Optical Deposition of Carbon Nanotubes for Fiber-based Device Fabrication

Abstract

Since carbon nanotubes (CNTs) have been applied to a passive mode-locker, or a saturable absorber (Set et al., 2004a; Set et al., 2004b), applications of CNTs in photonics field have been intensively investigated. Their quasi-one-dimensional structures produce their distinctive characteristics, their strong third-order nonlinearity and ultrafast recovery time, shorter than 1ps (Chen et al., 2002; Ichida et al., 2002). These characteristics are attractive for future ultrafast photonic networks and can be used in ultrafast photonic devices, such as ultrafast all-optical switches and all-optical logic gates. Since CNTs are nano-sized material, one of the largest problems to realize CNT-based devices is handling issue. Optical deposition method has an advantage in efficiency over the other handling methods, such as spraying (Set et al., 2004a; Set et al., 2004b), direct synthesis (Yamashita et al., 2004), and polymer embedding methods (Sakakibara et al., 2005). These methods mostly require complicated processes, large-scale setups, and dissipate significant numbers of CNTs. A simpler and more cost effective handling technique of CNTs is required for mass productive CNT-based optical devices. We proposed optical CNT deposition technique to solve the problem. The light injection into CNT-dispersed solution from an optical fiber end deposited CNTs onto core region of the optical fiber end. We realized a passively mode-locked fiber laser using the CNT-deposited fiber as a saturable absorber, or a passive mode-locker. However, the technique requires very precise control of the light injection power to deposit uniform and less scattering CNT layer, because highly uniform CNT solution, which has very small CNT entanglements, is required. Smaller CNT entanglements require higher injection power. High power injection makes the CNT layer around the core, not on the core. The upper limit of optical intensity depends on the flow speed caused by the injected light. Additional technique is, accordingly, needed to optimize injection power for each solution. We introduced in-situ optical reflectometry to monitor the deposition process, and area-selectively deposited CNTs onto core regions of optical fiber ends. The area-selectivity was confirmed by field emission scanning electron microscope (FE-SEM). The technique has been applied only to deposition of CNTs onto fiber ends, though it has potentially versatile applications. We propose and demonstrate optically induced deposition of CNTs around microfibers by injecting light through the fibers. We also demonstrate a passively mode-locked fiber laser using a CNT-deposited microfiber fabricated by the technique.