Surface-magnetoplasmon nonreciprocity effects in noble-metal/ferromagnetic heterostructures

Abstract

We demonstrate magnetic field control of surface plasmon excitations in noble-metal/ferromagnetic/noble metal trilayers, analogous to the effects previously observed in semiconductor structures. We show that the coupling of an external magnetic field to the surface plasmon-polariton wave vector is greatly enhanced in the metallic structure due to the ferromagnetic nature of one of its constituents. The observed coupling could be used to modulate the surface plasmon response in ultrasensitive spectroscopic applications. DOI: 10.1103/PhysRevB.76.153402 PACS number͑s͒: 78.20.Ls, 73.20.Mf, 78.66.Bz, 42.25.Bs Surface plasmon-polariton ͑SPP͒ modes are electromagnetic excitations localized at the interface between two media, one with positive and the other with negative dielectric constant. These modes may appear at the interface between a degenerate semiconductor and a dielectric or between a metal and a dielectric. In the former case, due to the low value of the plasma frequency of the semiconductor, the frequencies of the SPPs are restricted to the far infrared range, whereas in the second case the SPP modes can have frequencies varying from the far infrared to the visible range. The propagation characteristics of the SPPs and their EM field distribution depend strongly on the optical properties and interface morphology of the system. This dependence has been exploited in different optical contexts such as light guiding at the subwavelength scale, 1-3 optical switching, 4 biochemical sensing, 5 or nanometer resolved far-field optical microscopy. 6 To date SPPs are commonly considered as passive, i.e., insensitive to the magnetic field and just depending on the optical and geometrical properties of the system. In this work we demonstrate the control of SPP excitations in metallic trilayer structures by means of an external magnetic field. We show that the coupling of the magnetic field to the wave vector of the plasmon is greatly enhanced by the ferromagnetic nature of the trilayer structure. This effect was first studied theoretically in semiconductor-based SPPs 7-9 and in metals. 10 The effect of the magnetic field on the properties of the SPP modes depends on the relative orientation of the applied magnetic field with respect to the wave vector of the SPP. In particular, we will show that when the magnetic field is applied perpendicular to the direction of propagation of the SPP and parallel to the interface, it modifies the dispersion relation of the SPP mode in such a way that the dispersion relation depends on the k direction ͓i.e., w͑k͒ w͑−k͔͒. Experimentally this magnetic field induced nonreciprocity has been observed on semiconductor-based SPPs, 11 but not yet in metallic systems. This is due to the high magnetic field needed to observe magnetic field induced effects on metallic based SPPs.One way to reduce the required external magnetic field is to incorporate ferromagnetic metals. Due to the magnetooptical ͑MO͒ activity that many ferromagnetic materials exhibit at low magnetic fields, surface magnetopl...

Intertwined magneto-optical and plasmonic effects in Ag/Co/Ag layered structures

“…Both deviations of the actual Co thickness with respect to the nominal one and the intrinsic interface roughness present in the actual structures, may be responsible for this difference. The observed MO enhancement in the Ag/Co/Ag can be compared with that previously obtained in the Au/Co/Au system, 16 exploring the effect of the substitution of Au by Ag and its associated improvement of plasmonic properties. In Fig.…”

“…6͑a͒, data not shown here͔ both R and ⌬R / R basically do not exhibit any angular dependence. It has previously been shown that the values for ⌬R / R in Au/Co/Au under SPP excitation conditions are one order of magnitude larger than those without SPP excitation 16,19 due both to a simultaneous reduction in the reflectivity of the system and to an enhancement of the electromagnetic field at the MO active layer, i.e., the Co layer. For the specific case of 5 nm Au/5 nm Ag/8 nm Co/7 nm Ag, shown in Fig.…”

“…[12][13][14] An interesting option for being used as SPP external control agent is the magnetic field. 15,16 The magnetic field ability to affect the properties of surface-plasmon polaritons in highly doped InSb semiconductors has been known for a long time. 17 This effect is governed by the nondiagonal elements of the dielectric tensor, ij , responsible for the magneto-optical ͑MO͒ activity and whose value is controlled by the magnetic field, which in this kind of semiconductors turns out to be relatively high under the presence of moderate magnetic fields.…”

“…For example, in the Au/Co/Au system, the MO activity can be greatly enhanced when the SPP resonance is excited: 16,[18][19][20][21] in this situation the light absorption is maximum and, for specific layer thickness, the electromagnetic field of the light is greatly enhanced at the MO active layer ͑Co in this case͒. This leads, respectively, to a reduction in the system reflectivity and to an enhancement in the magnetic component of the MO activity 16,22 which altogether are responsible of the enhanced global MO response. Moreover, it has also been demonstrated that the SPP wavevector in Au/Co/Au trilayers can be modified by an external magnetic field due to its dependence with the off-diagonal elements of the dielectric tensor, which in turn are magnetic field dependent in these systems.…”

“…Moreover, it has also been demonstrated that the SPP wavevector in Au/Co/Au trilayers can be modified by an external magnetic field due to its dependence with the off-diagonal elements of the dielectric tensor, which in turn are magnetic field dependent in these systems. 16 Most of the previous studies have been done by using Au as the plasmonic component of the magnetoplasmonic material. However, compared to gold, silver exhibits narrower and more intense plasmon resonances in the visible range due to its lower absorption losses and its higher plasmafrequency value.…”