1. Introduction

Carbon nanotubes (CNTs) are receiving increasing attention in photonics research field because of their exceptional properties after the reports of their ultrafast recovery time[1] and an ultrafast saturable absorber application, or a passive mode-locker[2]. Furthermore, they have high optical nonlinearity[1], and they are expected to become future optical nonlinear materials.

However, one of the biggest problems is difficulty of handling. Since they are spontaneously entangled with each other, it is difficult to disperse them in common solvents. To incorporate them into photonic devices, several approaches have been reported, such as spraying[2], polymer composition[3], direct synthesis[4]. We have proposed and demonstrated, for the first time, optical deposition technique[5,6]. Subsequently to our proposal, another group have separately demonstrated the technique using a different solvent[7,8]. We can deposit CNTs onto an optical fiber end using a simple process and a simple setup by propagating a light from the optical fiber end into a CNT dispersed solution. The technique permits most efficient use of CNTs for photonic applications among the techniques that have been already proposed.

Although CNTs were successfully deposited, they could be placed only onto the fiber ends. We found that sphere-shaped super-structures can be fabricated by simply changing the experimental condition using the setup similar to the optical deposition technique. Since super-structures could be physically manipulated, constructing super-structures made of CNTs is one of the important tasks. Conventional techniques to fabricate super-structures after synthesis of CNTs mainly utilize templates of super-structures and chemical functionalization[9]. In this study, we demonstrate optical formation of sphere-shaped super-structures made of CNTs without chemical functionalization. We fabricated the spheres using higher optical intensity than that of the optical deposition technique. Realization of a passively mode-locked fiber laser using the sphere showed that CNTs were incorporated in the sphere.

2. Optical formation of carbon nanotube spheres

Figure 1 shows a schematic of our experimental setup. Light at a wavelength of 1560 nm was amplified by a high-power erbium-doped fiber amplifier (EDFA). The light was incident from a cleaved fiber end into a droplet of CNT-dispersed dimethylformamide (DMF) solution. The setup was similar to the experimental setups used for the optical deposition technique[5–8]. In contrast to the previous reports, the microscope was used to observe formation of CNT spheres in the current work.

By precisely controlling the power of the injected light, we can deposit CNTs onto optical fiber ends. In Fig. 2, microscope images of the CNTs deposited fiber ends are shown. Optimum optical power for depositing CNTs onto a fiber core region depends on the sizes of CNT bundles in a solution. Generally, high optical power is needed to deposit small bundles. In contrast, low power is sufficient to deposit large bundles. For the case of our solution, CNTs were deposited onto a core of an optical fiber at the power of 21.5 dBm (Fig. 2(a)). The margine of the optical power to deposit onto the core was 1.0 dB. In contrary, CNTs were deposited around the core by higher injection power (22.5 dBm) (Fig. 2(b)). We found that a sphere-shaped CNT super-structure can be fabricated by much higher optical power.

 

Fig. 1. Schematic diagram of experimental setup for CNT spheres formation.

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Fig. 2. Microscope images of CNTs deposited optical fiber ends. (a) CNTs are deposited onto the core (b) CNTs are deposited around the core.

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Fig. 3. Microscope images of a growing CNT sphere (every 10sec.).

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Fig. 4. Mechanism of a CNT sphere formation using light injection.

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Fig. 5. Physical manipulation of a CNT sphere

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Figure 3 shows a cleaved fiber end and a CNT sphere which was fabricated by high intensity light at 28.0 dBm. First, a small sphere was formed (Fig. 3(a)). It gradually became larger, and finally its diameter reached 100 µm (Figs. 3(b) ~ (f)). The images in Fig. 3 were taken every 10 sec. A diameter of a CNT sphere could be controlled by optical intensity and a time period of the injected light.

Most probable mechanism of the CNT sphere formation is that CNTs were adsorbed onto a surface of a micro-bubble (Fig. 4). First, CNTs are optically deposited onto a fiber end as reported[5–8], and are heated by the light. Evaporated DMF become a micro-bubble which has a sphere shape. A micro-bubble in liquid adsorb small particles, such as CNTs, to their surfaces by electrostatic force[10]. CNTs which have been dispersed in DMF solution are gathered and deposited on the micro-bubble, and a sphere-shaped super-structure is fabricated.

Relatively small spheres were unstable because the spheres were based on micro-bubbles of evaporated DMF. In contrast, large spheres which had larger diameter than 15µm were stable, and could be physically manipulated. Figure 5 displays manipulation of a CNT sphere from an optical fiber to another optical fiber. Adsorption force between the fiber and the sphere was sufficiently strong and the sphere did not spontaneously detach from the fibers. This technique allows us to manipulate CNTs and deposit them onto desired positions by a simple process.

3. Mode-locked fiber laser using an optically formed carbon nanotube sphere

In the previous section, we have reported the optical formation of CNT spheres. However, we could not take them out from the DMF solution. It was difficult to maintain the shape when they passed through the boundary between liquid and air, and the spheres collapsed at the boundary. Because of difficulty of taking their Raman spectra or scanning electron microscope (SEM) images in liquid, we inserted one into a fiber ring laser cavity as a saturable absorber to confirm that the sphere incorporated CNTs. Realization of passive mode-locking verifies an existence of CNTs in the sphere.

Figure 6(a) shows an experimental setup for a passively mode-locked fiber ring laser using a CNT sphere. In the laser cavity, a small CNT sphere was inserted between two fiber ends. The gap was around 5µm, and was immersed in a CNT-dispersed DMF droplet. A microscope image of the sphere is shown in Fig. 6(b). An EDFA was used as a laser gain medium, and an isolator was inserted to prevent back reflection in the cavity. We controlled the polarization state inside the ring cavity using a polarization controller (PC). A 20-m-long single mode fiber (SMF) was inserted to control the total dispersion of the cavity to be nearly zero. The output light came out from a 3 dB coupler, and its optical spectrum was measured using an optical spectrum analyzer whose resolution was 0.1 nm.

We controlled the gain of the EDFA to maintain both the shape of the small sphere and the lasing. The fiber alignment was slightly changed after the sphere formation because the sphere changed the optical path between the two fibers. We obtained a broad lasing spectrum which is a feature of pulsed laser output as shown in Fig. 7. However, the spectrum does not seem like a soliton pulse spectral shape. Although we inserted SMF into the cavity, residual dispersion in the cavity prevented to form a soliton pulse. The pulsed oscillation was not stable enough to measure an autocorrelation trace, because the pulse inside the cavity had high peak power, and disturbed the stability of the CNT sphere. The disturbance resulted in the instability of the laser oscillation and the soliton pulse formation.

 

Fig. 6. (a) Schematic of an experiment setup for a passively mode-locked fiber laser using a CNT sphere. (b) Microscope image of a CNT sphere inserted between two fiber ends.

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Fig. 7. Output spectrum of a passively mode-locked fiber laser using a CNT sphere (0.1 nm resolution)

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During the lasing, CNT deposition onto the fiber core was prevented by the sphere which stuck on the core region. After the mode-locking experiment, we removed the CNT sphere and observed the optical fiber end with a microscope. CNTs were only deposited around the core, same as Fig. 2(b). The amount of CNTs for a passive mode-locking requires at least the amount that can be seen with a microscope like as Fig. 2(a). Thus, there was not enough amount of CNTs on the fiber core for a passive mode-locking in this experiment, although the deposition of the CNT bundle onto a fiber is the first step of a CNT sphere formation. Therefore, we concluded the mode-locking was realized by the CNT sphere and CNTs were included in the sphere.

4. Conclusion

In this letter, we reported the fabrication of sphere-shaped CNT super-structures without chemical functionalization. The super-structures were fabricated by the setup similar to that for optical deposition of CNTs. Higher optical power than that of optical deposition technique was used. By the light injection, CNTs were deposited onto the optical fiber end and were heated up by the light. The heat evaporated DMF and made a micro-bubble. Finally, a sphere-shaped CNT super-structure was formed by adsorbing CNTs onto the surface of micro-bubble. A diameter of a CNT sphere could be controlled by changing optical intensity and a time period of the injected light. We physically manipulated the CNT sphere and were able to put CNTs at any desired place.

To obtain a direct evidence which shows CNTs were incorporated in the sphere, we used the sphere as a saturable absorber and constructed a passively mode-locked fiber laser. The broad lasing spectrum was obtained which indicated the realization of the passive mode-locking. This result showed that CNTs were surely incorporated in the sphere.