Mechanism of Single-Cycle THz Pulse Generation and X-ray Emission: Water-Flow Irradiated by Two Ultra-Short Laser Pulses

Abstract

The interaction of two subsequent ultra-short sub-milli-Joule laser pulses with a thin water flow results in an emission of a strong single-cycle THz pulse associated with enhanced soft X-ray emission. In this paper, a chain of processes produced in this interaction is analyzed and compared with other THz generation studies. It is demonstrated that the enhanced THz and X-ray emissions are produced by an energetic electron beam accelerated in the interaction of a main laser pulse with liquid water ejected from the surface by the pre-pulse. This scheme thus provides an efficient laser energy conversion in a THz pulse, avoiding laser self-focusing and filamentation in air.

Keywords: Bremsstrahlung emission; THz emission; coherent transient radiation; dipole radiation; water ablation.

Figure 1

Two-pulse irradiation on the micro-thin water flow. Time domain spectroscopy (TDS) was used to detect THz radiation from the irradiation zone. See Figure 2 for the detailed geometry of the light interaction with the water jet.

Figure 2

Two-pulse irradiation of the water micro-sheet at different orientations. (a) A backlit time-resolved shadowgraphy ($yz$-plane or a front-view) by white light continuum (WLC) at $\Delta t=4.7$ ns after a 0.1 mJ pre-pulse (the main-pulse of 0.4 mJ is not irradiated when this shadowgraph image was taken). (b) Top-view ($xz$-plane) positions of laterally separated pre- and main-pulses. The Inset photo shows water micro-sheet formed from colliding water jets. (c) The timing of the second (main) pulse is linked to the surface perturbation arrival at the point of irradiation from the ablation caused by the first pulse. Schematics of experiment of two-pulse irradiation of water flow at 60° angle of incidence; note, a tilted coordinate frame $x^{\prime }{y}^{\prime }{z}^{\prime }$ is shown in (c) to make water surface as a $x^{\prime }{y}^{\prime }$-plane as used for discussion of surface shockwaves. The image used for the schematics is a rendered slide glass ablation with one fs-laser pulse. It is used to illustrate the generic phenomenon of ablation crater formation, side walls, and microflows/droplets. Inset shows the focal volume of laser pulses in air.

Figure 3

Temporal evolution of the diameter of a shockwave front as a function of delay time, $\Delta t$, after the energy $E_1$ deposition of the pre-pulse. The inset shows a shadowgraph at the delay of 4.7 ns; the pre-pulse energy is 0.1 mJ (same as Figure 2a). The back-light illumination through a 17-$\mathsf{\mu }$m-thick water sheet is made by the white light continuum (WLC) produced by the main pulse $E_2$. The apparent thickness of the 17-$\mathsf{\mu }$m water flow is twice larger in projection, i.e., 34 $\mathsf{\mu }$m (front view) due to the 60° angle of incidence onto the water sheet. The time evolution of the shockwave radius according to the Sedov–Taylor model in the case of spherical ($i=3$) and cylindrical ($i=2$) explosion follows $R\propto t^{2/(i+2)}$; see the dashed and dash-dotted lines (Equation (1)). Slope of 7.6 $\mathsf{\mu }$m/ns is shown (red line as an eye guide); the error bars are 10%.

Figure 4

Polarization of THz radiation depends on the $(x,y)$ position of the pre-pulse with respect to the main pulse (0, 0). (a) Schematics of interaction on the surface of thin water flow at the moment of arrival of the main-pulse at a time delay $\Delta t$ after pre-pulse; the plane of incidence in $xz$-plane. The n is the normal to the front of shockwave with different projections (n_x, n_y) dependent on the location. Ejected droplets and shockwave in pre-surface air induced by pre-pulse are clearly discernible in shadowgraphy (Figure 2a) but not shown here for simpler schematics. The diameter of laser pulse on the xy-plane is comparable with the perturbation of shocked water. Shadowgraphy insets show the focal volume and air densification. (b) Left hand circular and right hand circular (LHC and RHC) polarizations of THz emission at different half-planes in respect to the positive and negative y position values for pre-pulse focus [47]. Projections of electric vector traces shown for reflected THz radiation (adopted from ref. [47]). The top-inset is a rendered SEM image of a glass ablation site by a single fs-laser pulse (same as in Figure 2c). Thumbnail images of time-integrated optical emission are shown side-by-side with THz polarization traces.

Figure 5

Images of plasma plume at the surface of melted Ga target obtained at a time delay of 12.5 ns in two perpendicular directions in the target plane. The yellow arrow shows the direction of pre-pulse beam. Images are adapted from the paper by Uryupina et al. [65].