Development of Holographic Sensors for Monitoring Relative Humidity and Temperature

Photonic structures capable of responding to an analyte with an easily identifiable change in their optical properties have generated wide interest due to their possible application as holographic sensors. Holographic sensors are considered a low-cost, lightweight and disposable technology, and have potential for application in different areas ranging from medical diagnostics to environmental sensing including the monitoring of environmental temperature and relative humidity. In spite of the existing wide range of temperature and humidity sensors, holographic sensors are of special interest as they can provide fast, real-time, reversible or irreversible, visual colorimetric or electronic readouts. The main objective of this project was the development of holographic sensors with response to relative humidity and/or temperature. Holographic humidity sensors were fabricated by holographic recording of volume phase transmission gratings in acrylamide /diacetone acrylamide-based photopolymers containing polyvinyl alcohol as a binder. The diffraction efficiency and the Bragg angle were found to be humidity dependent in the relative humidity range 20 - 90 %. It was shown that reversibility of the response, sensitivity and operation range of the sensor can be tuned by varying the photopolymer composition. Best sensitivity up to 3%DE/%RH was observed for diacetone acrylamide-based gratings in the relative humidity range 70 - 90 %. A novel thermosensitive photopolymer containing N-isopropylacrylamide as the main monomer was developed. The diffraction efficiency up to 80 % in transmission mode and 20 % in reflection mode was achieved. It was demonstrated that the temperature switchable swelling/shrinking of the novel N-isopropylacrylamide-based photopolymer can be implemented in the development of holographic temperature sensors, temperature visual indicators and holographic optical elements with temperature controlled direction of the diffracted light and diffraction efficiency. Best sensitivity up to 2%DE/oC and 4.3nm/oC was obtained for sensors based on volume phase transmission and reflection gratings, respectively. For the first time, an optical sensor based on the surface relief structure with reversible response to relative humidity in the range 35 - 97 % was developed. The device comprises the Aztec grating as a substrate and a coating thin polymer film as a sensing medium. It was demonstrated that alteration of the coating layer composition allows tuning the operation range and sensitivity of the device