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PhD thesis (EN)

2017 (EN)
Ισχυρά πεδία ακτινοβολίας τεραχέρτζ (THz) από αλληλεπιδράσεις λέιζερ πλάσματος
Broadband intense THz fields from laser plasma interactions

Κουλουκλίδης, Αναστάσιος

Κοέν, Σαμουήλ
Παναγιώτης, Λουκάκος
Τζωρτζάκης, Στέλιος
Κιοσέογλου, Γεώργιος
Κοπιδάκης, Γεώργιος
Πελεκάνος , Νικόλαος
Παπάζογλου, Δημήτριος

The THz frequency range is one of the most interesting regions of the electromagnetic spectrum. THz radiation lies between the far-infrared and the microwave region, in a frequency interval from 0.1 THz to 75 THz. Historically, this part of the spectrum remained in the dark and it was known as the "THz gap" due to the lack of coherent sources in this frequency range. Nevertheless since the last two decades numerous works have been done on coherent sources and making it possible to access the THz frequency region of the electromagnetic spectrum and reveal its unique properties. Many non-metallic and non-polar materials are transparent to THz radiation, while at the same time THz radiation is non-ionizing (photon energies of few meV) and is absorbed heavily by water. Apart form water, many other molecules have strong absorption features in this frequency range, due to vibrational and rotational transitions. These transitions are specific to each molecule exhibiting this way fingerprints which allow their spectroscopic identification. These unique properties of this radiation, suggest the development of promising applications ranging from spectroscopic characterization materials to imaging for medical and security reasons.Moreover, nowadays sources capable to produce intense THz pulses are available paving the way towards nonlinear studies and electron acceleration to relativistic velocities taking advantage of the high field strength and long wavelengths. This dissertation is focused on the generation of intense, broadband THz pulses through laser plasma interactions. The scheme used is two-color filamentation in air, under which the fundamental and the second harmonic of an ultrashort pulsed laser are combined and focused into air forming a filament, which produces intense THz pulses in the far field. We propose a comprehensive physical model explaining the conical character of broadband terahertz generation from such sources while at the same time we experimentally investigate its validity by tuning in a controlled way the emission properties. To fully characterize our sources, we devise a method to reconstruct broadband THz wavepackets that are strongly distorted during propagation in dispersive and lossy media. Our approach implements an experimental procedure along with an algorithm that allows one to gain access to the lostBeyond the characterization of the sources we propose novel approaches to enhance the THz emission. These include,firstly the use of abruptly autofocusing beams for generating strong THz beams at well-defined remote locations, having first verified the required harmonics generation from such beams. We show that indeed, such a beam can lead to efficient THz generation resulting in a 5.3-times enhanced THz wave pulse energy compared to normal Gaussian-beam-induced plasma under the same conditions. We believe this work will inspire a new direction for controlling THz radiation from laser-induced plasma and pave the way for THz remote spectroscopy. Secondly, THz emission through two color filamentation scheme in transparent solids is for the first time studied. We present our experimental results that verify the above statement. Finally, we examine how the effect of the repetition rate of the excitation laser source can affect the plasma and consequently the THz emission. We present an enhancement of 70% in the emitted THz amplitude for higher repetition rates. (EN)

Τύπος Εργασίας--Διδακτορικές διατριβές




Σχολή/Τμήμα--Σχολή Θετικών και Τεχνολογικών Επιστημών--Τμήμα Επιστήμης και Τεχνολογίας Υλικών--Διδακτορικές διατριβές

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