Magneto-optics refers to changes in the properties of light when it is transmitted or reflected in the presence of a magnetic field (externally applied or from a magnetized material medium). These phenomena were first discovered by Michael Faraday in 1845
[1]. He noticed that a linearly polarized light beam undergoes a polarization rotation when it propagates parallel to an externally applied magnetic field. Just over 30 years later, the Reverend John Kerr observed the corresponding effect when he studied the reflection of linearly polarized light from the surface of a magnet
[2][3]. These magneto-optics (MO) effects, conventionally called Faraday and Kerr MO effects (in transmission and reflection, respectively), not only helped establish the theory of electromagnetism in the late 19th century, but also laid the foundation for several technological achievements. The Faraday effect, for example, has the unique property of breaking time-reversal symmetry and Lorentz’s reciprocity theorem
[4], which has been utilized for the development of non-reciprocal optical devices and laser systems
[5][6]. The Kerr effect, on the other hand, allows a way to measure or visualize the magnetic state of material media, and has therefore been used for spectroscopy
[7][8][9][10] and data-storage applications
[11][12][13]. More recently, the advent of nanotechnology and modern nanofabrication tools motivated the exploitation of MO phenomena at micro- and nano-scale levels for a new era of devices, with their optical properties actively manipulated by applied magnetic fields. However, the latter is challenged by weak MO effects at visible and infrared wavelengths
[14].
2. A Brief Review on Magnetoplasmonics
Due to the importance of nanotechnology and magnetophotonics, and also to provide the reader with a historical context, researchers prefer to start with a brief history of plasmonics. The first experimental evidence of surface plasmon resonance (SPR) phenomenon was observed by Wood in 1902
[15] while measuring the reflectance spectra of a metallic grating. In particular, Wood observed a sharp discontinuous change in the reflectance (around a specific frequency), which was called the Wood’s anomaly. Then, in 1907, Rayleight discovered that the frequencies at which anomalies occur are intimately related to the incident angle and wavelength (as well as to the angle of emergence of the diffracted waves), providing a qualitative explanation for the Wood’s anomaly
[16][17]. However, it was not until 1941 that Fano demonstrated that the physical principle behind the Wood’s anomaly stems from the excitation of surface waves at the metallic/dielectric interface
[18]. Indeed, Fano indicated the similarity between the Wood’s anomaly in the grating and the surface waves in the attenuated total reflection (ATR) effect. Furthermore, in the early 1950s, Pines and Bohm
[19][20][21][22] showed in a series of papers that conduction electrons in a metal (considered as a free electron gas) can describe collective oscillations, whose quantizations were called plasmons. Based on these previous achievements, Ritchie derived the dispersion relation of SPRs in 1957
[23], which was experimentally demonstrated two years later by Powell and Swan
[24]. Merging the ATR analogy (described by Fano) with the SPR dispersion relation, Kretschmann
[25] and Otto
[26] proposed the use of prism couplers (high-refractive-index incident media) for the excitation of SPRs in flat planar metallic surfaces. On the other hand, the time harmonic oscillation of the electric field component of light forces the density of free charges in metallic nanoparticles to oscillate between upper and lower boundaries. This oscillation exhibits a maximum amplitude at the resonant frequency, analogous to a forced harmonic oscillator
[27]. Although in 1904 Maxwell–Garnett used an effective dielectric permittivity theory to explain the colors of glasses containing small metallic nanoparticles
[28], the full electromagnetic theory of light scattering and absorption by colloidal metallic nanoparticles was only developed in 1908 by Mie
[29]. Plasmonic resonances in metallic nanoparticles are radiative localized (non-propagating) SPRs (LSPRs), contrary to SPRs in gratings or flat planar metallic surfaces, which are surface-guided modes achieved under phase-matching conditions.
Since the first experimental demonstrations of SPR applications in gas sensing and biosensing, made four decades ago by Nylander and Liedberg et al.
[30][31], there has been intense research activity to enable this technology for the detection of small analytes or in very dilute solutions
[32]. However, due to very small changes in the refractive index (RI) of the analyte medium, the resolution of SPR (bio)sensing devices is largely limited by the overlap between nearby resonances. In the search for alternatives to surpass this last drawback, magnetoplasmonic biosensing, i.e., merging plasmonics with magnetism and spectroscopical techniques, has emerged as a promising alternative. The first experimental demonstration of magnetoplasmonic sensing was made by Guo et al.
[33], in 1999, who used MO modulation to monitor the phase shift resulting from the minute change of the angle of incidence, demonstrating improved sensing resolutions compared to the conventional SPR approach. Then, in 2006, Sepúlveda et al.
[34] demonstrated resolution improvements of up to three orders of magnitude in biosensing applications when using the sharp curves from plasmonically enhanced transverse MO Kerr effect (TMOKE) instead of broad reflectance SPR lines. The MOSPR sensor is composed by a Au (5 nm)/Co (3 nm)/Au (21 nm) tri-layer film on glass substrate and covered by air (the superstrate). A prism coupler of glass is also used for the ATR mechanism. The reflectance (
R) associated with the demagnetized system (
M=0), represented with solid blue circles (see the left axis), is comparatively plotted with the corresponding
TMOKE=ΔRR=R(+M)−R(−M)R(+M)+R(−M) values (represented by solid orange circles in relation to the right axis) around a minima of
R. From this last result it can be clearly seen that TMOKE curves are sharper than SPR ones, which is used to improve the quality factor and the sensing performance of the structure, as previously mentioned. The superstrate is then successively injected with concentrations of 1%, 2%, 3%, and 4% of ethanol gas diluted with nitrogen, with the corresponding time transient measurements, respectively. As noticed, the signal-to-noise ratio is better for
ΔR/R than that for reflectivity
R, showing improved sensitivity by using the MOSPR sensing. It is worth mentioning that further resolution improvements can be achieved through combined advanced modeling and nanopatterning methods
[35][36][37][38][39]. Nevertheless, the MOSPR sensing approach may be inappropriate for detection of very high analyte concentrations. This last behavior is explained through the close relationship of the optimized TMOKE amplitudes with the SPR phase-matching condition, considering that the latter is lost when the changes in the refractive index are too high.
Achievements in SPR magnetoplasmonics are not limited to (bio)sensing. For example, the plasmonically enhanced magneto-optic effect is also being actively exploited for new integrated active nanophotonic devices. In particular, high-speed non-reciprocal magnetoplasmonic waveguides have been actively developed during the last years
[40][41][42]. These latter devices enable unidirectional light propagation by actively manipulating the insertion losses for forward and backward modes in a magnetoplasmonic waveguide. This active tuning is achieved by using an external magnetic field to manipulate the magnetization state of a building ferromagnetic material in the waveguide, which in turn alters the insertion loss level in the waveguide. An important application of non-reciprocal waveguides is the stable operation of semiconductor lasers, preventing them from unwanted backward reflection in the optical transmission path. Instead of free space optical isolators consisting of Faraday rotators (magnetic garnet) and two polarizers, semiconductor optical isolators based on the non-reciprocal loss, have been studied, where ferromagnetic metal thin films are deposited at a part of the waveguides. Transversely magnetized ferromagnetic metal provides different propagation loss depending on the magnetization direction or propagation direction, owing to the time-inversion asymmetry. Significantly, on-chip optical isolators can be realized through monolithic integration of semiconductor optical isolators with semiconductor lasers, simplifying the system and avoiding the work of physical alignment between the lasers and isolators
[43]. Researchers show a cross-section scanning electron microscopy (SEM) micrograph of a semiconductor optical isolator, comprising a ferromagnetic Co thin film on an InP substrate. The vertically magnetized Co thin film provides optical isolation for transverse electric (TE) mode light by using the TMOKE. The experimental setup for optical isolation measurements is shown in
Figure 1b, and results showing optical isolation of up to 45 dB/mm are presented in
Figure 1c.
Figure 1. (a) A cross-sectional SEM image of the TE mode semiconductor optical isolator. (b) A schematic diagram of the experimental setup for measuring the characteristics of semiconductor optical isolators. (c) Forward and backward propagation loss as a function of the length of the semiconductor optical isolators.
The magnetic control of chiroptical activity is a new and fluorishing application of magnetoplasmonics that is increasingly gaining attention
[44][45][46][47][48][49]. The term chirality, used for the first time in 1894 by Lord Kelvin
[50], refers to objects that are non-superimposable on their mirror images. Our hands are, perhaps, the most universal example. Because of this analogy, both enantiomorphs (from the Greek’s composition enantios = opposite + morphe = form) are conventionally classified as L- (left) and R-enantiomorphs (right) or, when referring to molecules, L- and R-enantiomers. In contrast to achiral (without chirality) environments, where both enantiomers of a chiral molecule (sharing the same stoichiometric molecular formula) exhibit the same physical and chemical properties, molecular chirality plays a key role in the biochemical and biological activity of molecules. In fact, because the elementary building blocks of living organisms (e.g., amino acids and sugars) are chiral, biochemical reactions are inherently chiral
[51][52]. The latter is crucially important in pharmacology, where therapeutic effects are associated with a single enantiomer and the other is ineffective or induces serious side effects
[53][54][55], as was demonstrated by the thalidomine disaster in the late 1950s
[56][57][58].
Although physical and chemical techniques (e.g., circular dichroism and chromatography) are currently used for recognition and separation of enantiomers in the pharmaceutical industry, their performance is still limited to high concentrations or large analyte sizes
[59][60]. On the other hand, the Faraday rotation effect is itself chiral. In fact, the physics behind magnetic circular dichroism (MCD), i.e., the differential absorption of left and right circularly polarized light, stems from the Faraday effect
[61]. Therefore, through the unique sensing capabilities of plasmonics (using chiral or achiral geometrical designs
[62]) with an active CD modulation mechanism
[63], the combination of the magnetoplasmonic Faraday effect with circular dichroism (CD) spectroscopy (commonly known as magneto-chiroptical effect) may allow improved detection limits with higher resolutions
[64][65].