sentences:  Another approach to improving media sensitivity has been to employ resonant two-photon absorption (also known as "1+1" or "sequential" two–photon absorbance).Nonresonant two-photon absorption (as is generally used) is weak since in order for excitation to take place, the two exciting photons must arrive at the chromophore at almost exactly the same time.This is because the chromophore is unable to interact with a single photon alone.