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Nonlinear Metasurface with Magnetic Response
Dragomir N. Neshev
Nonlinear Physics Centre, Research School of Physics and Engineering,
The Australian National University, Canberra, ACT, Australia
[email protected]
Abstract— The nonlinear properties of optical metasurfaces can be dramatically enhanced in
the vicinity of their magnetic resonances. Here we present our recent studies on enhancement
of the nonlinear frequency conversion of plasmonic and all-dielectric metasurfaces with magnetic
resonant properties. We characterize the nonlinearity enhancement and the origin of the nonlinear
frequency conversion.
Introduction
The nonlinear optical properties of nanostructures are known to differ substantially from those
of bulk media because they are affected by strong confinement and local resonances. It is well
established that the strong field enhancement through formation of ‘hot spots’ can dramatically
boost nonlinear effects in metallic nanoparticles [1]. Importantly, in the case of metasurfaces, the
nanopatterning can lead not only to more efficient nonlinear interaction, but also to completely new
nonlinear regimes due to the magnetic optical response of meta-atoms. For example, the magnetic
nonlinearities can provide strong nonlinear enhancement [2] or directional emission of harmonics [3].
As such the harvesting of the magnetic nonlinearities in metamaterials and metasurfaces can open
the way for highly efficient nonlinear devices, including frequency converters and optical switches.
Here we present our recent studies on nonlinear response of optical metasurfaces with nonlinear
magnetic response. In particular, we show how the second harmonic generation (SHG) in metaldielectric-metal nano-disks is enhanced by the presence of magnetic dipolar resonances at the
fundamental and second harmonic frequencies. We also show that due to the low loses and purity
of the magnetic response in high-index dielectric silicon metasurfaces, we can enhance the third
harmonic generation (THG) from an ultra-thin silicon layer by two orders of magnitude. Our
results represent the first steps towards the rigorous engineering of the nonlinear response of optical
metasurfaces and the development of highly efficient ultra-thin nonlinear devices.
Nonlinear plasmonic metasurfaces
For our experiments with plasmonic metasurfaces, we employ a regular array of metal-dielectricmetal disks (meta-atoms) as shown in Fig. 1(a). We use two metal disks separated by a dielectric
layer to obtain two distinct resonant modes of the meta-atom at wavelengths of ∼ 680 nm and
∼ 800 nm. The two resonances are associated with co- and counter-propagating electric currents
Figure 1: (a) SEM image of a fabricated nonlinear plasmonic metasurface. Inset shows a side view of a
single three-layer meta-atom made of Au/MgF2 /Au layers. (b) Geometry of the SHG experiment for TMand TE-polarized pump waves. (c) Schematic of THG from silicon nanodisks on a glass substrate. (d) A
photograph of the experiment, showing how the enhanced THG is visible with a bare eye.
excited in the parallel metal disks and are referred as an electric and magnetic resonance, respectively.
We study the second-order optical response of the metasurface for both TM and TE polarizations
of the pump beam and different angles of incidence ϕ, as shown schematically in Figs. 1(b). The
sample is illuminated from the top side (fundamental beam spot size of ∼ 50 µm) and the TM
polarized second harmonic signal is collected from the substrate side. We have experimentally
observed that the intensity of the second-harmonic signal exhibits strong enhancement near the
magnetic resonance of the nanoparticles, which agrees well with numerical calculations. We have
shown that this strong enhancement of the second-harmonic intensity is a result of the interference
of the higher-order multipoles, magnetic dipole and electric quadrupole modes induced by the pump
wave at oblique incidence.
Nonlinear dielectric metasurfaces
Recently, high-permittivity nanoparticles have emerged as a promising alternative to metallic
nanoparticles for a wide range of nanophotonic applications that utilize optically-induced localized
magnetic resonant modes. Such nanoparticles offer unique opportunities for the study of nonlinear
effects due to very low losses in combination with multipolar characteristics of both electric and
magnetic resonant optical modes. More importantly, the nonlinear optical effects of magnetic origin
can have fundamentally different properties compared with those of electric origin.
To test these ideas we have experimental characterized the nonlinear optical response of resonant silicon nanodisks on a glass substrate through THG microscopy and spectroscopy techniques
[Figs. 1(c)]. We have observed enhanced third-order optical nonlinearities of the silicon nanodisks
at the vicinity of the magnetic dipole resonances pumped by femtosecond laser pulses [Figs. 1(d)].
The efficiency of the IR-to-visible conversion was found to be enhanced by two orders of magnitude
with respect to the unstructured bulk silicon slab. The conversion efficiency was found to be limited
only by two-photon absorption in the substrate [4].
Conclusions
In conclusion, we have shown that strong enhancement of harmonic generation from ultra-thin
metasurfaces can be achieved by incorporating the artificial magnetic response in optical metasurfaces. We believe that our results pave the way to establishing novel efficient platforms of nanoscale
resonant nonlinear optical surfaces driven by optically-induced magnetic response of high-index and
composite nanoparticles.
ACKNOWLEDGMENT
I acknowledge financial support by the Australian Research Council. I would like to thank to a
number of collaborators, especially to S. Kruk, L. Wang, A. S. Shorokhov, M. R. Shcherbakov,
B. Hopkins, M. Decker, I. Staude, M. Weismann, A. Yu. Bykov, E. V. Melik-Gaykazyan, E. A.
Mamonov, A. A. Ezhov, I. A. Kolmychek, T. Murzina, A. E. Miroshnichenko, A. A. Fedyanin,
N. C. Panoiu, I. Brener, and Yu. Kivshar.
REFERENCES
1. M. Kauranen and A. V. Zayats, “Nonlinear plasmonics,” Nat. Photon. 6, 737–748 (2012).
2. A. A. Zharov, I. V. Shadrivov, and Y. S. Kivshar, “Nonlinear properties of left-handed metamaterials,” Phys. Rev. Lett. 91, 037,401 (2003).
3. A. Rose, D. Huang, and D. R. Smith, “Nonlinear interference and unidirectional wave mixing
in metamaterials,” Phys. Rev. Lett. 110, 063,901 (2013).
4. M. R. Shcherbakov, D. N. Neshev, B. Hopkins, A. S. Shorokhov, I. Staude, E. V. MelikGaykazyan, M. Decker, A. A. Ezhov, A. E. Miroshnichenko, I. Brener, A. A. Fedyanin, and
Y. S. Kivshar, “Enhanced third-harmonic generation in silicon nanoparticles driven by magnetic response,” Nano Lett. 14, 6488–6492 (2014).