THz wave generation in laser-included plasma has attracted considerable attention recently and the intense THz waves generated from air-induced plasma, serving as broadband THz source for sensing and imaging applications, has attracted more and more researchers’ interests in recent years. In our experiment, terahertz is detected using THz Air-Biased Coherent Detection (THz-ABCD) method. This method can achieve the third-order nonlinear susceptibility tensor to produce field-induced optical second harmonic photons. In the THz-ABCD system, red-shift is observed in frequency spectra with enhanced pump power and decreased bandwidth. After changing the probe power, the red-shift in frequency spectral can also observed with enhanced probe power, but the bandwidth is broadening as the prober power increasing. We further explore of these phenomena through intense self-phase modulation of the optical pulse in the plasma and the collision behavior. This study reveals that we can control THz intensity and bandwidth by changing pump power and probe power in the ABCD system.
Terahertz wave which can provide innovative sensing and imaging techniques can obtain spectroscopic
information unavailable at other wavelengths. The terahertz air-biased-coherent-detection (ABCD) method can achieve
the third-order nonlinear susceptibility tensor to produce field-induced optical second harmonic photons. Therefore, the intense terahertz wave generated and detected by the laser-induced air plasma provides a promising ultra-broadband terahertz source and sensor for spectroscopy and imaging technique. Aiming at that purpose, an understanding of the
frequency spectrum characterization of terahertz pulse is crucial. In this work, we investigated the variation of the THz
pulse bandwidth measured through the third harmonic generation using the coherent detection scheme, by increasing the
optical probe pulse power and biased electric field. A bandwidth broadening of the measured THz pulse is observed by
increasing either the probe pulse power or the bias voltage strength. We speculate that a pulse shape change of the probe beam and a saturation effect during the second-harmonic generation might cause the bandwidth broaden with probe
power. To further investigate the mechanism, we fixed the power of probe laser at 150mW and changed the bias voltage.
The results show that the frequency spectrum width becomes wider gradually with the increasing of the bias voltage. A theoretical explaination shows that the bandwidth broadening with bias field might be introduced by a pulse shape
change of the bias field induced second harmonic wave. This study reveals that we can control THz intensity and
bandwidth by changing probe power and bias voltage in the ABCD system.
The characterization of the emission of terahertz (THz) waves generated by four-wave mixing in the presence of laser-induced air plasma under different pump power is presented. In our experiment, terahertz is detected using THz Air-Biased Coherent Detection (THz-ABCD) method. Red-shift is observed in frequency spectra with enhanced pump power and the bandwidth is narrowing down. Then the localized terahertz radiation along the plasma was studied, by constricting the emission area with a pinhole. The spatio-frequency dependent of THz emission from laser-induced air plasma is observed. These phenomena are explained by intense self-phase modulation of the optical pulse in the plasma. This study reveals that terahertz spectrum can be controlled by changing the pump power to get useful frequency range.