Hyper–Rayleigh scattering optical activity ( ), a form of chiroptical harmonic scattering, is a

nonlinear optical Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typic ...

physical effect whereby chiral scatterers (such as nanoparticles or molecules) convert

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

(or other

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...

) to higher frequencies via harmonic generation processes, in a way that the intensity of generated light depends on the chirality of the scatterers. "Hyper–Rayleigh scattering" is a nonlinear optical counterpart to Rayleigh scattering. "

Optical activity Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...

" refers to any changes in light properties (such as intensity or polarization) that are due to

chirality Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from ...

History

The effect was theoretically predicted in 1979, in a mathematical description of hyper Raman scattering optical activity. Within this theoretical model, upon setting the initial and final frequencies of light to the same value, the mathematics describe the hyper Rayleigh scattering optical activity. The theory was well in advance of its time, and the effect remained elusive for 40 years. Its author David L. Andrews referred to it as the "impossible theory". However, in January 2019, an experimental demonstration was reported by Ventsislav K. Valev and his team. The team investigated the hyper Rayleigh scattering (at the

second harmonic generation Second-harmonic generation (SHG, also called frequency doubling) is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with twice the energy of ...

frequency) from chiral nanohelices made of silver. Valev and his team observed that the intensity of the hyper Rayleigh scattering light depended on the direction of circularly polarized light and that this dependence reversed with the chirality of the nanohelices. Valev's work unambiguously established that the effect is physically possible, opening the way for nonlinear chiroptical investigations of a variety of chiral light-scattering materials; including molecules, plasmonic metal nanoparticles and semiconductor nanoparticles.

Significance

Hyper Rayleigh scattering optical activity (HRS OA) is arguably the most fundamental nonlinear chiral optical (chiroptical) effect; since other nonlinear chiroptical effects have additional requirements, which make them conceptually more involved, i.e. less fundamental. HRS OA is a scattering effect and therefore it does not require the frequency conversion process to be coherent, contrary to other nonlinear chiroptical effects, such as second harmonic generation circular dichroism or second harmonic generation optical rotation. Moreover, HRS OA is a parametric process: the initial and final quantum mechanical states of the excited electron are the same. Because the excitation proceeds via

virtual state In quantum physics, a virtual state is a very short-lived, unobservable quantum state. In many quantum processes a virtual state is an intermediate state, sometimes described as "imaginary" in a multi-step process that mediates otherwise forbi ...

s, there is no restriction on the frequency of incident light. By contrast, other nonlinear scattering effects, such as two-photon circular dichroism and hyper-Raman are non-parametric: they require real energy states that restrict the frequencies at which these effects can be observed.

In molecules

Soon after the first demonstration of hyper Rayleigh scattering optical activity in metal nanoparticles, the effect was replicated in organic molecules, specifically aromatic oligoamide foldamers.

At the third harmonic

Whereas the initial experimental demonstration of hyper-Rayleigh scattering optical activity was observed at the second harmonic of the illumination frequency of light, the effect is general and can be observed at higher harmonics. The first demonstration of hyper-Rayleigh scattering optical activity at the third harmonic was reported by Valev's team in 2021, from sliver nanohelices.

References

External links

New physical effect demonstrated by University of Bath scientists after a 40 year search
official press release by the University of Bath.
Bath University has something to twist and shout about after 40-year search
by the EPSRC.
“Impossible” UEA physics theory proven in practice after 40 years
official press release by

University of East Anglia The University of East Anglia (UEA) is a public research university in Norwich, England. Established in 1963 on a campus west of the city centre, the university has four faculties and 26 schools of study. The annual income of the institution f ...

.
Hyper-Rayleigh scattering
published in the scientific journal ''

Nature Photonics ''Nature Photonics'' is a peer-reviewed scientific journal published by the Nature Publishing Group. The editor-in-chief is Oliver Graydon. The journal covers research related to optoelectronics, laser science, imaging, communications, and other as ...