Document Type

Theses, Ph.D

Disciplines

1. NATURAL SCIENCES, 1.3 PHYSICAL SCIENCES

Abstract

Volatile organic compound (VOC) monitoring at parts-per-million (ppm) concentration levels is essential for occupational safety, indoor air quality, and regulatory compliance. Although holographic gas sensors have been widely investigated, most operate in reflection mode as qualitative visual indicators, with limited quantitative evaluation of sensing mechanisms and scarce systematic demonstrations of repeatable multi-cycle validation for VOCs at ppm concentrations. A physics-based optimisation framework for transmission-mode holographic gas sensors which can bridge this gap has remained largely undeveloped.

This thesis presents the development and optimisation of real-time transmission-mode volume phase transmission holographic grating (VPTHG) sensors for ppm-level VOC detection. A comprehensive theoretical framework based on Kogelnik’s coupled-wave theory was established to quantify and deconvolute the individual contributions of dimensional variation (Δd), refractive index modulation change (Δn₁), and average refractive index variation (Δnavg) to diffraction efficiency under fixed-angle interrogation. The resulting parameter maps defined physically meaningful perturbation ranges and enabled systematic design-space optimisation of spatial frequency, grating thickness, slant angle, and interrogation geometry. Optimisation was performed within these deconvoluted response regimes, allowing sensor configurations to be tuned theoretically according to application-dependent requirements such as enhanced Bragg-detuning sensitivity, dynamic range, or interrogation stability.

A dedicated gas testing system was designed and constructed to provide stable and repeatable exposure conditions tailored for holographic sensor evaluation. Experimental validation demonstrated repeatable toluene sensing over multiple exposure–evacuation cycles across a concentration range of 100–10,000 ppm. Toluene, a widely used industrial solvent and established benchmark aromatic VOC, is particularly relevant due to its documented health risks within the low-ppm exposure regime. The transmission-mode platform exhibited a stable, concentration-dependent diffraction efficiency response and an extended dynamic range compared to conventional visual reflection-mode approaches. Comparative studies with methanol confirmed analyte-dependent behaviour, while zeolite-functionalised photopolymer gratings demonstrated enhanced modulation characteristics, highlighting the role of porous nanostructuring in tailoring sensor performance.

By linking material properties, geometric design parameters, and interrogation strategy to measurable diffraction efficiency response, this work establishes a quantitative, physics-based optimisation framework for transmission-mode holographic gas sensors. The results advance holographic sensing from qualitative indication toward electronically interrogated, applicationengineered VOC detection platforms suitable for real-time environmental and occupational monitoring, and provide a foundation for future multiplexed and material-enhanced sensing architectures.

DOI

https://doi.org/10.21427/nbb9-8722

Creative Commons License

Creative Commons Attribution-NonCommercial 4.0 International License
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License

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