THz From Laser Induced Ionization of Air
Intense THz pulses are required for the fundamental investigation of nonlinear light-matter interaction and several techniques could be adopted to generate an intense THz wave. For instance, when a two-color laser pulse (fundamental and second harmonic) ionizes a gas, an intense and linearly polarized THz pulses is emitted [1,2]. Although peak fields as high as 500 kV/cm have been reported with this technique , saturation effects limit the intensity scaling of the THz emission and increasing the pump pulse energy does not always results in higher THz fields. We are currently investigating possible ways to enhance the THz emission from gas ionization.
One of our main achievements in this field is related to the demonstration of a significant enhancement of the terahertz generation efficiency, up to 10-3. This has been achieved by investigating the wavelength dependence of the THz emission via two-color driven gas ionization. We indeed observed a remarkably favorable dependence of the THz generation efficiency for increasing pump wavelengths. Our measurement shows that conversion efficiencies up to 10-3 can be readily achieved and peak fields in the MV/cm range can be obtain with <1 mJ pump pulses at 1.8 μm .
We recorded the THz emission by air ionization for a wavelength range between 800 nm and 2 μm. The pump pulse, delivered by a commercial Optical Parametric Amplifier was kept at nearly 60 fs duration, 400 uJ energy and 6 mm diameter for the whole tuning range and was focused by a 4” focal length parabolic mirror. A Beta Barium Borate crystal was inserted in the beam-path before the focus in order to generate the second harmonic component required to break the symmetry of the ionizing field. The emitted THz radiation was then collected by a parabolic mirror and focused back to a spot by another mirror. THz filters were placed between the mirrors, to remove the intense pump, and the THz signal was recorded by a calibrated pyroelectric detector.
The measured THz energies as a function of the pump pulse wavelength are reported in Fig. 1a. The data show a strong increase of the THz generation efficiency that peaks to 10-3 at 1.8 μm and then drops. This observation is not consistent with the predicted quadratic behavior, yet a simplified numerical model, accounting for the wavelength dependencies of the experimental parameters, well reproduces the experimental trend. In the numerical model we indeed considered that the emission volume scales as the linear focused volume of the input radiation, i.e. as the product of the wavelength dependent beam-waist and Rayleigh range (both linearly dependent from the wavelength for a fixed aperture input beam). Furthermore, we introduced the dependency of the peak intensity from the wavelength, following the geometrical consideration above. With these corrections, the numerical prediction properly fits the experimental data (red curve in Fig. 1a).
We have also recorded the THz pulse electric field trace and spot size to estimate the peak field at maximum conversion efficiency (measurement shown in Fig. 1b). From the experimentally measured quantities, we evaluate a peak field in the focus of the last mirror as high as 4.4 MV/cm.