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

Subject Area Experimental Condensed Matter Physics
Synthesis and Properties of Functional Materials
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Term from 2018 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 392070342
 
Final Report Year 2023

Final Report Abstract

This project was devoted to one representative of the newly developed class of Ba-containing, non-chalcopyrite symmetry, chalcogenide nonlinear crystals applicable in the mid-IR part of the optical spectrum. The main activities encompassed characterization of relevant properties and demonstration of specific advantages in parametric frequency down conversion from laser sources operating in the 1-µm spectral range to the mid-IR beyond 5 µm. The characteristics studied include transparency and bandgap, refractive indices and birefringence versus wavelength and temperature, components of the second order susceptibility tensor with their relative signs, thermo-mechanical properties such as hardness, Young’s modulus, anisotropic thermal expansion and anisotropic thermal conductivity, and nonlinear refractive index. On the basis of realized three-wave interaction processes, Sellmeier equations and thermo-optic relations were fitted to better describe the phase-matching properties. Interesting configurations for non-critical, 90° phase-matching and broadband parametric amplification have been identified. Specific approaches were developed to reveal the relative signs of the nonlinear coefficients in low symmetry crystals, essential for reproducible orientation in order to ensure maximized effective nonlinearity for a given parametric process. BaGa4S7 was applied for the first time in a femtosecond optical parametric amplifier at 100 kHz seeded by white light continuum and pumped by an Yb-laser system at 1028 nm. Sub-4 cycle, phase-stable pulses have been directly generated near 10 µm, making profit from the superior dispersive properties of the nonlinear crystal for this parametric process. To achieve high single pulse energy in the nanosecond regime, an original cascaded optical parametric oscillator scheme with intracavity pumping of the second stage was employed, starting with a Nd-laser system at 1 µm and 100 Hz. Maximum energies exceeding 1 mJ at 10 ns pulse duration were obtained with tunability in the 6.62 - 11.34 µm range, and narrow band operation was realized by type-2 phase-matching and a volume Bragg grating. The extensive dissemination policy contributed greatly to increase the visibility of this new class of non-oxide crystals for nonlinear frequency conversion in the mid-IR, which are free of the main technological issues with the well-known chalcopyrites, such as chemical instability and the requirement for post growth annealing, making them attractive for future commercialization.

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