Polymer top covered Bragg reflectors as optical humidity sensors

Bragg reflectors are multi-layered systems of sequentially deposited low and high refractive index materials with quarter-wavelength optical thicknesses exhibiting structural colors due to the presence of high reflection band in the visible spectral range. If Bragg reflector is covered with film sensitive to external stimuli then the change in the environment could be easily detected by monitoring the change of Bragg reflector’s transmittance or reflectance spectra.

In the present study as a sensitive material we use linearly branched copolymer containing poly(N,N-dimethyl acrylamide) and Poly(ethylene oxide) blocks deposited in the form of thin films by spin-coating method. The required thickness is pre-optimized through theoretical modelling in order to achieve the highest sensitivity. Two types of Bragg reflectors, having different optical contrast and operating wavelengths are implemented as transducers of humidity changes. The first one consists of sequentially deposited SiO₂ and Nb₂O₅ films prepared by sol-gel and spin-coating methods. The second one comprises alternating dense and porous Nb₂O₅, the last prepared with addition of organic template. Single films and Bragg reflectors are characterized by transmittance measurements at different humidity levels in the range from 5 to 95 % relative humidity (RH). The influence of number of the layers in the stack, the operating wavelength and optical contrast on the sensitivity is studied. The potential and advantages of using top covered Bragg reflector as humidity sensor with simple optical read-out are demonstrated and discussed.

Acknowledgments: The financial support of Bulgarian National Science Fund, grant number DN08-15/14.12.2016 is highly appreciated. R. Georgiev acknowledges World Federation of Scientists for fellowship and project DFNP-17-97/28.07.2017 of the Program for career development of young scientists.