Polymer Top-Covered Bragg Reflectors as Optical Humidity Sensors

Thin films from double hydrophilic copolymer of complex branched structures containing poly(N,N-dimethyl acrylamide) and poly(ethylene oxide) blocks were used as humidity sensitive media, and two types of Bragg reflectors with different optical contrast and operating wavelengths were implemented as transducers of humidity changes. The required film thickness was pre-optimized through theoretical modelling in order to achieve the highest sensitivity. Single films and Bragg reflectors were characterized by transmittance measurements at different humidity levels in the range from 5% to 95% relative humidity. The influence of the number of the layers in the stack, the operating wavelength and optical contrast on sensitivity was studied. The potential and advantages of using top-covered Bragg reflectors as humidity sensors with simple optical read-out are demonstrated and discussed.