TW class attosecond X-ray pulse laser

TW class attosecond X-ray pulse laser
Attosecond X-ray pulse laser with high power and short pulse duration are the key to achieve ultrafast nonlinear spectroscopy and X-ray diffraction imaging. The research team in the United States used a cascade of two-stage X-ray free electron lasers to output discrete attosecond pulses. Compared with existing reports, the average peak power of the pulses is increased by an order of magnitude, the maximum peak power is 1.1 TW, and the median energy is more than 100 μJ. The study also provides strong evidence for soliton-like superradiation behavior in the X-ray field.High-energy lasers have driven many new areas of research, including high-field physics, attosecond spectroscopy, and laser particle accelerators. Among all kinds of lasers, X-rays are widely used in medical diagnosis, industrial flaw detection, safety inspection and scientific research. The X-ray free-electron laser (XFEL) can increase the peak X-ray power by several orders of magnitude compared to other X-ray generation technologies, thus extending the application of X-rays to the field of nonlinear spectroscopy and single-particle diffraction imaging where high power is required. The recent successful attosecond XFEL is a major achievement in attosecond science and technology, increasing the available peak power by more than six orders of magnitude compared to benchtop X-ray sources.

Free electron lasers can obtain pulse energies many orders of magnitude higher than the spontaneous emission level using collective instability, which is caused by the continuous interaction of the radiation field in the relativistic electron beam and the magnetic oscillator. In the hard X-ray range (about 0.01 nm to 0.1 nm wavelength), FEL is achieved by bundle compression and post-saturation coning techniques. In the soft X-ray range (about 0.1 nm to 10 nm wavelength), FEL is implemented by cascade fresh-slice technology. Recently, attosecond pulses with a peak power of 100 GW have been reported to be generated using the enhanced self-amplified spontaneous emission (ESASE) method.

The research team used a two-stage amplification system based on XFEL to amplify the soft X-ray attosecond pulse output from the linac coherent light source to the TW level, an order of magnitude improvement over reported results. The experimental setup is shown in Figure 1. Based on the ESASE method, the photocathode emitter is modulated to obtain an electron beam with a high current spike, and is used to generate attosecond X-ray pulses. The initial pulse is located at the front edge of the spike of the electron beam, as shown in the upper left corner of Figure 1. When the XFEL reaches saturation, the electron beam is delayed relative to the X-ray by a magnetic compressor, and then the pulse interacts with the electron beam (fresh slice) that is not modified by the ESASE modulation or FEL laser. Finally, a second magnetic undulator is used to further amplify the X-rays through the interaction of attosecond pulses with the fresh slice.

FIG. 1 Experimental device diagram; The illustration shows the longitudinal phase space (time-energy diagram of the electron, green), the current profile (blue), and the radiation produced by first-order amplification (purple). XTCAV, X-band transverse cavity; cVMI, coaxial rapid mapping imaging system; FZP, Fresnel band plate spectrometer

All attosecond pulses are built from noise, so each pulse has different spectral and time-domain properties, which the researchers explored in more detail. In terms of spectra, they used a Fresnel band plate spectrometer to measure the spectra of individual pulses at different equivalent undulator lengths, and found that these spectra maintained smooth waveforms even after secondary amplification, indicating that the pulses remained unimodal. In the time domain, the angular fringe is measured and the time domain waveform of the pulse is characterized. As shown in Figure 1, the X-ray pulse is overlapped with the circularly polarized infrared laser pulse. The photoelectrons ionized by the X-ray pulse will produce streaks in the direction opposite to the vector potential of the infrared laser. Because the electric field of the laser rotates with time, the momentum distribution of the photoelectron is determined by the time of electron emission, and the relationship between the angular mode of the emission time and the momentum distribution of the photoelectron is established. The distribution of photoelectron momentum is measured using a coaxial fast mapping imaging spectrometer. Based on the distribution and spectral results, the time-domain waveform of attosecond pulses can be reconstructed. Figure 2 (a) shows the distribution of pulse duration, with a median of 440 as. Finally, the gas monitoring detector was used to measure the pulse energy, and the scatter plot between the peak pulse power and the pulse duration as shown in Figure 2 (b) was calculated. The three configurations correspond to different electron beam focusing conditions, waver coning conditions and magnetic compressor delay conditions. The three configurations yielded average pulse energies of 150, 200, and 260 µJ, respectively, with a maximum peak power of 1.1 TW.

Figure 2. (a) Distribution histogram of half-height Full width (FWHM) pulse duration; (b) Scatter plot corresponding to peak power and pulse duration

In addition, the study also observed for the first time the phenomenon of soliton-like superemission in the X-ray band, which appears as a continuous pulse shortening during amplification. It is caused by a strong interaction between electrons and radiation, with energy rapidly transferred from the electron to the head of the X-ray pulse and back to the electron from the tail of the pulse. Through in-depth study of this phenomenon, it is expected that X-ray pulses with shorter duration and higher peak power can be further realized by extending the superradiation amplification process and taking advantage of pulse shortening in soliton-like mode.