Improving the Angular Visibility of Photopolymer-Based Reflection Holograms for Sensing Applications

Volume reflection hologram-based sensors are designed to visibly change colour in response to a target stressor or analyte. However, reflection holograms fabricated in thick photopolymer films are highly angularly selective, making these sensors challenging to view and interpret by non-experts. Here, the use of speckle holography to improve the visibility of reflection holograms is presented. A novel recording approach combining speckle recording techniques with Denisyuk reflection recording geometry is described. The recorded speckle reflection grating operates as a series of multiplexed reflection gratings with a range of spatial frequencies, capable of reflecting light at a wider range of angles. A comparative study of the angular and wavelength selectivity of speckle and standard reflection gratings was conducted. The FWHM of the angular selectivity curves of the speckle reflection gratings is doubled (4 degrees) in comparison to standard 4500 lines/mm reflection gratings (2 degrees). The wavelength selectivity FWHM is also doubled from 4.2 to 8.6 nm. The comparative ability of the speckle and standard reflection gratings to act as colour-changing compressional pressure sensors in the 0.88-5.31 MPa range is described. Finally, we present a prototype reflection hologram viewer which enables the easy observation of angularly specific reflection holograms by non-experts.

Automated summary

Volume reflection hologram-based sensors change color in response to stressors or analytes, but thick photopolymer films used to fabricate reflection holograms have high angular selectivity, making them difficult for non-experts to view and interpret. This study presents the use of speckle holography to improve the visibility of reflection holograms. A novel recording approach combining speckle recording techniques with Denisyuk reflection recording geometry is described, enabling the creation of speckle reflection gratings with a wider range of angles and wavelengths. The comparative performance of speckle and standard reflection gratings as color-changing compressional pressure sensors is also discussed.