Normal-incidence reflectance difference spectroscopy based on a liquid crystal variable retarder

We propose liquid crystal variable retarder-based reflectance difference spectroscopy for normal-incidence measurements. Principles, instrumentation, data collection and reduction, and calibration procedures are provided. The signal noise is better than 10−3, and the spectral range is from 1.6 to 2.4 eV with 346 photon energy channels. As a demonstration, reflectance difference signals of a multilayer pentacene film on poly (ethylene terephthalate) (PET) film are presented with different polarization azimuths. The characteristic peaks at 1.8 and 1.97 eV, corresponding to the Davydov splitting of pentacene crystal, are observed, which indicate well-ordered in-plane anisotropic structure of pentacene crystal film on PET. Thanks to normal incidence, this design is immune to adjusting the optical structure for the measurements with different working distances, and the objective lens is easily integrated to realize microarea measurements.