Cross-polarized common-path temporal interferometry for high-sensitivity strong-field ionization measurements.

Absolute density measurements of low-ionization-degree or low-density plasmas ionized by lasers are very important for understanding strong-field physics, atmospheric propagation of intense laser pulses, Lidar etc. A cross-polarized common-path temporal interferometer using balanced detection was developed for measuring plasma density with a sensitivity of ∼0.6 mrad, equivalent to a plasma density-length product of ∼2.6 × 1013 cm-2 if using an 800 nm probe laser. By using this interferometer, we have investigated strong-field ionization yield versus intensity for various noble gases (Ar, Kr, and Xe) using 800 nm, 55 fs laser pulses with both linear (LP) and circular (CP) polarization. The experimental results were compared to the theoretical models of Ammosov-Delone-Krainov (ADK) and Perelomov-Popov-Terent'ev (PPT). We find that the measured phase change induced by plasma formation can be explained by the ADK theory in the adiabatic tunneling ionization regime, while PPT model can be applied to all different regimes. We have also measured the photoionization and fractional photodissociation of molecular (MO) hydrogen. By comparing our experimental results with PPT and MO-PPT models, we have determined the likely ionization pathways when using three different pump laser wavelengths of 800 nm, 400 nm, and 267 nm.

In Situ Generation of High-Energy Spin-Polarized Electrons in a Beam-Driven Plasma Wakefield Accelerator.

In situ generation of a high-energy, high-current, spin-polarized electron beam is an outstanding scientific challenge to the development of plasma-based accelerators for high-energy colliders. In this Letter, we show how such a spin-polarized relativistic beam can be produced by ionization injection of electrons of certain atoms with a circularly polarized laser field into a beam-driven plasma wakefield accelerator, providing a much desired one-step solution to this challenge. Using time-dependent Schrödinger equation (TDSE) simulations, we show the propensity rule of spin-dependent ionization of xenon atoms can be reversed in the strong-field multiphoton regime compared with the non-adiabatic tunneling regime, leading to high total spin polarization. Furthermore, three-dimensional particle-in-cell simulations are incorporated with TDSE simulations, providing start-to-end simulations of spin-dependent strong-field ionization of xenon atoms and subsequent trapping, acceleration, and preservation of electron spin polarization in lithium plasma. We show the generation of a high-current (0.8 kA), ultralow-normalized-emittance (∼37 nm), and high-energy (2.7 GeV) electron beam within just 11 cm distance, with up to ∼31% net spin polarization. Higher current, energy, and net spin-polarization beams are possible by optimizing this concept, thus solving a long-standing problem facing the development of plasma accelerators.

Extremely Dense Gamma-Ray Pulses in Electron Beam-Multifoil Collisions.

Sources of high-energy photons have important applications in almost all areas of research. However, the photon flux and intensity of existing sources is strongly limited for photon energies above a few hundred keV. Here we show that a high-current ultrarelativistic electron beam interacting with multiple submicrometer-thick conducting foils can undergo strong self-focusing accompanied by efficient emission of gamma-ray synchrotron photons. Physically, self-focusing and high-energy photon emission originate from the beam interaction with the near-field transition radiation accompanying the beam-foil collision. This near field radiation is of amplitude comparable with the beam self-field, and can be strong enough that a single emitted photon can carry away a significant fraction of the emitting electron energy. After beam collision with multiple foils, femtosecond collimated electron and photon beams with number density exceeding that of a solid are obtained. The relative simplicity, unique properties, and high efficiency of this gamma-ray source open up new opportunities for both applied and fundamental research including laserless investigations of strong-field QED processes with a single electron beam.

Erratum: In Situ Generation of High-Energy Spin-Polarized Electrons in a Beam-Driven Plasma Wakefield Accelerator [Phys. Rev. Lett. 126, 054801 (2021)].

This corrects the article DOI: 10.1103/PhysRevLett.126.054801.

Photon deceleration in plasma wakes generates single-cycle relativistic tunable infrared pulses.

Availability of relativistically intense, single-cycle, tunable infrared sources will open up new areas of relativistic nonlinear optics of plasmas, impulse IR spectroscopy and pump-probe experiments in the molecular fingerprint region. However, generation of such pulses is still a challenge by current methods. Recently, it has been proposed that time dependent refractive index associated with laser-produced nonlinear wakes in a suitably designed plasma density structure rapidly frequency down-converts photons. The longest wavelength photons slip backwards relative to the evolving laser pulse to form a single-cycle pulse within the nearly evacuated wake cavity. This process is called photon deceleration. Here, we demonstrate this scheme for generating high-power (~100 GW), near single-cycle, wavelength tunable (3-20 µm), infrared pulses using an 810 nm drive laser by tuning the density profile of the plasma. We also demonstrate that these pulses can be used to in-situ probe the transient and nonlinear wakes themselves.

A pulse-to-pulse timing jitter measurement between two synchronized amplified laser beams for TTX.

In China, Tsinghua Thomson Scattering X-ray Source (TTX) is the dedicated hard X-ray source based on the Thomson scattering between a terawatt ultrashort laser and a relativistic electron beam. In the TTX, two synchronized Ti: sapphire laser systems generate the terawatt ultrashort infrared scattering laser and the ultraviolet driving laser for the photocathode RF gun to produce the electron beam; measuring the timing jitter between the electron beam and the laser beam is an essential task for the X-ray source. In the present study, we report on a single shot, non-collinear cross correlator with fs resolution and measured the timing jitter between the two synchronized laser systems with a pulse-to-pulse method, which is beneficial to estimate the jitter of the X-ray yield in the TTX system. Although it is more important to synchronize the scattering laser to the electron beam and not of the driving laser, the laser-laser jitter measurement would be a good first step towards that goal, and the result generated can be considered as the error signal for the potential feedback stabilization.

Diffraction based method to reconstruct the spectrum of the Thomson scattering x-ray source.

As Thomson scattering x-ray sources based on the collision of intense laser and relativistic electrons have drawn much attention in various scientific fields, there is an increasing demand for the effective methods to reconstruct the spectrum information of the ultra-short and high-intensity x-ray pulses. In this paper, a precise spectrum measurement method for the Thomson scattering x-ray sources was proposed with the diffraction of a Highly Oriented Pyrolytic Graphite (HOPG) crystal and was demonstrated at the Tsinghua Thomson scattering X-ray source. The x-ray pulse is diffracted by a 15 mm (L) ×15 mm (H)× 1 mm (D) HOPG crystal with 1° mosaic spread. By analyzing the diffraction pattern, both x-ray peak energies and energy spectral bandwidths at different polar angles can be reconstructed, which agree well with the theoretical value and simulation. The higher integral reflectivity of the HOPG crystal makes this method possible for single-shot measurement.

110 W 1678 nm laser based on high-efficiency optical parametric interactions pumped by high-power slab laser.

This Letter presents a high-efficiency optical parametric amplifier pumped by a high-power slab laser with approximate uniform rectangular distribution. By optimizing the overlapping, spectrum matching, and pulse synchronization for the pump and signal lasers, output power of 110.8 W at 1678 nm with corresponding conversion efficiency of 32.3% was achieved in addition to sufficient usage of the effective area in MgO doped periodically poled lithium niobate crystal. It could also provide a designable and tunable wavelength of the amplified laser in a wide infrared region.

High-power and widely tunable mid-infrared optical parametric amplification based on PPMgLN.

We report on a high-power and widely tunable optical parametric amplifier (OPA) based on PPMgLN and pumped by a pulsed 1.064 µm MOPA laser. The operating wavelength of the OPA system is continuously tunable from 2.68 to 3.07 µm by adjusting the temperature of PPMgLN crystals, with average output power varying from 74.6 to 66.7 W for 310 W of pump power, corresponding to an optical-to-optical conversion efficiency of ∼22.8% at 2.68 µm and ∼20.5% at 3.07 µm, respectively. Output beam quality factor (M2) of the OPA was measured to be <4 over the whole tuning range.

High-power, narrow-bandwidth mid-infrared PPMgLN optical parametric oscillator with a volume Bragg grating.

We report on a high-power, narrow spectral bandwidth 2.907 µm PPMgLN optical parametric oscillator (OPO) pumped by a 1.064 µm pulsed Nd:YAG MOPA laser source. Free-running operation of the OPO exhibits maximum average output power of 71.6 W at 2.907 µm with a slope efficiency of 26.7%. Broad 2.907 µm spectral bandwidth of the free-running OPO was suppressed from ~9 nm to less than 0.7 nm by using a VBG as one cavity mirror. The maximum average power was 51.7 W at 2907.55 nm for the spectrum-narrowed OPO, corresponding to a slope efficiency of 22.5%. Continuously tunable ranges of ~8 nm around 2.907 µm had been achieved via adjusting the temperatures of the VBG and PPMgLN accordingly.