1. Introduction

Since carbon nanotubes (CNTs) were adopted as a saturable absorber1, their photonic applications have been intensively investigated. They have strong third-order nonlinearity [2], and ultrafast recovery time which is shorter than 1ps [3] owing to their one-dimensional structures. Their potential applications include ultrafast all optical switches and all optical logic gates. However, handling of CNTs is a large issue and several methods have been proposed and demonstrated: spraying1, direct synthesis [4], and polymer composition [5,6].

To overcome the problems, we have proposed an optically manipulated CNT deposition method [7,8] and another group separately demonstrated the technique subsequent to our proposal [9]. The method requires only a lightsource to deposit CNTs onto a core region of an optical fiber end. Light injection from the fiber end into CNT-dispersed solution preferentially deposits CNTs onto core regions of optical fiber ends, resulting in efficient CNT-use. Optical pass alignment, therefore, is not necessary to this technique. The principle of optical deposition is not well confirmed, but one possible mechanism would be flow of the solution induced by the light injection. The light injection thermally causes convection and swirl nearby the core, and result in the CNT deposition. Another probable mechanism is optical tweezer effect, which traps micro- and nano-sized objects by the optical intensity diversion in the solution. The optical trapping of CNTs with a single focused beam in a solution has been reported [10,11].

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. With the increase of the light intensity, flow speed becomes too high to trap the CNTs onto the core. High power injection makes the CNT layers around the core, not on the core, as we have already reported [7,8]. 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. In this paper, we employed optical reflectometry to simplify the optimization process and deposit CNTs onto very small areas.

2. Experimental setup for in-situ monitoring of optical CNT deposition

The introduction of optical reflectometry produces important functionalities to the system: deposition starting time detection and in-situ layer uniformity evaluation. The thickness of CNT layer can be roughly adjusted by controlling duration of injection even without the reflectometry starting. Since entangled CNTs which may flow to a fiber end are trapped and deposited, CNT deposition does not start just after the light injection. This is the reason why it is difficult to control the number of CNTs. First deposition of CNT entanglement drastically increases reflectivity at the fiber end due to the high index contrast between CNTs and silica-glass. It, then, become a seed of a CNT layer and the deposition continues because of strong Van der Waals force among CNTs. The duration of the deposition, therefore, controls the layer thickness by introducing optical reflectometry. As the layer become more uniform, the reflection became less affected by the solution flow. The fluctuation of the reflectivity gradually decreased.

The experimental setup for the in-situ optical reflectometry of CNT deposition is shown in Fig. 1. Light at a wavelength of 1560 nm from a laser diode was used for both the optical deposition and optical reflectometry. The light was amplified by a high-power erbium-doped fiber amplifier (EDFA), and subsequently was split into two by a 10:90 coupler. The 10 % of the light was monitored for reference by a power meter after 20 dB attenuation. The light of 90 % was injected from a cleaved fiber end into a dimethylformamide (DMF) solution, where purified CNTs were uniformly dispersed. The power of the reflected light from the fiber end was measured by another power meter through a circulator. The reference and the reflected light powers were measured at every 500 msec. The refractive indices of DMF and silica-glass are 1.42 and 1.44, respectively. Since the refractive index difference between DMF and silica-glass was small, the reflection was suppressed before CNT deposition. On the contrary, semiconducting CNTs had the refractive indecies of around 3.0 [12], though the refractive indecies of CNTs depend on their chiralities. The reflectivity drastically increased after the first deposition of an entanglement. The deposition was achieved by the optimization of the injection power with monitoring the reflection. Even if we repeat the experiment by changing the injection power with the highly uniform CNT solution, we could not deposit the CNTs only onto the core without the reflectometry. It was because there was very small margin of the injection power when we used the highly uniform CNT solution. Moreover, the solution condition, especially the sizes of the CNT bundles, changed in time and this prevented us from the preferential deposition without the reflectoemtry. The optical reflectometry offered the detection capability of the starting time of CNT deposition to the system and, consequently, controllability of the number of CNTs by adjusting the light injection duration after the deposition started. Subsequent to the process, we took microscope images and field emission scanning electron microscope (FE-SEM) images of the fiber ends.

 

Fig. 1. Experimental setup for optically manipulated CNT deposition with optical reflectometry monitoring.

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3. Experimental results and discussions

Figure 2 shows in-situ optical reflectometry data series which describe the deposition processes of thin and thick layers. Once the EDFA was turned on, its output was kept constant at around 20 dBm. It took a short period of time (10-100 s) to start the deposition. The start timings were not the same in the two cases because the solution was not completely uniform. The reflectivity was as small as -40 dB because of the small refractive index difference between DMF and the fiber as mentioned before. Figure 2 clearly show the drastic increase of the reflectivities about 20 dB due to the first entanglement deposition. The thicknesses were controlled by changing the light injection period after the increase of reflectivity. To deposit a thin layer, the EDFA was turned off at 8 seconds after the start of deposition as shown in Fig. 2(a). In the case of the thick layer deposition, the EDFA kept on for about 4 minutes after the deposition starting. During the deposition, the fluctuation of reflectivity gradually decreased (Fig. 2(b)). It indicates that the CNT layer gradually became uniform.

Optical microscope images of the thin and thick layers are shown in Fig. 3. The figures describe the thickness difference between the two layers. To directly observe the existence of CNTs, the layer thicknesses and the sizes of the deposition regions, we took the FE-SEM images of the fiber ends shown in Fig. 4 and 5. The thin CNT layer was clearly observed only at the core region in Fig. 4, whereas the deposited layer was very thick in Fig. 5. The thick layer was preferentially deposited only onto ~15μm diameter circular region. This result was achieved the using highly uniform solution and the precise optimization of injection power which was enabled by the in-situ optical reflectometry.

 

Fig. 2. Data series of optical reflectometry of CNT deposition (a)thin layer deposition process (b)thick layer deposition process.

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Fig. 3. Optical microscope images of fiber ends with CNT layers on the core region (a) thin CNT layer (b) thick CNT layer.

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Fig. 4. FE-SEM images of the fiber end with the thin layer (a) whole fiber end (b) magnified around the core region.

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Fig. 5. FE-SEM images of the fiber end with the thick layer (a) whole fiber end (b) magnified around the core region.

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4. Conclusion

In this paper, for the first time, we proposed and demonstrated the application of optical reflectometry for the in-situ monitoring of the optical CNT deposition processes. We successfully observed the start timing of the deposition and controlled the CNT layer thicknesses by the optical reflectometry. The FE-SEM images support the controllability of the number of the CNTs on the fiber ends. These results are the first observation of FE-SEM images of deposited CNTs onto the fiber end, which is the direct evidence of the area-selective deposition, and the thickness controllability. Using the highly uniformly dispersed solution, we realized preferential deposition onto ~15μm diameter circular region.