Recently, an exponentially decaying waveform (the time-dependence of detection probability) of a Mössbauer γ-ray photon was transformed into a regular sequence of short pulses in a sinusoidally vibrating recoilless resonant absorber [Nature, 508, 80-83 (2014)]. In the present paper, we show that the peak amplitude of the pulses can be considerably increased via joint adjustment of optical depth of the absorber and the initial phase of its vibration. This is due to reduction of the photoelectric absorption and maximizing the constructive temporal interference of spectral content of the single-photon wave packet in optically deep absorber. The ultimate capabilities for transforming a waveform of 14.4 keV photon from 57 Co radioactive source into a regular train of pulses in a harmonically vibrating 57 Fe recoilless resonant absorber are discussed. We show that the shortest pulse duration, produced by this technique, is limited by the highest available vibration frequency of a piezo-transducer and at present can be as short as 7.7 ps. The maximum achievable detection probability of the transformed photon at the experimentally feasible conditions is more than two times higher than peak detection probability of the photon emitted by the source and nearly 5.5 times higher than obtained in the above reference.

In this paper, we present the analytical theory of attosecond pulse formation via optical modulation of an active medium of the hydrogen-like C 5+ plasma-based X-ray laser at 3.4 nm wavelength in the "water window" range, taking into account a variation of the population inversion caused by radiative decay of the upper lasing states. We derive an analytical solution for the X-ray field amplified by an X-ray laser with time-dependent population inversion, which is simultaneously irradiated by a strong optical laser field, and use it to find the optimal conditions for the attosecond pulse formation from a narrowband seeding X-ray field. We show that the shape of pulses can be improved at the cost of reduced pulse peak intensity (i) via external attenuation of the resonant spectral component of the amplified X-ray field or (ii) by using a resonantly absorbing medium (the active medium of the X-ray laser after the change of sign of the population inversion) for the pulse formation. The results of the analytical theory are in a good agreement with the numerical solutions of the Maxwell-Bloch equations which account for the nonlinearity, as well as the amplified spontaneous emission, of the active medium. Both analytically and numerically we show the possibility to produce a train of attosecond pulses with sub-200 as duration and the peak intensity exceeding 10 12 W/cm 2 at the carrier wavelength 3.4 nm in the "water window" range, which makes them attractive for the biological and medical applications.

We derive the analytical theory describing the process of sub-femtosecond pulse formation from a quasimonochromatic seeding extreme ultraviolet (XUV) radiation, which propagates in active medium of a hydrogen-like plasma-based X-ray laser dressed by a strong infrared laser field. We discuss the ultimate capabilities and limitations of this process on the basis of the derived analytical solution and extensive numerical studies for the case of Li 2+ plasma-based X-ray laser with a carrier wavelength 13.5nm. We analyze the role of plasma dispersion and find the optimal conditions for the formation of attosecond pulses with the highest contrast. Under the optimal conditions, the influence of amplified spontaneous emission from the active medium is negligible. The peak intensity of the produced XUV pulses can exceed 10 10 -10 11 W/cm 2 , while the duration of pulses varies in the range of 400-600 as.

No abstract

The induced transparency of opaque medium for resonant electromagnetic radiation is a powerful tool for manipulating the field-matter interaction. Various techniques to make different physical systems transparent for radiation from microwaves to x-rays were implemented. Most of them are based on the modification of the quantum-optical properties of the medium under the action of an external coherent electromagnetic field. Recently, an observation of acoustically induced transparency (AIT) of the 57Fe absorber for resonant 14.4-keV photons from the radioactive 57Co source was reported. About 150-fold suppression of the resonant absorption of photons due to collective acoustic oscillations of the nuclei was demonstrated. In this paper, we extend the AIT phenomenon to a novel phase-locked regime, when the transmitted photons are synchronized with the absorber vibration. We show that the advantages of synchrotron Mössbauer sources such as the deterministic periodic emission of radiation and controlled spectral-temporal characteristics of the emitted photons along with high-intensity photon flux in a tightly focused beam, make it possible to efficiently implement this regime, paving the way for the development of the acoustically controlled interface between hard x-ray photons and nuclear ensembles.

We study the process of propagation of high harmonics of optical radiation in an active medium of a plasma-based X-ray laser, simultaneously irradiated by an intense optical field of fundamental frequency. It is shown that for moderate plasma dispersion of the active medium at the frequency of the modulating optical field, the energy and relative amplitudes of the harmonics at the output of the medium are determined by their phases at the entrance to the medium, as well as by the time-delay of the harmonics with respect to the modulating field. These dependences are due to interference of high-order harmonics with a set of multi-frequency fields generated by each of the harmonics in the process of coherent scattering in a modulated active medium. The possibilities of using these effects to increase the efficiency of harmonic amplification, to control the harmonic spectrum, and determine the relative phases at the entrance to the medium are discussed on the example of the active medium of hydrogen-like Li2+ ions (with a 13.5 nm wavelength of an inverted transition).

The transformation of exponentially decaying waveform (the time dependence of detection probability) of a Mössbauer γ-ray photon into a regular sequence of short nearly bandwidthlimited pulses in a vibrating recoilless resonant absorber is studied. The case of dual-tone vibration at the fundamental frequency and its second harmonic under experimentally feasible conditions is considered. We show that this technique allows one to shorten pulses and increase the ratio of pulse separation to pulse length as well as to enlarge pulse amplitude compared to the pulses produced in harmonically vibrating absorber. The obtained results pave the way to the development of the time-resolved Mössbauer γ-ray spectroscopy and manipulation of quantum information in hard x-ray/γ-ray range.

We report an observation of a 148-fold suppression of resonant absorption of keV photons from exp (-5.2) to exp(-0.2) with preservation of their spectral and temporal characteristics in an ensemble of the resonant two-level 57 Fe nuclei at room temperature. The transparency was induced via collective acoustic oscillations of nuclei. The proposed technique allows extending the concept of induced optical transparency to a hard x-ray/-ray range and paves the way for acoustically controllable interface between x-ray/-ray photons and nuclear ensembles, advancing the field of x-ray/-ray quantum optics.The induced transparency of opaque medium for resonant electromagnetic radiation is a powerful tool for manipulating the field-matter interaction. The study of phenomena associated with transparency induced in natural and artificial quantum and classical systems for resonant electromagnetic radiation in a wide spectral range from microwaves to γ-rays, as well as their applications, is an extremely broad area of research [1][2][3][4][5][6][7][8][9][10][11][12], extended also to acoustic waves [13,14]. Self-induced transparency [1], transparency via Autler-Townes splitting (ATS) [2,3], and electromagnetically induced transparency (EIT) [4,5] including their analogs in various quantum and classical systems [6][7][8][9][10][11], are just several examples of numerous techniques for suppression of resonant absorption. Important applications of induced optical transparency such as high refractive index and nonlinearities at the few-photon level, slow and stored light, optical quantum information processing, etc. stimulate a development of similar techniques in the hard x-ray/γ-ray domain. Highenergy 10˗100-keV photons can be easier detected and tighter focused than optical photons [15], while corresponding resonant recoilless nuclear transitions have orders of magnitude narrower linewidths at room temperature than optical atomic transitions at a comparable solid density [12,16]. These features are promising for realization of very compact and efficient interfaces between single hard x-ray/-ray photons and nuclear ensembles. However, the common tools for controlling quantum optical interfaces, such as intense spectrally narrow coherent sources and highfinesse cavities are still unavailable in hard x-ray/-ray range, preventing from a direct realization of the basic optical transparency techniques such as EIT and ATS-transparency, for high-energy photons. Several different techniques to control resonant interaction between hard x-ray/-ray photons and nuclear ensembles were developed, based on variation of hyperfine or external magnetic field [10,17,18], mechanical displacement (periodic or non-periodic) of an absorber or source with respect to each other including acoustic vibration [12,16,[19][20][21][22][23][24][25][26][27], and placing nuclei into a spatial sandwich-like nano-structure [11]. The 25% reduction in absorption of 14.4-keV photons was observed via anti-crossing of the upper energy sublevels of 57 Fe nuclei...