In this work, we investigate the plasmon-induced optical and magneto-optical anisotropies in the large-area square-ordered Co antidots film. It shows that both the outline of reflectivity spectrum and Kerr spectrum are significantly modified by surface plasmon polarition (SPP) resonances. Moreover, the magnitude of Kerr angle reaches to about 10 minutes at the azimuthal angle 45°, which is over 3 times of that of pure Co film. These phenomena are attributed to the SPP resonances with different diffraction orders of reciprocal lattice vectors.
© 2014 Optical Society of America
Surface plasmons (SPs) are the collective electronic oscillation at the interface between metal and dielectric materials . In recently years, SPs have derived various interdisciplines and applications, for example, nano-lasers [2, 3], bio-sensors [4, 5], plasmonic tweezers [6–9], chiral and negative refractivity materials [10, 11], etc. By introducing surface plasmons into magneto-optics, the so-called magneto-plasmonics has become a hotspot of research in last decade. The combination of ferromagnetism and plasmonics in the magneto-plasmonic materials can result in many novel experimental phenomena. For example, by changing the way SPs excited, in turn changing the way electrons and photons oscillate and interact, we can manipulate the magneto-optical (MO) response of the magneto-plasmonic materials. In the magneto-optical Kerr effect (MOKE) experiments, the excitation of SPs can significantly enhance either the magnitude of Kerr angle in the longitudinal  and polar  configuration or the relative reflectivity change in the transversal  configuration. Moreover, by manipulating the in-phase or out-phase relationship between localized SPs and the electromagnetic wave, the sign of Kerr angle can reverse . On the other hand, the properties of SPs are tunable in the ferromagnetic materials due to the time-reversal symmetry breaking by an applying magnetic field along the light propagation direction .
Among the several methods to excite SP resonances, utilizing the periodically patterned structures [12, 14] which can support propagating surface plasmon polaritons by fulfilling the wavevector match condition from the reciprocal lattice vectors, is the most convenient way.
The anisotropic effect, that originated from the breaking of system symmetry in the periodically patterned ferromagnetic materials, namely magneto-plasmonic crystals , is another way to manipulate the MOKE through SPs.
Researchers preferred to utilize the nanosphere lithography to fabricate the periodic patterned samples [18–21]. M.V. Sapozhnikov  and Z.L. Han  both studied the Co corrugated film by depositing metal onto the close-packed polystyrene spheres. The latter found that the anisotropy of magneto-optical response in the range of tens of micrometers at wavelength λ = 633nm. E.T. Papaioannou  and J.F. Torrado  also investigated the anisotropic effects in the hexagonal Ni and Fe antidots arrays. However, due to the intrinsic properties of nanosphere lithography, the samples are limited to hexagonal symmetry and because of the lack of centimeter-level order, the anisotropic effects are usually covered by the average effect of short-range domains with different packing orientations. Other fabrication methods were also used, like anodic alumina template [13, 24], ion beam etching , etc. However, most of them are incapable to reach both large-area order and nanoscale size feature in a convenient and inexpensive way.
In the present work 1cm × 1cm square-order cobalt antidots film with 412nm × 412nm period was prepared. The interference photolithography [24, 26–28], which is a fast way to produce designed patterns over a large area without any defects, is chosen to fabricate our antidots sample. We will demonstrate that the optical and magneto-optical response of Co antidots film is strongly anisotropic, and the Kerr angle is significantly enhanced by SPPs.
First, broadband anti-reflection(BAR, Brewer Science, WiDE-15B) layer was spin-coated on a silicon substrate at 3000rpm, in order to prevent any unnecessary light reflection from the substrate, otherwise it would deform and blur the interference pattern. Then the coated film was baked at 180 °C for two minutes on a hot plate, allowing for the second spin-coating of the negative photo resist (NPR) (Allresist, AR-4740) at 4000 rpm. After that the film with 180nm NPR was baked at 95 °C for one minute. To obtain periodical NPR antidots pattern, a Lloyd’s Mirror interference lithography system with a 325nm wavelength He-Cd laser was used. The NPR film was exposed in the interference illumination twice before and after a 90° rotation normal to the sample surface in order to produce square lattice pattern. The Co antidots film with 60nm height was produced by depositing Co on the NPR antidots pattern using DC magnetron sputtering. The SEM images of the final Co antidots pattern are shown in Fig. 1(a) (small-scale) and Fig. 1(b) (large-scale). The film has good periodicity with highly reproduced NPR antidots pattern. The inter-antidots space and antidots diameters can be easily controlled by the interference parameters, and the thickness of Co antidots film can be controlled by sputtering time. Figure 1(c) depicts the structure diagram of the cross-section of the films. There are double layers in the sample, the top layer is the Co antidots array and the bottom layer is the Co disks array. Since Co is high damping, the top layer can effectively prevent light from penetrating to the bottom layer, thus we simply exclude the bottom layer from discussion. It has also been confirmed by the COMSOL simulations in Fig. 4.
3. Experiments and discussion
SPPs can be excited by light either in Kretschmann or Otto configuration because the enlarged wavevector of attenuated wave at the total reflection can match the wavevector of SPPs. However, the more convenient way to excite SPPs is to introduce periodic structure where the reciprocal vectors will do the compensation. The dispersion relation of SPPs at the interface between Co film and dielectric can be written as14, 29], the matching condition for the 2D periodic structures is more complicated as the azimuthal angle φ of incident light changes. Figure 2 shows the simulation result of the relationship between the directional angles (θ, φ) and the wavelength of light, which excites SPPs with different diffraction orders in the Co antidots film. When φ = 0°, the result is same as the 1D periodic structure, as shown in Fig. 2(a). Only the (−1, 0) diffraction order related SPPs can occur in the visible range, while SPPs of higher diffraction orders can only be excited by ultraviolet light, no matter what the incident angle θ is. When we fix the incident angle at 45° and rotate the azimuthal angle, there are three diffraction orders involved in the visible range: (−1, 0), (0, −1), (−1, −1), as shown in Fig. 2(b). The light wavelength that excites SPPs of (−1, 0) diffraction order decreases with increasing φ (from 0° to 45°), and the (0, −1) diffraction order related SPPs increase with φ from (0° to 45°). They eventually meet with each other around 570nm at φ = 45°. The (−1, −1) diffraction order related SPPs show a relatively weak dependence on the azimuthal angle and overlap with (0, −1) diffraction order related SPPs around φ = 32.4°.
To prove the excitation of SPPs, the reflectivity spectrum of p-polarized light [Fig. 1(c)] with different incident angles (θ = 45°, 50°, 55°, 60°, 65°) and different azimuthal angles (φ = 0°, 30°, 45°) (corresponding to the vertical dash lines in Fig. 2), was investigated. The optical anisotropy is clearly observed. When φ = 0°, there are two reflectivity minima (Wood’s anomalies ) appearing in the visible range [Fig. 3(a)]. According to Fig. 2(a), the Wood’s anomaly around 403nm comes mainly from the (−1, −1) diffraction order related SPPs and partly from the (0, −1) diffraction order related SPPs, while the strong reflectivity minimum of Wood’s anomaly round 698nm is distinctively attributed to the (−1, 0) diffraction order related SPP resonance.
According to Fig. 3(a), when the incident angle rotates from 45° to 65°, the Wood’s anomaly around wavelength 403nm seldom shifts. This is coincident with the simulation result in Fig. 2(a), which shows that, when incident angle is larger than 45°, the incident wavelengths that excite (−1, −1), (0, −1) diffraction orders related SPPs almost don’t change. However, the Wood’s anomaly around 698nm changes significantly, though it is also in keeping with the (−1, 0)-diffraction-order spectrum in Fig. 2(a). That is because the SPPs are more vulnerable to the x directional diffraction order when y directional diffraction order is absent (see red line in Fig. 2(a)).
When φ = 30°, the two minima get closer as shown in Fig. 3(b). This is coincident with the simulation results in Fig. 2(b), in which the (0, −1) and (−1, −1) diffraction orders’ spectra get closer with increasing φ. When φ reaches to 45°, the two minimum nearly coincide. Thus, we clearly confirm that the minimum around 490nm in Fig. 3(b) is caused by the combination of (−1, −1) and (0, −1) diffraction orders’ SPP resonances. For the same reason, in Fig. 3(c), the minimum at shorter wavelength 502nm is related to (−1, −1) order distinctively, and the minimum at longer wavelength 555nm is contributed by the combination of (−1, 0) and (0, −1) diffraction orders.
The COMSOL simulation results of the intensity of E field at two resonant positions also prove the excitation of SPPs, as shown in Fig. 4. The geometric model for simulation is exactly the same as the sample except for the absence of the BAR layer. At the surface of antidots film [Fig. 4(b)(d)], the patterns of |E| distribution show a wave-like configuration which is clearly originated from SPPs. In addition, since the |E| at the bottom layer is much less than that of top layer, thus it is reasonable to neglect the effect of the bottom layer [Fig. 4(c), 4(e)].
It is known that the electric field near the surface of metal is redistributed when SPPs are excited, leading to the energy accumulation inside the metal, consequently strengthening the overall MO responses, such as enhancing the Kerr angle in longitudinal MOKE (LMOKE). Previous works mainly focused on the combination of noble metals and magnetic materials, which induces SPPs and provides magnetic moment individually. Here we simply use Co film with antidots arrays pattern to excite SPP resonances.
The magneto-optical response of the Co antidots film induced by p-polarized incident light in LMOKE is shown in Fig. 5 which shows the Kerr angle spectrum with different azimuthal angles in the spectral range from 430nm to 710nm. When φ = 0° and there is no SPP resonance, as depicted in Fig. 5(a), the Kerr spectrum of Co antidots film is smooth, just like the pure Co film. However, a peak appears at about 700nm, which is related to the (−1, 0) diffraction order’s SPPs.
With azimuthal angle φ increasing, there are two Kerr angle peaks appear in the visible range. One is at shorter wavelength, and the other is at longer wavelength [Fig. 5(c)]. The two peaks move towards each other [Fig. 5(b) and 5(c)], and at last coincide in the middle wavelength around 525nm when φ reaches to 45° [Fig. 5(e)]. According to Fig. 2(b) and Fig. 3, peak 1 mainly comes from the (0, −1) diffraction order and partly from the (−1, −1) order, due to relatively weak resonance amplitudes of higher diffraction orders’ SPPs. On the other hand, peak 2 is exclusively induced by (−1, 0)-diffraction-order SPPs. The two peaks merge into a single peak around 525nm at φ = 45°. The reason why the two reflectivity minima in Fig. 3(c) (θ≤65°) don’t coincide with each other, maybe that the wave band breadth of resonances for LMOKE is wider than that of the reflectivity’s minima. While increasing azimuthal angle, the Kerr angles of the two peaks gradually increase, and a maximum appears at wavelength 550nm at φ = 45°, the Kerr rotation angle reaches to 10 minutes, almost 3 times stronger than pure Co film. The result indicates that the interplay between SPP and LMOKE can significantly change the outline of LMOKE spectrum, and shows strong anisotropic effect.
In conclusion, we have fabricated large-area Co antidots film with long-range order. The absorption dips in reflectivity spectra and the Kerr angles in the MOKE spectra show significant anisotropic phenomena when changing the azimuthal angle. These phenomena are directly related to the resonance of SPPs with different diffraction orders, at the same time, Kerr angles are significantly enhanced at these resonances.
This work is supported by the National Key Project of Fundamental Research of China (Grant Nos. 2012CB932304 and 2010CB923404), the Natural Science Foundation of China (Grant Nos. 11374146 and U1232210) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
References and links
2. P. Berini and I. De Leon, “Surface plasmon-polariton amplifiers and lasers,” Nat. Photonics 6(1), 16–24 (2011). [CrossRef]
3. M. A. Noginov, G. Zhu, A. M. Belgrave, R. Bakker, V. M. Shalaev, E. E. Narimanov, S. Stout, E. Herz, T. Suteewong, and U. Wiesner, “Demonstration of a spaser-based nanolaser,” Nature 460(7259), 1110–1112 (2009). [CrossRef] [PubMed]
5. B. Sepúlveda, A. Calle, L. M. Lechuga, and G. Armelles, “Highly sensitive detection of biomolecules with the magneto-optic surface-plasmon-resonance sensor,” Opt. Lett. 31(8), 1085–1087 (2006). [CrossRef] [PubMed]
7. M. Righini, G. Volpe, C. Girard, D. Petrov, and R. Quidant, “Surface plasmon optical tweezers: tunable optical manipulation in the femtonewton range,” Phys. Rev. Lett. 100(18), 186804 (2008). [CrossRef] [PubMed]
8. J. Prikulis, F. Svedberg, M. Kall, J. Enger, K. Ramser, M. Goksor, and D. Hanstorp, “Optical spectroscopy of single trapped metal nanoparticles in solution,” Nano Lett. 4(1), 115–118 (2004). [CrossRef]
9. M. L. Juan, M. Righini, and R. Quidant, “Plasmon nano-optical tweezers,” Nat. Photonics 5(6), 349–356 (2011). [CrossRef]
12. V. I. Belotelov, D. A. Bykov, L. L. Doskolovich, A. N. Kalish, and A. K. Zvezdin, “Extraordinary transmission and giant magneto-optical transverse Kerr effect in plasmonic nanostructured films,” J. Opt. Soc. Am. B 26(8), 1594–1598 (2009). [CrossRef]
13. J. B. González-Díaz, A. García-Martín, G. Armelles, D. Navas, M. Vázquez, K. Nielsch, R. B. Wehrspohn, and U. Gösele, “Enhanced magneto-optics and size effects in ferromagnetic nanowire arrays,” Adv. Mater. 19(18), 2643–2647 (2007). [CrossRef]
14. A. A. Grunin, A. G. Zhdanov, A. A. Ezhov, E. A. Ganshina, and A. A. Fedyanin, “Surface-plasmon-induced enhancement of magneto-optical Kerr effect in all-nickel subwavelength nanogratings,” Appl. Phys. Lett. 97(26), 261908 (2010). [CrossRef]
15. V. Bonanni, S. Bonetti, T. Pakizeh, Z. Pirzadeh, J. Chen, J. Nogués, P. Vavassori, R. Hillenbrand, J. Åkerman, and A. Dmitriev, “Designer magnetoplasmonics with nickel nanoferromagnets,” Nano Lett. 11(12), 5333–5338 (2011). [CrossRef] [PubMed]
16. J. Y. Chin, T. Steinle, T. Wehlus, D. Dregely, T. Weiss, V. I. Belotelov, B. Stritzker, and H. Giessen, “Nonreciprocal plasmonics enables giant enhancement of thin-film Faraday rotation,” Nat Commun 4, 1599 (2013). [CrossRef] [PubMed]
17. G. Armelles, A. Cebollada, A. García-Martín, and M. U. González, “Magnetoplasmonics: combining magnetic and plasmonic functionalities,” Adv. Opt. Mater. 1(1), 10–35 (2013). [CrossRef]
18. Z. Liu, L. Shi, Z. Shi, X. H. Liu, J. Zi, S. M. Zhou, S. J. Wei, J. Li, X. Zhang, and Y. J. Xia, “Magneto-optical Kerr effect in perpendicularly magnetized Co/Pt films on two-dimensional colloidal crystals,” Appl. Phys. Lett. 95(3), 032502 (2009). [CrossRef]
19. Z. L. Han, J. H. Ai, P. Zhan, J. Du, H. F. Ding, and Z. L. Wang, “Strong in-plane anisotropy of magneto-optical Kerr effect in corrugated cobalt films deposited on highly ordered two-dimensional colloidal crystals,” Appl. Phys. Lett. 98(3), 031903 (2011). [CrossRef]
20. M. V. Sapozhnikov, S. A. Gusev, V. V. Rogov, O. L. Ermolaeva, B. B. Troitskii, L. V. Khokhlova, and D. A. Smirnov, “Magnetic and optical properties of nanocorrugated Co films,” Appl. Phys. Lett. 96(12), 122507 (2010). [CrossRef]
21. A. A. Grunin, N. A. Sapoletova, K. S. Napolskii, A. A. Eliseev, and A. A. Fedyanin, “Magnetoplasmonic nanostructures based on nickel inverse opal slabs,” J. Appl. Phys. 111, 07A948 (2012).
22. E. T. Papaioannou, V. Kapaklis, E. Melander, B. Hjörvarsson, S. D. Pappas, P. Patoka, M. Giersig, P. Fumagalli, A. Garcia-Martin, and G. Ctistis, “Surface plasmons and magneto-optic activity in hexagonal Ni anti-dot arrays,” Opt. Express 19(24), 23867–23877 (2011). [CrossRef] [PubMed]
23. J. F. Torrado, E. T. Papaioannou, G. Ctistis, P. Patoka, M. Giersig, G. Armelles, and A. Garcia-Martin, “Plasmon induced modification of the transverse magneto-optical response in Fe antidot arrays,” Phys. Status. Solidi. RRL 4(10), 271–273 (2010). [CrossRef]
24. J. Oh and C. V. Thompson, “Selective barrier perforation in porous alumina anodized on substrates,” Adv. Mater. 20(7), 1368–1372 (2008). [CrossRef]
25. S. Wu, Z. Zhang, Y. Zhang, K. Zhang, L. Zhou, X. Zhang, and Y. Zhu, “Enhanced rotation of the polarization of a light beam transmitted through a silver film with an array of perforated S-shaped holes,” Phys. Rev. Lett. 110(20), 207401 (2013). [CrossRef]
26. M. Farhoud, J. Ferrera, A. J. Lochtefeld, T. E. Murphy, M. L. Schattenburg, J. Carter, C. A. Ross, and H. I. Smith, “Fabrication of 200 nm period nanomagnet arrays using interference lithography and a negative resist,” J. Vac. Sci. Technol. B 17(6), 3182–3185 (1999). [CrossRef]
28. C. A. Ross, H. I. Smith, T. Savas, M. Schattenburg, M. Farhoud, M. Hwang, M. Walsh, M. C. Abraham, and R. J. Ram, “Fabrication of patterned media for high density magnetic storage,” J. Vac. Sci. Technol. B 17(6), 3168–3176 (1999). [CrossRef]
29. A. V. Chetvertukhin, A. A. Grunin, A. V. Baryshev, T. V. Dolgova, H. Uchida, M. Inoue, and A. A. Fedyanin, “Magneto-optical Kerr effect enhancement at the Wood's anomaly in magnetoplasmonic crystals,” J. Magn. Magn. Mater. 324(21), 3516–3518 (2012). [CrossRef]
30. R. Wood, “XLII. On a remarkable case of uneven distribution of light in a diffraction grating spectrum,” Philos. Mag. 4(21), 396–402 (1902). [CrossRef]