Two-time and two-frequency correlators

The main problem of 2P physics under incoherent continuous pump is to determine the most suitable quantity to characterize the 2P processes against the 1P. Therefore, the interest of two-time and two-frequency correlation functions in this system is greater than in any other. Here, these functions find the practical application of quantifying the quality of the 2P generation vs 1P. This information is not in $S^{(2)}(\omega)$ but rather in the frequency resolved two-photon counting, given by $S^{(2)}(t_1,\omega_1;t_2,\omega_2)$ in Eq. (2.110). Computing and understanding this general function is one of my main present objectives.