Two-dimensional van der waals heterojunction and preparation method and application thereof

Abstract

The application relates to the field of two-dimensional materials, and specifically provides a two-dimensional van der Waals heterojunction and a preparation method and application thereof. The heterojunction comprises a first material layer and a second material layer, and the first material layer and the second material layer interact with each other through van der Waals force; the first material layer is a saturated absorption material, and the second material layer is an anti-saturated absorption material. In the preparation, the second material layer is prepared on a substrate first, and then the first material layer is prepared on the second material layer; the first material layer and the second material layer are prepared by using one of physical vapor deposition, chemical vapor deposition, liquid phase stripping and magnetron sputtering. The heterojunction disclosed in the application can be used for all-optical diodes. The heterojunction comprises saturated absorption and anti-saturated absorption materials; when laser irradiation exists, the nonlinearity of non-reciprocal absorption and the charge transfer effect of the heterojunction interface exist simultaneously, so that the non-reciprocal factor F and the symmetry S of transmittance are increased, and therefore the performance of the heterojunction for all-optical diodes is good.

Description

Technical Field

[0001] This application relates to the field of two-dimensional materials, and more specifically, to a two-dimensional van der Waals heterostructure, its preparation method, and its application. Background Technology

[0002] With the advent of the big data era, the demand for data transmission speed and bandwidth is constantly increasing, mainly due to the rapid growth in data volume and data processing complexity. 5G technology, fiber optic communication, and data center interconnection technology are all continuously developing. Improving data transmission speed and bandwidth is crucial to ensuring efficient data flow and processing.

[0003] Diodes are crucial devices for information transmission and processing. They are used for rectification, modulation and demodulation, voltage regulation, and are widely used in communication systems and signal processing. Silicon-based electronic diodes consist of P-type and N-type semiconductors forming a PN junction. When a forward voltage is applied to the P-terminal, electrons flow from the N-terminal to the P-terminal, forming a current; under reverse voltage, the current flows almost nonexistent, exhibiting unidirectional conductivity. However, silicon-based electronic diodes suffer from an "electron bottleneck"—the limited speed of electrons restricts their movement within the diode, thus affecting its performance. This also limits the transmission speed of electrons in communication and data processing, making it difficult to increase bandwidth. Therefore, to improve diode performance and speed, scientists continuously research and develop new technologies and materials to overcome the electron speed limitation. In other words, due to the relatively low propagation speed and frequency of electrons, silicon-based electronic diodes face speed and bandwidth limitations in high-speed data transmission.

[0004] Because photons are immune to electromagnetic interference and possess high speed and wide bandwidth, all-optical diodes, using photons as an information transmission medium, exhibit high speed and wide bandwidth transmission characteristics. Recent research has shown that two-dimensional materials have strong interactions with light fields; however, single materials cannot achieve non-reciprocal transport, thus failing to realize the function of a diode. High-performance all-optical diodes based on two-dimensional materials are currently lacking in the market. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of the prior art by providing a two-dimensional van der Waals heterojunction, its fabrication method, and its applications. The applicant discovered that the paper titled "Emerging Low-Dimensional Materials for Nonlinear Optics and Ultrafast Photonics" published in the journal *Advanced Materials* discloses the nonlinear optical absorption properties of two-dimensional materials. Excitation at the band gap exhibits a negative nonlinear absorption coefficient (saturation absorption characteristics), while excitation below the band gap exhibits a positive nonlinear absorption coefficient (anti-saturation absorption characteristics). Two-dimensional materials possess good nonlinear absorption characteristics; however, forming an all-optical diode requires achieving unidirectional optical path conduction. Through research and experimental verification, the applicant identified two two-dimensional materials with opposite nonlinear absorption coefficients at the characteristic excitation wavelength and constructed a van der Waals heterojunction with them to achieve non-reciprocal light transmission. All-optical diodes use photons as a medium for information transmission, resulting in faster transmission speeds and solving the "electronic bottleneck" problem in electronic diodes. Electronic diodes have poor high-frequency characteristics, as the frequency of light is higher than that of electrons. Therefore, this invention achieves information transmission speeds that electronic diodes cannot reach and overcomes the bandwidth limitations of silicon-based electronic diodes.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] This application provides a two-dimensional van der Waals heterostructure, which includes a first material layer and a second material layer, and the first material layer and the second material layer interact with each other through van der Waals forces; the first material layer is a saturated absorber material, and the second material layer is an anti-saturated absorber material.

[0008] Furthermore, the material of the first material layer is one of NbC, graphene, and MXene, and the material of the second material layer is one of GaS, transition metal sulfides, and metal oxides.

[0009] Furthermore, the thickness of the first material layer is 4-200 nm, the thickness of the second material layer is 4-200 nm, and the sum of the thicknesses of the first and second material layers is 8-400 nm.

[0010] The above-mentioned method for preparing a two-dimensional van der Waals heterostructure includes: firstly, preparing a second material layer on a substrate, and then preparing a first material layer on the second material layer; wherein the first material layer is prepared using one of physical vapor deposition, chemical vapor deposition, liquid phase exfoliation, and magnetron sputtering, and the second material layer is prepared using one of physical vapor deposition, chemical vapor deposition, liquid phase exfoliation, and magnetron sputtering.

[0011] Furthermore, the second material layer is made of GaS and is prepared by physical vapor deposition; the first material layer is made of NbC and is prepared by liquid phase exfoliation, with sapphire as the substrate; the first material layer is transferred onto the second material layer.

[0012] Furthermore, the liquid phase stripping method includes: first, thoroughly mixing NbC powder with alcohol, and then sonicating the mixture for at least 90 minutes; then, centrifuging the sonicated solution and taking the supernatant at a speed of 3000-7000 rpm; finally, vacuum filtering the supernatant onto a filter membrane to obtain the first material layer, with a pore size of 1-5 μm.

[0013] Furthermore, the physical vapor deposition method includes placing precursor powder and substrate in a glass tube, sealing the glass tube, and introducing a carrier gas. The precursor powder is placed in the middle of the heating zone, and the substrate is placed downstream of the carrier gas. The temperature of the heating zone is raised to a preset temperature, the precursor powder sublimates, and the gaseous precursor is deposited on the substrate along with the carrier gas to form a second material layer.

[0014] Furthermore, the precursor powder is GaS powder, the preset temperature is 930-950℃, and the carrier gas is argon.

[0015] The aforementioned two-dimensional van der Waals heterojunction is used in an all-optical diode. The heterojunction is placed in the optical path, and the optical field is transmitted through the heterojunction. When the incident laser is incident from the first material layer side of the heterojunction and exits from the second material layer side, the optical field is turned on; when the incident laser is incident from the second material layer side of the heterojunction and exits from the first material layer side, the optical field is turned off.

[0016] Furthermore, the optical path can be formed by optical elements or by optical fibers; the light source is a femtosecond laser with a center wavelength of 580-2600nm.

[0017] Compared with the prior art, the beneficial effects of this invention are as follows: The heterojunction of this application is formed by saturated absorber and anti-saturated absorber; when incident light irradiates, both non-reciprocal nonlinear absorption and heterojunction interface charge transfer effects exist simultaneously, and these two effects interact. The larger linear and nonlinear absorption coefficients, as well as the interface charge transfer, increase the non-reciprocity factor F and transmittance symmetry S, thus resulting in better performance of the heterojunction of this application when used in all-optical diodes. The all-optical diode of this application has a faster response speed, and its bandwidth, depending on the material selection, is less susceptible to limitations. Attached Figure Description

[0018] Figure 1 A schematic diagram of a two-dimensional van der Waals heterostructure provided by the present invention;

[0019] Figure 2A schematic diagram illustrating a method for preparing a two-dimensional van der Waals heterostructure provided by the present invention;

[0020] Figure 3 A schematic diagram of a two-dimensional van der Waals heterojunction used in an all-optical diode, provided by the present invention;

[0021] Figure 4 The present invention provides experimental results on the normalized transmittance as a function of light intensity of a two-dimensional van der Waals heterostructure under 800 nm femtosecond laser excitation; wherein, data with normalized transmittance greater than 1 are forward transmission data, and data with normalized transmittance less than 1 are reverse transmission data.

[0022] Figure 5 A schematic diagram (a) of a two-dimensional van der Waals heterojunction used in an all-optical diode, and a heterojunction band structure (b) provided for the present invention;

[0023] Figure 6 The simulation results show the response of the all-optical diode based on the NbC / GaS heterojunction without considering interface effects.

[0024] Figure 7 The simulation results of the all-optical diode response based on the NbC / GaS heterojunction take into account the interface effect;

[0025] Figure 8 The effect of different parameter variations on the performance of all-optical diodes; Figure 8 (a) Figure 8 (c) Figure 8 (e) Figure 8 (g) are respectively β SA β RSA Simulation results of the effect on the normalized transmittance-light intensity curve; Figure 8 (b) Figure 8 (d) Figure 8 (f) Figure 8 (h) represents the changes in the corresponding non-reciprocal factor F and transmittance symmetry S;

[0026] Figure 9 The effect of different parameter variations on the performance of all-optical diodes; Figure 9 (a) Figure 9 (c) Figure 9 (e) Figure 9 (g) are respectively σ TPA σ 12 σ s σ e Simulation results of the effect on the normalized transmittance-light intensity curve; Figure 9 (b) Figure 9 (d) Figure 9 (f) Figure 9 (h) represents the changes in the corresponding non-reciprocal factor F and transmittance symmetry S.

[0027] Icons: 1-Base; 2-First material layer; 3-Second material layer. Detailed Implementation

[0028] To make the implementation process of this invention clearer, a detailed description will be provided below in conjunction with the accompanying drawings.

[0029] This invention provides a two-dimensional van der Waals heterostructure, such as Figure 1 As shown, the heterojunction includes a first material layer 2 and a second material layer 3. The first material layer 2 and the second material layer 3 can be disposed on a substrate 1, which can be sapphire, glass, etc., possessing high light transmittance and not generating nonlinear effects with the light field. The first material layer 2 and the second material layer 3 interact through van der Waals forces. The first material layer 2 is a saturable absorber, and the second material layer 3 is an anti-saturable absorber. The saturable and anti-saturable absorption characteristics of the materials are related to the wavelength of the incident laser. Generally, when the photon energy of the incident laser is greater than the band gap of the material, it exhibits saturable absorption characteristics; when the photon energy of the incident laser is less than the band gap of the material, it exhibits anti-saturable absorption characteristics. Therefore, under the same incident laser, that is, at the same wavelength or band, the first material layer 2 needs to exhibit saturable absorption characteristics, and the second material layer 3 needs to exhibit anti-saturable absorption characteristics. Specifically, the materials of the first material layer 2 and the second material layer 3 can be NbC and GaS, graphene and TMD (transition metal sulfides), MXene (a two-dimensional material composed of carbides and nitrides) and GaS, MXene and TMD, graphene and metal oxides, or MXene and metal oxides. In this application, NbC and GaS are used as examples. NbC is a two-dimensional metallic material that exhibits saturated absorption characteristics in the ultraviolet to infrared band and has strong stability. GaS has a large band gap (2.2 eV), and exhibits anti-saturated absorption characteristics when the incident laser wavelength is greater than 563 nm. Furthermore, the thickness of the first material layer 2 is 4-200 nm, the thickness of the second material layer 3 is 4-200 nm, and the sum of the thicknesses of the first material layer 2 and the second material layer 3 is 8-400 nm. This provides good non-reciprocity and a higher damage threshold.

[0030] The aforementioned method for preparing a van der Waals heterojunction includes: first, preparing a second material layer 3 on a substrate 1, and then preparing a first material layer 2 on the second material layer 3; wherein the first material layer 2 and the second material layer 3 can be prepared using one of physical vapor deposition, chemical vapor deposition, liquid phase exfoliation, or magnetron sputtering, and the preparation methods of the first material layer 2 and the second material layer 3 can be the same or different. Specifically, in this application, GaS is prepared on sapphire using physical vapor deposition as the second material layer 3, and an NbC thin film is prepared using liquid phase exfoliation as the first material layer 2; the first material layer 2 is then transferred onto the prepared second material layer 3 to obtain the heterojunction.

[0031] Specifically, a two-dimensional van der Waals NbC / GaS heterojunction was prepared using a combination of liquid phase exfoliation and physical vapor deposition, as illustrated in the schematic diagram below. Figure 2 As shown, where Figure 2 (a) describes the process of preparing NbC thin films using a liquid-phase exfoliation method. This method is simple, efficient, and suitable for batch production. The specific NbC thin film preparation process is as follows: 200 mg of NbC powder is mixed with 150 mL of ethanol and sonicated for 90 minutes. After sonication, NbC nanosheets are obtained. Gaps exist between adjacent nanosheets, forming a porous structure that easily localizes the light field, resulting in stronger interaction between the light field and the material. Subsequently, the sonicated NbC solution is centrifuged at 3000-7000 rpm (preferably 3000 rpm) for 10 minutes to obtain a supernatant of NbC nanosheets. The nanosheets in the supernatant are smaller and have a larger specific surface area. After filtration, the resulting film has more porous structures. This enhances the interaction between the light field and the material, and also increases the charge transfer paths at the heterojunction interface, which is beneficial for improving the non-reciprocity of the heterojunction. Finally, 30 mL of the supernatant of NbC nanosheets was filtered through a vacuum filter with a pore size of 1-5 μm (preferably 2 μm) to obtain an NbC film with a thickness of 4-200 nm (the thickness of the NbC film in this embodiment is 81 nm). Figure 2 (b) The process for preparing GaS thin films by physical vapor deposition (PVD) is described, which can produce high-quality samples. The specific preparation process of the GaS thin film is as follows: 10 mg of GaS powder is placed in the heating zone of a glass tube, and the temperature is increased to 930-950°C (preferably 940°C) over 30 minutes and maintained for 40 minutes, during which argon gas is passed through at a flow rate of 40 sccm. The sublimated GaS gas flows to the sapphire substrate 1 under the action of argon gas and forms a GaS thin film. The thickness of the GaS thin film can be 4-200 nm (in this embodiment, the thickness of the GaS thin film is 37 nm). In this application, the method transfers the NbC thin film obtained by liquid phase stripping to the GaS thin film prepared by physical vapor deposition, avoiding the damage of the NbC thin film at high temperatures during physical vapor deposition.

[0032] Furthermore, the first material layer 2 and the second material layer 3 have different thicknesses. The first material layer 2 is thicker in the middle region and thinner at the edges, while the second material layer 3 is the opposite, with a thinner middle region and thicker edges. This increases the asymmetry of the light propagation path. During forward incidence, in the middle region, the light field first passes through the thicker saturated absorber material. Under high light intensity, the saturated absorber material becomes transparent, allowing light to pass smoothly through the middle region. In the edge region, because the saturated absorber material is thinner at the edges and the light intensity is lower, some light is absorbed. When passing through the thinner anti-saturated absorber material, the light intensity is insufficient to significantly increase its absorption, and some light still passes through. Therefore, the middle region dominates light propagation, and the transparency effect of the saturated absorber material enhances light transmission, thus conducting the light field. When light is incident in reverse, in the middle region, the light first passes through a thinner anti-saturable absorbing material. Even if the light intensity is not high, some of it will be absorbed initially. Subsequently, it encounters a thicker saturable absorbing material, which still absorbs a significant amount of light even at low light intensities. In the edge region, the light encounters a thicker anti-saturable absorbing material. When the light intensity is high, the anti-saturable absorption effect is more pronounced, increasing absorption. When passing through the thinner saturable absorbing material, although some light is absorbed, most of it has already been absorbed by the anti-saturable absorbing material. Similarly, in the middle region, where light propagation is dominant, the absorption effect of the anti-saturable absorbing material enhances light absorption, resulting in light field cutoff. On the other hand, the nonlinear optical effect is enhanced. This asymmetric thickness design causes the light intensity distribution to differ when the light propagates in the forward and reverse directions. When propagating in the forward direction, the light intensity gradually increases in the saturated absorbing material, while when propagating in the reverse direction, the light intensity gradually decreases in the anti-saturated absorbing material. This light intensity dependence further enhances the nonlinear optical effect, leading to asymmetric light propagation. The change in thickness causes the light to continuously experience changes in refractive index and absorption coefficient during propagation, enhancing the change in local light intensity and making the nonlinear effect more significant. This change is manifested differently in the forward and reverse propagation, thereby increasing non-reciprocity.

[0033] This application describes a van der Waals heterojunction that can be used in all-optical diodes. The realization of an all-optical diode requires the aforementioned van der Waals heterojunction and incident laser light. The heterojunction is positioned in the optical path, and a sample can be directly fixed in the optical path using components such as a sample holder. The light field transmits through the heterojunction. When the incident laser light enters from the side of the first material layer 2 of the heterojunction and exits from the side of the second material layer 3, the optical field is conductive; when the incident laser light enters from the side of the second material layer 3 of the heterojunction and exits from the side of the first material layer 2, the optical field is cut off. The optical path can be formed by optical elements or by optical fibers. The wavelength of the light source needs to be greater than 563 nm. In this embodiment, the light source is a femtosecond laser with a center wavelength of 580-2600 nm, preferably with a center wavelength of 800 nm. Specifically, as shown... Figure 3As shown, when the incident laser is incident from the anti-saturated absorber side (first material layer 2, NbC), the transmittance of the outgoing light field on the saturated absorber side (second material layer 3, GaS) increases, and the optical path is opened; when the incident laser is incident from the anti-saturated absorber side, the transmittance of the outgoing light field on the saturated absorber side decreases, and the optical path is cut off; thus, unidirectional conduction of the optical field is achieved.

[0034] To obtain the light transmission characteristics of the heterojunction, a Z-scan system is used to probe its nonlinear properties. Specifically, the Z-scan system includes a wavelength-tunable femtosecond pulsed laser, an optical translation stage, a lens with a focal length of 17.5 mm, several mirrors, an optical attenuator, a silicon detector (200-1100 nm), a germanium detector (800-1800 nm), and a lock-in amplifier. The laser acts as the light source, illuminating the lens and generating a Gaussian beam, with the beam reaching its maximum intensity at its center. The optical translation stage is used to fix the heterojunction sample and simultaneously move it a certain distance back and forth around the waist of the Gaussian beam. The optical attenuator attenuates the light intensity to prevent damage to components in the optical path. The detector detects the transmitted light intensity of the heterojunction. The lock-in amplifier controls the movement of the optical translation stage. The Gaussian beam illuminates the heterojunction, and the transmitted light is detected. The resulting curves reveal the interaction between the light field and the heterojunction. When the incident laser wavelength is 800 nm, which is greater than 563 nm, GaS exhibits anti-saturation absorption characteristics. The normalized transmittance curve obtained under 800 nm incident laser light is shown below. Figure 4 As shown, when the first material layer 2 is close to the lens and the second material layer 3 is close to the detector, it is forward bias, and the light field is conducted. When the second material layer 3 is close to the lens and the first material layer 2 is close to the detector, it is reverse bias. Figure 4 The upper data points represent forward transmission, and the lower data points represent reverse transmission. During forward transmission, the normalized transmittance increases with increasing pump intensity, and the optical path is open. During reverse transmission, the normalized transmittance decreases with increasing pump intensity, and the optical path is closed, resulting in non-reciprocal transmission and achieving unidirectional conduction of the all-photodiode. Since the horizontal axis is less than 8.4 GW / cm... 2 When the pump intensity is less than 8.4 GW / cm², the transmittance remains almost unchanged, exhibiting a linear relationship. 2 The NbC / GaS heterojunction exhibits linear absorption, making non-reciprocal transport impossible and thus preventing it from functioning as an all-optical diode. This limitation is further exacerbated by pump intensities exceeding 8.4 GW / cm². 2 The NbC / GaS heterojunction exhibits nonlinear absorption; that is, under 800 nm femtosecond laser, the pump intensity is greater than 8.4 GW / cm². 2This enables the realization of all-optical diodes. The heterojunction of this application can be used in all-optical diodes, which is related to the non-reciprocal nature of nonlinear absorption and the synergistic effect of charge transfer at the heterojunction interface.

[0035] Analyzing the light field propagation process, the incident laser first passes through the first material layer 2, which saturates and absorbs, and then through the second material layer 3, which saturates and absorbs in the opposite direction, representing forward propagation. Conversely, the incident laser first passes through the second material layer 3, which saturates and absorbs in the opposite direction, and then through the first material layer 2, which saturates and absorbs in the opposite direction, representing reverse propagation. Figure 5 As shown in (a), the propagation equations of the incident laser in the first material layer 2 of saturated absorption (SA) and the second material layer 3 of anti-saturated absorption (RSA) are equations (1) and (2), respectively:

[0036]

[0037] in, and These are the linear absorption coefficients of saturated and antisaturated absorbers, respectively; β SA and β RSA These are the nonlinear absorption coefficients of the saturated and antisaturated absorbing materials, respectively. I represents the pump intensity, and z represents the distance the light travels within the material.

[0038] Figure 5 (b) is the energy level model for charge transfer at the heterojunction interface. GaS excites electrons from the ground state to the excited state through two-photon absorption. At this point, some electrons in the GaS excited state relax to the ground state, while others transfer to the NbC excited state within the τ2 timescale, subsequently relaxing together with the electrons in the NbC excited state to the NbC ground state. Meanwhile, electrons in the NbC ground state are not only excited to higher energy levels, but some also transfer to the GaS ground state within the τ3 timescale. In the interface charge transfer model, N0 (N1) is the particle number density of the first (second) excited state of GaS, and σ... TPA and σ 12 These are the two-photon absorption cross section and excited-state absorption cross section of GaS, respectively; N s (N e ) is the particle number density of the first (second) excited state of NbC, σ e (σ s τ₁ is the absorption cross section of the ground state (excited state) of NbC; τ₁ and τ₄ are the relaxation times of carriers in the excited states of GaS and NbC, respectively; τ₂ and τ₃ are the time scales for the transfer of carriers from the excited state of GaS to the excited state of NbC and the transfer of carriers from the ground state of NbC to the ground state of GaS, respectively. The corresponding rate equations are as follows:

[0039]

[0040] N = N0 + N1 + N s +N e (7)

[0041]

[0042] Where N0 is the particle number density of the first excited state of GaS, N1 is the particle number density of the second excited state of GaS, and N is the total ion number density; σ TPA It is the two-photon absorption cross section of GaS, σ 12 It is the excited-state absorption cross section of GaS; N s It is the particle number density of the first excited state of NbC, N e It is the particle number density of the second excited state of NbC; σ e It is the ground-state absorption cross section of NbC, σ s τ is the excited-state absorption cross section of NbC; τ1 is the relaxation time of carriers in the excited state of GaS, τ4 is the relaxation time of carriers in the excited state of NbC; τ2 is the time scale for the transfer of carriers from the excited state of GaS to the excited state of NbC, τ3 is the time scale for the transfer of carriers from the ground state of NbC to the ground state of GaS; I is the pump intensity, z is the distance the light travels within the material, α is the linear absorption coefficient, t is time, h is Planck's constant, and ν is the frequency. Solving equations (3)-(8), we obtain the propagation equation of light at the interface:

[0043]

[0044] in, σ TPA and σ 12 It is the two-photon absorption cross section and excited-state absorption cross section of the antisaturable absorber; σ s and σ eτ1 represents the ground-state absorption cross section and the excited-state absorption cross section of the saturated absorbing material; I is the pump intensity; z is the distance the light travels within the material; N is the total ion number density; τ1 is the relaxation time of carriers in the excited state of GaS, τ4 is the relaxation time of carriers in the excited state of NbC; τ2 is the timescale for the transfer of carriers from the excited state of GaS to the excited state of NbC, and τ3 is the timescale for the transfer of carriers from the ground state of NbC to the ground state of GaS; h is Planck's constant, and ν is the frequency. When light is incident in the forward direction, charge transfer causes electrons in the saturated absorbing material to transfer to the antisaturated absorbing material, resulting in charge accumulation. This charge accumulation can partially shield the built-in electric field, making it easier for light to pass through. The electric field shielding effect caused by charge accumulation enhances light transmission during forward propagation and reduces reflection and absorption. During reverse propagation, charge transfer causes charge to accumulate at the interface in the antisaturated absorbing material, enhancing the electric field at the interface. This enhanced interfacial electric field increases light absorption and reflection, making it more difficult for light to pass through during reverse propagation. The local electric field enhancement effect caused by charge transfer is more pronounced during reverse propagation, further increasing light absorption and reflection. This improves the non-reciprocity of light field transmission, resulting in better performance for all-optical diodes.

[0045] The all-optical diode response of a heterojunction was simulated using Matlab software. Specifically, the data was fitted to obtain the nonlinear absorption coefficient, and the nonlinear absorption coefficient β corresponding to the NbC thin film was calculated. SA The nonlinear absorption coefficient β of the GaS thin film is -282 cm / GW. RSA The value is 812 cm / GW. Firstly, interface effects are not considered in the simulation; only β is taken as an example. SA = -282cm / GW and β RSA Substituting 812cm / GW into formulas (1) and (2) for simulation, the simulation results are as follows: Figure 6 As shown. Figure 6 This shows the variation of normalized transmittance with light field intensity. The upper curve represents the simulation results for forward transmission, and the lower curve represents the simulation results for reverse transmission. (Comparison) Figure 4 The experimental results and Figure 6 Compared to the experimental results, the simulation results show that the forward propagation curve rises more slowly and the reverse propagation curve falls more quickly, which differs significantly from the experimental results. Therefore, in addition to the non-reciprocity of nonlinear absorption, there are other effects in the physical mechanism of NbC / GaS heterojunction all-optical diodes. Including interface charge transfer in the simulation, that is, incorporating the propagation equation (9) describing the interface into the simulation, the simulation results are as follows: Figure 7 As shown. Figure 7 The upper curve represents the simulation results of forward transmission, and the lower curve represents the simulation results of reverse transmission. The trend of the curves is similar to... Figure 4The experimental results are consistent with those in the previous paper; this indicates that the all-optical diode of the NbC / GaS heterojunction is caused by the non-reciprocity of nonlinear absorption and the interface charge transfer, and both effects are indispensable. That is, the propagation equation of the light field in the saturated absorbing material is expressed by formula (1), the propagation equation of the light field in the anti-saturated absorbing material is expressed by formula (2), and the propagation process of the light field between the saturated absorbing material and the anti-saturated absorbing material is expressed by formula (9). The two materials forming the heterojunction in this application can exhibit saturated absorption and anti-saturated absorption at the same wavelength, respectively. Under photoexcitation, the above two effects exist simultaneously inside, therefore, it can be used for all-optical diodes.

[0046] By using fourth-order Runge-Kutta equations (1), (2), and (9), simulation results for different parameters during forward and reverse propagation of incident light can be obtained. The specific parameters are as follows: β SA β RSA σ TPA σ 12 σ s and σ e Furthermore, the influence of each parameter on the all-optical diode can be determined. Further simulation results are as follows: Figure 8 and Figure 9 .

[0047] To clearly describe the performance of an all-optical diode, the non-reciprocity factor and transmittance symmetry are defined as F = 10 × log₂(10 / 10). 10 (Tforward / Treverse) and S = (Tforward-1) / (1-Treverse), where T forward and T reverse These are the normalized transmittance for forward and reverse transmission, respectively. The closer S is to 1 and the larger F is, the better the performance of the all-optical diode; when S < 0, the all-optical diode cannot function and unidirectional light conduction cannot be achieved. Figure 8 Showing β SA β RSA Simulation results of the effect on the response of all-optical diodes. Figure 8 (a) and Figure 8 (c) are different (3×10 4 -8×10 4 cm -1 )and (1.0×10 4 -7.7×10 4 cm -1 The value represents the diode's performance. For a more intuitive observation... and The performance of the all-optical diode depends on the different and The F and S values ​​under the given value are calculated and displayed. Figure 8 (b) and Figure 8 (d) in. By Figure 8 (b) and Figure 8 As shown in (d), where F represents the non-reciprocity factor and S represents transmittance symmetry. The F value increases with... and The value increases with the increase of T, which is due to the higher T caused by the large linear absorption. forward and lower T reverse The S value, however, remained almost unchanged, which means and It only affects the non-reciprocity factor of all-optical diodes.

[0048] Figure 8 (e) and Figure 8 (g) describes different β SA (-500--200cm / GW) and β RSA The response of all-optical diodes with values ​​of (600-1000 cm / GW) demonstrates the change in normalized transmittance with light intensity, clearly revealing the changes in the diode's non-reciprocity and symmetry. For example... Figure 8 (f) and Figure 8 As shown in (h), β SA The resulting variations in the non-reciprocity factor F and transmittance symmetry S range from 1.4 to 3.2 dB and 0.1 to 56.4 dB, respectively; β SA The resulting variations in the non-reciprocity factor F and transmittance symmetry S range from 1.75 to 1.9 dB and 4.8 to 0.3, respectively. Figure 8 (e) Figure 8 (g) Figure 8 (f) Figure 8(h) In comparison, both the saturation absorption coefficient and the anti-saturation absorption coefficient are relatively large. As the nonlinear absorption coefficient increases, the non-reciprocity factor F also gradually increases. This indicates that the nonlinear absorption coefficients of the saturation and anti-saturation absorption materials determine whether an all-optical diode can be successfully realized. Specifically, the saturation absorption coefficient ranges from 200 to 500 cm / GW, and the anti-saturation absorption coefficient ranges from 600 to 1000 cm / GW. The all-optical diode is determined by both the saturation and anti-saturation absorption coefficients. Within these parameter ranges, fixing one parameter—for example, when the saturation absorption coefficient is -300 cm / GW, the anti-saturation absorption coefficient ranges from 600 to 1000 cm / GW, allowing for the realization of an all-optical diode; when the anti-saturation absorption coefficient is 700 cm / GW, the saturation absorption coefficient ranges from -200 to -500 cm / GW, also allowing for the realization of an all-optical diode; as the anti-saturation absorption coefficient increases, the non-reciprocity performance of the all-optical diode improves. However, the symmetry performance is best only when the anti-saturation absorption coefficient is at a specific value, that is, when S = 1.

[0049] The effect of interface charge transfer is reflected in σ TPA σ 12 σ s and σ e Among the parameters, diode simulation based on interface effects is as follows: Figure 9 As shown. Figure 8 and Figure 9 A curve with a normalized transmittance greater than 1 indicates forward transmission, while a curve with a normalized transmittance less than 1 indicates reverse transmission. Figure 9 (a) and Figure 9 (c) shows different σ TPA (2.3×10 -19 –5.7×10 -17 cm 4 / GW) and σ 12 (1.7×10 -19 –1.7×10 -16 cm 2 The all-optical diode response with different σ values ​​is shown in the results. TPA and σ 12 The response of the all-optical diodes is completely consistent. To visually observe this phenomenon, σ TPA and σ 12 The dependent F and S values ​​are calculated and displayed in Figure 9 (b) and Figure 9 In (d), the values ​​of F and S change with σ. TPA and σ 12 The change remains constant, revealing that σ TPA and σ 12 It does not contribute to the performance of the all-optical diode. Different σ values... s(1.1×10 -17 –10×10 -17 cm 2 ) and σ e (1.4×10 -19 –2.3×10 -17 cm 2 Simulation results of the all-optical diode with a value of ) are as follows Figure 9 (e) and Figure 9 As shown in (g), different σ values ​​are revealed. s and σ e The performance of all-optical diodes varies significantly depending on the value. Similarly, to clearly observe σ... s and σ e The impact of different σ values ​​on the overall viewing diode. s and σ e The F and S values ​​under the given values ​​are calculated and displayed. Figure 9 (f) and Figure 9 (h), where the spherical pattern represents the non-reciprocity factor F data, and the block pattern represents the transmittance symmetry S. It was observed that as σ... s and σ e As the value of σ increases, both the F and S values ​​show an increasing trend, indicating that σ s and σ e The larger the value, the higher the non-reciprocity factor F and transmittance symmetry S of the all-photodiode, and the better its performance; especially σ e The value has a greater impact on the performance of all-optical diodes.

[0050] In summary, the heterojunction formed by the saturated and antisaturated absorbing materials in this application can be used in all-optical diodes, and the key to its superior performance lies in the simultaneous presence of two effects during incident light irradiation: non-reciprocal nonlinear absorption and charge transfer at the heterojunction interface. These two effects work synergistically to result in superior all-optical diode performance. Specifically, in this application, the saturated absorbing material NbC and the antisaturated absorbing material GaS exhibit strong linear absorption, leading to higher To. forward and lower T reverseThe high non-reciprocity factor contributes to the superior performance of the all-photon diode. The saturated absorber NbC and the anti-saturated absorber GaS exhibit large nonlinear absorption, resulting in a large non-reciprocity factor and thus superior all-photon diode performance. Interface charge transfer exists between NbC and GaS. GaS excites electrons from its ground state to an excited state via two-photon absorption. At this point, some electrons from the GaS excited state relax to the ground state, while others transfer to the NbC excited state within the τ2 timescale, subsequently relaxing together with the electrons from the NbC excited state to the NbC ground state. Meanwhile, electrons in the NbC ground state are not only excited to higher energy levels but also transfer to the GaS ground state within the τ3 timescale. In the interface charge transfer model, N0 (N1) is the particle number density of the first (second) excited state of GaS, and σ... TPA and σ 12 These are the two-photon absorption cross section and excited-state absorption cross section of GaS, respectively; N s (N e ) is the particle number density of the first (second) excited state of NbC, σ e (σ s σ0 is the ground-state (excited-state) absorption cross section of NbC; τ1 and τ4 are the relaxation times of charge carriers in the excited states of GaS and NbC, respectively; τ2 and τ3 are the time scales for the transfer of charge carriers from the excited state of GaS to the excited state of NbC and the time scales for the transfer of charge carriers from the ground state of NbC to the ground state of GaS, respectively. Charge transfer at the interface causes the absorption cross section σ0 in NbC to change. s and σ e The increase in σ enhances the non-reciprocity factor F and transmittance symmetry S, especially the absorption cross section σ. e The increase in the non-reciprocity factor F improves the performance of the all-optical diode. Therefore, the heterojunction in this application can be used in all-optical diodes.

[0051] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A two-dimensional van der Waals heterostructure, characterized in that, The heterojunction includes a first material layer and a second material layer, which interact with each other through van der Waals forces; the first material layer is a saturated absorber material, and the second material layer is an anti-saturated absorber material. The first material layer is made of NbC thin film, and the second material layer is made of GaS thin film; the first material layer has a thicker middle region and a thinner edge region, and the second material layer has a thinner middle region and a thicker edge region.

2. The two-dimensional van der Waals heterostructure according to claim 1, characterized in that, The thickness of the first material layer is 4-200 nm, and the thickness of the second material layer is 4-200 nm; the sum of the thicknesses of the first material layer and the second material layer is 8-400 nm.

Images