Exploring Novel Parametric Phenomena for Tunable THz-Visible

Parametric generation and amplification in non-centrosymmetric transparent second-order nonlinearity crystals is well known since 1962, shortly after the dawn of the laser era. Yet it took until the early 1990s before parametric devices began to supplant dye lasers as sources of wavelength-tunable pulses owing to the progress in the solid- state ultrafast laser development. However, the tunability of nonlinear-crystal-based optical parametric generators/amplifiers (OPG/OPA) is still limited due to a limited set of pump wavelengths available and, more significantly, due to the narrow transparency windows of crystalline media. In addition, one of the problems for scaling up the intensity of parametrically generated pulses is optical damage of nonlinear crystals. Ultrafast lasers producing femtosecond pulses with tens and even hundreds of mJ of energy are currently being developed for a variety of applications, such as ultrafast spectroscopy, high-field and attosecond physics, nuclear energy physics, environmental and atmospheric sensing, public security and defense. These and other applications demand broadly tunable femtosecond OPG/OPAs in the new intensity and wavelength regimes that require alternatives to the established second-order nonlinear crystal OPAs. Driven by the urgent necessity do address the lack of adequate multi-mJ femtosecond tunable pulse generation technology, this project aims to explore new exciting opportunities arising from four-wave mixing (4WM) in gases and develop a high-intensity source tunable in an unprecedented spectral range- from the THz range to the visible. The main advantages of using third-order-nolinearity centro-symmetric gases over nonlinear crystals are as follows: -far superior intensity handling and absence of optical damage; -far broader transparency than solids; -very low dispersion enabling extremely broadband phasematching and tunability. The disadvantage of having a low effective nonlinearity can be readily overcome by using optical filaments and/or gas-filled waveguides in order to increase the nonlinear interaction length to tens of centimeters. So far mainly 4WM-based parametric generation in gas filaments/waveguides the VUV region, inaccessible for nonlinear crystal, has been targeted by several groups [OL 24, 1565 (1997); OL 32, 2481 (2007)]. The goal of this project is to achieve tunability in the red spectral wing, extending it into the THz regime. The project encompasses three main objectives: -development of a unique cryogenically cooled directly diode-pumped 10-mJ ~150-fs Ytterbium booster as a pump source for 4WM experiments; -extension of the 4WM visible few-cycle pulse OPA demonstrated in the J range [PRL 97, 023904, (2006)] into a multi-mJ pump range; -demonstration of the world`s first seeded THz pulse parametric amplifier (THzPA) and investigation of THz wave guiding in the surface plasmon mode regime around the optical filament. Whereas the 4WM OPA would represent an (improved) addition to the existing crystal-based solutions for visible tunability, the seeded single-cycle THzPA promises a really revolutionary step overtaking the intensity records currently held by semiconductor THz emitters and offering a virtually unlimited way to scale the THz pulse energy without increasing the beam cross-section. A particularly attractive feature of this THz OPA is the pin-point delivery of the THz beam facilitated by the guiding laser beam. Projecting experiments with the THzPA source at TU Vienna in the future beyond the scope of this two-year project, such powerful shaped / compressed THz pulses can be used for ground-state time-resolved rotational spectroscopy in small molecules, investigation of collective modes in proteins and of electronic resonances in semiconductors and their nanostructures.