Shape-enhanced magneto-optical activity: Degree of freedom for active plasmonics

Abstract

Magnetoplasmonic nanostructures with enhanced magneto-optical ͑MO͒ activity hold great promise for the active manipulation of plasmon propagation. In this Rapid Communication, we demonstrate that the shape of elliptical nanocylinder plays an essential role on the large enhanced MO activity: ͑1͒ saturated MO activity component depends on the angle of polarization plane of incident light with respect to the semimajor axis of elliptical nanocylinder and ͑2͒ a low magnetic field induces MO activity with nonferromagnetic origin exhibiting a giant linear component added to the saturated component in the MO loops. The intriguing optical and MO properties were well interpreted within one model by assigning a biaxial effective permittivity tensor to the elliptical nanocylinder array. It is suggested that the shape enhanced MO activity can be maximized by elaborately tuning the plasmon resonance as well as increasing the aspect ratio of elliptical nanocylinder.The control of light propagation at nanoscales has gained intense interests resulting in the rapid emerging field of plasmonics. New concepts such as light beaming, 1 miniature plasmonic wave plates, 2 and magnetically controlled nanowaveguiding 3 have suggested that metallic nanoparticles incorporating magneto-optical ͑MO͒ properties provide active route to focus and direct light along a really tiny way down to a few tens nanometers. Attempts in magnetophotonics 4 and magnetoplasmonic biosensor 5 have revealed that giant MO activity ͑Faraday rotation or ellipticity͒ can be obtained when tailoring conventional MO thin film into artificial plasmonic nanostructures. [6][7][8][9] Simply, the nanosphere possessing MO activity can be described by an explicit polarizability tensor ␣ , having ␣ xy =3⑀ xy / ͑⑀ xx +2⑀ d ͒ 2 , where ⑀ xx , ⑀ xy are the components from the bulk permittivity tensor of bulk materials and ⑀ d the permittivity of surrounding dielectrics. 10 At the spectral position of localized surface-plasmon resonance ͑LSPR͒, the real part of the denominator Re͑⑀ xx +2⑀ d ͒ approaches zero, giving rise to giant MO activity. An interesting question will be whether the shape of nanostructures can further enhance the MO activities in plasmonic nanostructures and how. Elliptical nanocylinder possesses simple shape but with intriguing optical properties such as very large local-field enhancement 11 and largely reduced plasmon damping 12 has attracted much attention, recently, in a variety of nano-optic devices such as label-free biosensing. 13 In this work, we found that in addition to the previously verified LSPR enhancement, shape of elliptical nanocylinder provides a different degree of freedom to further modify and enhance the MO activity. A simple model was suggested which interpreted well both the optical and MO properties of the elliptical nanocylinder array. The model suggests that the MO activity can be substantially amplified by elaborately tuning the LSPR resonance as well as increasing the aspect ratio of elliptical nanocylinder.Elliptical nanocylinder squ...

The effect of shape anisotropy on the spectroscopic characterization of the magneto-optical activity of nanostructures

“…Both nanodisks were arranged in a square array with grating constant of 250 nm. The electromagnetic coupling between nanodisks, termed as Bragg diffraction in periodic array has been extensively discussed [14,8]. However, with the small grating constant, this effect is absent in the measurement wavelength range from 530 to 960 nm.…”

Tailoring the Faraday effect by birefringence of two dimensional plasmonic nanorod array

“…Similar results were obtained for nanorods with different shapes and sizes using different laser wavelengths (data not shown). Unlike our previous study 24 in which an elliptical nanosandwich was approximated by a single layer having both optical anisotropy and MO activity, the gold nanorods function as a polarizer and the results are described by a two-layer model. To illustrate how the transmission coefficient ratio governs the polarizer angle dependence of the FR, the effects of the amplitude and the phase of the ratio are discussed separately.…”

Magneto-optical response in bimetallic metamaterials

“…In recent years, magnetoplasmonics, the study of systems that combine plasmonic and magnetic properties of matter, holds promise to both enhance MO effects and to enable non-reciprocal, magnetic-field-controlled plasmonic devices [2][3][4][5][6][7]. Ferromagnetic metals, such as Fe, Ni, and Co, are known to be magneto-optically active but have poor plasmonic properties in the near-infrared (NIR) spectral range due to high Joule losses.…”

“…In particular, enhanced Faraday effect (FE) and magneto-optical Kerr effect (MOKE) have been observed in hybrid devices that simultaneously support plasmonic resonances and are MO active [8][9][10]. Furthermore, nanostructuring of magnetoplasmonic systems provides control over the frequency dispersion and phase of MO effects [3,6,7,[11][12][13][14] while maintaining a compact form factor. In bulk homogeneous media, strong MO effects require long interaction lengths between light and matter.…”