Abstract
Simultaneous emission of the THz wave and hard X-ray from thin water free-flow was induced by the irradiation of tightly-focused femtosecond laser pulses (35 fs, 800 nm, 500 Hz) in air. Intensity measurements of the THz wave and X-ray were carried out at the same time with time-domain spectroscopy (TDS) based on electro-optic sampling with a ZnTe(110) crystal and a Geiger counter, respectively. Intensity profiles of the THz wave and X-ray emission as a function of the solution flow position along the incident laser axis at the laser focus show that the profile width of the THz wave is broader than that of the X-ray. Furthermore, the profiles of the THz wave measured in reflection and transmission directions show different features and indicate that THz wave emission is, under single-pulse excitation, induced mainly in laser-induced plasma on the water flow surface. Under double-pulse excitation with a time separation of 4.6 ns, 5–10 times enhancements of THz wave emission were observed. Such dual light sources can be used to characterise materials, as well as to reveal the sequence of material modifications under intense laser pulses.
Highlights
An intense (>1013 W/cm2) femtosecond laser at near-IR and matter interaction [1,2,3] induce highly-nonlinear optical processes and result in photon conversion to an X-ray of several-keV [4], as well as to a THz wave at meV [5] in association with the white light continuum in the visible spectral range [6]
This study reports the demonstration of the dual X-ray and THz wave simultaneous emission from water flow irradiated by focused femtosecond laser pulses in air
Different characteristic features of THz wave emission, which are associated with X-ray emission, in the reflection and the transmission under the single-pulse excitation were clearly revealed
Summary
An intense (>1013 W/cm2) femtosecond laser at near-IR (photon energy ∼1 eV) and matter interaction [1,2,3] induce highly-nonlinear optical processes and result in photon conversion to an X-ray of several-keV [4], as well as to a THz wave at meV [5] in association with the white light continuum in the visible spectral range [6]. Fast electron motions accelerated by laser ponderomotive forces [15,18,19] and the four-mixing process induced by two-colour excitation [17] have been proposed as THz emission mechanisms from laser-induced plasmas Such laser-induced plasmas of the solid targets generate a powerful THz wave because of the high plasma densities, there are problems with the instability of the intensity and a limitation in the successive delivery of laser irradiation to the target. Experiments on such simultaneous emission with a Al-coated glass substrate [15] and gas targets, He [15] or Ar [20], in vacuum chambers were reported This type of work, though the numbers of such reports are still quite limited, will be the first step for the discussion of the conversion of near-IR (eV) laser photon energy into photons at the two ends of the spectrum, several-keV (X-ray) and meV (THz wave), and will contribute well to other fields like material sciences. A simultaneous emission of the THz wave and hard X-ray in air using distilled water as a target irradiated by tightly-focused near-IR femtosecond laser pulses and THz wave emission enhancements under double-pulse excitation are presented
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