中央研究院 原子與分子科學研究所

By focusing conventional 1-TW 40-fs laser pulses into a dense 450-um-long nitrogen gas cell, we demonstrate the feasibility of routinely generating electron beams from laser wakefield acceleration (LWFA) with primary energies scaling up to 10 MeV and a high charge in excess of 50 pC. When electron beams are generated with a charge of ~30 pC and a beam divergence of 40 mrad from the nitrogen cell having a peak atom density of 7.6*10^18/cm3, increasing the density inside the cell by 25%—controlled by tuning the backing pressure of fed nitrogen gas—can induce defocusing of the pump pulse that leads to a twofold increase in the output charge but with a trade-off in beam divergence. Therefore, this LWFA scheme has two preferred regimes for acquiring electron beams with either lower divergence or higher beam
charge depending on a slight variation of the gas/plasma density inside the cell. Our results identify the high potential for implementing submillimeter nitrogen gas cells in the future development of high-repetition-rate LWFA driven by sub-TW or few-TW laser pulses.

A scheme for ion-based high-harmonic generation from water window to keV x-ray is investigated. He1⁺ ions with 54.42-eV ionization potential extend the harmonic cutoff energy to 1 keV. The transverse selective-zoning method of quasi-phase-matching is utilized to overcome the severe plasma dispersion in a highly ionized medium. The calculated conversion efficiency reaches about 15% of the perfect phase-matching condition. Wavelength tunability is achieved by incorporating a programmable spatial-light modulator to control the quasi-phase-matching pattern.

This paper discusses numerical and experimental results on frequency downshifting and upshifting of a 10-um infrared (IR) laser to cover the entire wavelength (frequency) range from wavelength=1-150 um (frequency=300-2 THz) using two different plasma techniques. The first plasma technique utilizes frequency downshifting of the drive laser pulse in a nonlinear plasma wake. Based on this technique, we have proposed and demonstrated that in a tailored plasma structure, multi-millijoule energy, single-cycle, long-wavelength IR (3-20 um) pulses can be generated by using an 810 nm Ti:sapphire drive laser. Here, we extend this idea to the THz frequency regime. We show that sub-joule, terawatts, single-cycle terahertz (2-12 THz or 150-25 um) pulses can be generated by replacing the drive laser with a picosecond 10 um CO₂ laser and a different shaped plasma structure. The second plasma technique employs frequency upshifting by colliding a CO₂ laser with a rather sharp relativistic ionization front created by ionization of a gas in less than half cycle (17 fs) of the CO₂ laser. Even though the electrons in the ionization front carry no energy, the frequency of the CO₂ laser can be upshifted due to the relativistic Doppler effect as the CO₂ laser pulse enters the front. The wavelength can be tuned from 1 to 10 um by simply changing the electron density of the front. While the upshifted light with 5 < wavelength(um) < 10 propagates in the forward direction, that with 1 < wavelength(um) < 5 is back-reflected. These two plasma techniques seem extremely promising for covering the entire molecular fingerprint region.

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.

The availability of intense, ultrashort coherent radiation sources in the infrared region of the spectrum is enabling the generation of attosecond X-ray pulses via high-harmonic generation, pump–probe experiments in the molecular fingerprint region and opening up the area of relativistic infrared nonlinear optics of plasmas. These applications would benefit from multi-millijoule single-cycle pulses in the mid- to long-wavelength infrared region. Here, we present a new scheme capable of producing tunable relativistically intense, single-cycle infrared pulses from 5 to 14 μm with a 1.7% conversion efficiency based on a photon frequency downshifting scheme that uses a tailored plasma density structure. The carrier-envelope phase of the long-wavelength infrared pulse is locked to that of the drive laser to within a few per cent. Such a versatile tunable infrared source may meet the demands of many cutting-edge applications in strong-field physics and greatly promote their development.

We show that a high-energy electron bunch can be used to capture the instantaneous longitudinal and
transverse field structures of the highly transient, microscopic, laser-excited relativistic wake with
femtosecond resolution. The spatiotemporal evolution of wakefields in a plasma density up ramp is
measured and the reversal of the plasma wake, where the wake wavelength at a particular point in space
increases until the wake disappears completely only to reappear at a later time but propagating in the
opposite direction, is observed for the first time by using this new technique.