Nanofabrication via laser interference lithography and integration of various optical systems for remote sensing applications

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Khalid, Muhammad (2019). Nanofabrication via laser interference lithography and integration of various optical systems for remote sensing applications. University of Birmingham. Ph.D.

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Abstract

Nanophotonic devices help to manipulate light at nanometric scale through various optical phenomena in near infrared and visible regions of the electromagnetic spectrum. This research aims to present the fabrication, modelling, optical characterisation and real life applications of optical devices based on materials such as ink, soft polymer, gelatine, leuco dye and liquid crystals. Laser interference-based ablation is utilised to generate phase conjugate nanostructures on ink, gelatine based edible nanostructures for food decoration purposes, and flexible polymeric nanostructures on polydimethylsiloxane substrate to demonstrate their use for remote sensing applications. Replication of Cornercube Retroreflector array and diffusing surface is conducted in this research to construct flexible force and temperature sensors.
CCRs are mainly exploited in this research due to their retroreflection property. Reflected light from CCRs is sent back towards the source through total internal reflection and is independent of the incident angle. A silver coated CCR is used to fabricate a 2D conjugate periodic gratings structure on ink coated glass substrate through Denisyuk reflection holography. Diffractive gratings (super prism) fabricated from simple mirror-based interference reflection have less features to manipulate as compared to the conjugate diffractive gratings made by using interference obtained from CCRs. Nanometric holographic CCR showed somewhat similar optical properties as shown by master centimetric CCR e.g. phase conjugation. Predictions through computational modelling were also in good agreement with the experimental (optical characterisation) results.
CCR array structures are most commonly encountered in everyday life activities such as traffic signals, vehicle safety systems and nightwear clothing. The use of brittle optical devices is limited due to their rigidity. In this research, PDMS was used to replicate rigid CCRs array structures into a flexible form. Polymeric CCRs array was examined and compared to the stencil by utilising optical microscopy. Optical characterisations were performed under various mechanical and thermal stress levels. Optical properties dependent on structure’s dimension were tuned based on the external stimuli such as force. It is concluded in this study that polymeric optical structures have a potential to be employed in numerous sensing applications for stretch, temperature, pH, and humidity.
Combination of CCRs and thermochromatic materials can yield remote temperature sensors based on active components. This research also demonstrates two different systems including liquid crystals and leuco dyes to record temperature changes within a region of interest. Glass based CCRs were coated with leuco dye and liquid crystals and were treated at various known temperatures under continuous monochromatic light illumination. Reflected power from thermochromatic CCR was tuned based on supplied temperature and was found to be dependent on the colour scheme. These novel systems may help to monitor environmental conditions such as temperature changes within hazardous areas, where human access is restricted.
Finally, edible, flexible and multi-layered materials were engineered with photonic structures to examine the flexibility of Nd:YAG laser ablation in Denisyuk reflection mode. Fabricated structures were examined by scanning electron microscopy and optically characterised with monochromatic and broadband light sources. Various shapes of nanostructures were achieved by utilising provided parameters for fabrication. It is hence concluded that laser interference-based ablation is simple, fast, cost-effective and flexible technique to copy reflective objects in nanometric scale.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):