Emission Of Coherent Radiation Research Articles

Radiation Reaction Cooling as a Source of Anisotropic Momentum Distributions with Inverted Populations.

Under the presence of strong electromagnetic fields and radiation reaction, plasmas develop anisotropic momentum distributions, characterized by a population inversion. This is a general property of collisionless plasmas when the radiation reaction force is taken into account. We study the case of a plasma in a strong magnetic field and demonstrate the development of ring momentum distributions. The timescales for ring formation are derived for this configuration. The analytical results for the ring properties and the timescales for ring formation are confirmed with particle-in-cell simulations. The resulting momentum distributions are kinetically unstable and are known to lead to coherent radiation emission in astrophysical plasmas and laboratory setups.

Improved Smith–Purcell free-electron laser based on quasi-bound states in the continuum

In this article, a method to enable efficient emission of coherent radiation by using an intense electron beam coupled with a quasi-bound state in the continuum (quasi-BIC) is investigated. We present an analytical solution providing an intuitive round-trip phase condition to explain the origin of quasi-BICs of dielectric gratings. Numerical study of the beam–wave interaction shows that the electrons can be bunched by the synchronous space harmonic enhanced by the quasi-BIC, resulting in self-excited coherent oscillation and consequently efficient Smith–Purcell radiation. This work presents an interesting solution for coherent radiation sources, and may find application in communications and physics.

Proposal to generate a pair of intense independently tunable attosecond pulses from undulator radiation

We propose and numerically evaluate two schemes to generate a pair of extreme-ultraviolet monocycle pulses with gigawatt-level peak power, whose time delay and central wavelengths can be precisely controlled. The methods are based on coherent emission of radiation by an ultrarelativistic electron beam with a current profile given by a chirped sinusoid, which is generated through the interaction with a conventional broadband laser. The possibility to produce phase-locked attosecond pulses with independently tunable properties in the extreme-ultraviolet spectral region has the potential to significantly advance studies of charge dynamics in molecules of biological interests.

Different types of coherence: Young-type interference versus Dicke superradiance

Dicke superradiance, i.e., the enhanced spontaneous emission of coherent radiation, is often attributed to radiation emitted by synchronized dipoles coherently oscillating in phase. At the same time, Dicke derived superradiance assuming atoms in entangled Dicke states which do not display any dipole moment. To shed light on this apparent paradox, we study the intensity distribution arising from two identical two-level atoms prepared either in an entangled Dicke state or in a separable atomic state with non-vanishing dipole moment. We find that the two configurations produce similar far field intensity patterns, however, stemming from fundamentally distinct types of coherence: while in the second case the atoms display coherence among the individual particles leading to Young-type interference as known from classical dipoles, atoms in Dicke states possess collective coherence leading to enhanced spontaneous emission. This demonstrates that the radiation generated by synchronized dipoles and Dicke superradiance are fundamentally distinct phenomena and have to be interpreted in different ways.

Wave-Kinetic Approach to Collective Atomic Emission

We study the collective scattering of radiation by a large ensemble of Na≫1 atoms, in the presence of a pump field. We use the wave-kinetic approach where the center-of-mass position of the moving atoms is described by a microscopic discrete distribution, or alternatively, by a Wigner distribution. This approach can include thermal effects and quantum recoil in a natural way, and even consider atomic ensembles out of equilibrium. We assume two-level atoms with atomic transition frequency ωa very different from the frequency ω0 of the pump field. We consider both the quasi-classical and quantum descriptions of the center-of-mass motion. In both cases, we establish the unstable regimes where coherent emission of radiation can take place.

Superradiant and stimulated-superradiant emission of bunched electron beams

We outline the fundamental coherent radiation emission processes from a bunched charged particles beam. In contrast to spontaneous emission of radiation from a random electron beam that is proportional to the number of particles, a pre-bunched electron beam can emit spontaneously coherent radiation proportional to the number of particles - squared, through the process of (spontaneous) superradiance (SP-SR) (in the sense of Dicke's). The coherent SP-SR emission of a bunched electron beam can be even further enhanced by a process of stimulated-superradiance (ST-SR) in the presence of a seed injected radiation field. In this review, these coherent radiation emission processes for both single bunch and periodically bunched beams are considered in a model of radiation mode expansion. The general model of coherent spontaneous emission is extended to the nonlinear regime, particularly for undulator (wiggler) interaction: Tapering-Enhanced Stimulated Superradiant Amplification (TESSA). Processes of SP-SR and TESSA take place in tapered wiggler seed-injected FELs. In such FELs, operating in the X-Ray regime, these processes are convoluted with other effects. However these fundamental emission concepts are useful guidelines for the strategy of wiggler tapering efficiency and power enhancement. Based on this model, we review previous theories and experiments on coherent radiation sources based on SP-SR (coherent undulator radiation, synchrotron radiation, Smith-Purcell radiation etc.), in the THz regime and on-going works on tapered wiggler efficiency-enhancement concepts in all optical frequency regimes up to X-Rays.

Random anti-lasing through coherent perfect absorption in a disordered medium.

Non-Hermitian wave engineering is a recent and fast-moving field that examines both fundamental and application-oriented phenomena1-7. One such phenomenon is coherent perfect absorption8-11-an effect commonly referred to as 'anti-lasing' because it corresponds to the time-reversed process of coherent emission of radiation at the lasing threshold (where all radiation losses are exactly balanced by the optical gain). Coherent perfect absorbers (CPAs) have been experimentally realized in several setups10-18, with the notable exception of a CPA in a disordered medium (a medium without engineered structure). Such a 'random CPA' would be the time-reverse of a 'random laser'19,20, in which light is resonantly enhanced by multiple scattering inside a disorder. Because of the complexity of this scattering process, the light field emitted by a random laser is also spatially complex and not focused like a regular laser beam. Realizing a random CPA (or 'random anti-laser') is therefore challenging because it requiresthe equivalent of time-reversing such a light field in all its degrees of freedom to create coherent radiation that is perfectly absorbed when impinging on a disordered medium. Here we use microwave technology to build a random anti-laser and demonstrate its ability to absorb suitably engineered incoming radiation fields with near-perfect efficiency. Because our approach to determining these field patterns is based solely on far-field measurements of the scattering properties of a disordered medium, it could be suitable for other applications in which waves need to be perfectly focused, routed or absorbed.

Superbunching and Nonclassicality as new Hallmarks of Superradiance.

Superradiance, i.e., spontaneous emission of coherent radiation by an ensemble of two-level atoms in collective states introduced by Dicke in 1954, is one of the enigmatic problems of quantum optics. The startling gist is that even though the atoms have no dipole moment they radiate with increased intensity in particular directions. Following the advances in our understanding of superradiant emission by atoms in entangled W-states we examine the quantum statistical properties of superradiance. Such investigations require the system to have at least two excitations in order to explore the photon-photon correlations of the radiation emitted by such states. We present specifically results for the spatially resolved photon-photon correlations of systems prepared in doubly excited W-states and give conditions when the atomic system emits nonclassial light. Equally, we derive the conditions for the occurrence of bunching and even of superbunching, a rare phenomenon otherwise known only from nonclassical states of light like the squeezed vacuum. We finally investigate the photon-photon cross correlations of the spontaneously scattered light and highlight the nonclassicalty of such correlations. The theoretical findings can be implemented with current technology, e.g., using ions in a linear rf-trap, atoms in an optical lattice or quantum dots in a cavity.

Linear model formulation for superradiant and stimulated superradiant prebunchede-beam free-electron lasers

We present a unified model and analysis of radiation modes excitation in free-electron lasers with a periodically premodulated electron beam. The formulation characterizes superradiant coherent radiation emission from electron beam prebunched at the synchronous frequency, including the case of stimulated superradiant where in addition to a prebunched electron beam an electromagnetic wave is injected into the undulator region. The derived formulation of radiation mode excitation by currents is applicable to general structures that can support orthogonal eigenmodes.

The physics behind free electron lasers (FELs) based on magnetostatic and optical undulators

AbstractBy explicitly writing the equations of motion for highly relativistic electrons in an electromagnetic field we show the equivalence between the magnetostatic and optical (laser) undulator‐based free electron lasers (FELs). In order to gain insight into the physics behind both FEL mechanisms we use a simplified approach developed in Ref. 1 to describe the optical amplification process. By using elementary physics we explain the phenomenon of microbunching, caused by the interaction of electrons with the emitted wave, which leads to coherent emission of radiation and is the essential ingredient in the lasing effect of a FEL. With our simplified model we derive the two main FEL parameters – the gain and saturation lengths. Finally, we discuss the potential advantages of using an optical instead of a magnetostatic undulator in a FEL. With our approach the physics behind FELs becomes evident and accessible to researchers outside the specialized field.

Synchronization of Self-Heated Josephson Junctions by a Superconducting Resonator

Abstract Experiments on emission of coherent radiation from thick mesa structures of high-temperature superconductors showed that the structure acts like a resonator and that there is strong self-heating of intrinsic Josephson junctions. We considered analytically basic conditions of synchronization of the self-heated stack of two junctions in a resonator using the method of slowly varying amplitudes. We calculated the changes of the temperatures of junctions and the changes of their IV-characteristics due to self-heating in the region of the resonance frequency.

Coherent radiation by two-level quantum systems with permanent dipole moments under multiphoton resonant laser excitation

The theory of coherent radiation emission in two-level quantum systems with permanent dipole moments at the multiphoton resonant excitation by moderately strong laser fields is presented. A simple analytic formula for the expectation value of a dipole moment operator is obtained. The spectrum corresponding to harmonic generation and low-frequency radiation on the generalized Rabi frequency is investigated both analytically and numerically. The effect of dynamic Stark shift on a dipole spectrum under direct multiphoton resonant excitation is considered.

Coherent radiation from extensive air showers

The generic properties of the emission of coherent radiation from a moving charge distribution are discussed. The general structure of the charge and current distributions in an extensive air shower are derived. These are subsequently used to develop a very intuitive picture for the properties of the emitted radio pulse. Using this picture can be seen that the structure of the pulse is a direct reflection of the shower profile. At higher frequencies the emission is suppressed because the wavelength is shorter than the important length scale in the shower. It is shown that radio emission can be used to distinguish proton- and iron-induced air showers.

Further Investigation of Coherent Thermal Emission from Single Negative Materials

Planar heterogeneous structures have been proposed to achieve selective spectral and directional emission by using layers of negative permittivity (ϵ) paired with layers of negative permeability (μ), the so-called single negative materials. The coherent emission of radiation is achieved by the excitation of surface polaritons at the interface between the negative-ϵ and negative-μ layers and can be obtained for both p and s polarization. In the present work, a detailed study is carried out to investigate the effects of layer thicknesses and other important parameters on the emission characteristics of the planar heterogeneous structures. Furthermore, it is found that proper selection of the parameters of the layered structures can result in quasi-monochromatic but nearly diffuse emission of radiation; this may have potential applications in, for example, thermophotovoltaic devices.

DC electric field effects on the electron dynamics in double rectangular quantum dots

The effect of a dc electric field on the temporal evolution of an electron in a double rectangular quantum dot is explored in this work. In the framework of the effective mass approximation, first-order scattering rates for interaction between confined electron-''free'' electron and electron-longitudinal acoustic phonon at room temperature are calculated in the high tunneling regime, and used to evaluate the dynamics of the population and coherence in the first three confined levels under a short ac electric field pulse. Small values of these rates dependent upon the bias field make feasible the emission of coherent radiation at terahertz range.

Multiple-barrier and nanoscale superconducting devices

Since the pioneering studies of Josephson and Giaever, the development of electronicdevices has been a key research area in superconductivity. In conventional metallicsuperconductors, highly reliable tunnel junction fabrication techniques have enabled theapplication of superconducting devices in astronomy and metrology and have allowed thedevelopment of large scale integrated circuits for fast digital processing. These sub-micronheterostructure fabrication processes have also made possible the study of superconductingdevices containing two or more closely spaced barriers. Such structures allow theinvestigation of a wide range of novel effects and have led to the development of basicresearch areas and applications as diverse as electronic cryocoolers, coherent radiationemission and quantum computing. This paper will review the science which can beinvestigated using multiple-barrier superconducting devices and describe recent progress inthe area.

Electron dynamics in rectangular double quantum dots

The temporal evolution of an electron in a double rectangular quantum dot in the presenceof an electric field pulse is explored in this work. In the framework of the effective massapproximation, first-order scattering rates for electron–electron and electron–longitudinalacoustic phonon interaction at room temperature are calculated in the high tunnellingregime, and used to evaluate the dynamics of the population and coherence of the firstthree confined levels under an electric field pulse. Small values of these rates dependentupon the coupling barrier make feasible the emission of coherent radiation near 0.1 THz.

Planar heterogeneous structures for coherent emission of radiation

The coupling of excited surface polaritons with thermal radiation through diffraction by gratings results in coherent thermal emission in a certain frequency range toward well-defined directions for p polarization. A planar coherent source is proposed that uses multilayers of negative permittivity (epsilon) and negative permeability (mu) materials. Owing to the excitation of surface polaritons at the interface between the negative-epsilon and negative-mu layers, coherent emission can be achieved for both s and p polarization. Moreover, one can control the emission frequency and direction by adjusting the layer thicknesses.

Program for Experimental Investigations of the Mechanisms for the Emission of Coherent Radiation by Relativistic Electrons with Energies of 10–500 MeV in Oriented Crystals

A program of experimental investigations of the mechanisms for the emission of coherent radiation by electrons in periodic structures is proposed. The program is aimed at developing sources of energy-tunable quasimonochromatic x radiation and γ radiation.

Phase transition in a radiation-matter interaction with recoil and collisions.

The standard model introduced to describe the collective atomic recoil of an ensemble of atoms interacting with a strong electromagnetic field has been here extended by the inclusion of collisions with a buffer gas. As a result, we find that in the thermodynamic limit the coherent emission of radiation exhibits a continuous phase transition upon increasing the pump intensity. The output laser field is strictly larger than 0 only above a critical value. We find that the transition is not associated with the onset of spatial ordering but rather with the onset of a synchronization between the polarization phase and spatial position. A coherence parameter is introduced to characterize the phase transition.