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We have investigated third-order nonlinear optical properties of charge-transfer (CT) complexes of C 60 with various aromatic amines with different ionization potentials. The third-order nonlinear optical susceptibility |χ (3) | measured by degenerate four-wave mixing shows a resonant enhancement at the absorption band due to the CT transition. We have found that the second hyperpolarizability γ of CT complexes increases with an increase of the oscillator strength, which depends on the ionization potential. The highest hyperpolarizability among CT complexes studied here is 5.2 × 10 -28 esu for C 60 /aniline CT complex.

We have experimentally investigated the diameter dependence of Coulomb effects on the optical transition in semiconducting single-walled carbon nanotubes. The absorption band due to the lowest band-to-band transition shifts to the higher-energy side compared to the transition energy calculated by a tight-binding model. The blueshift originating from the Coulomb effect increases with decreasing tube diameter d as d Ϫ1.3. We have found that this dependence is attributed to both the increase of the band gap and the enhancement of the exciton binding energy with a decrease of the diameter.

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The origin of near-infrared (NIR) luminescence from graphene oxide (GO) is investigated by photoluminescence (PL) excitation spectroscopy, time-resolved PL spectroscopy, and density functional theory based many body perturbation theories. The energy of experimentally observed NIR PL peak depends on the excitation energy, and the peak broadens with increasing excitation energy. It is found that the PL decay curves in time-resolved spectroscopy show build-up behavior at lower emission energies due to energy transfer between smaller to larger graphene nanodisc (GND) states embedded in GO. We demonstrate that the NIR PL originates from ensemble emission of GND states with a few nanometers in size. The theoretical calculations reveal the electronic and excitonic properties of individual GND states with various sizes, which accounts for the inhomogeneously broadened NIR PL. We further demonstrate that the electronic properties are highly sensitive to the protonation and deprotonation processes of GND states using both the experimental and theoretical approaches.