Mid-IR Single-Cylce Nonlinear Optics

Attosecond science develops tools and methods to observe and control electronic dynamics in atoms, molecules and other strongly confined quantum systems, which naturally occur on an attosecond (1 as = 10-18 s) time scale. The base technology of this field is certainly the generation of waveform-controlled laser pulses in the strong-field regime, i.e. with an electric field comparable to that binding valence electrons to their atoms. Shortening the laser pulse envelope to a duration close to period of its carrier wave leads to the extreme regime of single- cycle nonlinear optics, where tunnel-ionization launching the laser-matter interaction can effectively be confined to a single attosecond time window around the strongest field crest. To date, this single-cycle regime of nonlinear optics is accessible with visible to short-wavelength infrared wavelengths (0.7-1.8 m). Pushing to ever longer carrier wavelengths, i.e. towards the mid-IR region (2.46 m), is one of the prevailing current trends in attosecond science due to the much higher achievable kinetic energies of the laser-field-driven electrons and the resulting opportunity to generate attosecond x-ray pulses with much higher photon energies and ever shorter duration. In this project, we will apply a method for the generation of singe-cycle laser pulses that stands out from the commonly used methods for laser post-compression due to its astonishing simplicity and its scalability to the mid-infrared region. This method, pulse self-compression in a specialty Kagome-type hollow-core photonic crystal fiber with a large hollow core filled with a noble gas as a nonlinear optical medium, has been recently demonstrated in our lab to let ultra- short short-wavelength infrared pulses with energies of several tens of microjoules undergo, in a single step, a 20-fold nonlinear self-compression to reachover the whole beam diametera sub-cycle duration and gigawatt peak power at the fiber exit. We will scale this method to produce and characterize the first ever high-quality single-cycle mid-infrared pulses. We will then optimize the output power and stability of these pulses, making them ready to drive the generation of attosecond x-ray pulses. Based on this milestone, we will develop and characterize a source of isolated attosecond pulses with at least 200 eV photon energies. This source will reach into a hitherto inaccessible spectral region for isolated attosecond pulses, enabling new applications involving x-ray absorption edges. Furthermore, the attosecond source will become unprecedentedly compact and simple, enabled by the high laser intensities reached already in the fiber core and a clever differential pumping scheme.

Attosecond physics develops tools and methods to observe and control the dynamics of electrons in atoms, molecules and other strongly localized quantum systems. This dynamics typically proceeds on attosecond time scales (1 as = 10^18 s). The principle technology in this research field is the generation of laser pulses with a controlled waveform in the so-called strong field regime, implying that the electric field of the pulse is similar in strength to the field binding of the valence electrons to their atoms. Shortening the duration of the pulse envelope to the duration of a single oscillation period of its carrier wavelength ushers the regime of single-cycle nonlinear optics. This regime confines the process of tunnel ionization, corresponding to the first step in the light-matter, to a well-defined attosecond time interval near the peak of the strongest half-cycle of the pulse electric field. Previously, such a single-cycle regime of extreme nonlinear optics was only reachable in the visible or short infrared (IR) wavelength ranges (0.7-1.8 m). The push toward increasingly longer carrier wavelengths-into the mid IR range (2.4-6 m)-is one of the dominant trends in the technology of ultrashort laser pulses. This project relied on a combination of methods for the generation of extremely short few-cycle laser pulses. First, employing the method of optical parametric frequency conversion, the pulse wavelength was shifted from the near-IR laser wavelength into the mid IR range. Two different laser systems were expanded during the project to reach, step by step, high pulse energies: first, at the wavelength of 3.2 m by using KTP/KTA crystals and, afterwards, to shift the central wavelength further into the 5-6 m by implementing parametric conversion in ZGP crystals. Subsequently, using the method of pulse post compression in various new types of hollow waveguides, themed-IR pulses were compressed to yield peak powers of several 100 GW which are record-level in this wavelength range. These newly developed laser sources were employed to examine previously inaccessible regimes of temporal and spatial evolution of strong laser fields in air and solids. As part of this research, new mechanisms of nonlinear-optical self-compression and the possibility for diffraction-free propagation of energetic mid-IR light bullets in a gas were identified. Convincing advantages of using mid-IR pulses for driving strong-field dynamics were demonstrated in the experiments on highly efficient emission of secondary coherent radiation in the terahertz range from a mid-IR-driven laser plasma.