Asymmetric sub-wavelength Fabry-Perot resonators for gas sensing applications.

Abstract

This thesis is a report of investigation on resonance-enhanced absorption of light in ultra-thin optically lossy films on metal substrates for gas sensing applications. Planar thin-film structures comprising lossy coatings on metal substrates with a film thickness far less than the wavelength of the probing light form sub-wavelength thickness asymmetric Fabry-Perot resonators. Recently, it was demonstrated that under suitable illumination conditions such thin-film structures can sustain wave-interference effects which lead to significant enhancement in the light absorption inside the ultra-thin coatings. We have proposed that, if such thin-film structures are made of gas sensitive materials then the resonance enhanced absorption feature can be used for gas sensing applications. Experimental investigations on three kinds of lossy thin-film cavities studied in this thesis demonstrate that the proposed sensing scheme can adopt polymers, metal-oxides, and metal-organic frameworks as the sensing materials, and utilize changes in their film thickness, refractive index and absorbance to generate measurable shifts in the reflection spectra of the samples.

Wave interference effects in transparent films with thickness greater than quarter-wavelength of the probing light have been studied since 1665, but similar effects were not observed in sub-wavelength thickness lossy films until 2012 due to lack of suitable mechanisms to sustain wave interferences. Future applications of thin-film structures exhibiting wave interference effects in lossy films would require a comprehensive understanding of the roles of key parameters: absorbance, film thickness, metal substrates and illumination conditions. We have explored such parameters using angle-resolved reflection spectroscopy and transfer matrix method. The reflection spectra of such thin-film structures show strong dependence on the polarization and the angle of the incident light, for weakly lossy films. Metal substrates play a key role by providing a non-trivial, close to 2 pi reflection phase-shift, and also participate in energy dissipation.

In the context of gas sensing applications, thin-_lm structures made of dye-doped polyvinyl alcohol, sub-stoichiometric titanium-dioxide and HKUST-1 metal-organic framework films on various metal substrates were selected to study their response towards water-vapour and carbon-dioxide. The motivation to adopt these sensing materials was to determine the wide applicability of the proposed sensing scheme. Two of these materials possess native optical losses, and hence have not been studied in asymmetric Fabry-Perot cavity configuration in the past. In contrast, the dye-doped polymer system is representative of composite media where a transparent gas sensitive dielectric host is made lossy by using dye molecules demonstrating that the existing knowledge of transparent sensing materials can be transferred to this sensing scheme with some alterations. In addition, the dye-doped polymer films on silver substrates exhibit coupling between weak Fabry-Perot resonances and the dye-molecular excitations leading to an interesting absorption behaviour that has been analysed in detail in this thesis. Finally, the sensing response of HKUST-1 films based sensors indicates that changes in film absorbance make the data analysis more complex than that of transparent films based systems and future implementation of this sensing scheme would require suitable algorithms to extract useful information from the measured spectral shifts.