Selective reflection of a laser beam from a dilute rubidium vapor

We study the selective reflection of the laser beam from rubidium atomic vapor at the D-2 line (wavelength lambda = 780 nm) at different atomic densities. We use a tunable free-running diode laser. We observed a measurable signal at a low atomic density N when the mean distance between resonance atoms reached two wavelengths. In our experiment, the dimensionless parameter N (1/3) lambda varied from 0.5 to 2.8. The reflectivity increased with density monotonically. It is interesting to perform experiments when the parameter N (1/3) lambda a parts per thousand(a) 1.