RU2620026C1 - Method for modulation of electromagnetic radiation intensity using magnetoplasmonic crystal - Google Patents

Abstract

FIELD: physics, instrumentation.

SUBSTANCE: invention refers to optics, namely to a light intensity modulation method for optical and near infrared ranges. The invention can be used in the applied magneto-optics, optoelectronics, photonics, as well as sensor technology. The method for intensity modulation of transmitted or reflected electromagnetic radiation using a magnetoplasmonic crystal includes creation of a bidimensional magnetoplasmonic crystal consisting of a transparent dielectric substrate, a two-dimensional array of noble metal particles of subwave size immersed into the thin magnetic dielectric layer with a thickness not smaller than the particle size; lighting of magnetoplasmonic crystal by TM-polarized electromagnetic radiation upon application of a magnetic field in the geometry of the equatorial magneto-optical Kerr effect.

EFFECT: invention provides intensity modulation of transmitted and reflected optical radiation via structures with dimensions smaller than the wavelength of the applied radiation.

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Description

FIELD OF THE INVENTION

The invention relates to optics, and in particular to methods of modulating the light intensity of the optical and near infrared ranges. The invention can be used in applied magneto-optics, in optoelectronics, photonics, as well as in sensor technology.

State of the art

Magnetoplasmonics as a branch of magnetooptics was developed due to the possibility of enhancing magnetooptical effects in periodically nanostructured magnetoplasmon materials due to resonant excitation of plasmons (Belotelov, VI, Akimov, IA, Pohl, M., Kotov, VA, Kasture, S., Vengurlekar, AS, ... & Bayer, M. Enhanced magneto-optical effects in magnetoplasmonic crystals. Nature Nanotechnology, 6 (6), 370-376, (2011)).

A known method and device for modulating the phase of the reflected light signal based on the Kerr effect (US 4246549, IPC: H01S 3/10). The device implements controlled phase control of the reflected signal when reflected from a layer of a ferromagnetic or ferrimagnetic garnet placed on a surface with a reflection coefficient preferably above 95%, which can be implemented, for example, as a set of dielectric layers. The device uses the improvement associated with the use of a layer of ferromagnetic or ferrimagnetic garnet, in contrast to the known similar devices using the iron layer for the Kerr effect, which has a high absorption and a tendency to oxidation.

However, this device has a large thickness, including the required thickness of the magnetic grenade, as well as a reflective layer.

There is also known a method and device (US 5477376, IPC: G02F 1/09) for modulating the intensity of transmitted radiation based on the magneto-optical Faraday effect by changing the magnetization of a magnetic garnet layer using an electromagnet or a mechanically biased permanent magnet. The device includes a structure with alternating magnetization domains in a direction perpendicular to the propagation of modulated radiation. In the absence of a magnetic field, domains with the opposite direction of magnetization form a diffraction grating, diverting part of the energy of the main beam to diffracted beams. Thus, modulation or attenuation of the initial beam is achieved.

This method does not have pronounced selectivity for wavelengths, and the device is poorly applicable as a reflective modulating magneto-optical element of integrated photonics, since it has large dimensions, in particular, thickness.

Also known is a method and apparatus for controlling an optical signal based on pumping optical energy into a surface plasmon polariton (US 8879138, IPC: G02F 1/01). The device uses an active medium, the optical properties of which can vary depending on an external electric or magnetic field. To control using a magnetic field, a dielectric with impurities of magnetic metals (Ni, Co) or ferromagnetic garnets is used.

This method is used to modulate the radiation in the transmission geometry and the device is not suitable for use as a modulating reflective element. In addition, the device has large geometric dimensions, which does not allow its use as an element of integrated optics.

There is also a known method of rotation of the plane of polarization of light and a method for manufacturing a device for this method is described (US 7965436, IPC: G02F 1/09). The device consists of a non-magnetic dielectric waveguide and a magnetic shell, the waveguide thickness is about 50-400 nm. The device carries out a circular rotation of polarization by 45 ° when radiation passes through 2 μm of the medium.

This method cannot be used in reflection geometry, and, in addition, the device is large.

Closest to the claimed method is a method of enhancing the magneto-optical Kerr effect through the use of photonic crystalline periodic structures on the surface and transfer of incident radiation energy to surface plasmons (US 9110316, IPC: G02F 1/09). In turn, the equatorial magneto-optical Kerr effect allows you to modulate the intensity of the reflected signal. At least a five-fold enhancement of the equatorial Kerr effect in comparison with unstructured material is claimed. In the known solution, in contrast to the proposed method, a photonic crystal structure with a thickness of about 300 nm is used for the magneto-optical Kerr effect. Nickel is used as a ferromagnetic metal, and the method itself allows limited use of, for example, iron for magneto-optical modulation. Also, the two-dimensional ordered nanostructure considered in this method is not suitable for use as a modulating transmitting element.

Disclosure of invention

The objective of the invention is to provide a method for magneto-optical modulation of the intensity of optical radiation using a thin-film plasmonic nanostructure with a thickness of the active part of less than 200 nm with the possibility of the device working in geometry both for transmission and reflection.

The technical result is a magnetic field-controlled modulation of the intensity of optical radiation when reflected from or transmitted through a structure whose optical thickness is less than or of the order of the wavelength of the modulated radiation, which allows the method to be used in compact devices.

The problem is solved in that the method of modulating the intensity of electromagnetic radiation is carried out using a magnetoplasmon crystal, which includes a transparent dielectric substrate with a two-dimensional array of noble metal particles with subwave sizes located on it in the layer of magnetic garnet, immersed in a dielectric magnetic layer with a thickness not less than the particle size, in this case, the magnetoplasmon crystal is illuminated with TM-polarized radiation of the optical or near-IR ranges when applied magnetic dielectric layer of alternating magnetic field in the equatorial geometry magnetooptical Kerr effect provides the possibility of modulation of the radiation intensity as a transmissive or reflective.

A two-dimensional array of particles is a strictly periodic structure with a period of particle arrangement of at least 200 nm. To obtain maximum light modulation, the magnitude of the applied external magnetic field should be in the saturation region of the magnetization of the dielectric magnetic layer. As particles, gold or silver particles with sizes from 50 nm to 200 nm can be used.

The optimal result is realized in geometry, where the external magnetic field is directed perpendicular to the plane of incidence of the radiation on the structure of the magnetoplasmonic crystal and parallel to the plane of the magnetoplasmonic crystal, the magnetization vector of the magnetic layer lies in the plane of the magnetoplasmonic crystal and is perpendicular to the plane of incidence of light, the angle of incidence of light is 0 at least 10 degrees , the azimuthal angle lies in the range from 0 to 360 degrees.

The advantage of magnetoplasmon structures is the possibility of creating devices with dimensions not exceeding the radiation wavelength. Therefore, the use of magnetoplasmonic materials as devices in which magneto-optical effects are possible and which, moreover, are compact, makes it possible to widely use such materials in various fields of physics, optics, and electronics. These advantages of magnetoplasmonic materials made it possible to develop a method for modulating the intensity of transmitted and reflected optical radiation using a structure with dimensions smaller than the wavelength of the radiation used (for example, see Table 1).

Brief Description of the Drawings

The invention is illustrated by drawings, where in FIG. Figure 1 shows a schematic representation of a magnetoplasmon crystal: a two-dimensional array of gold nanoparticles (2) located on a fused silica substrate (3) and immersed in a layer of magnetic garnet (1). In FIG. 2 shows an image of a magnetoplasmon crystal obtained using an atomic force microscope. FIG. Figure 3 shows a schematic optical diagram of a method for modulating the intensity of electromagnetic radiation, where (4) is a system for generating TM polarized optical radiation, (5) is a magnetoplasmon crystal, and (6) is a system for detecting optical radiation. In FIG. 4 is a graph showing the transmission spectrum of a magnetoplasmon crystal (curve 1) for the angle of incidence of radiation on the crystal equal to 20 degrees, as well as the spectrum of the transverse magneto-optical Kerr effect (curve 2) for the angle of incidence of radiation equal to 20 degrees.

The implementation of the invention

To implement a method of modulating the intensity of electromagnetic radiation using magneto-optical and plasmon effects, a two-dimensional magnetoplasmon crystal is required, consisting of a dielectric substrate, a two-dimensional array (with a period of at least 200 nm) of noble metal particles (for example, gold, silver) with subwavelengths from 50 nm up to 200 nm immersed in a dielectric magnetic layer with a thickness of at least a particle diameter; the magnetoplasmon crystal is illuminated with TM-polarized radiation with a wavelength of 400 nm to 3000 nm when a saturating external magnetic field is applied, for example, using inductors. An external magnetic field is directed perpendicular to the plane of incidence of light on the structure and parallel to the plane of the magnetoplasmonic crystal, the magnetization vector of the magnetic layer lies in the plane of the magnetoplasmonic crystal and is perpendicular to the plane of incidence of light. The angle of incidence of radiation θ on the sample is at least 10 degrees, the azimuthal angle lies in the range from 0 to 360 degrees. In such a magnetoplasmon lattice, it is possible to excite the quasi-waveguide and plasmon modes in the spectral range of the incident radiation. Due to mode excitation, energy is redistributed between the incident electromagnetic radiation and the modes excited in the crystal, and, as a result, resonance features appear in the spectra of reflected and transmitted radiation - local maxima / minima are observed.

The indicated parameters of the magnetoplasmon crystal and the optical scheme are necessary and sufficient to obtain the claimed technical result.

The following is an example implementation of a method for modulating light intensity using a square lattice of gold particles in yttrium iron garnet. The method is based on the use of a two-dimensional magnetoplasmon crystal (Fig. 1), consisting of a quartz substrate (1), a “square” array of gold particles with a size of 110 nm and a period of d = 600 nm (2), immersed in a film of 100 yttrium iron garnet nm (3). For a crystal with this design, it is possible to excite local plasmons in gold nanoparticles, quasi-waveguide modes localized inside the magnetic metal between the rows, and also coupled plasmon modes. A magnetoplasmonic crystal can be obtained by technology known from the prior art (see, for example, N. Uchida, Y. Mizutani, Y. Nakai, AA Fedyanin, M. Inoue, Garnet composite films with Au particles fabricated by repetitive formation for enhancement of Faraday effect, J. Phys. D: Appl. Phys. 44 (2011) 064014). An array of gold nanodisks on a quartz substrate is made using electron beam lithography after magnetron sputtering of a gold film. To obtain gold particles, the array is annealed for 10 minutes at a temperature of 1000 ° C. Next, using magnetron sputtering, an upper layer of magnetic garnet is obtained, followed by annealing of the structure. The magnetization of the structure lies in the plane of the structure. This method of manufacturing a magnetoplasmon crystal demonstrates a good periodicity of the lattice (Fig. 2). To implement this method of light modulation, the initial radiation, the wavelength of which belongs to the range of 400-3000 nm, should be directed in reflection / transmission geometry to a magnetoplasmon crystal placed in a saturating alternating magnetic field. An external magnetic field is directed perpendicular to the plane of incidence of light on the structure and parallel to the plane of the magnetoplasmonic crystal, the magnetization vector of the magnetic layer lies in the plane of the magnetoplasmonic crystal and is perpendicular to the plane of incidence of light. The angle of incidence of light is selected so as to satisfy the phase-matching conditions between the projection of the wave vector of the incident optical radiation, the surface plasmon polariton vector and the reciprocal lattice vector of the magnetoplasmon crystal. As a result, the reflected / transmitted radiation will be modulated at the frequency of the magnetic field.

The crystal includes a layer of magnetic garnet, for which the magneto-optical Kerr effect is observed, and due to the excitation of the quasi-waveguide and plasmon modes, this effect is enhanced. Changes in the properties of the modes of the gold lattice due to magnetization lead to the appearance of a resonant dependence of the response in the far field. During the experiment, a clear relationship was found between the position of the resonance of the modes excited in the crystal and the resonance of the transverse magneto-optical Kerr effect (Fig. 4). The equatorial magneto-optical Kerr effect consists in changing the intensity and phase of electromagnetic radiation when interacting with a magnetized medium, for which the magnetization vector lies in the plane of the sample and is perpendicular to the plane of incidence of light. In the proposed method, a TM-polarized wave is incident on a magnetoplasmon crystal, an alternating saturating magnetic field with an amplitude of 1 kOe is used to modulate the response in the far field. The frequency of the magnetic field should not coincide with the frequency of the mechanical resonances of the magnetoplasmon crystal or the frequency used in the electrical network (50 Hz) to reduce noise.

A feature of the geometry used in this invention is that the equatorial magneto-optical Kerr effect is observed not only in reflection geometry, but also in transmission geometry (Fig. 3). In the latter case, the magnitude of the effect is determined as follows:

where H is the magnitude of the applied magnetic field, T (H) is the intensity of the transmitted electromagnetic radiation when applying a magnetic field of H, T (0) is the magnitude of the intensity of the transmitted light without applying a magnetic field. This effect is even in magnetization, i.e., δ changes its sign when the direction of the external magnetic field is reversed or when the angle of incidence θ changes by - θ.

In FIG. Figure 4 shows the transmission spectrum of a magnetoplasmon crystal for the angle of incidence of electromagnetic radiation on the structure equal to 20 ° (curve 1) and the spectrum of the magneto-optical Kerr effect (curve 2) in the transmission geometry for the same angle. A dip in the transmittance spectrum at a wavelength of 840 nm corresponds to the plasmon mode, and features in the transmittance spectrum at a wavelength near 560 nm are associated with the excitation of a quasi-waveguide mode. When modes are excited in a magnetoplasmon crystal, a long interaction of incident radiation with the medium occurs, which leads to an increase in the magneto-optical response. The graph shows that in the range from 525 nm to 575 nm, due to the excitation of the quasi-waveguide mode, the magneto-optical Kerr effect intensifies. Thus, the application of an external magnetic field to a magnetoplasmon crystal allows us to change the intensity of the transmitted radiation by δ, which is not less than 0.04% per 100 nm of the structure thickness (Fig. 4).

Since this invention uses a transparent ferrimagnetic material, which not only transmits incident radiation, but also reflects it, the proposed method for modulating the radiation intensity can work both for transmission and reflection.

The present invention is presented in the form of a specific example, which, however, is not the only possible, but clearly demonstrates the possibility of achieving the desired technical result.

As a result of the fact that by periodic structuring of magnetoplasmonic crystals at microscales it is possible to modulate the intensity of optical radiation, and the use of dielectric magnetic material, such as, for example, yttrium iron garnet, it is possible to register both transmitted and reflected radiation, the possibility of applying the claimed invention as universal compact magneto-optical materials controlled by an external magnetic field, which operate as pass-through And reflection.

Thus, a method for modulating the intensity of transmitted or reflected electromagnetic radiation using a structure with dimensions smaller than the wavelength of the radiation used is proposed, which consists in the fact that the surface of a magnetoplasmonic crystal in the form of periodically nanostructured ferromagnetic and noble metals is illuminated by optical radiation while applying alternating magnetic field perpendicular to the plane of incidence of light and parallel to the plane of the magnetoplasmon crystal la The design of the structure is determined by the working wavelength of the optical radiation (Table 1). The intensity of transmitted or reflected light is modulated by changing the amplitude and sign of the applied magnetic field (by changing the strength and direction of the current in the electromagnets that create this field).

Thus, the inventive method allows you to modulate the intensity of electromagnetic radiation in two geometries using a magnetoplasmon crystal, the active part of which is smaller than the wavelength of the radiation used.

Claims (5)

1. A method for modulating the intensity of electromagnetic radiation using a magnetoplasmonic crystal, including a transparent dielectric substrate, a two-dimensional array of noble metal particles with subwavelength dimensions, immersed in a dielectric magnetic layer with a thickness not less than the particle size, characterized in that the magnetoplasmonic crystal is illuminated with TM-polarized optical radiation or near IR ranges when an alternating magnetic field is applied to the dielectric magnetic layer in the geometry of the magneto pticheskogo transverse Kerr effect, which provides the possibility of modulation of the radiation intensity as a transmissive or reflective. 2. The method according to p. 1, characterized in that the two-dimensional array of particles is a periodic structure with a period of arrangement of particles of at least 200 nm. 3. The method according to p. 1, characterized in that the magnitude of the applied external magnetic field corresponds to the saturation region of the magnetization of the dielectric magnetic layer. 4. The method according to p. 1, characterized in that the particles are particles of gold or silver with sizes from 50 nm to 200 nm. 5. The method according to p. 1, characterized in that the external magnetic field is directed perpendicular to the plane of incidence of the radiation on the structure of the magnetoplasmon crystal and parallel to the plane of the magnetoplasmon crystal, the magnetization vector of the magnetic layer lies in the plane of the magnetoplasmon crystal and is perpendicular to the plane of incidence of light, the angle of incidence of radiation θ on the sample is at least 10 degrees, the azimuthal angle of the magnetoplasmon crystal lies in the range from 0 to 360 degrees.

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