Structural origins of circular dichroism in surface second harmonic generation

Circular dichroic (CD) ratios often exceeding 100% have been reported in previous surface second harmonic generation (SHG) measurements of chiral surface films, offering promise for the development of unique characterization methods to study biologically interesting surface systems. In this work, the molecular and surface structural origins of these large dichroic differences were explored in theoretical treatments and modeling calculations. Several new conclusions were formed regarding chirality in electric dipole allowed SHG measurements; (1) SHG-CD is allowed even in uniaxial films of achiral chromophores, provided they assemble with asymmetry in the twist angle, (2) for systems dominated by one or more chiral tensor element, this same asymmetry in surface packing is required for SHG-CD to be observed, and (3) large SHG-CD ratios are predicted in standard reflection and transmission measurements of ultrathin films in systems with spectral overlap between multiple excited states. The methodology described provides a formalism to aid in extracting structural information from nonlinear optical measurements of chiral films.