Chirality conferral enables the observation of hyper-Raman optical activity

In life as in human-made self-assembly processes, chirality is conferred between building blocks and across hierarchically organized assemblies with emerging and increasing complexity1,2,3. Chirality conferral can occur, for instance, within chemical reactions4, from chiral molecules to achiral inorganic materials, or from chiral inorganics to achiral molecules5,6,7,8,9. Molecular chirality has been conferred to achiral plasmonic silver nanoparticles10, gold nanorods11, gold nanoparticles12 and solid-state lattices in organic–inorganic perovskites13,14. Conversely, the chirality of chiral nanotubes has been conferred to organic molecules15. The chirality conferral process is not trivial16, and it should be noted that some achiral molecules that adsorb on inorganic surfaces can become chiral or form chiral organic–inorganic entities17,18. Chirality can also be conferred from the macroscale to the nanoscale, for example via macroscale twist of a material that contains plasmonic nanoparticles19. Several previous works, including work from some of the co-authors here, have focused on the chirality conferral from plasmonic nanoparticles to achiral molecules, but crucially, they have not established the dependence on intrinsic chirality by showing rotation invariance of the results20,21,22. Moreover, while previous studies have explored photon-to-matter chirality transfer23, chirality conferral from the electromagnetic field of nanoparticles to molecules has hardly been explored.Here, we unambiguously demonstrate chirality conferral from the electromagnetic field of chiral plasmonic nanoparticles (Au nanohelices) to crystal violet (CV) molecules that are achiral (on average, as we explain below). This conferral results in the discovery of a new chiral optical effect—hyper-Raman optical activity—that was predicted 45 years ago by one of the co-authors (et al.)24. Previous attempts to demonstrate this phenomenon have been reported unsuccessful, due to polarization artefacts and due to thermal effects associated with high-power laser illumination25. We recorded that, upon illumination at 1,064 nm, there is a difference of hyper-Raman spectra (that is, those around 532 nm) for right- and left-handed circularly polarized light (RCP and LCP, respectively), known as the circular intensity difference (CID)26. The CID spectrum from the achiral CV changes sign, depending on the handedness of the nanohelices. Importantly, we demonstrate that our data correspond to intrinsic chirality27. The arrays of nanohelices exhibit extrinsic chirality in linear reflection, in second-harmonic generation circular dichroism (CD)28 and in second-harmonic generation optical rotation29. The extrinsic chirality results in a sign change of the ‘chiroptical’ response upon rotating the sample; such sample rotation effects have also been shown to affect Raman scattering30. By contrast, sample rotation that does not affect the Raman spectra is associated with antisymmetric Raman tensors, which Barron introduced to explain true Raman optical activity31,32,33. We demonstrate that the sign of the Raman and hyper-Raman peaks in our CID spectra does not depend on sample rotation, which is an unambiguous signature of chirality. We also show that no hyper-Raman optical activity is observed from achiral gold nanostructures coated with CV, thereby confirming the role of the nanohelices’ geometry. While in our observation we make use of chirality conferral by plasmonic nanohelices to achiral molecules, the original paper on hyper-Raman optical activity focused on chiral molecules alone. To take this difference into account, here we expand the original theory to surface-enhanced hyper-Raman scattering (SEHRS) and we include the chirality conferral mechanism. We present the leading light–matter interaction processes and their quantum amplitudes, whose modulus square yields the measured hyper-Raman signal intensity. We show how the chiroptical response emerges from the sum of electric dipole quantum amplitude contributions and terms that include contributions from magnetic dipoles or electric quadrupoles. Our work demonstrates that chirality conferral can lead to new physics, as illustrated by the observation of a new phenomenon.Figure 1a shows a diagram of our experiment, where LCP and RCP are incident on an array of left-handed plasmonic nanohelices, made of gold. The dimensions of the helices and the chemical formula for CV are shown in inset. As can be seen, the CV molecule’s formula is achiral; however, it is energetically unfavourable for the three branches to remain coplanar. Due to steric hindrance, the molecule exhibits rotational flexibility, with the branches rotating to adopt left-handed and right-handed orientations, in an overall propeller-like configuration. Each molecule can transition between these configurations. Therefore, the molecule is achiral because of its dynamic behaviour, which allows a continuous conversion between the left- and right-handed configurations, at room temperature. Strictly speaking, a CV molecule is achiral ‘on average’. With many molecules, at any moment in time, there is a racemic mixture of left- and right-handed configurations. Depending on the handedness of circularly polarized light, the intensity of the Raman scattered light from CV changes. The nanohelices were designed to exhibit a peak in their scattering cross-section at the fundamental frequency of laser illumination (1,064 nm) but not at the second-harmonic frequency (532 nm), as it is shown in Fig. 1b and Supplementary Fig. 1. Details of the simulation parameters are provided in Methods. Conversely, the CV molecules lack resonances at 1,064 nm but exhibit pronounced energy states around 532 nm, as can be seen in Fig. 1c. This combined system is therefore doubly resonant, which favours the expression of hyper-Raman scattering, by Fermi’s golden rule.Fig. 1: CV molecules on Au nanohelices constitute a doubly resonant system for hyper-Raman scattering.a, Schematic diagram of the reported hyper-Raman optical activity effect: illumination with LCP and RCP results in different intensities of hyper-Raman scattering. b, The nanohelices have strong resonances at 1,064 nm and not at 532 nm, as demonstrated by simulations of the Rayleigh scattering cross-section spectra. c, The crystal violet (CV) molecules have strong resonances at 532 nm and not at 1,064 nm, as shown by their absorbance spectra. d, Surface-enhanced Raman scattering (SERS) (top) and surface-enhanced hyper-Raman scattering (SEHRS) (bottom) circular intensity sum (CIS) spectra of CV. e, Scanning electron microscopy (SEM) and scanning electron transmission microscopy (STEM) images of Au nanohelices. Left: SEM images present left- and right-handed nanohelix arrays on Si wafer. Middle: STEM images present a single nanohelix isolated and deposited on a STEM grid. Right: SEM images present top-down views of left-handed (left) and right-handed (right) Au nanohelices.The surface-enhanced Raman scattering (SERS) and SEHRS spectra of the CV can be obtained by summing the spectra recorded for LCP and RCP illumination, on both left- and right-handed nanohelices, as shown in Fig. 1d. All the Raman spectra were acquired with a Raman spectrometer (InVia, Renishaw), and the CV spectra are in agreement with previous reports34. An important feature of the plasmonic nanohelices is that they are oriented—all are facing the same direction, as demonstrated by the scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) images in Fig. 1e. This orientation suggests that strong extrinsic chirality is present in the system.In chiroptical studies, it is important to distinguish intrinsic/true from extrinsic chirality, anisotropy effects on the polarization and artefacts35. In the case of ordered materials, such as our metal nanostructures, the ordering itself can produce chirality-like effects—a behaviour that has been referred to as ‘pseudo chirality’36 or ‘extrinsic chirality’37. To address this question, we characterized the reflectance of our samples, using the setup described in Fig. 2a. For each of them, we calculated the ellipticity (θ), defined by$$\theta =\frac{180}{\uppi }{\tan }^{-1}\left[\left(\sqrt{{I}^{{\mathrm{RCP}}}}-\sqrt{{I}^{{\mathrm{LCP}}}}\right)\Big{/}\left(\sqrt{{I}^{{\mathrm{RCP}}}}+\sqrt{{I}^{{\mathrm{LCP}}}}\right)\right],$$
(1)
where \({I}^{{\mathrm{RCP}}}\) and \({I}^{{\mathrm{LCP}}}\) represent the intensities of detected light for RCP and LCP illumination, respectively. Figure 2b shows the rotationally averaged ellipticity at 532 nm and 1,064 nm, respectively. The data were extracted from the CD spectra shown in Supplementary Fig. 2. We note that at 532 nm and at 1,064 nm the sign of the ellipticity is opposite for left- and right-handed helices. Although for left- and right-handed nanohelices, the ellipticity plots in Supplementary Fig. 2 are close to mirror images, at each wavelength, a very strong sample rotation dependence can be observed, as seen in Fig. 2c. Figure 2c shows the ellipticity as a function of sample rotation for 532 nm and for 1,064 nm. The plots were obtained from averaging 11 curves in the 528–536 nm range and in the 1,056–1,072 nm range, respectively. We note that there is little spectral variation across these ranges and that the Ti:Sapphire pulse used in the hyper-Raman experiments at 1,064 nm is about 15 nm in width. For all curves in Fig. 2c, the ellipticity changes sign four times upon sample rotation. Because chirality is invariant upon rotation, this ellipticity cannot be measuring chirality alone. There are several physical mechanisms that affect these measurements.Fig. 2: Extrinsic chirality that is clearly present in reflectivity measurements with circularly polarized light does not seem to affect the rotationally averaged Raman spectra.a, Diagram of the reflective microscope setup (FL, filament lamp; P, polarizer; λ/4, quarter-waveplate; CPL, circularly polarized light; BS, beam splitter; A, analysing polarizer (analyser); M, mirror). b, Rotationally averaged ellipticity from reflection spectroscopy are not mirror images for the two enantiomorphs. c, Reflection ellipticity versus sample angle (rotation) at 532 nm and 1,064 nm reveal extrinsic chirality—the ellipticity depends strongly on the sample rotation and changes sign several times. d, Diagram of the hyper-Raman instrument (λ/2, half-waveplate; L1 and L2, collimation lenses; M, mirror; P, polarizer; DM, dichroic mirror; λ/4, quarter-waveplate; A, analysing polarizer (analyser)). e, Illustration of the mapping technique used—illumination was performed in 5-by-6 arrays, with alternating right- and left-handed circularly polarized light (RCP and LCP, respectively) illumination for each spot. f, The difference of SERS and SEHRS spectra, acquired for RCP and LCP illumination, from CV molecules deposited on left- and right-handed Au nanohelices. g, The sum and difference of hyper-Raman spectra acquired for RCP (top) and LCP (bottom) illumination, from CV molecules on commercial achiral SERS substrates. The inset shows an SEM image of the sample surface.These nanohelices are arranged in arrays that exhibit a local hexagonal lattice. Because of the way they are fabricated (that is, all at once), the nanohelices are oriented in the same way, with terminations pointing towards the source of deposition. Thus, the samples also exhibit unidirectional anisotropy. Moreover, it is possible that the nanohelices are locally slightly tilted, with their main axis deviating from the z axis, which could be another source of unidirectional anisotropy. When illuminated, electromagnetic resonances are excited both within each helix and between helices, depending on the wavelength of illumination. The intrinsic chirality of the helices plays a role, but the extrinsic chirality due to unidirectional anisotropy is a strongly competing effect. Additionally, because we use achromatic waveplates, the retardation is not identical across the full spectrum of operation—there is a slight linearly polarized component and it depends on wavelength. At each wavelength, for LCP and RCP positions of the waveplate, the linearly polarized components are at 90° to each other. Therefore, rotating the sample for LCP and RCP illumination affects several wavelength-dependent mechanisms. Distinguishing and optimizing these individual mechanisms is very important for the field of metasurfaces—for instance, to produce ultrathin polarizers for circularly polarized light (CPL). In the present context, the key point is that the ellipticity is observed to change sign several times, as the sample rotates. It is clear that, in these reflection experiments, polarization imperfection (also identified as a problem in previous attempts to observe hyper-Raman optical activity)25 and material anisotropy are present. But would they affect the (hyper-)Raman scattering of CV?The diagram of the Raman and hyper-Raman scattering setup can be seen in Fig. 2d. Circularly polarized light was obtained with the help of a polarizer and a quarter-wave plate (λ/4). Possible contributions from experimental artefacts, such as light circularity (Supplementary Fig. 3), quenching or photodecomposition of CV, variations in the number of illuminated molecules and so on, have been addressed (Supplementary Information).In Fig. 2e, the rotationally averaged SERS and SEHRS spectra were obtained from averaging a total of 360 RCP (IRCP) and 360 LCP (ILCP) spectra. Within each sample, the plotted CID (IRCP − ILCP) is therefore free from artefacts related to the number of molecules or quenching. Because the left- and right-handed nanohelices are grown separately, there are geometrical (batch) difference between them. Moreover, the left- and right-handed nanohelices were spin-coated with CV separately; hence, a difference in molecular concentration exists. Accordingly, the amplitude of the CID peaks in Fig. 2f differs between left- and right-handed nanohelices. However, the sign of all the peaks is opposite, for both SERS and SEHRS, which is an indication of the hyper-Raman optical activity effect.To confirm the role of the nanohelices, we repeated our experiments using achiral nanostructures from a commercial (Silmeco), SERS substrate, made of Au. An SEM image of the substrate surface is displayed in the inset. Figure 2g shows SEHRS results for (IRCP + ILCP) and (IRCP − ILCP) in the top and bottom panels, respectively. The former is simply the hyper-Raman spectrum of CV, as also shown in Fig. 1d (bottom). The latter demonstrates that there is no measurable CID on these achiral substrates, at stark contrast to the sign-changing peaks observable with nanohelices. Now, the key question is: does this sign change upon rotating the sample? If it does, the sign change would be indicative of extrinsic chirality and, hence, not a signature of hyper-Raman optical activity.Figure 3a,b shows the SERS-CID and SEHRS-CID spectra of CV molecules on left- and right-handed nanohelices, for eight sample rotation angles (\(\phi\)). Clearly, there are some fluctuations of the peak heights. These are attributed to millimetre-scale non-uniformity across the samples that results in variable CV concentration and quenching rates. Importantly, though, in both Raman and hyper-Raman scattering, the sign of the CID spectra is independent of sample rotation.Fig. 3: The sign of the Raman spectra does not change upon sample rotation, which indicates that the effect originates from intrinsic chirality.a,b, The SERS (a) and SEHRS (b) CID spectra are plotted for eight different samples rotation angles. The CID spectra are consistently negative for CV on left-handed nanohelices (in red) and positive for CV on right-handed nanohelices (in blue).The data in Fig. 2c indicate four sign changes that are periodic. It is impossible for a set of eight equally spaced points (45° step) to exhibit the same sign with such a periodicity. Therefore, we conclude that the sign of the SERS-CID and SEHRS-CID spectra is independent of the sample rotation. This conclusion excludes extrinsic chirality, and it is consistent with the presence of an antisymmetric Raman tensors, characteristic of Raman optical activity and of hyper-Raman optical activity, respectively. For clarity, Fig. 3a,b focus on the most pronounced Raman and hyper-Raman peaks, in the 1,500–1,700 cm−1 region. The full 1,000–1,800 cm−1 region comprised three hyper-Raman peaks, and they all keep the same sign upon rotation, as shown in Supplementary Fig. 4. Moreover, while the data in Supplementary Fig. 4 were acquired in May 2023, rebuilding and recalibrating the experiment to repeat the measurements in February 2024 produced very similar results, as shown in Supplementary Fig. 5. The observed effects are robust, and they originate from a chirality conferral from the electromagnetic field of the plasmonic nanohelices to the achiral molecules. But how?In the following, we identify all the lead mechanisms for the light–matter interactions that play a role in our experiments (Fig. 4) and we calculate their quantum amplitudes, whose modulus square relates to the hyper-Raman signal intensity that we measure. First, we consider the hyper-Raman process in the CV molecules. It involves the annihilation of two incident photons (at ω) and the emission of a Stokes frequency-shifted second-harmonic (at \(\omega ^{\prime} =2\omega -{\omega }_{{\rm{vib}}}\)), where ωvib is the vibrational frequency of the CV molecule. In the electric dipole approximation, the molecular response depends on the vibrational derivative of the molecular hyperpolarizability that can be written as38$${{\beta}^{\prime} }_{ijm}\equiv {Q}^{1\leftarrow 0}{[\partial {\beta }_{ijm}(\omega )/\partial Q]|}_{{Q}_{0}}.$$
(2)
Fig. 4: Feynman diagrams of the leading electrodynamic interactions that contribute to SEHRS.a, The 12 possible interactions when the two incident photons are annihilated at the nanohelix and a Raman-shifted photon is emitted at the CV molecules. Time progresses upwards. The Au nanohelix is represented by the vertical gold-coloured world line on the left in each case, its thickness signifying bulk response. The world line for the CV is represented by the black line on the right, its final state in purple exhibiting vibrational excitation. The input photons are shown as black wavy lines and the signal output photon as red, and each radiation field–matter interaction is shown as a green dot. The horizontal dashed line represents dynamic coupling in the near-zone limit. Line intervals between interaction dots on either side represent virtual states. b, The six possible permutations of annihilated and emitted photons. ii–vi illustrate different cases, with the same temporal ordering in which at least one of the input photon annihilations takes place within the CV, with iv–vi further distinguished by the emitted photon originating from the nanohelix. c, Leading forms of mixed-multipole counterparts to b(i). Here, the green dot signifies an E1 interaction, orange M1 and blue E2. For clarity, the M1 and E2 interactions are highlighted with an asterisk and a triangle, respectively. Similar counterparts exist for each of the remaining subpanels within b.Here and below, the prime symbol denotes a vibrational transition. The molecular hyperpolarizability tensor \(\beta\) for the input frequency \(\omega\) (i, j and m are the Cartesian coordinates) comprises three terms corresponding to the three possible sequences of absorbing two photons and emitting one at the second-harmonic, in a molecule. Equation (2) uses the Taylor series expansion of the tensor in terms of the normal mode coordinate Q for the activated molecular vibration, whose matrix element \(\langle 1|Q|0\rangle\) for a fundamental \(1\leftarrow 0\) transition is represented as Q1←0. The hyperpolarizability derivative in square brackets is evaluated at the equilibrium configuration Q0. The expression on the right-hand side of equation (2) couples with a product of input and output polarization vectors, \({e}_{i}(\omega ){e}_{j}(\omega ){\bar{e}^{\prime} }_{m}(\omega {^\prime} )\), the overbar denoting complex conjugation where necessary for circular polarizations. Hence, for the CV molecules, the quantum amplitude takes the form \({{\beta}^{\prime} }_{ijm}{e}_{i}(\omega ){e}_{j}(\omega ){\bar{e}^{\prime} }_{m}(\omega ^{\prime} )\), using the implied summation convention for repeated subscript indices. Next, we take the nanohelix into account.In our data, the observed CID in the hyper-Raman signals can only be generated through coupling between the electromagnetic field generated by chiral Au nanohelices and the achiral CV molecules, where nearest-neighbour interactions can be assumed the strongest. Considering the nanohelix + CV molecule as an entity, each of the three-photon events can occur at either site, within the span of an optical wavelength. The dynamic coupling may be represented as a quasi-instantaneous two-body interaction. We consider the lowest non-vanishing order of perturbation theory to derive the quantum amplitude for the overall process—fourth order, comprising third order in single-centre light–matter interactions and first order in a two-centre coupling.As our system is doubly resonant, we now consider the case where the two incident photons are annihilated in the nanohelix, and the hyper-Raman photon emerges from the CV; energy is conserved by near-field dynamic coupling between the two entities. There are 12 contributory pathways for this specific form of interaction, represented by the 12 topologically distinct diagrams shown in Fig. 4a. For gold, the world line is represented with a width as a visual aid, and its tensor response is a nonlinear susceptibility (\({\chi }_{{\rm{Au}}}^{(2)}\)) in contrast to the molecular response of the CV (\({\tilde{\alpha ^{\prime} }}_{{\rm{CV}}}\)), where tilde denotes values associated with input at the optical harmonic (\(2\omega\)). For this form of interactions, we can write \({\chi }_{{\rm{Au}}}^{(2)}\begin{array}{c}\cap \\ \cup \end{array}\,{\tilde{\alpha ^{\prime} }}_{{\rm{CV}}}\), where the symbol \(\begin{array}{c}\cap \\ \cup \end{array}\) denotes electrodynamic coupling. Corrective factors in the coupling, associated with plasmonic effects in the gold nanoparticles, are assimilated within the coupling tensors, as is shown for energy transfer by the methods of Schatz et al.39,40,41. In the electric dipole approximation, summing over all 12 contributions and over the repeated indices k, l, we obtain$$\begin{array}{l}\left\{{\chi }_{{\rm{Au}}}^{(2)}\begin{array}{c}\cap \\ \cup \end{array}{\tilde{\alpha ^{\prime} }}_{{\rm{CV}}}\right\}_{ijm}\equiv {\chi }_{ijk}^{{\rm{Au}}(2)}(-2\omega ;\omega ,\omega ){V}_{kl}\left(2\omega /c^{\prime},\right.\\\qquad\qquad\qquad\qquad\quad\left.{{\mathbf{R}}}_{{\rm{CV}}}-{{\mathbf{R}}}_{{\rm{Au}}}\right){Q}^{1\leftarrow 0}{\left[\partial {\alpha }_{lm}^{{\rm{CV}}}(2\omega )\Big/\partial Q\right]\Big|}_{{Q}_{0}},\end{array}$$
(3)
where \({{\mathbf{R}}}_{{\rm{CV}}}\) and \({{\mathbf{R}}}_{{\rm{Au}}}\) represent the local position vectors of the CV and nanohelix, respectively, and where the \({c}^\prime\) denotes the local permittivity-corrected speed of light42. The expression on the right-hand side of equation (3) again couples with a product of input and output polarization vectors to give the first estimate of the quantum amplitude:$${\left\{{\chi }_{{\rm{Au}}}^{(2)}\begin{array}{c}\cap \\ \cup \end{array}{\tilde{\alpha ^{\prime} }}_{{\rm{CV}}}\right\}}_{ijm}(\omega ){e}_{j}(\omega ){\bar{e}^{\prime} }_{m}(\omega ^{\prime} ).$$
(4)
In equation (3), \(V\) designates the retarded resonance dipole–dipole interaction, a second rank tensor whose argument exhibits explicit dependence on frequency and the vector displacement of the CV molecule from its counterpart nanohelix. However, within the near-zone the leading term of the tensor is frequency independent43 and we can approximate the result in terms equivalent to static coupling \({V}_{kl}(2\omega /c^{\prime} ,{{\mathbf{R}}}_{{\rm{CV}}}-{{\mathbf{R}}}_{{\rm{Au}}})\simeq {V}_{kl}^{\;0}({{\mathbf{R}}}_{{\rm{CV}}}-{{\mathbf{R}}}_{{\rm{Au}}})\). Equation (3) features the Raman tensor for the CV, the quantity in square brackets denoting the mode derivative of the polarizability at the equilibrium configuration.Figure 4b highlights the fact that, although we expect the doubly resonant system to favour photon annihilations at the helix and emission at the CV molecule, other annihilation-emission photon events are possible and they cannot be distinguished, under much less than the span of a wavelength. Each of the 12 diagrams in Fig. 4a has four counterparts with the same temporal sequence but differently positioned photon events. The full forms of associated general equations are the subject of a follow-up publication, where we further discuss additional quantum amplitudes contributions and the reasons why they have been omitted.Within the E1 approximation, no CID can arise. The tensor response of the complete material system is an eigenstate of the spatial parity operation P with eigenvalue −1; it does not register chirality because it is specifically odd parity44. Therefore, we now need to focus on additional terms arising from electric and magnetic multipole expansions and the symmetry-based selection rules. We have the following leading terms in the Power–Zienau–Woolley interaction Hamiltonian:$${H}_{\mathrm{int}}^{{\mathrm{mult}}}=\underbrace{-{\mathbf{\upmu }}\cdot {{\mathbf{e}}}^{\perp }}_{{\mathbf{E}}1}\underbrace{-{\mathbf{m}}\cdot {\mathbf{b}}}_{{\mathbf{M}}1}\underbrace{-{\boldsymbol{\Theta }}:{\boldsymbol{\nabla }}{{\boldsymbol{e}}}^{\perp }}_{{\mathbf{E}}2}-\ldots ,$$
(5)
where \({{\mathbf{e}}}^{\perp }\) is the transverse electric field and b the magnetic field. The magnetic dipole (M1) and electric quadrupole (E2) terms, whose moments are denoted by m and \({\boldsymbol{\Theta }}\), respectively, connect to the first order of expansion in the vector potential45,46. The transverse electric field \({{\bf{e}}}^{\perp }\) is a polar vector, odd under P, as is the electric dipole m with which it couples. In the M1 term, both the field b and the magnetic dipole m are pseudovectors, even under P; so too is the electric quadrupole \({\boldsymbol{\Theta }}\), a second-rank tensor coupled to the gradient of the electric field. M1 and E2 represent interactions whose associated magnitudes are small compared with E1 if selection rules simultaneously allow both, which requires symmetry breaking. In consequence, both are commonly engaged, together with E1, in chiral interactions.Each field–matter interaction (green dot) entails all of the contributions given in equation (5)—not just the first. Moreover, the hyper-Raman signal intensity arises from the modulus square of the quantum amplitude. In consequence, the leading contribution to any chiroptically responsive, enantioselective effect comprises a sum of cross-terms between the leading, fully E1 quantum amplitude contribution associated with equation (2) and terms of the forms exhibited by equation (4) but in which one of the E1 interactions is substituted by an M1 or E2 coupling. Those substitute interactions must occur at the Au site since that is the sole chiral constituent. In this case, the relevant counterparts to Fig. 4b(i), for example, are as shown in Fig. 4c.Each of the graphs in Fig. 4c again has a counterpart within the other 71 time orderings. When the quantum amplitudes for the full set are summed, in the back-directed Raman signal, we obtain the terms whose sign changes with the handedness of circularly polarized light and upon substituting the opposite enantiomorph of the nanohelix. It is these terms that contribute to the signal measured in our experiments (for the full details of these calculations, see ref. 47).Because hyper-Raman scattering (and therefore hyper-Raman scattering optical activity) probes different physical quantities (hyperpolarizabilities) than Raman scattering, it offers complementary vibrational information48. For instance, it can reveal vibrational modes that are silent in Raman scattering49. In microscopy of biological systems, it combines increased penetration depth from convenient excitation in the near-infrared (NIR) transparent region with detection in the visible47,50; the process also offers reduced (one-photon) autofluorescence and lower photochemical damage, since illumination is away from most electronic resonances51. Importantly, hyper-Raman scattering benefits from better enhancement at the surface of plasmonic nanoparticles and nanostructures52, because it scales as a power law of the local electric fields53. Hyper-Raman scattering is particularly suited for studying molecules with strong hyperpolarizabilities, such as push–pull organic chromophores54; it offers a broader range of pH sensitivity for molecular imaging inside cells55, and because it requires two incident photons, it can benefit from entangled photon states56.Similarly to Raman optical activity, hyper-Raman optical activity can be expected to reveal the absolute configuration or conformation, in proteins, in synthetic chiral polymers and in similarly intricate molecules. This is important in chiral molecules, especially in pharmaceuticals, agrochemicals, natural products and bioactive compounds are often large, with complex geometries, containing numerous chiral centres that each require analysis. Typically, these centres are chiral carbons bonded to functional groups, such as nitriles57 or amines58,59,60. Experimental methods that can distinguish and characterize the chirality within such molecules are highly desirable61.In conclusion, we report chirality conferral from the electromagnetic field of gold nanohelices to achiral molecules, the former being over 100 times larger than the latter. This conferral has enabled the experimental observation of the hyper-Raman optical activity effect that was theoretically predicted 45 years ago. The observation has now been demonstrated in a doubly resonant system, consisting of plasmonic gold nanohelices and CV molecules, where the chirality of the nanohelices was conferred to the achiral (on average) molecules. Care has been taken to eliminate possible artefacts, for example, from extrinsic chirality in the nanohelices, from the degree of light circularity, from photodecomposition and from differences in the number of illuminated molecules. When achiral nanoparticles were used as the SERS substrate (also see Supplementary Fig. 6), the hyper-Raman optical activity effect was absent, confirming the role of chirality transfer from the nanohelices. The origin of the effect was revealed by identifying the lead light–matter interaction processes and by calculating their quantum amplitudes, whose modulus square produces the hyper-Raman signal intensity observed. We show that the chiroptical response originates from the sum of cross-terms between the leading, fully electric dipole quantum amplitude contribution and terms that include contributions from magnetic dipoles or electric quadrupoles.Our findings show that the chirality conferral mechanism between chiral nanoparticles and achiral molecules is highly promising for exploring new science at the organic–inorganic interface, as demonstrated by the discovery of a new physical effect. We anticipate that hyper-Raman optical activity will be sensitive to the absolute configuration of chiral molecules that are key for synthetic biology, for human-made self-assembling nanotechnology, and for pharmaceutical synthesis. Naturally, as it has taken 45 years between the prediction of this effect and its first observation, the technological development of the associate experimental technique and its commercialization will take time. Important milestones will be to observe hyper-Raman optical activity in chiral molecules and in those that are of biomedical relevance. Further optimization of SERS materials and substrates can lead to even greater enhancements in Raman signals, enabling the detection and analysis of even lower concentrations of analytes. Exploring new fabrication techniques and materials with enhanced plasmonic properties can contribute to this optimization. Exploring machine learning algorithms and chemometric methods can aid in the design and synthesis of SERS materials as well as intrinsically chiral molecules that meet the above criteria. Optimizing the light sources for this emerging technique will also be a key advance.