Two-color Air Plasma Research Articles

Frequency-resolved measurement of two-color air plasma terahertz emission

We investigated the far-field terahertz beam profile generated from an air plasma induced by two-color femtosecond laser pulses. Under our experimental conditions (filament length shorter than the dephasing length between the two-color pulses), using electro-optic sampling in both ZnTe (0.2-2.2 THz) and GaP (0.4-6.8 THz) crystals, and ultra-broadband ABCD technique (1-17.5 THz), we determined that the THz beam exhibits a unimodal beam pattern below 4 THz and a conical one above 6 THz. This experimental finding is consistent with theoretical studies based on the unidirectional pulse propagation equation, which predict the transition of THz emission from a flat-top profile to a conical one due to the destructive interference of THz waves emitted from the plasma filament. Our results also underscore the importance of accounting for experimental artifacts, such as photo-excited losses in silicon resulting in on-axis THz absorption along with the influence of drilled mirrors, in characterizing the complex spatial and frequency-dependent behavior of two-color plasma-induced terahertz emission.

Frequency-resolved characterization of broadband two-color air-plasma terahertz beam profiles.

The frequency-resolved terahertz (THz) beam profile characteristics of a two-color air-plasma THz source were investigated in the broadband frequency range (1-15 THz). The frequency resolution is achieved by combining THz waveform measurements and the knife-edge technique. Our results show that the THz focal spot size is strongly frequency dependent. This has important implications on nonlinear THz spectroscopy applications where accurate knowledge of the applied THz electrical field strength onto the sample is important. In addition, the transition between the solid and hollow beam profile of the air-plasma THz beam was carefully identified. Far from the focus, the features across the 1-15 THz range have also been carefully examined, revealing the characteristic conical emission patterns at all frequencies.

Active stabilization of terahertz waveforms radiated from a two-color air plasma

Intense laser fields focused into ambient air can be used to generate high-bandwidth current densities in the form of plasma channels and filaments. Excitation with bichromatic fields enables us to adjust the amplitude and sign of these currents using the relative phase between the two light pulses. Two-color filamentation in gas targets provides a route to scaling the energy of terahertz pulses to microjoule levels by driving the plasma channel with a high-energy laser source. However, the structure of plasma channels is highly susceptible to drifts in both the relative phase and other laser parameters, making control over the waveform of the radiated terahertz fields delicate. We establish a clear link between the phase dependence of plasma currents and terahertz radiation by comparing in situ detection of current densities and far-field detection of terahertz electric fields. We show that the current measurement can be used as a feedback parameter for stabilizing the terahertz waveform. This approach provides a route to energetic terahertz pulses with exceptional waveform stability.

Scaling up and parametric characterization of two-color air plasma terahertz source

The plasma produced by frequency mixing of an ultrafast pulsed laser and its second-harmonic field in air is a promising, table-top source of terahertz radiation. To maximize the generation efficiency of such a source, multi-parameter optimization is essential. A parametric study involving various external controls is presented here. Starting from the optimization of phase and polarization of the second-harmonic field, the effect of laser intensity on the terahertz flux is reported. Moreover, the effect of external focusing conditions, such as focal length of the lens and effect of laser beam aperture, is explored in detail. Contrary to the previous reports, we find that with the increase of laser intensity, terahertz (THz) flux increases with more conversion efficiency; this is attributed to the multiple ionization of the air, leading to stacked release of contributory electrons. Exploration of beam convergence reveals that relatively weaker focusing leads to increased terahertz production compared to sharp focusing conditions. Moreover, temporal characterization of the emitted terahertz pulse conducted using electro-optic sampling provides pulse duration of 368 fs with corresponding frequency spectra up to 3.6 THz. The maximum electric field of the terahertz pulse is estimated to be 1.12 MV cm−1 in our experimental conditions. Results of 2D particle-in-cell simulations are also presented with the objective of validating the role of transient photocurrent mechanism in THz generation from two-color laser-induced gas plasma. We believe the broadband, bright THz source presented here will be useful in the imaging of bigger samples as well as nonlinear spectroscopy.

Strong Terahertz Radiation from a Liquid-Water Line

Terahertz generation from liquid, in particular water, was long considered impossible, because of strong absorption. While terahertz radiation from liquid water was finally observed in 2017, the mechanism has remained unclear, and the efficiency low. Here experiments show that when a column of flowing water is used, the efficiency can be enhanced by three orders of magnitude---as high as for a typical two-color air plasma source, yet using a single-color laser pump here. The authors explain the mechanism with a model of current induced by laser ponderomotive force, which is supported by particle-in-cell simulations.

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

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.