Two-photon absorption spectrum of CdS in theA- andB-excitonic region at 20 °K by the new twin-laser technique

The $A$ and $B$ excitonic spectrum of CdS was measured at 20 °K using a new variant of two-photon spectroscopy, the twin-laser technique. This method has two collinear lasers (a fixed-wavelength high-power laser and a low-power tunable laser) which are incident simultaneously on the crystal. Polarization measurements were performed in which sharp $2P$ excitons were clearly resolved, but no $1S$ excitons were seen. It is shown that a many-band theory is necessary to correctly interpret the experimental results.