Chirality is a fundamental property of molecules and materials leading to very interesting optical activity effects. Classical optical effectivity effects such as optical rotation and circular dichroism have been known for many years and are of crucial importance to investigate the structure of molecules and to elucidate the secondary structure of biomolecules. Other optical activity effects also exist, both in linear and nonlinear optics, and are becoming increasingly important. Furthermore, new chiral materials based on nanoscale building blocks show very peculiar and new optical effects that can have important applications. The 16 papers in this feature issue focus on several new aspects of the role of chirality in optical phenomena in a variety of materials.
© 2014 Optical Society of America
Chiral materials are materials that lack mirror symmetry. They occur in two different forms, usually called enantiomers, that are each other’s mirror images. While most physical and chemical properties of both enantiomers are very similar, their optical properties differ significantly. For example, chiral materials exhibit optical activity effects in which the refractive index of each enantiomer is different for one particular handedness of circularly-polarized light. As a consequence, chiral materials give rise to effects such as optical rotation (or optical rotatory dispersion) and circular dichroism . Chirality is a well-known concept in the field of chemistry, biology and pharmacology. Most biological agents are chiral and therefore the enantiomeric purity of pharmacological agents is very crucial. Furthermore, all proteins and DNA are chiral, and as a consequence optical characterization techniques such as optical rotation, circular dichroism (including vibrational circular dichroism and Raman optical activity) are of crucial importance. In addition, the design of chiral molecules and materials is often accompanied by numerical simulations, based on quantum mechanics, because they can help to understand experimental phenomena and deduce structure-property relationships. In this issue, Cappelli et al. describe the current state-of-the-art for predicting the optical rotatory dispersion in aqueous solution for the challenging case of methyloxirane .
Lately, several other characterization techniques have attracted interest, most of them based on nonlinear optical techniques. Examples are second-harmonic generation – circular dichroism (SHG-CD) and second-harmonic generation – optical rotatory dispersion (SHG-ORD), both techniques that are sensitive to surface chirality . In this feature issue, Bruyère et al. uses SHG to study the formation of chiral supramolecular aggregates at an air-water interface . Huttunen et al. explore SHG-CD in complex 3D structures and show that third-harmonic generation – circular dichroism (THG-CD) effects could occur in biological materials .
The interplay between chirality and magnetism is also of great interest, not only from a fundamental perspective but also from an applications point of view since it could lead to the development of a whole new range of magneto-optical devices . Magneto-chiral dichroism is such an effect where the absorption coefficient of a chiral molecule for unpolarized light depends on the direction of a longitudinally applied magnetic field. In this issue, Hattori et al. give an overview of this phenomenon in aromatic π-conjugated compounds and discusses its role in the origin of homochirality of life and possible applications in asymmetric synthesis and new magneto-optical devices . Mathevet et al. on the other hand exploit magnetic circular dichroism to investigate the polarization states in the focal region of Gaussian beams .
Chirality is also an important factor in liquid crystals (LC). Especially cholesteric liquid crystal phases are of crucial importance for the design of opto-electronic devices . In this feature issue, Chen et al. investigates the dielectric properties and phase behavior of Blue phase liquid crystals while Lin et al. explore the role of a chiral dopant on the LC properties under in-plane switching in nonuniform fields [10, 11].
Most of the contributions to this feature issue are in the field of chiral metamaterials, nanomaterials and photonic crystals. It is clear that such materials display a variety of interesting optical effects such as negative refraction and strongly enhanced chiro-optical responses, and give rise to a whole range of new applications, such as biological sensors, optical circular polarizers, negative refractive index materials, etc. . In this issue, Potravkin et al. and Chadha et al. investigate helical photonic metamaterials, both metallic and dielectric, and their interaction with ultrashort pulses [13,14]. Kaya et al. on the other hand explores windmill-shaped planar structures and found an enhanced near-field chiro-optical response . Oates et al. focuses on the mid IR optical response in achiral split ring resonators due to extrinsic chirality (or pseudochirality) . A somewhat different topic was explored by Moocarme et al. who numerically investigated the optomechanical response of nanowires and demonstrates that the excitation of chiral plasmon modes leads to the strongest mechanical forces . Finally, Lee et al. studied the propagation of electromagnetic waves in one-dimensional photonic crystals with a chiral defect layer and found that under oblique incidence this gives rise to two defect modes that depend on the strength of the chirality index of the defect layer .
There are also three contributions related to the optical properties of the exocuticle of scarab beetles. Although at first sight this may seem unrelated to the topics mentioned in the previous paragraphs, Ching et al. found that the structure and optical properties of the exocuticle of the scarab beetle Rhomborhina Gigantea is very similar to cholesteric liquid crystals . Along the same line, Mendoza-Galván et al. found that the dispersion relation of optical modes in the exocuticle of Cotinis mutabilis is similar to that found in cholesteric liquid crystals and Hernández-Jiménez et al. showed that light reflected from the cuticle form Chrysina aurigans is mainly left-hand circularly-polarized [20, 21].
References and links
1. L. D. Barron, Molecular Light Scattering and Optical Activity (Cambridge University Press, 1982).
2. F. Egidi, I. Carnimeo, and C. Cappelli, “The optical rotatory dispersion of methyloxirane in aqueous solution: assessing the performance of density functional theory in combination with a fully polarizable QM/MM/PCM approach,” Opt. Mater. Express 5(1), 196–209 (2015).
3. T. Verbiest, K. Clays, and V. Rodriguez, Nonlinear Optical Characterization Techniques: An Introduction (CRC press, 2009).
4. A. Bruyère, E. Benichou, L. Guy, A. Bensalah-Ledoux, S. Guy, and P.-F. Brevet, “Reversibility of the supramolecular chirality of bridged binaphtol derivatives at the air-water interface,” Opt. Mater. Express 4(12), 2516–2524 (2014). [CrossRef]
5. M. J. Huttunen, M. Partanen, G. Bautista, S.-W. Chu, and M. Kauranen, “Nonlinear optical activity effects in complex anisotropic three-dimensional media,” Opt. Mater. Express 5 (1), 11–21 (2015).
6. G. L. J. A. Rikken and E. Raupach, “Observation of magneto-chiral dichroism,” Nature 390(6659), 493–494 (1997). [CrossRef]
7. S. Hattori and K. Ishii, “Magneto-chiral dichroism of aromatic π conjugated systems,” Opt. Mater. Express 4(11), 2423–2432 (2014). [CrossRef]
8. R. Mathevet and G. Rikken, “Magnetic circular dichroism as a local probe of the polarization of a focused Gaussian beam,” Opt. Mater. Express 4, 2574–2585 (2014).
9. P. G. de Gennes, J. Prost, The Principle of Liquid Crystals (Oxford, 1993).
10. S. Chen, P. Wu, and W. Lee, “Dielectric and phase behaviors of blue-phase liquid crystals,” Opt. Mater. Express 4(11), 2392–2400 (2014). [CrossRef]
11. G. Lin, T. Chen, Y. Lin, J. Wu, and Y. Yang, “Effects of chiral dopant on electro-optical properties of nematic liquid crystal cells under in-plane switching and non-uniform vertical electric fields,” Opt. Mater. Express 4(12), 2468–2477 (2014). [CrossRef]
12. V. K. Valev, J. J. Baumberg, C. Sibilia, and T. Verbiest, “Chirality and chiroptical effects in plasmonic nanostructures: fundamentals, recent progress, and outlook,” Adv. Mater. 25(18), 2517–2534 (2013). [CrossRef] [PubMed]
13. N. Potravkin, E. Cherepetskaya, I. Perezhogin, and V. Makarov, “Ultrashort elliptically polarized laser pulse interaction with helical photonic metamaterial,” Opt. Mater. Express 4(10), 2090–2101 (2014). [CrossRef]
14. A. Chadha, D. Zhao, and W. Zhou, “Comparative study of metallic and dielectric helix photonic metamaterial,” Opt. Mater. Express 4(12), 2460–2467 (2014). [CrossRef]
15. S. Kaya, “Circular dichroism from windmill-shaped planar structures operating in mid-infrared regime,” Opt. Mater. Express 4(11), 2332–2339 (2014). [CrossRef]
16. T. W. H. Oates, T. Shaykhutdinov, T. Wagner, A. Furchner, and K. Hinrichs, “Mid-infrared gyrotropy in split-ring resonators measured by Mueller matrix ellipsometry,” Opt. Mater. Express 4(12), 2646–2655 (2014).
17. M. Moocarme, B. Kusin, and L. Vuong, “Plasmon-induced Lorentz forces of nanowire chiral hybrid modes,” Opt. Mater. Express 4(11), 2355–2367 (2014). [CrossRef]
18. K. Lee, J. Wu, and K. Kim, “Defect modes in a one-dimensional photonic crystal with a chiral defect layer,” Opt. Mater. Express 4(12), 2542–2550 (2014). [CrossRef]
19. S. Ching, G. Li, H. Tam, D. Goh, J. Goh, and K. Cheah, “Chirality in Rhomborhina Gigantea beetle,” Opt. Mater. Express 4(11), 2340–2345 (2014). [CrossRef]
20. A. Mendoza-Galván, E. Muñoz-Pineda, K. Järrendahl, and H. Arwin, “Evidence for a dispersion relation of optical modes in the cuticle of the scarab beetle Cotinis mutabilis,” Opt. Mater. Express 4(12), 2484–2496 (2014). [CrossRef]
21. M. Hernández-Jiménez, D. E. Azofeifa, E. Libby, C. Barboza-Aguilar, Á. Solís, L. Arce-Marenco, I. García-Aguilar, A. Hernández, and W. E. Vargas, “Qualitative correlation between structural chirality through the cuticle of Chrysina aurigans scarabs and left-handed circular polarization of the reflected light,” Opt. Mater. Express 4(12), 2632–2645 (2014).