中央研究院 原子與分子科學研究所

Using rare-gas ions as the interacting medium, phase-matched high-order harmonic generation up to 1-keV
hard x ray can be achieved by balancing the negative plasma dispersion and Gouy phase shift with the
positive, intensity-dependent, intrinsic dipole phase variation. The time-dependent phase-matching condition
automatically gates the harmonic output to an isolated attosecond pulse. The estimated pulse duration is as short
as 120 as.

We demonstrate what we believe to be a new approach to energetic picosecond
10.2-μm pulse generation based on nonlinear mixing of subnanosecond single-frequency 1338-
nm pulses and broadband 1540-nm chirped pulses in a BGGSe crystal followed by a grating
compressor for the purpose of seeding high-power CO2 amplifiers. The energy of the 10.2-μm
pulses exceeding 60 μJ with 3.4%-rms fluctuation can be routinely obtained. Single-shot pulse
duration measurement, performed by Kerr polarization rotation time-resolved by a streak camera,
together with the pulse spectrum, indicates the pulse width is between 2.7-3 ps. Numerical
calculations show that power broadening and dynamic gain saturation with Rabi-flopping can
be induced with such an intense seed in a multi-atmospheric CO2 amplifier. These nonlinear
effects greatly suppresses pulse splitting due to the comb-like spectrum of the CO2 molecule. A
peak power exceeding 1 TW is expected after multipass of amplification while maintaining an
appropriate high intensity by controlling the beam size along the path.

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.