Femtosecond spontaneous parametric upconversion and downconversion in a quadratic nonlinear medium

Spontaneous parametric upconversion (SPUC) and spontaneous parametric downconversion (SPDC) have been observed in the β-barium borate (BBO) crystal, and their mechanisms are experimentally and theoretically investigated. SPUC, tuned from 530 to 600 nm with a FWHM of about 25 nm, can be attributed to the sum frequency between the quantum noise and the fundamental laser pulse in the type II phase matching condition (e + o →e). SPDC, tuned from 480 to 520 nm with a FWHM of about 15 nm, can be attributed to the difference frequency between the quantum noise and the fundamental laser pulse in the type I phase matching condition (e→o + o).