Superradiant Regime Research Articles

ON THE WAVE GENERATION IN A MAGNETOACTIVE WAVEGUIDE

The conditions for generation in the superradiance regime and the formation of a waveguide field taking into account reflection from the ends are discussed. In different generation modes, as shown in this work, both the waveguide field, determined by reflection effects and the geometry of the system, and the field of the total radiation of core particles, that is, the superradiance field, can dominate.

Three-dimensional theory of superradiant free-electron lasers

The solitonlike superradiant regime of free-electron lasers (FEL) offers a promising path towards ultrashort pulses, beyond the natural limit dictated by the bandwidth of the high-gain FEL instability. In this work we present a three-dimensional theory of the superradiant regime, including the effects of beam emittance and energy spread. Our work takes advantage of recent developments in nonlinear FEL theory to provide a fully analytical description of solitonlike superradiance. Our theory proves the existence of a diffraction-dominated steady-state regime in which the superradiant peak power grows indefinitely while leaving the pulse duration and on-axis intensity almost unchanged. These results are in excellent agreement with three-dimensional simulations and are supported by recent experimental results at the Linac Coherent Light Source. This work advances nonlinear FEL theory and provides a theoretical framework for the next generation of attosecond x-ray FELs. Published by the American Physical Society 2024

Super-radiant backward-wave oscillator with two-period slow-wave structure

In order to significantly increase the peak power of the output wave pulse of a Cherenkov backward-wave oscillator (BWO) operating in the super-radiance regime, it is proposed to use a slow-wave structure in the form of a waveguide with a two-period corrugation. This approach is especially effective in a hybrid system in which the super-radiant wave pulse generated in a BWO with traditional single-period corrugation is amplified in a BWO with a two-period corrugation. Simulations based on the simplest 1D spatiotemporal theory predict a significant enhancement (by 4–5 times compared with the traditional super-resonant BWO version) of the peak power of the output wave pulse.

Threshold studies for a hot beam superradiant laser including an atomic guiding potential

Motivated by the outstanding short time stability and reliable continuous operation properties of microwave clock masers, intense worldwide efforts target the first implementation of their optical analogues based on narrow optical clock transitions and using laser cooled dilute atomic gases. While as a central line of research large efforts are devoted to create a suitably dense continuous ultracold and optically inverted atom beam source, recent theoretical predictions hint at an alternative implementation using a filtered thermal beam at much higher density. Corresponding numerical studies give encouraging results but the required very high densities are sensitive to beam collimation errors and inhomogeneous shifts. Here we present extensive numerical studies of threshold conditions and the predicted output power of such a superradiant laser involving realistic particle numbers and velocities along the cavity axis. Detailed studies target the threshold scaling as a function of temperature as well as the influence of eliminating the hottest part of the atomic distribution via velocity filtering and the benefits of additional atomic beam guiding. Using a cumulant expansion approach allows us to quantify the significance of atom-atom and atom-field correlations in such configurations. We predict necessary conditions to achieve a certain threshold photon number depending on the atomic temperature and density. In particular, we show that the temperature threshold can be significantly increased by using more atoms. Interestingly, a velocity filter removing very fast atoms has only almost negligible influence despite their phase perturbing properties. On the positive side an additional conservative optical guiding towards cavity mode antinodes leads to significantly lower threshold and higher average photon number. Interestingly we see that higher order atom-field and direct atom-atom quantum correlations play only a minor role in the laser dynamics, which is a bit surprising in the superradiant regime.

Carrier-envelope phase stabilization in FEL oscillators

FEL oscillators can produce few-cycle optical pulses with a high-extraction efficiency when the oscillators are operated in the superradiant regime. Such FEL oscillators are unique light sources to explore intense light field science, especially in mid-infrared and longwave infrared where ultrashort pulses for the high-intensity applications are difficult to produce from conventional lasers. Since the laser-matter interaction in the intense field regime is described in terms of the oscillating electric field rather than the instantaneous intensity, the carrier-envelope phase (CEP) must be stabilized in many applications of few-cycle optical pulses to the intense light field science. Stabilization of CEP in FEL oscillators has been proposed with an external seed laser and coherent spontaneous emission from the electron bunches. In this paper, we study CEP stabilization in FEL oscillators assisted by coherent spontaneous emission from electron bunches with numerical simulations.

Nonlinear compression of naturally down-chirped superradiance pulses from a free-electron laser oscillator by thick germanium plates.

Naturally down-chirped superradiance pulses, with mirco-pulse energy, peak wavelength, and micropulse duration of 40 µJ, 8.7 μm, and 5.1 optical cycles, respectively, emitted from a free-electron laser (FEL) oscillator were nonlinearly compressed down to 3.7 optical cycles using a 30-mm-thick Ge plate. The peak power enhancement owing to nonlinear compression was found to be 40%. The achieved peak power and pulse duration were comparable to those of recently developed high-intensity and few-cycle long-wavelength infrared sources based on solid-state lasers. FEL oscillators operating in the superradiance regime can serve as unique tools for studying strong-field physics in long-wavelength infrared regions.

On Accounting for Own Fields of Emitters when Describing Generation Modes

The paper discusses three different modes of electromagnetic field generation by an ensemble of oscillators placed at the radiation wavelength in the one-dimensional case. The excitation of the resonator field is considered, which, as a rule, is determined by the geometry of the system, with and without taking into account the eigenfields of the emitters. The superradiance regime of the same ensemble of oscillators is also analyzed. In fact, superradiance is formed due to the emitters' own fields even in the absence of a resonator. It is noted that the maximum achievable amplitudes of induced fields both in the superradiance regime and in the regime of excitation of the resonator field are comparable. This makes us think about the role of the self-fields of emitters in electronic devices. It is shown that in describing the resonator excitation mode, in addition to the resonator field, it is also necessary to take into account the sum of the natural fields of the emitters in the active zone. Synchronization of emitters leads not only to an increase in the resonator field, but also, as in the superradiance regime, it significantly increases the amplitude of the sum of the oscillator fields. It is shown that in the practically interesting case of open systems (dissipative generation modes), taking into account the eigenfields of the emitters significantly reduces the characteristic time for the development of the generation process and increases the maximum achievable oscillation amplitude. This account also changes the conditions for achieving the maximum energy flow from the system. This can change the operating point of the generation process, which is determined by the requirement for the maximum rate of energy output from the system.

Superradiant energy extraction from rotating hairy Horndeski black holes

Adopting the manifestation of low frequency and low mass for the scalar perturbation, we perform a semiclassical analysis of the superradiance phenomenon for a rotating hairy Horndeski black hole (BH). For the spacetime under study enriched by the hairy Horndeski parameter $h$, in addition to the mass $M$ and spin $a$, we compute the amplification factor of scalar wave scattering indicating the energy extraction from the BH. We find that due to the addition of the hairy parameter $h$ in the geometry, the superradiance scattering and its frequency range enhance compared to the Kerr BH. This implies that Horndeski's gravity belongs to those alternative theories of gravity that make the amplification factor larger than the Kerr BH so that the energy extraction in its framework is more efficient than general relativity. Calculating the outgoing energy flux measured by an observer at infinity verifies the role of the hairy parameter $h$ in the increase in energy extraction efficiency from the rotating BH. By implementing the BH bomb mechanism, we present an analysis of the superradiant instability of the underlying BH spacetime against massive scalar fields. Our analysis indicates that the hairy Horndeski parameter leaves no imprint on the standard superradiant instability regime.

Correlations and linewidth of the atomic beam continuous superradiant laser

We propose a minimalistic model to account for the main properties of a continuous superradiant laser, in which a beam of atoms crosses the mode of a high-finesse Fabry-Perot cavity, and collectively emits light into the cavity mode. We focus on the case of weak single atom - cavity cooperativity, and highlight the relevant regime where decoherence due to the finite transit time dominates over spontaneous emission. We propose an original approach where the dynamics of atoms entering and leaving the cavity is described by a Hamiltonian process. This allows deriving the main dynamical equations for the superradiant laser, without the need for a stochastic approach. We derive analytical conditions for a sustained emission and show that the ultimate linewidth is set by the fundamental quantum fluctuations of the collective atomic dipole. We calculate steady-state values of the two-body correlators and show that the continuous superradiant regime is tied to the growth of atom-atom correlations, although these correlations only have a small impact on the laser linewidth.

SUPERRADIATION OF CLASSICAL OSCILLATORS AT CONSTANT PUMPING

The dependence of the generation efficiency in the superradiance regime of an ensemble of oscillators on the pump energy input rate, as well as on the characteristics of the change in the pump phase, is studied. It is assumed that the pumping phase can vary both in time and in space. The most efficient pumping method remains the mode with slow energy input into the system, in comparison with the characteristic generation time, and the nature of the distribution of the field phase along the system can be either random or with a certain spatial period, but slightly changing with time.

Lifetime of scalar cloud formation around a rotating regular black hole

Does circumventing the curvature singularity of the Kerr black hole affects the timescale of the scalar cloud formation around it? By definition, the scalar cloud, forms a gravitational atom with hydrogen-like bound states, lying on the threshold of a massive scalar field's superradiant instability regime (time-growing quasi-bound states) and beyond (time-decaying quasi-bound states). By taking a novel type of rotating hollow regular black hole proposed by Simpson and Visser which unlike its standard rivals has an asymptotically Minkowski core, we address this question. The metric has a minimal extension relative to the standard Kerr, originating from a single regularization parameter $\ell$, with length dimension. We show with the inclusion of the regularization length scale $\ell$ into the Kerr spacetime, without affecting the standard superradiant instability regime, the timescale of scalar cloud formation gets shorter. Since the scalar cloud after its formation, via energy dissipation, can play the role of a continuum source for gravitational waves, such a reduction in the instability growth time improves the phenomenological detection prospects of new physics because the shorter the time, the more astrophysically important.

Superradiance-Mediated Photon Storage for Broadband Quantum Memory

Superradiance, characterized by the collective, coherent emission of light from an excited ensemble of emitters, generates photonic signals on timescales faster than the natural lifetime of an individual atom. The rapid exchange of coherence between atomic emitters and photonic fields in the superradiant regime enables a fast, broadband quantum memory. We demonstrate this superradiance memory mechanism in an ensemble of cold rubidium atoms and verify that this protocol is suitable for pulses on timescales shorter than the atoms' natural lifetime. Our simulations show that the superradiance memory protocol yields the highest bandwidth storage among protocols in the same system. These high-bandwidth quantum memories provide unique opportunities for fast processing of optical and microwave photonic signals, with applications in large-scale quantum communication and quantum computing technologies.

Charged black holes in de Sitter space: Superradiant amplification of charged scalar waves and resonant hyperradiation

Black holes typically inhabit highly dynamical galactic environments and are frequently permeated by accretion media. The inevitable scattering of scalar, electromagnetic, and gravitational waves off rotating and charged black holes provides a remarkable demonstration of the energy extraction and subsequent amplification of scattered waves under the expense of the compact object's rotational and electromagnetic energy, a phenomenon known as superradiance. Certain circumstances tremendously favor the energy extraction process in such extent that linear superradiant instabilities are triggered. We examine the superradiant amplification of monochromatic charged scalar fields impinging Reissner-Nordstr\"om-de Sitter black holes which are known to exhibit superradiantly unstable quasinormal modes. We find that even though the frequency range of superradiance is reduced when a positive cosmological constant is incorporated, the amplification factors are significantly elevated with respect to those occurring in Reissner-Nordstr\"om spacetime. Intriguingly, we confirm that the long-lived quasinormal resonances that reside in the superradiant regime induce resonant peaks when the wave's frequency matches the real part of the quasinormal mode, regardless of the spacetime's modal stability.

Superradiant instability of a Kerr-like black hole in Einstein-bumblebee gravity

An exact Kerr-like solution has been obtained recently in Einstein-bumblebee gravity model where Lorentz symmetry is spontaneously broken. In this paper, we investigate the superradiant instability of the Kerr-like black hole under the perturbation of a massive scalar field. We find the Lorentz breaking parameter $L$ does not affect the superradiance regime or the regime of the bound states. However, since $L$ appears in the metric and its effect cannot be erased by redefining the rotation parameter $\tilde{a}=\sqrt{1+L}a$, it indeed affects the bound state spectrum and the superradiance. We calculate the bound state spectrum via the continued-fraction method and show the influence of $L$ on the maximum binding energy and the damping rate. The superradiant instability could occur since the superradiance condition and the bound state condition could be both satisfied. Compared with Kerr black hole, the nature of the superradiant instability of this black hole depends non-monotonously not only on the rotation parameter of the black hole $\tilde{a}$ and the product of the black hole mass $M$ and the field mass $\mu$, but also on the Lorentz breaking parameter $L$. Through the Monte Carlo method, we find that for $l=m=1$ state the most unstable mode occurs at $L=-0.79637$, $\tilde{a}/M=0.99884$ and $M\mu=0.43920$, with the maximum growth rate of the field $\omega_{I}M=1.676\times10^{-6}$, which is about 10 times of that in Kerr black hole.

Computer modeling of radiation of free oscillators

The article discusses a system of computer simulation of superradiance of a system of free oscillators described by a system of ordinary differential equations of complex variables. The system of equations is solved by the Runge-Kudt method modified for complex variables. The model's adequacy increases with an increase in the number of oscillators, which leads to an increase in the number of differential equations of complex variables, a linear increase in memory consumption and a cubic increase in time consumption. The developed program made it possible to increase the number of oscillators by orders of magnitude in comparison with previous simulations. Parallelization and vectorization of computations made it possible to obtain acceptable time parameters for this. The model describes in a one-dimensional approximation the generation of an electromagnetic field by oscillators located in an open resonator. In this case, the development of the so-called dissipative instability, the dissipative generation regime, is possible. It is assumed that the oscillators do not interact with each other and only the resonator field affects their behavior. If the resonator field is absent or small, the superradiance regime is possible, when the essential radiation of each oscillator and the field in the system is the sum of all the eigenfields of the oscillators. In the dissipative regime of instability generation, the system of oscillators synchronizes the induced resonator field. Synchronization of oscillators in the superradiance mode owes its existence to the integral field of the entire system of oscillators. The computer modeling system provides for the tasks of the initial conditions of the problem, the parameters of the system of equations, the time interval, step by time, etc. Visualization of the obtained solution of the system of equations has been implemented.

Cavity-immune spectral features in the pulsed superradiant crossover regime

Lasing in the bad cavity regime has promising applications in precision metrology due to the reduced sensitivity to cavity noise. Here we investigate the spectral properties and phase behavior of pulsed lasing on the $^1$S$_0 - ^3$P$_1$ line of $^{88}$Sr in a mK thermal ensemble, as first described in arxiv:1903.12593. The system operates in a regime where the Doppler-broadened atomic transition linewidth is several times larger than the cavity linewidth. We find that by detuning the cavity resonance, the influence of the cavity noise on the peak lasing frequency can be eliminated to first order despite the system not being deep in the bad cavity regime. Experimental results are compared to a model based on a Tavis-Cummings Hamiltonian, which enables us to investigate the interplay between different thermal velocity classes as the underlying mechanism for the reduction in cavity noise. These velocity-dependent dynamics can occur in pulsed lasing and during the turn-on behavior of lasers in the superradiant crossover regime.

DISSIPATIVE INSTABILITIES AND SUPERRADIATION REGIMES (CLASSIC MODELS)

The generation of an electromagnetic field by oscillators in an open resonator is discussed in a one-dimensional approximation. In this case, the development of the so-called dissipative instability  the dissipative generation regime. Such an instability with the generation of electromagnetic oscillations arises when the decrement of oscillations in an open resonator in the absence of oscillators turns out to be greater than the increment of the resulting instability of the system of oscillators placed in this resonator. It is assumed that the oscillators do not interact with each other, and only the resonator field affects their behavior. If the resonator field is absent or small, the superradiance regime is possible, when the radiation of each oscillator is essential and the field in the system is the sum of all the eigenfields of the oscillators. In the dissipative regime of instability generation, the system of oscillators is synchronized by the induced resonator field. The synchronization of the oscillators in the superradiance mode owes its existence to the integral field of the entire system of oscillators. With a weak nonlinearity of the oscillators, a small initiating external field is required to excite the generation regime. It is noteworthy that the maximum value of the superradiance field is approximately two times less than the maximum field that the same particles could generate if they were at the same point. In all cases, for a given open resonator, the superradiance field turned out to be somewhat larger than the resonator field. Nevertheless, for the same resonator, the increments and attainable field amplitudes in both cases are of the same order of magnitude.

Generalization of the Menegozzi and Lamb maser algorithm to the transient superradiance regime

ABSTRACT We investigate the application of the conventional quasi-steady state maser modelling algorithm of Menegozzi & Lamb (ML) to the high field transient regime of the one-dimensional Maxwell-Bloch (MB) equations for a velocity distribution of atoms or molecules. We quantify the performance of a first order perturbation approximation available within the ML framework when modelling regions of increasing electric field strength, and we show that the ML algorithm is unable to accurately describe the key transient features of R. H. Dicke’s superradiance (SR). We extend the existing approximation to one of variable fidelity, and we derive a generalization of the ML algorithm convergent in the transient SR regime by performing an integration on the MB equations prior to their Fourier representation. We obtain a manifestly unique integral Fourier representation of the MB equations which is $\mathcal {O}\left(N\right)$ complex in the number of velocity channels N and which is capable of simulating transient SR processes at varying degrees of fidelity. As a proof of operation, we demonstrate our algorithm’s accuracy against reference time domain simulations of the MB equations for transient SR responses to the sudden inversion of a sample possessing a velocity distribution of moderate width. We investigate the performance of our algorithm at varying degrees of approximation fidelity, and we prescribe fidelity requirements for future work simulating SR processes across wider velocity distributions.

6 MeV novel hybrid (standing wave - traveling wave) photo-cathode electron gun for a THz superradiant FEL

A novel 6 MeV hybrid photo-injector has been designed and constructed, based on a smaller-scale prototype previously built in UCLA’s Particle Beam Physics Laboratory. It has been commissioned at Ariel University in Israel as an on-going collaboration between the two universities. This unique, new generation design provides a radically simpler approach to RF feeding of a gun/buncher system, leading to a much shorter beam via velocity bunching owed to an attached traveling wave section of the photo-injector. This design offers better performance in beam parameters, providing a high quality electron beam, with energy of 6 MeV. The hybrid photo-injector was designed to provide emittance of approximately 3 μm, and pulse duration of 150 fs at up to 1 nC per pulse. The hybrid gun is driven by a SLAC XK5 klystron as the high power RF source, and third harmonic of a ∼35 fs IR Laser amplifier (266 nm) to extract electrons from the photo-cathode. The unique electron gun will produce a bunched electron pulse to drive a THz FEL, which will operate at the super-radiant regime, and therefore requires ambitious beam properties. This paper describes the gun and presents experimental results from the gun and its sub-systems, including energy and charge measurements, compared with the design simulations.

Regular and bistable steady-state superradiant phases of an atomic beam traversing an optical cavity

We investigate the different photon emission regimes created by a preexcited and collimated atomic beam passing through a single mode of an optical cavity. In the regime where the cavity degrees of freedom can be adiabatically eliminated, we find that the atoms undergo superradiant emission when the collective linewidth exceeds the transit-time broadening. We analyze the case where the atomic beam direction is slanted with respect to the cavity axis. For this situation, we find that a phase of continuous light emission similar to steady-state superradiance is established providing the tilt of the atomic beam is sufficiently small. However, if the atoms travel more than half a wavelength along the cavity axis during one transit time we predict a dynamical phase transition to a new bistable superradiant regime. In this phase the atoms undergo collective spontaneous emission with a frequency that can be either blue or red detuned from the free-space atomic resonance. We analyze the different superradiant regimes and the quantum critical crossover boundaries. In particular we find the spectrum of the emitted light and show that the linewidth exhibits features of a critical scaling close to the phase boundaries.