A cascaded active magneto-optical terahertz non-reciprocal deflection device

Abstract

The application discloses a kind of based on indium antimonide-medium composite super surface cascade active magneto-optical terahertz non-reciprocal deflection device, belong to novel artificial electromagnetic material and terahertz science and technology field.The application will combine indium antimonide, medium super surface and wave plate, and cascade two-layer structure, realize the multi-degree-of-freedom control and scanning of light beam.Different static magnetic field is applied by electromagnet, so that indium antimonide produces Faraday optical rotation effect to incident terahertz wave, to control the polarization angle of wave plate, further realize the spin state of incident to geometric phase super surface.Different deflection effects are produced to different spin states by geometric phase super surface.By the combination of the above-mentioned device, the application can realize the deflection of dynamic light beam, finally realize the active deflection device of terahertz in 0.4~0.55THz range, and the deflection angle range can reach 42° to 67.8°.And, due to the time reversal symmetry breaking introduced by magnetic field, the device also has non-reciprocity, protects the stability of system.The application has the advantages of active adjustable, integrable, good stability, etc., and has good application prospect in terahertz communication, radar and imaging fields.

Description

TECHNICAL FIELD

[0001] The application relates to a cascade active magneto-optical terahertz non-reciprocal deflector based on an indium antimonide-dielectric composite metasurface, and belongs to the fields of novel artificial electromagnetic materials and terahertz science and technology. BACKGROUND

[0002] With the growing demand for high-speed wireless communication and biological detection, terahertz (THz) technology has become a hot topic in recent research. Among these technologies, controlling THz waves, especially their trajectories and deflection directions, is crucial for today's THz systems. Unfortunately, due to the limited interaction between THz waves and natural materials, THz devices often encounter problems such as large volume and low efficiency. Unlike microwave band devices (such as phased array radars) that can independently control array elements, operating THz devices is more challenging due to their small wavelengths. So far, there is still a lack of effective THz devices for active beam control or deflection control.

[0003] Planar metamaterials can 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 achieve many functions once the cells are properly adjusted to form a phased array. However, due to the fixed cells or elements, their functions are always single and passive, hindering their further application in THz systems. Research on active THz metamaterial devices is limited by tunability and low efficiency.

[0004] In recent years, magneto-optical material indium antimonide has attracted attention in the terahertz wave band. Indium antimonide is a narrow-band semiconductor material that has a magneto-optical effect in the terahertz wave band. At low temperatures (such as between 70-100 Kelvin), it has a strong magneto-optical effect in the 0.2-0.8 THz band. The magneto-optical effect of indium antimonide can be divided into two types: one is the cyclotron resonance effect, which mainly manifests itself in different absorption rates for left and right circularly polarized light; the other is the Faraday optical rotation effect, which produces different phases for left and right circularly polarized light, causing the incident polarization state to rotate. In recent years, many excellent THz devices and works based on indium antimonide have emerged, such as THz isolators, THz 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 extend the advantages of dielectric metasurfaces such as high efficiency and rich electromagnetic resonance modes, while also giving them tunable characteristics. By using indium antimonide to control the polarization state, phase, non-reciprocity, etc. of light, it is expected to develop a new type of dynamic deflector, providing highly integrated multifunctional dynamic control devices for terahertz optical systems. SUMMARY

[0006] Purpose of the invention: The present invention proposes a cascaded active magneto-optical terahertz non-reciprocal deflection device based on an indium antimonide-dielectric composite metasurface, which changes the optical properties of indium antimonide 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 based on an active magneto-optical terahertz non-reciprocal deflection device of 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), exhibiting different responses to different magnetic fields. In the present invention, when a magnetic field is applied, indium antimonide produces a Faraday effect for different circularly polarized terahertz waves. The Faraday effect of indium antimonide in the present invention under a magnetic field manifests itself in having different phases for different circularly polarized light, thereby causing the polarization direction of incident linearly polarized light to rotate. The rotation angle can be adjusted from -45° to +45° using the magnetic field.

[0009] Furthermore, the quarter-wave plate combined with indium antimonide in the present invention performs polarization state conversion, with its principal axis forming a 45° angle with the spatial x-axis, thereby converting incident linearly polarized light in the x- and y-directions into circularly polarized light. Furthermore, the quarter-wave plate between the two cascaded structures in the present invention can also achieve polarization state conversion, efficiently converting incident circularly polarized light 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 a periodic structure composed of different rectangular arrays with different rotation angles, 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 left and right circular 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 circular 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 circular 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 periodically 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 direction of propagation of the same direction as the incident light as a positive magnetic field, and the direction of propagation of the opposite direction of light as a negative magnetic field.

[0013] Furthermore, the incident terahertz wave described in the present invention is a broadband terahertz signal, covering a frequency range of 0.4-0.55 THz. The quarter-wave plate exhibits high efficiency within this frequency range. Specifically, the polarization conversion efficiency (PCE) is maintained at over 70% within a certain frequency range. The PCE is defined as the ratio of the power in the target polarization state to the incident light power.

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

[0015] Furthermore, the geometric phase metasurface responds differently to different spin states. The first geometric phase metasurface can diffract left-handed circularly polarized light into the (0, +1) diffraction order and right-handed circularly polarized light into the (0, 0) diffraction order. The second geometric phase metasurface can diffract left-handed circularly polarized light into the (+1, 0) diffraction order and right-handed circularly polarized light into the (0, 0) diffraction order. Combining these two layers, the terahertz signal can be manipulated into 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 emitted from 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 of the incident light on the dielectric metasurface, which is then converted into different circular polarization states through a 1 / 4 wave plate. Finally, the light is incident on the geometric phase metasurface to regulate the terahertz deflection state, solving the technical problem of the existing terahertz deflection devices being single in function and non-tunable.

[0022] 3. Each magnetic field can be independently controlled, requiring a small magnetic field range of -0.25-0.25 Tesla. This provides a highly efficient, broadband, highly integrated, and controllable deflection control device for terahertz systems.

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

[0024] Figure 1 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 1 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 for circularly polarized light under different magnetic fields according to 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 This is the far-field pattern of the metasurface in different magnetic fields according to an embodiment of the present invention, which can achieve angle control;

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

[0031] The following is a further description of the specific embodiments of the present invention with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are intended only to explain the present invention and are not intended to limit the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall 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 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, and 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.

[0033] like Figure 1 The schematic diagram shows a device consisting of a two-layer cascade structure. A 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 by using a magnetic field, thereby controlling the exit angle. After passing through the first layer, the terahertz wave is incident on the second layer. The second structure is identical to the first, but rotated 90 degrees. When passing through the second layer, the outgoing terahertz light can also be adjusted to different diffraction orders by adjusting the magnetic field. Because the upper and lower magnetic fields can be adjusted independently, the device can deflect the terahertz wave in four different directions.

[0034] The indium antimonide used in the present application is a magneto-optical material in the terahertz wave band, and its propagation equation can be written as follows in the case of applying a longitudinal magnetic field:

[0035]

[0036] wherein,

[0037]

[0038] wherein, ω c is the cyclotron resonance frequency, which is in a certain proportion to the magnetic field ω c = eB / m * , B is the magnetic field intensity, e is the electronic charge, m * is the effective carrier mass. For indium antimonide, m * = 0.014m e , and m e is the electron mass. ε ∞ = 15.68 is the high-frequency cut-off 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, μ is related to temperature and can be written as μ = 7.7*10 4 (T / 300) -1.66 cm 2 ·V -1 ·s -1 . In the present application, we use indium antimonide at a temperature of 80K, and the carrier concentration is N = 0.5*10 14 cm -3 .

[0039] According to the propagation equation of indium antimonide, we can obtain two eigen solutions of indium antimonide, which are left-handed circularly polarized light and right-handed circularly polarized light. For the two eigen solutions, in the cyclotron resonance frequency band, indium antimonide shows different absorption rates for left-handed circularly polarized light and right-handed circularly polarized light, at this time only one circularly polarized component can be transmitted, such as the black area of Figure 3 (a); in the far cyclotron resonance frequency band, such as the white area of Figure 3 (b), indium antimonide has a strong Faraday effect, at this time the polarization state of the linearly polarized incident terahertz wave will be rotated after passing through indium antimonide. Through calculation, the relationship between the Faraday rotation angle and the magnetic field at different frequencies can be obtained, as shown in Figure 3 (c). This Faraday effect is non-reciprocal, that is, the Faraday rotation angles of linearly polarized light are opposite for positive and negative magnetic fields. Figure 3 (d) shows the magnetic field required to rotate linearly 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 linearly polarized light by +45° and -45°. Therefore, when linearly 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 point the main axis of the 1 / 4 wave plate is 45°, and the outgoing light is 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 rotations 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, the trend is uniform across the entire metasurface. This allows the geometric phase metasurface to respond differently to left-hand and right-hand rotations, thereby deflecting the light beam in different directions. The second layer is similar to the first, except that the first deflects the light beam along the x-axis, while the second deflects it along the y-axis. Between the two layers, we added an additional quarter-wave plate to convert the outgoing circularly polarized light into linearly polarized light, ensuring that the light incident on the second layer is also linearly polarized.

[0041] The deflection mentioned in the present invention means that the terahertz wave energy is mainly concentrated in 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 a periodic metasurface at different diffraction orders. It can be seen that the structure's diffraction efficiency is greater than 0.3 in the 0.4-0.55 THz range, with the primary diffraction order being +1. It's important to note that when the structure is rotated at a 0° angle, the diffraction orders are (0, +1), while when the structure is rotated at a 90° angle, the diffraction orders are (+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 order 0.

[0042] More specifically, Figure 6 When passing through the first layer, 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 point, the outgoing light is left-handed or right-handed circularly polarized. After passing through the intermediate quarter-wave plate, the light is converted back to a linearly polarized state and incident on the second layer. After passing through the second layer, a forward 0.17T magnetic field and a reverse 0.17T magnetic field are applied, and the incident (0, 0) and (0, +1) orders can be adjusted to the (0, 0), (+1, 0) and (0, +1), (+1, +1) orders, respectively. Because the magnetic field can be independently controlled by electromagnets, the overall structure has four outgoing deflection states. In the operating frequency range of 0.4-0.55THz, due to the metasurface period of 810 microns, its deflection angle range can reach 42° to 67.8°.

[0043] It is worth noting that because the magneto-optical material InSb breaks time reversal symmetry when a magnetic field is applied, when light of the same polarization state is reflected back, the beam is isolated. This makes the device non-reciprocal, protecting the stability and signal-to-noise ratio of the system.

Claims

1. A cascaded active magneto-optical terahertz non-reciprocal deflection device, characterized in that: The cascade active magneto-optical terahertz non-reciprocal deflection device comprises a two-layer cascade structure (1), wherein each layer of the structure (1) is composed of a magneto-optical material indium antimonide (2), a dielectric 1 / 4 wave plate (3) and a geometric phase metasurface (4) arranged in sequence; the geometric phase metasurface (4) is composed of a periodic structure made of silicon material, wherein three matrix units constitute one period, and the size of each period is 270 μm. ×710μm, 300μm in height, and 700μm in substrate thickness; the phase of each unit covers a range of 2π; a quarter wave plate (5) is placed between the two layers of the structure (1), and two sets of magnetic fields are applied to each cascade structure (1) respectively through electromagnets; the overall structural plane range of the device is between 1.42cm×1.42cm and 2.84cm×2.84cm; the magneto-optical material indium antimonide (2) needs to be subjected to a low temperature, between 70K-100K, and when a magnetic field is applied, the magneto-optical material indium antimonide (2) has a Faraday effect in the terahertz band, which is manifested in the opposite phase modulation 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°; the dielectric quarter wave plate (3) combined with the magneto-optical material indium antimonide (2) has an operating range of 0.4 THz -0.55THz, its main axis forms 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; each periodic structure of the geometric phase metasurface (4) is composed of three rectangular units with different rotation angles, and the period of each rectangular unit is 270μm, wherein each rectangular unit has different rotation angles and structural parameters, and the two geometric phase metasurfaces (4) diffract left-handed and right-handed circularly polarized light to the (0, +1), (0, 0) orders and the (0, 0), (+1, 0) orders respectively, thereby deflecting the terahertz wave in different directions; the 1 / 4 wave plate (5) located between the two layers of the cascaded structure (1) converts the circularly polarized light into linearly polarized light, and has a high polarization conversion efficiency in the 0.4THz-0.55THz band.

2. A cascaded active magneto-optical terahertz non-reciprocal deflection device according to claim 1, characterized in that The geometric phase metasurface (4) is composed of silicon with high transmittance in the terahertz band, and its refractive index is 3.

4.

3. The cascaded active magneto-optical terahertz non-reciprocal deflection device according to claim 1, characterized in that Two sets of magnetic fields are respectively applied to the two-layer cascade structure (1), and each set of magnetic fields is independently regulated by means of electromagnets, with the magnetic field range being -0.25T-0.25T.

4. The cascaded active magneto-optical terahertz non-reciprocal deflection device according to claim 1, characterized in that The device operates within the range of 0.4THz-0.55THz. By applying different magnetic fields, the polarization state of the light emitted from the indium antimonide is adjusted to the x and y polarization directions, thereby changing the polarization state of the emitted light. By controlling the different combinations of the magnetic fields of the upper and lower cascaded structures (1), the diffraction orders are regulated to four deflection states of (0, 0), (0, +1), (+1, 0) and (+1, +1), and the deflection angle range is 42° to 67.8°.

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