A power-squared sensor for two-photon spectroscopy and dispersion of second-order coherence

A method using second harmonic generation from potassium dihydrogen phosphate (KDP) powder was developed and used as a normalizing reference to obtain accurate relative two-photon fluorescence excitation spectra of naphthalene and several related compounds in the range 550–650 nm. The method removes almost all uncertainty in matching overlapping spectra obtained from different dyes when using a dye laser pumped by a pulsed YAG laser second harmonic. The resulting naphthalene spectrum, which was corrected for the dispersion of the second harmonic generation efficiency, differs only slightly from most previously reported two-photon spectra which used linearly responding references. The distortions introduced in the latter method during single dye scans due to the increase of second-order coherence in the low gain extremes of the tuning curve were verified as resulting mainly from shortening of the pulse width. Spatial changes in the beam profile as a function of wavelength were found to make a smaller, but significant contribution to the dispersion of the second-order coherence.