Superradiant Emission Research Articles

Superradiant Spin-Flip Radiative Emission of a Spin-Polarized Free-Electron Beam

Radiative emission from the magnetic moments of the spins of an electron beam has never been observed directly, because it is fundamentally much weaker than the electric charge emission. We show that the detectivity of spin-flip and combined spin-flip-cyclotron-resonance-emission radiation can be substantially enhanced by operating with ultrashort spin-polarized electron beam bunches under conditions of superradiant (coherent) emission. The proposed superradiant spin-flip radiative emission scheme can be used for noninvasive diagnostics of polarized electron or positron beams. Such beams are of relevance in important scattering experiments off nucleons in nuclear physics and off magnetic targets in condensed matter physics.

Analysis of super-radiant Smith-Purcell emission

Smith-Purcell emission is studied for a system with no incident field or electromagnetic feedback mechanism other than what is provided by the grating itself. A model is developed that analyzes the interaction between the electron beam and the grating electromagnetic modes. The system of equations is solved numerically, demonstrating linear emitted power versus current at low current levels and super-radiant power emission at higher currents. These results are compared to the previously published experimental data, and the model is used to gain some understanding of the physical mechanisms at work.

Superradiant emission of Smith-Purcell radiation

Smith-Purcell (SP) radiation is emitted when an electron passes close to the surface of a metallic grating. The radiation becomes coherent (fluence proportional to the square of the number of electrons) when the electrons are in bunches whose dimensions are smaller than the wavelength of the radiation. This has been observed in experiments in which the electrons are prebunched by an rf linac. The enhancement of the spectral intensity is accompanied by large changes in the angular and spectral distribution of the radiation when the electrons appear in periodic bunches. This is called superradiance. Recently, superradiant SP radiation has been observed from a so-called Smith-Purcell free-electron laser (SP-FEL) in which the electrons are bunched by the lasing process. As in other slow-wave structures, the electron beam in a SP-FEL interacts with an evanescent wave for which the phase velocity matches the electron velocity and amplifies it. The frequency of this wave lies below the range of SP radiation and the wave is not radiated except from the ends of the grating. However, the bunching of the electrons by the interaction with the evanescent wave enhances the ordinary Smith-Purcell radiation and changes the angular and spectral distribution due to superradiant effects. In this article, we introduce a new method for computing the SP radiation in three dimensions, including the effects of finite grating length and superradiance due to periodic electron bunching at an arbitrary frequency. We show that the SP radiation develops spectrally and angularly narrow peaks at the harmonics of the bunching frequency. In rf linacs, where the bunches are widely spaced, several closely spaced harmonics lie under the spectral envelope of the emission from a single electron. In a SP-FEL the harmonics are widely spaced and the SP radiation appears in narrow cones at the SP angles corresponding to the harmonics of the bunching frequency. Finally, we calculate the angular spectral fluence radiated by an electron passing over a lamellar grating of finite length, examine its coherent enhancement in SP-FELs and rf linacs, and compare the results with numerical simulations and available experimental data.

On-Demand Superradiant Conversion of Atomic Spin Gratings into Single Photons with High Efficiency

We create quantized spin gratings by single-photon detection and convert them on demand into photons with retrieval efficiencies exceeding 40% (80%) for single (a few) quanta. We show that the collective conversion process, proceeding via superradiant emission into a moderate-finesse optical resonator, requires phase matching. The storage time of 3 micros in the cold-atom sample, as well as the peak retrieval efficiency, are likely limited by Doppler decoherence of the entangled state.

Propagation effects in the quantum description of collective recoil lasing

The free electron laser and collective atomic recoil laser (CARL) are examples of collective recoil lasing, where exponential amplification of a radiation field occurs simultaneously with self-bunching of an ensemble of particles (electrons in the case of the FEL and atoms in the case of the CARL). In this paper, we discuss quantum and propagation effects using a model where the particle dynamics are described quantum-mechanically in terms of a matter-wave field, which evolves self-consistently with the radiation field. The model shows that the scattered radiation evolves superradiantly both in the case where the particle ensemble is short compared to the cooperation length of the system, and where the ensemble is long compared to the cooperation length. In both short and long pulse cases there exist a classical and quantum regime of superradiant emission. For short samples in both quantum and classical regimes the superradiant pulse has a low peak intensity and is said to exhibit ‘weak’ superradiance. For long pulses in both quantum and classical regimes of evolution, the dynamics at the rear edge of the sample is dominated by propagation. This produces a ‘strong’ superradiant pulse with much higher peak intensity than that predicted by ‘mean-field’ or ‘steady-state’ models in which propagation effects are neglected.

Superradiant and stimulated-superradiant emission in prebunched electron-beam radiators. II. Radiation enhancement schemes

Further enhancement of the intense coherent superradiant and stimulated-superradiant emission from prebunched electron beams is possible, in schemes of prebunched beam radiation devices, and particularly free electron laser (FEL). The enhancement of coherent power and spectral power by use of a waveguide, particularly at the zero-slippage condition, is evaluated. A special scheme of a stimulated-superradiance FEL oscillator is analyzed and is shown to feature ultimate radiative energy conversion efficiency (near 100%).

Superradiant and stimulated-superradiant emission in prebunched electron-beam radiators. I. Formulation

A formulation for the characterization of superradiant and stimulated-superradiant radiative emission from bunched electron beams is presented. The radiation is characterized in terms of power and spectral power per radiation mode, which provide a measure of the useful spatially coherent radiation power and spectral power emitted by a radiation source. When the bunched electron beam emits superradiantly, these parameters scale like the square of the number of electrons, orders of magnitude more than spontaneous emission. The formulation applies to emission from single bunches, a finite number of bunches in a macropulse, or periodic bunching. It can be employed on any kind of $e$-beam radiation scheme. Specific analytic expressions are derived for coherent synchrotron radiation and prebunched free-electron laser, providing a basis for comparing and understanding their connection.

Coherent emission in one-dimensional photonic band gap materials

We study the properties of super-radiant coherent emission from a two-level system of atoms embedded in a one-dimensional photonic band gap (1D-PBG) structure. A reduced system of equations is derived that enables us to explore the role of the 1D-PBG band edge in the emission characteristics of coherent pulses. The effect of the emitters' spatial location in the PBG structures is also studied in a finite 1D-PBG, where the super-radiant emission characteristics are also presented.

Proposal for high-precision atomic-parity-violation measurements by amplification of the asymmetry by stimulated emission in a transverse electric and magnetic field pump-probe experiment

Amplification by stimulated emission of radiation provides an intriguing means for increasing the sensitivity of Atomic Parity Violation (APV) measurements in a pump-probe configuration well adapted to the 6S-7S cesium transition. It takes advantage of the large number of atoms excited along the path of the pump beam. In the longitudinal E-field configuration currently exploited in an ongoing APV measurement, this number is limited only by the total voltage sustainable by the Cs vapor. In order to overcome this limit, we consider, both theoretically and experimentally, the possibility of performing the measurements in a transverse E-field configuration requiring a much lower voltage. We discuss the necessarily different nature of the observable and the magnetoelectric optical effects entering into play. They condition modifications of the experimental configuration with, in particular, the application of a transverse magnetic field. We suggest the possibility of rotating the transverse direction of the fields so as to suppress systematic effects. With a long interaction length, a precision reaching 0.1 percent in a quantum noise limited measurement can be expected, now limited only by the necessity of operating below the threshold of spontaneous superradiant emission of the excited medium. If we approached this limit, however, we could greatly amplify the asymmetry using triggered superradiance.

Atomic squeezing under collective emission

Atomic squeezing is studied for the case of large systems of radiating atoms, when collective effects are well developed. All temporal stages are analyzed, starting with the quantum stage of spontaneous emission, passing through the coherent stage of superradiant emission, and going to the relaxation stage ending with stationary solutions. A method of governing the temporal behaviour of the squeezing factor is suggested. The influence of a squeezed effective vacuum on the characteristics of collective emission is also investigated.

Shot-to-shot fluctuations in the directed superradiant emission from extended atomic samples

We study collective spontaneous emission from arbitrary distributions of N two-state atoms using quantum trajectory theory and without an a priori single-mode assumption. Assuming a fully excited initial state, we calculate the angular distribution of the average intensity, focusing on pencil- and disc-shaped samples. The formalism is developed around an unravelling of the master equation in terms of source mode quantum jumps. A modified boson approximation is made to treat the many-atom case, where it is found that strong directional superradiance with shot-to-shot fluctuations occurs for a few hundred to a few thousand atoms.

Coherent electronic coupling versus localization in individual molecular dimers.

We have investigated electronic excitation transfer in individual molecular dimers by time and spectrally resolved confocal fluorescence microscopy. The single molecule measurements allow for directly probing the distribution of the electronic coupling strengths due to static disorder in the polymer host. We find dimers where the excitation is delocalized (superradiant emission) while for others emission originates from a localized state. Transitions between delocalized and localized states as observed for a given dimer are attributed to structural fluctuations of the guest-host system.

Simulation of high power broadband cyclotron autoresonance maser amplifier and electron beam experiments

The design, simulation, and preliminary experimental implementation of an efficient, broadband cyclotron autoresonance maser (CARM) amplifier operating over the 9–13 GHz frequency band is presented. The amplifier is based on a high current accelerator capable of generating a ∼35 A pencil electron beam at an accelerating voltage of ∼450 kV. A full three-dimensional numerical model of the CARM amplifier has been constructed within the particle-in-cell code KARAT taking into account electron beam parameters derived from simulation and experiment. An electron beam current of 32A at an accelerating voltage of 400 kV was measured. Numerical simulations demonstrate the possibility of obtaining 37 dB gain and an interaction efficiency of 17%. In addition a viable amplification bandwidth of 9–13 GHz is apparent, with a minimum gain and efficiency of 25 dB and 10%, respectively, at the boundaries of the amplification band. The peak modeled efficiency and gain (17%, 37 dB) were obtained at a frequency of 12 GHz. Computational simulations have also revealed correlation between the instantaneous amplification bandwidth and the spectral width of cyclotron superradiant emission within the system.

Cooperative effects in atom field interactions in a Bose–Einstein condensate

Using a semiclassical theory we give a unified approach to a class of coherent atom field interactions in a Bose–Einstein condensate, including superradiant light scattering, coherent amplification of atoms, and coherent light amplification. The populations of the excited states are explicitly taken into account, and the role of the recoil kinetic energies of the atoms with different momenta is discussed. In addition to the known features of superradiant emission, we obtain a shift in the spectrum of the scattered light as a consequence of the atomic recoil. The same theoretical method yields results that are in agreement with the experimental findings for atomic beam amplification, as well as for light amplification. Considering the atomic beam amplification process, we obtain a single outgoing amplified atomic beam. In treating the coherent light amplification, the unexpectedly small group velocity seen in the experiments is reproduced, and connected to the double Λ configuration of the levels involved.

Exchange dipole interaction in a multilevel cooperative system of atoms

Atomic systems with three or more equidistant energy levels, in which a cascade process is possible, are considered. Hamiltonians obtained for such systems are analogous to Heisenberg Hamiltonians, but for systems with integral spins. For Dicke states in multilevel systems, quantum-mechanical mean values of the energy of a cooperative system are calculated taking into account weak interactions between atoms. The type of emission preceding superradiant avalanche emission of the system is analyzed. It can be expected that a coherent state may be formed, in which collective processes affect one another not only via population of the intermediate common layer, but also via phasing of pure quantum states. The single superfluorescence pulse that can be formed in this case is not a simple superposition of two or more pulses of sequential superradiant transitions in two-level systems.

Enhanced super-radiant emission of FEM near waveguide-cutoff and near zero-slippage conditions

We report on super-radiance obtained from the TAU FEM just above waveguide cutoff and near grazing intersection. Grazing intersection (or “Zero Slippage”) is defined as the point at which the two synchronous frequencies merge to one frequency. In this case, the radiated power frequency can be tuned over a very wide band by change of the pre-modulation frequency. Near the lower synchronous frequency, the super-radiance power is much greater and the spectral width is much narrower than those at the higher synchronous frequency. The super-radiance emission near cutoff (lower synchronous frequency) and near to the upper synchronous frequency was measured and compared to those predicted by an analytical model for a wide range of frequencies.

Generalized theory and simulation of spontaneous and super-radiant emissions in electron devices and free-electron lasers.

A unified formulation of spontaneous (shot-noise) and super-radiant emissions in electron devices is presented. We consider an electron beam with an arbitrary temporal current modulation propagating through the interaction region of the electronic device. The total electromagnetic field is presented as a stochastic process and expanded in terms of transverse eigenmodes of the medium (free space or waveguide), in which the field is excited and propagates. Using the waveguide excitation equations, formulated in the frequency domain, an analytical expression for the power spectral density of the electromagnetic radiation is derived. The spectrum of the excited radiation is shown to be composed of two terms, which are the spontaneous and super-radiant emissions. For a continuous, unmodulated beam, the shot noise produces only incoherent spontaneous emission of a power proportional to the flux eI(0) (DC current) of the particles in the electron beam. When the beam is modulated or prebunched, a partially coherent super-radiant emission is also produced with power proportional to the current spectrum /I(omega)/(2). Based on a three-dimensional model, a numerical particle simulation code was developed. A set of coupled-mode excitation equations in the frequency domain are solved self-consistently with the equations of particles motion. The simulation considers random distributions of density and energy in the electron beam and takes into account the statistical and spectral features of the excited radiation. At present, the code can simulate free-electron lasers (FELs) operation in various modes: spontaneous and self-amplified spontaneous emission, super-radiance and stimulated emission, in the linear and nonlinear Compton or Raman regimes. We employed the code to demonstrate spontaneous and super-radiant emission excited when a prebunched electron beam passes through a wiggler of an FEL.

Stochastic initiation of superradiance in a cavity: An approximation scheme within quantum trajectory theory

We investigate the stochastic initiation of superradiant emission from a collection of excited two-state atoms coupled through a low-Q cavity mode. Noncollective emission into other modes of the electromagnetic field is included, allowing us to describe the transition from predominantly noncollective to collective behavior as the number of atoms is increased. We examine the dependence of the photon statistics on the number of atoms and the ratio of collective to noncollective decay rates. Numerical results for a few to 10 6 atoms are obtained using an approximation scheme developed within the framework of quantum trajectory theory. Results are compared with those for a pure collective emission process and an earlier treatment of noncollective effects.

Two-photon absorption-induced optical properties of a new lasing dye in two solvents

Pumped by an infrared lasing and its frequency-doubling source, the fluorescence and superradiance emission properties of a new lasing dye trans-4-[4′-(N,N-diethylamino)styryl]-N-methyl pyridinium methyl sulfate (abbreviated DEASPS), have been studied in benzyl alcohol and chloroform. The two-photon absorption (TPA) and emission properties of DEASPSare influenced by the solvents used. The emission wavelength of the dye in benzyl alcohol is redshifted relative to that in chloroform. The lifetime of two-photon absorption-induced fluorescence is about 529 ps in benzyl alcohol, whereas it is 340 ps in chloroform. Correspondingly, the one-photon-excited fluorescence lifetimes in the two solvents are also given. The upconversion efficiency of DEASPS in chloroform is higher than in benzyl alcohol. Finally, the effective molecular two-photon absorption cross-sections were measured by the nonlinear transmittance method. It is found that the optical limiting effect of the dye in benzyl alcohol is stronger than that in chloroform.

Spatial photon confinement and super-radiation from dye cored dendrimer

This paper describes the preparation of a dendritic macromolecule having a rhodamine B chromophore, called Rh-dendrimer, and the emission properties from the neat films due to the spontaneous emission and super-radiative emission. An important concept to be investigated for material preparation, is the surrounding of a fluorescent chromophore by the dendritic branches. Therefore, a dendrimer becomes a core-shell structure, achieving a high efficiency of fluorescence with little energy transfer and self-quenching. The optical properties from dendrimer were very different from that of a bare chromophore. The Rh-dendrimer showed a high efficiency of fluorescence, while a bared rhodamine chromophore formed a charge transferred complex and exhibited less fluorescence. Under optical excitation with sufficient intensity ( I 0), the emission intensity was non-linearly increased as a function of the square of the number of molecules ( N), giving relation of I∝ N 2 I 0. This property can not be adequately explained within the simple picture of spontaneous emission. Results were discussed in terms of cooperative emission.