is sometimes also called Reflectance Difference Spectroscopy (RDS). Since there is another technique with the same RDS abbreviation, in our group we prefer to use RAS.
Principle RAS setup with two polarizers and photoelastic modulator
The basic setup is quite simple. A linear polarized light source is shown perpendicular on the sample. The light is reflected from the sample. The change of polarisation due the reflection is analysed by a photoelastic modulator (PEM) and an anlyser prim. The only requirement for the system is a transparent ambient and a window above the sample. This is easily fulfilled for MOVPE as form MBE, and even for etching cells.
Not every sample is suitable for RAS. The sample needs exactly two distict eigenaxis on the surface. Hexagonal surfaces (like GaAs (111) or GaN (0001)) cannot change the polarization due to symmetry reasons. The same is true for Si (001), where both axis are equal. Luckily, most III-V semiconductors (like GaAs, InP, ... ) have a fcc bulk lattice structure and can be investigated by RAS.
Further reading about RAS can be found on this korean webseite.