In recent years, the threat to pilots and other transport vehicles has increased through laser striking. This is due to the ease of accessibility and the low cost of ownership for handheld portable lasers. The reports from the Civil Aviation Authority (CAA) and their American counterpart, the Federal Aviation Administration (FAA), have outlined that visible green laser attacks consist of 83-91% of all incidents. This can lead to temporary impairment to the human eye, with consequences increasing with exposure time resulting in retinal and photo-chemical eye damage. Additionally, the magnitude of severity is increased and could lead to fatal collisions. Current efforts for laser protection devices consist of traditional thin film filters. This technology is well-established, but it has disadvantages related to the angular intolerance, where the transmission spectrum of the protection filter undergoes a continuous blue shift to shorter wavelengths as angle of the incident beam increases. The optical and transport industry, as well as government defence agencies, have a desired interest in developing a truly wide-angle (up to 60 degree) and shift-free laser protection system, which is the main drive behind this research. Furthermore, optical narrowband filters with wide-angle operation are highly sought after in other applications including Raman spectroscopy, multi-photon microscopy, and life science.
The research presented in this doctoral thesis employs a new theoretical approach to the problem and presents laser protection designs that can effectively block out any desired wavelength within the visible region through merging plasmonic physics with thin film interference theory. This allows for high transmission filters with a single rejection band at the plasmonic resonant wavelength. Filter designs can be established in quick succession with a specifically developed inverse design software tool that supplements industry standard optical thin film filter design software by integrating an effective Drude-Lorentz model to describe a ‘metafilm’, or supplements a full-wave electromagnetic simulator by incorporating the meta-atom design. The metafilm methodology provides reliable results when compared to the design of a full metamaterial model through a full-wave electromagnetic simulation software.
Attributing minimal parameters to the metamaterial allows for an inverse full metamaterial model to be designed through isotropic plasmonic particles. The functional responses of the designed filters can achieve optical densities (OD) within the range of 1OD to 4OD for all polarisation states over a wide range of angles up to 85+ degrees. The use of thin film interference theory enables wavelengths outside the plasmonic resonance to obtain high overall transmission allowing for a high integrated visual photopic response. Due to the narrowband, typically full-wave half-maximum below 50 nm, the filter colouration can be near-neutral.
The discussion for the applications of metafilms can also be applied to traditional thin film filters in order to create a combination filter. This work presents combination filters which provide a boost and enhancement to the optical density, and advanced angular sensitivity performance. Traditional thin films are still an integral part for optical systems, so allowing a gradual introduction to metamaterial filters is a functional starting point to convince the industry that the future of wide-angle shift-free bandstop filter lies with optical metamaterials. Furthermore, the advances for manufacturing optical metamaterials are not yet advanced enough for a direct and solo metamaterial filter. Nevertheless, with current fabrication techniques, a combinational filter is a plausible route.
The device’s active blocking layer structure is based on a three-dimensional plasmonic nanocomposite metamaterial, with a base element of aluminium/silver/gold nanoparticles arranged in a crystallographic primitive hexagonal Bravais lattice planar array, surrounded in a host dielectric medium. The active component is sandwiched between a transparent substrate and an anti-reflection coating. The lattice arrangement enables polarisation insensitivity, with a three-dimensional array density catering for an increased attenuation.
The full metamaterial wide-angle operating optical filters have to feature uniform structural sub-wavelength features, which is a highly difficult process over large areas with conventional lithographic techniques. Prototype filters have been fabricated with a shift-free wide-angle notch that is capable of filtering out blue light. The fabrication process takes advantage of self-assembled diblock copolymers (BCP) with metallic nanoparticle inclusions through selective impregnation. The BCP layers (PS-b-PEO) were annealed within a solvent vapour atmosphere (SVA) to induce a phase separation that forms a pentagonal periodic nanostructure of ~20 nm features with ~40 nm separation distance. Blue light filters were used for a proof of concept and provide a step towards desired blocking at any chosen wavelength.
The research undertaken within this Doctor of Philosophy programme provides a robust metamaterial filter design process with a method for developing and fabricating the devices as a proof-of-concept. The designed filters can be placed onto a number of devices ranging from personnel goggles to aircraft windows. This work can lead to the realisation for the next generation of laser protection devices that are currently in high demand.