Title: Sub-cycle control of light waves
Language: English
Authors: Balčiūnas, Tadas 
Qualification level: Doctoral
Keywords: Carrier envelope phase; ultrashort laser pulses; high-field phenomena
Advisor: Baltuska, Andrius 
Issue Date: 2015
Number of Pages: 129
Qualification level: Doctoral
Attosecond physics is becoming an established methodology for directly probing the fastest processes in nature. Attosecond optics is enabled by strong field phenomena and relies on ionization and control of electron wave-packets in a very rapidly oscillating optical field. Consequently, strong field ionization is the starting point of all typically studied strong field phenomena, such as above threshold ionization (ATI), coherent and incoherent Xray generation and THz emission from laser plasmas. When a strong periodic driving field suppresses the binding potential of an electron in an atom, molecule or crystal, the electron can tunnel out to become a quasi-free particle within a tiny fraction of the driving optical cycle of the laser pulse twice per optical cycle near the peak of each prominent half cycle. This thesis describes novel tools and methods for controlling the ionization via shaping of the light waveforms on a sub-cycle level. For the measurement of the ionization dynamics optical methods are developed that rely on the detection of secondary radiation emitted in the THz and XUV spectral ranges. As suggested by Brunel two decades ago, the sharp bursts of electrons ejected into the continuum at the peaks of optical half cycles correspond to many orders of optical harmonics of the driver frequency. For a symmetric optical cycle, the ionization bursts of equal strength occur twice per optical cycle leading to the emission of odd-numbered harmonics spaced at twice the driver frequency. By adding a field that is twice or half the frequency of the fundamental field it is possible to break the symmetry of the positive and negative field crests enabling the emission of even harmonics. A particularly interesting is the lowest-frequency emission peak that can be interpreted as a -zeroth- order harmonic sideband corresponding to the THz wave emission. In this thesis a novel scheme is described where incommensurate frequency two-color field can generate temporally modulated electric micro-currents in plasma which allow tuning of the central frequency of the THz sideband. The Brunel model was initially proposed to explain the high-order harmonic generation (HHG) in the XUV spectral range. However, the key process behind the emission of these very high order harmonics, well above the ii ionization potential of an atom, is the recollision with the parent ion. In the so-called simple man's model, the electron ejected into the continuum is accelerated by the laser field and upon its return to the parent ion a high energy photon is emitted. Optimization of HHG is a very active field of research, as it is a promising table-top source of coherent XUV and soft X-Ray radiation packed in extremely short pulses. However the main issue is low generation efficiency. The extremely short time window of this process that takes place within a fraction of half-cycle of the optical field, makes it difficult to control and tailor HHG process. We describe a method for the generation of intense waveforms that allows controlling the electric field on the attosecond time scale and it is based on Fourier synthesis of the light wave composed from several infrared color pulses. These cycle-sculpted waveforms are used for controlling the ionization bursts and subsequent trajectory of the electrons that are emitted in the continuum in a way that they are most efficiently accelerated and return to the ion therefore improving the efficiency of HHG and extending the cut-off toward higher photon energies. These multi-color waveform shaped pulses allow high degree of control over the HHG process, but it is repetitive and the attosecond pulse emission occurs many times per laser pulse. In order to limit the attosecond XUV bursts to just one per laser pulse and still be able to control the field of the pulse, a single cycle ultrashort pulses are necessary for driving HHG. We describe a novel pulse self-compression scheme that allows 20-fold shortening of pulses in the IR spectral range down to sub-cycle duration. The scheme is based on a Kagome lattice hollow core photonic crystal fiber. Upon nonlinear propagation of the pulse in the fibre due to anomalous dispersion of the waveguide that compensates the positive chirp induced by the self phase modulation, the pulse shortens by itself in a very compact setup and allows high degree of integration into a HHG setup. The main results presented in this thesis can be summarized as follows: A concept of multicolor driver pulse synthesis based on spatial and temporal superposition of waves from an optical parametric amplifier is proposed and demonstrated. Both the absolute (CEP) and relative phase of the constituent pulses at every carrier frequency is controlled. To this end, both an active phase-locking of the femtosecond pump laser and the passive phase lock resulting from the operation of a white-light-seeded OPA are employed. Continuous tunability of THz emission from a two-color-driven laser plasma is theoretically predicted and experimentally verified by means of a CEPlocked parametric generator of two incommensurate optical driver frequencies. A unified view on the generation of low-order sideband (THz emission) and higher-order optical sidebands (low-order harmonics emission) from a iii laser-driven plasma is developed based on a fully quantum-mechanical description of continuum-continuum transitions for ionized electrons as an extension of the earlier semi-classical Brunel model. Active CEP stabilization is realized for the first time on a multi-millijoule femtosecond Yb-doped amplifier. A new forward CEP stabilization technique is proposed and demonstrated on a multi-kHz repetition-rate Yb-doped amplifier. A possibility to optimize the HHG average spectral brightness and extend the spectral cut-off by means of engineering the electron trajectory in a multicolor-shaped optical cycle of a linearly-polarized driver pulse has been demonstrated experimentally and confirmed theoretically. Substantial increase of the peak intensity of the corresponding attosecond XUV bursts in the time domain is predicted. An efficiency and bandwidth improving concept for femtosecond OPA driving with shaped femtosecond Yb-pulses is proposed and demonstrated. Based on the phase-only pulse shaping, the technique yields a chirped top-hat pulse for OPA pumping while enabling, simultaneously, a fully compressed pulse required for efficient generation of the white-light seed for the OPA. Generation of externally and self-compressed pulses from a novel type of a gas-filled hollow waveguide - a Kagome-lattice photonic-crystal fiber - was explored as a means of increasing the pulse intensity and reducing the number of optical cycles of the driver laser. A sub-single-cycle transient was generated at the wavelength of 1.7 -m, which enabled XUV emission in a single isolated attosecond burst regime as evidenced by the observation of a continuous XUV spectrum free of harmonic modulation.
URI: https://resolver.obvsg.at/urn:nbn:at:at-ubtuw:1-78343
Library ID: AC12660736
Organisation: E387 - Institut für Photonik 
Publication Type: Thesis
Appears in Collections:Thesis

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