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1. Generation of attosecond pulses in “water window” range by a plasma-based X-ray laser

   https://doi.org/10.1088/1742-6596/1412/9/092012  

   Khairulin, I R; Antonov, V A; Kocharovskaya, O (January 2020, Journal of Physics: Conference Series)

   Synopsis We suggest a technique to generate a train of attosecond pulses in “water window” range by hydrogen-like C⁵⁺plasma-based X-ray laser with two sequentical active plasma channels irradiated by two different optical laser fields with orthogonal polarizations. We show also the possibility to transform the radiation of a plasma-based X-ray laser dressed by an optical laser field into a train of attosecond pulses in a resonant absorber irradiated by the different optical field. 

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2. Boosting the efficiency of high harmonic amplification by modulating a plasma-based X-ray laser with the second harmonic of the fundamental frequency laser field

   Khairulin, I.R.; Antonov, I.R.; Kocharovskaya, O. (October 2021, International conference on Ultrafast Optical Science)

   Attosecond pulses formed by high order harmonics (HHs) of an infrared (IR) laser field is a powerful tool for studying and controlling ultrafast dynamics of electrons in atoms, molecules and solids at its intrinsic time-scale. However, in the X-ray range the energy of attosecond pulses is rather limited. Their amplification is an important but very challenging problem since none of the existing amplifiers can support the corresponding PHz bandwidth. In our previous work [1] we proposed a method for the attosecond pulse amplification in hydrogen-like active medium of a recombination plasma-based X-ray laser dressed by a replica of the fundamental frequency IR field used for the HH generation. Due to the IRfield-induced sub-laser-cycle Stark shift and splitting of the lasing energy levels the gain of the active medium is redistributed over the combination frequencies, separated from the resonance by even multiples of the frequency of the IR field. If the incident HHs forming an attosecond pulse train are tuned in resonance with the induced gain lines and the active plasma medium is strongly dispersive for the modulating IR field, then during the amplification the relative phases of harmonics and (under the optimal choice of the IR field strength) the shape of the amplified pulses will be preserved. In the present work we show the possibility of boosting the efficiency of HH amplification by modulating the active medium of an X-ray laser with the second harmonic of the fundamental frequency IR field. We show that under the action of a laser field (with arbitrary frequency) the gain redistribution occurs not only over the even combination frequencies discussed in [1], but also over the odd frequencies separated from the resonance by odd multiples of the laser frequency. Besides, nearly half of the medium gain is contained in the even induced gain lines, and nearly half in the odd. If the modulating field is the second harmonic of the IR field, used for the generation the HHs and attosecond pulses, then the seeding HHs can be tuned in resonance with both even and odd gain lines simultaneously, which will make the overall gain much higher as compared to the previously considered case of the fundamental frequency modulating field (when only the even gain lines play the role). By the example of the C5+ X-ray laser with 3.38 nm wavelength of the inverted transition we show the possibility of increasing the efficiency of 430 as pulse amplification by 8.5 times when the active medium is modulated with the second harmonic of the fundamental frequency IR field with wavelength 2.1 µm. 

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3. All-optical sampling of few-cycle infrared pulses using tunneling in a solid

   https://doi.org/10.1364/PRJ.420916  

   Liu, Yangyang; Gholam-Mirzaei, Shima; Beetar, John E.; Nesper, Jonathan; Yousif, Ahmed; Nrisimhamurty, M.; Chini, Michael (May 2021, Photonics Research)

   Recent developments in ultrafast laser technology have resulted in novel few-cycle sources in the mid-infrared. Accurately characterizing the time-dependent intensities and electric field waveforms of such laser pulses is essential to their applications in strong-field physics and attosecond pulse generation, but this remains a challenge. Recently, it was shown that tunnel ionization can provide an ultrafast temporal “gate” for characterizing high-energy few-cycle laser waveforms capable of ionizing air. Here, we show that tunneling and multiphoton excitation in a dielectric solid can provide a means to measure lower-energy and longer-wavelength pulses, and we apply the technique to characterize microjoule-level near- and mid-infrared pulses. The method lends itself to both all-optical and on-chip detection of laser waveforms, as well as single-shot detection geometries. 

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4. Enhanced Amplification of Attosecond Pulses in a Hydrogen-like Plasma-Based X-ray Laser Modulated by an Infrared Field at the Second Harmonic of Fundamental Frequency

   https://doi.org/10.3390/photonics9020051  

   Khairulin, Ilias R.; Antonov, Vladimir A.; Ryabikin, Mikhail Yu.; Kocharovskaya, Olga (February 2022, Photonics)

   In a recent work (Antonov et al., Physical Review Letters 123, 243903 (2019)), it was shown that it is possible to amplify a train of attosecond pulses, which are produced from the radiation of high harmonics of the infrared field of the fundamental frequency, in the active medium of a plasma-based X-ray laser modulated by a replica of the infrared field of the same frequency. In this paper, we show that much higher amplification can be achieved using the second harmonic of the fundamental frequency for modulating of a hydrogen-like active medium. The physical reason for such enhanced amplification is the possibility to use all (even and odd) sidebands induced in the gain spectrum in the case of the modulating field of the doubled fundamental frequency, while only one set of sidebands (either even or odd) could participate in amplification in the case of the modulating field of the fundamental frequency due to the fact that the spectral components of the high-harmonic field are separated by twice the fundamental frequency. Using the plasma of hydrogen-like C5+ ions with an inverted transition wavelength of 3.38 nm in the water window as an example, it is shown that the use of a modulating field at a doubled fundamental frequency makes it possible to increase the intensity of amplified attosecond pulses by an order of magnitude in comparison with the previously studied case of a fundamental frequency modulating field. 

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5. Spatiotemporal coupling of attosecond pulses

   https://doi.org/10.1073/pnas.1817626116  

   Wikmark, Hampus; Guo, Chen; Vogelsang, Jan; Smorenburg, Peter W.; Coudert-Alteirac, Hélène; Lahl, Jan; Peschel, Jasper; Rudawski, Piotr; Dacasa, Hugo; Carlström, Stefanos; et al (March 2019, Proceedings of the National Academy of Sciences)

   The shortest light pulses produced to date are of the order of a few tens of attoseconds, with central frequencies in the extreme UV range and bandwidths exceeding tens of electronvolts. They are often produced as a train of pulses separated by half the driving laser period, leading in the frequency domain to a spectrum of high, odd-order harmonics. As light pulses become shorter and more spectrally wide, the widely used approximation consisting of writing the optical waveform as a product of temporal and spatial amplitudes does not apply anymore. Here, we investigate the interplay of temporal and spatial properties of attosecond pulses. We show that the divergence and focus position of the generated harmonics often strongly depend on their frequency, leading to strong chromatic aberrations of the broadband attosecond pulses. Our argument uses a simple analytical model based on Gaussian optics, numerical propagation calculations, and experimental harmonic divergence measurements. This effect needs to be considered for future applications requiring high-quality focusing while retaining the broadband/ultrashort characteristics of the radiation. 

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