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We analytically describe the strong-field light-electron interaction using a quantized coherent laser state with arbitrary photon number. We obtain a light-electron wave function which is a closed-form solution of the time-dependent Schrödinger equation (TDSE). This wave function provides information about the quantum optical features of the interaction not accessible by semi-classical theories. With this approach we can reveal the quantum optical properties of high harmonic generation (HHG) process in gases by measuring the photon statistics of the transmitted infrared (IR) laser radiation. This work can lead to novel experiments in high-resolution spectroscopy in extreme-ultraviolet (XUV) and attosecond science without the need to measure the XUV light, while it can pave the way for the development of intense non-classical light sources.

We demonstrate a tool for quantitative measurements in the extreme ultraviolet (EUV) spectral region measuring spatially resolved atomic ionization products at the focus of an EUV beam. The ionizing radiation is a comb of the 11th–15th harmonics of a Ti:Sapphire femtosecond laser beam produced in a Xenon gas jet. The spatial ion distribution at the focus of the harmonics is recorded using an ion microscope. Spatially resolved single- and two-photon ionization products of Argon and Helium are observed. From such ion distributions single- and two-photon generalized cross sections can be extracted by a self-calibrating method. The observation of spatially resolved two-EUV-photon ionization constitutes an initial step towards future single-shot temporal characterization of attosecond pulses.

High-order harmonics in the extreme-ultraviolet spectral range, resulting from the strong-field laser-atom interaction, have been used in a broad range of fascinating applications in all states of matter. In the majority of these studies the harmonic generation process is described using semi-classical theories which treat the electromagnetic field of the driving laser pulse classically without taking into account its quantum nature. In addition, for the measurement of the generated harmonics, all the experiments require diagnostics in the extreme-ultraviolet spectral region. Here by treating the driving laser field quantum mechanically we reveal the quantum-optical nature of the high-order harmonic generation process by measuring the photon number distribution of the infrared light exiting the harmonic generation medium. It is found that the high-order harmonics are imprinted in the photon number distribution of the infrared light and can be recorded without the need of a spectrometer in the extreme-ultraviolet.

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The development of ultra-short intense laser sources in the visible and extreme ultraviolet (XUV) spectral range led to fascinating studies in laser-matter interactions and attosecond science. In the majority of these studies the system under investigation interacts with a focused laser beam, which ionizes the system. The ionization products are usually measured by devices, which spatiotemporally integrate the ionization signal originating from the entire focal area, discarding in this way valuable information about the ionization dynamics that take place in the interaction volume. Here, we review a recently developed approach in measuring the spatially resolved photoionization yields resulting from the interaction of infrared (IR)/XUV ultra-short laser pulses in gas phase media. We show how this approach enables a) the in-situ focus diagnostic, b) quantitative studies of linear and non-linear ionization processes in the IR/XUV regime, c) single-shot XUV-pump-XUV-probe studies and d) single-shot 2 nd -order XUV autocorrelation measurements. The article has been published in

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Ο σκοπός αυτής της διατριβής, είναι η κβαντική–οπτική περιγραφή της αλληλεπίδρασης ισχυρού πεδίου λέϊζερ–ατόμου. Χρησιμοποιώντας μία πλήρη κβαντομηχανική προσέγγιση (πιο συγκεκριμένα, θεωρώντας σύμφωνη κατάσταση για το πεδίο του λεϊζερ) για την περιγραφή των αρμονικών υψηλής τάξης, παραγώμενες από την αλληλεπίδραση ενός ισχυρού υπέρυθρου πεδίου λεϊζερ με άτομα, καταγράψαμε το φάσμα των αρμονικών χωρίς τη χρήση συμβατικού φασματόμετρου ακραίας υπεριώδους φασματικής περιοχής. Αυτό επιτεύχθηκε, με τη χρήση στατιστικής φωτονίων στην κατάσταση του υπέρυθρου φωτός που εξέρχεται από το αέριο μέσο. Βρέθηκε ότι η ακτινοβολία έχει μια ξεχωριστή μη–κλασσική συμπεριφορά, καθώς η κατανομή φωτονίων αποτελείται από μία σειρά καλά καθορισμένων κορυφών, οι οποίες αντιστοιχούν στο φάσμα των αρμονικών υψηλής τάξης. Αυτή η εργασία συνθέτει τα πεδία της φυσικής ισχυρού πεδίου και κβαντικής οπτικής και ανοίγει το δρόμο για μελέτες στην επιστήμη των αττοδευτερολέπτων, χρησιμοποιώντας συμβατικά διαγνωστικά υπέρυθρης ακτινοβολίας.