Chip-Scale Aligned Chiral Carbon Nanotubes Exhibiting Giant Second Harmonic Generation
Chiral carbon nanotubes (CNTs) are direct-gap semiconductors with optical properties governed by one-dimensional excitons with enormous oscillator strengths. Each species of chiral CNTs has an enantiomeric pair of left- and right-handed CNTs with nearly identical properties, but enantiomer-dependent phenomena can emerge, especially in nonlinear optical processes. Theoretical studies have predicted strong second-order nonlinearities in chiral CNTs, but no experimental quantitative verification has been reported due to the lack of macroscopically ordered assemblies of single-enantiomer chiral CNTs. Here, we report the synthesis of centimeter-scale, densely packed, aligned single-enantiomer chiral CNT films that are microfabrication-compatible. We observe giant second harmonic generation (SHG) emission from the chiral CNT film, which originates from the intrinsic chirality and inversion symmetry breaking of the atomic structure of chiral CNTs. The observed nonlinear susceptibility of the as-fabricated film reaches $4.9\times 10^2$\,pm/V at a pump wavelength of 1030\,nm, corresponding to the lowest-energy excitonic resonance, indicating $\chi_{xyz} = 1.6\times 10^3$\,pm/V for a perfectly aligned CNT crystal. Our calculations based on many-body theory correctly estimate the spectrum and magnitude of such excitonically enhanced optical nonlinearity. These results are promising for the development of scalable chiral-CNT electronics and nonlinear photonics.