Nanoscale magnetophotonics

This Perspective surveys the state-of-the-art and future prospects of science and technology employing the nanoconfined light (nanophotonics and nanoplasmonics) in combination with magnetism. We denote this field broadly as nanoscale magnetophotonics. We include a general introduction to the field and describe the emerging magneto-optical effects in magnetoplasmonic and magnetophotonic nanostructures supporting localized and propagating plasmons. Special attention is given to magnetoplasmonic crystals with transverse magnetization and the associated nanophotonic non-reciprocal effects, and to magneto-optical effects in periodic arrays of nanostructures. We give also an overview of the applications of these systems in biological and chemical sensing, as well as in light polarization and phase control. We further review the area of nonlinear magnetophotonics, the semiconductor spin-plasmonics, and the general principles and applications of opto-magnetism and nano-optical ultrafast control of magnetism and spintronics.


I. Introduction
During the past two decades our ability to control materials at the nanoscale allowed a more aware study of nanoscale light-matter interactions, leading to the advent of nanophotonics and nano-optics.
One particularly efficient way of confining light into subwavelength volumes is by using the collective electromagnetic-induced electronic excitations known as plasmons. Unlike conventional optics, plasmonics enables the unrivalled concentration and enhancement of electromagnetic radiation well beyond the diffraction limit of light [1][2][3][4]. Besides its fundamental scientific importance, manipulation of light at the nanoscale is of great interest due to its potential exploitation towards real-life applications such as energy harvesting and photovoltaics, wave-guiding and lasing, optoelectronics, biochemistry and medicine.
To achieve new functionalities, the combination of plasmonics with other material properties has become increasingly appealing. In particular, magnetoplasmonics and magnetophotonics are emerging areas that aim at combining magnetism, plasmonics and photonics [5 -11] to find new ways of controlling the properties of plasmons using magnetic fields or vice-versa, to control magnetic properties with light. Nanoscale magnetophotonics entails the fundamental studies of photonelectronic spin interactions in nanostructured materials [12]; the enhancement of magneto-optical (MO) activity in materials [13][14][15], including dielectrics [16], 2D materials [17], nanoparticledecorated graphene [18] and graphene-based metasurfaces and their topological transformations [19]; the active control of plasmons with weak magnetic fields [20]; topological photonics and gyromagnetic photonic crystals [21]; magnetoplasmonics-based bio-and chemical sensing [22] and magnetophotonic and magnetoplasmonic crystals (MPCs) as modulators of light transmission, reflection and polarization [23][24][25].
Since the early 1970s, the investigation of the interaction between magnetism and plasmons has been a topic of high interest. In 1972 Chiu and Quinn showed that an external static magnetic field could control the properties of surface plasmon polaritons (SPPs) such as their propagation or localization [26] [ Fig. 1(a)]. The exponential growth of fabrication techniques in semiconductor 3 technology during the last two decades boosted engineering of photonic band gap materials and plasmonic systems operating at optical frequencies. MO properties of photonic crystals and their potential use in integrated optics were thoroughly investigated in 2005 by Belotelov and Zvezdin [27]. Shortly after, extraordinary transmission and plasmon-enhanced giant Faraday and Kerr effects were demonstrated in noble metal-dielectric plasmonic system made of Au films with either a subwavelength hole [28] or slit [29,30] array on top of a magnetic Bi:YIG layer. In parallel, the theoretical study by Yu et al. predicted that a waveguide formed at the interface between a photonic crystal and a metal under a static magnetic field possesses unique dispersion relations resulting in modes propagating in only one allowed direction [31]. In 2007 Gonzalez-Diaz et al. demonstrated that the coupling of an external magnetic field to the surface propagating plasmon wave vector can be greatly enhanced in noble metal/ ferromagnetic/ noble metal trilayers, which allows magnetic control of surface plasmon analogously to semiconductors [32]. At the end of 1990s, Martín-Becerra et al. showed that magnetic modulation of SPP wave vector could be significantly improved by depositing a dielectric overlayer in such geometries [33] and, later on, that both the real and imaginary parts of SPP wave vector are affected by the magnetic field in noble/ ferromagnetic/ noble metal films resulting in spectrally dependent modulation [34]. Unfortunately, for noble metal-based plasmonic structures, the magnetic field required to achieve proper control of surface plasmon properties is too high for application purposes. With nanoengineering of complex systems combining ferromagnetic materials and noble metals, which exhibit simultaneously magnetic and plasmonic properties, it became possible to control the plasmon wave vector with a weak (100 mT regime) external magnetic field [35,36], generate ultrashort SPP pulses [37] and produce SPP-induced magnetization in nickel with effective magnetic field of 100 Oe by femtosecond laser pulse [38]. Hybrid magnetoplasmonic systems combining noble metal and iron garnets that are typically highly transparent compared to ferromagnetic metals provide magnetic modulation of light transmittance. Enhanced MO effects and strong magnetic modulation of light intensity were found in metallic nanostructures integrated with iron garnet film [39 -43]. Furthermore, plasmon mediated MO transparency was observed in 4 magnetophotonic crystal formed by gold grating stacked on top of bismuth-substituted rare-earth iron garnet deposited on top of gadolinium gallium garnet [25]. In similar architectures, a shift of plasmon polariton resonance was manipulated by femtosecond laser pulses [44]. Finally, in systems that combine plasmonic crystals and magnetic semiconductors the MO effects could be dramatically enhanced in both transmission and reflection [45].
In parallel to the studies on propagating plasmons, the current rapid advances in nanofabrication enable the broadening of our understanding of optics at the nanoscale with nanostructures supporting also localized surface plasmon resonances (LSPRs) [ Fig. 1(b)]. Also here hybrid structures were proposed to combine all-in-one the advantages of noble-metals and magnetic materials [46,47]. Also, pure ferromagnetic nanostructures were demonstrated to support LSPRs [48] and SPPs [49,50] and at the same time exhibit sizeable magnetic effects under low magnetic fields, leading to a large tunability of the MO response [51]. The strong coupling between SPP and the MO activity leading to a significant enhancement and tunability of the Kerr effect as a result of lattice design were observed also in pure ferromagnetic (Fe, Co, Ni) 2D hexagonal lattices [52][53][54].  (c) MOKE configurations and coordinate system used in their description. 6 This Perspective covers a plethora of intriguing effects and phenomana associated with lightmatter interactions in nanoscale geometries in the presence of magnetic field. Section I contains a brief introduction to the field of magnetophotonics and highlighs important discoveries in geometries supporting propagating and localized plasmons. For smooth navigation through this Perspective, the reader can always refer to the classification of the MOKE configurations that is given in Fig. 1(c) of Sec. I. Section II of this Perspective mainly covers MO effects in magnetoplasmonic and magnetophotonic nanostructures in different configurations. We start with Part (a) of Sec. II, that presents the overview of fundamental works that theoretically explored the origin of MO by analytical models and explained the role of spin-orbit (SO) coupling in the MO activity in nanostructures supporting localized plasmons. We then proceed to Sec. II B where we explain the origin and the resonant enhancement of MO effects in MPCs and derive the dispersion relations. In both Secs. II B and II B we discuss the fundamental limitations and the main strategies used to maximize the MO enhancement in magnetoplasmonic nanostructures and MPCs. In Sec. II C we discuss transversely magnetized MPCs and plasmonic nonreciprocity, specifically focusing on the variety of materials and geometries that provide strong light modulation by the transversely applied magnetic field. Section II D delves into MO effects in longitudinal magnetization and introduces the longitudinal magnetophotonic intensity effect (LMPIE). Finally, we devote Sec. II E to MO effects in dot-and antidot periodic arrays and consider special light illumination conditions associated with Wood's anomalies and second harmonic generation. In Sec. III we give an overview of applications of nanoscale magnetoplasmonics and magnetophotonics in biological and chemical sensing and light's polarization and phase control. Section IV is entirely focused on nonlinear-optical processes attainable in the vicinity of SP resonances in the presense magnetic fields. We continue with Sec. V that introduces the emerging field of magnetically induced spin-polarization in semiconductors.
Section VI of this Perspective is devoted to ultrafast magnetism and fundamental understanding of the relationship of spin orbital momentum and orbital angular momentum of light and nanoscale 7 magnetism giving a special attention to the inverse Faraday effect and helicity-dependent all-optical magnetization switching. We conclude by giving our outlook on the field and by summarizing the recent advances that pave the way to practical magnetophotonic devices.

II. Magneto-optical effects in magnetoplasmonic and magnetophotonic nanostructures a. Localized plasmons in magnetoplasmonic nanostructures
Magnetoplasmonic nanostructures and nanostructured magnetophotonic crystals support surface plasmon resonances (localized and/or propagating). Therefore, they exhibit strongly enhanced MO activity at low magnetic fields. Regarding the systems supporting LSPRs, Sepulveda et al. first explained intuitively this phenomenon in 2010 [13]. They showed that in pure gold nanodisks the large MO response comes from an increase of the magnetic Lorentz force induced by the large collective movement of the conduction electrons when a LSPR is excited in the presence of a static magnetic field [see Figs. 2(a) and 2(b)]. Lorentz oscillator model for dielectrics, but with the addition of a static magnetic field, which exerts a Lorentz force on the bound electrons (adapted from Ref. [9]). (b) Schematic of the MO effect induced by the Lorentz force in a metal nanoparticle [13]. (c) A ferromagnetic disk modeled with two orthogonal damped harmonic oscillators coupled by the SO interaction; m represents the mass of the conduction electrons; the spring constants kx and ky originate from the electromagnetic restoring forces due to the displacements of the conduction electrons; βx and βy are the damping constants [56]. yielding analytical expressions for the resonantly enhanced MO response. All these models can be transferred to other complex and hybrid nano-optical systems and can significantly facilitate device design. However, the magnetic field-induced modulation of light polarization achieved in magnetophotonic crystals so far is only in the order of a fraction of degree, which is insufficient for any practical purposes. When using conventional ferromagnets, the main obstacles are the exiguity of MO activity arising from the SO coupling and the rather inefficient excitation and/or propagation of plasmon modes, due to their high dissipative losses. One of the key challenges is indeed to increase the strength of SO-coupling without increasing the plasmon damping. The main strategies currently pursued with conventional ferromagnetic materials, namely without increasing the intrinsic SO-coupling, are (i) periodic arrangements of magnetoplasmonic nanoantennas [58,59]; 9 (ii) 3D ferromagnets [60] and composite ferromagnetic/noble metal [61] and ferromagnetic/dielectric/noble metal nanostructures [62], and (iii) heterogeneous units comprising multiple nanoantennas placed in proximity to enable their near-field interaction [63 -67]. Initial investigations have shown that the enhancement of polarization rotation by one order of magnitude can indeed be achieved following these strategies. Finally, it is worth noticing that exploting highindex all-dielectric nanostructures one can reduce the high losses, which are inherent in magnetic materials [16]. The use of these materials can lead to peculiar novel phenomena where magnetic dipoles are responsible for the MO activity, thus opening interesting perspectives in the engineering of novel nanoscale MO effects.

b. Magnetoplasmonic crystals
Periodically nanostructured metal-dielectric systems allow excitation of propagating plasmonic modes by incident light. On the other hand, their periodicity is of the order of wavelength of SPPs propagating at the metal-dielectric interface and at some frequency range constructive interference takes place and band gaps appear. Therefore, such kind of structures can be referred to the plasmonic crystals in analogy to photonic crystals. If some magnetic substances are involved, then such periodic structure is called magnetoplasmonic crystal (MPC).  Generally, excitation of plasmonic resonance provides enhancement of MO effects. A noble metal plasmonic crystal without any magnetic media can be also made MPC if a high external magnetic field is applied. If the magnetic field is in-plane and transverse with respect to SPP propagation then it provides some enhancement of the T-MOKE [68]. In this case the MO properties are due to Lorentz force acting on free electrons in a magnetic field. A resonant increase of the T- 11 MOKE was reported for one-dimensional Co, Fe and Ni gratings [69,70]. Pronounced resonance of T-MOKE in a sample of 1D trilayer SiO2/Fe/Ag MPC fabricated on a commercial blue-ray disc also allowed to consider a refractive index sensor on its basis [71]  respectively. Since the overall optical losses for such systems are lower than for pure ferromagnetic metals the effect of resonant increase of the T-MOKE due to propagating SPPs in these structures is more pronounced. It also allows to consider these structures as highly sensitive plasmonic biosensors [71,79]. Concept of MPC works not only with transverse magnetization. Recently, Maccaferri et al. The MPC structures can also be referred as magnetophotonic metasurfaces, though the term of metasurface is more general and also includes all-dielectric and semiconductor materials consisting of substrates covered with cylinders and spheres sustaining Mie resonances [80,81]. An example of magnetoplasmonic metasurface is represented by two-dimensional arrays of Si nanodiscs covered by a thin Ni film [82]. Optical resonances in such samples lead to enhanced MO response like Faraday rotation of 0.8 deg. which is reasonably large taking in mind that the magnetic part is only 5 nm thick.
The main disadvantage of most of the aforementioned approaches is that the optical losses associated with the presence of a ferromagnetic metal are still relatively high. This fact limits exploiting fully the potential gain of the combined concepts of nanostructuring and plasmonics in magneto-optics. If the ferromagnetic metals were avoided as in cases of pure semiconductors or noble metal systems, huge external magnetic fields exceeding several Tesla would be necessary to make 12 the T-MOKE at least comparable with the effect in ferromagnets. That is why it seems that the plasmonic crystals containing low-loss ferromagnetic dielectrics and noble metals can provide even better results [9,14,83]. The most pronounced enhancement of the MO effects takes place for highquality resonances that are achieved if the ferromagnetic metal is substituted by a low absorptive noble one and the dielectric layer is magnetized. Probably, the best candidates for magnetic dielectric are bismuth rare-earth iron garnet films of composition BixR3-xFe5O12, where R is a rare-earth element [84]. Therefore, we will consider main properties of MPCs taking this kind of structures as exemples and study their properties in detail.
Let us consider an MPC consisting of smooth magnetic dielectric on a substrate and noble metal film periodically perforated with subwavelength array(s) of slits and holes. In such structure SPPs can propagate either along the upper interface, the air/metal interface, or along the bottom interface between the metal and magnetic dielectric. Though the metal film is not continuous, the SPP can still propagate along the structure if the air gap size is notably smaller than the SPP wavelength and air takes relatively small part of the MPC crystal lattice. During SPP propagation some part of its energy continuously leaks in the far-field due to the SPP scattering on the metal grating. This mechanism also contributes to the SPP energy decrease in MPCs together with conventional energy dissipation in lossy metal and dielectric layers.
On the other hand, metal perforation provides a very efficient way to excite SPPs by using light.
The metal grating provides diffracted light with different in-plane wavevector components. If some of them coincide with the SPP wavevector then the light will be coupled to SPPs. In this case the momentum conservation law is written as where 0 is the wavevector of light in vacuum, is the SPP wavenumber along the metal-dielectric interface, 3 is the dielectric constant of the medium above the metal/dielectric structure, is the angle of incidence, and are two reciprocal lattice vectors, | | = 2 ⁄ , � � = 2 ⁄ ; and are the periods of the grating along the x-and y-directions; ( ) , are two in-plane unit vectors along the plane of light incidence and along the SPP propagation direction, respectively, and 1 and 2 are integers. In the grating configuration, SPPs can be excited on both the metallic interfaces.
Strictly speaking, the absolute value of the wavevector of the grating SPP in Eq. (1) deviates from the one for the smooth metal-dielectric interface determined by = 0 � substituted rare-earth iron garnets have rather low MO activity at 1.55 µm. In this case, the use of cerium substituted iron-garnets seems to be more preferable [85]. is characteristic of media with a toroidal moment τ whose transformation properties are identical to those for M × N [86]. Thus, the propagation of SPP is similar to the propagation of a wave in a medium with a toroidal moment along its direction. In electrodynamics, the presence of a toroidal moment is known to give rise to optical nonreciprocity. In the case under consideration, the latter is manifested in a difference between the wave vectors of the electromagnetic wave as it propagates in the direction along the vector τ and in the opposite direction [87]: Similar optical nonreciprocity takes place for a SPP in the case of a transversally magnetized medium = 0 (1 + g) where 0 = 0 ( 1 2 ( 1+ 2 ) ⁄ ) 1/2 and = (− 1 2 ) −1/2 (1 − 2 2 1 2 ⁄ ) −1 ; 1 and 2 are the dielectric constants of metal and dielectric, respectively, and gyration g is a parameter linear in the magnetization that is responsible for the MO properties of the material (in terms of the dielectric tensor, g=iεzx=-iεxz if the magnetization is directed along y-axis). It follows from Eq. (2) that, in the first approximation, the wavenumber of the surface wave depends linearly on the film gyration g, which confirms the nonreciprocity effect. Equation where I(M) and I(0) are the intensities of the reflected or transmitted light in the magnetized and nonmagnetized states, respectively [88].
Due to the T-MOKE light intensity can be controlled by magnetic field without any polarizers or other additional optical elements. T-MOKE is mostly determined by interface between nonmagnetic and magnetic media and therefore is highly sensitive to the magnetization near the sample surface and can sustain decent values even for ultra-thin films. Moreover, its inverse counterpart is of primary importance in ultrafast magnetic phenomena [36].
The T-MOKE for a bare iron-garnet film is very small (δ~10   Reprinted with permission from Ref. [90]. Copyright 2018 The Optical Society.
The T-MOKE configuration is of particular interest since it provides also new ways of routing the directivity of light emission by using an external magnetic field. The routing of emission for excitons in a diluted-magnetic-semiconductor quantum well was demonstrated in hybrid plasmonic semiconductor structures [91]. In that case a CdMnTe quantum well sandwiched between a CdMgTe buffer and spacer layers was covered with 1D gold grating to allow SPP excitation which l led to enhanced light emission directionality of up to 60%.

d. Magneto-optical effects in longitudinal magnetization of a MPC
As we have discussed so far, the implementation of nanostructured hybrid materials provides a remarkable increase of the T-MOKE. Interestingly, plasmonic structures can give origin to novel MO phenomena as well [25,40] In particular, the plasmonic crystal consisting of a 1D gold grating on top of a magnetic waveguide layer allows observing the MO intensity effect in longitudinal configuration, where a magnetic field is applied in the plane of the magnetic film and perpendicular to the slits in the gold grating [ Fig. 6]. Longitudinal magnetization of the structure modifies the field distribution of the optical modes and thus changes the mode excitation conditions. In the optical far-field, this manifests in the alteration of the optical transmittance or reflectance when the structure is magnetized.
Thus, this effect is described similarly to the T-MOKE by relative change of the transmittance or  a second-order effect in gyration (as it is an even function of the magnetic field). The modulation level can be increased even further by using materials with higher MO response, thus enabling the use of the LMPIE in modern telecommunication devices. Furthermore, the effect of mode switching is of great interest in the framework of active plasmonics and metamaterials [92,93]. Recently, the LMPIE in an MPC was used for magnetometry [94]. The experimental study revealed that such an approach allows to reach the nT sensitivity level, which was limited by the noise of the laser.
Moreover, the sensitivity can be improved up to fT/Hz 1/2 and micrometer spatial resolution can be reached.
e. Magneto-optical effects in dot-and antidot periodic arrays 22 Metallic grating-like structures can provide the basis for the excitation of both SPPs and surface lattice resonances (SLRs) [95][96][97][98]. By employing modern nanolithography techniques, 2D grating geometries can be designed at will, thus facilitating further tunability in the launching and control of the momentum of such plasmon modes.
For the case of SPPs, the grating can provide the additional momentum needed for triggering these evanescent surface waves according to the master equation: with �⃗ being the SPP wave vector, �⃗ // the component of the incident light wave vector parallel to the grating plane and ⃗ a reciprocal lattice vector corresponding to the grating geometry.
Furthermore, by performing the transformation into the reciprocal space of the grating geometry, it is possible to adapt the established Ewald geometrical representation, often utilized in scattering studies [99], to discuss the coupling of the incident light to plasmon resonances. However, it is important to note here that, due to the momentum gap between the dispersion relations of light propagating in vacuum and SPPs, the situation resembles that of an "inelastic"-like process, where the grating effectively adds the missing momentum to the light, thus enabling the launching of a SPP.
To illustrate the use of this geometric construction and its utility in designing an MPC, we refer to the well-studied case of hexagonal antidot arrays [50,[52][53][54], schematically shown in Fig. 7(a). resonances. Whenever such a condition is met by tuning the angle of incidence and wavelength of the light, abrupt changes in the reflectivity are observed, which are commonly referred to as Wood's anomalies [103]. If the metallic particle array is composed of ferromagnetic materials, the Wood's anomalies are accompanied by an enhancement of the MOKE, in a similar fashion to the case of SPPs [58,59], as shown in Fig. 8.
26 Fig. 8. (a) Schematic of the system studied in ref [59]. In the presence of magnetic material, the system response is governed not only by the induced dipole moment dy parallel to the driving field Ey and the lattice period px (direction of dipole radiation), but also by the spin-orbit-induced and magnetic-field tunable dipole moment dx and lattice period py. here that, compared to the SPP case, this case resembles an "elastic"-like process. Therefore, the momentum added by the grating is used to change the direction of the light wave vector, while the length of the latter is the same for the incident and scattered light. Introducing magnetic metallic particles with anisotropic shapes further allows distinct and anisotropic in-plane SLRs, determined by the particle polarizabilities and the spectral relation between localized resonances and Bragg modes [58]. Such MPCs could yield new metamaterials and optical devices, like magnetismcontrolled nonreciprocal optical isolators, notch-filters for the light polarization, or bio-sensors [58].
An interesting situation arises when the grating period Λ is such that /4 < Λ < /2 (or 2 0 < < 4 0 ), where in the non-linear optical regime the sample can be treated as a grating structure, while in the linear regime the sample behaves as a metasurface [102,104,105]. This situation is graphically depicted in Fig. 9, for the case of a sample composed of Ni nanodimers, having different 28 periodicities along the x and y directions, defining grating-and metasurface-like behavior in the nonlinear and linear response, respectively. The second harmonic generation exhibits a grating behavior, with associated Wood's anomalies. A decrease in specularly reflected intensity, of an order of magnitude larger than the linear effect (in the grating regime) is observed, accompanied by a sizeable magnetic contrast [102]. with higher sensing performance in terms of sensitivity and lower limit of detection with respect to traditional SPR sensors [112]. [ Fig. 10(d)]. In another work, they showed that high-quality factor surface modes in photonic crystal/iron-garnet film heterostructures can be used also for sensor applications [118]. to random distributions of pure Ni nanodisks and/or their random counterpart [128]. Finally, it is worth mentioning that, depending on the specific application, it could be more appropriate to exploit systems supporting either SPPs or LSPRs. Specific binding events can be detected using either SPPs [113] and LSPRs [126], but LSPRs or localized modes in general have been proved to be superior in molecular sensing of single molecules [129]. , graphene/noble metal nanowires [131], and in a ferromagnetic metal/dielectric nanoparticle system where non-metallic high-refractive index semiconductor ('all-dielectric') nanoantennas support optical Mie resonances [132]. Theoretical systems of pure all-dielectric and ferromagnetic nanoantennas with strongly enhanced MO has been proposed [133]. Furthermore, another intriguing and interesting advance in the field is also represented by the manipulation of structured light, namely light carrying orbital and/or spin angular momentum information [134][135][136]. In these works, either the spin or the orbital angular momenta were shown to be actively tuned by applying an external magnetic field. In particular, an interesting direction might be the merging of the strong chiral response and the angular momentum selectivity reported in Ref. [135] with the strong magnetic field modulation (beyond 100%) reported in [136], where on the contrary the overall chiral response was very weak due to the 2D geometry of the system.
Along this direction, it is worth mentioning that a detailed analysis which might help to reach this goal was reported by Feng et al. [137], where they analyzed the contributions from optical chirality, optical anisotropy and magnetic modulation of circular dichroism (CD) to the global optical response in Au/Co split-ring geometries. In this particlaur case they showed a system which have a strong chiral response with a MO-mediated magnetic modulation of CD of about 25% [ Fig. 11(d)]. anisotropy and a split ring/ring structure with an Au/Co multilayer, with perpendicular magnetic anisotropy. As it can be seen, magnetic saturation along surface normal requires a much smaller magnetic field for the multilayer case. Reprinted with permission from Ref. [137]. Copyright 2017 The Optical Society.
The ability to externally control optical states could be a key feature in such nanophotonic applications as nanoscale local polarization detection, chirality recognition and polarization spectroscopy, as well as magnetic field sensing [94] or tunable near-field emission of a desired optical state [138]. MO properties of CoPt nanostructures with antiparallel magnetic alignment combined with noble metal (Au and Ag) fine grains were recently investigated by Yamane et al. [139] revealing the enhancement of MO effects via LSPRs in the grains. Previously, the same group achieved an 37 impressive rotation of 20° in the visible spectral region by using CoPt/ZnO/Ag multilayered structure that works like a MO Fabry-Pérot cavity [140]. Almpanis et al. predicted also similar impressive values of the polarization rotation in the near-IR in magnetic garnet film sandwiched between two metallic layers, patterned with periodically spaced parallel grooves on their outer sides [141]. An intriguing case of magnetic field-assisted dynamic alignment resulting in enhancement/cancellation of plasmon optical response rather than polarization was demonstrated utilizing multisegmented Au/Ni/Au nanorods [24]. The recent comparison study on hybrid magnetoplasmonic gold-magnetite nanoparticles with core-shell, dumbbell-like and cross-linked geometries suggests the improvement of tunability, light scattering enhancement and local field enhancement at the interface between magnetic and plasmonic constituents [142].
It is worth mentioning that a magnetic manipulation of propagating plasmons in MPCs made of magnetic garnets [27] can also lead to strongly enhanced MO activity which gives rise to exotic optical properties such as MO transparency [143] and extraordinary transmission in sub-wavelength nanohole arrays [144]. In similar garnet materials, Subkhangulov Fig. 11(b)].
The SPPs and the waveguide modes of smooth semiconductors in the presence of an external magnetic field were considered in [148,149]. In these works, Kushwaha and Halevi have undertaken a theoretical study of magnetoplasma waves in a thin, semiconducting film, and they showed that the 38 magnetic field does not introduce any linear magnetization terms in the modes dispersion but it induces transverse electromagnetic field components and the appearance of modes with a negative group velocity, which are a magnetoplasma generalization of the Fuchs-Kliewer modes.
The polarization rotation MO effects were studied in different types of smooth multilayered metal/dielectric structures with either metallic or dielectric magnetized components [147,[150][151][152][153]176]. Probably, one of the first experimental demonstration of the influence of the plasmonic modes on the Faraday effect was published in [154]. Without making reference to surface plasma waves, author of [154] reported an optically enhanced Kerr rotation in thin iron films, magnetized in the longitudinal orientation, near what has become identified as the plasmon angle.
In some papers [152,153], plasmon-induced P-or L-MOKE enhancement was claimed but it was usually accompanied by decrease in the intensity of the signal. The SPPs-assisted pronounced increase of the Faraday effect was reported in the Bi-substituted iron garnet film covered with thin corrugated silver and gold layers [151]. It was assumed that the main contribution in the enhancement of the Faraday effect in such systems is made by the polarization rotation of the SPPs excited on the metal/dielectric interface.
Faraday and Kerr effects in periodic metal-dielectric structures were also considered recently [70,72,74,144,[155][156][157][158]. In particular, Diwekar et al. [155] experimentally investigated the Kerr effect upon reflection of visible light from a perforated cobalt film magnetized perpendicular to the surface. It was revealed that, in the vicinity of the region of anomalous transmission of light, the Kerr effect is reduced by one order of magnitude. There is a number of works dealing with the metaldielectric structures characterized by a considerable enhancement of the Faraday effect [156,157,158]. In those works, a magnetic medium was placed either inside holes in the metal [156], or the metal itself was ferromagnetic [157,158].
The plasmonic crystals of perforated gold on top of the smooth thick ferromagnetic layer were also investigated by measuring the cross-polarized transmission and polar Kerr rotation as a function of external magnetic field [144]. Although the effects of plasmons on these processes were observed, the enhancement of the MO effects via SPPs was not clearly demonstrated.
Though most of the periodic structures were fabricated by means of electron beam lithography and subsequent etching some other fabrication approaches were also used. Sapozhnikov et al. Ni SPPs modes is reported. However, the effect of disorder was shown to decrease the amount of that enhancement. One more magnetoplasmonic periodic structure was fabricated by depositing Co/Pt multilayers on arrays of polystyrene spheres [159].
It should be noted that the increase of the Faraday and Kerr rotation was reported recently for pure dielectric systems at the wavelengths of waveguide mode resonances [160,161], and in plasmonic structures containing graphene in THz frequency range [162].
In what follows we focus on the Faraday effect in MPC based on iron-garnet films. At the nonresonant frequencies, the Faraday rotation is close to that of a single magnetic film and is defined by Let the incident wave be TM-polarized. First, at the frequency ωTM, where either a TM mode or a SPP can be excited, the TM field is partly converted in a TE mode. But, since the excitation condition for the TE mode is not fulfilled at this frequency, the TE field component is re-emitted contributing to the far field. Moreover, the enhancement of the Faraday effect is due to the fact that the effective path of either the TM mode or the SPP is larger than in the nonresonant case. Second, at the frequency ωTE, the electromagnetic field re-emitted by the structure is partially converted in the TE mode. Also, at this frequency, the TE mode has a large effective path that causes the enhancement of the Faraday effect. Thus, the mechanism for the Faraday rotation enhancement depends on the type of the excited eigenmode.
If the magnetic film thickness is comparable to wavelength of the incoming light, the waveguide modes become essential [30]. As shown in Fig. 12(a), the Faraday rotation displays both negative and positive peaks. Furthermore, the positive Faraday rotation peak at λ = 883 nm corresponds to more  Reprinted with permission from Ref. [23] Copyright 2013 Springer-Nature.
In [29] it was emphasized that the Faraday rotation in the periodic systems is strongly related with the group velocity and gets its maximum values when vg is zero. In the case of an MPC the dependence of the Faraday angle on the group velocity can be written as follows where 〈 〉 is the matrix element of the MO parameter 〈 〉 = � calculated in the volume of the single lattice cell of the system. Eq. (7) demonstrates the strong correlation between the Faraday effect enhancement and the slow light effect. In the case of plasmonic crystals the mechanism is similar. At the normal incidence the eigenmodes are excited at the Γ point of the Brillouin zone that corresponds to the bandgap edges. The excited modes experience decrease of the group velocity and the effective time of the interaction of a mode with the magnetic media and the conversion to the opposite mode increases, and therefore, the Faraday effect is enhanced.
The experimental demonstration of the Faraday effect enhancement in MPCs similar to the one considered above was done in [23] [ Fig. 12(b)]. The spectra of the Faraday rotation exhibit resonant features. The spectra of the Faraday rotation exhibit resonant features. The sample with 495 nm lattice period reaches a maximum Faraday rotation of 0.80° at λ = 963 nm, which is 8.9 times larger than the −0.09° Faraday rotation of the bare iron-garnet film. As seen from Fig.12(c), the same sample shows also a 36% transmittance at the resonant wavelength.

IV. Nonlinear magnetophotonics
Strong localization and enhancement of electromagnetic fields represents one of the most prominent feature of plasmonics. Obviously, this enhancement can be exploited to boost up the efficiency of a plethora of nonlinear-optical processes, such as, to name a few, Raman scattering or second harmonic generation (SHG), constituting the core of nonlinear plasmonics [163].  [164 -170] will not be discussed here.
Instead, we will overview the possiblities of nonlinear magnetoplasmonics exemplified by magneto-induced SHG (mSHG) as the lowest-order nonlinear-optical process. Most of the formalism shown here is directly applicable to the difference and sum frequency generation (DFG and SFG, respectively) too, which is important, for instance, in THz spectroscopy [171]. The SHG radiation is produced by the nonlinear polarization at the double frequency 2 which originates from the 43 anharmonicity of the optical response of the system to the externally applied electromagnetic field ( ): where (2) is the second-order nonlinear susceptibility tensor. In magnetized media, the mSHG intensity variations can be characterized by the so-called magnetic contrast 2 : where 2 (± ) are the SHG intensities measured at the two opposite directions of magnetization .
Here, the mSHG intensity variations are governed by the interference of the (2 ) contributions originating in the non-magnetic (crystallographic) and magnetization-induced second-order susceptibility tensors, respectively: (2) = (2, ) ± (2, ) [172]. Oftentimes, Eq. (9) can be further simplified by considering the ratio of these two (complex) effective susceptibilities = � (2, ) (2, ) ⁄ � and their phase difference Δ : It is thus clear that the role of SPP resonances on the variations of magnetic contrast can be restricted to their modification of either or Δ . Indeed, despite boosting the total SHG output, the prominent SPP-induced enhancement of the local fields alone is unable to change 2 , as both crystallographic and magnetic SHG contributions are equally enhanced. Reported rather long ago [173], the first experimental evidence for this might have resulted in delaying the development of nonlinear magnetoplasmonics..
The plasmon-induced variations of can originate in the anisotropy of the (2) tensor. Indeed, the excitation of LSPRs in anisotropic nanostructures results in unequal resonant enhancement of various (2) components responsible for the crystallographic and m SHG, respectively. Absent in spherical nanoparticles, this effect has been demonstrated in anisotropic Ni nanopillars [174]. In the chosen combination of polarizations, crystallographic and magnetic SHG contributions are given by (2) and (2) components, respectively. LSPR modes in these structures facilitates strong enhancement of (2 ) along the main axis of the pillars, which is equivalent to the resonance in (2) (but not in (2) ) components. As such, the effective ratio is modified, giving rise to the LSPRinduced variations of 2 . Although this particular system allows for a very clear demonstration of the anisotropy mechanism, the mSHG-LSPR effects in more complicated geometries can be understood in a similar way [175].
At the same time, the experimentally observed propagating SPP-induced variations of the SHG magnetic contrast have been ascribed to the non-locality of the nonlinear-optical response [176 -180].
Yet, large number of interfaces and respective non-zero components of the (2) tensor did not allow for a clear picture of relevant non-linear magnetoplasmonic effects in the studied trilayer films.

Shortly after, Kirilyuk et al. demonstrated SPP-induced variations of the SFG magnetic contrast in
the near-field spectral region [181], pointing out the high promise of this technique for studying magnetic surface excitations.
Preliminary indications of the resonant variations of ∆ as the origin of the propagating (either on gratings or using prism coupling) SPP-induced mSHG modification were found by Newman et al. [182,183]. It took, however, about a decade until this has been clearly verified by direct SHG interferometry [184,185] and complex polarization analysis [186,187]. Interestingly, similar phase behavior has been reported upon excitation of a collective plasmonic mode in an array of nanodisks 45 [188]. Apparently, the SPP-induced variations of ∆ and are not always possible to disentangle, as, for example, both are present in many practical situations. For instance, mSHG yield from an isotropic magnetic interface in P-in, P-out combination of polarizations is governed by crystallographic (2) , (2) , (2) = (2) and magneto-induced (2) , (2) , (2) = (2) complex tensor components, so that the effective (2, ) , (2, ) are given by the interference effects. All of them contribute to the total SHG output, resulting in strong intertwining of the amplitude and phase variations originating in the SPP-induced electric field enhancement. Yet, the sign change of magnetic contrast clearly indicates the importance of the SPP-induced phase shift between the (2) components.
Interestingly, the most characteristic feature of nonlinear plasmonics is its sensitivity to the resonances at frequencies different to the fundamental one (2 for SHG) [189,190]. This can be exploited for novel mSHG effects where the SPP at the frequency 2 results in stronger mSHG contrasts than the fundamental SPP resonance [191,192]. Importantly, the system has to support SPP resonances at both ,2 , which is not the case for purely Au-based structures and typical 1.55 eV photon energy excitation. Large values of 2 (up to 33%) can be further optimized by adjusting thickness of the plasmonic Ag layer [191] and the excitation wavelength, thus shifting the SPP resonances at the fundamental and SHG frequencies closer to each other due to the SPP dispersion.
The latter opens an interesting perspective on the study of resonances overlapping at multiple frequencies to get stronger magnetic modulation of nonlinear-optical effects.
We emphasize the large magnitude of MO effects in nonlinear optics as compared to their linear counterparts. Indeed, if linear magnetoplasmonics typically deals with 0.5-1% reflectivity modulation, in SHG one can quite easily obtain an order of magnitude enhancement. These large effects are not bound to one particular class of objects but are ubiquitous in ferromagnetic metalbased structures, ferromagnetic-noble metal multilayers as well as hybrid noble metal-magnetic dielectric systems [193]. Yet, for sensing and switching applications not only magnetic modulation but also total efficiency of nonlinear-optical conversion is important. However, the strongest SHG modulation is often observed at the minima of the total SHG yield, originating in the destructive 46 interference of multiple contributions. Designing a novel system with overlapping large nonlinearoptical signals and their strong modulation upon magnetization reversal remains one of the open challenges of nonlinear magnetoplasmonics.

V. Spin-polarization in semiconductors using plasmons
A recent and intriguing development in the field combining plasmons and magnetism, is the extension of the activities towards material systems, incorporating semiconductors. It has been long suggested [194], that future electronic technologies will be relying not only on the control of the charge of the electrons, but also their spin degree of freedom. Already in the 90's, various routes were explored to induce magnetic order or spin-polarization in semiconductors, utilizing light [195][196][197]. In these first studies, no particular weight was placed on the effect plasmon resonances might have on the interaction of light with magnetism in semiconductors, as the majority of investigated systems were thin films [195,195]. Indications of interesting physical effects being present in semiconducting particle systems, emerged in works by J. A. Gupta et al. [197] and by R. Beaulac et al. [198]. In the latter, the photomagnetic effects in the form of photoexcited exchange fields leading to strong Zeeman splittings in the band structure, were reported to persist up to room temperature. Ref. [202]). Copyright 2018 Springer-Nature.
Existing approaches for achieving spin polarization in semiconductors, have mostly focused on researching dilute magnetic semiconductors or magnetic oxides [199,200]. The approach involving light to achieve spin-polarization or spontaneous magnetization in semiconductors, could potentially add an extra route, adding tunability into the scheme, via the control of size and shape of semiconductor nanoprticles. As an example, ZnO nanocrystals, have been shown to exhibit plasmon resonances in the near-infrared, supported by the observation of a strong magnetic circular dichroism (MCD) signal, which is temperature independent and linearly dependent on the applied magnetic field [201]. More specifically, the temperature independent MCD was attributed to a Pauli-like paramagnetic behavior of the nanocrystals, more common for alkali or noble metals. Recently, nonresonant coupling between cyclotron magnetoplasmonic modes and excitonic states was reported, leading to spin polarization and Zeeman splitting of the excitonic states under externally applied magnetic fields [202] [ Fig. 13]. Surprisingly, also for this case the effects persist up to room temperature. Beyond the generation of spin polarized carrier populations in semiconducting materials, recent works have also been reporting on schemes for optical detection of spin currents in hybrid devices. These so far have been based on molecular semiconductors, such as fullerenes [203].
These recent developments open up the way for a fresh look on the plasmon-exciton and plasmon-spin interaction in semiconductors. Combined, they offer the possibility for optical→spin and spin→optical conversion, necessary for a complete framework for an emerging new technology.
Expanding this approach also to metal/oxide systems, where plasmon resonances have also been shown to generate and transfer spin currents [204,205], opens up a completely new landscape. Surely, magnetophotonics and magnetoplasmonics will play a crucial role in upcoming developments, concerning material and device designs, holding promises for applicability in information processing and technology [206].

VI. Opto-magnetism: towards an ultrafast control of magnetism and spintronics
The ability to manipulate optical pulses on timescales well into the femtosecond regime has opened the door for attempts to control magnetism in an unprecedented, ultrafast way. In 1996, Beaurepaire and collaborators [207] made a seminal discovery when they observed that a short laser pulse (60 fs, =620 nm) could demagnetize a thin Ni film by 50% on a sub-picosecond timescale. This timescale was much shorter than what was expected from the spin-lattice relaxation at that time. The discovery of the ultrafast quenching of the magnetic order initiated much research and eventually led to the birth of a new scientific branch, ultrafast magnetism, poised on the intersection of magnetism and photonics. The microscopic mechanism of the ultrafast suppression of the spin magnetization gave rise to much debate and controversy in the scientific community (see [208,209] for reviews).
Obviously, a detailed microscopic understanding of how spin angular momentum can be controlled ultrafast can have far reaching consequences for the future development of ultrafast spintronics, i.e.
spintronic devices that can operate at THz frequencies or faster.
While the initial discovery of Beaurepaire et al. [207] demonstrated the ultrafast decay of spin moment, a second discovery, made by Stanciu et al. [210] showed that optical laser pulses could be used to deterministically reverse the spin moment. Investigating a particular ferri-magnetic alloy, Gd22Fe74.6Co3.4, they found that the magnetization direction of magnetic domains could be reverted just by applying continuous radiation or by short laser pulses. This discovery could have important technological implications, since, for example, switching the spin magnetization by an ultrashort pulse could lead to much faster writing of magnetic bits in magnetic recording media. The origin of the all-optical switching (AOS) was initially thought to be linked to the helicity of the laser pulse, i.e., the injected photonic spin moment, but subsequent investigations showed that solely fast laser heating was sufficient to trigger the magnetization reversal [211]. The origin of the helicityindependent switching in GdFeCo alloy was then analyzed to be related to the presence of a magnetization compensation point (antiparallel and nearly compensating moments on Fe, Co and Gd) and the quite different spin-dynamics timescales of the laser excited 3d spin moments on Fe, Co, and the rather slow dynamics of the localized 4f moment on Gd [212,211,213]. Investigating other ferromagnetic compounds and multilayer systems, Mangin and collaborators [214,215] could show that helicity-dependent all-optical switching (HD-AOS) was achievable for a broad range of ferromagnetic materials, even for the hard-magnetic recording material FePt that does not exhibit any compensation point. This discovery prompted that there must exist suitable, but as yet poorly known, ways to employ the photon spin angular momentum (SAM) to act on the material's spin moment to trigger spin reversal of the latter.
One of the possible ways for the photon to act on the spin moment could be through an opto-magnetic effect, the inverse Faraday effect (IFE). This non-linear MO effect, discovered in the sixties [216], describes the generation of an induced magnetic moment by a circularly polarized electromagnetic wave = ( × * ) where κ is a materials' dependent constant. The generated magnetization is proportional to the intensity | | 2 and induced along the photon's wavevector. Reversing the helicity from left-to rightcircular polarization reverses the direction of the induced magnetization. A first theoretical model for the IFE was proposed by Pitaevskii in the sixties [217]. This model was however based on the assumption that the medium is non-absorbing, a condition which is not met for the metallic materials and nanostructures that have come into the focus in recent years. As it is essential to be able to treat metallic systems, an improved theoretical model that accounts for both effects of photon absorption 50 and photon helicity has been formulated by Battiato et al. [218] and Berritta et al. [219] (see below).
To explain all-optical switching, dichroic heating was proposed as an alternative mechanism that could play a role [220]. This mechanism is based on the somewhat different absorption of left and right circularly polarized light in a ferromagnet which implies that the electrons are heated to a somewhat different temperature for left and right circularly polarized radiation, something which could assist switching when the electron temperatures are close to the Curie temperature.
Irrespective of what the deeper origin of the photon-spin interaction is, the spatial resolution of the area where the magnetization could be switched was limited by the light focal spot to domain sizes mostly larger than 10x10 µm 2 . It was consequently realized that, to reach ultrafast lightmagnetism operations at the nanoscale a further aspect needed to come in. Plasmonics offers precisely the ability to concentrate and enhance electromagnetic radiation much below the diffraction limit, which is essential for opto-magnetic applications in spintronics, where a major goal is deterministic control of nanometer sized magnetic bits. While plasmonics and magnetoplasmonics (see Ref. detection scheme for label-free refractometric sensing [121]. Thermal effects associated with LSPR in nanoparticles such as hot-electron generation and its dynamics were studied by Saavedra et al. [221]. A first attempt to utilize plasmonics to achieve all-optical spin switching on the nanometer scale was made in 2015 by Liu et al. [222] who deposited 200 nm Au nano-antennas on a ferrimagnetic TbFeCo film. In this way they could use a high local heating and concentrate the area that exhibits spin switching to about 50 nm diameter. However, it was observed that some areas switched and others didn't. This could be related to a composition inhomogeneity of the TbFeCo film on a sub-100 nm scale. Earlier investigations of AOS in GdFeCo films showed that the switching depends sensitively on the Gd/Fe concentration ratio [223]. An X-ray diffraction study by Graves et al. [224] on GdFeCo showed that a composition inhomogeneity could lead to local variations in the switching 51 behavior. A different investigation was made by Kataja et al. [225], who could observe both plasmoninduced demagnetization and magnetic switching in a Ni nanoparticle array, excited by a femtosecond laser pulse, which they explained by the plasmonic local heating of the nanoparticles above the Curie temperature. A next step in this direction could be the fabrication of GdFeCo nanoparticle arrays. To explain the IFE in bulk materials several models have been proposed recently. can generate a substantial local magnetization via the IFE. It is clear that not only an enhanced intensity is needed, but that the local field must be circularly polarized, too. This implies that a special design of the nanostructures is needed (see, e.g. [231,232]), to ensure that a high magnetic induction pulse is generated. Such modeling can be carried out with Maxwell solvers such as COMSOL or Lumerical. Tsiatmas et al. [231] predicted in this way that Ni-Au nanorings excited with a fluence of ~0.1 J/cm 2 at plasmon resonance could sustain thermoelectric currents that cause a magnetic induction pulse of ~0.2 T. A different route has been followed by Hurst et al. [233], who used a quantum hydrodynamic model to study the magnetic moment induced by circularly polarized radiation in individual Au nanoparticles [ Fig. 14(a)]. Circularly polarized radiation can excite electric dipole-like LSPRs in two orthonormal directions on the nanoparticle with a phase difference between them. An orbital magnetization density, ( ) ∝ ( ) × ( ) , appears in the nanoparticle as a result of the free electron motion, leading to a non-vanishing electron current density J on the surface of the nanoparticle, see Fig. 14(b). Consequently, the free electron cloud will rotate around the nanoparticle.
The magnetic induction B due to the circulating current, computed with the Biot-Savart law in the center of the nanoparticle and shown in Fig. 14(c), is predicted to reach 0.3 T for laser intensities of 10 3 GW/cm 2 . The magnetic moment M induced by this plasmonic IFE can reach ~0.6 µB per Au atom, depending on the size of the nanoparticle and laser intensity, see Fig. 14(d). Even though the assumed laser intensities are very high, the induced moments and generated magnetic fields predicted for the plasmonic IFE [233] of nanoparticles are notably much larger than those computed ab initio for the IFE in bulk materials [219]. This strongly increased magnetic moment of the Au nanoparticle nicely illustrates the huge impact that plasmonics could potentially have in the area of optomagnetism. Specifically, the collective motion of the free electron cloud in the surface plasmon resonance can lead to a much larger total induced magnetization than the excited motions of individual bound electrons. All of the above considerations built upon exploiting the spin angular momentum functionality of the electromagnetic field. A number of years ago it was realized that an optical beam can also carry a 54 well-defined optical angular momentum (OAM) [234][235][236][237]. For years the OAM has been considered as an exotic, yet benign feature, but more recently its potential usefulness is becoming realized [238,239]. Beams with high OAM values can nowadays be made in the lab (see e.g. [240][241][242]).
Combining OAM with plasmonics, it has been demonstrated that subfemtosecond dynamics of OAM can be realized in nanoplasmonic vortices [239]. Hence, plasmonic vortices carrying OAM can be confined to deep subwavelength spatial dimension and could offer an excellent time resolution.
The OAM can, therefore, be expected to soon enter the developing area of magnetophotonics, where the OAM could offer a new functionality to control the nanoscale magnetism [135]. There are however many open questions that will have to be solved before this ultimately can be achieved. On short lengthscales comparable to the wavelength of light, the spin angular momentum (SAM) and OAM of a light beam become strongly coupled [243] and it will be difficult to separate their respective contributions. Also, although there is an emerging understanding of the IFE coupling of the SAM of a beam to the electron spin, a similar understanding of the interaction of OAM with spin or orbital magnetism has still to be established. Recently, a first observation of interaction of magnetism and an OAM vortex beam in the THz regime was reported [244]. It can already be perceived that taking both spin and orbital degrees of freedom of photonic beams into account will become paramount for the future development of magnetophotonics.

Conclusions and future perspectives
Research on linear and non-linear magnetoplasmonic nanoantennas and nanoscale magnetophotonics has up to now clearly demonstrated the feasibility of active magnetic manipulation of light at the nanoscale. An impact of such active control on applications has been so far hindered by the weak coupling between magnetism and electromagnetic radiation and the high dissipative losses in the used materials. Several strategies, the most promising of which are summarized in this Perspective, have been identified to overcome these limitations. Thereby, this rapidly developing 55 field holds great promise to provide a smart toolbox for actively tunable optical materials and devices in a variety of future disruptive technologies, such as flat nanophotonics, ultrasensitive detection, alloptical and quantum information technologies and spintronics. Nanoscale magnetophotonics could play a prominent role in the design of next-generation technology for computer memory, as the hard disk drive industry is facing a major challenge in continuing to provide increased areal density, driven by the ever-increasing data storage requirements. The heat-assisted magnetic recording approach provides a combination of high coercive field magnetic materials with local heating by a plasmon nanoantenna [245,246]. This approach currently allows up to record-breaking 1 Tb inch 2 storage densities. Another practical application of nanoscale magnetophotonics is the use of nanoparticles in medicine, diagnostic techniques and drug delivery due to their potential for direct magnetic manipulation [120,247]. In this regard, solutions of chemically synthetized magnetoplasmonic nanoparticles [248] is fundamental, also in view of potential applications which go beyond nanomedicine, such as the manipulation of the thermal properties of such nanoparticles and/or their environment [249]. Magnetoplasmonic Au-Fe alloy nanoparticles were proved to provide high sensitivity and high resolution in magnetic resonance imaging (MRI), X-ray tomography (CT) and surface enhanced Raman scattering (SERS) [250]. It has been recently demonstrated that magnetochromic hydrogels can be synthetized and be used as magnetic field-modulated color displays [251]. Eventually, multiband MO response would represent another advance in the field [90]. Furthermore, magnetoplasmonic effects can be used for metrology and recently many works pointing in this direction has appeared [89,252,253]. We also foresee that the control of the many degrees of freedom of light (specifically, the optical orbital angular momentum) is within the reach with nanoscale magnetophotonics.
The combination of nanophotonics, magnetoplasmonics and spintronics opens new horizons for practical implementation of magnetic-field controllable nanoscale devices for ultrafast information processing and storage. Newly emerged designs and concepts may help to overcome some of the limitations including plasmon dissipation losses, low efficiency of plasmon excitation in magnetic 56 materials and high magnetic fields required for sufficient modulation. Recent demonstration of tunable multimode lasing modes demonstrated with magnetoplasmonic nanoparticles in combination with organic gain material paves the way for loss-compensated magnetoplasmonic devices [254].
Ultrafast optical excitation also provides means for more efficient excitation of plasmons via the subpicosecond thermal diffusion of hot electrons due to the formation of nanometer-sized hotspots [255].
Ultrafast control of optical response with spintronics and optical generation of spin waves are very recent advances in the field of nanoscale magnetophotonics, as well. Optical excitation of spin waves [256,257] and optical control of magnetization dynamics [258,259] in GdFeCo and TbFeCo films and magnetic dielectrics by circularly polarized femtosecond laser pulses opens the route for spin wave based devices. Spintronic platforms typically operating with very weak magnetic fields may become next candidates for high-speed photonic devices in mid-and far-IR via the change in resistivity due to the giant magnetoresistance [260]. Local manipulation of the magnetic moments at submicron scale in MO nanodevice with electrically-driven domain wall was recently experimentally implemented [261]. Overall, creating practical magnetophotonics devices will require all-optical and plasmon-assisted control of the magnetic spin and magnetic control of light-matter interactions with low magnetic fields on the nanoscale.