A metamaterial terahertz modulator based on negative differential resistance effect

Abstract

The invention provides a metamaterial terahertz modulator based on negative differential resistance effect, belonging to the field of terahertz technology. The structure of the modulator comprises a metamaterial layer, a substrate layer, a metal plate and a metal electrode; the metamaterial layer is composed of a periodic array structure, the structural unit is electrically connected to a metal electrode through a gold trace, the substrate layer is a III-V semiconductor with negative differential resistance effect, and the metal plate serves as another metal electrode; by applying a high bias voltage to the two electrodes, a strong longitudinal electric field is generated inside the semiconductor substrate, causing the negative differential resistance effect of the semiconductor material, changing the material properties of the semiconductor substrate, thereby modulating the resonant frequency and resonance strength of the device, forming a voltage-adjustable active terahertz metamaterial device, and providing a new idea for realizing an active metamaterial terahertz device.

Description

Technical Field

[0001] The invention relates to a metamaterial terahertz modulator based on negative differential resistance effect, belonging to the technical field of terahertz device manufacturing. Background Art

[0002] Terahertz waves are electromagnetic waves with a frequency between 0.1-10THz, which are between the microwave and infrared bands in the electromagnetic spectrum. Compared with microwave technology, terahertz technology has higher resolution, and compared with light wave technology, terahertz waves have lower energy and stronger penetration ability, and can be widely used in radar, security inspection, imaging and other fields. However, because many natural materials have very weak electromagnetic responses to terahertz waves, the development of terahertz technology has been subject to certain restrictions. Metamaterials, as an artificial electromagnetic material, are composed of periodically or non-periodically arranged metal structures. They can effectively control the amplitude, phase, polarization, propagation mode, etc. of terahertz waves, and can realize many terahertz functional devices, such as absorbers, optical switches, polarization devices, etc.

[0003] At present, many studies at home and abroad are focused on actively adjustable terahertz metamaterial devices. There are generally three ways to realize actively adjustable terahertz metamaterial devices. The first is to integrate some nonlinear elements (such as Gunn diodes, Schottky diodes, varactor diodes) or some nonlinear materials at the opening of the open resonant ring. The second is a mechanically reconfigurable metamaterial based on micro-nano electromechanical systems, which changes the shape or arrangement of structural units through mechanical control. The third is to make materials whose dielectric properties are adjustable under the action of external forces (such as light, electricity, heat, etc.), such as semiconductors, liquid crystals, ferroelectric materials, phase change materials, etc., into metamaterials or metamaterial substrates.

[0004] As mentioned above, there are various ways to realize actively adjustable metamaterial devices, but so far, there has been no use of the negative differential resistance effect of III-V semiconductors (such as GaAs, GaN, InP, etc.) to achieve dynamic electrical regulation of the electromagnetic properties of metamaterial devices. Summary of the invention

[0005] Technical problem: The present invention provides a metamaterial terahertz modulator based on negative differential resistance effect. By applying a high bias longitudinal voltage to a III-V semiconductor substrate (equivalent to forming a Gunn diode), a negative differential resistance effect is generated inside the semiconductor. Due to the electron transfer mechanism, the carrier concentration, mobility, and effective mass distribution in the substrate will change, thereby changing the resonant characteristics of the device, forming a voltage-adjustable active terahertz metamaterial device.

[0006] Technical solution: The metamaterial terahertz modulator proposed in the present invention utilizes the negative differential resistance effect of III-V semiconductors to modulate the absorption characteristics of the device for the first time, providing a new idea for realizing active metamaterial terahertz devices.

[0007] A metamaterial terahertz modulator based on negative differential resistance effect of the present invention comprises a metamaterial layer, a substrate layer, a metal plate and a metal electrode; a first highly doped semiconductor layer is disposed below the substrate layer, a metal plate is disposed below the first highly doped semiconductor layer, a second highly doped semiconductor layer is disposed above the substrate layer, and a metal electrode and a metamaterial layer including structural units and metal wiring are disposed above the second highly doped semiconductor layer.

[0008] The metamaterial layer includes periodically arranged structural units, each structural unit is longitudinally connected by metal wiring and is electrically connected to a metal electrode by the metal wiring.

[0009] The structural unit is one of a square structure, an open resonant ring or a ring structure.

[0010] The substrate layer is a III-V group semiconductor, which is a semiconductor material with a negative differential resistance effect.

[0011] The metal plate serves as an electrode below the substrate layer.

[0012] The material of the metamaterial layer, the metal plate and the metal electrode is one of gold, silver, copper or aluminum.

[0013] A second highly doped semiconductor layer is provided between the metal electrode and the structural unit and the substrate layer, and a first highly doped semiconductor layer is provided between the metal plate and the substrate layer, so that the electrode, the metamaterial layer, the metal plate and the substrate layer form ohmic contact.

[0014] The semiconductor material having a negative differential resistance effect is one of gallium arsenide, gallium nitride or indium phosphide.

[0015] The III-V semiconductor material utilizes the negative differential resistance effect of the III-V semiconductor to modulate the absorption characteristics of the device. A high bias voltage is applied to the substrate layer through electrodes and metal plates. At this time, the electrodes, metal plates, metamaterial structural units and the substrate layer form a Gunn diode, generating a negative differential resistance effect inside the semiconductor. Due to the electron transfer mechanism, the distribution of carrier concentration, mobility and effective mass in the substrate layer will change, thereby changing the resonance characteristics of the device.

[0016] The working mode of the Gunn diode is one of an ideal mode, a dipole domain transit time mode, an electron accumulation domain mode, a confined space charge accumulation mode or a quenched dipole layer mode.

[0017] Beneficial effects: The metamaterial terahertz modulator proposed in the present invention uses the negative differential resistance effect of III-V semiconductors to modulate the absorption characteristics of the device for the first time, providing a new idea for realizing active metamaterial terahertz devices. In addition, compared with dynamic regulation using light, heat, magnetism and other methods, modulation using voltage is easier to achieve, does not require complex equipment and experimental conditions, and is more convenient to use. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A three-dimensional schematic diagram of the structure of the metamaterial terahertz modulator based on the negative differential resistance effect of the present invention;

[0019] Figure 2 It is a top two-dimensional schematic diagram of the structure of the metamaterial terahertz modulator based on the negative differential resistance effect of the present invention;

[0020] Figure 3 It is a bottom two-dimensional schematic diagram of the structure of the metamaterial terahertz modulator based on the negative differential resistance effect of the present invention;

[0021] Figure 4 A schematic diagram of a highly doped gallium arsenide layer of a metamaterial terahertz modulator structure based on a negative differential resistance effect according to the present invention;

[0022] Figure 5 It is the absorption characteristic curve of the device under different voltages in the ideal mode;

[0023] Figure 6 This is the oscillating current waveform in the electron accumulation domain mode;

[0024] Figure 7 This is the absorption characteristic curve of the device in the electron accumulation domain mode.

[0025] The figure includes: a metal electrode 1 , a structural unit 2 , a metal trace 3 , a substrate layer 4 , a metal plate 5 , a first highly doped semiconductor layer 6 , and a second highly doped semiconductor layer 7 . DETAILED DESCRIPTION

[0026] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

[0027] Embodiment 1:

[0028] like Figure 1 As shown in FIG. 1 , the metamaterial terahertz modulator based on negative differential resistance effect of the present invention is composed of a metamaterial layer, a substrate layer, a metal plate and a metal electrode. Figure 2 As shown, the metamaterial layer is composed of periodically arranged structural units, and each structural unit is electrically connected to the metal electrode 1 through a metal trace. The substrate layer is a semiconductor material with a negative differential resistance effect. Figure 3 As shown, the metal plate forms another metal electrode. Figure 4 As shown in the grid pattern area, there is a highly doped semiconductor layer on the patterns corresponding to the electrode 1, the metal plate 5 and the metamaterial, so that the electrode 1, the metal plate 5, the metamaterial and the substrate layer form an ohmic contact.

[0029] The structural unit of the metamaterial layer can adopt a square structure, an open resonant ring, a ring structure, etc. In this embodiment, a square structure is preferred.

[0030] The material of the metamaterial layer and the metal plate can be gold, silver, copper, or aluminum, and gold is preferred in this embodiment.

[0031] In this embodiment, the metamaterial structure unit period is preferably 50 μm, the side length is 32 μm, the thickness is 150 nm, the substrate layer thickness is preferably 8 μm, and the substrate layer doping concentration is preferably 1×10 16 cm -3 , the preferred metal plate thickness is 2um.

[0032] Tuning the material properties of the semiconductor substrate is a means of dynamically controlling the electromagnetic response of the metamaterial. In this embodiment, the substrate layer adopts a III-V semiconductor, and the material properties of the semiconductor are modulated based on the negative differential resistance effect. The semiconductor material is one of gallium arsenide, gallium nitride, and indium phosphide. In this embodiment, gallium arsenide is preferred. A high bias voltage is applied to the gallium arsenide substrate through two electrodes, and the metal electrode 1 serves as the anode and the metal plate 5 serves as the cathode. At this time, the metal electrode 1, the metamaterial, the metal plate 5 and the gallium arsenide substrate form a Gunn diode. The Gunn diode based on the negative differential resistance effect has five operating modes: ideal mode, dipole domain transit time mode, electron accumulation domain mode, and electron accumulation domain mode. In this embodiment, it is assumed that there are no defects or unevenness inside the semiconductor, and an ideal mode will be generated at this time; in the ideal mode, the high electric field is evenly distributed throughout the semiconductor, so it can be considered that the movement of carriers everywhere inside the semiconductor is the same, and the material properties everywhere inside the semiconductor are also the same, and no oscillating current will be generated; in this ideal mode, the electrons in the central valley of the semiconductor get enough energy to jump to the satellite valley, which will cause the electron mobility of the semiconductor material to become smaller, the effective mass of the electron to become larger, and the real part of the dielectric constant to increase, thereby causing the resonant frequency of the device to redshift. The change in the resonance intensity of the device is mainly affected by the degree of impedance matching. The properties of the semiconductor material change with the change of voltage, which will affect the equivalent impedance of the device.

[0033] like Figure 5 The absorption characteristic curve of the device under different voltages is shown in Figure 2. 0 =0 <V 1 <V 2 <V 3<V 4 As can be seen from the curve, as the voltage continues to increase, the resonant frequency of the device continues to decrease, and the absorption intensity of the device first increases and then decreases, but compared with the device without voltage, the absorption intensity always increases. The maximum frequency modulation range of the first peak is 0.18THz, and the maximum frequency modulation range of the second peak is 0.06THz.

[0034] Therefore, in an ideal mode, the resonant frequency and resonant amplitude of the metamaterial device can be controlled in a time-invariant manner by applying longitudinal voltages of different magnitudes. However, this mode has very high requirements for the contact between the semiconductor material and the electrode, so it is difficult to achieve. In practice, this device is more likely to produce an electron accumulation domain mode.

[0035] Embodiment 2:

[0036] Compared with Example 1, this embodiment has the following changes:

[0037] In actual situations, there are always various defects and impurities inside semiconductor samples, and charge domains are generally generated near the cathode due to fluctuations in carrier concentration and other reasons. The device described in the present invention is more likely to generate an electron accumulation domain mode. In the electron accumulation domain mode, the electron accumulation domain will be generated from the cathode, slowly grow and move toward the anode, and finally be absorbed by the anode, and then continue to generate electron domains at the cathode, and so on and so forth, generating a corresponding oscillating current. The current oscillation period of the preferred device of the present invention is 14ps, such as Figure 6 Shown is the oscillating current waveform.

[0038] The electron concentration in the electron domain increases, and the electron mobility decreases; the electron concentration outside the electron domain remains unchanged, and its mobility distribution is relatively uniform, but its mobility is also reduced compared to the device without voltage. Therefore, compared to semiconductor materials without electric field, the generation of electron domains will affect the parameters such as electron mobility and electron concentration of the semiconductor, thereby further affecting the real and imaginary parts of the dielectric constant of the material. In addition, after the electron domain is generated, it slowly grows and moves toward the anode until it is absorbed by the anode. This process is dynamic and changes with time. During this period, the material parameters of the semiconductor are also constantly changing, so the absorption rate of the device should also be time-varying.

[0039] Figure 7The absorption spectrum of the device at different time points in a cycle is shown. It can be found that the resonance absorption intensity of the device increases after the voltage is applied, and the resonance absorption point has red-shifted, which is mainly affected by the properties of the semiconductor material. The resonance point of the first peak red-shifted by 134.08GHz, and the absorption intensity increased by 5.84%. The resonance point of the second peak red-shifted by 47.82GHz, and the absorption intensity increased by 5.58%. The absorption rate of the device is time-varying, but this change is very weak and almost negligible. General equipment cannot achieve such a high resolution, so it can also be used as a steady-state regulation method.

[0040] The embodiments are preferred implementations of the present invention, but the present invention is not limited to the above-mentioned implementations. Any obvious improvements, substitutions or modifications that can be made by those skilled in the art without departing from the essential content of the present invention belong to the protection scope of the present invention.

Claims

1. A metamaterial terahertz modulator based on negative differential resistance effect, characterized in that: The terahertz modulator comprises a metamaterial layer, a substrate layer, a metal plate and a metal electrode; below the substrate layer (4) is a first highly doped semiconductor layer (6), below the first highly doped semiconductor layer (6) is a metal plate (5), above the substrate layer (4) is a second highly doped semiconductor layer (7), above the second highly doped semiconductor layer (7) is a metal electrode (1) and the metamaterial layer comprises a structural unit (2) and a metal wiring (3); The metamaterial layer comprises periodically arranged structural units (2), each structural unit (2) being longitudinally connected via a metal wiring (3) and electrically connected to a metal electrode (1) via the metal wiring (3); The substrate layer (4) is a III-V group semiconductor, which is a semiconductor material having a negative differential resistance effect.

2. The metamaterial terahertz modulator based on negative differential resistance effect according to claim 1, characterized in that: The structural unit (2) is one of a square structure, an open resonant ring or a ring structure.

3. The metamaterial terahertz modulator based on negative differential resistance effect according to claim 1, characterized in that: The metal plate (5) serves as an electrode below the substrate layer (4).

4. The metamaterial terahertz modulator based on negative differential resistance effect according to claim 1, characterized in that: The material of the metamaterial layer, the metal plate (5) and the metal electrode (1) is one of gold, silver, copper or aluminum.

5. The metamaterial terahertz modulator based on negative differential resistance effect according to claim 1, characterized in that: A second highly doped semiconductor layer (7) is provided between the metal electrode (1) and the structural unit (2) and the substrate layer (4), and a first highly doped semiconductor layer (6) is provided between the metal plate (5) and the substrate layer (4), so that the electrode (1), the metamaterial layer, the metal plate (5) and the substrate layer (4) form ohmic contact.

6. The metamaterial terahertz modulator based on negative differential resistance effect according to claim 1, characterized in that: The semiconductor material having a negative differential resistance effect is one of gallium arsenide, gallium nitride or indium phosphide.

7. The metamaterial terahertz modulator based on negative differential resistance effect according to claim 1, characterized in that: The III-V semiconductor material utilizes the negative differential resistance effect of the III-V semiconductor to modulate the absorption characteristics of the device. A high bias voltage is applied to the substrate layer (4) through the electrode (1) and the metal plate (5). At this time, the electrode (1), the metal plate (5), the metamaterial structure unit (2) and the substrate layer form a Gunn diode, generating a negative differential resistance effect inside the semiconductor. Due to the electron transfer mechanism, the distribution of carrier concentration, mobility and effective mass in the substrate layer will change, thereby changing the resonance characteristics of the device.

8. The metamaterial terahertz modulator based on negative differential resistance effect according to claim 7, characterized in that: The working mode of the Gunn diode is one of an ideal mode, a dipole domain transit time mode, an electron accumulation domain mode, a confined space charge accumulation mode or a quenched dipole layer mode.

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