Introduction to two-photon absorption
Two-photon absorption means that generally electrons must absorb an energy equivalent to the difference between these two energy levels in order to transition from a low energy level to a high energy level.
Two-photon absorption occurs mainly at the focal point of the super-strong laser generated by the pulse laser. The laser intensity in other places on the optical path is not sufficient to produce two-photon absorption. Due to the longer wavelength and lower energy used, the corresponding single-photon process It cannot happen, therefore, the two-photon process has good spatial selectivity and less damage to the sample.