Focusing and delivery of laser radiation for nano- and microfabrication

Al-Jarwany, Qassim Ahmed

Abstract

The recent advances in nanotechnology and nanofabrication motivate the drive to achieve a tighter focusing of light; this requires a high numerical aperture (NA) optical system. The need for high optical resolution has led scientists to discover the use of optical microlens for improving the performance of high numerical aperture (NA) optical systems. By focusing the laser beam through a microlens, the width of the beam can be reduced according to the needs of the application. In this work, the laser beam was focused by a microspherical lens (NA=0.7) into 150 nm or by tapered fibre into 4 μm diameter spots. The measurements indicate the strong influence of tightly focused beams. This thesis comprises of three parts; the first results chapter investigates the choice of material by considering the material properties and feasibility of fabrication (chapter 2). It has been shown in previous studies that the glass transition temperature of the polymer is an important factor in determining the laser ablation rate. High glass transition temperatures make it a good material candidate for optical waveguides. Polycarbonate (PC), polymethylmethacrylate (PMMA), negative photoresist SU-8, and chitosan have been characterised to choose suitable material as a substrate for soft nanolithography (chapter 3). The choice of material due to the glass transition temperature of the material (from literature), material optical properties are investigated experimentally at the range of wavelength from 190 nm to 1000 nm. Laser ablation experiments on PC, PMMA, SU- 8 and chitosan using a 193 nm ArF laser over a fluence range of 10 mJcm−2 –1000 mJcm−2. The ablation threshold at 193nm was found to be 24, 110, 40, and 95 mJ.cm-2 for PC, PMMA, SU-8, and chitosan respectively. The photoresist SU-8 and chitosan were chosen as both materials are biocompatible, and have a high glass transition temperature. Optical properties measured for these materials found that both materials have much higher absorption coefficients (αSU-8 ~ 4.2×105m-1 and αchitosan ~3.3×105m-1) compared with PC and PMMA (αPC =1×105m-1 and αPMMA=2×105m-1 )at 193 nm.

The second part of this thesis reports experimental and computational results of an irradiated laser microsphere supported on biocompatible materials; SU-8 photoresist and chitosan (chapter 3). An ArF excimer laser (193 nm wavelength) was used with 11.5 ns pulse width to modify the underlying substrate, producing a single concave dimple. Atomic force microscopy and scanning electron microscope measurements have been used to quantify the shape and size of laser inscribed dimple. The dimple has a diameter of 150 ± 10 nm FWHM and a depth of 190 ± 10nm on SU-8 compared to 180 ± 10 nm FWHM and a depth of 350 ± 10nm on chitosan due to the optical properties of the materials. Finite-difference time-domain (FDTD) simulations were carried out to simulate the propagation of 193 nm laser radiation, focussed by a 1 µm diameter silica sphere. Finite Element Method (FEM) simulations were carried out to calculate laser- induced temperature rise of the both SU-8 and Chitosan layer beneath the microsphere. The SiO2 microsphere acts as a small ball lens tightly focussing the laser radiation. Delivery of the focussed laser radiation locally heats the substrate beneath the microsphere. As a consequence, mass transport takes place, forming a nano dimple.

The third part of this thesis presents the use of a CO2 laser (10.6 μm wavelength) for producing microlenses at the end of silica optical fibre (chapter 4). By focused CO2 laser beam, silica optical fiber is irradiated and heated to the softening points (1800 K) of the silica material. Surface tension and the parameters of the fabrication system shape the melted material into a spherical micro-lens or tapered fiber that remains joined to the optical fiber. Different core diameters (125, 400, 600, 1000, and 1500 μm) of multimode fibres have been used for this fabrication. The roughness of the microlens was reduced to less than 20 ± 1 nm roughness by polishing the surface with a CO2 laser at low power (1- 2 W). Throughout this work, different microlenses (ball/parabolic) and tapered fibres were fabricated at the end of silica optical fibre. The minimum spot diameter at FWHM was close to 160 μm and 110 μm for microball and parabolic lenses, respectively. While the tapers had the minimum waist diameters down to 4 μm and maximum taper length of ~ 3.5 mm using silica multi-mode fibre. Finally, the knife-edge technique and He-Ne laser beam (632.8 nm wavelength) were coupled into a fibre to investigate the properties of the microlenses which produced a minimum spot size of 5 ±1 μm at FWHM in the focal region of the tapered fibre lenses of 125, 400 and 600 μm core diameter of the fibre.

As a result, Chitosan and SU-8 have been used as substrate materials for recording tightly focussed focal regions, 193nm ArF laser has been used to realise extremely small, 150nm diameter, Photonic Nano Jets (PNJ’s). FDTD optical simulations accurately predict the spatial properties of microsphere PNJ’s emitting at 193. CO2 laser (10.6 μm) radiation has been used to form tapers and spherical lenses on the distal end of optical fibres. Finally, tight focusing using microspheres and lensed optical fibres could be integrated on lab-on- chip platforms for applications such as optical trapping and cell membrane modifications. An important application related to the results of this study is that focusing laser light produces a force that can be used to remove or trap selected cells or large tissue areas from living cell culture down to a resolution of individual single cells and subcellular components similar to organelles or chromosomes, respectively.

The nanostructures fabricated in this chapter can be refined to achieve specific dimensions in; diameter, depth, shape, and periodicity so they can be used as antireflective surfaces for solar-cell applications [1].or could be used in drug delivery [2]. While laser microbeams are frequently used for measurement or imaging of biological parameters as well as using the optical tweezer system for trapping or moving of cells, the future medical applications will be focused on micromanipulation or microdissection methods for delivering molecules or nano drugs into a cell [3]. Delivering such nano- drugs into cancer cells requires overcoming the cell membrane by focusing the laser. This phenomenon is named photoporation which is based on the generation of localized transient pores in the cell membrane using the photonic nano jet [4].

Citation

Al-Jarwany, Q. A. Focusing and delivery of laser radiation for nano- and microfabrication. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4223093

• Thesis Type: Thesis
• Deposit Date: Apr 9, 2021
• Publicly Available Date: Feb 23, 2023
• Keywords: Physics
• Public URL: https://hull-repository.worktribe.com/output/4223093
• Additional Information: Department of Mathematics and Physical Sciences, The University of Hull
• Award Date: Dec 1, 2020