The Study Of Gas Photonic Terahertz Wave Generation And Detection

Recently, gas plasma, produced by focusing an intense laser beam to ionize atoms or molecules, was demonstrated to be promising for broadband terahertz (THz) waves source and sensor. Unlike solid state materials commonly used in THz time domain spectroscopy (THz-TDS) system, such as electro-optical crystals and photon-conductive antennas, gas medium have no phonon resonance or echoes due to THz waves or optical reflection. Therefore, an gas plasma can be used to generate strong (E-field-MV/cm), broadband (0.1-40THz), and coherent THz waves through a complex nonlinear physical process. On the other hand, plasma in ambient air or selected gases can also be used as THz wave sensors. Moreover, some gaseous medium, from the point of use-cost, is a better choice than the solid state materials mentioned above since there is no concern about their damage.Focused on the broadband THz generation, detection and applications based on the gas plasma, this thesis is composed of four part:Firstly, the methods and theories for the THz generation and detection with laser induced gas plasma is introduced. Based on the widely used photocurrent model, the influence of prepared plasma induced by a prepulse on the THz emission generated by main two-color femtosecond lasers (fs-lasers) have been numerically and analytically investigated. Observed results in previous report, such as the suppression of THz emission from main pulse due to the existence of prepulse, can be easily reproduced. Moreover an exact expression between E-field of THz waves generated by main pulse and the residual electrons’density of prepared plasma has been obtained. Besides the cases of large time interval between the two pulses, THz generation from two overlayed pulses is also investigated. Studies show that the Cross-phase modulation would lead to the variation of the THz output, which is promising for the high-speed all-optical THz switching.Secondly, the transient photocurrent model for THz detection process is developed to investigate the missed microscopic process of the THz Air Breakdown Coherent Detection. The observed results in experiments, such as transition of detection mode and SH intensity clamping, can be numerically reproduced. Besids the direct THz detection, we also systematically studied the THz heterodyne detection process with transient photocurrent model, results are agree with the experiments. Our results demonstrate that FWM model does not appear to account for the THz Air Breakdown Coherent Detection, but the transient photocurrent model does.Thirdly, the few cycle laser pulses is studied for the broadband THz detection for the first time. The time behavior of gas plasma induced by a few-cycle laser pulse with a simultaneously incident terahertz pulse is investigated. Results show that if the probe pulse has a fixed carrier envelop phase (CEP), pulse energy and asymmetric pulse shape, coherent THz detection could be realized with very low laser energy and without a bias field, and may be very useful in some fields such as remote THz sensing. More importantly, we demonstrated that heterodyne THz coherent detection method with few cycle laser pulse is possible. Since heterodyne detection can greatly reduce the noise from the jitter of probe pulse energy and CEP, which is promising in ultrawide band THz pulse coherent detection.Fourthly, the intense THz field is firstly studied on the single shot CEP measurement for the few cycle laser pulse. It is found that the THz electric field can significantly adjust the SH emission intensity and the increment is sensitive to the CEP of the few cycle laser pulses. With this THz adjusted SH signal, we could retrieve the absolute value of the CEP. Further studies show that one can add a similar signal channel with a fixed phase difference with respect to the other one and map the CEP on a two-dimensional parametric plot with the two signals to directly obtain the CEP of the consecutive few cycle laser pulses. Besides, a30-100fs laser can simultaneously pump the generation of few cycle laser pulse and THz pulse, therefore the whole system is all-optical. Such an method may be useful for some relevant experiments and applications.The researches presented in this thesis provides a critical understanding of broadband terahertz source and sensor and introduces a new direction for scientific application of terahertz wave gas photonics.