Conditions for the onset of superradiance regime in a relativistic electron bunch

Problems associated with the formation of coherent oscillations of an ensemble of classical oscillators and their superradiance instability are considered. The dispersion properties of an electron bunch and the conditions for the generation of nonequilibrium radiation are determined in the quasi-steady anharmonic oscillator approximation.     

Reconciling the eigenmode analysis with the Maxwell-Bloch equations approach to superradiance in the linear regime

The superradiance from a slab of inverted two-level atoms is theoretically analyzed in the linear regime from both the perspective of the expansion in eigenfunctions of the integral equation with the Lienard-Wiechert potential as kernel, and that of linearizing the Maxwell-Bloch equations. We show the equivalence of both approaches. We show that the so-called Reduced Maxwell-Bloch equations do not yield even approximately the correct solution when applied in the obvious way, but that they can be made to give the correct solution by adding a fictitious input field.     

Stimulated superradiance

If N atoms simultaneously interact with quasiresonant classical and quasiresonant quantized fields, the modes exchange photons. This processes exhibits cooperative properties, i.e., the number of photons in the quantized mode oscillates, and the amplitude of these oscillations is proportional to N ². Zh. éksp. Teor. Fiz. 116, 858–869 (September 1999)     

Effect of the near-lying level on the kinetics of populations in the superradiance of a thin layer: Local-field regime

The effect of the local field on the competition of transitions in the superradiance of an incompletely excited thin layer of three-level atoms Λ-diagram of working transitions) is studied. The analysis made proves that the upper sublevel of the lower doublet is initially unpopulated. It is shown that the Raman transition, in the local-field regime, is ahead of the development of the superradiance pulse and causes the transfer of populations between sublevels of the lower doublet.     

On the theory of vibronic superradiance

The Dicke superradiance on vibronic transitions of impurity crystals is considered. It is shown that parameters of the superradiance (duration and intensity of the superradiance pulse and delay times) on each vibronic transition depend on the strength of coupling of electronic states with the intramolecular impurity vibration (responsible for the vibronic structure of the optical spectrum in the form of vibrational replicas of the pure electronic line) and on the crystal temperature through the Debye-Waller factor of the lattice vibrations. Theoretical estimates of the ratios of the time delays, as well as of the superradiance pulse intensities for different vibronic transitions well agree with the results of experimental observations of two-color superradiance in the polar dielectric KCl:O ₂ ^? . In addition, the theory describes qualitatively correctly the critical temperature dependence of the superradiance effect.     

Temperature dependence of superradiance intensity

Using the idea of exchange interaction in a system of two-level atoms participating in a superradiance process, we derive from first principles the superradiance Hamiltonian of such a system, which is found to be analogous to the Heisenberg Hamiltonian. We consistently calculate the coupling constant of the interaction that leads to the emergence of a superradiance state in the system. We also predict the existence of isospin excitations in the superradiance state, whose presence reduces the intensity of the corresponding superradiance pulse. Finally, we calculate the temperature dependence of the intensity of the superradiance pulse and find it be analogous to the Bloch T ^3/2-law for spin systems. Zh. éksp. Teor. Fiz. 116, 1148–1160 (October 1999)     

Enhanced superradiance effect in a system of interacting two-level atoms and crossover from coherent to many-atom multiphoton relaxation regime

We study effects of direct interatomic interaction on cooperative processes in atom-photon dynamics. Using a model of two-level atoms with Ising-type interaction as an example, it is demonstrated that interparticle interaction can promote cooperative radiative relaxation. For small number of atoms this results in inhibition of incoherent spontaneous decay leading to the regime of collective pulse relaxation. Above superradiance threshold increase in delay time and enhancement of superradiance is occurred. In the case of strong interaction (as compared to excitation energy of an atom) transition to the regime of multiphoton relaxation occurs, which we discuss using a simple model of two atoms in a high-Q single mode cavity. It is shown that such transition is accompanied by Rabi oscillations involving many-atom multiphoton states. Dephasing effect of dipole-dipole interaction and solitonic mechanism of relaxation are discussed as well.     

The counter-rotating terms in the superradiance

Summary It is pointed out that the counter-rotating terms play an important role in the superradiance of the first kind, in which the atomic system is initially in an uncorrelated state with maximal dipole moment. If the counter-rotating terms are added to the master equation for the Dicke model derived by Agarwal and Bonifacioet al., the numerical solutions for the atomic-population inversion and radiation rate show strong damped oscillations (damped recurrence effects) in their time behaviour. A new limiting relation for the cut-off frequency, which depends on the number of atoms, is also obtained for the treated model.

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Riassunto Si evidenzia che i termini controrotanti svolgono un importante ruolo nella superradianza del primo tipo, in cui il sistema atomico è inizialmente in uno stato incorrelato con impulso dipolare massimo. Se i termini controrotanti sono aggiunti all’equazione principale per il modello di Dicke derivato da Agarwal e Bonifacioet al., le soluzioni numeriche per l’inversione di popolazione atomica e il valore di radiazione mostrano forti oscillazioni smorzate (effetti di ricorrenza smorzati) nel loro comportamento nel tempo. Si ottiene anche una nuova relazione per la frequenza di taglio per il modello trattato che dipende dal numero di atomi.

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Резюме Отмечается, что члены с противоположным вращением играют важную роль в суперсветимости первого рода, где атомная система первоначально находится в некоррелированном состоянии с максимальным дипольным моментом. Если члены с противоположным вращением добавляются в ?управляющие? уравнение для модели Дикка, которое было получено Агарвалом, Ъонифачио и др., то численные решения для инверсии атомной заселенности и интенсивности излучения обнаруживают сильные затухающиее осцилляции (затухаюцие рекуррентные эффекты), в их временном поведении. Для рассматриваемой модели получено новое предельное соотношение для частоты обрезания, которая зависит от числа атомов.

     

Induced superradiance in activated crystals

The observation of superradiance in activated crystals induced by propagation of a short coherent pulse through an inverted sample is proposed. The theoretical estimation of the induced superradiance in Nd: YAG is given.     

Recombination of a classical Coulomb plasma under the action of external stochastization

Different types of stochastic action on the stationary state of a classical Coulomb plasma, manifested in earlier studies, are investigated from first principles by numerical simulation. The effect on the electron distribution resulting from the following is investigated: errors of numerical simulation, rough walls, adjustable stochastization of electron velocities, and random electron collisions with two-level atoms. It is shown that a substantial deformation of the electron distribution in total energy is generated by stochastic action alone, varying the energy of individual electrons (adjustable stochastization, inelastic collisions, and so on).Institute of General Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 76–88, November, 1992.     

Stochastic structure of superradiance pulses

We have made a theoretical study of the statistics of superradiant pulses excited by pumping pulses. We describe experiments on the superradiance of a dye (Rhodamine 6G) that confirm the theory.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, No. 4, pp. 73–78, April, 1976.     

Experimental observation of cyclotron superradiance

Intense (several hundreds of kilowatts) subnanosecond coherent microwave radiation — cyclotron superradiance of an electron swarm moving in a uniform magnetic field — was recorded. The maximum power of the radiation was observed under group synchronization conditions, when the translational velocity of the swarm is equal to the group velocity of the waves in the waveguide channel. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 322–325 (10 March 1996)     

Model calculation of the laser-semiconductor interaction in subpicosecond regime

We develop a model calculation to investigate the interaction of a subpicosecond laser pulse with a direct-gap semiconductor. In this problem, carrier distributions are extremely far from equilibrium and nonequilibrium LO phonons are generated at the center of the Brillouin zone. We solve numerically a system of three transport equations, namely one for the conduction band, one for the valence band and one for the LO phonons. Contrary to all previous work, we do not assume that carrier distributions are nondegenerate so that we can investigate the following problems: (a) conditions of internal thermalization in degenerate nonequilibrium plasma (thermalization time, influence of screening on the energy-loss rate, formation of nonequilibrium LO phonons) and (b) absorption saturation and transmission of the pumplaser pulse.     

The single photon superradiance from the eigenmode analysis

Using the eigenmode analysis of the scalar photon theory, I compute the probability of the atoms remaining excited and the probability for the atoms remaining in the initial quantum state of a system of two-level atoms cloud in a sphere initially prepared to radiate in the forward direction, i.e., the single photon superradiance problem. The convergence in the results obtained for increasingly larger radii for the sphere suggests that the asymptotic limits for these quantities are obtained for a sphere with a radius equal to six times the resonant wavelength. I predict the maximal value of the probability of secondary excited states from large spheres at 17.1%.     

Entanglement and the phase transition in single-mode superradiance

We consider the entanglement properties of the quantum phase transition in the single-mode superradiance model, involving the interaction of a boson mode and an ensemble of atoms. For an infinite size system, the atom-field entanglement diverges logarithmically with the correlation length exponent. Using a continuous variable representation, we compare this to the divergence of the entropy in conformal field theories and derive an exact expression for the scaled concurrence and the cusplike nonanalyticity of the momentum squeezing.     

Effect of the initiating-pulse area on induced superradiance

The formation of reflected and transmitted waves in induced superradiance is studied. It is shown that the relationship between reflected and transmitted waves periodically varies with increasing initiating-pulse area. The effect is attributed to the periodic cooperative exchange between forward and backward waves in a sample and its termination at the moment corresponding to the pulse delay time.     

Equilibrium superradiance in a Bose gas

A model of free₄He atoms interacting with radiation exhibits an equilibrium phase transition in which the atomic ground-state Bose condensation is coupled to condensations of virtual photons and virtually excited atoms of the same macroscopic wavelength. The condensed phase has a twofold polarization degeneracy. It is suggested that this might furnish a mechanism for a discrete symmetry-related phase degeneracy of superfluid liquid⁴He required to explain the transition according to Tisza's generalized Gibbsian thermodynamics. A more realistic model would require inclusion of repulsive interactions.Supported in part by the National Science Foundation, Grant DMR76-17467.