Table-Top Lasers for Resonant Infrared Deposition of Polymer Films

Lead Research Organisation: University of Southampton
Department Name: Optoelectronics Research Centre

Abstract

Thin films of organic molecules and polymers play a critical role in a huge number of electronic, photonic, mechanical, and medical technologies that are crucial to modern life. Significant examples include antistiction coatings for micro-mechanical systems such as computer magnetic disk drives, multi-layer films for light-emitting devices and flexible displays, thin film transistors for computer and TV displays, and biodegradable coatings for time-release drug delivery. Current industrial technologies for deposition of such films have some significant drawbacks. For example, thin films of conjugated polymers, used in organic light-emitting diodes and photovoltaic solar cells, have been fabricated by a variety of methods based on solution casting processes, but this leads to solvent induced conformational defects that adversely influence the optoelectronic behaviour. As a solution-free alternative, thermal evaporation is viable for short chain oligomers and small organic molecules, but is very challenging for long-chain conjugated polymers. As another example, thin films of PTFE are desirable for a large number of applications due to its biocompatibility, low frictional resistance, chemical inertness, and low dielectric constant, and again many techniques have been developed for its deposition but each has certain drawbacks. For example, spin coating is problematic due to a lack of suitable solvents and a need for post-annealing that can be undesirable for microelectronic structures, while plasma polymerisation of fluorocarbon monomers and sputtering techniques produce fluorine deficient PTFE films. It is for these reasons that laser-based deposition of polymer films has become important. UV lasers may be used for pulsed laser deposition of polymers but it is difficult to grow a film with the same chemical structure as the starting material. Typically, the polymer is converted to monomers and small oligomeric fragments in the plume and repolymerisation occurs upon deposition. However, if repolymerisation is incomplete or if there are missing groups due to direct scission photoreactions then the film will be chemically modified. Matrix-assisted pulsed-laser evaporation aims to resolve this issue by dissolving the polymer in a volatile solvent, which is then frozen to create a solid target. Ideally the polymer would be transparent to the incident light and the host highly absorbent, thereby limiting direct interaction between the laser and the polymer. However, this ideal situation is not easily accomplished using UV lasers.These problems have led to the development of resonant infrared pulsed laser deposition (RIR-PLD) where excitation of vibrational resonances can lead to the breaking of relatively weak intermolecular bonds and deposition of polymer films with unmodified chemical structure. This technique can be applied directly to the polymer or in a matrix-assisted format. However, the relevant vibrational modes lie within the molecular fingerprint region of the IR spectrum (2-10um) where there is an unfortunate dearth of appropriate laser sources. Consequently, the vast majority of RIR-PLD experiments to date have been performed using a free-electron laser. While this source is ideal for demonstration purposes it is certainly not suitable for a commercial processing facility.We therefore propose to build a novel, compact and efficient source of high-energy picosecond pulses with broad tunability in the mid-IR. The source is based on an synchronously pumped optical parametric oscillator with a fibre feedback arm to conveniently allow long cavity lengths, relatively low repetition rates, and hence high pulse energies. It will be pumped by a simple gain switched diode laser, scaled to high average powers by an Yb-doped fibre amplifier. This table-top replacement for the FEL will revolutionise thin-film polymer deposition and consequently impact strongly upon a number of important applications.

Planned Impact

Photonics, electronics, medical and other industries will benefit from the various aspects of the research outputs of this proposal. They will learn about our advances in ultrafast mid-IR laser sources and polymer thin film deposition from our publications, conference papers, web sites, and articles in a 6-monthly newsletter (the Light Times) circulated to a broad range of academics and industrialists. We will target conferences that have both high academic standing and a large industrial attendance, in a range of disciplines to cover both laser development and polymer thin film deposition. This will include CLEO Europe and CLEO US as the major international laser conferences and the EMRS spring meetings as major materials-research conferences. All these meetings have accompanying industrial exhibitions offering the opportunity to reach and network with both academic and industrial beneficiaries. We will also contribute to the COLA conference in the last year of the project to target a specialist laser ablation audience and to PHOTON 2012 to specifically impact upon UK academics and industrialists. UK and internationally-based industry will have the opportunity to commercialise various aspects of our work (the fibre laser, the OPO, the RIR-PLD technology) as we will assemble a highly-skilled impact panel that will inform us of end-user requirements and aid knowledge transfer. In the first instance, we will look to our project partners, Covesion Ltd (a commercial sector SME), as a route for transferring technology to UK industry, giving them the opportunity to license the generated intellectual property in accordance with our collaboration agreement. We believe the timescale of our project (3-years) is realistic in order for us to realise this immediate benefit through technology transfer. The impact panel will also have members who are more concerned with the application of the proposed laser source such as Prof. Haglund of Vanderbilt University and AppliFlex LLC (as a potential exploiter of the technology for fabrication of polymer thin films and as a means to access the many companies interested in polymer thin films that he has built contacts with during his studies on RIR-PLD), Dr Zergioti of the National Technical University of Athens (as a user of polymer thin films for electronic devices), and Dr. Malcolm Watson of BAE Systems (as a user of mid-IR OPOs in defence applications). Through our regular meetings with the panel we will be informed of their end-user requirements, which will guide our research so that it is of the most benefit to them and other researchers and industrialists in their fields. The timescale for these users to benefit will be longer (~5 years), as in some cases further work may be required to optimise the lasers to particular applications. However, these users could take our work as a starting point for further investigation or indeed further collaborative funding could be applied for to realise these applications. In the longer term (10 years and beyond) the general public will also be beneficiaries as the application areas for our ultrafast laser system and the high-quality polymer-film deposition technology that we will develop are of relevance to healthcare, environmental monitoring, and many of the modern electronic, photonic, and mechanical devices that are vital to our modern quality of life. The staff working on the project will have a full range of professional development courses made available to them. In terms of research skills, the postdoctoral researchers will benefit from gaining experience in ultrafast photonics, fibre lasers, OPOs and their application to pulsed laser deposition of polymers. They will also benefit from involvement in the various collaborative aspects of the project, which has strong potential for knowledge transfer activities. These research and professional skills are of high relevance to potential future employers in all sectors.

Publications

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Description We have investigated the limits of amplifying near-infrared picosecond pulses to high energies in standard all-solid fibres. We have produced a compact and robust source of pulses with energies up to 17 micro-Joules. This source was then used to pump an optical parametric oscillator (OPO), generator (OPG) and amplifier (OPA) and produce pulses with high energies that are tunable across the mid-infrared part of the spectrum. Several novel configurations of the OPO with have been investigated in order to match the needs of synchronous pumping to the low pump repetition rate, while maintaining a compact cavity. Such sources have applications in ablation and deposition of high quality polymer films for a number of applications but this requires further investigation.
Exploitation Route The high-power fibre laser source could be useful for materials processing applications, and the high-pulse mid-IR OPOs are suitable for resonant mid-IR pulsed laser deposition for a number of applications involving high-quality polymer thin films.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Healthcare,Manufacturing, including Industrial Biotechology