Strong Nonlinear Optical Chirality Induced by Lattice Surface Modes on Plasmonic Metasurface

Optical metasurfaces, consisting of spatially variant meta-atoms, represent a new kind of optical platforms for controlling the wavefront of light, with which many interesting applications such as metalens, optical holography have been successfully demonstrated. Further extension of the optical functionalities of metasurfaces into nonlinear optical regime has led to unprecedented control over the local optical nonlinear generation processes. It has been shown that the nonlinear optical metasurface with achiral geometries could exhibit intrinsic optical chirality in second- and third-harmonic generations. In this work, we propose an alternative approach for achieving giant nonlinear optical chirality in achiral plasmonic metasurfaces by exploiting the lattice surface modes of plasmonic metasurfaces. Specifically, we theoretically and experimentally demonstrate the strong circular dichroism for second harmonic generation (SHG) on plasmonic metasurfaces consisting of split ring resonator meta-atoms. The giant nonlinear circular dichroism is attributed to the contribution from lattice surface modes at fundamental wavelengths. Our findings may open new routes to design novel nonlinear optical devices with strong optical chirality. resonance mode a The proposed nonlinear metasurface devices 40-42 pave new routes for designing novel chiral-optical functionalities in nonlinear optical regimes.

required to generate large extrinsic optical chirality, and thus putting a limit to the miniaturization of the optical systems. Recently, it was shown that linear optical chirality could be greatly enhanced by optimizing the mode coupling effects, such as the lattice plasmon mode supported by a plasmonic metasurface consisting of split ring resonators (SRRs). 27 Interestingly, the SRR metaatom also exhibits strong nonlinear optical response in SHG process, (1), [28][29][30][31][32][33][34][35] partially due to the fact that the gold SRR meta-atom is capable of inducing strong magnetic resonance when interacting with the FW (1), [28][29][30][31][32][33][34][35] It is expected that the SHG efficiency from the SRR metasurface can be further improved by considering the surface lattice resonances 27 either at the fundamental or second harmonic wavelengths 33 .
In this work, we realize a strong SHG-CD effect by utilizing lattice surface modes (LSM) on the plasmonic metasurface consisting of split ring resonator (SRR) meta-atoms. The SRR metasurface does not have planar chirality and exhibits negligible optical chirality as in threedimensional metamaterials. Here, we demonstrate a new strategy of realizing strong nonlinear CD by exploiting both LSMs on the SRR metasurface and extrinsic optical chirality introduced by the oblique incidence of the FW. The spin sensitive LSMs arise from the strong coupling between localized plasmon resonance and the Rayleigh anomaly (RA) mode. It is shown that the maximum nonlinear CD at SHG frequencies is around three times higher than for the FW in the linear optical regime. Thus, it is expected that the spin sensitive LSMs hold great potentials for designing novel nonlinear chiral-optical devices for bio-sensing or information processing. Figure 1a and Figure 1b show the schematic diagrams for the nonlinear optical chirality that arises from the SRR metasurface under both RCP and LCP fundamental waves with ±10 degree oblique incident angle. Figure 1c illustrates the experimental setup for the SHG-CD measurement.
For normal incident, the metasurface plane is kept parallel to the x-y plane as shown in Figure 1c.
When the incident angle is titled to ±10 degree, the metasurface plane is rotated ± 10 degree along the y-axis which is parallel to the vertical arm of the SRR meta-atom. After passing through the metasurface, the transmitted white light or SHG waves are collected by an objective lens and finally measured by using an EMCDD equipped spectrometer. Figure 1d shows the geometrical parameters of the SRR meta-atom and the scanning electron microscopy image of the SRR metasurface used in this work. The plasmonic metasurface consists of 30 nm thick gold SRRs that are fabricated on a glass substrate and then coated by a 150 nm thick MgF2 film. For the U-shaped SRR meta-atom, the length of the horizontal and the vertical nanorods are 220 nm and 120 nm, respectively. These SRR meta-atoms are arranged in a square lattice with a period of 540 nm in both x-and y-directions. The strong localized surface plasmon resonance (LSPR) from a single SRR meta-atom originates from the oscillations of surface charges induced by incident light. In the linear optical regime, there are three main resonant modes supported by an individual SRR meta-atom excited by either TM (p-) or TE (s-) linearly polarized incident waves. 36 As shown in Figure 2a and Figure 2b, for TM polarization, the direction of the incident electric field is parallel to the x-axis and the long arm of the bottom nanorod, thus the dipole and quadrupole modes can be excited at longer (~1200 nm) and shorter wavelengths (~ 850 nm), respectively. In comparison, for TE polarization, the direction of the polarization is perpendicular to the incident plane (x-z plane), which gives the incident electric field parallelling to the long arm of the vertical nanorod.
In this case, only the dipole mode can be excited, and it can be very close to the quadrupole mode if the arm length of the vertical nanorod is judiciously chosen.
By deliberately assembling these SRR meta-atoms into a square lattice with period of P, the plasmonic resonant modes arising from each meta-atom can be coupled with the collective mode induced by the Rayleigh anomaly, leading to either constructive or destructive interferences between the optical modes near the localized plasmon resonant wavelengths of the meta-atoms.
This lattice surface resonance mode, or Rayleigh anomaly mode, can be excited when the following condition is met. (1) Where λRA is the wavelength of the RA resonance mode, P is the lattice constant of the metasurface, is the incident angle of light, n is the refractive index of the surrounding medium, the '+' and '-' sign corresponds to the positive and negative diffraction order of the RA resonance mode, respectively. In this work, the RA resonance occurs at a wavelength of 724 nm under normal incident and is shifted to 850 nm with an incident angle of +10 degree. In parts c-d of Figure 2, we plot the near-field distributions of the real part of Ey to characterize the two LSPR modes of the SRR meta-atoms under excitation of TM and TE waves. It is found that both the quadrupole mode and the vertical dipole mode are located around the wavelength of 850 nm, indicating that under certain oblique incident angle the combined dipole-quadrupole modes can be coupled with the lattice resonance, and finally leading to a strong optical chiral optical response on the plasmonic metasurface.
As shown in Figure 3, we further characterize the linear optical properties of the SRR metasurface by using circularly polarized (CP) FW under both normal incidence and ±10 degree oblique incidence. For the incident angle θ = ±10 degree, the metasurface is rotated in the incident It is well known that for chiral materials, the second harmonic generation circular dichroism (SHG-CD) is significant more sensitive than linear CD effects. [15][16][17][23][24][25][26] Therefore, in the following, we study case if the chirality can be enhanced in second harmonic generation process with the spin sensitive LSR mode on the metasurface. The SHG signal generated from the SRR metasurface is measured by using a spectrally tuneable femtosecond-OPO laser (repetition an Andor spectrometer (SP300i) with EMCCD detector is used to record the SHG spectrum. In Figure 4a, we first plot the wavelength dependent SHG intensity by scanning the wavelength of the circularly polarized FW from 800 nm to 900 nm under normal incidence. It is clearly observed that for both LCP (blue dot line) and RCP (black dot line) excitation, the peak response of SHG occurs at 850 nm, which corresponds to the dipole-quadrupole plasmon resonance wavelength shown in Figure 3a. Theoretically, it is expected that the CD of SHG waves is zero since no linear and nonlinear chirality can be introduced under normal incidence of the FW. This is reflected by the experimental results. The small discrepancy from theoretical prediction may be attributed to the imperfections of the sample and the experimental setup. As shown in Figure 4b, when the incident angle of the FW changes to +10 degree, the intensity of the SHG under excitation of RCP FW approaches its maximum value at the wavelength of 820 nm, which is almost 3 times higher than that in the case of LCP-FW excitation. On the other hand, when the incident angle of the FW is -10 degree, the SHG intensity under excitation of LCP FW approaches its maximum value at the same wavelength as shown in Figure 4c. By defining the CD value for the SHG waves as SHG-CD = (IRCP -ILCP)/(IRCP + ILCP), we can obtain the CD spectrum shown in Figure 4d. In the experiment, the measured maximum value of the SHG-CD is up to 0.6, which is six times higher than the maximum measured CD value (Figure 3f) in the linear optical measurement.
To better understand the measured nonlinear CD results from the gold SRR metasurface device under normal and oblique incidence of the FW, numerical simulations for the SHG process are conducted. The nonlinear polarization distribution that contributes to the SHG is calculated based on the linear optical responses at the fundamental and second harmonic wavelengths. 38,39 Specifically, the nonlinear optical response of the metasurface at the SHG wavelength is described by a spatially varying surface polarization , which depends on the complex field and the nonlinear susceptibility of the material at the location , respectively. Finally, the contribution to the far-field signal from the nonlinear polarization of the SHG at each local point can be calculated by the following equation: (2) As shown in Figure 4, the spectral response of the SHG from the SRR metasurface is calculated for the FW incident for zero ( Figure 4a) and ± 10 degrees (Figure 4b and 4c). It is found that under both normal and oblique incidence, the nonlinear optical calculations agree with the experimental results. However, the calculated maximum values of the nonlinear CD is higher than that of the measured ones due to the imperfection of the nanofabrication. In principle, the performance of the nonlinear chirality could be further improved by optimizing the geometrical parameters and quality of metasurface device.
In conclusion, we designed an SRR metasurface which exhibits strong linear optical chirality mediated by the lattice surface resonance and oblique incidence of light. We showed that the optical chirality in linear optical regime is greatly enhanced in the SHG process by deliberately aligning the frequency of fundamental wave with the lattice resonance mode under a small oblique incident angle. The proposed nonlinear metasurface devices [40][41][42] pave new routes for designing novel chiral-optical functionalities in nonlinear optical regimes.