Magneto-optical nonreciprocal meta-structure surface device based on magnetic second-order nonlinearity

Abstract

The invention belongs to the field of optical metasurfaces, and particularly relates to a magneto-optical nonreciprocal metasurface device based on magnetic second-order nonlinearity. According to the invention, the silicon disc structure layer with the sub-wavelength size is arranged on the magneto-optical material layer of the near-infrared band according to the matrix period, the phase and amplitude of electromagnetic waves are regulated and controlled by changing the length, width and height of the structure units of the structure layer, and the nonlinear magneto-optical material with high nonlinear coefficient contrast is adopted. The function of non-reciprocal transmission of circularly polarized light is realized in an optical frequency band by using a magnetic second-order nonlinear effect. Finally, a nonlinear nonreciprocal device is realized through a magneto-optical material based on a magnetic second-order nonlinear effect, and a sub-wavelength size structure is adopted, so that miniaturization and integration of the device are facilitated. The method has important application prospects in the fields of free space isolators, nonreciprocal lenses, nonreciprocal holographic imaging and the like, and a new design thought is provided for quantum nonreciprocal super-structure surface devices in the future.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of optical super-structured surfaces, and particularly relates to a magneto-optical non-reciprocal super-structured surface device based on magnetic second-order nonlinearity. BACKGROUND

[0002] A super-structured surface is a two-dimensional subwavelength flat-plate structure that can effectively regulate the amplitude, polarization and wavefront of electromagnetic waves, and has become a research hotspot in recent years. Its main function is to realize various optical devices such as lenses and wave plates on an interface. Compared with traditional optical devices, a super-structured surface has a subwavelength thickness and higher flexibility to design various electromagnetic wave devices. A nonlinear super-structured surface has attracted much attention due to its important applications in holographic imaging and quantum fields. Meanwhile, if non-reciprocity is introduced into a nonlinear super-structured surface, the application of the super-structured surface in signal processing, information transmission and quantum technology will become more extensive and diversified.

[0003] At present, the harmonics generated by a traditional nonlinear super-structured surface mainly come from the nonlinear material itself or from micro-nano structures with different symmetries. If such characteristics are used to realize non-reciprocity, most of them use the mode of asymmetric waveguide structure to realize a diode-like function. Only under the condition of a specific input energy, the non-reciprocity is achieved. However, the design structure is too complex, the conditions are harsh, and it is difficult to dynamically control the non-reciprocal function of the super-structured surface.

[0004] Although intensity-type non-reciprocal devices (transmission intensity is different) have been studied in the microwave and terahertz frequency bands, in the optical frequency band, the existing intensity-type non-reciprocal devices based on magneto-optical materials have only been theoretically studied, and there is no experimental conclusion. According to the theory, the preparation of such devices needs to meet the low loss of magneto-optical materials and specific high-Q structures to achieve related performance. However, there is no existing magneto-optical material and mature high-Q structure processing technology. SUMMARY

[0005] In view of the above problems or deficiencies, to solve the blank that the existing intensity-type non-reciprocal devices based on magneto-optical materials in the optical frequency band only stay in the theoretical stage, the application provides a magneto-optical non-reciprocal super-structured surface device based on magnetic second-order nonlinearity.

[0006] A magneto-optical non-reciprocal super-structured surface device based on magnetic second-order nonlinearity is composed of a substrate layer, a nonlinear magneto-optical material layer and a subwavelength structure layer.

[0007] The substrate layer is a material layer with a refractive index of 1.95 in the near-infrared wave band (gadolinium gallium garnet substrate GGG).

[0008] The nonlinear magneto-optical material layer is a magneto-optical material with magnetic second-order nonlinearity.

[0009] The subwavelength structure layer is composed of cylindrical silicon disk structural units arranged in a matrix period, with the matrix period being equal in both row and column directions. This material is prepared using a magnetron sputtering system, and the specific structure is fabricated using high-precision electron beam lithography equipment and an etching machine. The phase and amplitude of the circularly polarized electromagnetic waves are controlled by changing the height and radius of the cylindrical silicon disk structural units. Ultimately, at the fundamental frequency (f0: near-infrared band), the forward and reverse transmission efficiencies are almost identical, with a contrast of almost zero, achieving weak non-reciprocity in intensity. At the second harmonic (2f0: visible light band), the forward and reverse transmission efficiencies differ at the resonance point, with a second harmonic generation efficiency contrast of 3.36, achieving strong non-reciprocity in intensity.

[0010] The overall device size is D1×D2, and the device responds to circularly polarized incident electromagnetic waves; the dimensions satisfy the following relationships: D1=20λ0~25λ0, D2=20λ0~25λ0, λ0=c / f0, where c is the speed of light in vacuum.

[0011] Furthermore, the magneto-optical material possessing second-order magnetic nonlinearity is a single-crystal Ce-doped iron garnet magneto-optical thin film.

[0012] The aforementioned magneto-optical materials with second-order magnetic nonlinearity (such as single-crystal Ce-doped iron garnet magneto-optical thin films) have an out-of-plane saturation magnetic field strength Hs≈2kOe. The second-order nonlinear polarizability of such materials is... This can be expressed as the superposition of crystallographic and magnetostrictive contributions, and its second-order nonlinear coefficients satisfy the formula... In this expression, The second-order nonlinear magnetic susceptibility, representing the origin of pure crystallography, and This represents the nonlinear polarizability induced by the magnetization M. The positive and negative signs in the formula not only represent the direction of the magnetic field, but also the rotation direction of the incident light (such as left-handed LCP and right-handed RCP).

[0013] According to the above formula, under the condition of maintaining the magnetization state in the target frequency band, the nonlinear coefficient of the magneto-optical material under the condition of left-handed circularly polarized light incidence is... ≈4.41 pm / v, the nonlinear coefficient under right-handed circularly polarized light incidence conditions. ≈2.49 pm / v. Therefore, after the magneto-optical material is saturated with an external magnet, the intensity non-reciprocity can be achieved at the harmonics using the second-order nonlinear theory of magneto-optics.

[0014] Further, the substrate layer thickness is L1=0.5 mm; the magneto-optical material layer thickness is L2=200 nm; the height of the cylindrical structure unit is h=175 nm, and the radius is R=237 nm, and the entire subwavelength structure layer is formed by a matrix with equal periods p=855 nm in the row and column directions.

[0015] In the optical frequency band, due to the long-term lack of low-loss magneto-optical materials and mature micro-nano processing technology of similar devices of the traditional magneto-optical mechanism, such optical frequency devices have so far only remained in the theoretical discussion stage and lack experimental verification. In order to break through the above technical bottleneck, the present application introduces the magnetic second-order nonlinear theory as a new path to construct intensity-type non-reciprocal devices in the optical frequency band, and prior to this, the theoretical and experimental research in this field is still blank.

[0016] Therefore, the present application prospectively proposes and realizes a magneto-optical non-reciprocal metasurface device based on magnetic second-order nonlinearity. The scheme periodically integrates an array of subwavelength-sized silicon discs on a magneto-optical material layer in the near-infrared frequency band, and realizes deep modulation of electromagnetic wave phase and amplitude by precisely controlling the three-dimensional geometric parameters of the structure unit. At the same time, relying on the contrast of the nonlinear coefficient of the nonlinear magneto-optical material, the present application ingeniously excites the magnetic second-order nonlinear effect, and finally successfully realizes the non-reciprocal transmission for circularly polarized light in the optical frequency band. The present application not only fundamentally avoids the dependence on extremely high-Q structures in traditional design, but also greatly promotes the miniaturization and on-chip integration of nonlinear non-reciprocal devices. The architecture shows great application potential in the fields of free-space isolators, non-reciprocal lenses, and non-reciprocal holographic imaging, and further provides a pioneering design paradigm for the research and development of future quantum non-reciprocal metasurface devices. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 It is a structural schematic diagram of the embodiment.

[0018] Figure 2 It is a base frequency transmission spectrum of the embodiment.

[0019] Figure 3 It is a frequency-doubled energy intensity spectrum of the embodiment. DETAILED DESCRIPTION

[0020] The present application will be further described in detail below in combination with the drawings and embodiments.

[0021] As described in the part of the background art, very high input power is required for a nonlinear nonreciprocal device based on a nonlinear nonreciprocal device, and there is a dynamic reciprocity problem; and the present application proposes a magneto-optical nonreciprocal device based on magnetic second-order nonlinearity, which has the advantages of low power consumption, stability, dynamic regulation by a magnetic field, and provides a new design idea for future quantum nonreciprocal super surface devices. Therefore, it is of great significance to realize a magneto-optical nonreciprocal super surface device based on magnetic second-order nonlinearity.

[0022] The magneto-optical nonreciprocal super surface device based on magnetic second-order nonlinearity provided by the embodiment includes a substrate layer with a thickness of L1, a magneto-optical material layer with a thickness of L2, and a cylindrical structure layer with a height of h, a radius of R, and a period of p. The size of the entire device is D1 x D2. The device responds to circularly polarized incident electromagnetic waves. The magnetization direction is parallel to the incident direction of the electromagnetic wave. By changing the radius and height of the structural unit of the structural layer, the phase and amplitude of the electromagnetic wave are regulated. At the fundamental frequency (f0: near-infrared waveband), weak nonreciprocity in intensity is realized; at the second harmonic frequency (2f0: visible waveband), strong nonreciprocity in intensity is realized.

[0023] Ce:YIG material with high magnetic second-order nonlinear contrast and high magneto-optical effect in the near-infrared waveband is used as the substrate; at the same time, a high-directional cylinder is used as the structural unit for circular polarization design. By designing the Mie resonance unit of the subwavelength period structure, the function of nonreciprocal transmission of circularly polarized light at the second harmonic frequency is realized.

[0024] For the circularly polarized intensity type nonreciprocal device of the embodiment, one preferred embodiment includes a 0.5 millimeter thick GGG substrate layer and a 200 nm thick magneto-optical material Ce:YIG layer; a cylindrical a-Si with a height of 175 nm, a radius of 234 nm, and a period of 855 nm is used as the structural unit of the subwavelength structure layer, and the overall device is 50 um x 50 um. The device responds to circularly polarized incident electromagnetic waves, and its structural schematic diagram is as shown in Figure 1 The incident wave is a circularly polarized wave with different handedness, and the intensity of the second harmonic frequency is detected at the transmission end. The magnetization direction is parallel to the incident direction, so that the device is magnetized in the out-of-plane direction.

[0025] In the embodiment, the single crystal Ce-doped iron garnet magneto-optical film is first deposited on the surface of the substrate in a thickness of 200 nm by means of magnetron sputtering, and finally crystallized by means of rapid thermal annealing to prepare the magneto-optical film.

[0026] The device prepared in the embodiment is tested and analyzed as follows: in the near-infrared waveband, the fundamental frequency incident wavelength is 1415 nm ± 25 nm, and the transmittance of the sample is tested, and the results are as shown in Figure 2As shown, the forward incidence almost coincides with the reverse incidence, realizing weak non-reciprocity in the fundamental frequency band. In the nonlinear test, the generation efficiency of the frequency-doubled transmission end is detected, and the results are as shown in Figure 3 As shown, in the target second-harmonic wave band (707.5 nm), the contrast of the second-harmonic generation efficiency is 3.36, realizing strong non-reciprocity of the frequency doubling.

[0027] As can be seen from the above, when the electromagnetic wave is incident to the magneto-optical material layer with magnetic second-order nonlinearity, due to the magnetic second-order nonlinearity of the material, in the magnetization state of the device, the second-harmonic generation efficiency of the forward incident circularly polarized electromagnetic wave and the reverse incident circularly polarized electromagnetic wave will be different, realizing the function of weak non-reciprocity of the electromagnetic wave fundamental frequency and strong non-reciprocity of the frequency doubling. At the same time, due to the innovative design concept of combining magnetic second-order nonlinearity and optical superstructure, on the basis of realizing magnetic second-order nonlinearity and non-reciprocal optical response, the compactness of the device structure and the functional integration are significantly improved, providing a feasible implementation path for the miniaturization and integration of the on-chip optoelectronic system.

[0028] In addition, the design idea and working mechanism of the present application not only provides a new device basis for nonlinear non-reciprocal holographic imaging, dynamic optical encryption and intelligent optical sensing applications, but also brings a new design perspective and physical carrier for quantum non-reciprocal superstructure devices, topological photonic structures and non-Hermite optical systems in the field of quantum information processing. Therefore, the present application has important scientific significance and wide application prospect in promoting the development of non-reciprocal photonics, integrated nonlinear optics and quantum optics devices.

Claims

1. A magneto-optical nonreciprocal metasurface device based on second-order magnetic nonlinearity, characterized in that: It consists of a substrate layer, a nonlinear magneto-optical material layer, and a subwavelength structure layer; The substrate layer is a material layer with a refractive index of 1.95 in the near-infrared band; The nonlinear magneto-optical material layer is a magneto-optical material with second-order magnetic nonlinearity; The subwavelength structure layer is composed of cylindrical silicon disk structure units arranged in a matrix period, with the matrix period being equal in both row and column directions. The phase and amplitude of the circularly polarized electromagnetic wave can be controlled by changing the length, width, and height of the cylindrical silicon disk structure units. Ultimately, weak non-reciprocity in intensity is achieved in the fundamental near-infrared band f0; and strong non-reciprocity in intensity is achieved in the second harmonic visible band 2f0. The overall device size is D1×D2, and the device responds to circularly polarized incident electromagnetic waves; the dimensions satisfy the following relationships: D1=20λ0~25λ0, D2=20λ0~25λ0, λ0=c / f0, where c is the speed of light in vacuum.

2. The magneto-optical non-reciprocal metasurface device based on second-order magnetic nonlinearity as described in claim 1, characterized in that: The magneto-optical material possessing second-order magnetic nonlinearity is a single-crystal Ce-doped iron garnet magneto-optical thin film.

3. The magneto-optical non-reciprocal metasurface device based on second-order magneto-nonlinearity as described in claim 2, characterized in that: The single-crystal Ce-doped iron garnet magneto-optical thin film is first prepared by depositing a 200 nm thick Ce-doped amorphous garnet film on the substrate surface by magnetron sputtering, and finally by crystallizing the magneto-optical thin film through rapid thermal annealing.

4. The magneto-optical non-reciprocal metasurface device based on second-order magnetic nonlinearity as described in claim 1, characterized in that: The substrate is a gadolinium gallium garnet material, GGG(111).

5. The magneto-optical non-reciprocal metasurface device based on second-order magnetic nonlinearity as described in claim 1, characterized in that: The substrate layer has a thickness of L1=0.5 mm; the magneto-optical material layer has a thickness of L2=200 nm; the cylindrical structural unit has a height of h=175 nm and a radius of R=237 nm, and the entire subwavelength structural layer is composed of a matrix period of p=855 nm with equal periods in the row and column directions.

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