Third-order nonlinear optical properties of calcite crystal in UV region

Abstract

Different from cascaded second-order nonlinear frequency conversion which is a common way to obtain ultraviolet (UV) laser only through noncentrosymmetric crystal, the optical properties of the phase-matched direct third-harmonic generation (THG) in the centrosymmetric calcite crystal was studied. A femtosecond laser was taken as the pumping source in the type-I (ooo→e) and type-II (ooe→e) phase-matched experiment, and a nanosecond laser was also used for comparison. Polarization angle ρ, phase-matching angle θ and azimuth angle φ in direct THG were determined and verified. Strong third-harmonic at 266 nm was observed in calcite crystal pumped by femtosecond laser. Compared with type-I phase-matched, Type-II was more efficient for energy conversion. The shortest harmonic wavelength obtained was about 213 nm, which was close to deep-UV region. The maximal output energy was 19.3 μJ (average power, ～19.3 mW) and the maximal conversion efficiency was 1.25% at 266 nm, which was the highest conversion efficiency at present. These results may indicate a wider application field for centrosymmetric crystals and another method to produce laser in UV and even deep-UV region.