Χρονικός Χαρακτηρισμός Στενού Παλμού Υπέρθεσης Χαμηλής Τάξης Αρμονικών Λέιζερ

Nowadays, it is well established that the superposition of higher-order harmonics (HOH), resulting from the non-linear response of matter to intense laser pulses (greater than 1013 W/cm2), comprises an avenue towards ultra-short pulse generation reaching the attosecond time-scale. Although this field has progressed significantly there are still a number of difficulties one has to surmount. An arbitrary superposition of harmonics may not depict close temporal confinement. Thus, it is of great importance to know the temporal characteristics of the superposition. Towards this goal an experimental method has been previously proposed the measurement of the relative phase distribution of the spectral components of a superposition of higherorder harmonics or the phase distribution of individual ones. This method is based on the phase-control principles of the excitation probability of an excited atom by the harmonic radiation and its fundamental frequency. The work of this thesis is focused on implementing this proposed method in order to directly measure the phase distributions of a short pulse produced by the superposition of the third and fifth harmonics of a Ti:Sapphire laser system generated in Xe gas. For In this measurement we used a previously proposed dispersionless experimental set-up based on a transmission grating interferometer. From the retrieved phase and the measured spectral amplitude distributions the temporal profile of the pulses could be reconstructed and was found in good agreement with the simulated one. This work opens-up a new route for the characterization of harmonics, for the temporal characterization of XUV pulses of ultra-short duration

Γένεση ισχυρών παλμών υπεριώδους κενού μικρής χρονοδιάρκειας

The exploration of electronic motion in real time as well as the control of electronic dynamics requires the development of new light sources in the XUV spectral region and with pulse durations of the order of attoseconds. One of the basic requirements for the examination and observation of such dynamics is to control the generation of these pulses. High-harmonic generation provides a powerful source of ultrashort coherent radiation in the XUV and soft-­x-ray range. For time domain spectroscopic applications,the exploitation of isolated attosecond pulses is more advantageous for the interpretation of the acquired data than a train of attosecond pulses. Isolated attosecond pulses are generated when XUV emission is confined with in half a cycle of the IR driving pulse. The non-linear medium is then emitting only one XUV light burst, i.e.a coherent XUV continuum. Nowadays, two possible solutions are proposed towards the generation of intense isolated attosecond pulses. The first approach requires the development of high-peak-power few-cycle laser systems, which are not commercially available, while the second approach utilizes the already commercially available high-peak-power many-cycles laser systems. In this thesis a new technique, which is called Interferometric Polarization Gating (IPG) is proposed, developed and implemented, based on the second approach. By appling this technique, the appropriate control of the high harmonic generation process is feasible leading to the generation of intense coherent XUV radiation. The energy content of this radiation is in the sub 100-nJ regime, which is currently the highest ever achieved energy in coherent XUV continuum generation. The temporal characterization of this radiation is made by means of 2nd order volume autocorellation (2-IVAC). The exploitation of these intense XUV continua in tracing ultrafast electronic dynamics in atomic systems is presented. This is the first experimental realization of an XUV-­ump-XUV-probe sequence. These intense coherent XUV continua allow for time-resolved linearand non-linear spectroscopy in this spectral region and thus are of great importance to a broad field of scientific disciplines

Generation of Attosecond Light Pulses from Gas and Solid State Media

Real-time observation of ultrafast dynamics in the microcosm is a fundamental approach for understanding the internal evolution of physical, chemical and biological systems. Tools for tracing such dynamics are flashes of light with duration comparable to or shorter than the characteristic evolution times of the system under investigation. While femtosecond (fs) pulses are successfully used to investigate vibrational dynamics in molecular systems, real time observation of electron motion in all states of matter requires temporal resolution in the attosecond (1 attosecond (asec) = 10−18 s) time scale. During the last decades, continuous efforts in ultra-short pulse engineering led to the development of table-top sources which can produce asec pulses. These pulses have been synthesized by using broadband coherent radiation in the extreme ultraviolet (XUV) spectral region generated by the interaction of matter with intense fs pulses. Here, we will review asec pulses generated by the interaction of gas phase media and solid surfaces with intense fs IR laser fields. After a brief overview of the fundamental process underlying the XUV emission form these media, we will review the current technology, specifications and the ongoing developments of such asec sources

Transverse Electromagnetic Mode Conversion for High-Harmonic Self-Probing Spectroscopy

We report on high-order harmonic (HHG) two-source interferometry (TSI) in molecular gases. We used a 0-π phase plate to create two bright spots at the focus of a lens by converting a Gaussian laser beam into a TEM please define 01 Transverse Electromagnetic Mode. The two bright foci produce two synchronized HHG sources. One of them is used to probe on-going dynamics in the generating medium, while the other serves to heterodyne the signal. The interference of the emissions in the far–field gives access to the phase difference between the two sources. In self–probing HHG phase spectroscopy, one of the two sources is used as a reference while the other one probes some on goin dynamics in the generating medium. We first compute overlap integrals to investigate the mode conversion efficiency. We then establish a clear relation between the laser phase-front curvature and the far-field overlap of the two HHG beams. Both Fresnel diffraction calculations and an experimental lens position scan are used to reveal variations of the phase front inclination in each source. We show that this arrangement offers λXUV100 precision, enabling extremely sensitive phase measurements. Finally, we use this compact setup for TSI and measure phase variations across the molecular alignment revival of nitrogen and in vibrating sulfur hexafluoride. In both gases, the phase variations change sign around the ionization threshold of the investigated molecule