Chirality is a general phenomenon in nature. Many biomolecules in our body such as DNA and enzymes are chiral. The enantiomers existing in oranges and lemons cause different smells. More importantly, while one chirality forms a powerful medication, the other may cause very serious side effects, for instance in the case of left- and right-handed Thalidomide. It is hence of crucial significance to understand chirality for the purpose of interpreting chirality in biology as well as employing chirality for sensing applications in chemistry, pharmacy, etc.
Chiral plasmonics holds great potential in the sense that it has a large range of flexibility to mimic natural chiral substances and simultaneously exhibits a giant chiroptical response arising from the strongly confined and enhanced electro-magnetic field. Nonlinear chiral plasmonics is even more desired since the nonlinear chiroptical effects might be orders of magnitude higher than their linear counterparts. Until now, both linear and nonlinear chiroptical properties in tailored chiral plasmonic systems have been investigated. However, the underlying physical mechanism for nonlinear plasmonic chirality is far from being understood and further quantitative modelling is particularly missing.
Here we study the third-order chiroptical responses of a 3D chiral structure consisting of identical corner-stacked gold nanorods, the so-called plasmonic Born-Kuhn analog. The structures were fabricated by a multi-layer electron-beam lithography (EBL) technique. First, the glass substrate was covered by a dielectric spacer layer (IC1-200, Futurex) via spin-coating. Second, EBL processing procedures (electron-beam exposure of a PMMA resist, development, evaporation of a gold film, and subsequent lift-off) were implemented to fabricate one layer of gold nanorods. The rod lengths were varied to tune the plasmonic resonances in the range of 920-1150 nm, which match the spectral window of our ultrafast laser source. Third, another IC1-200 spacer layer was spin-coated above the layer of gold nanoantennas. Fourth, employing a second EBL cycle assisted by precise positioning, the second layer of gold nanorods was finished. Finally, a third IC1-200 spacer layer was planarized on top in order to create isotropic environment for the gold nanostructures. The thickness of the IC1-200 spacer layer was selected to ensure strong coupling via optical near-field between the two layers of gold nanorods. A C4 geometrical symmetry was designed to eliminate linear birefringence in the structures.
A home-made wavelength-tunable laser source with 60 fs pulse duration was used as the pump for nonlinear frequency conversion. Circularly polarized fundamental light was realized by combination of a polarizer and a broadband quarter waveplate. The third-harmonic-generation signals were recorded by a CCD camera attached to a spectrometer. Nonlinear chiroptical spectroscopy was performed by tuning the wavelength and switching the handedness of the fundamental light.
To interpret the nonlinear chiroptical responses, we utilized a coupled anharmonic oscillator model, in which the coupling term of the two layers of gold nanorods and the phase retardation of the incoming fundamental and outgoing generated wave are fully considered. In this way, we achieve good agreement between experimental measurement and analytical prediction. This quantitative model addresses the origin of the nonlinear chiroptical effects and is very instructive for the efficient design of plasmonic chiral structures for giant nonlinear circular dichroism. Our research extends the present understanding of chiral plasmonic systems and paves the way towards ultrasensitive nonlinear chiral sensing.