Interconversion of Optical to Terahertz Pulses in Nonlinear Beta Barium Borate Crystal

ABSTRACT Low‐temperature β‐phase barium borate (β‐BBO) is an important nonlinear crystal, widely utilized for optical second‐harmonic (SH) generation. Recent re‐measurements of refractive indices in the terahertz (THz) range reveal that β‐BBO is a positive uniaxial crystal. Here, we report THz optical rectification (OR) and electro‐optic (EO) sampling responses in a 0.5‐mm‐thick β‐BBO crystal. Two temporally separated, polarization‐tunable THz pulses were observed and identified as forced and free THz pulses, respectively. We further demonstrate that distinct effective second‐order susceptibilities enable phase matching for SH and THz generation. Additionally, β‐BBO can be employed to probe intense THz‐induced birefringence in the probe laser pulses. This study establishes β‐BBO as a reliable material for THz emission and characterizes its THz EO sampling performance for high‐field THz detection.