Development of the crystal growth and application of the novel, wide-bandgap, non-oxide nonlinear optical crystal BaGa4S7 in optical parametric amplifiers

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This project is devoted to the development of the crystal growth, optical characterization and applications of one recently discovered nonlinear optical crystal, BaGa4S7 or shortly BGS, transparent in the mid-IR up to about 12 micron. Crystal growth by the vertical Bridgman-Stockbarger method with oriented seeds and increased temperature gradient will ensure larger sizes in the desired crystallographic orientation for phase-matching while implementation of the Horizontal Gradient Freeze technique will reduce the concentration of defects and the residual absorption, improving the damage resistivity of BGS. Post-growth treatment (annealing) will be carried out in both cases and the residual losses will be quantified by absorption measurements. The optical damage threshold of the newly grown crystals and the Kerr coefficient will be measured with ultrashort (ps and fs) pulses at 1.03 micron under similar conditions as in the nonlinear frequency down-conversion experiments envisaged. Extraordinary properties of BGS, such as the extremely wide band-gap (3.54 eV, i.e. in the UV) for a chalcogenide (non-oxide) material and the resulting high damage threshold and low group velocity mismatch and dispersion enable some unique applications with short and ultrashort laser pulses to convert the wavelength of widely spread and power-scalable diode-pumped laser sources operating in the 1 micron spectral range to the mid-IR above 5 micron. Optical parametric amplification (OPA) and in particular chirped-pulse OPA will be studied with BGS pumped by 1 ps long pulses and seeded by femtosecond white light continuum to generate idler pulses with unprecedented energy, peak and average (of the order of 1 W) powers near 8.7 micron at a repetition rate of 100 kHz, interesting for strong-field physics and attoscience. The appr. 100 fs long, few-cycle mid-IR pulses will be intrinsically carrier-envelope-phase stabilized. While such short pulses will be achieved employing the stretching-compression concept, also pure OPA will be investigated at sub-ps pulse durations because this particular magic idler wavelength corresponds to equal group velocities of all the three, pump, signal, and idler interacting waves. The availability of large size optical elements of BGS with low residual losses and uniform quality is a prerequisite for reduction of the pump threshold and increase of the conversion efficiency in optical parametric oscillators (OPOs) pumped again near 1 micron by ns pulses and generating idler pulses of yet higher (on the mJ level) energy. The demonstration of damage-free long-term operation of such devices at a repetition rate of 100 Hz will open the way for their commercialization and application in minimally invasive laser surgery near 6.5 micron.