Waveguide integrated dual-side planar nanometer air channel terahertz light frequency mixer

Abstract

The application provides a waveguide integrated double-sided planar nano-air channel terahertz light mixer, which utilizes the ballistic transport characteristics of electrons in the nano-air channel to obtain high-speed response and super-large bandwidth, and can be used to generate millimeter wave terahertz signals; the light mixer comprises a substrate, a photocathode, an ohmic contact electrode, a waveguide, a left anode and a right anode, the waveguide coupling mode is butt coupling or bottom layer evanescent wave coupling, the device adopts a waveguide incidence structure, compared with a traditional surface incidence structure, the quantum efficiency and the response speed are no longer restricted to each other, and has a unique advantage in miniaturization and integration; in addition, the device is composed of double-sided nano-air channels, so that the effective emission length of field photoelectrons can be increased without increasing the device area, and the saturated photocurrent is improved while the high bandwidth is maintained.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor photodetectors and millimeter wave / terahertz devices, and in particular relates to a waveguide-integrated double-sided planar nanometer air channel terahertz optical mixer. Background Art

[0002] With the development of wireless communication technology, millimeter-wave and terahertz technologies have gained widespread application due to their wide spectrum and high transmission rate. Optical mixers are currently a key method and means of generating continuous millimeter-wave and terahertz waves. By converting two beat-frequency optical signals with a certain frequency difference into a continuous coherent electromagnetic wave, they can achieve on-chip integrated millimeter-wave and terahertz sources with narrow linewidth, high coherence, wide-range continuous tunability, low temperature dependence, and no limitations imposed by phase matching and the Manley-Row relation in optical mixing in nonlinear media. Currently, the main types of optical mixers include photoconductive antennas, PIN photodiodes (PIN-PDs), and single-column carrier photodiodes (UTC-PDs). However, because each photon in the device generates at most one electron-hole pair, and because the utilization of photogenerated carriers is sacrificed to achieve wide bandwidth performance, traditional solid-state optoelectronic mixers face technical bottlenecks in achieving high responsivity, wide bandwidth, and high saturation output power simultaneously.

[0003] Ideally, electrons in a photomixer should be transported ballistically at near the speed of light to maximize response speed and bandwidth. However, due to carrier scattering caused by lattice interactions, and the fact that the introduction of an external electric field in solid-state devices to increase electron transport speed also leads to increased scattering and carrier transport saturation, ballistic transport is difficult to achieve in traditional solid-state photodiodes. Vacuum is an ideal medium for electron ballistic transport. The saturated electron transport velocity in a vacuum approaches the speed of light, obeying Newton's second law of motion. This enables faster switching speeds and greater power efficiency. Compared to traditional solid-state semiconductor optoelectronic devices, vacuum optoelectronic devices, due to the lack of lattice collisions and energy dissipation, have low dark current and theoretically offer higher speed, power, and efficiency, and can operate in extremely harsh environments. However, traditional vacuum photonic devices, which rely on the external photoelectric effect to emit electrons from the cathode surface, suffer from issues such as low emission current, high operating voltage, low responsivity and quantum efficiency, and the requirement for a vacuum environment. This poses challenges to device miniaturization and integration.

[0004] In addition to the aforementioned devices, nano-air-channel photodiodes are a new type of optoelectronic device that has emerged in recent years. They combine the advantages of semiconductor and vacuum devices, utilizing a nano-air channel smaller than the mean free path of electron scattering to enable ballistic electron transport. They also offer the advantages of ease of integration and compatibility with existing semiconductor processes. Theoretically, these devices can operate at frequencies exceeding THz and can operate in extremely harsh environments such as strong radiation and high and low temperatures, making them a promising approach to realizing terahertz optical mixers. However, most reported nano-air-channel photodiodes utilize a surface-incident structure, which has certain limitations. For a given absorption coefficient, the device's quantum efficiency is proportional to the thickness of the absorber layer, while the bandwidth is inversely proportional to the thickness. To achieve higher quantum efficiency, increasing the absorber layer's thickness reduces the bandwidth. This interplay of these two constraints makes it difficult for surface-incident devices to simultaneously achieve both high responsivity and wide bandwidth. Furthermore, in surface-incident devices, the effective electron emission region is confined to the edge of the photocathode. The intermediate dielectric region does not contribute to photoelectron emission, but instead increases the device's capacitance and limits the operating bandwidth. Summary of the Invention

[0005] In response to the above-mentioned shortcomings of the existing technology, the present invention proposes a waveguide-integrated double-sided planar nano-air channel terahertz optical mixer, aiming to provide a new type of optical mixer that can operate without a vacuum environment, has a large bandwidth, high responsiveness, high output power, high conversion efficiency, is suitable for large-scale silicon photonic integration processes, and can be mass-produced on large-area wafers.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions:

[0007] A waveguide-integrated double-sided planar nano-air channel terahertz photoelectric mixer, the photomixer comprising a substrate, a photocathode, an ohmic contact electrode, a waveguide, a left anode, and a right anode;

[0008] The waveguide coupling mode of the mixer is butt coupling or bottom evanescent wave coupling;

[0009] When the waveguide coupling mode is butt-jointing coupling, a photocathode, a waveguide, a left anode, and a right anode are provided at different positions on the substrate. In addition to being coupled to the butt-jointing waveguide layer, the photocathode is butt-jointed with an ohmic contact electrode on the other side. The left and right anodes are distributed on both sides of the ohmic contact electrode. Nano-air channels are provided between the edge of the photocathode and the anodes on both sides.

[0010] When the waveguide coupling mode is bottom evanescent wave coupling, there are mesas of different heights on the substrate, the lower one is the first mesa, and the higher one is the second mesa. The first mesa has a waveguide, a left anode, and a right anode, and the second mesa has a photocathode and an ohmic contact electrode. The photocathode is above the waveguide, and the left and right anodes are connected at the end surface of the photocathode. There are nano air channels between the photocathode and the left and right anodes, as well as between the photocathode and the connection between the two anodes.

[0011] Furthermore, the function of the photocathode is to absorb photons to generate electron-hole pairs, and its length is shorter than the waveguide; when a semiconductor film is used as a photocathode, p-type doping is adopted and a negative bias is applied thereto, and when a semiconductor film is used as a photoanode, n-type doping is adopted and a positive bias is applied thereto.

[0012] Furthermore, the photocathode is selected from one of semiconductor thin film, graphene, metal or metalloid.

[0013] Furthermore, the left anode, the right anode and the ohmic contact electrode are made of conductive materials, the left and right anodes are made of light-transmitting or light-opaque materials, and the ohmic contact electrode is made of metal with strong reflectivity.

[0014] Furthermore, due to the presence of the nano-air channel, when the bias voltage applied to the left and right anodes and the ohmic contact electrode is low, a local large electric field is generated in the photocathode, and photogenerated carriers are multiplied in the photocathode, which exponentially increases the photocurrent, response and terahertz output power.

[0015] Furthermore, when the photomixer operates under reverse bias, an inversion layer is generated in the photocathode, forming a two-dimensional electron gas (2DEG) or a two-dimensional hole gas (2DHG). Under the influence of Coulomb repulsion and an external electric field, electrons (holes) achieve zero-barrier emission.

[0016] Furthermore, the waveguide and the photocathode are suitable for both butt coupling and bottom evanescent wave coupling incidence. When bottom evanescent wave coupling is adopted, there is also a nano air channel on the end face of the photocathode, thereby further increasing the effective emission side length.

[0017] The basic working principle of the waveguide-integrated double-sided planar nano-air channel terahertz optical mixer proposed in the present invention is: two beams of mixed light with a frequency difference in the terahertz band are injected from the end face, coupled into the waveguide and continuously transmitted inward, and gradually transmitted to the photocathode and absorbed through the periodic mode coupling between the waveguide and the photocathode; under the action of the mixed light and the external electric field, the photocathode excites photoelectrons, or accumulates a high concentration of holes on its surface, inducing the metals at both ends to generate and emit two-dimensional electron gas. The emitted electrons are ballistically transported at a speed close to the speed of light in the nano-air channel, and finally reach the anode to be received to generate high-frequency photocurrent, which is transmitted through the electrode to a load such as a dipole antenna or a coplanar waveguide to radiate millimeter waves or terahertz waves.

[0018] When a semiconductor film made of p-type doped material is used, the semiconductor film serves as the photocathode, and its contact electrode serves as the negative electrode. When a negative bias is applied to the semiconductor film contact electrode, the semiconductor energy band bends into a reverse depletion state. When the voltage is sufficiently high, a two-dimensional electron gas layer forms near the metal ends of the semiconductor film. Under near-infrared light with a wavelength of 1550nm and a frequency difference in the terahertz band, and under the action of an external electric field, photons are absorbed and generated in the semiconductor depletion region. Under the strong electric field, a photogenerated carrier multiplication effect occurs, multiplying the photogenerated electrons and significantly increasing the two-dimensional electron gas concentration. The Coulomb repulsion between the two-dimensional electron gas and the applied electric field lowers the barrier to electron emission, making it easier for electrons to be emitted into the nano-air channel for ballistic transport. From there, they reach the anode, where they are absorbed and generate a high-frequency current. This high-frequency current is transmitted through the electrodes to a load such as a dipole antenna or coplanar waveguide, radiating a millimeter-wave terahertz signal.

[0019] When using a semiconductor film made of n-type doped materials, the semiconductor film serves as the photoanode, and its contact electrode serves as the positive electrode. When a positive bias is applied to the semiconductor film contact electrode, the semiconductor band bends into a reverse depletion state. When the voltage is sufficiently high, a two-dimensional hole gas layer forms near the metal ends of the semiconductor film. Under near-infrared light with a wavelength of 1550nm and a frequency difference in the terahertz band, and under the action of an external electric field, photons are absorbed and generated in the semiconductor depletion region. Under the strong electric field, a photogenerated carrier multiplication effect occurs, multiplying the number of photogenerated electrons and significantly increasing the concentration of the two-dimensional electron gas. The high concentration of the two-dimensional hole gas induces the generation of a high concentration of two-dimensional electron gas in the metal ends. The Coulomb repulsion between the two-dimensional electron gases and the applied electric field lower the barrier to electron emission, making it easier for electrons to be emitted into the nano-air channel for ballistic transport. There, they are absorbed by the anode and generate a high-frequency current. This high-frequency current is transmitted through the electrodes to a load such as a dipole antenna or coplanar waveguide, radiating a millimeter-wave terahertz signal.

[0020] The beneficial technical effects of the present invention are as follows:

[0021] 1. In the optical mixer proposed by the present invention, the absorption layer can be either a semiconductor or graphene, metal or metalloid (such as TiN), which has a wider range of applications compared to traditional semiconductor optical mixers;

[0022] 2. The optical mixer proposed in this invention is composed of double-sided nano-air channels, which can increase the effective emission side length of photoelectrons (i.e., the amount of field-emitted electrons) without increasing the device area. Since the current size is proportional to the emission side length, the saturation photocurrent is increased.

[0023] 3. The optical mixer proposed in this invention adopts a waveguide-incidence structure. Compared with the traditional surface-incidence structure, quantum efficiency and response speed are no longer mutually restricted. In addition, this structure has unique advantages in device miniaturization and integration.

[0024] 4. The optical mixer proposed in the present invention is suitable for both butt coupling and bottom evanescent wave coupling. When bottom evanescent wave coupling is adopted, there are also nano-air channels on the end face of the absorption layer, thereby further increasing the effective emission side length. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0026] Figure 1 The double-sided planar nano-air channel terahertz optical mixer with a butt-coupled waveguide integrated with a semiconductor film as a photocathode provided in an embodiment of the present invention includes: 11, insulating Si substrate; 12, Si waveguide layer; 13, p-type Ge thin film; 14, left ITO (indium tin oxide) transparent anode; 15, left nano-air channel; 16, cathode Ti / Pt / Au; 17, right nano-air channel; 18, right ITO transparent anode;

[0027] Figure 2 The invention provides a double-sided planar nano-air channel terahertz optical mixer with a butt-coupled waveguide integrated with a semiconductor film as a photoanode: wherein: 21, insulating Si substrate; 22, Si waveguide layer; 23, n-type Ge thin film; 24, left cathode TiAu; 25, left nano-air channel; 26, anode Au; 27, right nano-air channel; 28, right cathode TiAu;

[0028] Figure 3The embodiment of the present invention provides a double-sided planar nano-air channel terahertz optical mixer with an evanescent wave coupled waveguide integrated with a metal-like TiN film as the bottom layer of the photocathode: wherein, 31, insulating quartz substrate; 32, Si3N4 waveguide layer; 33, photocathode TiN film; 34, left anode Ni / Au; 35, left nano-air channel; 36, right nano-air channel; 37, right anode Ni / Au; 38, middle end face nano-air channel;

[0029] Figure 4 The embodiment of the present invention provides a double-sided planar nano-air channel terahertz optical mixer integrated with an underlying evanescent wave coupled waveguide and a semiconductor thin film as a photoanode: wherein 41 is an insulating SiO2 substrate; 42 is a waveguide layer; 43 is an n-type GaAs thin film; 44 is a left cathode Hf / Au; 45 is a left nano-air channel; 46 is an anode Au; 47 is a right nano-air channel; 48 is a right cathode Hf / Au; 49 is a middle end face nano-air channel. DETAILED DESCRIPTION

[0030] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0031] The present invention provides a waveguide-integrated double-sided planar nanometer air channel terahertz optical mixer, the optical mixer comprising a substrate, a photocathode, an ohmic contact electrode, a waveguide, a left anode and a right anode;

[0032] The waveguide coupling mode of the mixer is butt coupling or bottom evanescent wave coupling;

[0033] When the waveguide coupling mode is butt-jointing coupling, a photocathode, a waveguide, a left anode, and a right anode are provided at different positions on the substrate. In addition to being coupled to the butt-jointing waveguide layer, the photocathode is butt-jointed with an ohmic contact electrode on the other side. The left and right anodes are distributed on both sides of the ohmic contact electrode. Nano-air channels are provided between the edge of the photocathode and the anodes on both sides.

[0034] When the waveguide coupling mode is bottom evanescent wave coupling, there are mesas of different heights on the substrate, the lower one is the first mesa, and the higher one is the second mesa. The first mesa has a waveguide, a left anode, and a right anode, and the second mesa has a photocathode and an ohmic contact electrode. The photocathode is above the waveguide, and the left and right anodes are connected at the end surface of the photocathode. There are nano air channels between the photocathode and the left and right anodes, as well as between the photocathode and the connection between the two anodes.

[0035] The following is a detailed description of the technical solutions for the butt-coupled waveguide integration of semiconductor thin films as photocathodes, the butt-coupled waveguide integration of semiconductor thin films as photoanodes, the bottom-layer evanescent wave coupled waveguide integration of metal-like TiN thin films as photocathodes, and the bottom-layer evanescent wave coupled waveguide integration of semiconductor thin films as photoanodes.

[0036] Example 1

[0037] A double-sided planar nano-air channel terahertz optical mixer with a semiconductor film as a photocathode and a butt-coupled waveguide integrated circuit. Figure 1 As shown, the substrate in this embodiment is an insulating Si substrate, and a 300nm P-type Ge film is placed above the substrate; ITO transparent anodes are provided on the upper and lower sides of the Ge film, a 300nm thick cathode Ti / Pt / Au is provided on the left, and a 300nm thick Si waveguide is provided on the right; there are nano-air channels between the Ge film and the left and right anodes, and the size of the nano-air channels is greater than 0nm and less than 1000nm.

[0038] The working process of the optical mixer is as follows: a negative bias is applied to the photocathode, and the p-type Ge film generates a depletion region under the reverse bias, and an inversion layer is generated near its surface to form 2DEG; two beams of near-infrared light with a wavelength near 1550nm and a wavelength difference in the terahertz band are coupled and transmitted to the absorption layer through the Si waveguide. The photogenerated electrons in the depletion region of the Ge film drift to the 2DEG layer under the action of a large electric field to increase its concentration, and ionization collisions and multiplication occur in the process, and the Coulomb repulsion between the electrons also increases; the electrons at the edge of the 2DEG are repelled by the internal electron Coulomb repulsion and the yin and yang Under the action of the large electric field between the electrodes, electrons at other locations are directly emitted into the nano-air channel under the large electric field and quickly cross to the anode to be collected, forming a photocurrent. This photocurrent is modulated by the superimposed light field of the two near-infrared beams. Its oscillation frequency is equal to the frequency difference of the two incident near-infrared beams, and the changes in amplitude and phase are also synchronized with the amplitude and phase of the superimposed light field of the two near-infrared beams. The high-frequency photocurrent is transmitted through the electrodes to a load such as a dipole antenna or coplanar waveguide, radiating terahertz waves, thereby realizing the generation of optical mixing terahertz signals. In this embodiment, the 3dB bandwidth limited by the carrier transit time can reach 2THz.

[0039] Example 2:

[0040] A double-sided planar nano-air channel terahertz optical mixer with a semiconductor film as a photoanode and a butt-coupled waveguide integration, such as Figure 2As shown, the substrate in this embodiment is an insulating Si substrate, and above the substrate is a 300nm n-type Ge film; TiAu cathodes are provided on the upper and lower sides of the Ge film, a 300nm thick Au anode is provided on the left, and a 300nm thick Si waveguide is provided on the right; there are nano-air channels between the Ge film and the left and right cathodes, and the size of the nano-air channels is greater than 0nm and less than 1000nm.

[0041] The working process of the optical mixer is as follows: a positive bias is applied to the ohmic contact electrode of the n-type Ge film, and the n-type Ge generates a depletion region under reverse bias, and an inversion layer is generated near its upper surface to form 2DHG; two beams of near-infrared light with a wavelength near 1550nm and a wavelength difference in the terahertz band are coupled and transmitted to the absorption layer through the Si waveguide. The photogenerated holes in the depletion region drift to the 2DHG layer under the action of a large electric field to increase their concentration, and ionization collisions and multiplication occur in this process. At the same time, 2DEG is induced on the lower surface of the TiAu cathode on the other side of the nano-insulating layer and the air channel, and the Coulomb repulsion between the electrons therein also increases accordingly; 2DE Under the action of the internal electron Coulomb repulsion and the large electric field between the cathode and anode, the electrons at the edge of G are emitted across the surface potential barrier into the nano-air channel. Surface electrons at other positions are directly field-emitted into the nano-air channel under the large electric field and quickly cross to the photoanode to be collected, forming a photocurrent. This photocurrent is modulated by the superimposed light field of two near-infrared beams. Its oscillation frequency is equal to the frequency difference of the two incident near-infrared beams, and the changes in amplitude and phase are also synchronized with the amplitude and phase of the superimposed light field of the two near-infrared beams. The high-frequency photocurrent is transmitted through the electrode to a load such as a dipole antenna or a coplanar waveguide to radiate millimeter waves or terahertz waves, thereby realizing the generation of optical mixing millimeter wave signals. In this embodiment, the 3dB bandwidth limited by the carrier transit time can reach 225GHz.

[0042] Example 3:

[0043] A double-sided planar nano-air channel terahertz optical mixer with a bottom layer evanescent wave coupled waveguide integrated with a metal-like TiN film as the photocathode, such as Figure 3 As shown, the substrate in this embodiment is an insulating quartz substrate, and on the substrate are a 100nm thick Si3N4 waveguide and a 200nm thick Ni / Au anode. On the Si3N4 waveguide is a 100nm thick photocathode TiN film. When the TiN film is used as a photocathode, no additional ohmic contact electrode is required; the left and right Ni / Au anodes are connected above the Si3N4 waveguide, and there are nano-air channels with a spacing of 20nm between the TiN film and the left and right Ti / Au anodes, as well as between the TiN film and the two anodes.

[0044] The working process of the optical mixer is as follows: a negative bias is applied to the photocathode. Under the illumination of two near-infrared light beams with a wavelength difference near 1550nm and in the terahertz band, the Si3N4 waveguide couples and transfers light to the TiN film for absorption. The photogenerated electrons on the TiN side surface overcome the surface potential barrier under the action of the large electric field, are field-emitted into the nano-air channel, and quickly cross laterally to reach the anode for collection, forming a photocurrent. The photocurrent is modulated by the superimposed light field of the two near-infrared light beams, and its oscillation frequency is equal to the frequency difference of the two incident near-infrared light beams. The changes in amplitude and phase are also synchronized with the amplitude and phase of the superimposed light field of the two near-infrared light beams. The high-frequency photocurrent is transmitted through the electrode to a load such as a dipole antenna or a coplanar waveguide to radiate millimeter waves or terahertz waves, thereby realizing the generation of optical mixing millimeter waves and terahertz waves.

[0045] Example 4:

[0046] A double-sided planar nano-air channel terahertz optical mixer with a bottom layer evanescent wave coupled waveguide integrated with a semiconductor film as a photoanode, such as Figure 4 As shown, the substrate in this embodiment is an insulating SiO2 substrate, on top of which are a 100nm thick waveguide, a 200nm thick left and right cathodes composed of Hf / Au, and a 200nm thick anode Au. On the waveguide is a photoanode composed of a 100nm thick n-type GaAs film; the left and right Hf / Au cathodes are connected above the waveguide layer, and there are nano-air channels with a spacing of 10nm between the GaAs film and the left and right Hf / Au cathodes, as well as between the GaAs film and the two cathodes.

[0047] The working process of the optical mixer is as follows: a positive bias is applied to the ohmic contact electrode of the n-type GaAs thin film. Under the illumination of two near-infrared light beams with a wavelength difference near 850nm and in the terahertz band, the waveguide couples and transfers light to the GaAs thin film, where it is absorbed and a large number of photogenerated holes are generated and accumulated on the GaAs side surface. This induces the generation of a large number of electrons on the Hf / Au cathode side surface opposite the nano-air channel. Under the action of the large electric field, the electrons on the Hf / Au cathode side surface overcome the surface potential barrier and are field-emitted into the nano-air channel. They then rapidly cross laterally to the GaAs photoanode and are collected, forming a photocurrent. This photocurrent is modulated by the superimposed optical field of the two near-infrared light beams, with its oscillation frequency equal to the frequency difference between the two incident near-infrared light beams. The changes in amplitude and phase are also synchronized with the amplitude and phase of the superimposed optical field of the two near-infrared light beams. The high-frequency photocurrent is transmitted through the electrode to a load such as a dipole antenna or coplanar waveguide to radiate millimeter waves or terahertz waves, thereby realizing the generation of optical mixing millimeter wave and terahertz signals. In this embodiment, the 3dB bandwidth limited by the carrier transit time can reach 20THz.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A waveguide-integrated double-sided planar nano-air channel terahertz optical mixer, characterized in that: The optical mixer includes a substrate, a photocathode, an ohmic contact electrode, a waveguide, a left anode and a right anode; The waveguide coupling mode of the mixer is butt coupling or bottom evanescent wave coupling; When the waveguide coupling mode is butt-jointing coupling, a photocathode, a waveguide, a left anode, and a right anode are provided at different positions on the substrate. In addition to being coupled to the butt-jointing waveguide layer, the photocathode is butt-jointed with an ohmic contact electrode on the other side. The left anode and the right anode are distributed on both sides of the ohmic contact electrode. Nano-air channels are provided between the edge of the photocathode and the anodes on both sides. When the waveguide coupling mode is bottom evanescent wave coupling, there are mesas of different heights on the substrate, the lower one is the first mesa, and the higher one is the second mesa. The first mesa has a waveguide, a left anode, and a right anode. The second mesa has a photocathode and an ohmic contact electrode. The photocathode is above the waveguide, and the left anode and the right anode are connected at the end surface of the photocathode. There are nano air channels between the photocathode and the left anode and the right anode, as well as between the photocathode and the connection between the two anodes.

2. The optical mixer according to claim 1, wherein The function of the photocathode is to absorb photons to generate electron-hole pairs, and its length is shorter than the waveguide; when a semiconductor film is used as a photocathode, p-type doping is adopted and a negative bias is applied thereto; when a semiconductor film is used as a photoanode, n-type doping is adopted and a positive bias is applied thereto.

3. The optical mixer according to claim 1, wherein Semiconductor thin film is used as the photocathode.

4. The optical mixer according to claim 1, wherein The left anode, the right anode and the ohmic contact electrode are made of conductive materials. The left anode and the right anode are made of light-transmitting or light-impermeable materials, and the ohmic contact electrode is made of metal with strong reflectivity.

5. The optical mixer according to claim 1, wherein: Due to the existence of the nano-air channel, when the bias voltage applied to the left anode, right anode and ohmic contact electrode is low, a local large electric field is generated in the photocathode, and photogenerated carriers are multiplied in the photocathode, which exponentially increases the photocurrent, response and terahertz output power.

6. The optical mixer according to claim 1, wherein: When the photomixer operates under reverse bias, an inversion layer is generated in the photocathode, forming a two-dimensional electron gas or a two-dimensional hole gas, thereby achieving zero-barrier electron emission.

7. The optical mixer according to claim 1, wherein: The waveguide and the photocathode are suitable for both butt coupling and bottom evanescent wave coupling incidence. When bottom evanescent wave coupling is adopted, there is also a nano air channel on the end face of the photocathode, thereby further increasing the effective emission side length.

8. The optical mixer according to claim 1, wherein The working principle of the optical mixer is as follows: two mixed light beams with a frequency difference in the terahertz band are injected from the end face, coupled into the waveguide and continuously transmitted inward, and gradually transmitted to the photocathode through the periodic mode coupling between the waveguide and the photocathode and absorbed; Under the action of mixed light and an external electric field, the photocathode excites photoelectrons or accumulates a high concentration of holes on its surface, inducing the metals at both ends to produce and emit two-dimensional electron gas. The emitted electrons are ballistically transported at a speed close to the speed of light in the nano-air channel, and eventually reach the anode to be received to generate high-frequency photocurrent. The high-frequency photocurrent is transmitted through the electrode to the dipole antenna or coplanar waveguide to radiate millimeter waves or terahertz waves.

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