Sub-wavelength Gratings For Long Wavelength Sensing
Authors:   ,   Cork Institute of Technology, Hogan Brian 
Published: August 2024

This Ph.D. thesis is concerned with the design and use of guided mode resonance (GMR) reflectors for applications in optical absorption sensing at wavelengths around 10 μm. These grating based reflectors comprise appropriately designed optical nanostructures which resonantly reflect a wide range of incident wavelengths. Because of the periodic modulation of the refractive index in the grating layer, the guided modes which exist in this region leak out of the waveguide and arc more appropriately termed leaky or quasi guided modes. Resonant reflection occurs when phase matching of the incident light and these leaky modes cancels any transmitted light. The wideband nature of this device results from multiple resonances which are spectrally close to one another. By appropriately tuning the geometric parameters of the device as well as selection of the appropriate materials, wideband reflectivity can be obtained. The resonances which support the wideband reflectivity are shown to be highly sensitive to changes in the refractive index in the regions which overlap with the leaky or quasi-guided mode making them ideal candidates for optical sensing transducers. While the impact of changes in the real part of the refractive index is well known these GMR devices can also be designed with sensitivity to changes in the imaginary part of the refractive index such that absorbing species can be measured. To date, these sub-wavelength grating structures have been extensively researched for applications at telecommunications wavelengths using Silicon and Silicon Dioxide as the grating and substrate materials respectively. In the 10 μm wavelength range however, both of these materials suffer from optical loss which is unsuitable in a resonant optical structure. To overcome this issue a new material pair is proposed and a fabrication process suitable to this new material pair is developed and refined. Using the design principles outlined above a wideband reflector will be designed and fabricated for operation in the 10 μ wavelength range. These reflectors will then be used as absorption based optical sensors measuring concentration of analyte materials with absorption fingerprints in the wavelength range. Using Strontium Fluoride and Germanium as the new material pair, standard optical contact lithography and inductively coupled plasma etching are used to fabricate sub-wavelength structures operating as wideband reflectors in the 10 μm wavelength range. The reflectors were characterised using both a global’ light source in an FTIR interferometer as well as a QCL light source with an msl-12 HgCdTe detector. These reflectors are subsequently used as absorption based optical sensors to measure ethanol concentrations in a water solution by targeting the large absorption peak of ethanol at 9.54 μm. Two distinct variations of sub-wavelength gratings were designed and fabricated to operate as wideband reflectors. These were both characterised using the optical devices described above and there was good agreement for both types of grating structure between theoretically predicted and experimentally measured reflectivity values. One of these wideband reflectors was then used to verify the suitability of these structures to operate as absorption based optical sensors. Using the reflectivity from the grating for a pure water solution as a reference the normalised reflectivity for various ethanol concentrations were measured with good agreement being observed between theoretically predicted and experimentally measured values. Sub-wavelength grating structures are highly versatile devices which can be designed to operate as polarisers, filters, reflectors, focusing lenses or focusing mirrors simply by altering the device geometry. The development of wideband reflectors using this multi-purpose structure has great potential to enable the development of many future optical components. The Attenuated Total reflectance sensor has long been the absorption based sensor of choice however the sub-wavelength structures developed in this thesis have the potential to supplant the established ATR sensor owing to superior performance as well as the ability to incorporate the structure to an integrated photonic device. The ability to use these structures in integrated photonic devices will allow them to be key drivers in the push towards integrated enabling hand-held sensing devices.