Multi-junction electric field enhanced unbiased photoconductive terahertz source and terahertz emitter

Abstract

The invention provides a multi-junction electric field enhanced unbiased photoconductive terahertz source and a terahertz emitter, and the terahertz source comprises a substrate, and a distributed Bragg reflector, a photoconductive layer and a metal grating electrode which are sequentially arranged on the substrate from bottom to top. The metal grating electrode, the photoconductive layer and the distributed Bragg reflector jointly form a plasmon resonant cavity which is used for absorbing incident light and generating photon-generated carriers; the photoconductive layer comprises a plurality of semiconductor layers which are sequentially arranged along the thickness direction, and is used for constructing a superposed multi-junction built-in electric field in the photoconductive layer; the multi-junction built-in electric field photon-generated carriers are used for driving the photon-generated carriers to move directionally to generate light current, and the light current is finally fed into an antenna structure to generate terahertz waves; on the basis of enhancing semiconductor absorption, through regulation and control of a built-in electric field and improvement of light field-electric field overlapping, light-terahertz conversion efficiency is remarkably improved finally.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic terahertz technology, and more specifically, to a multi-junction electric field enhanced unbiased photoconductive terahertz source and terahertz emitter. Background Technology

[0002] Photoconductive terahertz sources have broad application prospects in THz imaging, communication, and sensing. By irradiating the semiconductor gap between electrodes with a femtosecond laser pulse of a suitable wavelength, and under the influence of a bias electric field applied between the two electrodes, transient photogenerated carriers generated in the semiconductor are accelerated under the influence of the electric field, thus generating a THz-varying current, which can be transmitted through antenna radiation or waveguides. Photoconductive terahertz sources typically use low-temperature GaAs material with an excitation wavelength of 800 nm. In contrast, In... 0.53 Ga 0.47 Photoconductive terahertz sources with an excitation wavelength of 1550 nm, represented by As, serve as a link between terahertz technology and communication technology. They also have the advantages of low cost and high integration, making them a future development direction.

[0003] To achieve high output power, photoconductive devices typically require a high DC bias voltage. However, In 0.53 Ga 0.47 As materials inherently possess low dark resistivity, which generates significant dark current under an applied bias voltage. This not only leads to Joule heating but may also cause thermal breakdown of the device. Furthermore, the presence of an external bias voltage increases the complexity and power consumption of the device. Therefore, unbiased photoconductive terahertz sources have gradually attracted the attention of researchers. Unbiased photoconductive devices rely on a built-in electric field to achieve efficient carrier separation, thereby eliminating the need for an applied bias voltage. Compared with traditional photoconductive terahertz sources, they have the following advantages: (1) The unbiased design eliminates dark current and reduces Joule heating, thereby improving device stability and reliability; (2) Under an external bias voltage, a short circuit in a single micro-nano gap in the electrode array can cause the entire device to fail, while the unbiased structure can still work even with a local short circuit, avoiding the risk of short circuits between electrodes; (3) It has higher integration and practicality without the need for complex bias circuits.

[0004] Currently, conventional unbiased photoconductive terahertz sources suffer from the following problems: First, the low output power of photoconductive devices is due to insufficient efficiency in coupling incident light into the semiconductor material and low carrier transport efficiency. Second, although plasmonic electrode designs can enhance optical field localization and optical coupling efficiency, thereby increasing output power, their built-in electric field is usually weak and covers a small area. Furthermore, the light field may penetrate deep into the semiconductor, causing some photogenerated carriers far from the electrodes to be unable to be effectively accelerated by the electric field. Therefore, the overall photoelectric conversion efficiency of unbiased terahertz sources remains limited and needs further improvement. Summary of the Invention

[0005] To overcome the shortcomings of the low overall photoelectric conversion efficiency of unbiased terahertz sources in the prior art, this invention provides a multi-junction electric field enhanced unbiased photoconductive terahertz source and terahertz emitter. Based on enhancing semiconductor absorption, it significantly improves the light-to-terahertz conversion efficiency by regulating the built-in electric field and enhancing the overlap of the light field and electric field.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: A multi-junction electric field-enhanced unbiased photoconductive terahertz source includes: a substrate, and a distributed Bragg reflector, a photoconductive layer, and a metal grating electrode disposed sequentially from bottom to top on the substrate; The metal grating electrode, the photoconductive layer, and the distributed Bragg reflector together constitute a plasmonic resonant cavity, which is used to absorb incident light and generate photogenerated carriers. The photoconductive layer includes a plurality of semiconductor layers arranged sequentially along the thickness direction, which are used to construct a superimposed multi-junction built-in electric field within the photoconductive layer. The built-in electric field of the multi-junction is used to drive the directional movement of the photogenerated carriers to generate photocurrent.

[0007] Preferably, the photoconductive layer includes a first semiconductor layer, a second semiconductor layer, and a third semiconductor layer disposed sequentially from bottom to top along the thickness direction; The upper surface of the first semiconductor layer contacts the metal grating electrode to form a Schottky junction, generating a first built-in electric field. The upper surface of the second semiconductor layer contacts the lower surface of the first semiconductor layer to form a homojunction, generating a second built-in electric field; The upper surface of the third semiconductor layer contacts the lower surface of the second semiconductor layer to form a heterojunction and generate a third built-in electric field. The lower surface of the third semiconductor layer is in contact with the distributed Bragg reflector; The first, second, and third built-in electric fields are aligned in direction and together constitute the superimposed multi-junction built-in electric field.

[0008] Preferably, the material of the first semiconductor layer is p-type doped InGaAs; the material of the second semiconductor layer is p+ type doped InGaAs; and the material of the third semiconductor layer is p+ type doped InP.

[0009] Preferably, the doping concentration of the p+ type doped InGaAs is in the range of 10. 16 ~10 21 cm -3 The p+ type doped InP has a doping concentration of at least 10.21 cm -3 .

[0010] Preferably, the formation position of the homojunction is changed by altering the thickness of the first semiconductor layer and / or the second semiconductor layer, thereby expanding the overlap rate between the light absorption region and the built-in electric field of the multijunction and increasing the amplitude of the photocurrent.

[0011] Preferably, the distributed Bragg reflector adopts a periodic stacked structure of InGaAsP and InP, wherein the start and end layers of the periodic stacked structure are both made of InGaAsP, and the number of InGaAsP and InP pairs is not less than 40.

[0012] Preferably, the material of the metal grating electrode is Au, Ti / Au, or Cr / Au.

[0013] Preferably, the substrate is a SI-InP substrate.

[0014] Preferably, the structure of the terahertz source further includes a buffer layer disposed between the substrate and the distributed Bragg reflector.

[0015] The present invention also provides a multi-junction electric field enhanced unbiased photoconductive terahertz transmitter, comprising: the above-mentioned terahertz source, and an antenna structure; The terahertz source is irradiated by pulsed laser or by continuous optical beat frequency irradiation. The resulting transient or continuously changing photocurrent is fed into the antenna structure to generate, radiate or transmit terahertz waves.

[0016] Compared with the prior art, the beneficial effects of the technical solution of the present invention are: This invention provides a multi-junction electric field enhanced unbiased photoconductive terahertz source and terahertz emitter. A metal grating electrode, a photoconductive layer, and a distributed Bragg reflector together constitute a plasmonic resonant cavity for absorbing incident light and generating photogenerated carriers. The photoconductive layer includes multiple semiconductor layers arranged sequentially along the thickness direction to construct a superimposed multi-junction built-in electric field within the photoconductive layer. The photogenerated carriers in the multi-junction built-in electric field are used to drive the directional movement of the photogenerated carriers, generating transient or continuously changing photocurrents. Finally, the photocurrents are fed into an antenna or transmission line structure to radiate or transmit terahertz waves. This invention, based on enhanced semiconductor absorption, significantly improves the light-to-terahertz conversion efficiency by modulating the built-in electric field and enhancing the overlap of the light and electric fields. Specifically, by constructing a plasmonic resonant cavity coupled with a metal grating and a digital beamforming circuit (DBR), the light absorption efficiency is increased to 86%. Furthermore, by forming Schottky junctions, p-p+ homojunctions, and heterojunctions within this resonant cavity, a larger self-driving electric field is provided for the photogenerated carriers throughout the absorption layer. Compared to conventional photoconductive terahertz radiation sources, the average built-in electric field within the absorption layer is increased by 6.8 times, thereby enabling carrier drift towards the grating electrode and promoting rapid carrier transport to the nanoelectrode within a sub-picosecond timescale, thus improving the conversion efficiency from input optical power to photocurrent. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of a multi-junction electric field enhanced unbiased photoconductive terahertz source structure provided in Example 1.

[0018] Figure 2 This is a comparison diagram of the unbiased plasmonic radiation source without a resonant cavity and the unbiased plasmonic radiation source with a resonant cavity provided in Example 2.

[0019] Figure 3 This is a comparison diagram of the photocurrent and radiation field generated by Au / n-InGaAs and Au / p-InGaAs provided in Example 2.

[0020] Figure 4 (a) is a schematic diagram of the unbiased radiation source structure with plasmon-resonant cavity coupling provided in Example 2; Figure 4 (b) is a graph showing the variation of the absorptivity under 1550 nm wavelength illumination with grating period P and photoconductive layer thickness Hs. Figure 4 (c) Comparison of absorption spectra of plasmon-resonant cavity coupled structure and conventional plasmon structure.

[0021] Figure 5 The graphs shown in Example 2 illustrate the relationship between spectral absorption and L, and the relationship between spectral absorption and Hm.

[0022] Figure 6 (a) and Figure 6 (b) The electric field diagrams of the conventional unbiased plasmon radiation source and the plasmon-resonant cavity coupling structure provided in Example 2 under 1550nm excitation, respectively; Figure 6 (c) Comparison of photocurrents between the two; Figure 6 (d) shows the peak photocurrent and THz radiation field of the plasmon-resonant cavity coupling structure under different p-InGaAs doping concentrations.

[0023] Figure 7 (a) and Figure 7(b) are the built-in electric field comparison diagram and radiation power spectrum of the plasmon-resonant cavity coupled structure provided in Example 2 before and after the introduction of the unbiased radiation source into the InGaAs / InP heterojunction.

[0024] Figure 8 This refers to the built-in electric field of InGaAs with different doping concentrations in the 0-200 nm depth region provided in Example 2.

[0025] Figure 9 (a) Photocurrent peaks are generated for different p+-InGaAs doping concentrations in the plasmonic resonant cavity structure provided in Example 2; Figure 9 (b) Generating radiation power spectra for different p+-InGaAs doping concentrations; Figure 9 (c) shows the peak radiation power and radiation bandwidth for different p+-InGaAs doping concentrations; Figure 9 (d) represents different 10 16 cm -3 p-InGaAs thickness generates photocurrent; Figure 9 (e) and (f) are 10 respectively 16 cm -3 Electric field diagrams for p-InGaAs with thicknesses of 180 nm and 40 nm.

[0026] Figure 10 The electric field distribution diagrams for p-InGaAs with thicknesses of 180 nm and 40 nm provided in Example 2 are shown. Detailed Implementation

[0027] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this application. To better illustrate this embodiment, some parts in the accompanying drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions; It will be understood by those skilled in the art that certain well-known structures and their descriptions may be omitted in the accompanying drawings.

[0028] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] Example 1 like Figure 1 As shown, this embodiment provides a multi-junction electric field enhancement type unbiased photoconductive terahertz source, including: a substrate, and a distributed Bragg reflector, a photoconductive layer and a metal grating electrode arranged sequentially from bottom to top on the substrate; The metal grating electrode, the photoconductive layer, and the distributed Bragg reflector together constitute a plasmonic resonant cavity, which is used to absorb incident light and generate photogenerated carriers. The photoconductive layer includes a plurality of semiconductor layers arranged sequentially along the thickness direction, which are used to construct a superimposed multi-junction built-in electric field within the photoconductive layer. The built-in electric field of the multi-junction is used to drive the directional movement of the photogenerated carriers to generate photocurrent.

[0030] In the specific implementation process, the incident light is first injected from the metal grating electrode into the plasmonic resonant cavity, where the incident light is absorbed and photogenerated carriers are generated. The photoconductive layer is provided with multiple semiconductor layers, and superimposed multi-junction built-in electric fields are constructed inside it. The multi-junction built-in electric fields are used to drive the directional movement of photogenerated carriers to generate transient or continuously changing photocurrents. Finally, the photocurrents are fed into an antenna structure to generate terahertz waves. This embodiment, based on enhanced semiconductor absorption, achieves a significant improvement in optical-terahertz conversion efficiency by regulating the built-in electric field and enhancing the overlap of the optical and electric fields.

[0031] Example 2 This embodiment provides a multi-junction electric field enhancement type unbiased photoconductive terahertz source, including: a substrate, and a distributed Bragg reflector, a photoconductive layer and a metal grating electrode arranged sequentially from bottom to top on the substrate; The metal grating electrode, the photoconductive layer, and the distributed Bragg reflector together constitute a plasmonic resonant cavity, which is used to absorb incident light and generate photogenerated carriers. The photoconductive layer includes a plurality of semiconductor layers arranged sequentially along the thickness direction, which are used to construct a superimposed multi-junction built-in electric field within the photoconductive layer. The built-in electric field of the multi-junction is used to drive the directional movement of the photogenerated carriers to generate photocurrent; The photoconductive layer includes a first semiconductor layer, a second semiconductor layer, and a third semiconductor layer arranged sequentially from bottom to top along the thickness direction; The upper surface of the first semiconductor layer contacts the metal grating electrode to form a Schottky junction, generating a first built-in electric field. The upper surface of the second semiconductor layer contacts the lower surface of the first semiconductor layer to form a homojunction, generating a second built-in electric field; The upper surface of the third semiconductor layer contacts the lower surface of the second semiconductor layer to form a heterojunction and generate a third built-in electric field. The lower surface of the third semiconductor layer is in contact with the distributed Bragg reflector; The first built-in electric field, the second built-in electric field, and the third built-in electric field are in the same direction and together constitute the superimposed multi-junction built-in electric field. The first semiconductor layer is made of p-type doped InGaAs; the second semiconductor layer is made of p+ type doped InGaAs; and the third semiconductor layer is made of p+ type doped InP. The doping concentration range of the p+ type doped InGaAs is 10. 16 ~10 21 cm -3 The p+ type doped InP has a doping concentration of at least 10. 21 cm -3 ; By changing the thickness of the first semiconductor layer and / or the second semiconductor layer, the formation position of the homojunction is changed, thereby expanding the overlap rate between the light absorption region and the built-in electric field of the multijunction and increasing the amplitude of the photocurrent. The distributed Bragg reflector adopts a periodic stacked structure of InGaAsP and InP, wherein the start and end layers of the periodic stacked structure are both made of InGaAsP, and the number of InGaAsP and InP pairs is not less than 40. The metal grating electrode is made of Au and has only a single-sided metal electrode. The substrate is specifically a SI-InP substrate; The structure of the terahertz source also includes a buffer layer disposed between the substrate and the distributed Bragg reflector.

[0032] In practical implementation, the metal grating electrode is ultimately connected to an antenna (e.g., a helical antenna). One end of the helical antenna is connected to the metal grating electrode on the photoconductor layer, and the other end is grounded to guide excess carriers and avoid carrier shielding effects. When the device is excited by pulsed laser or continuous light beat frequency, the generated photogenerated carriers are collected by the nano-grating electrode and transmitted to the antenna, thereby radiating terahertz waves. These terahertz waves can then be coupled out to free space through a silicon lens.

[0033] like Figure 2 As shown, Figure 2 A comparison was made between a conventional unbiased radiation source with plasmon electrodes and an unbiased radiation source with plasmon-resonant cavity coupling proposed in this embodiment. For example... Figure 2Figures (a), (b), and (c) show schematic diagrams of a conventional plasmon unbiased radiation source, energy band diagrams calculated before and after Au / p-InGaAs contact, and schematic diagrams of photogenerated carrier motion and electric field. The work function of Au is lower than that of p-InGaAs, causing electrons to diffuse from Au to p-InGaAs. After equilibrium, a built-in electric field (light green) perpendicular to the surface and pointing towards the semiconductor is formed at the semiconductor interface. Therefore, photogenerated electrons generated in the InGaAs layer drift towards the metal electrode above the semiconductor under the influence of the built-in electric field and are collected by the electrode. In contrast, when the InGaAs doping type is n-type, the energy bands in the semiconductor bend upwards, resulting in a built-in electric field perpendicular to the surface and pointing towards the metal. This causes photogenerated electrons to drift towards the semiconductor side and holes to drift towards the metal. Due to the low hole mobility, the photocurrent is smaller compared to p-type semiconductors. Figure 3 As shown.

[0034] from Figure 2 (c) It can be seen that under the excitation of pulsed laser, the SPP effect excited by the metal grating structure makes the light field localized in the region below the metal, and the localized light field can promote the generation of more photogenerated electrons and holes. Figure 2 (c) The green area represents the region with a strong built-in electric field generated near the p-InGaAs side of the Au / p-InGaAs interface. When the incident laser irradiates the InGaAs of the photoconductive antenna, the number of photogenerated carriers increases rapidly. These photogenerated carriers can be effectively separated under the influence of the built-in electric field, forming a transient photocurrent. According to the Duvillaret analysis model, the THz radiation electric field is proportional to the first derivative of the transient photocurrent, while the THz radiation power is proportional to the square of the THz radiation electric field.

[0035] like Figure 2 (d) shows a schematic diagram of a non-biased photoconductive radiation source structure with plasmon-resonant cavity coupling. Figure 2 (e) and (f) show the energy band diagrams and photogenerated carrier schematics of Au / p-InGaAs / p+-InGaAs / P+-InP before and after contact, respectively. Figure 2 As shown in (d), a plasmonic resonant cavity is formed between the metal grating and the distributed Bragg reflector (DBR) below. This structure can confine incident photons to a greater extent within the photoconductive material, while also achieving a more uniform light field distribution (such as...). Figure 2(f) shows the yellow area). On the other hand, by designing a three-layer structure of p-InGaAs / p+-InGaAs / P+-InP between the metal grating and the DBR, a hybrid junction is formed within the cavity: including an Au / p-InGaAs Schottky junction, a p-InGaAs / p+-InGaAs homojunction, and a p+-InGaAs / P+-InP heterojunction. Compared with conventional unbiased plasmonic radiation sources, the introduction of p-InGaAs / p+-InGaAs homojunction and p+-InGaAs / P+-InP heterojunction can generate additional electric fields, such as... Figure 2 (f) As shown in the green area. These additional electric fields cause photogenerated electrons to drift away from the metal grating, thereby increasing the transient photocurrent.

[0036] In this embodiment, the DBR uses a 50.5-pair InGaAsP / InP periodic structure, with a high-refractive-index InGaAsP layer as the start and stop layers, achieving a high reflectivity of 0.995. The specific calculation method is shown in Supplementary Material Figure S2.

[0037] Figure 4 (a) is a schematic diagram of a non-biased photoconductive radiation source with exciton-resonant cavity coupling. The metal grating spacing, grating height, and grating period are represented by L, H, and H, respectively. m In P, a p-InGaAs layer is used between the metal grating and the DBR, and in H... s This indicates the thickness of the layer. Figure 4 (b) shows the absorptivity of a plasmon-resonant cavity coupled unbiased photoconductive radiation source at 1550 nm as a function of p-InGaAs layer thickness Hs and grating period P. The figure shows that when the grating period is less than 390 nm, calculations reveal that the absorptivity exhibits a narrow absorption peak related to Fabry-Perot (FP) cavity resonance with Hs. When the grating period is greater than 410 nm, the incident light excites the metal grating, generating a plasmon effect and coupling with the FP cavity, thus enhancing light absorption. The relationship between the generated surface plasmons and the incident wave when the incident light interacts with the metal grating is as follows:

[0038]

[0039]

[0040]

[0041] Where, k sp It is the surface plasma wave vector, θ is the angle between the incident electromagnetic wave and the surface, and k gHere, p is the wave vector of the grating structure, and p is the period of the grating structure. This refers to the incident wavelength. Calculations show that when the grating order is 1, the grating period for vertical excitation to produce the plasmon effect is approximately 420 nm, which is similar to the simulation results. Figure 4 As can be seen from (b), when P = 460 nm, H s At 390 nm, the absorption of InGaAs reaches 0.82, and this point will be used for subsequent experiments in this embodiment. Figure 5 (a) is a graph showing the spectral absorption of a plasmonic-cavity coupled unbiased photoconductive radiation source as a function of grating spacing L. Where P = 460 nm, H... s =390nm, H m =290 nm. Absorption first increases and then decreases as L increases, reaching a maximum absorbance of 0.85 when L = 160 nm. Figure 5 (b) is the spectral absorption of an unbiased photoconductive radiation source in a plasmonic resonator and H m The function graph. At P=460 nm, H s When the wavelength is 390 nm and the wavelength is L = 160 nm, the absorption increases with the increase of H. m First increase then decrease, when H m At 310 nm, the maximum absorption rate reaches 0.86.

[0042] Figure 4 (c) shows the absorption spectra of the plasmonic-cavity coupled radiation source and the conventional plasmonic radiation source. As can be seen from the figure, the InGaAs material absorptivity at the strongest absorption peak of 1550 nm in the plasmonic cavity radiation source is 0.86, which is 2.46 times that of the conventional plasmonic radiation source. During 1550 nm excitation, the SP generated in the plasmonic cavity radiation source reaches a "FP-like" cavity resonance condition during longitudinal propagation, allowing the InGaAs layer to capture more light energy and enhancing the spectral absorption of the ultrathin device.

[0043] like Figure 6 (a) and (b) are the normalized optical field diagrams of the conventional plasmon structure and the plasmon-resonant cavity structure. The optical field of the plasmon semiconductor portion is mainly localized below the metal, exhibiting a decreasing trend from the surface to the interior. The optical field of the plasmon-resonant cavity coupled structure is significantly stronger than that of the conventional plasmon structure. Under no bias conditions, the radiation performance of the two structures was calculated in this embodiment. Figure 6In (c), the photocurrent of both structures begins to rise at the left and right. The appearance of the rising edge of the photocurrent in the figure corresponds to the pulse center time of the 900mW femtosecond pulsed laser, indicating that the incident laser is the main factor causing the increase in photocurrent. The plasmon-resonant cavity coupling structure generates more photogenerated carriers, increasing the photocurrent from 41mA to 71mA, an increase of 73.2%.

[0044] Furthermore, this embodiment also investigates in detail the relationship between the doping concentration of the InGaAs absorption layer in the plasmonic-resonant cavity coupling structure and the photocurrent and THz intensity, such as... Figure 6 As shown in (d). When the doping concentration is 10... 16 cm -3 At a concentration of 10, the maximum photocurrent and THz radiation field were obtained. 14 -10 16 cm -3 As the Schottky barrier formed at the Au-InGaAs interface increases with increasing doping concentration, the photocurrent and THz radiation field show an upward trend. At a concentration of 10... 16 -10 18 cm -3 As the concentration of doped carriers increases, more doped carriers participate in the redistribution of carrier concentration through drift. More doped carriers drift in the opposite direction to electron diffusion, generating a shielding effect, which weakens the photocurrent and THz radiation. Therefore, the photocurrent and THz radiation field show a decreasing trend.

[0045] To reduce the transit time of photogenerated carriers to the grating electrode and improve the terahertz radiation power, this embodiment also introduces a 30 nm p-type highly doped InP layer (p+ type InP) at the bottom of the absorption layer, with a doping concentration of 10. 21 cm -3 . Figure 7 (a) The built-in electric field diagrams of InGaAs and InGaAs / InP structures obtained from simulation calculations show that the construction of InGaAs / InP heterojunctions significantly enhances the built-in electric field of InGaAs layers with a depth of 200-360 nm. Figure 7 (b) Plasmon-resonant cavity coupling structure, radiation power diagrams before and after the introduction of P+InP, it can be seen that the radiation power of the radiation source with InGaAs / InP heterojunction is increased by 50%.

[0046] Furthermore, this embodiment introduces a homojunction of p-InGaAs and p+-InGaAs, forming a vertically downward built-in electric field within the InGaAs layer, thereby further enhancing the built-in electric field and improving the photogenerated electron transport velocity. The built-in electric field of the absorption layer varies with the doping concentration of the underlying p+-InGaAs as shown below. Figure 8 As shown, in 1020 cm -3 The integrated strength of the electric field is higher at higher doping concentrations. Figure 9 (a) and (b) are graphs showing the effect of p+-InGaAs doping concentration on photocurrent and radiant power, respectively. When the doping concentration of p+-InGaAs is 10... 20 cm -3 At that time, the radiation source had a photocurrent peak of 0.49 A, which is consistent with... Figure 8 The built-in electric field enhancement shown is consistent. Figure 9 (c) Peak radiation power and radiation bandwidth for different p+-InGaAs doping concentrations. By adjusting the doping concentration, the radiation power was increased by 446%, demonstrating the effectiveness of introducing the p-InGaAs / p+-InGaAs homojunction.

[0047] Based on this, this embodiment further enhances the photocurrent by expanding the overlap between the strong light absorption region and the strong built-in electric field, thereby improving the matching degree between the light field and the electric field. Therefore, this embodiment conducts an in-depth study on the thickness of p-InGaAs. Figure 9 (d) shows the time-varying THz photocurrent generated by a non-biased photoconductive radiation source with plasmon-cavity coupling under different p-InGaAs thicknesses. The peak photocurrent is 0.58 A when the p-InGaAs thickness is 40 nm. Here, this embodiment introduces the overlap ratio of the optical and electric fields, defining it as 75% of the normalized intensity of the optical field and a field strength greater than 5 × 10⁻⁶. 6 The percentage of the area of ​​the V / m region relative to the cross-sectional area of ​​the photoconductive layer. Figure 9 (e) and 9(f) show the strong electric fields (greater than 5 × 10⁻⁶) when the thickness of p-InGaAs / p+-InGaAs is 180 nm / 180 nm and when the thickness of p-InGaAs / p+-InGaAs is 40 nm / 320 nm, respectively. 6 The overlap between the electric field (V / m) and the strong light field region (>75%) can be observed. It can be seen that by adjusting the position of the homojunction formed by p-InGaAs and p+-InGaAs, the overlap area between the strong electric field and the strong light field can be increased, which is beneficial to improving the terahertz radiation power.

[0048] like Figure 10 As shown, Figure 10 (a) shows the electric field distribution of p-InGaAs and p+-InGaAs with thicknesses of 180 nm and 180 nm, respectively. It can be seen that the electric field strength is within 5 × 10⁻⁶ nm. 6 V / m to 2×10 6 V / m range variation; Figure 10Figure (b) shows the electric field distribution when the thicknesses of p-InGaAs and p+-InGaAs are 40 nm and 320 nm, respectively. It can be seen that the electric field intensity is within 1.2 × 10⁻⁶. 7 V / m to 2×10 6 V / m range varies.

[0049] Table 1 shows a comparison of the light absorptivity, photocurrent, bandwidth, peak value, average built-in electric field, and radiated power performance of THz radiation sources with different structures. It can be seen that by introducing a p-InGaAs / P+InP heterojunction, the built-in electric field and output power are significantly improved. Further introduction of a p-InGaAs / p+-InGaAs homojunction and improved matching of the light and electric fields result in an average built-in electric field that is 6.8 times higher than that of a conventional unbiased plasmonic radiation source, a bandwidth increase of 62%, and an output power increase of 21.2 times.

[0050] Table 1 Performance comparison of THz radiation sources with different structures

[0051] This embodiment proposes a non-biased photoconductive terahertz radiation source based on plasmon-cavity coupling enhanced by a multi-junction electric field, effectively solving the core problems of low light absorption efficiency and insufficient carrier transport efficiency in traditional non-biased devices. Through a metal grating-DBR resonant cavity design, the absorption rate of 1550 nm excitation light is increased to 86%, which is 2.46 times that of conventional plasmon structures. An Au / p-InGaAs Schottky junction and a p-InGaAs / p-InGaAs junction are innovatively constructed. + -InGaAs homojunction, p + -InGaAs / P + The multi-junction electric field structure of the InP heterojunction results in an average built-in electric field strength of 6.13 × 10⁻⁶. 6 The V / m ratio is increased by 6.8 times compared to the traditional structure, achieving efficient separation and accelerated transport of charge carriers in the absorbing layer. This is achieved through optimization of p... + -InGaAs doping concentration (10²) 0 cm - ³) and p-InGaAs / p + The placement of the InGaAs homojunction significantly improves the spatial overlap efficiency between the strong optical and electric field regions, thereby enhancing carrier drift. The resulting device exhibits a peak light output 14 times higher than conventional plasmon structures, a 21.2-fold increase in terahertz radiation power, and a bandwidth extended to 6.8 THz. This device requires no external bias voltage, providing an effective pathway for high-power, high-stability, and easily integrated terahertz sources.

[0052] Example 3 This embodiment provides a multi-junction electric field enhanced unbiased photoconductive terahertz transmitter, including: the terahertz source in embodiment 1 or 2, and an antenna structure; The photocurrent generated by the terahertz source is fed into the antenna structure to generate and radiate terahertz waves.

[0053] In the specific implementation process, the terahertz source in Example 1 or 2 is used to generate photocurrent, which is then fed into the antenna structure to generate and radiate terahertz waves. In this example, the antenna structure can be a helical antenna or an antenna of other structures, and no specific restrictions are made here.

[0054] The same or similar labels correspond to the same or similar parts; The terms used to describe positional relationships in the accompanying drawings are for illustrative purposes only and should not be construed as limiting this application. Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A multi-junction electric field enhancement type unbiased photoconductive terahertz source, characterized in that, include: A substrate, and a distributed Bragg reflector, a photoconductive layer and a metal grating electrode disposed sequentially on the substrate from bottom to top; The metal grating electrode, the photoconductive layer, and the distributed Bragg reflector together constitute a plasmonic resonant cavity, which is used to absorb incident light and generate photogenerated carriers. The photoconductive layer includes a plurality of semiconductor layers arranged sequentially along the thickness direction, which are used to construct a superimposed multi-junction built-in electric field within the photoconductive layer. The built-in electric field of the multi-junction is used to drive the directional movement of the photogenerated carriers to generate photocurrent.

2. The multi-junction electric field enhancement type unbiased photoconductive terahertz source according to claim 1, characterized in that, The photoconductive layer includes a first semiconductor layer, a second semiconductor layer, and a third semiconductor layer arranged sequentially from bottom to top along the thickness direction; The upper surface of the first semiconductor layer contacts the metal grating electrode to form a Schottky junction, generating a first built-in electric field. The upper surface of the second semiconductor layer contacts the lower surface of the first semiconductor layer to form a homojunction, generating a second built-in electric field; The upper surface of the third semiconductor layer contacts the lower surface of the second semiconductor layer to form a heterojunction and generate a third built-in electric field. The lower surface of the third semiconductor layer is in contact with the distributed Bragg reflector; The first, second, and third built-in electric fields are aligned in direction and together constitute the superimposed multi-junction built-in electric field.

3. The multi-junction electric field enhancement type unbiased photoconductive terahertz source according to claim 2, characterized in that, The first semiconductor layer is made of p-type doped InGaAs; the second semiconductor layer is made of p+ type doped InGaAs; and the third semiconductor layer is made of p+ type doped InP.

4. The multi-junction electric field enhancement type unbiased photoconductive terahertz source according to claim 3, characterized in that, The doping concentration range of the p+ type doped InGaAs is 10. 16 ~10 21 cm -3 The p+ type doped InP has a doping concentration of at least 10. 21 cm -3 .

5. A multi-junction electric field enhancement type unbiased photoconductive terahertz source according to any one of claims 2 to 4, characterized in that, By changing the thickness of the first semiconductor layer and / or the second semiconductor layer, the formation location of the homojunction is altered, thereby expanding the overlap rate between the light absorption region and the built-in electric field of the multijunction and increasing the amplitude of the photocurrent.

6. The multi-junction electric field enhancement type unbiased photoconductive terahertz source according to claim 1, characterized in that, The distributed Bragg reflector adopts a periodic stacking structure of InGaAsP and InP, wherein the start and end layers of the periodic stacking structure are both made of InGaAsP, and the number of InGaAsP and InP pairs is not less than 40.

7. The multi-junction electric field enhancement type unbiased photoconductive terahertz source according to claim 1, characterized in that, The material of the metal grating electrode is Au, Ti / Au, or Cr / Au.

8. The multi-junction electric field enhancement type unbiased photoconductive terahertz source according to claim 1, characterized in that, The substrate is specifically an SI-InP substrate.

9. A multi-junction electric field enhancement type unbiased photoconductive terahertz source according to claim 1, characterized in that, The structure of the terahertz source also includes a buffer layer disposed between the substrate and the distributed Bragg reflector.

10. A multi-junction electric field enhancement type unbiased photoconductive terahertz transmitter, characterized in that, include: The terahertz source and antenna structure as described in any one of claims 1 to 9; The photocurrent generated by the terahertz source is fed into the antenna structure to generate and radiate terahertz waves.

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