Multi-phonon absorptions of the nonlinear optical barium borate crystal

The nonlinear optical crystal barium borate (BBO) is known to have an IR transmission cutoff of 3.3 μm and multiple absorption peaks starting from 2.1 μm to the cutoff. Despite laser output with wavelength as long as 2.7 μm being achieved in BBO, the absorption mechanisms seem not to have been well understood. By a joint theoretical and experimental investigation, we uncover that the absorptions from 2.1 to 3.3 μm are due to the 3-phonon absorptions of the ordinary (o) ray, while those from 3.3 to 6 μm originate from 2-phonon absorptions of the o-ray. In contrast, BBO is highly transparent up to about 5 μm for the extraordinary (e) ray with weaker 2-phonon absorptions above that. The new understanding may help to extend the useful mid-IR band by exploitation of the wide transparency range of e-ray for BBO in laser generation or detection of short pulses.