Lock-in-free ATR characterization of thin-film pockels coefficients using the prism coupler method

The advancement of thin-film electro-optic (EO) materials and devices depends on reliable characterization and a thorough understanding of structure–property relationships that govern their EO performance. Here, we reinvent the attenuated total reflection (ATR) technique using the prism coupler method, which was proposed over three decades ago but remained largely underdeveloped, for standard characterization of thin-film EO coefficients (r33 and r13). The ATR technique detects the optical modes of a sharp drop in reflectivity (R) at specific critical angles in the spectrum, caused by the evanescent wave at the prism/air interface coupling into the slab waveguide through optical tunneling. For EO waveguides, substantial reflectivity change (ΔR) of ATR modes can be induced by small refractive index changes (Δn of 10-4 to 10-3 ) of the Pockels effect at the extrema of first or second derivatives, enabling efficient lock-in-free detection and determination of r-coefficients. Further multi-mode and frequency-selective ATR measurements on benchmark EO materials, including thin-film lithium niobate (TFLN) and EO polymer SEO100, led to the accurate determination of r-coefficients. The study validates the Gaussian function as a better model for representing the ATR spectra of waveguide films, providing a global approximation of optical modes than the traditional Taylor expansions. Compared to the Teng-Man reflection technique, the new ATR technique stands out for its improved accuracy, high traceability, effective self-calibration, simple instrumentation, rapid data processing, quantitative determination of both surface plasmon resonance mode and dielectric slab waveguide modes, and for organic electro-optic materials, obtaining primary molecular properties of push-pull chromophores for materials development.