Phys. Rev. X 9, 011024 (2019)

Chiral molecules and nanostructures are known to exhibit interesting polarization phenomena such as optical activity (rotation in the polarization of a linear light beam as it passes through a medium) and circular dichroism (differential absorption of left- and right-handed circularly polarized light). Now, Joel Collins and co-workers from the UK, Germany and Belgium have observed circular dichroism in second-harmonic hyper-Rayleigh scattering (HRS) for the first time. For the observation, silver nanohelices with a helical diameter (50 nm) much smaller than the wavelength of illumination (720–780 nm) and a wire radius of 12.5 nm were suspended in water. Left- or right-circularly polarized pulses were focused inside a glass cuvette filled with the liquid sample. The observed HRS signal was markedly stronger than the signal from multi-absorption effects. Importantly, clear differences in HRS intensity between left- and right-circularly polarized pulses was observed: (1) right-circularly polarized pulses generated HRS from the sample with left-handed nanohelices more strongly than the left-circularly polarized pulses; and (2) the magnitude relation in the HRS intensity changed when the sample with left-handed nanohelices was measured.