カーボンナノチューブの非線形発光特性: 一次元における高密度励起子の挙動

抄録

This paper describes nonlinear photoemission properties of carbon nanotubes both in terms of experiment and theory. In experiments, we performed photoluminescence excitation spectroscopy where carbon nanotubes were excited with high-intensity optical pulses. We found that, as the intensity of excitation pulses increases, all the photoemission peaks from different structure carbon nanotubes showed clear saturation in intensity. Each peak exhibited a saturation value that was independent of excitation wavelength, indicating that there is an upper limit on the density of excitons. We propose that this behavior is a result of efficient exciton-exciton annihilation through which excitons decay nonradiatively. In order to explain the experimental results, we have developed a model taking into account diffusion-limited limited exciton-exciton annihilation and spontaneous decays of excitons in one dimension. The solution of the model reproduced the experimental results well, from which the exciton densities in carbon nanotubes were estimated. The validity of the model was confirmed by its comparison with Monte Carlo simulations. We also show that the conventional rate equation for exciton-exciton annihilation fails to fit the experimentally observed saturation behaviors, especially at high excitonic densities. Finally, we discuss possible influences of the existence of the upper density limit on some photoelectric applications of carbon nanotubes.