Frequency response of an atomic resonance driven by weak free-electron-laser fluctuating pulses

Abstract

Abstract. Motivated by recent experiments pertaining to the interaction of weak SASE-FEL pulses with atoms and molecules, we investigate the conditions under which such interactions can be described in the framework of a simple phase-diffusion model with decorrelated atom-field dynamics. The nature of the fluctuations that are inevitably present in SASE-FEL pulses is shown to play a pivotal role in the success of the decorrelation. Our analysis is performed in connection with specific recent experimental results from FLASH in the soft X-ray regime.

A streak camera study of superfluorescence at multiple wavelengths from helium atoms excited using free electron laser pulses

“…Highlights include electromagnetically induced transparency [1,2], stimulated emission [3][4][5][6], stimulated scattering [7], and other quantum optics effects [8][9][10]. In particular, experimental and theoretical studies of the driving of transitions in few-level atomic systems with free-electron laser (FEL) pulses [11][12][13][14][15][16][17] are currently advancing our understanding. Such studies are also important for understanding FEL experiments which do not rely on high intensity [18,19].…”

Multi-photon enhancement of the Schwinger pair production mechanism under strong FEL radiation

“…The effects of this result on pair creation are discussed in the next section. A brief clarification should be made at this point regarding the connection with FEL radiation: As is generally known, apart from amplitude fluctuations, FEL radiation does also exhibit fluctuations in phase [48,50,51], making the standing wave hypothesis [5,6,8,9,16,24,36] even more challenging to realise experimentally. The reason we do not include this type of fluctuations in the formulation is twofold: First, the problem of vacuum e − e + pair creation in the presence of fields that undergo fluctuations both in amplitude and phase is considerably more complex, while the tools that have been developed for treating analogous problems in atomic transitions [62][63][64] are not directly applicable to the problem at hand.…”

“…Briefly, their stochastic properties are akin to those of chaotic (thermal) radiation [60]. The feature of that radiation of direct relevance to our considerations is the strong intensity fluctuations, implying spiky behavior during the pulse, which is known to affect the yield of processes whose rate depends on the intensity in a non-linear fashion [50,51,61].…”

“…That is the possibility of enhancement of the Schwinger mechanism owing to the intrinsic stochastic fluctuations of FEL sources, and if yes, over which range of intensities such enhancement could be expected to be significant. The stochastic properties of self-amplified spontaneous emission (SASE) FEL sources have been documented quite extensively through theoretical [48][49][50][51][52][53] as well as experimental studies [54][55][56][57][58][59]. Briefly, their stochastic properties are akin to those of chaotic (thermal) radiation [60].…”