Abstract

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.

Highlights

  • Bragg reflectors are multilayered systems comprising layers with alternating low and high refractive index and quarter-wavelength optical thickness, i.e., the film thickness multiplied by the film’s refractive index equals a quarter of the operating wavelength of the Bragg reflectors

  • We proved the concept of implementing top-covered Bragg reflectors as optical sensors for humidity

  • The sensor under investigation consisted of polymer film as a sensitive element that is spincoated on top of a multi-layered stack (Bragg reflector) regarded as a transducing element

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Summary

Introduction

Bragg reflectors are multilayered systems comprising layers with alternating low and high refractive index and quarter-wavelength optical thickness, i.e., the film thickness multiplied by the film’s refractive index equals a quarter of the operating wavelength of the Bragg reflectors (λBR). Due to the quarter-wavelength thickness of all layers in the reflectors, all multiple reflected waves are in phase, interfere constructively and a band of high reflectance centered at λBR appears in the spectra. If Bragg reflectors are designed in such a way that the operating wavelength is in the visible part of the spectra, a distinctive color called structural color is observed, transparent and colorless layers are used for building the reflector [1,2]. We have already demonstrated [10,15] that a possible solution is to Proceedings 2019, 3, 12, doi:10.3390/IOCN_2018-1-05478 www.mdpi.com/journal/proceedings

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