ABSTRACT
Optical nonlinear effects in particular four-wave mixing (FWM) are experimentally observed using the fabricated CNT-based devices. FWM-based wavelength conversion applications are further achieved with good performance. 16.1 IntroductionCNTs have recently drawn much research attention owing to their unique optical properties [1-3]. CNTs are mainly categorized as single-wall or multiwall. The structure of a SWCNT can be simply understood by wrapping a one-atom-thick layer of graphene into a cylinder. A multiwall CNT (MWCNT) is two or more CNTs nested within one another in a coaxial form. As the properties of MWCNTs are determined by contribution of all individual shells with different structures, they are usually more defective than SWCNTs. In the case of single-wall ones, due to their pure one-dimension properties (with typical diameter of ~1 nm and length of ~1 μm) as well as well-defined structures, they exhibit properties that are not shared by MWCNTs [1]. The structure of the SWCNTs is basically determined by a single parameter called chirality. Depending on the chirality, they exhibit two different electrical properties: metallic or semiconducting. Semiconducting CNTs have energy bandgaps like those in ordinary semiconductors, and photons having corres-ponding optical frequency can be absorbed by the CNTs. This absorption is exicitonic and saturable, and its recovery is very fast in order of femtoseconds. The bandgap energy in semiconducting CNTs can be controlled by the tube diameter. In particular, a tube diameter of around 1.2 nm gives absorption at a wavelength around the 1550 nm optical communication band [4,5]. However, it is not yet possible to synthesize only one selected chirality using any fabrication methods and further processing of sorting a narrow chirality distribution of CNTs is necessary. During recent years, much research effort had been focused on a number of photonics applications and studies of SWCNTs, including mode-locked lasers [4-6], ultra-fast optical response [7,8], and optical noise suppression [9]. In particular, such CNTs can be employed as an optical nonlinearity medium owing to the theoretically estimated ultra-
high third-order nonlinearity [10-12]. It is believed that the third-order nonlinearity of the CNTs is originated from the inter-band transitions of the π-electrons causing nonlinear polarization. In this sense CNTs can be regarded to have similar properties with other organic optical materials such as polyacetylene or polydiacetylenes, which exhibit extremely high third-order nonlinearity. Experimental investigation of such optical nonlinearities in CNTs has been initiated by a Kerr shutter-based optical switching using a few centimeters of CNT-deposited D-shaped fiber [13,14] as well as optical loop mirror incorporated with CNT-loaded planar waveguide [15], which indicated the possibilities of practical nonlinear CNT-based devices. In this chapter, we describe the optical properties of SWCNTs and the considerations of CNT diameters in order to obtain suitable operation wavelength. The design and fabrication of CNT-based photonics devices with different platforms are introduced. Also, the experimental measurements of nonlinear effects generated from the fabricated CNT-based devices are discussed. In particular, the observation of four-wave mixing (FWM) in a CNT-deposited planar lightwave circuit (PLC) waveguide and its application in wavelength conversion are addressed. Such PLC waveguides can have much flexibility of integration and fabricating special waveguide structures. As a material for generating nonlinear effects, CNTs are deposited on the over-cladding removed PLC waveguide for CNT-light interaction. Tunable FWM-based wavelength conversion is obtained with the CNT device and a power penalty of 3 dB at 10-9 level is measured for 10 Gb/s non-return-to-zero (NRZ) wavelength converted signal in the bit-error-rate (BER) measurements. 16.2 Nonlinear Optical Properties of
Carbon NanotubesSince its discovery, CNT can be synthesized with many techniques such as chemical vapor deposition, laser ablation, etc. Among different methods, SWCNTs can be made by a bulk production method called high-pressure CO conversion (HiPCO). Since the isolation of individual CNTs from each other is critical to obtain the maximum nonlinearity, the diameter and the diameter
distribution of the CNTs are well controlled in the process. In order to further select the CNTs with suitable tube diameter, the fabricated CNT powder are dispersed in dimethylformamide (DMF), an effective solvent for separating and suspending individual CNTs. After several steps of centrifugal separations, only the homogeneous part of the CNT in DMF solution is adopted. Figure 16.1 shows the absorption spectrum of the CNTs measured by a spectrophotometer scanning from 400 nm to 2000 nm. By controlling the HiPCO process thus the nanotube diameters and the diameter distribution, the CNTs show a desirable absorption peak near 1550 nm. In the experiment, such properties of the CNTs with an absorption near 1550 nm are found to be highly suitable for generating nonlinear effects such as FWM at this wavelength range. The prepared CNT in DMF solution can be either directly sprayed on a waveguide surface with a spray gun or undergo optically-driven deposition to deposit onto an optical fiber. Figure 16.2 depicts the scanning electron microscope (SEM) image of some deposited CNTs on the surface of a waveguide, showing that the CNTs are successfully deposited on the surface.
