In traditional linear spectroscopy, a light pulse interacts with a sample, causing it to absorb or emit light at specific frequencies. However, in nonlinear optical spectroscopy, the light pulse is so intense that it induces nonlinear effects, such as changes in the sample’s refractive index or absorption coefficient.
Nonlinear optical spectroscopy experiments typically involve the use of ultrafast lasers, which provide high-intensity light pulses with durations of femtoseconds to picoseconds. In traditional linear spectroscopy, a light pulse interacts
where E is the electric field, and χ(1), χ(2), and χ(3) are the linear, second-order nonlinear, and third-order nonlinear susceptibilities, respectively. In traditional linear spectroscopy