Photoconductive antenna for reducing space charge shielding effect

Abstract

The invention discloses a photoconductive antenna for reducing a space charge shielding effect, relates to a terahertz transmission technology, and provides a scheme for solving the problems of overhigh shielding voltage and the like in the prior art. The structure comprises a photoconductive substrate, a first electrode arranged on one side of the upper end face of the photoconductive substrate, and a second electrode arranged on the other side of the upper end face of the photoconductive substrate. A gap nano metal array is arranged between the first electrode and the second electrode; the gap nano metal array comprises unit cells which are distributed in an array; the metal sheets in the unit unit cells and / or between the unit unit cells excite local plasmon polaritons to enhance the light field intensity of corresponding areas when irradiated by incident laser. The terahertz photoconductive antenna has the advantages that the nanometer array structure is introduced, a high radiation enhancement area is generated through metal plasmon coupling, the terahertz photoconductive antenna obtains better radiation characteristics, meanwhile, the nanometer array structure is structurally optimized, the distribution condition of carriers in the photoconductive antenna is improved, and space charge shielding voltage is effectively reduced.

Description

Technical Field

[0001] The present invention relates to terahertz transmission technology, and in particular to a photoconductive antenna for reducing the space charge shielding effect. Background Art

[0002] Terahertz technology has received increasing attention due to its applications in medical imaging, security inspection, long-distance communication, material characterization [1-3], etc. Terahertz radiation sources are the key to the application of terahertz systems. The photoconductive antenna technology has become the most widely used method for terahertz generation and emission due to its advantages such as easy use, small size, and direct connection with optical fibers. Compared with other terahertz generation technologies, photoconductive emitters can generate terahertz waves in a wide bandwidth range, have relatively low equipment costs, and also have advantages such as high gain, small size, and easy tuning [4].

[0003] Although photoconductive antennas exhibit good performance in terahertz radiation and provide advantages for terahertz wave generation, during the process of terahertz radiation generation, when the laser power or bias electric field reaches a certain level, space charge shielding [5] will occur in the photoconductive antenna, that is, the Coulomb electric field generated during the electron-hole separation process inhibits the externally applied bias voltage field. This space charge shielding weakens the externally applied bias electric field, hinders the accelerated movement of photo-generated carriers, and limits the increase in radiation power.

[0004] In the past few decades, in order to reduce the influence of the space charge shielding voltage in different scenarios, researchers have made great efforts in material properties, working conditions [6], and structural design [7]. A widely used method is to use material optimization (such as selecting photoconductive materials with low carrier mobility or specific doping), which can solve the shielding problem caused by the rapid movement of carriers from the material without significantly changing the external structure of the antenna, and can select appropriate impurities and doping concentrations according to specific requirements to effectively control the shielding voltage. However, too low carrier mobility may reduce the response speed of the antenna and affect the generation efficiency of terahertz radiation; at the same time, too high or too low doping concentration will also bring uncontrollable effects on parameters such as the carrier mobility and light absorption efficiency of the material, and may bring additional defects, affecting the stability and reliability of the material. Other methods, such as changing the working conditions of the photoconductive antenna (such as controlling the light intensity, optimizing the bias voltage, etc.), although can weaken the influence of the space shielding voltage to a certain extent, will lead to a decrease in terahertz radiation power, have a greater impact on the radiation performance of the photoconductive antenna, and result in very limited usage scenarios.

[0005] References

[0006] [1] Qi Jin, E. Yiwen, and Xi-Cheng Zhang 2020 Terahertz aqueous photonics Front. Optoelectron. 14 37-63;

[0007] [2] R.A. Lewis 2014 A review of terahertz sources J. Phys. D: Appl. Phys. 47 374001;

[0008] [3] Albert Redo-Sanchez, Norman Laman, Brian Schulkin, and Thomas Tongue 2013 Review of Terahertz Technology Readiness Assessment and Applications J. Infrared Millim. Terahertz Waves 34 500-518;

[0009] [4] Ehsan Rahmati and Mehdi Ahmadi-Boroujeni 2018 Improving the efficiency and directivity of THz photoconductive antennas by using a defective photonic crystal substrate Opt. Commun. 412 74-79;

[0010] [5] Rone-Hwa Chou, Chan-Shan Yang, and Ci-Ling Pan 2013 Effects of pump pulse propagation and spatial distribution of bias fields on terahertz generation from photoconductive antennas J. Appl. Phys. 114 043108;

[0011] [6]E.Nisha Flora Boby, Jitendra Prajapati, Vaisshale Rathinasamy, Shouvik Mukherjee, and Shyamal Mondal 2022 Parametric Investigation of Interdigitated Photoconductive Antenna for Efficient Terahertz Applications Arab.J.Sci.Eng. 47 3597-3609;

[0012] [7] Mohammadreza Khorshidi, Sajad Zafari, and Gholamreza Dadashzadeh 2019 Increase in terahertz radiation power of plasmonic photoconductive antennas by embedding buried three-stepped rods in electrodes Opt.Express 27 22327。 Summary of the Invention

[0013] The object of the present invention is to provide a photoconductive antenna that reduces the space charge shielding effect to solve the problems existing in the above-mentioned prior art.

[0014] The photoconductive antenna for reducing the space charge shielding effect in the present invention operates in the frequency band with an incident laser wavelength of 900 nm to 2600 nm; the structure includes a photoconductive substrate, a first electrode provided on one side of the upper end surface of the photoconductive substrate, and a second electrode provided on the other side; a gap nano-metal array is provided between the first electrode and the second electrode; the gap nano-metal array includes unit cells distributed in an array; the metal sheets inside and / or between the unit cells excite local surface plasmons when irradiated by the incident laser to enhance the light field intensity in the corresponding region.

[0015] The photoconductive antenna for reducing the space charge shielding effect in the present invention has the advantages that a nano-array structure is introduced, and a high radiation enhancement region is generated by metal surface plasmon coupling, so that the terahertz photoconductive antenna obtains better radiation characteristics. At the same time, by optimizing the structure of the nano-array structure, the distribution of carriers in the photoconductive antenna is improved, and the space charge shielding voltage is effectively reduced. Brief Description of the Drawings

[0016] Figure 1It is a schematic structural diagram of the first embodiment of the photoconductive antenna described in the present invention.

[0017] Figure 2 It is a schematic diagram of the distribution of the gap nano-metal array in the first embodiment of the photoconductive antenna described in the present invention.

[0018] Figure 3 It is a heat map of the electric field distribution in the first embodiment of the photoconductive antenna described in the present invention.

[0019] Figure 4 It is a heat map of the carrier distribution in the first embodiment of the photoconductive antenna described in the present invention.

[0020] Figure 5 It is a schematic structural diagram of the second embodiment of the photoconductive antenna described in the present invention.

[0021] Figure 6 It is a schematic diagram of the distribution of the gap nano-metal array in the second embodiment of the photoconductive antenna described in the present invention.

[0022] Figure 7 It is a heat map of the electric field distribution in the second embodiment of the photoconductive antenna described in the present invention.

[0023] Figure 8 It is a heat map of the carrier distribution in the second embodiment of the photoconductive antenna described in the present invention.

[0024] Figure 9 It is a schematic structural diagram of the third embodiment of the photoconductive antenna described in the present invention.

[0025] Figure 10 It is a schematic diagram of the distribution of the gap nano-metal array in the third embodiment of the photoconductive antenna described in the present invention.

[0026] Figure 11 It is a heat map of the electric field distribution in the third embodiment of the photoconductive antenna described in the present invention.

[0027] Figure 12 It is a heat map of the carrier distribution in the third embodiment of the photoconductive antenna described in the present invention.

[0028] Figure 13 It is a comparison chart of the time-domain photocurrent curves of the three embodiments of the present invention and the prior art.

[0029] Figure 14 It is a curve graph of the shielding voltage values of the three embodiments of the present invention.

[0030] Reference numerals:

[0031] 100 - Photoconductive substrate;

[0032] 200 - Gap nano - metal array, 201 - First electrode, 202 - Second electrode, 210 - Unit cell, 211 - First equilateral - triangle metal sheet, 212 - Second equilateral - triangle metal sheet, 213 - Third equilateral - triangle metal sheet, 214 - Circular metal sheet. Detailed implementation mode

[0033] Example 1

[0034] In the present invention, a photoconductive antenna for reducing the space - charge shielding effect operates in the frequency band with an incident laser wavelength ranging from 900 nm to 2600 nm. The structure is as Figure 1 and Figure 2 shown, including a photoconductive substrate 100, a first electrode 201 disposed on one side of the upper end surface of the photoconductive substrate 100, and a second electrode 202 on the other side.

[0035] A gap nano - metal array 200 is provided between the first electrode 201 and the second electrode 202. The gap nano - metal array 200 includes unit cells 210 distributed in an array. The metal sheets inside and / or between the unit cells 210 excite local surface plasmons when irradiated by the incident laser to enhance the light - field intensity in the corresponding region. The material of the photoconductive substrate 100 is InGaAs. The material of the gap nano - metal array 200 is Au. The unit cell 210 is composed of a first equilateral - triangle metal sheet 211. The distance d1 between the apex angles of each first equilateral - triangle metal sheet 211 and the apex angles of adjacent first equilateral - triangle metal sheets 211 is equal. In this embodiment, the specific dimension is selected such that the side length L1 of the second equilateral - triangle metal sheet 212 is 240 nm. The distance d1 is 50 nm. The thickness of the photoconductive substrate 100 is 310 nm. The thicknesses of both the first electrode 201 and the second electrode 202 are 20 nm. The thickness of the gap nano - metal array 200 is 50 nm.

[0036] The InGaAs material exhibits extremely remarkable wide - spectrum response characteristics. Its response range usually spans from the near - infrared to the mid - infrared band, and it can sensitively capture and respond to light with wavelengths ranging from about 900 nm to 2600 nm. This excellent characteristic endows the InGaAs - based photoconductive antenna with excellent adaptability, enabling it to be well - matched with common 1550 - nm lasers. At the same time, InGaAs has a high carrier mobility, which helps to improve the terahertz radiation efficiency and response speed of the photoconductive antenna to obtain higher terahertz radiation. The electronic structure of gold enables free electrons to undergo strong collective oscillations under the action of a suitable light field, efficiently exciting metal plasmons. The incident laser wavelength λ is 1550 nm, and the repetition frequency f pA femtosecond pulsed laser with a frequency of 100 MHz is incident vertically from directly above the photoconductive antenna onto the gap nano-metal array 200. At the same time, a positive voltage of 20 V is applied to the first electrode 201, and the second electrode 202 is grounded, which can excite the generation of photocurrent.

[0037] When the incident laser irradiates the gap nano-metal array 200, the energy of the photons is transferred to the free electrons in the metal. When the frequency of the photons matches the collective oscillation frequency of the free electrons in the metal, the induced resonance causes the free electrons to oscillate collectively, thereby exciting the generation of local surface plasmons. Metal plasmons can localize the optical field in a very small area around the nanostructure, significantly enhancing the optical field intensity in this area. This enables the photoconductive antenna to absorb the incident light more effectively and improve the generation efficiency of photo-generated carriers. When the electromagnetic wave reaches the optimal coupling with the plasmons, the specific period of the nanostructure can be expressed as:

[0038]

[0039] where υ is an integer representing the plasmon mode number, ε 1 is the relative permittivity of the dielectric, and ε 2 is the relative permittivity of the metal.

[0040] When the incident laser irradiates the photoconductive substrate 100, free carriers are generated and accelerated under the action of the DC bias voltage. The formed transient photocurrent decays completely within a very short time. The transient photocurrent induces an electric field and a magnetic field, thereby generating and radiating terahertz waves. The carrier generation rate G varying with time is obtained by the following formula:

[0041]

[0042] where k pc is the imaginary part of the refractive index of the semiconductor, c is the speed of light, D t is the laser pulse duration, h is Planck's constant, and P s is the total power flux density; the total power flux density can be obtained by solving the Maxwell equations.

[0043] However, when the femtosecond pulsed laser irradiates the photoconductive antenna, due to the spatial separation of electrons and holes, a self-induced electric field opposite to the direction of the bias electric field will be generated, thereby shielding the bias electric field. Therefore, as the power of the femtosecond pulsed laser increases, the radiation power of the terahertz photoconductive antenna shows saturation. Considering the existence of the electric field shielding effect, the expression of the current density j(t) and the shielding electric field E sc (t) in the photoconductive antenna can be approximated as:

[0044] j(t) = eμ e n(t)(E bias - Esc (t))

[0045] where e and μ e are the charge and mobility of electrons respectively, and n(t) and E bias are the carrier density and bias electric field generated in each unit cell respectively. The space charge shielding voltage value can be obtained according to the above formula in combination with the carrier density distribution in the photoconductive antenna. Under the excitation of a 1550 nm incident laser, the electric field distribution of the gap nano-metal array 200 is as Figure 3 shown, while the carrier distribution is as Figure 4 shown.

[0046] Example Two

[0047] The difference from Example One is that, as Figure 5 and Figure 6 shown, the unit cell 210 is composed of a second regular triangular metal sheet 212 and an inverted third regular triangular metal sheet 213. The side lengths of both the second regular triangular metal sheet 212 and the third regular triangular metal sheet 213 are L2. The vertex angle at the lower end of the second regular triangular metal sheet 212 corresponds to the vertex angle at the upper end of the third regular triangular metal sheet 213, and the distance therebetween is d2. The distance between each unit cell 210 and the adjacent unit cells 210 on the left and right is d3, and the distance between each unit cell 210 and the adjacent unit cells 210 above and below is d4. The distance relationship satisfies: d3 > d2 = d4. In this example, the specific dimensions are selected as the side length L2 being 240 nm, the distance d3 being 50 nm, and the distances d2 and d4 being 20 nm. Under the excitation of a 1550 nm incident laser, the electric field distribution of the gap nano-metal array 200 is as Figure 7 shown, while the carrier distribution is as Figure 8 shown.

[0048] Example Three

[0049] The difference from Example One is that, as Figure 9 and Figure 10 shown, the unit cell 210 is composed of a circular metal sheet 214. The distance between the centers of each circular metal sheet 214 and the circular metal sheets 214 adjacent to it on four sides is d5. In this example, the specific dimensions are selected as the radius R of the circular metal sheet 214 being 120 nm. The distance d5 is 290 nm. Under the excitation of a 1550 nm incident laser, the electric field distribution of the gap nano-metal array 200 is as Figure 11 shown, while the carrier distribution is as Figure 12 shown.

[0050] Analysis by integrating the three examples: As Figure 13As shown, the photocurrents of the photoconductive antennas in the three embodiments are basically the same, starting to rise at about 1 ps, corresponding to the pulse center time of the femtosecond pulsed laser. When the incident laser irradiates the semiconductor of the photoconductive antenna, the number of photo-generated carriers rapidly increases and moves under the action of the electric field, thus forming a rapidly increasing transient photocurrent. Compared with the traditional planar-structured photoconductive antenna, the pulsed photocurrent of the photoconductive antenna of the present invention can be increased by up to 4.9 times.

[0051] The shape and distribution position of the nanoarray structure affect the generation and recombination of carriers, and further affect the magnitude of the shielding electric field. Compared with the cylindrical nanoarray in Embodiment 3, the triangular arrays in Embodiments 1 and 2 have the advantage that their sharp corners will bring a strong lightning rod effect, making it easier for electrons in the metal at the sharp points to accumulate. At higher laser powers, the spatial distribution of carriers also affects the space charge shielding effect. The spatial condition of Embodiment 1 can make carriers more evenly distributed at various positions between the electrodes compared with Embodiment 2, thus reducing the probability of space charge shielding caused by excessive accumulation of carriers at local positions, and finally having a lower space charge shielding voltage.

[0052] From the electric field distribution and carrier distribution in the three embodiments, it can be seen that the local hot spot of the electric field intensity in the triangular nanoarray appears at the edge of the tip angle, which is also the main position where carriers are generated. This helps to increase the drift current density under bias voltage. At the same time, compared with the cylindrical array structure, the electric field intensity and the number of carriers are greatly improved. The structure of Embodiment 1 concentrates the charge density generated by the initial photo-generated carriers near the device surface. At the same time, the extremely strong local field enhancement near the sharp corners can generate more carriers. This will result in a stronger local field enhancement factor and a higher non-equilibrium carrier concentration near the array, obtaining a higher efficiency of light-to-terahertz conversion. At the same time, under the action of an externally applied bias field, the generated carriers can reach the electrodes in an extremely short time, reducing the probability of carrier recombination during transmission and reducing the space charge shielding electric field.

[0053] Through the formula of current density and shielding electric field, the relationship between the space charge shielding voltage intensity and time of different nanoarray plasmonic terahertz photoconductive antennas can be simulated and calculated using COMSOL software, as Figure 14 shown. The calculation results show that after the selection of a specific structure, Embodiment 1 can reduce the space charge shielding voltage by 12.6% and 28.7% respectively compared with Embodiment 3 and Embodiment 2.

[0054] For those skilled in the art, various corresponding changes and deformations can be made according to the technical solutions and concepts described above, and all these changes and deformations should fall within the protection scope of the claims of the present invention.

Claims

1. A photoconductive antenna for reducing space charge screening effect, operating in a frequency band where the incident laser wavelength is 900nm to 2600nm; the structure comprises a photoconductive substrate (100), and a first electrode (201) arranged on one side of the upper end surface of the photoconductive substrate (100) and a second electrode (202) on the other side; It is characterized in that A gap nanometal array (200) is provided between the first electrode (201) and the second electrode (202); the gap nanometal array (200) comprises unit cells (210) distributed in an array; metal sheets within the unit cells (210) and / or between the unit cells (210) excite localized plasmons when irradiated by incident laser light to enhance the light field intensity of the corresponding area.

2. A photoconductive antenna for reducing space charge screening effect according to claim 1, characterized in that: The material of the photoconductive substrate (100) is InGaAs; the material of the gap nanometal array (200) is Au.

3. A photoconductive antenna for reducing space charge screening effect according to claim 2, characterized in that: The photoconductive substrate (100) has a thickness of 310 nm.

4. The photoconductive antenna for reducing space charge screening effect according to claim 2, characterized in that: The thickness of the first electrode (201) and the second electrode (202) are both 20 nm; the thickness of the gap nanometal array (200) is 50 nm.

5. The photoconductive antenna for reducing space charge screening effect according to claim 1, characterized in that: The unit cell (210) is composed of a first equilateral triangular metal sheet (211); the distance d1 between the vertex angle of each first equilateral triangular metal sheet (211) and the vertex angle of an adjacent first equilateral triangular metal sheet (211) is equal.

6. The photoconductive antenna for reducing space charge screening effect according to claim 5, characterized in that: The side length L1 of the second equilateral triangular metal sheet (212) is 240 nm; and the distance d1 is 50 nm.

7. The photoconductive antenna for reducing space charge screening effect according to claim 1, characterized in that: The unit cell (210) is composed of a second equilateral triangular metal sheet (212) and an inverted third equilateral triangular metal sheet (213), the side lengths of the second equilateral triangular metal sheet (212) and the third equilateral triangular metal sheet (213) are both L2; the vertex angle of the lower end of the second equilateral triangular metal sheet (212) corresponds to the vertex angle of the upper end of the third equilateral triangular metal sheet (213), and the distance is d2; the distance between each unit cell (210) and the unit cells (210) adjacent to the left and right is d3, and the distance between each unit cell (210) and the unit cells (210) adjacent to the top and bottom is d4; the distance relationship satisfies: d3>d2=d4.

8. The photoconductive antenna for reducing space charge screening effect according to claim 7, characterized in that: The side length L2 is 240 nm, the distance d3 is 50 nm, and the distance d2 and the distance d4 are 20 nm.

9. The photoconductive antenna for reducing space charge screening effect according to claim 1, characterized in that: The unit cell (210) is composed of a circular metal sheet (214); the center distances between each circular metal sheet (214) and the four adjacent circular metal sheets (214) are all d5.

10. The photoconductive antenna for reducing space charge screening effect according to claim 9, characterized in that: The radius R of the circular metal sheet (214) is 120 nm; and the distance d5 is 290 nm.

Images