Chiral second-order nonlinear optics

Organic second-order nonlinear optical (NLO) materials can be efficiently utilized in photonics devices for optical telecommunications and signal processing due to their high nonlinear response, processability, and low cost. Media with polar order are usually used to ensure the absence of the inversion symmetry required in second-order NLO. This study presents a method that results in creation of efficient chiral non-polar axially aligned NLO media. The stages of design and optimization include molecular engineering, quantum-mechanical theory of the molecular hyperpolarizability, chromophore characterization by means of Kleinman-disallowed hyper-Rayleigh scattering (KD-HRS), and analysis of the macroscopic alignment of the NLO chromophores in the bulk. Chiral systems belonging to non-polar symmetry groups D_∞ and D₂ with high nonlinear response can be easily fabricated in a number of polymeric and liquid-crystalline materials. The NLO susceptibility χ(2) of such media are defined by the Kleinman-disallowed irreducible component of the hyperpolarizability tensor β that transforms as a traceless symmetric second-rank tensor. Using KD-HRS, one can determine the figures of merit of all irreducible components of β including the second-rank part, which is relevant in chiral non-polar media. It is shown that conjugated Λ-shaped chromophores containing one electron acceptor and two donors (or vice versa) can have large second-rank component of β. Quantum-mechanically, this can be attributed to existence of low lying electronic states of B-symmetry, that is those that change sign upon rotation around the Λ's two fold axis. A propeller-like molecule Crystal Violet is also shown to have large second-rank response and is believed to be similar in this sense to a Λ-molecule. The analysis of alignment schemes for Λ-shaped and propeller-like chromophores shows that the optimized configuration can be achieved in uniaxially and biaxially stretched chiral polymers and in noncentrosymmetric liquid crystal phases. Potential applications for such media in photonics devices are discussed.