A simple yet highly-effective means for sensitivity enhancement of polymer-based effective index sensors using tailored high-index thin films is presented. Simulation and experimental results show that an appropriately-placed PTFE/Ta2O5 thin film can lead to enhanced sensitivity of asymmetric Mach–Zehnder interferometers to external refractive index changes of factors between 3 and 8. The films are deposited on the polymer using ion beam sputtering (IBS) at low temperatures (<80 °C) thereby avoiding detrimental heating effects on the substrate. It is shown that the limit of detection of the interferometer sensors is enhanced by a factor 20 (down to Δn ≤ 10−4) using these films.