Tailoring Air-plasmas Toward Advanced Terahertz Pulse Remote Generation and Detection PDF Download

Author: Kang Liu
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Languages : en
Pages : 119

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Book Description
"With the increasing demands for remote spectroscopy in homeland security and environmental monitoring, terahertz (THz) spectroscopy has drawn a significant amount of attention because of its capability to acquire chemical spectral signatures non-invasively. However, the development of advanced THz remote sensing techniques is hindered by three factors: (1) strong absorption of THz wave by the water vapor in the ambient air, (2) difficulty to generate intense broadband coherent THz source remotely, and (3) difficulty to pass the THz waveform information remotely without losing the signal to noise ratio. In the past, we have investigated different THz air-photonics techniques to overcome the difficulties of THz remote sensing. Still, many problems remained unsolved, and the gap between proof-of-concept studies and state-of-the-art product has yet to be fulfilled. In this thesis, the work focuses on exploring how to improve THz generation and detection method for the purpose of remote sensing through tailoring the generation and detection media, air-plasma. For the THz generation part, auto-focusing ring-Airy beam was used to enhance the THz wave generation yield from two-color laser induced air plasma. For the THz detection part, the author first demonstrated THz remote sensing at 30 meters using THz Radiation- Enhanced-Emission-of-Fluorescence (REEF) technique, greatly improved from the 10 meters demonstration last reported, and then explored the THz-REEF technique in the counter-propagation geometry, which is more practical for stand-off detections than co-propagation geometry. Plasma-based THz emitters excel in many aspects thanks to their broad radiation bandwidth, high output pulse energy and immunity to high power damage. Various excitation wavelengths and gases were explored to seek for a stronger THz wave emission from laser-induced plasmas, whereas few attempts were made for this purpose with artificially modulated exotic wavepackets that have fascinating properties. In this thesis, it is demonstrated that abruptly auto-focusing beam induced air-plasma can give a 5.3-time-enhanced THz wave pulse energy compared to normal Gaussian beam induced plasma under the same conditions. At the same time, a red shift on the THz emission spectrum is also observed. We are able to perform a simulation using the interference model to qualitatively describe these behaviors. Remote sensing is one of the major challenges for THz radiation applications, due to the THz wave attenuation by the atmosphere water vapor during its propagation. THz-REEF is a THz air-photonics technique that has the potential to bypass this issue, by having the sought-after THz spectral fingerprints carried from the target to the operator by ultraviolet light, which experiences low absorption in the atmosphere. This technique has been previously demonstrated when the THz radiation and the laser excitation are focused collinearly, namely the co-propagating geometry. However, the co-propagating geometry is not a favorable configuration for practical stand-off detection. Therefore, further exploration on alternative sensing geometries is still required. Herein we report the interaction of broadband THz radiation with plasmas induced by a counter-propagating laser beam, which is a more desirable geometry for remote sensing. We have found that in the counter-propagating geometry the maximum amplitude of the REEF signal is comparable to that in the co-propagating case, whereas the time resolved REEF trace significantly changes. By performing the study with different plasmas, we observed that in the counter-propagating geometry the shape of the REEF trace depends strongly on the plasma length and electron density. A new theoretical model suggesting that the densest volume of the plasma does not contribute to the fluorescence enhancement is proposed to reproduce the experimental measurements. Our results further the understanding of the THz-plasma interaction and highlight the potential of THz-REEF technique in the plasma diagnostic applications."--Pages xii-xiv.