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Nonreciprocal Plasmonics Enables Giant Enhancement of Thin-Film

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Nonreciprocal Plasmonics Enables Giant Enhancement of Thin-Film ARTICLE Received 17 Nov 2012 | Accepted 14 Feb 2013 | Published 19 Mar 2013 DOI: 10.1038/ncomms2609 OPEN Nonreciprocal plasmonics enables giant enhancement of thin-film Faraday rotation Jessie Yao Chin1, Tobias Steinle1, Thomas Wehlus2, Daniel Dregely1, Thomas Weiss1, Vladimir I. Belotelov3, Bernd Stritzker2 & Harald Giessen1 Light propagation is usually reciprocal. However, a static magnetic field along the propagation direction can break the time-reversal symmetry in the presence of magneto-optical materials. The Faraday effect in magneto-optical materials rotates the polarization plane of light, and when light travels backward the polarization is further rotated. This is applied in optical isolators, which are of crucial importance in optical systems. Faraday isolators are typically bulky due to the weak Faraday effect of available magneto-optical materials. The growing research endeavour in integrated optics demands thin-film Faraday rotators and enhance- ment of the Faraday effect. Here, we report significant enhancement of Faraday rotation by hybridizing plasmonics with magneto-optics. By fabricating plasmonic nanostructures on laser-deposited magneto-optical thin films, Faraday rotation is enhanced by one order of magnitude in our experiment, while high transparency is maintained. We elucidate the enhanced Faraday effect by the interplay between plasmons and different photonic waveguide modes in our system. 1 4th Physics Institute and Research Center SCOPE, University of Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany. 2 Institute of Physics, University of Augsburg, Universita¨tsstrae 1, 86135 Augsburg, Germany. 3 Faculty of Physics, Lomonossov Moscow State University, Leninskie Gory, Moscow 119991, Russia. Correspondence and requests for materials should be addressed to J.C. (email: [email protected]). NATURE COMMUNICATIONS | 4:1599 | DOI: 10.1038/ncomms2609 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2609 lasmonics provides abundant routes to manipulate light– External magnetic field matter interaction by nanostructures that can localize Plight and alter the field distribution at sub-wavelength scale1. Combining plasmonics with magneto-optics is of particular interest2–8, as it enables sophisticated control of magneto-optical material properties, which can lead to many optical devices where magnetism is applied. For example, by hybridizing ferromagnetic and noble metal materials, the magneto-optical polar Kerr effect was enhanced, which was y termed ‘magneto-plasmonics’9,10. In this realm, Temnov 11,12 et al. reported plasmon interferometers, which utilized z x propagating surface plasmons controlled by a static magnetic TM field due to an adjacent ferromagnetic layer. Meanwhile, one-way waveguiding13 and one-way extraordinary optical 14 transmission were proposed by integrating magnetic Period materials with a photonic crystal and spoof surface plasmons, respectively. Among the various magneto-optical effects, Faraday rotation is of special interest, as it is the key technology in optical isolation15,16. Previously, plasmonic heterostructures17 and planar photonic crystals18 were proposed theoretically to enhance the nonreciprocal Faraday effect, but have not yet 500 nm been realized. In this work, we experimentally demonstrate large enhance- Figure 1 | Magneto-plasmonic photonic crystal. (a) Faraday rotation by a ment of the Faraday effect over a rather broad bandwidth. It is magneto-plasmonic photonic crystal for TM-polarized incident light, achieved by incorporating nonreciprocal magneto-optical where f is the Faraday rotation angle. At normal incidence, TM-polarized material with plasmonic photonic crystal19–21. Specifically, light has the electric field perpendicular to the gold wires, and TE-polarized we fabricate plasmonic nanowire arrays on magneto-optical light has the electric field parallel to the wires. (b) Schematic of the thin films and characterize the Faraday rotation by polarimetry. magneto-optical photonic crystal, where BIG film (dark red) is deposited Faraday rotation is increased by up to 8.9 times compared with on glass substrate (blue) and periodic gold nanowires (golden) are sitting the bare film, while high transparency is maintained. We simulate atop. (c) A scanning electron microscopy image of one sample. the hybrid system by numerical tools and find good agreement between simulation and experiment. Our results reveal the mechanism behind the enhancement of Faraday effect in analogy to the classical coupled oscillator model, which can be used to 1 engineer the properties of the nonreciprocal plasmonic system. 0.8 We expect our concept to find applications in nonreciprocal 0.8 thin-film optical devices. 0.4 e 0 0.6 Results (deg) tion rota Magneto-plasmonic photonic crystal. We utilize periodic gold smittanc 0.4 nanowires structured on a thin layer of magneto-optical material, ay –0.4 which is deposited on a glass substrate, as depicted in Fig. 1a,b. Tran Farad BIG film Such a system allows simultaneous oscillation of plasmons and –0.8 400 nm period 0.2 400 nm period photons. A localized plasmon resonance is excited by incoming 450 nm period 450 nm period light polarized perpendicular to the wires, defined as TM-polar- 495 nm period 495 nm period –1.2 0 ized incident light. (TE and TM polarizations are specified in 700 800 900 1,000 700 800 900 1,000 Fig. 1.) Besides the plasmon resonance, the nanowires introduce Wavelength (nm) Wavelength (nm) periodicity to the hybrid structure and hence enable light to Figure 2 | Measured Faraday rotation and transmittance. (a) Measured couple into the magneto-optical thin film, which acts as a planar 20,21 Faraday rotation of the three samples at normal incidence photonic crystal waveguide for photons . The localized (TM polarization), compared with measured Faraday rotation of the bare plasmon resonance and the waveguide resonance interact BIG film. (b) Measured transmittance of the three samples at normal strongly, and this interaction can be tailored to engineer the incidence (TM polarization). magneto-optical properties. Fabrication and characterization. Three samples of plasmonic compared with Faraday rotation of the reference BIG film photonic crystals were fabricated, each of which consists of is shown in Fig. 2a. 120-nm wide and 65-nm thick periodic nanowires structured on The spectra of Faraday rotation display a resonant feature. As top of a 150-nm thick bismuth iron garnet (BIG) thin film the period increases, the resonance shifts to longer wavelength (Fig. 1b). The three samples have different periods of the and the maximum Faraday rotation increases. The sample of nanowires, being 400, 450 and 495 nm, respectively. Figure 1c 495 nm period reaches a maximum Faraday rotation of 0.80° at shows a scanning electron micrograph of one sample. A magnetic l ¼ 963 nm, which is 8.9 times enhancement of the À 0.09° field of 140 mT was applied normal to the BIG film, and Faraday Faraday rotation of the BIG film without gold wires. The 0.80° rotation f of the samples was measured at normal incidence Faraday rotation has a half-value bandwidth of 22 nm. As high using a rotating analyser polarimeter. For TM-polarized incident transparency is favourable for the potential application of Faraday light, Faraday rotation of the plasmonic photonic crystals rotators, spectra of transmittance were measured for all the 2 NATURE COMMUNICATIONS | 4:1599 | DOI: 10.1038/ncomms2609 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2609 ARTICLE samples (Fig. 2b). The 495-nm period sample shows 36% trans- magnetization, the permittivity of BIG can be described by a mittance at l ¼ 963 nm. In comparison, the sample of 450 nm tensor23 period exhibits Faraday rotation of 0.45° at l ¼ 907 nm. Com- 2 3 e À ig 0 pared with À 0.11° Faraday rotation of the bare BIG film, it is an 4 11 5 enhancement of 4.1 times. The sample of 400 nm period shows eBIG ¼ ig e22 0 ; Faraday rotation of 0.33° at the resonant peak of l ¼ 831 nm. 00e33 Compared with Faraday rotation of the bare BIG film, À 0.17 °, there is 1.9 times enhancement. The resonant shape of the where g in the off-diagonal elements describes the magnetization- Faraday rotation spectra and transmittance spectra is directly induced gyration of the material. The off-diagonal elements have related to the waveguide–plasmon polaritons20,21. Cross-coupling a similar role as the elastic spring in the two orthogonal of the TM and TE modes accounts for the enhancement of oscillators model, and give rise to an additional electric field Faraday rotation. component orthogonal to the electric field of the incident light. This secondary component of the electric field oscillates and emits TE-polarized light. Consequently, the optical far-field signal Two orthogonal coupled oscillators model. The responsible acquires a TE-polarized component and the plane of polarization coupling of the TM and TE modes can be described by two is rotated. Large enhancement of Faraday rotation requires strong orthogonal harmonic oscillators bound together by an elastic coupling between the TE and TM modes, which is most efficient spring22 (Fig. 3a). If an oscillator is excited in one direction, the when the two modes are spectrally close and therefore can be oscillation is transferred to the other oscillator via the elastic excited simultaneously. This intuitive model is supported by spring, and thus causes reradiation of waves in the orthogonal numerical modelling of the plasmonic photonic crystal. direction. In the magneto-plasmonic photonic crystal, TM- Figure 3b–e shows the simulated diagrams of absorbance and polarized incident light induces a TM waveguide–plasmon the enhancement of Faraday rotation for TE- and TM-polarized hybrid mode, with the electric field in the BIG film oscillating incident light in dependence of the wavelength of the incident in the plane perpendicular to the wires and the magnetic field light and the period of the nanowires. The white and red dashed oscillating along the wires20.
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