Visual Methods for Interpreting Optical Nonlinearity at the Molecular Level

Abstract

The emergence of nonlinear optical (NLO) measurement approaches has provided new windows into molecular and macromolecular structure within thin films and materials. The greatest barriers in mining this structural information increasingly appear in meaningfully relating these macroscopic results back to molecular-level descriptions, driven largely by the increasing complexity of the molecular systems and interfacial architectures under interrogation. As NLO methods continue their expansion into increasingly diverse disciplines, so grows the need for tools to guide this evolution without sacrificing the mathematical rigor of more traditional tensor representations. Recent developments reviewed in this Account are designed to facilitate interpretation of complex assemblies using relatively simple but still quantitatively accurate visual representations of the polarization-dependent optical nonlinearity, both for individual chromophores and for polymeric assemblies of coupled chromophores. Although the primary focus of this Account is on second-order nonlinear optical effects, including second harmonic generation and sum frequency generation, many of these same concepts also directly apply to higher-order phenomena.

Modeling the SHG activities of diverse protein crystals

“…1. The relationships between the visual representations and the tabulated tensor elements have been detailed previously (Moad et al, 2007;Wampler et al, 2007). In this case, the relatively large and positive xxx SHG tensor element corresponds to the amplitude of the lobe directed in the x direction in the hyperellipsoid representation and the x component of the vector intersecting the unit sphere along the x axis.…”

Imaging the Nonlinear Susceptibility Tensor of Collagen by Nonlinear Optical Stokes Ellipsometry

“…While it may initially appear overly simplistic to represent the hyperpolarizability of collagen using calculations for just the NMA building block, previous computational and experimental studies support the validity of this approach (37,(49)(50)(51). For all but the aromatic amino acids, the hyperpolarizabilities of polypeptides formed from the common amino acids are dominated by the amide unit connecting amino acid groups, which exhibits a relatively large hyperpolarizability.…”

Second-Order Nonlinear Optical Imaging of Chiral Crystals

“…The process of SHG can be qualitatively illustrated with a relatively simple anharmonic oscillator model (Figure 1) (7). Consider the linear interactions between a molecule and an incident oscillating field within the same framework: A polarization is induced by light within the medium through the harmonic “sloshing” of the electron cloud associated with the molecule.…”