5S—5D Two-Photon Spectroscopy in Warm Rubidium Vapour

In this paper we present a two-photon absorption spectroscopy experiment for investigating the 5S \(_{1/2}\) \( \rightarrow \) 5P \(_{3/2} \) \(\rightarrow \) 5D (5S to 5D) energy level structure of rubidium 87. This apparatus allows us to study in detail the hyperfine structure of the 5D level in rubidium. The experiment is part of the setup of a four-wave mixing experiment that is used to generate entangled photon pairs. We probe the 5S to 5D multi-level structure using a set of 780 nm and 776 nm external cavity diode lasers. These laser beams pass through a rubidium gas cell where the two-photon absorption process occurs; the intensity of one of the beams is measured at the exit of the cell using a photodetector. By varying the wavelength of one beam, various absorption spectra can be observed at the photodetector output signal. Measuring and understanding these spectra contributes to the field of atomic spectroscopy and is important to the correct operation of the four-wave mixing experiment.