Investigation of Optical Limiting Mechanism in Multiwalled Carbon Nanotubes
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ABSTRACT We report an investigation into the mechanism responsible for the optical limiting behavior in multiwalled carbon nanotubes. We conducted energy-dependent transmission measurements, picosecond time-resolved pump-probe, and nonlinear scattering experiments at 532-nm wavelength on multiwalled carbon nanotube suspension. For comparison, C60-toluene solution and carbon black suspension were also studied in the same experiments. The similarities that we observed between the multiwalled carbon nanotubes and carbon black suspension suggest that nonlinear scattering, which is known to be responsible for the limiting action in carbon black suspension, should play an important role in the limiting effect in multiwalled carbon nanotubes. INTRODUCTION
As a new one-dimensional structure of the carbon family [1], carbon nanotubes have attracted considerable attention because of their unique structure, which may lead to various electronic and optical applications [2-4]. Our previous study showed that multiwalled carbon nanotubes, which consist of multi-layers of graphene sheets and roll up like a hollow cylinder, are a promising candidate for broadband optical limiting in the visible and infrared region [5,6]. The limiting action was also observed in single-walled carbon nanotubes [7], which are made of one layer of graphene sheet. However, so far no direct experimental evidence was reported on the physical origin of the limiting behavior in carbon nanotubes. Here we present an experimental investigation into the mechanism responsible for the optical limiting phenomena in the multiwalled carbon nanotubes. The origins of the optical limiting properties of Cr,, solution and of carbon black suspension (CBS), which are amorphous aggregates of carbon particles, have been extensively studied [817]. Two different mechanisms, excited-state absorption and nonlinear scattering, were employed to interpret their respective limiting behavior [8-17]. It was generally accepted that excited-state absorption is dominant for C..... which can be described by a five-level model [8-13]. On the other hand, nonlinear scattering was found to be responsible for the limiting action in CBS. In the nonlinear scattering model, the limiting action results from vaporization and ionization of absorbing particles [14]. Due to the heating by intense laser pulses, the carbon particles heat up, vaporize and ionize to form micrometer-sized plasmas. As a consequence, plasma absorption and scattering by expansion of the induced micro-plasmas take place, which give rise to the limiting effect. It is also possible that the heat is transferred from the particles to
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Mat. Res. Soc. Symp. Proc. Vol. 597 ©2000 Materials Research Society
the surrounding liquid and form micro-meter-sized bubbles, which subsequently scatter light [1417]. To investigate the mechanism for the limiting behavior in multiwalled carbon nanotube (MWNT) suspension, we performed the following experiments at 532-nm wavelength. (1) Energy-dependent transmission measured with pico- and na
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