Coincidence detection of inharmonic pulses in a nonlinear crystal

Two trains of light pulses at periods that are equally shifted from the harmonics of a missing fundamental are combined in a nonlinear crystal. As a result of a noncollinear phase-matched second-order nonlinear generation, a new train of pulses is obtained. When the temporal width of the input pulses is large, the frequency of the resulting pulse train follows the observations from classical experiments on the perception of virtual pitch by the brain. On the other hand, when the width of the input pulses is small, the generated pulse train exhibits much lower frequencies, analogous to those observed in the motor neural system. Our experimental setup allows us to explore, systematically and continuously, the transition between these two regimes, while at the same time to demonstrate that the functionalities that have been observed in the nervous system are similar to the ones we observe from coincidence detection in quadratic nonlinear systems.