Efficient Second-Harmonic Generation from Molecular Monolayers

Organic nonlinear optical (NLO) materials offer excellent opportunities for high-density integration of flexible photonics and optoelectronics that are relevant to communication and computing technologies. Most organic NLO materials nowadays rely on permanent dipole moments in NLO molecules, which require complicated asymmetric molecular structures, bonding schemes, and alignment procedures. Here, we report on the observation of efficient second-harmonic generation (SHG) from a supramolecular nanostructure self-assembled from a basic chromophore. Using polarization-dependent SHG measurements in combination with transient absorption spectroscopy and Monte Carlo simulations, we identify the origin of the SHG to be light-induced transient dipole moments associated with charge-transfer states in the nanostructures. These findings highlight opportunities to harness NLO responses across a broad class of organic molecular assemblies by exploiting light-induced transient effects.