Prospects for chiral nonlinear optical media

Abstract

This paper describes the development and optimization of chiral, nonpolar media with large second-order nonlinear optical responses. We employ molecular engineering, quantum-mechanical sum-overstates theory, and measurements of molecular hyperpolarizability by means of Kleinman-disallowed hyper-Rayleigh scattering in order to understand molecular properties. Then we analyze the appropriate arrangement of the chromophores that produce an optimum axial nonlinear optical medium. Chromophores with large Kleinman disallowed traceless symmetric second-rank tensor hyperpolarizabilities can be aligned so as to result in large susceptibilities (2) in structures that lack polar order. We found that 3-shaped chromophores with 2 or similar symmetry are good candidates for these materials, as they can exhibit large second-rank components of the hyperpolarizability tensor. A wide variety of techniques can be used to fabricate bulk materials belonging to the chiral nonpolar symmetry groups and 2. The microscopic chromophore alignment schemes that optimize the nonlinear optical response in such materials are deduced from general symmetry consideration for both molecules and bulk. We also speculate on the possible application of such materials as high-bandwidth spatial light modulators.

Molecular Origins of the Remarkable Chiral Sensitivity of Second‐Order Nonlinear Optics

“…In this case, explicit evaluation of Equation (3) yields remarkably simple relationships for the four symmetry-allowed nonzero independent elements of the macroscopic second-order nonlinear polarizability tensor c (2) describing SHG [Equations (7a)-(7d)]. [7,8,43,74,75] c ZZZ ¼ N s ½hcos 3 qib zzz þ hsin 2 qcosqcos 2 yiðb zxx þ 2b xxz Þ ð7aÞ…”

“…The generation of macromolecular chiroptical activity in chiral assemblies of achiral chromophores (referred to as structural rotary power) is reasonably well-understood in absorbance measurements of chiral media, [71,72] but has only relatively recently been applied to the interpretation of chiroptical effects in SHG and SFG of oriented systems. [7,8,42,43,45,46,62] Although predictions of possible chiroptical effects in SHG from oriented assemblies of planar chromophores with C 2v symmetry were reported as early as 1987, [73] most of the work in characterizing the role of molecular orientation in driving chiroptical phenomena in SHG and SFG of uniaxial films and interfaces has been performed within the last few years. [7,8,42,43,45,46,62] In both planar chromophores with C 2v symmetry and in "two-state" chromophores, in which the nonlinear optical properties of the chromophore are reasonably well-described by considering interactions between the ground state and a single excited state, only four elements within the molecular tensor are significant in electronically resonant SHG and SFG: b zzz , b zxx , b xzx , and b xxz .…”

“…[7,8,42,43,45,46,62] Although predictions of possible chiroptical effects in SHG from oriented assemblies of planar chromophores with C 2v symmetry were reported as early as 1987, [73] most of the work in characterizing the role of molecular orientation in driving chiroptical phenomena in SHG and SFG of uniaxial films and interfaces has been performed within the last few years. [7,8,42,43,45,46,62] In both planar chromophores with C 2v symmetry and in "two-state" chromophores, in which the nonlinear optical properties of the chromophore are reasonably well-described by considering interactions between the ground state and a single excited state, only four elements within the molecular tensor are significant in electronically resonant SHG and SFG: b zzz , b zxx , b xzx , and b xxz . [7,8] Clearly, chirality is absent within the molecular nonlinear polarizability tensor in such chromophores (i.e., the s xz operation leaves the b (2) tensor unchanged).…”

“…[7,[10][11][12][13][14][15] More recently, theoretical and experimental evidence suggests that macromolecular orientational effects alone can dominate the chiroptical responses of uniaxial systems without the need for multiple coupled oscillators or even chiral chromophores at all (Figure 1 c). [7,8,[42][43][44][45][46] [a] Prof. G. J. Simpson Department of Chemistry Purdue University, West Lafayette IN 47907 Fax:(+ 1) 765-494-0239 E-mail: gsimpson@purdue.edu Figure 1.…”

“…[34][35][36][37][38][39][40][41] Additionally, the novel symmetry properties of SHG and SFG in chiral media suggest that new nonlinear optical materials can be constructed that exploit these chiroptical effects. [42][43][44][45][46] A critical bottleneck in all of these emerging inquiries is the development of intuitive models for connecting the macroscopic chiroptical response with the molecular and supramolecular structure. A brief review of recent fundamental advances in the area is presented.…”