Direct third-harmonic generation of <i>β</i>-BBO crystal to generate a tunable ultraviolet laser

Direct third harmonic generation (DTHG) is a third-order nonlinear process in which energy and momentum are exchanged between different modes of the optical field without being restricted by the crystal symmetry. For the first time (to our knowledge), tunable ultraviolet (UV) output is achieved in the birefringent crystal via DTHG. Using a 1030-nm femtosecond laser source to pump a <i>β</i>-BaB₂O₄ (<i>β</i>-BBO) crystal, a broadband supercontinuum spectrum of 400-1600 nm is generated by the self-phase modulation effect. Then, by rotating this <i>β</i>-BBO crystal to satisfy different phase-matching (PM) conditions, a UV tunable output of 300-415 nm is obtained via the DTHG process, with a maximum optical conversion efficiency of 1.03% at 340 nm. Under this condition, the third-order nonlinear susceptibility <i>χ⁽</i>³⁾ is determined to be 88 pm²/V². This method can directly convert near-infrared laser into UV spectral region in one step and can be used as an extremely convenient and effective frequency conversion method to obtain broadband tunable UV lasers.