Theoretical and Experimental Investigation of Slanted Volume Transmission Gratings for Holographic Gas Sensing

Slanted photopolymer-based diffraction gratings can undergo instant and quantifiable analyte-induced changes in both grating diffraction efficiency and peak reconstruction wavelength. This makes them an attractive option for gas sensing applications. The underlying mechanism for the sensor response is, however, poorly understood due to its complexity; the gaseous analytes produce simultaneous changes in multiple grating parameters (refractive index, thickness, refractive index modulation, and slant angle). Here, a novel and robust approach to the experimental characterization and theoretical analysis of holographic gas sensors based on slanted volume transmission gratings is presented. Slanted transmission gratings were fabricated in both undoped and MFI nanozeolite-doped photopolymer layers. The sensor response to the target analyte toluene was experimentally evaluated by real-time measurement of the cyclical change in grating diffraction efficiency and peak reconstruction wavelength. The experimental data was then theoretically analyzed using a novel approach based on Kogelnik's Coupled Wave Theory, enabling the calculation of the simultaneously occurring changes in the slanted grating thickness, effective refractive index, and refractive index modulation. Key findings include: 1) analyte-induced changes in refractive index modulation are the primary contributor to the overall sensor response and 2) Bragg angle detuning can enhance the sensing ability of slanted diffraction gratings, but in certain scenarios this effect is diminished/negated by analyte-induced changes in layer refractive index. This comprehensive procedure for sensor characterization and analysis facilitates the optimization of the slanted grating sensor design and improves the fundamental understanding of holographic gas sensor operation.