Polarization independent Fano resonances in arrays of core-shell nanoparticles

Andrey E. Miroshnichenko, Wei Liu, Dragomir N. Neshev, and Yuri S. Kivshar

���Nonlinear Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra, ACT 0200, Australia. ���

Studies of Fano resonances in periodic metallic structures have recently attracted surging attention due to the promising applications, including near-field amplification, sensing, and modification of spontaneous emission [1, 2]. The Fano resonance originates from an interference of the geometrical resonance of the structure (Wood’s anomaly) with plasmon resonances of metallic particles [1, 2]. However, due to the electric nature of the plasmon resonance, the sharp Fano resonances are usually strongly polarization dependent. In this work we demonstrate for the first time polarization-independent Fano resonances in arrays of core (metal)-shell (dielectric) nanoparticles (CSNPs) through the hybridization of magnetic and electric Mie resonances and their interference with the geometric resonance of the array.

It is shown that the hybrid structure, CSNP can support both electric and magnetic Mie resonances [3]. In Fig. 1(a) we show the total extinction efficiency spectra of a silver core dielectric shell particle of inner radius 38 nm and outer radius 150 nm. We also show the contribution from both electric (a1) and magnetic (b1) dipoles. Here only a1 and b1 contribute dominantly to Qext in the spectral regime of 1100 nm-1180 nm, therefore the CSNP can be viewed as a pair of orthogonal electric dipole (ED) and magnetic dipoles (MD) coinciding spectrally with the same strength. Here we show the results of one dimensional (1D) periodic particle arrays as shown in Fig. 1(b). We fix the wavevector k of the incident plane wave perpendicular to the array axis. This is because when the direction of k deviates from the normal of the array axis, the Fano resonance will split into several resonances with reduced strengths [4]. In the array of CSNPs the Fano resonance comes from the interference of three resonances (ED, MD and geometric resonance of the array) rather than two. Both ED and MD can interference simultaneously with the geometric resonance of the array, leading to polarization independent Fano resonances, irrespective of different orientations of the EDs and MDs. This is confirmed by both the results obtained using coupled dipole approximations (CDA) shown in Fig. 1(c) (solid line), and the FDTD simulation results in Fig. 1(c) (dashed line with circles) which agree well with the CDA results.

Polarization independent Fano resonances in arrays ofcore-shell nanoparticles

Wei Liu, Andrey E. Miroshnichenko∗, Dragomir N. Neshev, and Yuri S. KivsharNonlinear Physics Centre, Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS), Research School of Physics and

Engineering, Australian National University, Canberra, ACT 0200, Australia.∗ Email: aem124@physics.anu.edu.au

Abstract Summary

We reveal the existence of polarization independent Fano resonances inarrays of core-shell nanoparticles due to the interference of electric andmagnetic Mie resonances of each single particle with geometric reso-nance of the array.

Keywords: core-shell nanoparticles; Mie scattering; coupling ofelectric and magnetic dipoles; Fano resonance.

I. INTRODUCTION

Studies of Fano resonances in periodic metallic structures haverecently attracted surging attention due to the promising applica-tions, including near-field amplification, sensing, and modifica-tion of spontaneous emission [1, 2]. The Fano resonance orig-inates from an interference of the geometrical resonance of thestructure (Wood’s anomaly) with plasmon resonances of metallicparticles [1, 2]. However, due to the electric nature of the plas-mon resonance, the sharp Fano resonances are usually stronglypolarization dependent. In this work we demonstrate for the firsttime polarization-independent Fano resonances in arrays of core(metal)-shell (dielectric) nanoparticles (CSNPs) through the hy-bridization of magnetic and electric Mie resonances and their in-terference with the geometric resonance of the array.

Qext (total)Qext (a ) 1

Qex

t

0.8 1.0 1.2 1.4λ (µm)

4

8

12

16

Qext (b ) 1

5

15

25

45 75 900 25 1 1.1 1.21.3

Qex

t

θ (degree)λ (µ

m)

(c) CSNP array, d=1.15 µm

... ...d

k //

(b)

y

xE0

H0

θ

z

R = 38 nmR = 150 nm

1

2

(a)

n=3.5

CDA

FDTD

Figure 1: (a) Extinction efficiency spectra for the CSNP, which canbe approximated as a pair of orthogonal ED and MD coinciding spec-trally with the same strength. (b) The schematic geometry of the 1Darray of CSNPs under consideration. The incident plane wave is prop-agating along z and the polarization angle (electric field with respect tox direction) is θ. (c) Theoretical (solid line) and FDTD (dashed linewith circles) results of the extinction efficiency spectra for a 1D array ofCSNPs with d = 1.15 µm for different polarization angles.

II. RESULTS AND DISCUSSIONS

It is shown that the hybrid structure, CSNP can support bothelectric and magnetic Mie resonances [3]. In Fig. 1(a) we showthe total extinction efficiency spectra of a silver core dielectricshell particle of inner radius 38 nm and outer radius 150 nm.We also show the contribution from both electric (a1) and mag-netic (b1) dipoles. Here only a1 and b1 contribute dominantly toQext in the spectral regime of 1100 nm-1180 nm, therefore theCSNP can be viewed as a pair of orthogonal electric dipole (ED)and magnetic dipoles (MD) coinciding spectrally with the samestrength. Here we show the results of one dimensional (1D) peri-odic particle arrays as shown in Fig. 1(b). We fix the wavevectork of the incident plane wave perpendicular to the array axis. Thisis because when the direction of k deviates from the normal ofthe array axis, the Fano resonance will split into several reso-nances with reduced strengths [4]. In the array of CSNPs theFano resonance comes from the interference of three resonances(ED, MD and geometric resonance of the array) rather than two.Both ED and MD can interference simultaneously with the geo-metric resonance of the array, leading to polarization independentFano resonances, irrespective of different orientations of the EDsand MDs. This is confirmed by both the results obtained usingcoupled dipole approximations (CDA) shown in Fig. 1(c) (solidline), and the FDTD simulation results in Fig. 1(c) (dashed linewith circles) which agree well with the CDA results.

It is worth mentioning that the polarization independent fea-tures of the CSNPs originate from the simultaneously equal elec-tric and magnetic responses of each particle itself, rather thanthe overall symmetry of the whole structure. So this feature willbe preserved in other 2D and 3D structures. We expect thatthe CSNP arrays we investigate can work as an effective plat-form with much more flexibilities to study nonlinear and lasingeffects when nonlinear and/or gain materials are incorporated.Such mechanism of superimposing orthogonal electric and mag-netic modes to achieve polarization independent Fano resonancesis not confined to the field of optics and can be easily applied inother fields like atom physics, nuclear physics, and so on.

REFERENCES

[1] A. E. Miroshnichenko, S. Flach, and Y. S. Kivshar, Rev. Mod. Phys., 82,2257-2298 (2010).

[2] S. L. Zou, N. Janel, and G. C. Schatz, J. Chem. Phys., 20, 10871-10875(2004).

[3] R. Paniagua-Dominguez, F. Lopez-Tejeira, R. Marques, and J. A. Sanchez-Gil, New. J. Phys. 13, 123017 (2011).

[4] V. A. Markel, J. Phys. B.: Mol. Opt., 38, L115-L121 (2005).

REFERENCES: [1] A. E. Miroshnichenko, S. Flach, and Y. S. Kivshar, Rev. Mod. Phys., 82, 2257-2298 (2010). [2] B. Luk`yanchuk et al., Nature Materials 9, 707 (2010). [3] R.Paniagua-Dominguez, F.Lopez-Tejeira, R.Marques, and J.A.Sanchez- Gil, New. J. Phys. 13, 123017 (2011). [4] V.A.Markel, J.Phys.B.:Mol.Opt. 38, L115-L121(2005).

Figure 1: (a) Extinction efficiency spectra for the CSNP, which can be approximated as a pair of orthogonal ED and MD coinciding spec- trally with the same strength. (b) The schematic geometry of the 1D array of CSNPs under consideration. The incident plane wave is propagating along z and the polarization angle (electric field with respect to x direction) is θ. (c) Theoretical (solid line) and FDTD (dashed line with circles) results of the extinction efficiency spectra for a 1D array of CSNPs with d = 1.15 μm for different polarization angles.

 

Speaker: Andrey MiroshnichenkoSession: Singular Optics and Fano ResonancesSee program for placement.

PQE-2013 Abstract Processed 18 December 2012 0