Abstract
We use a broadband microbolometer array to measure the full three-dimensional (3D) terahertz (THz) intensity profile emitted from a two-color femtosecond plasma and subsequently focused in a geometry useful for nonlinear spectroscopic investigations. Away from the immediate focal region we observe a sharp, conical intensity profile resembling a donut, and in the focal region the beam collapses to a central, Lorentz-shaped profile. The Lorentzian intensity profile in the focal region can be explained by considering the frequency-dependent spot size derived from measurements of the Gouy phase shift in the focal region, and the transition from the donut profile to a central peak is consistent with propagation of a Bessel–Gauss beam, as shown by simulations based on a recent transient photocurrent model (You et al 2012 Phys. Rev. Lett. 109 183902). We combine our measurements to the first full 3D visualization of the conical THz emission from the two-color plasma.
Highlights
Influence of the transient currents in the plasma on the THz generation mechanism [25, 26], and the finite length of the plasma was shown to lead to conical emission of THz radiation in a preferred angle from the plasma due to constructive interference caused by the phase matching conditions between the fundamental and the second harmonic pump beams [29]
With offset in the theoretical and experimental findings on the generation mechanism in a two-color plasma discussed above, we investigate the full three-dimensional (3D) beam profile emitted from a two-color femtosecond plasma
We discuss the observed 3D beam profile in terms of measurements of the Gouy phase shift in the focal region, and simulations based on the transient photocurrent model put forward by You et al [29] show that the focusing of the beam profile is well described by propagation of a Bessel–Gauss (BG) beam
Summary
Influence of the transient currents in the plasma on the THz generation mechanism [25, 26], and the finite length of the plasma was shown to lead to conical emission of THz radiation in a preferred angle from the plasma due to constructive interference caused by the phase matching conditions between the fundamental and the second harmonic pump beams [29]. It has been predicted [30] and recently experimentally verified [31] that excitation of the plasma with longer-wavelength pump pulses leads to strong enhancement of the generated THz energy, with resulting peak field strengths in the MV cm−1 range, and pulse energies approaching the μJ range. We discuss the observed 3D beam profile in terms of measurements of the Gouy phase shift in the focal region, and simulations based on the transient photocurrent model put forward by You et al [29] show that the focusing of the beam profile is well described by propagation of a Bessel–Gauss (BG) beam
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