Accurate, Multimode, and Lock-In Free ATR Detection of Benchmark Electro-Optical Materials

Abstract

The prism-coupled attenuated total reflection (ATR) technique has recently been revitalized for measuring the electro-optical (EO) properties of poled polymers. In this study, the technique is optimized to evaluate the optical and EO properties of thin-film lithium niobate (TFLN) and a benchmark poled polymer. The differential reflectivity (ΔR) in ATR modes contains significantly overlapping peak regions with both large first (R′) and second (R″) derivatives over the effective refractive index (N_eff) range on the order of 10^–4. While large R′ and R″ are essential for obtaining quantitative ΔR with low noise, thereby achieving high sensitivity and accuracy of EO measurement, their spectral coalescence is detrimental, especially in a range comparable to or smaller than the index changes of EO materials. To overcome this challenge, a multimode, frequency selective, lock-in free ATR measurement was implemented on a 5.5 μm TFLN on silicon and a poled polymer on ITO glass. For EO analysis at low frequencies, broader modes with lower N_effs, dominated by R′, were selected. For sharp modes with higher N_effs, high-frequency measurements beyond the Nyquist limit isolate the EO effect in the R″-only region. This protocol exerts a unique pull-push-pull force to tune evanescent field coupling and split the ATR mode into a distinct doublet mode, separated exactly by twice the index change. Comprehensive analysis of these measurements gave consistent Pockels coefficients for both TFLN and the EO polymer, including results from traditional Taylor linear approximation and an improved model that accounts for mode splitting and broadening using a Gaussian model.