Method for enhancing second harmonic emission based on nonlinear energy transfer mechanism

Abstract

The application provides a method for enhancing second harmonic generation based on a non-linear energy transfer mechanism, obtaining a heterojunction composed of a donor two-dimensional material A and an acceptor two-dimensional material B, and then exciting by a two-photon pumping light source to induce non-linear energy transfer, thereby enhancing the second harmonic generation of the acceptor two-dimensional material B; in the heterojunction, the donor two-dimensional material A is (LA)2(A) n‑1 Pb n I 3n+1 ; wherein LA is a C3-C6 alkylammonium ion or a phenylammonium ion; A is a C1-C2 alkylammonium ion or a formamidate ion; n is 1-4; and the acceptor two-dimensional material B includes at least one of MoS2, MoSe2, WSe2 and h-BN. The application can effectively enhance the SHG of the acceptor material.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano optoelectronic integration of low-dimensional semiconductor materials, and specifically relates to a method for enhancing second harmonic emission (SHG) of two-dimensional materials based on a nonlinear energy transfer mechanism. Background Technology

[0002] Nonlinear optics is a field of research that explores certain nonlinear effects arising from the interaction between strong light and matter. These nonlinear optical effects can be applied to devices such as pulsed lasers, optical modulators, optical switches, photodetectors, and optical memories, offering unique advantages compared to traditional electronic devices. However, due to the limitations of the optical diffraction limit, the development of high-performance, small-size, on-chip integrated nonlinear light source devices has been greatly restricted, becoming a bottleneck for improving the performance of photonic chips. Therefore, it is necessary to find a small-size material with strong optical nonlinearity to keep pace with the current trend of high integration in photonic devices.

[0003] Two-dimensional materials, such as transition metal dichalcogenides (TMDCs), exhibit strong optical nonlinearity due to the breaking of their inversion symmetry, making them ideal for nonlinear applications in nanophotonics. However, the weak photomatter interaction and light absorption of these two-dimensional materials limit their practical applications, thus necessitating the exploration of suitable solutions.

[0004] Previously, the optical nonlinearity of two-dimensional materials could be significantly improved by constructing plasmonic nanocavities, waveguides, and metamaterials. However, these methods can only locally enhance the nonlinear optical emission intensity, and achieving uniform nonlinear optical enhancement over a large area remains a field worth exploring. Energy transfer (ET) has been proven to be a feasible method to simultaneously and significantly enhance both linear and nonlinear optical responses. This technique has evolved from interactions between zero-dimensional quantum dots to current two-dimensional systems, significantly improving both the transmission distance and effective area of ​​the interactions.

[0005] In summary, nonlinear optics, due to its correlation with strong light-matter interactions, shows great potential in condensed matter physics and optoelectronics. However, the currently weak light-matter interactions of two-dimensional materials hinder its practical applications. Therefore, this study aims to demonstrate a general method for enhancing two-dimensional material SHGs based on a nonlinear energy transfer mechanism by selecting a suitable van der Waals heterojunction system and employing two-photon nonlinear pumping technology combined with energy transfer methods. This has significant guiding and reference value for future fundamental research on micro- and nano-semiconductor materials and the application of nonlinear light sources in photonic integrated circuits. Summary of the Invention

[0006] The purpose of this invention is to provide a general method for enhancing the second harmonic emission (SHG) of two-dimensional materials based on a nonlinear energy transfer mechanism, aiming to enhance the SHG of two-dimensional materials based on nonlinear energy transfer.

[0007] Due to the weak light-matter interaction in two-dimensional materials, current methods for enhancing the nonlinearity of two-dimensional materials can only locally increase the intensity of nonlinear optics. Achieving uniform nonlinear optic enhancement over a large area remains to be explored. To address this technical problem, this invention provides the following solution:

[0008] A method for enhancing second harmonic emission based on a nonlinear energy transfer mechanism is proposed. A heterojunction composed of a donor two-dimensional material A and a acceptor two-dimensional material B is obtained, and then excited by a two-photon pump source to induce nonlinear energy transfer, thereby enhancing the second harmonic emission of the acceptor two-dimensional material B.

[0009] In the heterojunction described above, the donor two-dimensional material A is (LA)2(A). n-1 Pb n I 3n+1 Wherein, LA is a C3-C6 alkylammonium ion or phenylammonium ion; A is a C1-C2 alkylammonium ion or formamide ion; and n is 1-4.

[0010] The two-dimensional receptor material B includes at least one of MoS2, MoSe2, WSe2, and h-BN.

[0011] This invention provides a general method for enhancing the second harmonic emission (SHG) of two-dimensional materials based on a nonlinear energy transfer mechanism. Under the pumping of two-photon excitation light, the dipole absorption level in the donor two-dimensional material A is activated, and the donor energy is further transferred to the corresponding dipole level of the second harmonic in the acceptor through nonlinear energy transfer, thereby enhancing its SHG.

[0012] In this invention, based on the aforementioned donor two-dimensional material A-acceptor two-dimensional material B van der Waals heterojunction, further joint control of the thickness of the donor two-dimensional material A and the wavelength of the two-photon pump source helps to further improve the synergistic coupling effect of nonlinear energy transfer, helps to obtain tens of times the second harmonic emission enhancement of the acceptor, and at the same time realizes ultra-long-distance energy transfer behavior.

[0013] In this invention, in the donor two-dimensional material A, LA is butylammonium ion; A is methylammonium ion; and n is 2~3.

[0014] In this invention, the donor two-dimensional material A is (BA)2(MA)Pb2I7.

[0015] In this invention, the thickness of the donor two-dimensional material A is below 210 nm, preferably 70-80 nm. Research in this invention shows that controlling the donor two-dimensional material A within the preferred range can further synergistically improve the nonlinear energy transfer mechanism and enhance the intensity of the SHG signal of the two-dimensional material. For example, within a further preferred thickness range, the SHG signal of the MoS2 used can be enhanced by 50 times.

[0016] In this invention, the two-dimensional acceptor material B is h-BN. This invention demonstrates that using this material as the acceptor material to construct the heterojunction, combined with the excitation method described in this invention, can achieve a superior enhancement effect for SHG.

[0017] In this invention, the receptor two-dimensional material B can be a single-layer to multi-layer (e.g., 1 to 5 layers) two-dimensional material, and further, its thickness is 0.6 to 80 nm.

[0018] In this invention, the heterojunction is prepared by stacking a donor two-dimensional material B on a donor two-dimensional material A using a mechanical peeling method and a dry transfer technique.

[0019] For example, the heterojunction is formed on a substrate, wherein a donor two-dimensional material A is on the surface of the substrate, followed by the excitation. The substrate is, for example, a SiO2 / Si substrate.

[0020] In this invention, the two-photon pump source has a wavelength of 760-1000 nm, preferably 920-960 nm, and more preferably 935-945 nm. These preferred excitation conditions help to further enhance the SHG of the acceptor two-dimensional material.

[0021] The femtosecond pulse laser repetition frequency of the two-photon pump source is 1MHz, and the pulse width is 80~120fs;

[0022] Preferably, all optical tests of the present invention are performed at 240K.

[0023] Beneficial effects

[0024] 1. This invention successfully developed a general method for enhancing the second harmonic emission of two-dimensional materials based on a nonlinear energy transfer mechanism, filling a gap in the industry.

[0025] 2. The present invention has found that, based on the pumping of two-photon excitation light, a large number of two-photon absorbing dipoles in the donor two-dimensional material A are fully activated and transferred in the second harmonic response dipoles in the acceptor material through a nonlinear energy transfer mechanism, thereby enhancing the SHG signal of the acceptor material.

[0026] 3. This invention also found that by controlling the thickness of the donor two-dimensional material A, the proportion of organic components in the donor two-dimensional material A, and the wavelength of the two-photon pump source, the dark-state energy transfer efficiency can be artificially tunable, and the enhancement effect of the SHG signal of the acceptor material can be successfully controlled, providing important guidance for the subsequent design of more flexible and widely applicable near-infrared optoelectronic devices. Attached Figure Description

[0027] Figure 1 This is a schematic diagram and parameter diagram of the preparation of the donor two-dimensional material A ((BA)2(MA)Pb2I7, also referred to as RPP) in Example 1. Figure 1 a-1c is a schematic diagram of the donor two-dimensional material A prepared in Example 1; Figure 1 d-1h refers to the nanosheets obtained by mechanical exfoliation of the donor two-dimensional material A prepared in Example 1;

[0028] Figure 2 The image shows a test pattern of the MoS2 / RPP heterojunction prepared in Example 2. Figure 2 a is a side view of the MoS2 / RPP heterojunction prepared in Example 2 and the optical SHG process; Figure 2 b is an optical image of the MoS2 / RPP heterojunction prepared in Example 2. The white and blue dashed lines represent RPP and MoS2. Scale bar: 8 μm; Figure 2 c is a spatially resolved SHG intensity image of the MoS2 / RPP heterojunction prepared in Example 2 under 800nm ​​pulsed laser excitation; Figure 2 d represents the SHG spectra of pure MoS2 (red), RPP (green), and MoS2 / RPP heterojunction (blue) at 240 K in the MoS2 / RPP heterojunction prepared in Example 2. Figure 2 e represents the SHG intensity variation curves of the MoS2 and MoS2 / RPP heterojunctions prepared in Example 2, which both conform to the expected quadratic curves.

[0029] Figure 3 This is a test diagram from Example 3, in which... Figure 3 a is a schematic diagram of the two-photon excitation nonlinear energy transfer and SHG enhancement principle of the MoS2 / RPP heterojunction prepared in Example 3; Figure 3 b represents the wavelength-dependent SHG (orange) of MoS2 and the MoS2 / RPP heterojunction (blue) in the MoS2 / RPP heterojunction prepared in Example 3. Figure 3 c represents the wavelength-dependent SHG enhancement factor (orange) and RPP absorption (blue dashed line) of the MoS2 / RPP heterojunction prepared in Example 3.

[0030] Figure 4 This is a test diagram from Example 4, in which... Figure 4a shows optical (i), AFM (ii), and SHG intensity (iii) images of a typical heterostructure composed of ML-MoS2 of different thicknesses and donor two-dimensional material A prepared in Example 4. The white and blue dashed areas represent the donor two-dimensional material A and ML-MoS2. Scale bar: 5 μm. Figure 4 b is Figure 4 SHG spectra of the HS-1, HS-2, HS-3 and ML-MoS2 regions in a; Figure 4 c is Figure 4 The corresponding SHG intensity distribution and AFM height distribution extracted along the red dashed line in a; Figure 4 d represents the variation of the SHG strength of the ML-MoS2 / donor two-dimensional material A heterojunction prepared in Example 4 with the thickness of the donor two-dimensional material A; Figure 4 e is a schematic diagram of the bottom-up step transfer model of energy in the ML-MoS2 / donor two-dimensional material A heterojunction prepared in Example 4;

[0031] Figure 5 This is a test diagram from Example 5, in which... Figure 5 a-5b show the SHG spectra and enhancement coefficients of the same donor two-dimensional material A and MoS2 heterostructures with different numbers of layers prepared in Example 5; Appendix Figure 5 c-5d are the normalized SHG spectra of various monolayer TMDs and n=2 donor two-dimensional material A heterostructures prepared in Example 5; Appendix Figure 5 e-5g is the normalized SHG spectrum of donor two-dimensional material A with different n values ​​and ML-MoS2 heterostructure prepared in Example 5;

[0032] Figure 6 The SHG spectra of pure h-BN and HS in the h-BN / donor two-dimensional material A heterojunction prepared in Example 6 are shown.

[0033] Figure 7 The test image is for Comparative Example 1, where... Figure 7 a is a single-photon excitation interlayer CT image of the ML-MoS2 / donor two-dimensional material A heterostructure prepared in Comparative Example 1; (See attached image) Figure 7 b and 7c are the PL (fluorescence emission) lifetimes of donor two-dimensional materials A and ML-MoS2 prepared in Comparative Example 1, measured from the pure sample region and HS region.

[0034] Figure 8 The test image is for Comparative Example 2, where... Figure 8 Image a shows an optical image of the ML-MoS2 / CsPbBr3 heterojunction prepared in Comparative Example 2. Scale bar: 10 μm; (See attached image) Figure 8 b is a spatially resolved SHG intensity image of the ML-MoS2 / CsPbBr3 heterojunction prepared in Comparative Example 2; (See attached image) Figure 8 c represents the SHG spectra of pure ML-MoS2 and HS in the ML-MoS2 / CsPbBr3 heterojunction prepared in Comparative Example 2. Detailed Implementation

[0035] The excitation light source for the two-photon steady-state test is an 800nm ​​femtosecond laser with a repetition frequency of 1MHz and a pulse width of 80-120fs.

[0036] The excitation light source for the single-photon transient test is a 400nm femtosecond laser with a repetition frequency of 1MHz and a pulse width of 80-120fs.

[0037] The fluorescence testing instrument is a confocal microscope system, model: WITec, alpha-300.

[0038] The transient testing instrument is a streak camera, model: C10910, Hamamatsu.

[0039] The material thickness testing instrument is an atomic force microscope, model: Dimension Icon.

[0040] This invention, through further research on this van der Waals heterojunction system, successfully realized a general method for improving the SHG of two-dimensional materials based on a nonlinear energy transfer mechanism, and achieved the modulation of the SHG enhancement effect by adjusting the thickness of the donor two-dimensional material A and the wavelength of the two-photon pump source.

[0041] The technical solution of this invention innovatively discovers that, based on the pumping of two-photon excitation light, a large number of two-photon absorbing dipoles in the donor two-dimensional material A are fully activated and transferred through a nonlinear energy transfer mechanism to the second harmonic response dipoles in the acceptor material, thereby enhancing the SHG signal of the acceptor material.

[0042] The technical solution of this invention, based on the aforementioned nonlinear energy transfer mechanism, can increase the second harmonic emission of monolayer MoS2 by tens of times.

[0043] The technical solution of this invention, based on the aforementioned nonlinear energy transfer mechanism, can enhance the SHG signals of various two-dimensional materials.

[0044] In this invention, there are no special requirements for the solution synthesis method and conditions of the donor two-dimensional material A, and it can be achieved based on existing equipment and theories.

[0045] In this invention, both the donor two-dimensional material A and the acceptor two-dimensional material B are prepared by mechanical exfoliation.

[0046] In this invention, the van der Waals heterojunction is prepared by dry transfer technology.

[0047] In this invention, the two-photon pump source is, for example, an 800nm ​​femtosecond pulsed laser with a repetition frequency of 1MHz, a pulse width of 80~120fs, and a test temperature of 240K.

[0048] This invention has discovered that by changing the thickness of the donor two-dimensional material A, the nonlinear energy transfer effect can be artificially controlled, further enabling controllable modulation of SHG enhancement in two-dimensional materials.

[0049] In this invention, the thickness of the controllable donor two-dimensional material A is below 210 nm.

[0050] As a preferred option, the donor two-dimensional material A has the largest enhancement amplitude of second harmonic emission of monolayer MoS2 when the thickness is 100 nm, with a maximum enhancement factor of 50.

[0051] In the following cases, unless otherwise stated, the steps for preparing donor two-dimensional material A by solution method are as follows:

[0052] A certain amount of PbO powder was added to a mixed solution of IH and H3PO2, and the mixture was heated and stirred continuously to ensure that the precipitate was completely dissolved, forming a bright yellow liquid. Then, MAI powder was added to the bright yellow solution, and the mixture was heated and stirred continuously. BAI powder was then added, and the mixture was heated and stirred continuously to ensure that the precipitate was completely dissolved. The mixture was then cooled slowly, the precipitate crystals were filtered out, and after drying, donor two-dimensional material A crystal with the chemical formula (BA)2(MA)Pb2I7 was obtained.

[0053] Example 1

[0054] The donor two-dimensional material A ((BA)2(MA)Pb2I7; abbreviated as RPP) can be synthesized based on a conventional solution method. The steps are as follows: PbO powder, MAI powder, and BAI powder are added sequentially to IH and H3PO2 solutions, and the mixture is heated and stirred at each step to ensure complete dissolution. Then, the mixture is slowly cooled, the precipitated crystals are filtered out, and the (BA)2(MA)Pb2I7 crystals are obtained after drying. Specifically, the heating and stirring temperature after adding PbO powder to the mixed solution of IH and H3PO2 is 60℃, and the stirring is carried out for 4 hours; the heating and stirring temperature after adding MAI powder is 60℃, and the stirring is carried out for 2 hours; the heating and stirring temperature after adding BAI powder is 110℃, and the stirring is carried out for 4 hours; the volume ratio of PbO powder to MAI powder in the mixed solution of IH and H3PO2 is 9:1; and the mass ratio of PbO powder, MAI powder, and BAI powder is 2.7:1:1.8.

[0055] like Figure 1As shown in a-1e, a certain amount of PbO powder was added to a mixed solution of IH and H3PO2, and then stirred for about 4 hours on a magnetic stirring heating platform at 60°C to form a bright yellow liquid. MAI powder was then added to this yellow solution, and stirring and heating continued for 2 hours to ensure complete dissolution. The addition of BAI powder resulted in the formation of a precipitate. Heating and stirring continued at 110°C for about 4 hours to completely dissolve the precipitate. The solution was then slowly cooled, filtered to obtain the precipitate crystals, dried, and the donor two-dimensional material A crystals were obtained, with the chemical formula (BA)2(MA)Pb2I7. The organic content and purity of the synthesized donor two-dimensional material A can be changed by comprehensively adjusting the heating and stirring temperature and the dosage of different reagents. In this invention, under the aforementioned conditions, donor two-dimensional materials A of various thicknesses were further obtained by mechanical exfoliation.

[0056] Example 2

[0057] Nonlinear energy transfer research of ML-MoS2 / RPP (also simply MoS2 / RPP, where ML-MoS2 refers to monolayer MoS2 nanosheets) heterostructures:

[0058] See attached diagram for the test. Figure 2 , Figure 2 In this context, MoS2 refers to ML-MoS2, which is a single-layer MoS2 nanosheet, and EF refers to the reinforcement coefficient. ML-MoS2 and RPP nanosheets are obtained by mechanically exfoliating the corresponding bulk crystalline materials onto PDMS (polydimethylsiloxane), and then using a dry transfer technique to sequentially stack the RPP nanosheets and ML-MoS2 on a SiO2 / Si substrate.

[0059] Figure 2 a is a side view of the ML-MoS2 / RPP heterojunction and the optical SHG process. Figure 2 b represents a vertically stacked van der Waals heterojunction obtained by mechanically exfoliating and dry-transferring RPP and ML-MoS2 onto a SiO2 / Si substrate, with a scale bar of 8 μm. PL intensity mapping was then performed using 800 nm two-photon excitation light pumping. The heterojunction region exhibited a globally uniform MoS2 SHG signal enhancement phenomenon. Figure 2 c), and the SHG signals of ML-MoS2 / RPP, ML-MoS2 and RPP were measured respectively. Figure 2 d), and simultaneously fitted the curve of SHG intensity as a function of excitation power, with the slope of the curve close to 2 ( Figure 2 e). In summary, a comparison of SHG spectra ( Figure 2 c) and the power dependence fitting curve of the SHG signal ( Figure 2e) It can be determined that in the van der Waals heterojunction system, the SHG signal of MoS2 is strongly and uniformly enhanced, and the SHG signal exhibits second-order nonlinear optical emission.

[0060] Example 3

[0061] Compared with Example 2, the only difference is that the heterojunction is excited with two-photon pump light of different wavelengths, specifically: 760nm, 780nm, 800nm, 820nm, 840nm, 860nm, 880nm, 900nm, 920nm, 940nm, 960nm, 980nm, and 1000nm.

[0062] Figure 3 'a' shows the principle of two-photon excitation nonlinear energy transfer and SHG enhancement in an ML-MoS2 / RPP heterojunction. Figure 3 b represents second-harmonic generation (SHG) experiments conducted on pure ML-MoS2 and ML-MoS2 / RPP heterojunctions with excitation wavelengths ranging from 760 to 1000 nm to verify whether the SHG enhancement originates from nonlinear energy transfer. The maximum SHG intensity of pure ML-MoS2 appears at an emission wavelength of 440 nm, which corresponds to the resonant absorption of the indirect bandgap C exciton level. Figure 3 c shows a comparison between the SHG enhancement coefficient and the RPP absorption spectrum. The SHG enhancement coefficient reaches its peak at 406 nm and 465 nm, corresponding to the two deep energy levels of RPP at 3.08 eV (approximately 402 nm) and 2.65 eV (approximately 468 nm), respectively. When the excitation wavelength is 940 nm, the maximum SHG enhancement coefficient of MoS2 reaches 89 times. Therefore, it can be inferred that RPP directly transfers the two-photon excitation resonance energy absorbed at deep energy levels to MoS2 through the coupling interaction between dipoles, thereby enhancing SHG emission.

[0063] Example 4

[0064] Compared to Example 2, the only difference is that the thickness of the RPP is changed, roughly in the range of 15nm to 210nm.

[0065] RPPs of different thicknesses were obtained on PDMS by mechanical exfoliation. Nanosheets with a thickness range of 15~210 nm were selected and stacked with ML-MoS2 to construct ML-MoS2 / RPP van der Waals heterojunctions by dry transfer technology. Figure 4 Image a shows the optical (i), AFM (ii), and SHG intensity (iii) images of typical heterostructures composed of ML-MoS2 and RPP of different thicknesses. The white and blue dashed areas represent RPP and ML-MoS2. Scale bar: 5 μm. Figure 4 b is Figure 4SHG spectra of the HS-1, HS-2, HS-3 and ML-MoS2 regions in a, where HS-1, HS-2 and HS-3 are heterojunctions composed of ML-MoS2 and RPP with thicknesses of 73 nm, 58 nm and 15 nm, respectively. Figure 4 c is Figure 4 The corresponding SHG intensity distribution and AFM height distribution are extracted along the red dashed line in Figure a. It can be seen that RPP of different thicknesses will produce significantly different SHG outputs. Figure 4 Figure d shows the variation of SHG intensity in the ML-MoS2 / RPP heterojunction with RPP thickness. Figure e illustrates the step energy transfer in a heterojunction of RPP and ML-MoS2 with a certain thickness. This shows that energy is transferred layer by layer within the RPP. Energy transfer increases with RPP thickness, but when the thickness exceeds 100 nm, the region above 100 nm absorbs energy generated within the heterojunction region, reducing the nonlinear energy transferred upwards to MoS2 and thus decreasing the MoS2 SHG enhancement factor.

[0066] Example 5

[0067] Compared to Example 2, the only difference is that the composition of the acceptor two-dimensional material B or the donor RPP in the heterojunction was changed to verify the universality of this mechanism. The experimental groups were as follows:

[0068] Groups with altered receptor materials:

[0069] Group A: Acceptor material B is 1L-MoS2 (monolayer MoS2, also known as ML-MoS2), and donor material A is n=2 RPP (i.e., RPP prepared in Example 1); this group is labeled as 1L-MoS2 / RPP;

[0070] Group B: Acceptor material B is 3L-MoS2 (trilayer MoS2), and donor material A is RPP with n=2; this group is labeled as 3L-MoS2 / RPP;

[0071] Group C: Acceptor material B is 5L-MoS2 (five-layer MoS2), and donor material A is RPP with n=2; this group is labeled as 5L-MoS2 / RPP;

[0072] Group D: Recipient material B is WSe2 (monolayer WSe2), and donor material A is RPP with n=2; this group is labeled WSe2 / RPP;

[0073] Group E: Acceptor material B is MoSe2 (monolayer MoSe2), and donor material A is RPP with n=2; this group is labeled as MoSe2 / RPP;

[0074] Change the group of donor materials:

[0075] Group F: Acceptor material B is MoS2 (monolayer MoS2), and donor material A is n=1 RPP; this group is labeled as MoS2 / RPP (n=1);

[0076] Group G: Acceptor material B is MoS2 (monolayer MoS2), and donor material A is RPP with n=3; this group is labeled as MoS2 / RPP (n=3);

[0077] Group H: Acceptor material B is MoS2 (monolayer MoS2), and donor material A is RPP with n=4; this group is labeled as MoS2 / RPP (n=4);

[0078] Figure 5 a-5b shows the SHG measurements and comparisons of MoS2 with different thicknesses and heterojunctions composed of the same RPP. It is clear that the SHG signal enhancement coefficients of MoS2 with different thicknesses are basically the same. Figure 5 c-5d shows the SHG measurements and comparisons of heterojunctions composed of different receptor two-dimensional material B and RPP. It can be seen that for different receptor two-dimensional material B, there is a significant SHG signal enhancement after forming a heterojunction with RPP. Figure 5 SHG measurements and comparisons were performed on heterostructures composed of RPP with different organic ratios and ML-MoS2 in e-5g. It can be seen that RPP with different organic ratios significantly enhances the SHG signal of ML-MoS2. This indicates that the method of enhancing the SHG of two-dimensional materials through a nonlinear energy transfer mechanism has universality.

[0079] Example 6

[0080] Compared to Example 2, the only difference is that the acceptor material B is replaced with an h-BN film.

[0081] h-BN nanosheets of different thicknesses were obtained on PDMS by mechanical exfoliation. An h-BN nanosheet with a thickness of 80 nm was selected and stacked with RPP by dry transfer to construct an h-BN / RPP heterojunction. Figure 6 As can be seen, SHG measurements and comparisons were performed on Pure h-BN and h-BN / RPP heterojunctions, where Pure h-BN represents pure h-BN outside the heterojunction region. The attached test figures show a significant enhancement of the SHG signal, with an enhancement factor reaching 525. This further demonstrates the universality of the method for enhancing the SHG of two-dimensional materials through a nonlinear energy transfer mechanism.

[0082] Comparative Example 1

[0083] Compared to Example 2, the only difference is that single-photon excitation light is used for pumping.

[0084] In Example 2, both steady-state and transient tests of the van der Waals heterojunction were performed using 800nm ​​two-photon excitation optical pumping, while the transient test in Comparative Example 1 used 400nm single-photon excitation optical pumping.

[0085] The difference lies in the carrier dynamics at the heterojunction interface, such as... Figure 7 a is an interlayer CT image of an ML-MoS2 / RPP heterojunction under single-photon excitation. Figure 7 Figures b and 7c show the power pulsation (PL) lifetimes of Pure RPP and ML-MoS2 in the pure sample region and HS region of the ML-MoS2 / RPP heterojunction. Pure RPP and Pure MoS2 represent pure RPP and pure MoS2 outside the heterojunction region, respectively. The Pure RPP Fit, HS RPP Fit, Pure MoS2 Fit, and HS MoS2 Fit curves in the figures represent the fitted curves of the corresponding PL lifetimes. This indicates that under 400nm single-photon excitation, ML-MoS2 / RPP undergoes carrier transfer due to type II bandgap arrangement, leading to the dual quenching of MoS2 and RPP in the heterojunction region. This proves that the SHG signal enhancement is caused by energy transfer rather than interface effects.

[0086] Comparative Example 2

[0087] Compared to Example 5, the only difference is that the RPP containing organic components is replaced with the all-inorganic perovskite CsPbBr3.

[0088] Figure 8 a represents a van der Waals heterojunction of ML-MoS2 and CsPbBr3 stacked via dry transfer, with a scale bar of 10 μm. Figure 8 b is the spatially resolved SHG spectrum of the ML-MoS2 / CsPbBr3 heterojunction, scale bar 8 μm. Figure 8 c is a comparison of SHG intensity at the heterojunction of ML-MoS2 and ML-MoS2 / CsPbBr3. It can be found that the SHG signal on the heterojunction is significantly reduced, indicating that the all-inorganic perovskite CsPbBr3 cannot enhance the SHG signal of the two-dimensional material.

[0089] from Figure 1 It can be seen that each step of the solution method for preparing RPP, as well as the large-size, uniformly thick RPP obtained by mechanical exfoliation;

[0090] from Figure 2 It can be seen that the prepared ML-MoS2 / RPP has high and uniform SHG intensity in the heterojunction region. The SHG intensity of ML-MoS2 in the heterojunction is about 50 times higher than that of the pure ML-MoS2 region. Furthermore, the image slope of SHG signal and power in the two regions further illustrates the second-order nonlinear light emission of the SHG signal.

[0091] from Figure 3 It can be seen that by statistically analyzing a series of SHG enhancement coefficients of ML-MoS2 / RPP in heterojunction based on different excitation wavelengths, it is verified that the SHG enhancement originates from nonlinear ET, and the two SHG enhancement coefficient peaks are related to the deep energy levels of RPP, located at 3.08 eV and 2.65 eV, respectively.

[0092] from Figure 4 It can be seen that by statistically analyzing a series of ML-MoS2 / RPP SHG enhancement coefficients based on RPPs of different thicknesses in heterojunctions, this invention can regulate the enhancement degree of MoS2 SHG signal by controlling the thickness of the RPP.

[0093] from Figure 5 It can be seen that, by statistical analysis of heterojunctions composed of MoS2 with different thicknesses and different TMDs materials and RPP, as well as heterojunctions composed of ML-MoS2 and RPP with different organic ratios, under 800nm ​​two-photon excitation optical pumping, the SHG signal in the heterojunction region is stronger than that in the pure acceptor material region, and the enhancement coefficients obtained by different thicknesses of MoS2 on the same RPP are basically the same.

[0094] from Figure 6 It can be seen that the SHG enhancement factor obtained by the heterojunction composed of h-BN / RPP is about 525;

[0095] from Figure 7 It can be seen that under 400nm single-photon excitation, ML-MoS2 / RPP will undergo carrier transfer caused by type II band arrangement, which will lead to the double quenching of MoS2 and RPP in the heterojunction region.

[0096] from Figure 8 It can be seen that when inorganic perovskite CsPbBr3 is used to replace RPP and form a heterojunction with ML-MoS2, the SHG strength of MoS2 is weakened to a certain extent.

Claims

1. A method for enhancing second harmonic emission based on a non-linear energy transfer mechanism, characterized by: A heterojunction composed of a donor two-dimensional material A and an acceptor two-dimensional material B is obtained, and then excited by a two-photon pump light source to induce non-linear energy transfer, thereby enhancing the second harmonic emission of the acceptor two-dimensional material B; In the heterojunction, the donor two-dimensional material A is (LA)2(A) n-1 Pb n I 3n+1 ; wherein LA is a C3-C6 alkylammonium ion or a phenylammonium ion; A is a C1-C2 alkylammonium ion or a formamidium ion; and n is 1-4. The acceptor two-dimensional material B comprises at least one of MoS2, MoSe2, WSe2 and h-BN.

2. The method of claim 1, wherein the non-linear energy transfer mechanism is based on a second harmonic generation. The LA is a butylammonium ion; the A is a methylammonium ion; and the n is 2-3.

3. The method of claim 2, wherein the non-linear energy transfer mechanism is based on a second harmonic generation. The donor two-dimensional material A is (BA)2(MA)Pb2I7.

4. The method for enhancing second harmonic generation based on nonlinear energy transfer mechanism according to any one of claims 1-3, wherein: The thickness of the donor two-dimensional material A is less than 210 nm.

5. The method of claim 4, wherein the non-linear energy transfer mechanism is based on a second harmonic generation. The thickness of the donor two-dimensional material A is 70-80 nm.

6. The method of claim 1, wherein the non-linear energy transfer mechanism is based on a second harmonic generation. The acceptor two-dimensional material B is h-BN.

7. The method of enhancing second harmonic emission based on a non-linear energy transfer mechanism as claimed in claim 1 or 6, wherein: The thickness of the acceptor two-dimensional material B is 0.6-80 nm.

8. The method of claim 1, wherein the non-linear energy transfer mechanism is based on a second harmonic generation. The acceptor two-dimensional material B is stacked on the donor two-dimensional material A by a mechanical exfoliation method and a dry transfer technique to obtain the heterojunction.

9. The method of claim 8, wherein the non-linear energy transfer mechanism is based on a second harmonic generation. The heterojunction is formed on a substrate, wherein the donor two-dimensional material A is on the surface of the substrate, and then the excitation is performed.

10. The method of claim 9, wherein the non-linear energy transfer mechanism is based on a second harmonic generation. The substrate is a SiO2 / Si substrate.

11. The method of claim 1, wherein the non-linear energy transfer mechanism is based on second harmonic generation. The two-photon pump light source is a femtosecond pulse laser with a wavelength of 760-1000 nm.

12. The method of claim 11, wherein the non-linear energy transfer mechanism is based on second harmonic generation. The wavelength of the two-photon pump light source is 920-960 nm.

13. The method of claim 12, wherein the non-linear energy transfer mechanism is based on second harmonic generation. The wavelength of the two-photon pump light source is 935-945 nm.

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