MOU, Sen (2017) THz Time-Domain Spectroscopy of Ionic Liquids and THz-Pump/Second-Harmonic-Probe Experiment in Solids. [Tesi di dottorato]


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Item Type: Tesi di dottorato
Resource language: English
Title: THz Time-Domain Spectroscopy of Ionic Liquids and THz-Pump/Second-Harmonic-Probe Experiment in Solids
Date: 11 December 2017
Number of Pages: 102
Institution: Università degli Studi di Napoli Federico II
Department: dep06
Dottorato: phd028
Ciclo di dottorato: 30
Coordinatore del Corso di dottorato:
Marrucci, LorenzoUNSPECIFIED
Date: 11 December 2017
Number of Pages: 102
Keywords: Terahertz; Ionic Liquids: THz-Pump/Second-Harmonic-Probe
Settori scientifico-disciplinari del MIUR: Area 02 - Scienze fisiche > FIS/01 - Fisica sperimentale
Date Deposited: 17 Jan 2018 09:27
Last Modified: 20 Mar 2019 11:56

Collection description

THz indicates an electromagnetic wave whose spectrum locates between the microwave and infrared range. In the last two decades the advent of benchtop THz sources filled the so-called ‘THz gap’. The properties of THz radiation such as low photon energy, transparence to it of packaging materials and so on make it suitable for many applications in industrial and scientific fields. In this work we generated intensive broadband THz pulses radiated from femtosecond laser induced plasma via four-wave-mixing process coupling fundamental laser light and its second harmonic. In chapter 1 we generally introduce the principles of THz generation and detection. We also introduce the application of our THz source in static THz time-domain spectroscopy and time-resolved THz spectroscopy. In chapter 2 we present THz generation based on air plasma and THz detection through different methods and sensors, including ZnTe, GaSe, LAPC single crystals and air-based methods. The results show that the detected upper frequency edge with 500 µm ZnTe is limited by the phase mismatch between THz and probe beam. 250 µm ZnTe expands the detected frequency range but the detection efficiency is restricted by phonon absorption at 5.3 THz. GaSe can detect THz up to 30 THz with a deep gap due to phonon mode around 6.5 THz. Another dip around 19.1 THz due to silicon absorption is also found. LAPC is capable of measuring THz up to around 17 THz without gap. The air-based method results in ultra-broadband detection of short unipolar pulses. In chapter 3 we develop a precise contactless micro-distance measurement method based on THz pulses. The resolution of the measurement can achieve 1.5 µm with scanning step equal to 0.01 ps. A comparison between ZnTe detection sensor and air-based sensor indicates that the air-based detection technique has advantages that include the ability to measure smaller distance and the immunity to distortion introduced by vapor absorption in air. We also compare distance measurement with ZnTe in air and in nitrogen. We find that the measurement in nitrogen leads to a more straightforward extraction of the distance as compared to the measurement in air where additional analysis is needed to take into account the coherent oscillations produced by air absorption. In chapter 4 we investigate the mixing behavior of the dielectric function of mixtures of ionic liquids (ILs). In the investigation we measure the cell thickness with the method developed in chapter 3 and detect the THz radiation with electro-optic sampling in a 250 µm ZnTe crystal. The results show that the complex permittivity of ionic mixtures obeys linear mixing law. We also find that all the complex permittivity spectra of mixtures can be simulated with a combination of three oscillators which are assigned to different vibrational modes of the ionic liquids. In chapter 5 we discuss broadband THz time-domain spectroscopy applied to pure imidazolium-based ionic liquids. The broadband THz detection was realized thanks to the use of a LAPC crystal. We find in the absorption spectra of a series of pure ionic liquids clear signatures of both inter- and intra-molecular vibrational modes. This is the first time that inter- and intra-molecular modes of ionic liquids are measured in one shot without combining different techniques. We have assigned these vibrational modes to different processes in ionic liquids. We find an interesting correlation between the specific anion of IL and the peak frequency shift and bandwidth variation observed for the intermolecular absorption band. We discuss these correlations in terms of the ability of the specific anion in forming hydrogen bonds. In chapter 6 we discuss our experiments of THz-pump/second-harmonic-generation in single crystals of quartz. The experiments on quartz show that the second harmonic signal oscillates coherently at several frequencies in the THz frequency range and these oscillations depend on the specific combination of the probe and THz linear polarization, and orientation of quartz axis. We also find that the intensity and bandwidth of the spectrum of second harmonic are enhanced in presence of the THz, i. e., side-bands appears in the second harmonic spectrum.


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