Optical head of the RD spectrometer attached to a UHV chamber in front of a low strain window. The polarization optics (polarizer and analyzer) as well as the photoelastic modulator (PEM) are inside the optical head, whereas the light source (Xe lamp) and the optical spectrometer are connected via optical fibers.
Reflectance Difference Spectroscopy (RDS) is an optical technique which measures the difference in reflectivity at normal incidence for light linearly polarized along two orthogonal directions . RDS was initially developed as a reliable tool for monitoring the growth of cubic semiconductors: in this case the response of the optically isotropic bulk cancels out, leaving only the contributions of the surface and interfaces were the cubic symmetry is broken. At the Atomic Physics and Surface Science division (AOP), RDS has been successfully applied to surface science research and has become an indispensable tool for the investigation of surface electronic structures , adsorption/desorption kinetics , organic and inorganic thin film growth , ion and photon irradiation on single crystal surfaces , etc. Recently, RDS has also been applied to the study of nanomaterials like metal clusters  and nanopatterned surfaces. The pronounced sensitivity of RDS to surface adsorbates , orientation and conformation of organic molecules , surface strain , particle and surface plasmons , opens a broad field of applications for RDS - not only in fundamental scientific research but also in surface analytics and process control in industrial environments .
The spectrometer shown in Fig. 1 is a commercial instrument (UVISEL, Horiba Jovin-Yvon), in which the polarization of the light of a Xe lamp is periodically modulated by means of a photoelastic modulator (PEM) and directed onto the sample at normal incidence. Upon reflection, the modulated signal is spectroscopically analyzed by means of a diffraction grating monochromator and a photo-multiplier tube in the photon energy range from 1.5 to 5.5 eV. The present setup allows well resolved detection of optical anisotropies of the order of 10-4.
Besides in spectroscopic mode, data can also be acquired in kinetic mode by recording RD transients at a single photon energy, thus allowing the real-time monitoring of kinetic or time-critical processes such as adsorption/desorption, ordering phenomena, or phase transitions .
Inside the VT-STM chamber the sample can be magnetized, and the RDS can be used to characterize the magnetic properties via the (polar) magneto-optic Kerr effect (MOKE) . In contrast to conventional MOKE setups operating at a fixed single wavelength, RD-MOKE supplies the full spectral information.
Ex-situ, the instrument can be combined with a computer-controlled sample rotation stage to obtain so-called azimuth-dependent RDS (ADRDS) spectra, which can be used to precisely determine the orientation of the optical axes in complex anisotropic samples, such as semi-crystalline extruded polymers  or to remove interference fringes in partially transparent substrates or films .
Meanwhile, we have started to build our own RDS equipment and to develop new instruments and applications (see, for instance, the rotating-compensator based RCRDS design ).