Parametric Generation and Amplification of Optical and Terahertz Ultrashort Pulses in Dispersion-Managed Waveguides

In the framework of this joint Austro-Russian project we propose to perform a series of experimental and theoretical investigations aimed at the development of highly efficient parametric generation and amplification of ultrashort pulses in the optical and terahertz (THz) frequency ranges in waveguides with an actively shaped profile of dispersion. Our approach toward achieving high efficiency of parametric frequency conversion is to develop suitable micro-structured (MS) waveguides, also referred to as "photonic crystal" waveguides. The Russian partner will design and manufacture novel types of MS waveguides that should fulfill broadband phase matching conditions - a prerequisite for enhanced conversion efficiency of ultrashort laser pulses - for two types of laser platforms: (a) nJ - J energy level 100-200-fs pulses from Ytterbium fiber lasers systems and (b) J - mJ energy level 5-50-fs pulses from Titanium sapphire solid state laser amplifiers. The Austrian partner will perform experimental research on the newly developed waveguide types using the techniques of spectral interferometry (SI) and variations of frequency resolved optical gating (FROG) in order to fully characterize linear and nonlinear dispersion of the waveguides. By joining the forces in this project, the Russian partner will contribute its unique long-standing expertise in the engineering of photonic crystal structures and the access to the manufacturing capacities thereof, while the Austrian partner will bring in its unique experimental base offering a broad range of femtosecond lasers and the expertise in femtosecond nonlinear frequency conversion. The joint theoretical (Russian partner) and experimental (Austrian partner) studies will aim at (i) finding dispersion management solutions for broadband pulse tunability as well as (ii) for finding solutions for the generation of precisely controlled pulse trains as the result of parametric interactions in the waveguide, with both these issues being eagerly sought after in many real-life applications such as biomedical and chemical imaging, etc. For efficient generation and amplification of high-intensity THz pulses, we will employ the waveguide regime for parametric interaction between a THz pulse and a mJ-level femtosecond optical pump field. We will investigate whether or not optimum phase matching conditions for THz pulse production can be achieved by an interplay of the of the waveguide dispersion, plasma dispersion and plasmonic surface modes that we expect could assist THz wave guiding around an optical filament. Both partners, simultaneously applying for a joint project in their respective countries, have a proven track record of collaboration which has resulted in 7 journal publications since 2005 and numerous conference presentations. This project, if granted, would provide a formal basis for official cooperation, resources to hire personnel at the level of doctoral students and would allow the parties to carry out ground-breaking research that cannot be performed by either side individually.

In fundamental science, femtosecond (1 fs =10-15 s) few-cycle laser pulses are used for time-resolving structural dynamics of molecules and solids, controlling electron trajectories in ionization and recombination processes, acceleration of neutral particles, etc. Broad spectra of few-cycle pulses provide spectral coverage for sensing applications based on high-resolution vibrational spectroscopy and for high-precision atomic metrology based on stabilized frequency combs of mode-locked pulse trains. Techniques for the generation of few-cycle pulses around the fundamental frequencies of femtosecond laser amplifiers in the near-infrared (NIR) range are very mature. Nevertheless, efficient generation of tuneable pulses in the Mid-IR and Terahertz ranges remains very challenging because it requires nonlinear frequency down-conversion. Efforts of numerous research groups pursuing the development of long-wavelength femtosecond lasers are motivated by the demand for coherent sources needed for spectroscopic fingerprinting of molecules through vibrations (in the Mid-IR) and rotations (in THz). In strong-field applications, the interest in long-wavelength driver pulses is motivated by a series of wavelength dependent scaling laws and wavelength controlled regimes of photo-ionization. In the completed bilateral project, the joint team from PIVUT (Vienna) and MSU (Moscow) worked on exploring new regimes for nonlinear optical generation of long- wavelength pulses. The main aspects addressed during the lifetime of the project are related to a) the microscopic nature of the nonlinearity underlying the frequency conversion process and b) improvement of conversion efficiency through the use of (self-) waveguiding geometry. We explored experimentally the recent suggestion that non-scattering trajectories of laser field ionized electrons in an asymmetric laser field lead to efficient THz generation. The key achievement of the project was the demonstration of frequency-tuneable THz pulses that resulted from mixing two input laser fields with incommensurate optical frequencies, whereby the superposition of the two input oscillating fields allows controlling the instants of ionization and trajectories of the released electrons. To fulfill the technical prerequisites for THz frequency tuning, we successfully solved several technical challenges associated with the generation and mutually locking the so-called carrier-envelope phases of the two input optical fields of different colours. Investigating guiding geometry, we succeeded in the first demonstration of femtosecond filaments in gas using few-cycle Mid-IR fields. We also demonstrated that accurate description of lower-order harmonic generation in air requires inclusion of higher than conventionally anticipated optical nonlinearities. All experimental work with high-intensity pulses was performed in Vienna, the theoretical modelling was carried out by the collaborators in MSU.