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

Pernille Klarskov¹, Andrew C Strikwerda, Krzysztof Iwaszczuk and Peter Uhd Jepsen
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pkpe@fotonik.dtu.dk
DTU Fotonik—Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
¹ Author to whom any correspondence should be addressed. 

http://m.iopscience.iop.org/1367-2630/15/7/0

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.

GENERAL SCIENTIFIC SUMMARY 
Introduction and background. Nonlinear spectroscopy with intense terahertz (THz) light is an important tool for the understanding of anharmonicities and couplings in low-energy interactions in solids, liquids and gases. The light intensity is one of the key parameters in any nonlinear experiment. It is determined by the detailed shape of the beam in time and space. An intense femtosecond laser beam focused with its second harmonic in air forms a plasma which generates ultrashort THz pulses with high bandwidth and intensity. The THz radiation emitted from such air plasmas has intriguing propagation properties that we explore here.
Main results. We measure the full three-dimensional (3D) 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. This Lorentzian beam shape is due to weighted superposition of the Gaussian beam profiles at the individual frequencies within the bandwidth of the THz beam. We demonstrate that the transition from the donut profile to a central peak is consistent with propagation of a Bessel–Gauss beam.
Wider implications. The femtosecond two-color air plasma THz source has appealing properties such as ultrabroad frequency contents, ultrashort transform-limited pulse duration, and high peak field strength, three important properties for ultrafast, nonlinear spectroscopy. The focusing properties investigated here must be carefully considered when designing nonlinear or pump-probe experiments.

Figure. A donut-shaped THz beam profile emitted from a femtosecond two-color plasma is focused to a central spot due to its Bessel–Gauss propagation characteristics.