Two-photon transition (3S-5S,λ=6022A) in Na atom was investigated experimentally using a pulsed dye laser(Rhodamine-6G). Two-photon transition was identified by observing UV fluorescence.
When the circularly polarized light was used for pumping, only two photons of opposite direction was absorbed according to the selection rule. The signal was found to be proportional to 0.8th power of the density of Na atom, which deviated from ideal case. This deviation was due to non-radiative processes such as reabsorption and collision process.
The two-photon transition rate for various pumping intensity was measured and the result agreed well with that based on Bloch equations. It showed saturation effect at high pumping intensity region. The exponential index K which reflects the degree of saturation was measured in terms of a function of frequency for two different pumping intensities. The values of the K-index varied from 2.0 to 1.6 and from 1.9 to 0.41 as detuning changed from -200GHz to 200GHz for the pumping intensities of 3.5 MW/㎠ and 14 MW/㎠, respectively.
Stark shift in the two-photon transition was found to be 195 Hz/(W/㎠) which was the sum of Stark shifts of 3S-3P and 3P-5S transitions. A new technique of Stark shifts measurement is proposed.
