Second-harmonic generation (SHG) from plasmonic metals is normally governed by an “even” surface: 𝜒(2) term that survives in any centrosymmetric structure. Theory predicts that in a truly chiral plasmonic environment, an additional mirror-odd tensor 𝜒𝑜𝑑𝑑(3) can emerge and outgrow the even term so that the SHG output becomes a direct reporter of handedness.[1-3] Here we demonstrate that for helically patterned silver nanopillars fabricated by polarization-controlled 3D electrochemical printing on flat ITO, the mirror-odd chiral tensor element dominates the nonlinear response. Linear-polarized (0°) coupons show almost no SHG, whereas right- and left-handed films written with ±45° circular polarization produce >30-fold stronger back-scattered SH light under 820-nm femtosecond pumping. Two-photon–excited photoluminescence (2PPL), recorded with an 800-nm excitation Ti:sapphire Tsunami oscillator, follows the expected quadratic power law and yields an effective two-photon brightness contrast η2 = 6 ± 1 % which is consistent with the ratio 𝜒𝑜𝑑𝑑(3) 𝜒𝑒𝑣𝑒𝑛(3) predicted by Zhang et al.[3][4] Taken together, the data demonstrate that SHG in these samples is virtually on/off with chirality, while the 𝜒(3) channel is modulated but present in every film.
Because the ±45° enantiomers act as phase-conjugate sources with opposite second-order helicity, their SH photons are expected to exhibit polarization-path entanglement, offering a route to chirality-selected biphoton states without bulk crystals or periodic poling.[5] The combination of (i) readily patternable 3D chiral plasmonics, (ii) surface-localized nonlinearities, and (iii) compatibility with planar substrates suggests new integrated platforms for quantum light sources, enantio-specific biosensing, and ultrafast valley-photonics.
References
1. M. Kauranen & A.V. Zayats, “Nonlinear plasmonics,” Nat. Photonics 6, 737 (2012).
2. V.K. Valev et al., “Nonlinear detection of chiral plasmonic metamaterials,” Phys. Rev. Lett. 104, 127401 (2010).
3. C.-C. Zhang et al., “Plasmon-enhanced second harmonic generation of metal nanostructures,” Nanoscale 16, 5960 (2024).
4. J. Butet, P.F. Brevet & O.J.F. Martin, ACS Nano 9, 10545 (2015).
5. S. Kruk & Y. Kivshar, ACS Photonics 2, 1007 (2015).
