Experimental three-dimensional beam profiling and modeling of a terahertz beam generated from a two-color air plasma

Abstract

We use a broadband microbolometer array to measure the full three-dimensional (3D) terahertz (THz) intensity profile emitted from a twocolor 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.

Nitrogen plasma formation through terahertz-induced ultrafast electron field emission

“…The THz radiation has been characterized using electro-optic sampling [17][18][19] in a 300-μm-thick gallium phosphide (GaP) 110-cut crystal. At the sample position, THz peak electric fields of up to 200 kV/cm are obtained, with field strengths calibrated using combined information from free-space electro-optic sampling, pulse energy measurement, and spot size measurement [20]. The generated THz spectrum contains frequency components in the 0.1-2.5 THz range, with the amplitude reaching a maximum at 0.8 THz.…”

Ultra-broadband THz time-domain spectroscopy of common polymers using THz air photonics

“…The PMT current as a function of the time delay between the optical gate and the THz pulses is thus the observable of the ABCD measurement. We note here, that the airplasma filament produces a conically-shaped THz beam [22], which however can be described as a Bessel-Gauss beam with a bell-shaped profile at its waist when focused [23]. Therefore, the experimental geometry typical for a standard THz TDS system [4] is possible with THz air-photonics sources and detectors.…”

Ultrabroadband terahertz conductivity of highly doped ZnO and ITO

“…Ultrabroadband terahertz pulses are generated by focusing 35 fs, 600 µJ, 800 nm wavelength pulses and their second harmonic at 400 nm to induce an air plasma which emits a broadband THz transient in a conical pattern as a result of the transient photocurrent in the plasma [18,19]. The temporal shape of the transmitted THz transient is detected by the air biased coherent detection (ABCD) method [20].…”