Table-Top Lasers for Resonant Infrared Deposition of Polymer Films

有机分子和聚合物薄膜在对现代生活至关重要的大量电子、光子、机械和医疗技术中起着至关重要的作用。重要的例子包括用于微机械系统的防静电涂层，如计算机磁盘驱动器，用于发光器件和柔性显示器的多层薄膜，用于计算机和电视显示器的薄膜晶体管，以及用于定时释放药物的可生物降解涂层。目前用于沉积这种薄膜的工业技术存在一些明显的缺陷。例如，用于有机发光二极管和光伏太阳能电池的共轭聚合物薄膜已经通过基于溶液铸造工艺的各种方法制造，但这会导致溶剂诱导的构象缺陷，从而对光电行为产生不利影响。作为一种无溶液的替代方法，热蒸发对于短链低聚物和小有机分子是可行的，但对于长链共轭聚合物是非常具有挑战性的。另一个例子，PTFE薄膜由于其生物相容性、低摩擦阻力、化学惰性和低介电常数而被大量应用，并且已经开发了许多用于其沉积的技术，但每种技术都有一定的缺点。例如，由于缺乏合适的溶剂和需要对微电子结构不利的后退火，旋转涂层存在问题，而氟碳单体的等离子体聚合和溅射技术产生缺氟PTFE薄膜。正是由于这些原因，激光沉积聚合物薄膜变得非常重要。紫外激光可用于聚合物的脉冲激光沉积，但很难生长出与起始材料具有相同化学结构的薄膜。通常，聚合物在羽流中转化为单体和小的低聚碎片，并在沉积时发生再聚合。然而，如果再聚合不完全或由于直接裂解光反应而缺少基团，则薄膜将被化学修饰。基质辅助脉冲激光蒸发旨在通过将聚合物溶解在挥发性溶剂中来解决这个问题，然后将其冷冻以形成固体目标。理想情况下，聚合物对入射光是透明的，而宿主是高度吸收的，从而限制了激光和聚合物之间的直接相互作用。然而，这种理想的情况是不容易实现使用紫外激光器。这些问题导致了共振红外脉冲激光沉积（RIR-PLD）的发展，其中振动共振的激发可以导致相对较弱的分子间键断裂和沉积具有未修饰化学结构的聚合物薄膜。该技术可以直接应用于聚合物或在基质辅助格式。然而，相关的振动模式位于红外光谱（2-10um）的分子指纹区域内，在那里缺乏适当的激光源。因此，迄今为止绝大多数的RIR-PLD实验都是使用自由电子激光器进行的。虽然这个来源是理想的演示目的，但它肯定不适合商业处理设施。因此，我们建议建立一种新颖、紧凑和高效的高能皮秒脉冲源，在中红外波段具有广泛的可调性。该源是基于一个同步泵浦光学参量振荡器与光纤反馈臂，方便地允许长腔长度，相对较低的重复率，因此高脉冲能量。它将由一个简单的增益开关二极管激光器泵浦，通过掺镱光纤放大器缩放到高平均功率。这种桌面式自由电子激光器的替代品将彻底改变薄膜聚合物沉积，从而对许多重要应用产生重大影响。

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.