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Polymer top covered Bragg reflectors as optical humidity sensors
Katerina Lazarova * 1 , Rosen Georgiev 2 , Darinka Christova 3 , Tsvetanka Babeva 4
1  Institute of Optical Materials and Technologies “Acad. J. Malinowski”, Bulgarian Academy of Sciences
2  1Institute of Optical Materials and Technologies ‘‘Acad. J. Malinowski’’, Bulgarian Academy of Sciences
3  Institute of Polymers, Bulgarian Academy of Sciences
4  Institute of Optical Materials and Technologies ‘‘Acad. J. Malinowski’’, Bulgarian Academy of Sciences

Published: 23 August 2018 by MDPI AG in 1st International Online Conference on Nanomaterials session Devices & Energy

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.

Keywords: macromolecular architecture; poly(N,N-dimethylacrylamide) copolymers; thin films; humidity sensing; optical sensors; Bragg reflectors
Comments on this paper
Ana Maria  Diez Pascual
Comment on the paper
Thanks for your nice presentation!
One simple question: why you use films of thickness 290 nm if there is no enhancement of the sensitivity for films thicker than 250 nm?
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Tsvetanka Babeva
Comment answer
Hi, thank you for the question and nice comment.

You are right that after 250 nm the sensitivity is almost the same and it is independent of the thicknes. The only reason of using polymer film with thickness of 290 nm is to be on the save side

Best regards
Tsvetanka Babeva