In 1979, the concepts of chiroptical harmonic scattering and hyper-Raman optical activity were proposed,[1] but it took decades for scientists to demonstrate these effects experimentally. That search has finally ended. Chiroptical harmonic scattering can be considered in three regimes: Rayleigh scattering (for particles much smaller than light's wavelength), Mie scattering (for particles much larger than light's wavelength), and Tyndall scattering, which is intermediate. All three effects have nonlinear optical counterparts, where the scatterers are excited with light at one wavelength but scatter light at a harmonic frequency. Because the nonlinear effects originate from ‘hyperpolarizabilities’, these effects are referred to as ‘hyper’. Moreover, when such effects reveal the chirality of the scatterers they are named ‘chiroptical’ (chiral optical), a term often used interchangeably with ‘optical activity’. In 2019, our team showed chiroptical hyper-Rayleigh scattering using silver nanohelices.[2] Then, in 2022, we reported chiroptical hyper-Mie scattering with CdTe nanohelices.[3] Now, we present chiroptical hyper-Tyndall scattering with Si nanohelices.[4] These three nonlinear chiroptical scattering effects are elastic, occurring at harmonic frequencies of the light. In contrast, Raman scattering is inelastic, which sets it apart. Here, we also report hyper-Raman optical activity, surprisingly observed from achiral molecules – crystal violet. Previous attempts to observe this effect failed due to polarization artifacts and thermal effects from high-power laser illumination.[6] Our successful experiment was based on the chirality transfer between gold nanohelices and the molecules.

References:
[1] Andrews, D. L.; Thirunamachandran T., J. Chem. Phys. 1979, 70, 1027.
[2] Collins, J. T.; et al., Phys. Rev. X 2019, 9, 011024.
[3] Ohnoutek, L.; et al., Nat. Photonics 2022, 16, 126.
[4] B. J. Olohan, B. J.; et al., ACS Nano 2024, in press.
[5] Jones, R. R.; et al., Nat. Photonics 2024, in press.
[6] Marble, C. B.; et al. Proc. SPIE 2020, 11288, 1128829.