Lock-In Free ATR Detections of Organic Thin Film Pockels Effects within and beyond Nyquist

Rapid and reliable determination of thin film electro-optic (EO) coefficients is critically important to the future development of organic and polymeric nonlinear optical materials for photonic applications. It underscores the difficulties and complexities involved in homemade instrumentation and analysis algorithms, including Teng-Man (TM) reflection method and attenuated total reflection (ATR) technique, which do not adhere to standard specifications or calibration procedures. Here, we reinvent the ATR technique, which was proposed decades ago but remains largely underexplored, for rapid and reliable measurements of organic thin film EO coefficients (r₃₃ and r₁₃) at low to modest modulation frequencies. The study was motivated by the experimental observation of modulated ATR spectra of poled EO polymer films by a Metricon 2010/M prism coupler without a lock-in amplifier or oscilloscope. A further systematic study led to the development of a new ATR method by measuring the electric-field-induced reflectivity change in the ATR spectra of poled EO thin films for the quantitative analysis of r₃₃ and r₁₃ values. The method requires only two standard commercial instruments, a prism coupler and a functional generator. The results from our improved ATR method are in excellent agreement with those obtained from the TM method after careful error correction. This new electromodulated ATR spectroscopy without lock-in detection has integrated the significant progress in organic EO material research for r-coefficient improvement with the standard instrumentation of prism coupler and correct polarization-dependent analysis of modulated signals, which can be used routinely to characterize new thin film EO materials within and beyond the Nyquist sampling theorem. © 2024 American Chemical Society.