Structured photonic materials for multi-spectral imaging applications (2015)

Structured photonic materials are typically composed of periodic subwavelength elements where the unit cell geometries can impact the overall optical characteristics of the bulk material. By using micro and nanofabrication technologies it is possible to engineer the electromagnetic properties of structured photonic materials for a given application and create a variety of optical components such as band pass filters and absorbers. Two structured photonic materials that have gained substantial interest in recent years are plasmonic filters and metamaterials which are well suited for optical and terahertz imaging applications, respectively. In addition to imaging applications within individual wavebands, structured photonic materials, such as plasmonic filters and metamaterials, could be hybridised and combined with suitable sensors to create a multi-spectral imaging system capable of imaging at optical and terahertz wavebands simultaneously. These new hybrid structured photonic materials are known as synthetic multi-spectral materials, and their development will be presented in this work. 
To design synthetic multi-spectral materials it was necessary to optimise the plasmonic filter and metamaterial components independently. This involved electromagnetic simulation studies using finite-difference time-domain techniques, fabrication of the structured materials and characterisation using suitable techniques for the relevant spectral band. It was also necessary to ensure that all structures used the same materials and similar fabrication processing techniques as a means of simplifying hybridisation of the two structures.
Plasmonic filters exhibit extraordinary optical transmission due to coupling of light with surface plasmons at a metal-dielectric interface. A 16 colour plasmonic filter set, consisting of triangular hole arrays etched into an aluminium film, was optimised for imaging applications in the visible and near infrared spectral range. Initial work on the integration of synthetic multi-spectral materials with CMOS image sensors was undertaken by developing fabrication processes to integrate plasmonic colour filters with two different CMOS chips. Preliminary results from the characterisation of the optical filters fabricated on to the chips have been presented. The resonant wavelengths of the plasmonic colour filters were then scaled up to infrared wavelengths where it was necessary to consider the role of spoof surface plasmons on the extraordinary optical transmission phenomenon. This led to the fabrication of 8 short wave infrared plasmonic filters. 
Metamaterial band pass filters consist of a single metal film etched with a periodic complementary electric ring resonator unit cell structure. Metamaterial absorbers consist of an electric ring resonator, separated by a metallic ground layer by a dielectric spacer. In the course of this work, two metamaterial filters and four metamaterial absorbers were designed. The metamaterial structures exhibit resonant characteristics at terahertz frequencies.
Three synthetic multi-spectral materials, each consisting of hybrid plasmonic filter and terahertz metamaterial structures, have been simulated, fabricated and characterised. The first synthetic multi-spectral material combines 16 plasmonic filters with a terahertz metamaterial filter and is capable of filtering 15 optical wavelengths and a single near infrared wavelength, whilst simultaneously filtering a single terahertz frequency. The multi-spectral filter demonstrates that it is possible to engineer the optical passband characteristics of a thin metal film over several decades of wavelength using a single electron beam lithography step. The second synthetic multi-spectral material consists of 16 plasmonic filters hybridised with a terahertz metamaterial absorber and can filter 15 optical wavelengths and a single near infrared wavelength whilst simultaneously absorbing a single terahertz frequency. Plasmonic filters and metamaterial absorbers are promising components for use in the development of new optical and terahertz imaging systems, respectively, and therefore the second synthetic multi-spectral material represents a significant step forward in the development of a visible and terahertz multi-spectral camera. The third synthetic multi-spectral material combines 7 plasmonic filters with a low metal fill factor metamaterial absorber, to increase the measured transmission of the plasmonic filter components. The third synthetic multi-spectral material is capable of filtering three optical wavelengths, a single near infrared wavelength, a single short wave infrared wavelength and two mid infrared wavelengths, whilst simultaneously absorbing a single terahertz frequency. Such a synthetic multi-spectral material could aid in the development of a visible, infrared and terahertz multi-spectral camera.