Improvement of temperature-induced spectrum characterization in a holographic sensor based on N-isopropylacrylamide photopolymer hydrogel.

A novel thermo-sensitive N-isopropylacrylamide photopolymer was developed for improving the temperature and humidity responses of holographic sensors. Diffraction spectra of holographic volume gratings recorded in the materials were characterized to explore the sensing response capacity. A dependence of peak wavelength on the temperature was observed and provided a quantitative strategy for holographic sensing applications. Expansion of the humidity range induced a strong extension of wavelength shift. Finally, the temperature response reversibility was demonstrated experimentally. Our sensing results were completely different from the reported typical acrylamide polymer system. Compared with the former, we obtained a more sensitive temperature response and an evident shift expansion (>200  nm) at a relative humidity of 70% or higher. These results can obviously improve the thermo-sensitivity of a holographic sensor and expand the practical application area of the holographic sensing strategy.