Femtosecond mid-infrared filamentation in gases and solids

We propose an experimental investigation of femtosecond laser filamentation in gases and solids in the mid-infrared (mid-IR) spectral range. Extension of femtosecond filamentation into the mid-IR is a completely new research field which opens groundbreaking perspectives for many important applications such as high-resolution standoff spectroscopy of the atmosphere, strong field (attosecond) spectroscopy of atoms and molecules, generation of tunable high power terahertz emission, etc. Currently, only the group from the Vienna University of Technology has a high power mid-IR femtosecond laser source capable of achieving filamentation in gases. This places the applicant in a unique position to perform groundbreaking research in this project. One of the focuses of the project is a complete spectral, spatial and temporal characterization of the super-continuum generated in the mid-infrared filaments. An unprecedented multi-octave spanning continuum, which can be generated in mid-IR filaments, opens the possibility to produce few-optical- cycle laser pulses in different spectral regions ranging from the visible to the mid-IR. One of the main physical phenomena, underpinning the femtosecond filamentation and different applications of laser filaments, is gas ionization and plasma formation. Measurement of plasma parameters in the mid-infrared filament is one of the tasks of the project. It will provide insight into the complex physics of mid-IR filamentation and reveal the role of plasma in the filament formation and dynamics. Filamentation in solids with mid-IR laser wavelengths proceeds in a very new regime characterized by anomalous group-velocity dispersion, where nonlinear spatial dynamics is strongly coupled to temporal solitonic-like transformations of the laser field. In contrast to near-IR case, it also provides unique possibilities for investigations of photoionization and defects formation in the bulk of solids in the tunnel regime. Success of the project will have strong impact on different research fields related to ultrafast nonlinear optics and plasma physics. Knowledge about the spectral properties of the ultrabroadband emission generated in the mid-infrared filament will enable designing of a unique multi-color optical setup for time-resolved pump-probe spectroscopy. The possibility to reach KeV-energies of electrons in the process of high-order harmonics generation with long wavelength lasers makes few-optical-cycle mid- infrared laser pulses, enabled via spectral broadening in the mid-IR filament, of great importance for table-top sources of attosecond X-ray pulses. The hot and highly anisotropic plasma expected in the mid- IR filaments is a very attractive prerequisite for designing high efficient sources of terahertz radiation and a very important factor in atmospheric plasma-chemistry.

Femtosecond filamentation is a fascinating phenomenon of extreme nonlinear optics discovered around 20 years ago as a byproduct of the development of powerful ultrashort- pulse laser systems. Observation of this phenomenon and the subsequent experimental work in the near-infrared spectral range became only possible after the invention of chirped- pulse amplification, awarded the Physics Nobel Prize in 2018, and implementation of this amplification method in Ti:sapphire laser systems operating at the wavelength of 800 nm. By contrast, the Austrian team at the Photonics Institute of TU Wien developed intense ultrashort pulse laser sources operating at a considerably longer wavelength of 4 m and remains the world leader in terms of the peak power achieved in the mid-infrared spectral region. In the framework of this stand-alone FWF project, our group seized the advantage of its unique proprietary laser source and was able to pursue our core interest in strong-field processes. The main proposed and successfully achieved goal of this research was to investigate the quadratic wavelength scaling effect on various filamentation processes. The project has led to a number of breakthrough developments in the field of femtosecond filamentation. We have succeeded in identifying and experimentally and theoretically characterizing a novel regime of pulse self-compression, assisted by filamentation, in the so-called solitonic propagation regime in anomalously dispersive solids and in gases. This has led to the generation of record-breaking nearly 1-Terawatt peak power pulses in the targeted mid- infrared spectral region. The main novelty of the discovered regime is the possibility to operate solid materials at the peak power levels exceeding by three orders of magnitude the critical power of self-focusing. Under normal circumstances at shorter wavelengths, beams exceeding this critical power would break up into multiple filaments and loose properties of a single coherent laser beam. In the new regime developed in this project, it became possible to circumvent such a breakup through an interplay of dispersion and nonlinearity, opening an extremely attractive way to scale peak power through filamentation. We are also reporting several exciting breakthroughs in the formation and transmission of high-energy filaments through ambient air. This is particularly important because the broader international research community anticipates numerous advantages from the development of longwave beams as these, in principle, should allow the delivery of very high-energy pulses over considerable distances in a very tight spatial beam, without losing the beam quality and brightness to beam breakup and diffraction. To this end, we were able to demonstrate formation and propagation over distances of around 10 m, limited by the size of the laser lab, of temporally and spatially invariant light bullets in various characteristic regimes. Very significantly, we were able to show that the actual mechanisms responsible for the formation and sustained propagation of such bullets deviate very significantly from the theoretical predictions of leading international experts and that a host of additional effects related to linear and nonlinear interaction with air molecules must be taken into account. Also critically important for further development of the broader research field are our results on femtosecond filamentation in air at 4 m that show no apparent involvement of plasma. Although additional studies using complementary methods will still be necessary after this project to confirm the absence of transient plasma effects, it is already clear from our pioneering studies that mid-infrared filamentaiton can be realized using entirely different physical mechanisms compared to the well-documented filamentation behavior in the near- infrared spectral range.