On the Temporal Response of Instantaneous Two-Photon Processes

With the advent of shorter and shorter laser pulse resolution, the significance of two-photon transitions without an intermediate-state that shows all two-photon characteristics have grown in importance. We explore such systems in a theoretical context with experimental validation. We show that the case of a temporal scan between two laser pulses gives rise to the two-photon signal. Two-photon processes progress via intermediate virtual states and we have earlier shown that the virtual state model of multiphoton absorption is mathematically equivalent to the single-photon absorption model of an appropriate wavelength and phase. We probe the nature of the intermediate physical state of two-photon absorption by performing a temporal scan of the delay between two pulses generating two-photon process. We demonstrate the possibility of two-photon non-resolvable virtual state transitions when the two-photons are simultaneously incident on a two-level system with only two-photon transitions possible. These transitions continue to show the well-known characteristics of two-photon processes in terms of the control parameters of phase, polarization, and intensity. We note that, the significance of such transitions is that if a temporal scan is done on the two laser pulses causing the transition, there is no intermediate state whose signatures can be observed.