Efficient frequency conversion for cubic harmonic generation at 266  nm in centrosymmetric α-BBO crystal

Direct third-harmonic generation (THG) is a third-order nonlinear process without the restriction to the symmetric characteristic of crystals. It is of great interest for setting up a χ⁽³⁾ optical parametric oscillator which can be used in multiphoton quantum correlation. To obtain pure and strong THG, we proposed to elect the centrosymmetric crystal with delocalized π bond as the nonlinear media during the frequency conversion process. An unprecedented cubic harmonic energy 37.6 μJ (average power ∼37.6  mW, conversion efficiency ∼2.5%) at 266 nm was generated in α-BBO crystal (not the β-BBO), indicating direct THG in practical application. These results also demonstrated a succinct and efficient way to generate deep-UV laser and another development direction of nonlinear crystals in UV region.