Coherent Synthesis of Terahertz Radiation from Femtosecond Laser Filaments in Air

we report on the coherent synthesis of pulsed Terahertz radiation from femtosecond laser filaments organized in an array. The Terahertz intensity is proportional to the square of the number of the filaments, which provides a simple method for scaling up of the Terahertz energy with powerful femtosecond laser. Moreover, directional off-axis terahertz radiation can be achieved. This paves the way for applications of this Terahertz source for remote sensing. Electromagnetic radiation in the Terahertz (THz) domain has received much attention in the last two decades. THz spectroscopy is ideally suited for the identification of biological or complex inorganic molecules [1], the study of collective excitations in condensed media [24], for plasma diagnostics [5]. Applications of THz radiation range from medicine [6], food industry process control [7], to security concerns [8], telecommunication [9], etc. Impressive progress has been achieved in the last few years in the development of intense THz sources. Optical rectification of a short laser pulse in a nonlinear crystal yields THz sources with field amplitude reaching the MV/cm level [10]. Unfortunately, many applications which require illumination of a distant target are hampered by the poor transmission of THz radiation through atmosphere, due to the strong attenuation posed by water vapor. Fig. 1. Experimental setup. The two filaments are separated laterally by a variable distance of d. The adjustable temporal delay between the two filamentary pulses is p. Recently, significant progress has been achieved in remote THz generation. To alleviate the strong attenuation in air, one produces a THz emitting source close to the distant target. This can be readily achieved using intense infrared femtosecond laser pulses. A first scheme consists in simply launching a short laser pulse in air. If the initial peak power exceeds a critical value 2 0 2 0 8 72 . 3 n n Pcr    , the pulse undergoes filamentation [11]. Here, n0 and n 2 are the laser wavelength, the refractive index of air and the nonlinear refractive index of air, respectively. The plasma column created during this process is initially in an excited state of longitudinal oscillations; it produces a forward oriented, radially polarized conical THz emission [12]. By simple manipulation of the initial laser beam, it is possible to place the ionized region, and hence the THz source, at a distance from the laser, which can reach ha l-0 08 57 94 8, v er si on 1 4 Se p 20 13 Author manuscript, published in "Applied Physics Letters 102 (2013) 221107" DOI : 10.1063/1.4807917