Dual THz Wave and X-ray Generation from a Water Film under Femtosecond Laser Excitation

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.

Correlated emission of X-ray and sound from water film irradiated by femtosecond laser pulses

“…Effectively in these Z-scan experiments, the laser power on the water film continuously changes along the Z-axis as shown in Fig.4 (a). The bandwidth of the profile in X-ray is narrow at 37 µm, approximately twice as the thickness of water film, as reported previously [28,23]. Though the sound emission shows a wider profile with the bandwidth at 403 µm, the peak position of the sound emission is the same as the peak position of X-ray emission.…”

“…The sound intensity becomes independent on the focusing conditions due to filament formation. These findings are expected to contribute to future applications of combined emission of X-ray and sound as in the case of X-ray and THz wave synchronized emission [23] and further in the combination with THz wave and sound/ultrasound.…”

Giant Enhancement of THz Wave Emission under Double-Pulse Excitation of Thin Water Flow

“…In the case of flat water flow, when the thickness of jet was 100-240 µm at the delay time of ∼6 ps, THz emission increases 4-times [39], while in the case of water jet of 210 µm diameter at the delay time of 50 ps, THz pulse energy increases more than 8-times [38]. For thinner flat water flow with 20 µm thickness, the enhancement was about 100-times in |E| 2 estimated from TDS signal peak intensities at the nanosecond delays [32]. As the related-mechanisms have been already discussed in X-ray emission from aqueous solutions [33,40], the pre-pulse irradiation plays an important role in pre-plasma formation and in dynamic morphological changes due to ablation within picosecond to nanosecond time delays.…”

Enhancement of X-ray emission from nanocolloidal gold suspensions under double-pulse excitation

“…Recently, similar experiments with narrowly focussed fs-laser pulses for terahertz-wave emission from water flow were reported [ 51 ], while terahertz emission from various solutions under broad focusing was already known [ 52 ]. Simultaneous emission of X-ray and terahertz waves from gas clusters in vacuum [ 53 – 54 ], Al-coated glass substrates in vacuum [ 54 ], or a distilled water flow in air under double-pulse excitation [ 16 ] were independently reported. A simultaneous emission of X-ray and terahertz wave radiation is highly expected for novel pump–probe experiments [ 55 ].…”

“…An increase of the laser-absorption efficiency can be expected in plasmonic nanoparticles, which results in the efficient generation of highly ionized charge states [ 13 – 14 ]. It is expected that this characteristic interaction between intense femtosecond laser and plasmonic nanoparticles will contribute to a broad field of applications ranging from pulsed X-ray generation to energetic particle sources [ 14 ] or terahertz wave emission from aqueous solutions [ 16 ].…”