In a recent breakthrough, published in Nature Photonics (DOI: 10.1038/s41566-024-01486-z), the group of Prof. Ventsislav Valev from the University of Bath has demonstrated the first experimental realization of a 45-year-old prediction: the hyper-Raman optical activity. This discovery holds great potential for probing chiral molecular configurations—a critical aspect for fields like pharmaceutical development, biology, and nanotechnology.
Chirality refers to a property of a molecule where it has a non-superimposable mirror image, much like left and right hands. In biology, this “handedness” is essential because very often, only one specific chiral form of a molecule interacts correctly with biological targets, affecting processes like drug binding, enzyme function, and cellular signaling.
Chiral properties in molecules are usually induced chemically, but more recently, researchers were able to confer these properties to organic molecules through their interaction with specially designed nanostructures. The group of Prof. Ventsislav Valev managed to induce chirality in crystal violet molecules, a commonly used organic dye that can exist in both right- and left-handed forms, but is considered achiral as it can rapidly interconvert between both enantiomers. This was done by illuminating structured gold helices with circularly polarized light to generate a chiral electromagnetic field.
Artistic representation of hyper-Raman optical activity. Image: Ventsislav Valev and Kylian Valev
Consequently, hyper-Raman scattering could be considered as the nonlinear analog for Raman scattering, in the sense that two incident photons are needed for generating one scatter photon, as opposed to one incident photon for Raman scattering. This being essentially a two-photon process, the symmetry of the system is fundamental. The induced chiral properties imparted to the crystal violet molecules could be observed by hyper-Raman scattering, as it can reveal vibrational modes beyond what conventional Raman spectroscopy detects.
For this, the INSPIRE HF 100 laser was tuned at a wavelength of 1064 nm, in resonance with the gold nanohelices, while the crystal violet exhibited resonance at half that wavelength (i.e. 532 nm), creating a doubly resonant system. This innovative approach allowed for a significant amplification of the hyper-Raman signal.
The results of this study confirmed that the observed chiral intensity difference (CID) in the hyper-Raman spectrum depended on the polarization and handedness of the light and nanohelices, validating the effect as a true chiral phenomenon.
This marks an exciting milestone, not only in fundamental physics but also in the potential application of hyper-Raman scattering for chiral molecule analysis in advanced research fields.
In Prof. Ventsislav Valev’s words, “There is a long way ahead until the effect can be implemented as a standard analytical tool that other scientists can adopt, but we look forward to taking that journey.”
