Two-photon spectroscopy of rubidium using a grating-feedback diode laser

We describe an experiment for investigating the 5 S 1 ∕ 2 → 5 D 5 ∕ 2 two-photon transition in rubidium using a single grating-feedback diode laser operating at 778.1 nm ( 385 THz ) . Continuous tuning of the laser frequency over 4 GHz allows for the clear resolution of the Doppler-free spectral features and allows accurate measurement of the hyperfine ground-state splitting. A direct comparison between Doppler-broadened and Doppler-free spectral features is possible because both are distinctly evident in the two-photon spectra. By independently modifying the polarization state of the two laser fields, the impact of electric dipole selection rules on the two-photon transition spectra is investigated. This experiment is a valuable addition to the advanced undergraduate laboratory because it uses much of the same equipment as the single-photon saturated absorption spectroscopy experiment performed on the 5 S 1 ∕ 2 → 5 P 3 ∕ 2 transition in rubidium ( λ = 780.24 nm ) and provides students with an opportunity to investigate characteristics of atomic spectra not evident in the single-photon experiment.