Photonic devices for sensing and security applications
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The main aim of this thesis is the numerical and experimental verification of structured micro- and nano-scaled optical devices fabricated with e-beam, photolithography, reactive ion etching and embossing. Two separate themes were addressed; sensing of electromagnetic pulses by electro-optic non-linear guided wave photonic devices and free space photonic devices for security and anti-counterfeiting in polymer banknotes. The first theme was led by a comparison between two photonic devices used as extrinsic fibre optic sensors for the detection of short duration electromagnetic pulses (EMPs). A suitable electro-optic substrate was used for the fabrication of both micron sized waveguide-based evanescent coupling photonic devices, modelled using beam envelope methods, and its nano-structured surface Plasmon enhanced counterpart, modelled using FDTD. Both devices are capable of detecting EMPs with field strengths ranging from 50 – 500kV/m with pulse durations from 200ns – 2000ns. The surface enhanced plasmonic device showed improved device sensitivity and tunability, with a more linearized response along with greater ease of integration with optical fibres. In the second theme the photonic devices were used for image formation through diffractive optical methods with a view to polymeric mass replication. Multilevel 2μm - 8μm feature size diffractive optical elements, designed for dual colour operation in the scalar domain, were compared to 250nm feature size binary phase modulated Bragg gratings for single colour operation in the resonance domain. Both devices are capable of generating high fidelity images under appropriate illumination. The scalar domain elements, designed for 450nm and 650nm illumination, showed measured diffraction efficiencies of 37% and 55% in the 0th order for each of the respective illumination wavelengths. Operating at 532nm, the diffraction efficiency of the resonance domain element was measured to be 70%. The resonance domain element is significantly shallower than the scalar domain device with a reduced number of phase levels (2 compared to 16). With the final aim of this application area being replication on a flexible base substrate, the elements with larger feature sizes would represent challenging replication process (in terms of linearization of depth profile) whereas the smaller feature sizes would be more challenging in lateral dimensions. In both cases the potential for counterfeiting would be reduced and the addition of a second image at a different illumination wavelength for the scalar domain element would lead to a further enhanced degree of security.