Magneto-optical Substrate Research Articles

Polarization-multiplexed dual-band terahertz beam switching based on magneto-dielectric metasurfaces

Electrically tunable metasurfaces with pixel-level phase control are good candidates for dynamic manipulation of the beam direction in the terahertz band, which is a critical and highly desired technique for next generation communications. Yet the complex biasing, limited aperture and cross talk hinders its progress. In this study, we propose a magnetic-field-globally controlled metasurface for beam switching. The metasurface is composed of a bi-layer silicon metagrating on a magneto-optical substrate InSb. The beam direction is governed by two physical processes: the magnetically-controlled polarization and the polarization-controlled diffraction. By utilizing the circular dichroism and Faraday rotation effects in InSb, the metasurface can operate at two frequency bands. The beam is switched among four directions by properly choosing the magnetic field biasing and the frequency. This study offers a promising solution for beam management using magnetic field biasing and in a global manner.

Lateral optical force on linearly polarized dipoles near a magneto-optical surface based on polarization conversion

Novel lateral optical forces acting on dipoles near surfaces have been investigated in the past few years: circularly polarized dipoles experience lateral optical forces when in proximity to a surface due to the recoil of directionally excited modes. Recent work shows that even linearly polarized dipoles may experience lateral forces when nonreciprocal substrates are used, due to the asymmetric propagation of modes in the surface. We theoretically show that a linearly polarized particle emitting in close proximity to a magneto-optical substrate may experience a lateral optical force even if the external magnetic field is normal to the surface plane. The polarization conversion of the magneto-optic material introduces a gradient in the quasistatic fields reflected from the surface, resulting in a lateral optical force, and also gives surface plasmons a hybrid polarization character, which alters the surface plasmon directional excitation from the dipole, resulting in an optical recoil force. We envisage potential applications in nanomechanical devices, since similar magnetoplasmonic architectures have already been developed experimentally.

Radiative thermal diode driven by nonreciprocal surface waves

We demonstrate the possibility to rectify the nanoscale radiative heat flux between two nanoparticles by coupling them with the nonreciprocal surface modes of a magneto-optical substrate in a Voigt configuration. When the nonreciprocal medium supports a surface wave in the spectral window where heat exchanges take place, the rectification coefficient can reach large values opening so the way to the design of true thermal diodes.

Cascade replication of dissipative solitons.

We report a new effect of a cascade replication of dissipative solitons from a single one. It is discussed in the framework of a common model based on the one-dimensional cubic-quintic complex Ginzburg-Landau equation in which an additional linear term is introduced to account the perturbation from a particular potential of externally applied force. The effect is demonstrated on the light beams propagating through a planar waveguide. The waveguide consists of a nonlinear layer able to guide dissipative solitons and a magneto-optic substrate. In the waveguide an externally applied force is considered to be an inhomogeneous magnetic field which is induced by modulated electric currents flowing along a set of conducting wires adjusted on the top of the waveguide.

Graphene plasmons isolator based on non-reciprocal coupling.

In this paper we propose a graphene plasmons isolator based on non-reciprocal coupling within double graphene layer waveguide structure on a magneto-optical substrate. The difference in modal indices of graphene plasmons in opposite directions enables non-reciprocal coupling, which is theoretically investigated via coupled mode theory (CMT) and shows good agreement with numerical finite elements methods (FEM). The non-reciprocal coupling endows such system functionalities as magnetically controlled "plasmons circulator" or "plasmons isolator". For the latter case, systematical investigations of the insertion losses and isolation ratios with respect to the structural parameters, material properties, environmental parameters, fabrication errors as well as dielectric damping are presented. Theoretical investigation has shown an isolation ratio as 42 dB. The proposed plasmons circulator and isolator may serve as potential building blocks in graphene plasmons circuits.

Directional excitation of graphene surface plasmons

We propose a scheme to directionally couple light into graphene plasmons by placing a graphene sheet on a magneto-optical substrate. When a magnetic field is applied parallel to the surface, the graphene plasmon dispersion relation becomes asymmetric in the forward and backward directions. It is possible to achieve unidirectional excitation of graphene plasmons with normally incident illumination by applying a grating to the substrate. The directionality can be actively controlled by electrically gating the graphene, or by varying the magnetic bias. This scheme may have applications in graphene-based opto-electronics and sensing.

Effect of Gyrotropic Substrates on the Surface Plasmon Polaritons Guided by Metal Films of Finite Width

The propagation characteristics of surface plasmon polariton modes in metal-strip plasmonic waveguides with gyrotropic (magneto-optic) substrate material has been numerically studied. The method of line formalism has been adopted for computing the dispersion of fundamental symmetric and antisymmetric modes supported by the structure. The magneto-optic substrate has been chosen such that the resulting propagating modes exhibit nonreciprocal dispersion, enabling their application in many optical devices. Fundamental pair modes of surface plasmon polaritons supported by metal strips, which have degeneracy properties with isotropic dielectric substrates, have been found to be nondegenerate in our case. These modes show different trends in the dispersion with variation of metal film thickness, compared to that in an asymmetric strip waveguide with isotropic substrate. A nonreciprocal phase shift of 0.5-14 rad/mm is achievable for antisymmetric modes that are localized near the gyrotropic interface.

Metallic nonlinear magnetooptical nonreciprocal isolator

In advanced optical fiber communication systems where reflection light might cause degradation of the signal quality, optical waveguide isolators play an important role. The effect of a metal layer with negative permittivity on the behavior of nonlinear-magnetooptic isolator is studied. The isolator consists of metal film, nonlinear cladding, and magnetooptic substrate. It is found that difference between forward and backward propagation for TM 0 mode increases with increasing the absolute value of the tuning parameter which is the permittivity of the metal film, ɛ f . It is also found that the maximum cut-off thickness of the isolator occurs in self-defocusing case around η = 0.65 and at the highest assumed value of ɛ f = −8. These results are helpful in fabricating an isolator with high performance.

Kerr-resonance-condition-coupled enhancement in magneto-optic media

We derive an expression for cyclotron frequency ωc, which sets Re[ε+ε−]=1 in a magneto-optic (MO) substrate, at any incident photon energy. Thereby, at any desired part of the optical spectrum, large Kerr effects can be obtained, which are generally known to occur either at the free-charge-carrier-plasma-resonance frequency ωp, or at frequencies where active electronic transitions take place. Under these conditions, ωp is seen to play a very different role; it is seen that for any ω, the magnitude of the Kerr resonance in a single MO (InSb here) substrate increases with decreasing ωp. With the objective of achieving further Kerr enhancement, the effect of coating a thin film of this ωc-coupled InSb layer on a Ag substrate is numerically studied. Further Kerr enhancement, at the plasma-resonance frequency of Ag, is seen, which is dependent on the thickness of the MO layer and its ωp. In this configuration, giant resonances appear in the effective-dielectric-tensor spectra. The spectral locations of these resonances are dependent on the thickness of the MO layer. We interpret these resonance structures to be the effective cyclotron resonance. Our results suggest that there exists a strong correlation between the effective-cyclotron frequency and the plasma-resonance frequency of the noble.