Investigations into flexible ultrathin multi-level micro-optics using femtosecond laser pulses

• Main Author: Low, Mun Ji
• Other Authors: Murukeshan Vadakke Matham
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/143910
• Institution: Nanyang Technological University

Next-generation hybrid optics will provide superior performances over traditional optics by integrating the advantages of refractive, reflective, diffractive optics, and metasurfaces. Traditionally, hybrid optics have been realised by patterning diffractive optical structures on the surface of conventional bulk refractive or reflective optical elements. However, high-resolution manufacturing requirements for diffractive patterns over free-form refractive or reflective optical surfaces have hindered the widespread implementation of hybrid optics. In the context of complex integrated hybrid systems, the conventional solution is to stack or align several diffractive and refractive optical elements with varying surface profiles. Such combinations usually have challenging fabrication processes requiring custom-made fabrication equipment, are bulky and do not provide a unified and versatile approach that can be applied to arbitrary geometries. Alternatively, a conformal layer of planar diffractive lens (PDL) on a flexible substrate can be directly attached to the surface of any arbitrarily shaped object and helps to muffle the bulkiness of the hybrid system. Most importantly, the concept of combining arbitrary form factors could bring together the advantages of each optical element, or in some instances, intentionally introduce novel functionalities. In this context, the thesis aims to investigate flexible micro-optics with the value proposition of cost-effectiveness, simple manufacturing process and compact form factor. The development methodology and instrumentation scheme involve the patterning of the PDL based on graphene oxide (GO) thin films and subsequent transfer to a flexible substrate. The proposal is based on direct laser writing (DLW) process using a femtosecond (fs) pulsed laser as the energy source. The concept of flexible or stretchable graphene-based PDL is further investigated for applications in stackable hybrid optics. The initial investigation in the thesis involves fundamental research on the methodology for micro-optics patterning. A micro-optics patterning platform that incorporates a femtosecond pulsed laser as the energy source was proposed and all the required instrumentation including optical and opto-mechanical components (galvano scanner, f-theta lens, etc.), control hardware and software interfacing have been carried out during the investigation. The developed platform with well integrated hardware and software has allowed the user to assign parameters (such as laser, opto-mechanical and calibration) as required for the design to perform patterning with ease and flexibility. Subsequently, in-depth investigations on the photon-material interaction and heat-related effects with femtosecond laser pulses were carried out using the developed direct laser writing system. Insights gained from these investigations form the basis for optimising the parameters for realising graphene-based diffractive micro-optical elements. Various diffractive optical elements such as gratings and Fresnel zone plates (FZPs) were designed and fabricated through the combination of GO and reduced graphene oxide (rGO). It is demonstrated that diffractive optical elements with micrometer resolution can be patterned using the developed femtosecond laser direct writing (FsLDW) system. The investigations show the feasibility of constructing diffractive optical elements on GO thin films with high aspect ratio through the conceptualised one-step patterning methodology using the developed experimental configuration. The diffractive optical elements fabricated were further characterised for their optical performances. One of the most commonly used diffractive optical element is the Fresnel zone plate. Combining such a diffractive FZP to refractive optical elements to form a hybrid optical component is a promising strategy to achieve improved miniaturized devices with better performances compared to their refractive counterparts. Nonetheless, the design, simulation, and optimization of such hybrid optical element assemblies are complex, and finding the analytical solutions to these systems can be tedious and time consuming. In this context, the thesis also details a method for simulating the diffractive optical elements and hybrid optical systems using Zemax OpticStudio®. The designed optical performances of the hybrid optical system (realised by attaching an ultrathin flexible diffractive optics array onto a cylindrical lens) was analysed using the simulations and the results were compared with the optical performances achieved from actual experiments. Armed with the expertise and knowledge from these investigations, a novel, multi-level ultrathin diffractive optical element was fabricated by the tunable photoreduction of graphene oxides using the FsLDW platform. This demonstrated the first use of graphene oxides for generating multi-level transmittance and phase profiles in diffractive optical elements. Binary, four-level, and six-level diffractive FZPs were patterned to demonstrate the better light concentration to the main peak with suppressed multiple foci and side peaks by tunable photoreduction. The multi-level FZP was designed for a focal length of 15 mm at 638 nm wavelength and a spot size smaller than 14 µm was achieved with the suppression of diffractive side peaks by 14.9 % at the first-order and 10.8 % at the second-order (in comparison to a binary FZP). For the proof-of-concept for hybrid optics, the diffractive micro-optic array was transferred to a polydimethylsiloxane (PDMS) film of approximately 100 μm thickness, which was then attached on top of a bulk cylindrical lens. The hybrid optics enabled beam shaping of the transverse beam was investigated. The resulting optical characteristics agreed well with simulation results. The proposed method enables patterning of any arbitrary designed diffractive optics with multiple amplitude and phase modulations with lower cost and a short lead time on flexible and stretchable substrates. The contributions and findings from the thesis on the research of graphene-based PDL are expected to pave the way for applications requiring ultrathin and light weight optics. It is envisaged as enabler to novel hybrid optical components such as nature-inspired insect's micro-optic hybrid compound eyes for 3-Dimensional (3D) imaging, hybrid Shack-Hartmann wavefront sensors offering a wider dynamic range, compact heads-up displays for automobiles, higher performance optics with ultrathin aberration-tailored hybrid optics and efficient dispersion compensation for ultrafast photonics.