a method to study the plasmon excitations and energy band structure in a plasmonic crystal. It is shown that the presence of the periodic ion core potential leads to pseudo-resonance condition in the plasmon wavefunction and electrostatic potential profiles, quite analogous to the frequency resonance, leading to the gap formation in the energy dispersion profiles. It is found that the dual length scale character of plasmon excitations lead to occurrence of a critical value of $a_c=2\pi\lambda_p$ for the lattice constant ($\lambda_p$ being the plasmon wavelength) above and below which the energy band structure of plasmonic crystals becomes substantially different. It is also found that energy band gap positions of parabolic free electron energy dispersion relation are slightly higher compared to that of the plasmon excitations. Here based on the plasmon definition and due to the dual length scale character of plasmons compared to that of single electrons, we provide a simplified interpretation of the wave-particle duality in quantum physics and provide the length scale regime in which both wave and particle properties can be simultaneously examined in a single experiment, where, the complementarity principle breaks down. However, the dual characteristics of electron-plasmon coupled excitations, quite analogous to starling murmuration phenomenon, strongly suggests that the long south de Broglie-Bohm pilot wave of electron is nothing but the plasmon itself. The wavefunction and electrostatic potential solution for a one dimensional plasmonic lattice with a generalized periodic potential is also derived in this research.

Anisotropic optical properties of highly doped rutile SnO2: Valence band contributions to the Burstein-Moss shift

Abstract: The interband absorption of the transparent conducting semiconductor rutile stannic oxide (SnO2) is investigated as a function of increasing free electron concentration. The anisotropic dielectric functions of SnO2:Sb are determined by spectroscopic ellipsometry. The onsets of strong interband absorption found at different positions shift to higher photon energies with increasing free carrier concentration. For the electric field vector parallel to the optic axis, a low energy shoulder increases in prominence with increasing free electron concentration. We analyze the influence of different many-body effects and can model the behavior by taking into account bandgap renormalization and the Burstein-Moss effect. The latter consists of contributions from the conduction and the valence bands which can be distinguished because the nonparabolic conduction band dispersion of SnO2 is known already with high accuracy. The possible originsof the shoulder are discussed. The most likely mechanism is identified to be interband transitions at |k| > 0 from a dipole forbidden valence band.