Nonlinear Atomic Spectroscopy Inside A Random Porous Medium

We have studied the pump-probe spectroscopy of rubidium vapor confined to the micrometric interstices of a random porous glass. Due to the propagation in the highly scattering medium, the light fields are randomized inside the sample with significant consequences for the atomic spectra. A two-frequency modulation technique was used to isolate the atomic response proportional to the product of the intensities of the pump and probe fields. Unusual line shapes were observed which include relatively narrow structures that are present in spite of the Doppler broadening due to the atomic velocity distribution. A simple theoretical modeling of the light-atom interaction that assumes statistical isotropy of the diffuse light field and mutual temporal incoherence of the pump and probe fields is presented. Using a single adjustable parameter to account for the atomic confinement, the model successfully describes the diversity of the observed spectral line shapes.