Excitonic Second Harmonic Generation from Chirality in Aligned and Enantiomer Enriched Carbon Nanotubes

Second Harmonic Generation (SHG) is a sensitive probe to structural inversion symmetry breaking. Chiral materials also break inversion symmetry, but SHG is still forbidden unless the material is strongly anisotropic and dispersive. One-dimensional chiral carbon nanotubes (CNTs) with sharp excitonic resonances are ideal material for chiral SHG, and a giant resonance-enhanced 2nd nonlinear susceptibility χ(2) up to nm/V was theoretically predicted but never experimentally quantified. Here for the first time, we measured large SHG from single-chirality and aligned (11,-5) CNT in densely packed and centimeter-scale thin films. We verified that the SHG emission originates from the intrinsic structural chirality instead of surface or defects by the tilting-angle and polarization dependence of SHG. The exciton-enhanced χ(2) reaches 410 pm/V for an ideal (11, -5) CNT crystal at a fundamental pump wavelength of 1025 nm near the E11 resonance of the CNTs. . Our findings provide a fast and scalable method for characterizing the alignment and enantiomer purity of chiral films for nonlinear optics and spintronics.