Parametric attosecond pulse amplification far from the ionization threshold from high order harmonic generation in He+^+

Parametric amplification of attosecond coherent pulses around 100 eV at the single-atom level is demonstrated for the first time by using the 3D time-dependent Schr{\"o}dinger equation in high-harmonic generation processes from excited states of He$^+$. We present the attosecond dynamics of the amplification process far from the ionization threshold and resolve the physics behind it. The amplification of a particular central photon energy requires the seed XUV pulses to be perfectly synchronized in time with the driving laser field for stimulated recombination to the He$^+$ ground state and is only produced in a few specific laser cycles in agreement with the experimental measurements. Our simulations show that the amplified photon energy region can be controlled by varying the peak intensity of the laser field. Our results pave the way to the realization of compact attosecond pulse intense XUV lasers with broad applications

Construction of a magnetic bottle spectrometer and its application to pulse duration measurement of X-ray laser using a pump-probe method

To characterize the temporal evolution of ultrashort X-ray pulses emitted by laser plasmas using a pump-probe method, a magnetic bottle time-of-flight electron spectrometer is constructed. The design is determined by numerical calculations of a mirror magnetic field and of the electron trajectory in a flight tube. The performance of the spectrometer is characterized by measuring the electron spectra of xenon atoms irradiated with a laser-driven plasma X-ray pulse. In addition, two-color above-threshold ionization (ATI) experiment is conducted for measurement of the X-ray laser pulse duration, in which xenon atoms are simultaneously irradiated with an X-ray laser pump and an IR laser probe. The correlation in the intensity of the sideband spectra of the 4d inner-shell photoelectrons and in the time delay of the two laser pulses yields an X-ray pulse width of 5.7 ps, in good agreement with the value obtained using an X-ray streak camera.This work was supported by a Grant-in-Aid for Scientific Research B (No. 26286078) from the Japanese Society for the Promotion of Science. We thank the JAEA X-ray laser research group for laser operations. This work was performed under the shared use program of JAEA Facilities

Spallative ablation of dielectrics by X-ray laser

Short laser pulse in wide range of wavelengths, from infrared to X-ray, disturbs electron-ion equilibrium and rises pressure in a heated layer. The case where pulse duration $\tau_L$ is shorter than acoustic relaxation time $t_s$ is considered in the paper. It is shown that this short pulse may cause thermomechanical phenomena such as spallative ablation regardless to wavelength. While the physics of electron-ion relaxation on wavelength and various electron spectra of substances: there are spectra with an energy gap in semiconductors and dielectrics opposed to gapless continuous spectra in metals. The paper describes entire sequence of thermomechanical processes from expansion, nucleation, foaming, and nanostructuring to spallation with particular attention to spallation by X-ray pulse

High-order alloharmonics produced by nonperiodic drivers

High-order harmonics are ubiquitous in nature and present in electromagnetic, acoustic, and gravitational waves. They are generated by periodic nonlinear processes or periodic high-frequency pulses. However, this periodicity is often inexact, such as that in chirped (frequency-swept) optical waveforms or interactions with nonstationary matter -- for instance, reflection from accelerating mirrors. Spectra observed in such cases contain complicated sets of harmonic-like fringes. We encountered such fringes in our experiment on coherent extreme ultraviolet generation via BISER, and could not interpret them using currently available knowledge. Here, we present a comprehensive theory based on interference of harmonics with different orders fully explaining the formation of these fringes, which we call alloharmonics. Like atomic spectra, the complex alloharmonic spectra depend on several integer numbers and bear a unique imprint of the emission process, which the theory can decipher, avoiding confusion or misinterpretation. We also demonstrate the alloharmonics in simulations of gravitational waves emitted by binary black hole mergers. Further, we predict the presence of alloharmonics in the radio spectra of pulsars and in optical frequency combs, and propose their use for measurement of extremely small accelerations necessary for testing gravity theories. The alloharmonics phenomenon generalizes classical harmonics and is critical in research fields such as laser mode locking, frequency comb generation, attosecond pulse generation, pulsar studies, and future gravitational wave spectroscopy.Comment: 29 pages, 9 figures, 3 table

Numerical analysis of plasma medium of transient collisional excited X-ray laser

Two-dimensional (2D) radiation hydrodynamics simulations have been performed to investigate the refraction influence and the gain property in the plasma medium of the x-ray laser. The local energy deposition of the main pumping pulse generates a blast wave near the critical density surface, and the density dip structure is gradually formed behind the blast wave. The three-dimensional (3D) ray-trace calculation using the result of the 2D simulation shows the x-rays pass through the density dip with less refraction

Construction of a magnetic bottle spectrometer and its application to pulse duration measurement of X-ray laser using a pump-probe method

To characterize the temporal evolution of ultrashort X-ray pulses emitted by laser plasmas using a pump-probe method, a magnetic bottle time-of-flight electron spectrometer is constructed. The design is determined by numerical calculations of a mirror magnetic field and of the electron trajectory in a flight tube. The performance of the spectrometer is characterized by measuring the electron spectra of xenon atoms irradiated with a laser-driven plasma X-ray pulse. In addition, two-color above-threshold ionization (ATI) experiment is conducted for measurement of the X-ray laser pulse duration, in which xenon atoms are simultaneously irradiated with an X-ray laser pump and an IR laser probe. The correlation in the intensity of the sideband spectra of the 4d inner-shell photoelectrons and in the time delay of the two laser pulses yields an X-ray pulse width of 5.7 ps, in good agreement with the value obtained using an X-ray streak camera

Observation of gain coefficients of 15.47 nm Li-like Al soft x-ray laser in a recombining plasma pumped by a compact YAG laser

A Li-like Al soft x-ray laser oscillation (15.47 nm) by means of recombination plasma scheme was investigated to enhance the gain coefficent of lasing medium. In this study, we employed a Nd:YAG laser pulse composed of 8-pulse train, in which each pulse width was 400 ps and its interpulse time was 400 ps. The laser beam was tightly line-focused into 50 μm in height and 12 mm in length on Al slab target by using prism lens array. The created cylindrical plasma showed that with increasing plasma length the lasing intensity at 15.47 nm increased expoentially up to 3.5 mm. For the laser energy of 3 J, the gain coefficient was 8.3 cm−1. Above 3.5 mm in length, the signals seemed to increase linearly, indicating that the lasing action reached the saturation regime