Off-Axis Phase-Matched Terahertz Emission from Two-Color Laser-Induced Plasma Filaments

Abstract

We observe off-axis phase-matched terahertz generation in long air-plasma filaments produced by femtosecond two-color laser focusing. Here, phase matching naturally occurs due to off-axis constructive interference between locally generated terahertz waves, and this determines the far-field terahertz radiation profiles and yields. For a filament longer than the characteristic two-color dephasing length, it emits conical terahertz radiation in the off-axis direction, peaked at 4-7° depending on the radiation frequencies. The total terahertz yield continuously increases with the filament length, well beyond the dephasing length. The phase-matching condition observed here provides a simple method for scalable terahertz generation in elongated plasmas.

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.…”

Generation of Phase-Stable Sub-Cycle Mid-Infrared Pulses from Filamentation in Nitrogen

“…The MIR generation technique, FWM of the fundamental and SH of Ti:sapphire amplifier output, is essentially identical to those used for THz generation [23][24][25][26][27][28][29][30][31][32][33][34][35][36]. Here we discuss the difference between the MIR generation and the THz generation experiments.…”

“…The second point is the adjustment of the relative delay and polarization of the input pulses. At the THz generation experiments in the references [23][24][25][26][27][28][29][30][31][32][33][34][35], only the angles of the BBO crystal for the SHG can be adjusted to optimize the efficiency of the wavelength conversion. In this case, the freedom to control the relative delay and polarizations of the fundamental and SH pulses is seriously limited.…”

“…This means that the conversion efficiency of FWM can be significantly improved with a filamentation effect. Wavelength conversion of Ti:sapphire laser output by using FWM through filament to ultraviolet [13][14][15][16], visible [17], infrared [18][19][20][21][22], and terahertz (THz) [23][24][25][26][27][28][29][30][31][32][33][34][35][36] regions were demonstrated so far.…”

Analysis of THz generation through the asymmetry of photoelectron angular distributions

“…It is well-known that THz generation by a single-color laser is very small, but it is very efficiently generated by using a two-color pulse [10][11][12][13][14]. So far, the evidence supports the photocurrent model, in that ionization of the medium is indispensable [13,14], yet the mechanism behind the generation of THz remains unclear.…”