High intensity direct third harmonic generation in BBO

Generation of the third harmonic (THG) in a single process is an idea that has been around since the inception of nonlinear optics. Efficiencies for THG in solids have been limited to less than a percent with the highest conversion achieved using BBO. It has been suggested that unphasematched second-order processes can contribute significantly to THG, similar to the use of cascaded second order processes giving rise to an effective nonlinear refractive index. However, experimental uncertainties in efforts to assess this contribution by measuring conversion efficiency were too large to determine the relative contributions of the several processes. We were able to vary the azimuthal angle in BBO and measured directly the relative contributions of X(2) and X(3) to THG in BBO for both type I and type II phasematching.