Cascaded active magneto-optical terahertz non-reciprocal deflection device

Abstract

The invention discloses a cascade active magneto-optical terahertz non-reciprocal deflection device based on an indium antimonide-medium composite metasurface, and belongs to the technical field of novel artificial electromagnetic materials and terahertz science. According to the invention, indium antimonide, the medium metasurface and the wave plate are combined, and the two layers of structures are cascaded, so that multi-degree-of-freedom control and scanning of light beams are realized. Different static magnetic fields are applied through an electromagnet, so that indium antimonide generates a Faraday optical rotation effect on incident terahertz waves, the polarization angle of the incident terahertz waves is controlled, and the spin state of the incident terahertz waves on the geometric phase metasurface is further achieved. And meanwhile, the geometric phase metasurface generates different deflection effects on different spinning states. Through the combination of the above devices, the terahertz active deflection device can realize the deflection of dynamic light beams, and finally realizes the terahertz active deflection device which works in a range of 0.4-0.55 THz and has a deflection angle range of 42-67.8 degrees. Moreover, due to the fact that the time reversal symmetry introduced by the magnetic field is broken, the device further has nonreciprocity, and the stability of the system is protected. The device has the advantages of active adjustability, integration, good stability and the like, and has a good application prospect in the fields of terahertz communication, radar, imaging and the like.

Description

Technical Field

[0001] The invention relates to a cascade active magneto-optical terahertz non-reciprocal deflection device based on an indium antimonide-dielectric composite metasurface, belonging to the field of novel artificial electromagnetic materials and terahertz science and technology. Background Art

[0002] With the growing demand for high-speed wireless communications and bio-detection, terahertz (THz) technology has become a hot topic in recent research. Among these technologies, controlling THz waves, especially their trajectory and deflection direction, is crucial for today's THz systems. Unfortunately, due to the limited interaction between THz waves and natural materials, THz devices usually suffer from problems such as bulky size and low efficiency. Unlike devices in the microwave band (such as phased array radars), where array elements can be controlled independently, operating THz devices is more challenging due to the small wavelength. So far, there is still a lack of effective THz devices for active beam steering or deflection control.

[0003] Planar metamaterials may be a promising way to design THz devices, which can accelerate the development of high-performance THz functional metamaterial devices such as polarizers, superlenses, and polarizers. By utilizing the generalized Snell's law, metasurfaces can realize many functions once the units are properly adjusted to form a phased array. However, since the units or elements are fixed, their functions are always single and passive, hindering their further application in THz systems. Research on active THz metamaterial devices is limited by low tunability and efficiency.

[0004] In recent years, the magneto-optical material indium antimonide has attracted people's attention in the terahertz band. Indium antimonide is a narrow-bandgap semiconductor material with magneto-optical effect in the terahertz band. Under low temperature (such as between 70-100 Kelvin), it has a strong magneto-optical effect at 0.2-0.8 THz. The magneto-optical effect of indium antimonide can be divided into two types. One is the cyclotron resonance effect, which is mainly reflected in the different absorption rates of left-handed and right-handed polarized light; the other is the Faraday rotation effect, which produces different phases for left-handed and right-handed polarized light, thereby rotating the incident polarization state. In recent years, many excellent terahertz devices and works based on indium antimonide have emerged, such as terahertz isolators, terahertz unidirectional transmission waveguides, etc. However, there are few works combining indium antimonide with beam control, which are relatively immature.

[0005] In summary, combining indium antimonide with dielectric metasurfaces can give dielectric metasurfaces tunable properties while continuing their advantages such as high efficiency and rich electromagnetic resonance modes. Indium antimonide can be used to control the polarization state, phase, non-reciprocity, etc. of light, and it is expected to develop a new type of dynamic deflection device, providing a highly integrated multifunctional dynamic control device for terahertz optical systems. Summary of the invention

[0006] Purpose of the invention: The present invention proposes a cascaded active magneto-optical terahertz non-reciprocal deflection device based on InSb-dielectric composite metasurface, which changes the optical properties of InSb by applying an external magnetic field, thereby realizing non-reciprocal transmission and active deflection control of terahertz beams in different frequency ranges.

[0007] The technical solution of the present invention is as follows: the present invention is an active magneto-optical terahertz non-reciprocal deflection device based on a cascaded indium antimonide-dielectric composite metasurface, characterized in that the active cascaded magneto-optical terahertz non-reciprocal deflection device comprises a two-layer cascade structure (1), each layer of the structure is composed of a magneto-optical material indium antimonide (2), a dielectric 1 / 4 wave plate (3) and a geometric phase metasurface (4). The metasurface is composed of a silicon periodic structure, and the phase of each unit covers 2π. A 1 / 4 wave plate (5) is placed between the two layers of the structure, and two sets of magnetic fields are applied to each cascade structure by means of electromagnets.

[0008] Furthermore, the indium antimonide described in the present invention is a magneto-optical material in the 0.2-0.8 terahertz band at low temperatures (i.e., 70-100 Kelvin), and has different responses to different magnetic fields. In the present invention, indium antimonide will produce a Faraday effect on different circularly polarized terahertz waves when a magnetic field is applied. The Faraday effect of indium antimonide described in the present invention under a magnetic field is manifested in having different phases for different circularly polarized lights, thereby causing the polarization direction of the incident linearly polarized light to rotate, and the rotation angle can be adjusted from -45° to +45° through the magnetic field.

[0009] Furthermore, the 1 / 4 wave plate combined with indium antimonide in the present invention plays a role in polarization state conversion, and its main axis is at an angle of 45° with the spatial x-axis, so that the incident linear polarized light polarized in the x and y directions is converted into circularly polarized light. In addition, the 1 / 4 wave plate between the two cascade structures in the present invention can realize the conversion of polarization state, so that the incident circularly polarized light is efficiently converted into linearly polarized light.

[0010] Furthermore, the geometric phase metasurface described in the present invention is composed of a periodic structure, each period contains three units, each unit period is 270 microns, and the total period is greater than the wavelength. The geometric phase metasurface described in the present invention is composed of different rectangular arrays with different rotation angles in the periodic structure, and the rotation angles are 0°, 30° and 60° respectively. The parameters of each rectangular structure in one period are different, and the phases of 0, 0, 0 and 0, 60, 120° are generated for left and right circular light respectively. Therefore, the structure can diffract the left and right rotating light to the 0th and 1st diffraction orders respectively, and deflection occurs. If the structural periodic direction is the x direction, the left and right rotating light can be diffracted to the (0, 0) and (0, +1) orders; if the structural periodic direction is the y direction, the left and right rotating light can be diffracted to the (0, 0) and (+1, 0) orders. In the present invention, the geometric phase metasurfaces of the upper and lower cascade structures respectively adopt structures that extend the period in the x direction and the y direction.

[0011] Furthermore, the geometric phase metasurface described in the present invention is composed of high-resistance silicon. In the terahertz band, high-resistance silicon can be regarded as a lossless material with a refractive index of 3.4.

[0012] Furthermore, the magnetic field described in the present invention is generated by an electromagnet, and its magnetic field direction is longitudinal. The size and direction of the magnetic field can be controlled by the electromagnet. The present invention defines the forward magnetic field as the same direction as the propagation direction of the incident light, and the reverse magnetic field as the opposite direction to the propagation direction of the light.

[0013] Furthermore, the incident terahertz wave in the present invention is a broadband terahertz signal, and the frequency range needs to cover 0.4-0.55THz, and the 1 / 4 wave plate has high efficiency in these frequency ranges. That is, the polarization conversion efficiency can be maintained at more than 70% within a certain frequency range, wherein the polarization conversion efficiency is defined as the ratio of the power of the target polarization state to the incident light power.

[0014] Furthermore, by applying a magnetic field, the intensity of the Faraday effect of InSb is changed, so that InSb has a Faraday effect away from the cyclotron resonance frequency, thereby producing different rotation angles for the incident linear polarization state. Since InSb has non-reciprocity, the linearly polarized light can be rotated in the opposite direction by adjusting the direction of the magnetic field. When the rotation angle is +45°, the outgoing light is polarized in the x-direction, which is converted into left-handed polarized light after passing through a 1 / 4 wave plate; when the rotation angle is -45°, the outgoing light is polarized in the y-direction, which is converted into right-handed polarized light after passing through a 1 / 4 wave plate. When no magnetic field is applied, InSb has no response to the linear polarization state. At this time, InSb has only the function of absorbing and reflecting terahertz waves.

[0015] Furthermore, the geometric phase metasurface has different responses to different spin states. The first geometric phase metasurface can diffract left-handed circularly polarized light to the (0, +1) diffraction order and right-handed circularly polarized light to the (0, 0) diffraction order. The second geometric phase metasurface can diffract left-handed circularly polarized light to the (+1, 0) diffraction order and right-handed circularly polarized light to the (0, 0) diffraction order. The two-layer structure is combined to adjust the terahertz signal to four different deflection states through different magnetic field arrangements.

[0016] Furthermore, the above-mentioned geometric phase metasurface can achieve broadband diffraction, and its working range should be consistent with the frequency range of the incident terahertz wave.

[0017] Furthermore, by adjusting the magnetic field, the circular polarization state of the terahertz wave incident on the geometric phase metasurface can be adjusted, thereby further controlling the diffraction order of each layer.

[0018] Furthermore, the deflection device of the present invention has non-reciprocity when a magnetic field is applied, that is, reflected light of the same polarization state will be isolated when passing through the device again, thereby protecting the stability and signal-to-noise ratio of the device.

[0019] The beneficial effects and advantages of the present invention are:

[0020] 1. The indium antimonide layer has no pattern, and the overall structure is easy to design and process.

[0021] 2. The Faraday rotation effect of indium antimonide is used to change the polarization state incident on the dielectric metasurface, which is then converted into different circular polarization states through a 1 / 4 wave plate, and finally incident on the geometric phase metasurface to regulate the terahertz deflection state, solving the technical problem that the existing terahertz deflection devices have a single function and are not tunable.

[0022] 3. Each magnetic field can be controlled independently, and the required magnetic field is small, with a magnetic field range of -0.25-0.25 Tesla. Provide high-efficiency, broadband, highly integrated, and adjustable deflection control devices for terahertz systems.

[0023] 4. Due to the introduction of magneto-optical materials, the device has non-reciprocity, and the reflected light has little effect on the system, ensuring the stability and signal-to-noise ratio of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the structure and function of a cascaded active magneto-optical terahertz non-reciprocal deflection device provided by an embodiment of the present invention;

[0025] Figure 2 is a schematic diagram of the working state of an embodiment of the present invention;

[0026] Figure 3is the transmittance and Faraday rotation angle of indium antimonide to circularly polarized light under different magnetic fields described in the embodiment of the present invention;

[0027] Figure 4 The different output phase distributions of the metasurface for incident left-handed and right-handed polarized light according to the embodiment of the present invention;

[0028] Figure 5 is a diffraction efficiency diagram of different diffraction orders of the metasurface for incident left-handed and right-handed circularly polarized light according to an embodiment of the present invention;

[0029] Figure 6 It is the far-field pattern of the metasurface in different magnetic fields described in the embodiment of the present invention, which can realize angle control;

[0030] The figure includes: a two-layer cascade structure 1, magneto-optical material indium antimonide 2, a 1 / 4 wave plate 3, a dielectric geometric phase metasurface 4, and a middle 1 / 4 wave plate 5. DETAILED DESCRIPTION

[0031] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings. It is understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0032] An active magneto-optical terahertz non-reciprocal deflection device based on a cascaded indium antimonide-dielectric composite metasurface is characterized in that the active cascaded magneto-optical terahertz non-reciprocal deflection device comprises a two-layer cascade structure (1), each layer of the structure is composed of a magneto-optical material indium antimonide (2), a dielectric 1 / 4 wave plate (3) and a geometric phase metasurface (4). The metasurface is composed of a silicon periodic structure, and the phase of each unit covers 2π. A 1 / 4 wave plate (5) is placed between the two layers of the structure, and two sets of magnetic fields are applied to each cascade structure by means of electromagnets.

[0033] like Figure 1 As shown in the structural diagram, the device consists of a two-layer cascade structure. The terahertz wave with a frequency range of 0.4-0.55THz is incident on the first layer of the device. The outgoing light can be adjusted to different diffraction orders through the magnetic field, thereby controlling the outgoing angle. After passing through the first layer of the structure, the terahertz wave is incident on the second layer of the structure. The second structure is the same as the first structure, but rotated 90 degrees. When passing through the second layer of the structure, the outgoing terahertz can also be in different diffraction orders by adjusting the magnetic field. Since the upper and lower magnetic fields can be adjusted independently, the device can deflect the terahertz wave to 4 different directions.

[0034] Indium antimonide used in the present invention is a magneto-optical material in the terahertz band. When a longitudinal magnetic field is applied, its propagation equation can be written as follows:

[0035]

[0036] in,

[0037]

[0038] Among them, ω c is the cyclotron resonance frequency, which is proportional to the magnetic field ω c =eB / m * , B is the magnetic field strength, e is the charge of the electron, m * is the effective carrier mass. For InSb, m * =0.014m e , and m e is the mass of the electron. ∞ =15.68 is the high-frequency cutoff dielectric constant; ω is the angular frequency of the incident terahertz wave; ω p is the plasma frequency and ω p =(Ne 2 / m * μ) 1 / 2 ,γ is the carrier collision frequency,γ=4*e / (μm * ), and μ is the carrier mobility, which is temperature-dependent and can be written as μ = 7.7 × 10 4 (T / 300) -1.66 cm 2 ·V -1 ·s -1 In the present invention, we use InSb at 80K, with a carrier concentration of N = 0.5 × 10 14 cm -3 .

[0039] According to the propagation equation of InSb, we can get two eigensolutions of InSb, which are left-handed circularly polarized light and right-handed circularly polarized light. For these two eigensolutions, InSb exhibits different absorption rates for left-handed circularly polarized light in the cyclotron resonance frequency band, and can only transmit one circularly polarized state component at this time, such as Figure 3 (a) The black area; far away from the cyclotron resonance frequency band, such as Figure 3 In the white area of ​​(b), InSb has a strong Faraday effect. When the linearly polarized incident terahertz wave passes through InSb, its polarization state will rotate. Through calculation, it can be obtained that the relationship between the Faraday rotation angle and the magnetic field at different frequencies is as follows: Figure 3 (c) This Faraday effect is non-reciprocal, that is, the Faraday rotation angles of linearly polarized light are opposite to those of positive and negative magnetic fields. Figure 3 (d) shows the magnetic field required to rotate linear polarized light by +45° and -45° at different frequencies. It can be found that from 0.4-0.55THz, by adjusting the magnetic field, we can always rotate linear polarized light by +45° and -45°. Thus, when linear polarized light with a polarization state of 45° is incident, the outgoing light can be adjusted to the x and y polarization states through the magnetic field (at this time, the forward and reverse magnetic fields are applied respectively). The x and y polarization states then pass through a 1 / 4 wave plate, at which time the main axis of the 1 / 4 wave plate is 45°, and the outgoing light will be converted into left-handed circularly polarized light and right-handed circularly polarized light, respectively.

[0040] Furthermore, the dielectric metasurface structure designed in this article is composed of periodic units. In the present invention, it is set that there are three unit structures in each period. Each unit of the geometric phase metasurface is composed of a dielectric rectangular block with a period of 270 microns, a height of 300 microns, and a substrate thickness of 700 microns. Each large period is composed of three units. Through calculation and optimization, the rotation angles of the three units are 0°, 30°, and 60°, respectively. Each unit has a different response to left-handed and right-handed incident light. Its characteristics are: each unit is a half-wave plate, and each half-wave plate brings different phases to left-handed and right-handed light by design. For the entire metasurface, its left-handed phase distribution is as follows: Figure 4 As shown, it presents periodicity on the entire metasurface. For right-hand rotation, its phase distribution is as follows Figure 4 As shown, there is a uniform trend on the entire metasurface. The geometric phase metasurface can make different responses to left and right rotations, thereby deflecting the light beam to different directions. The second layer structure is similar to the first layer structure, the difference is that the first layer structure deflects the light beam on the x-axis, and the second layer structure deflects it in the y-axis direction. In the middle of the two layers, we added an additional 1 / 4 wave plate to convert the outgoing circularly polarized light into linearly polarized light, thereby ensuring that the light incident on the second layer structure is also linearly polarized light.

[0041] The deflection mentioned in the present invention means that the terahertz wave energy is mainly concentrated on different diffraction orders, and the diffraction is caused by the dielectric metasurface. Figure 5 The left figure shows the diffraction efficiency of left-handed light incident on the periodic metasurface at different diffraction orders. It can be seen that the diffraction efficiency of the structure is greater than 0.3 in the range of 0.4-0.55THz, and the main diffraction order is at +1. It should be noted that when the structural rotation angle is 0°, the diffraction order is (0, +1), and when the structural rotation angle is 90°, the diffraction order is (+1, 0). Figure 4 The right figure shows the diffraction efficiency of right-handed light at different diffraction orders after it is incident on the periodic metasurface. It can be seen that the diffraction efficiency of the structure is greater than 0.3 in the range of 0.4-0.55THz, and the main diffraction order is at the 0th order.

[0042] More specifically, Figure 6 , when passing through the first layer of structure, a forward 0.17T magnetic field and a reverse 0.17T magnetic field are applied, and the outgoing light can be adjusted to the (0, 0) and (0, +1) orders. At this time, the outgoing light is in a left-handed or right-handed circular polarization state. After passing through the middle 1 / 4 wave plate, the light becomes linearly polarized again and is incident on the second layer of structure. After passing through the second layer of structure, a forward 0.17T magnetic field and a reverse 0.17T magnetic field are applied, and the incident (0, 0) and (0, +1) order lights can be adjusted to (0, 0), (+1, 0) and (0, +1), (+1, +1) orders respectively. Since the magnetic field can be independently controlled by the electromagnet, the overall structure has four outgoing deflection states. In the operating frequency range, i.e. 0.4-0.55THz, since the metasurface period is 810 microns, its deflection angle range can reach 42° to 67.8°.

[0043] It is worth noting that since the magneto-optical material indium antimonide breaks the time reversal symmetry when a magnetic field is applied, when light of the same polarization state is reflected back, the beam will be isolated. As a result, the device has non-reciprocity, which protects the stability and signal-to-noise ratio of the system.

Claims

1. The present invention is based on a cascaded indium antimonide-dielectric composite metasurface active magneto-optical terahertz non-reciprocal deflection device, characterized in that The active cascade magneto-optical terahertz non-reciprocal deflection device includes a two-layer cascade structure (1), each layer of which is composed of a magneto-optical material indium antimonide (2), a dielectric quarter wave plate (3) and a geometric phase metasurface (4). The metasurface is composed of a silicon periodic structure, in which three units form a period, each period has a size of 270*710 microns, a height of 300 microns, and a substrate thickness of 700 microns. The phase of each unit covers 2π. A quarter wave plate (5) is placed between the two layers of the structure, and two sets of magnetic fields are applied to each cascade structure by means of electromagnets. The overall structural plane range is between 1.42cm*1.42cm and 2.84cm*2.84cm.

2. A cascade active magneto-optical terahertz non-reciprocal deflection device as claimed in claim 1, characterized in that The magneto-optical material indium antimonide needs to be subjected to low temperature, between 70-100 Kelvin. When a magnetic field is applied, indium antimonide has a Faraday effect in the terahertz band, which is manifested in the opposite phase of different circularly polarized light, thereby causing the polarization direction of the incident linearly polarized light to rotate, and the rotation angle can reach -45° to +45°.

3. A cascade active magneto-optical terahertz non-reciprocal deflection device as claimed in claim 1, characterized in that The operating range of the quarter wave plate combined with indium antimonide is 0.4-0.55THz, and its main axis is at an angle of 45° with the spatial x-axis, so that the incident linearly polarized light polarized in the x and y directions is converted into left-handed and right-handed circularly polarized light respectively.

4. A cascade active magneto-optical terahertz non-reciprocal deflection device as claimed in claim 1, characterized in that Each periodic structure of the geometric phase metasurface is composed of three different rectangular arrays with different rotation angles. The period of each rectangular unit is 270 microns, and each rectangle has different rotation angles and structural parameters. The two metasurfaces as a whole diffract left and right rotation to the (0, +1), (0, 0) order and (0, 0), (+1, 0) order respectively, thereby deflecting the terahertz wave in different directions.

5. The cascade active magneto-optical terahertz non-reciprocal deflection device according to claim 1, characterized in that The geometric phase metasurface is composed of a dielectric material with high transmittance in the terahertz band, such as silicon with a refractive index of about 3.

4.

6. A cascade active magneto-optical terahertz non-reciprocal deflection device as claimed in claim 1, characterized in that The quarter wave plate located between the two layers of the cascade structure converts circularly polarized light into linearly polarized light, and has a high polarization conversion efficiency in the 0.4-0.55THz band.

7. The cascade active magneto-optical terahertz non-reciprocal deflection device according to claim 1, characterized in that Two sets of magnetic fields are added to the two-layer cascade structure respectively. Each set of magnetic fields can be independently regulated by means of electromagnets. The magnetic field range is -0.25-0.25 Tesla.

8. The cascade active magneto-optical terahertz non-reciprocal deflection device according to claim 1, characterized in that The device operates in the range of 0.4-0.55THz. By applying different magnetic fields, the polarization state of the emission from InSb can be adjusted to x and y By controlling the different combinations of the magnetic fields of the upper and lower cascade structures, four deflection states can be controlled, with the diffraction orders being (0, 0), (0, +1), (+1, 0) and (+1, +1), and the deflection angle ranges from 42° to 67.8°.

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