Mid-IR carbon dioxide (CO<sub>2</sub>) gas sensing is critical for monitoring in respiratory care, and is finding increasing importance in surgical anaesthetics where nitrous oxide (N<sub>2</sub>O) induced cross-talk is a major obstacle to accurate CO<sub>2</sub> monitoring. In this work, a novel, solid state mid-IR photonics based CO<sub>2</sub> gas sensor is described, and the role that 1- dimensional photonic crystals, often referred to as multilayer thin film optical coatings , play in boosting the sensor’s capability of gas discrimination is discussed. Filter performance in isolating CO<sub>2</sub> IR absorption is tested on an optical filter test bed and a theoretical gas sensor model is developed, with the inclusion of a modelled multilayer optical filter to analyse the efficacy of optical filtering on eliminating N<sub>2</sub>O induced cross-talk for this particular gas sensor architecture. Future possible in-house optical filter fabrication techniques are discussed. As the actual gas sensor configuration is small, it would be challenging to manufacture a filter of the correct size; dismantling the sensor and mounting a new filter for different optical coating designs each time would prove to be laborious. For this reason, an optical filter testbed set-up is described and, using a commercial optical filter, it is demonstrated that cross-talk can be considerably reduced; cross-talk is minimal even for very high concentrations of N<sub>2</sub>O, which are unlikely to be encountered in exhaled surgical anaesthetic patient breath profiles. A completely new and versatile system for breath emulation is described and the capability it has for producing realistic human exhaled CO<sub>2</sub> vs. time waveforms is shown. The cross-talk inducing effect that N<sub>2</sub>O has on realistic emulated CO<sub>2</sub> vs. time waveforms as measured using the NDIR gas sensing technique is demonstrated and the effect that optical filtering will have on said cross-talk is discussed.
Raman spectroscopy is an important technique that has evolved into many advanced methods, used in a wide range of fields, ranging from artwork analysis to security. In this study, we have found through template-assisted glancing angle deposition, highly ordered silver nanostructures could be fabricated across a nano-imprinted polymer to produce surface enhanced Raman scattering substrates. Various nominal film thicknesses at 85° incident angle and then deposition angles were used to fabricate the substrates, which were then characterised by scanning electron microscopy and their Raman performance was assessed using trans-1, 2-bis (4-pyridyl) ethylene as the Raman probe. Observations show that the best thickness for glancing angle deposition at 85° to be 400 nm (relative to the quartz crystal microbalance) produced the best Raman signal enhancement. The nanostructures consisted of nanorods with 851 – 1360 nm average length. Scanning electron microscopy images reveal samples had good uniformity and consistency in all films grown by this method, as the surface features provide regularly spaced nucleation sties. These findings lead show that highly sensitive surface enhanced Raman scattering substrates can be reproduced consistently cheaply.
Mid infrared optical coatings are commonly designed & manufactured using typically electron beam evaporated films of Silicon and Silicon oxide. However the transparency of these coatings is limited by optical absorption in the films when producing coatings for wavelengths beyond 4um approximately. This work reports improvements in mid infrared (3 to 6um) filter transparency achieved by exploiting recent advances in microwave plasma assisted pulsed DC magnetron sputtering technology. Sputtered silicon compound films have been used to demonstrate efficient room temperature deposited optical coatings for gas sensing applications at wavelengths between 3 to 6um. This process technology allows a new selection of film materials to be used in design of mid infrared filters, with transmission and thermal drift characteristics enhanced compared with conventional electron beam evaporated coatings. The spectral location of the optical coatings is controlled by a non-optical method, which avoids the complex optical monitoring configurations normally required. Durable filters are obtained at room temperature, which would otherwise be required in conventional evaporation processes. Importance of water partial pressure control during deposition for mid infrared is emphasised.
Single wavelength (670 nm laser diode) optical monitoring of reflectance at 1 second intervals was used to observe the
surface oxidation of Ni and Hafnium metal films in-situ in a low pressure oxygen atmosphere and also in a microwave
plasma oxygen environment.
After depositing thin metal films by sputtering in an oxygen-free environment, the observed reflectance quickly
decreased when low pressure oxygen gas was introduced into the vacuum chamber and reached a stable value within a
few seconds, after formation of a thin oxide layer. An additional rapid fall in reflectance and increase in oxide thickness
was observed when a microwave plasma generator was used to produce an oxygen plasma containing atomic oxygen.
Based on pre-determined optical properties of the metal and metal oxide films, the optical monitoring data was fitted to
obtain the thickness of the metal oxide as a function of time. The fitting results showed that the exposure to low pressure
oxygen forms an equilibrium thickness of less than 0.5 nm of NiO<sub>x</sub> and 0.78 nm of HfO<sub>x</sub>, while the oxygen microwave
plasma treatment produces an equilibrium thickness of 1.5 nm for both NiO<sub>x</sub> and HfO<sub>x</sub>.