2023 Thomas Young Medal and Prize

Chirality is one of the most fundamental properties of nature, observable at scales ranging from the size of galaxies to that of subatomic particles. It is a property associated with the origin of life itself, as most biomolecules are chiral. Molecular chirality was discovered by Louis Pasteur by observing its effects on a light beam. The discovery of the laser revealed interactions with matter that can produce light at higher harmonics of the illumination frequency. Arguably, this marked the beginning of the field of nonlinear optics. Could the effects of chirality also be observed in light at higher harmonics?

In 1979, Professor David Andrews hypothesised that the chirality of scatterers could influence harmonic light. In his original paper, focusing on harmonic (or hyper) Raman scattering, the mathematical formalisms include harmonic Rayleigh scattering with explicit model calculations. Because of its fundamental character, the phenomenon predicted by Andrews would be the most direct expression of chirality in nonlinear optics. However, until recently, no one could actually observe this phenomenon, and Andrews later referred to his ideas as seemingly an “impossible theory”.

In the last two years, the situation has changed and chiroptical harmonic scattering has been observed across several materials, harmonics and scattering mechanisms. In 2019, Professor Ventsislav Valev’s team reported the first experimental observation of the effect in silver nanohelices randomly dispersed in a liquid environment. Valev’s team observed that upon illumination with circularly polarised ultrafast laser pulses, the intensity of light scattered as the second harmonic frequency of illumination changed, depending both on the chirality of the nanohelices and on the sense of circular polarisation. The paper, which appeared in Physical Review X, settled the 40 year-old scientific question and more experimental observations followed in quick succession.

In 2020, Valev’s team measured the new effect from chiral gold nanocubes, and reported the first experimental characterisation of a single nanoparticle evolving freely in a liquid environment. The same year, Valev’s experiments were replicated in molecules, by a research group in France. Demonstrating the general nature of the chiroptical harmonic scattering in 2021, Andrews and Valev jointly reported chiroptical third-harmonic Rayleigh scattering. Then in 2022, Valev’s team announced chiroptical third-harmonic scattering from chiral semiconducting nanoparticles.

In recognition of the scientific collaboration between optical theory and experiments coming together across four decades, Andrews and Valev shared the Royal Society of Chemistry’s 2022 Faraday Division Horizon Prize.

_{Images left to right: Professor David Andrews and Professor Ventsislav Valev}