SHEAR BIREFRINGENCE MEASUREMENTS ON POLYMER THIN-FILMS DEPOSITED ON QUARTZ RESONATORS

We have measured the second-order stress birefringence in a 2 mu m fil m of poly-(methyl-methacrylate) sandwiched between the top electrode o f a quartz resonator and a second? semitransparent gold layer. The gol d-film-gold system forms an optical Fabry-Perot resonator with charact eristic dips of reflectivity at certain angles of incidence, which are the coupling angles of the different optically resonant modes. When m odulating the driving voltage of the quartz resonator with a frequency in the kHz range and referencing the detection electronics to the mod ulation, shifts in the coupling angle in the range of 10(-3)degrees ar e easily detected. They result from a pseudo-de component in the stres s-optic signal, which is proportional to the square of the shear strai n. This second-order stress birefringence is related to normal stresse s in the film via the stress-optic law. We compare our findings to pre dictions from the theory of finite elasticity. (C) 1998 American Insti tute of Physics. [S0021-8979(98)03703-7].