Surface plasmon composite structure based on ultrathin metal film and preparation method thereof

Abstract

The invention discloses a surface plasmon composite structure based on an ultra-thin metal film and a preparation method of the surface plasmon composite structure, realizes high-momentum ultra-thin metal film surface plasmon mode efficient excitation, and solves the problem that the existing prism coupling excitation technology cannot realize high-momentum surface plasmon mode excitation. The high-momentum surface plasmon mode excitation provides a super-local electric field, the interaction between light and substances is remarkably enhanced, and a new technical approach is provided for a higher-performance optoelectronic device. According to the structural design, two-step coupling of momentum difference formed by a suspension area and a supporting area and a phase resonance mechanism are adopted, efficient excitation of a high momentum SPP mode is guaranteed, absorption peak adjustment in the range of 700-1600 nm can be achieved by adjusting the width and period of the strip and the thickness of the metal film, and when the polarization direction of incident light is perpendicular to the nanometer strip, the absorption rate of the absorption peak can be larger than 50%. The preparation of a high-quality continuous ultrathin metal film and a surface plasmon mode excitation structure is realized, ion pollution and glue residue are avoided, and the application is wide.

Description

Technical Field

[0001] This invention relates to the field of nano-optics and optoelectronic devices, specifically to a surface plasmon composite structure based on an ultrathin metal film and its preparation method. Background Technology

[0002] Surface plasmon polaritons (SPPs) are surface waves formed by the coupling of electron oscillations and electromagnetic waves at the metal-dielectric interface. Due to their subwavelength-scale electric field localization, SPPs hold significant application potential in biochemical sensing, nonlinear optics, and photoelectric detection. However, in traditional thick metal films, the coupling between SPP modes at the metal-dielectric interface is weak, the mode splitting is small, and the resulting short-range SPP mode wave vectors have limited growth, thus limiting their localization ability, typically less than twice the size of the dielectric wave vector. To achieve higher field confinement and thus enhance light-matter interaction, the metal film thickness needs to be reduced, for example, to below 10 nm, or even 1 nm. At this thickness, the SPP modes at the upper and lower interfaces are strongly coupled, the mode splitting is large, and short-range SPP modes with high wave vector sizes can reach more than 10 times the wave vector in the dielectric, significantly enhancing the field localization ability.

[0003] However, achieving surface plasmon resonance (SPR) in such thin metal films presents two major challenges. First, the fabrication process is difficult; traditional atomic deposition methods struggle to prepare continuous metal films with thicknesses below 10 nm. Furthermore, planar transfer of such thin films is prone to damage and contamination. Second, the plasmon wave vector of this mode is significantly increased, resulting in a substantial momentum mismatch with free-space light and consequently low excitation efficiency. Therefore, there is an urgent need for an ultrathin metal film structure with a rational design that can efficiently excite surface plasmon resonance modes, as well as a controllable fabrication method suitable for sub-10 nm metal film structures. Summary of the Invention

[0004] To address the challenges of efficient excitation of surface plasmons in ultrathin metal films and the difficulties in film fabrication, this invention proposes a surface plasmon composite structure based on ultrathin metal films and its preparation method. Specifically, it presents an ultrathin metal film surface plasmon composite structure excited by boundary scattering of nanostripes, and provides a method for preparing ultrathin metal films involving thick-film transfer followed by chemical thinning. This method is particularly suitable for high-sensitivity molecular sensing and optical field enhancement devices. The technical solution of this invention is as follows:

[0005] This invention discloses a surface plasmon composite structure based on an ultrathin metal film, comprising, from bottom to top:

[0006] High-refractive-index dielectric nanostrip periodic array;

[0007] An ultrathin continuous metal film covering a periodic array of nanostrips in a high refractive index medium;

[0008] The nanostrips are rectangular blocks arranged at intervals, with grooves formed between adjacent rectangular blocks;

[0009] The structure forms two regions: region 1, corresponding to the air / metal film / air in the nanostrip grooves, and region 2, corresponding to the air / metal film / / high refractive index medium in the nanostrip rectangular blocks. The two regions respectively form surface plasmon modes with different effective refractive indices.

[0010] As a further improvement, the high refractive index dielectric nanostrip periodic array and the ultrathin continuous metal film of the present invention further include:

[0011] A low-refractive-index medium interstitial layer is placed over the high-refractive-index medium nanostrips to form a periodic array of low-refractive-index nanostrips;

[0012] When a periodic array of low-refractive-index nanostrips is included, region 2 is air / metal film / low-refractive-index medium / high-refractive-index medium.

[0013] As a further improvement, the ultrathin metal film of the present invention is any one of the following metal materials: gold, silver, platinum, and copper.

[0014] As a further improvement, the thickness of the ultrathin metal film described in this invention is less than 10 nm.

[0015] As a further improvement, the high refractive index dielectric nanostrip periodic array of the present invention is silicon nitride, silicon, aluminum oxide, titanium dioxide or other dielectric materials with a refractive index greater than 1.8, and the thickness of the ultrathin metal film is preferably 1-5 nm.

[0016] As a further improvement, the low refractive index medium interstitial layer of the present invention is any one of silicon oxide, aluminum oxide, and polystyrene, and the thickness of the low refractive index medium interstitial layer is less than 5 nm.

[0017] As a further improvement, the nanostrips described in this invention excite surface plasmon modes in two regions through boundary scattering. Simultaneously, the two modes can interconvert at the boundary. To achieve efficient excitation, the following resonance condition must be satisfied:

[0018] ;

[0019] Where k1 and k2 are the wave vectors of the surface plasmon modes in region 1 and region 2, respectively, and w and s are the width of the strip rectangular block and the strip spacing groove, respectively. and These are the phase transitions during mode conversion in Region 1 and Region 2, respectively, where m is an integer, w is 50-800 nm, and s is 50-500 nm.

[0020] This invention also discloses a method for preparing a surface plasmon composite structure based on an ultrathin metal film, characterized by comprising the following steps:

[0021] (1) Periodic nanostrips are prepared on a high refractive index medium layered film or substrate to form a high refractive index medium nanostrip periodic array; or periodic nanostrips are prepared on a high refractive index medium layered film or substrate to form a high refractive index medium nanostrip periodic array, and a low refractive index medium interstitial layer is deposited or formed by natural oxidation on the high refractive index periodic nanostrips to form a low refractive index nanostrip periodic array with a thickness of less than 5 nm;

[0022] (2) A single crystal metal sheet with a thickness of 20–30 nm is transferred to the surface of a high refractive index medium nanostrip periodic array or a low refractive index nanostrip periodic array by dry transfer, so that the metal sheet covers the rectangular blocks and groove regions of the nanostrips;

[0023] (3) Using a chemical etching solution, the etching time, temperature and solution concentration are controlled to chemically thin the metal sheet to the required thickness, usually less than 10 nm, preferably 1-5 nm, to form an ultrathin continuous metal film; (4) Clean and dry with inert gas to obtain the final structure.

[0024] This invention also discloses a surface plasmon composite structure based on an ultrathin metal film for high-sensitivity biochemical sensing, near-infrared photoelectric detection, and field-enhanced second and third frequency-harmonic nonlinear optical applications.

[0025] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0026] (1) This invention realizes efficient excitation of high momentum ultrathin metal film surface plasmon modes, and solves the problem that existing prism coupling excitation technology cannot realize the excitation of high momentum surface plasmon modes.

[0027] (2) High-momentum surface plasmon mode excitation provides a superlocal electric field, which significantly enhances the interaction between light and matter, providing a new technical approach for higher performance optoelectronic devices.

[0028] (3) The structural design adopts a two-step coupling and phase resonance mechanism to form a momentum difference between the suspended and supported regions, which ensures the efficient excitation of the high momentum SPP mode.

[0029] (4) The absorption peak in the range of 700-1600 nm can be adjusted by adjusting the strip width, period and metal film thickness. When the polarization direction of the incident light is perpendicular to the nano strip, the absorption rate of the absorption peak can be greater than 50%.

[0030] (5) Reliable preparation method: The process of combining transfer and thinning is adopted to achieve the preparation of high-quality continuous ultrathin metal film and surface plasmon mode excitation structure, avoiding ion contamination and glue residue, and also avoiding the film cracking and discontinuity problems caused by direct transfer of sub-5 nm metal film, with good controllability and repeatability.

[0031] (6) Wide range of applications: It is suitable for high-sensitivity biochemical sensing, near-infrared photoelectric detection and second harmonic and third harmonic generation of nonlinear optics. Attached Figure Description

[0032] Figure 1 Schematic diagram of cross-section of structure, (a) is the structure without low refractive index medium layer; (b) is the structure with low refractive index medium layer.

[0033] Figure 2 It is an SPP dispersion curve of gold films of different thicknesses with air / gold film / silicon oxide / silicon structures;

[0034] Figure 3 This is an absorption spectrum curve of gold films of different thicknesses above silica / silicon nanoribbons.

[0035] Figure 4 This is a graph showing the absorption spectrum of the gold film above the silica / silicon nanoribbon as a function of the nanoribbon width.

[0036] Figure 5 This is a flowchart of the fabrication process for a low-refractive-index medium interstitial layer structure;

[0037] Figure 6 This is a flowchart of the fabrication process for a low-refractive-index interstitial layer structure.

[0038] 1 is a high refractive index dielectric layer or substrate; 2 is a high refractive index dielectric nanostrip periodic array; 3 is a low refractive index nanostrip periodic array; 4 is an ultrathin gold film; 5 is a photoresist; 6 is a single crystal gold wafer; 7 is PDMS; 8 is cysteine ​​chloroform solution. Detailed Implementation

[0039] This invention discloses a surface plasmon composite structure based on an ultrathin metal film 4, comprising, from bottom to top:

[0040] High refractive index dielectric nanostrip periodic array 2;

[0041] An ultrathin continuous metal film 4 is applied over a periodic array of high-refractive-index medium nanostrips 2;

[0042] The high refractive index medium nanostrip periodic array 2 consists of rectangular blocks arranged at intervals, with grooves formed between adjacent rectangular blocks;

[0043] The structure forms two regions: region 1, corresponding to the air / metal film / air of the groove of the high refractive index medium nanostrip periodic array 2, and region 2, corresponding to the air / metal film / / high refractive index medium of the rectangular block of the high refractive index medium nanostrip periodic array 2. The two regions respectively form surface plasmon modes with different effective refractive indices.

[0044] Between the high-refractive-index dielectric nanostrip periodic array 2 and the ultrathin continuous metal film 4, there is also:

[0045] A low-refractive-index medium interstitial layer is placed over the high-refractive-index medium nanostrips 2 to form a low-refractive-index nanostrip periodic array 3;

[0046] When the low-refractive-index nanostrip periodic array 3 is included, region 2 is air / metal film / low-refractive-index medium / high-refractive-index medium.

[0047] The ultrathin metal film 4 is any one of the metal materials of gold, silver, platinum and copper, and the thickness of the ultrathin metal film 4 is less than 10nm.

[0048] The high refractive index dielectric nanostrip periodic array 2 is made of silicon nitride, silicon, aluminum oxide, titanium dioxide or other dielectric materials with a refractive index greater than 1.8, and the thickness of the ultrathin metal film 4 is preferably 1-5 nm.

[0049] The low-refractive-index dielectric nanostrip periodic array 3 is any one of the low-refractive-index dielectric materials, such as silicon oxide, aluminum oxide, and polystyrene, with a thickness of less than 5 nm.

[0050] Nanostrip periodic arrays 2 and 3 excite surface plasmon modes in two regions through boundary scattering. Simultaneously, the two modes can interconvert at the boundary. To achieve efficient excitation, the following resonance condition must be satisfied:

[0051] ;

[0052] Where k1 and k2 are the wave vectors of the surface plasmon modes in region 1 and region 2, respectively, and w and s are the width of the strip rectangular block and the strip spacing groove, respectively. and These are the phase transitions during mode conversion in Region 1 and Region 2, respectively, where m is an integer, w is 50-800 nm, and s is 50-500 nm.

[0053] This invention also discloses a method for preparing a surface plasmon composite structure based on an ultrathin metal film 4, characterized by comprising the following steps:

[0054] 1) A periodic nanostrip array is prepared on a high-refractive-index medium layered film or substrate 1 to form a high-refractive-index medium nanostrip periodic array 2; or a periodic nanostrip array is prepared on a high-refractive-index medium layered film or substrate to form a high-refractive-index medium nanostrip periodic array 2, and a low-refractive-index medium interstitial layer is deposited or formed on the high-refractive-index periodic nanostrip array 2 by natural oxidation to form a low-refractive-index nanostrip periodic array 3 with a thickness of less than 5 nm;

[0055] 2) A single-crystal metal sheet 6 with a thickness of 20–30 nm is transferred to the surface of a high-refractive-index medium nanostrip periodic array 2 or a low-refractive-index nanostrip periodic array 3 by dry transfer, so that the metal sheet 6 covers the rectangular blocks and groove areas of the high-refractive-index medium nanostrip periodic array 2 or the low-refractive-index nanostrip periodic array 3.

[0056] 4) Using a chemical etching solution, the etching time, temperature and solution concentration are controlled to chemically thin the metal sheet 6 to the required thickness, usually less than 10 nm, preferably 1-5 nm, to form an ultrathin continuous metal film 4; 5) Clean and dry with inert gas to obtain the final structure.

[0057] This invention also discloses a surface plasmon composite structure based on an ultrathin metal film 4 for high-sensitivity biochemical sensing, near-infrared photoelectric detection, and field-enhanced second and third frequency-harmonic nonlinear optical applications.

[0058] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific implementation examples.

[0059] Example 1: Structure with a low refractive index dielectric layer

[0060] like Figure 1 As shown in (a), a silicon nanostrip periodic array is prepared on a silicon substrate with a width w of 100 nm, a strip spacing s of 200 nm, and a strip height of 250 nm. The surface is naturally oxidized to form a silicon dioxide nanostrip periodic array with a thickness of about 2 nm. An ultrathin metal film 4 is covered on the silicon dioxide nanostrip periodic array to form an air / gold film / air region 1 corresponding to the groove of the silicon nanostrip periodic array and an air / gold film / silicon dioxide / silicon region 2 corresponding to the rectangular block of the silicon nanostrip periodic array. Figure 2 The figure shows the dispersion curves of gold films with different thicknesses (4, 6, 8, 10 nm) in simulation calculation region 2. It can be seen that the wave vector magnitude (k) can be more than 10 times greater than that of free space (k0). Figure 3 To simulate and calculate the absorption curves of the structure with gold films of different thicknesses (4, 6, 8, 10 nm), the absorption rate reached a peak of 56% at a wavelength of 1380 nm when the gold film thickness was 4 nm. Figure 4The absorption curves are for different silicon strip widths (80-140 nm) when the gold film thickness is 6 nm. The absorption peak can be adjusted to 1060-1390 nm by adjusting the silicon strip width.

[0061] like Figure 5 The diagram shows the fabrication process of this structure, including the following steps:

[0062] (1) such as Figure 5 As shown in (a), silicon wafer 1 was ultrasonically cleaned with acetone, ethanol and deionized water in sequence, and then dried with nitrogen gas. Its surface was naturally oxidized to form a silicon dioxide layer with a thickness of about 2 nm.

[0063] (2) such as Figure 5 As shown in (b), PMMA photoresist 5 is uniformly coated on the silicon wafer using a spin coater, with a thickness of approximately 220 nm.

[0064] (3) such as Figure 5 As shown in (c), a nano-strip periodic array of photoresist 5 is formed by electron beam exposure patterning of photoresist 5, development for 35 seconds, and fixing for 35 seconds.

[0065] (4) such as Figure 5 As shown in (d), the entire structure was heated at 180 degrees Celsius for 90 seconds on a heating plate, and then etched by inductively coupled plasma (ICP) for 40 seconds to prepare periodic nanostrips on silicon dioxide and silicon substrate. The etching depth was about 250 nm. Then the photoresist 5 was removed to form a periodic array of silicon nanostrips and a periodic array of silicon dioxide nanostrips.

[0066] (5) such as Figure 5 As shown in (e), a single-crystal gold wafer 6 with a thickness of 20-30 nm prepared by chemical growth on a substrate is transferred and bonded to polydimethylsiloxane (PDMS7) 7 using a semi-dry method. The specific process is as follows: First, a PDMS7 sheet with a size of about 1 mm x 1 mm is cut and glued to a glass slide. Under a microscope, the PDMS7 is aligned with the single-crystal gold wafer 6 on the substrate. The PDMS7 is slowly placed to make it adhere to the target gold wafer 6. Then, a drop of deionized water is dropped from the side. After the water enters the gap between the PDMS7 and the substrate, the PDMS7 is slowly lifted, and the target gold wafer can be transferred from the substrate to the PDMS7.

[0067] (6) For example Figure 5As shown in (f), the single-crystal gold sheet 6 on the PDMS7 is aligned with the high-refractive-index nanostrip periodic array 2 and the low-refractive-index nanostrip periodic array 3 under a microscope. It is slowly lowered until the single-crystal gold sheet 6 adheres to the high-refractive-index nanostrip periodic array 2 and the low-refractive-index nanostrip periodic array 3, and then slowly lifted. This allows the single-crystal gold sheet 6 to be peeled off from the PDMS7 and transferred to the surfaces of the high-refractive-index nanostrip periodic array 2 and the low-refractive-index nanostrip periodic array 3.

[0068] (7) For example Figure 5 As shown in (g), the structure obtained in (6) was placed in cysteamine chloroform solution 8 (concentration of 20 mg / mL) for etching at a rate of about 8 nm / h. The etching time was controlled to reduce the thickness of the gold sheet 6 to about 5 nm, forming an ultrathin gold film 4.

[0069] (8) such as Figure 5 As shown in (h), the structure was removed from the etching solution, washed with deionized water, dried with nitrogen, and stored.

[0070] The above describes the preparation method only when silicon is used as the material for the high-refractive-index nanostrip periodic array 2. For other high-refractive-index materials (such as silicon nitride, titanium dioxide, and alumina), it is often necessary to first prepare a film layer (100-500 nm thick) on the substrate using processes such as atomic deposition and magnetron sputtering before preparing the high-refractive-index nanostrip periodic array 2. The subsequent steps are the same as the method described above. Furthermore, silicon and silicon nitride form a natural oxide layer of approximately 2 nm thick silicon dioxide in air as a low-refractive-index interstitial layer. For other highly stable materials, it is necessary to first prepare a thin layer of low-refractive-index material (such as silicon dioxide, polymers, etc., typically less than 5 nm thick) on the material surface using methods such as atomic deposition and magnetron sputtering before preparing the low-refractive-index nanostrip periodic array 3. The subsequent steps are the same as the method described above. These variations are readily conceived and implemented by those skilled in the art and are within the scope of protection of this invention.

[0071] Example 2: Structure without low refractive index dielectric layer

[0072] like Figure 1 As shown in (b), alumina is prepared into a periodic array structure of nano-strips (alumina is very stable, and no new dielectric layer material will form on its surface). An ultrathin gold sheet 4 is directly covered on top of the alumina nano-strip periodic array, forming region 1 (air / gold film / air) corresponding to the grooves of the alumina nano-strip periodic array and region 2 (air / gold film / high refractive index medium) corresponding to the rectangular blocks of the alumina nano-strip periodic array. Simulation calculations show that the absorption peak at a wavelength of 870 nm is approximately 40%, and the absorption peak position can also be adjusted by adjusting the strip width.

[0073] like Figure 6The diagram shows the fabrication process of this structure, including the following steps:

[0074] (1) such as Figure 6 As shown in (a), the surface of alumina 1 was ultrasonically cleaned with acetone, ethanol and deionized water in sequence, and then dried with nitrogen.

[0075] (2) such as Figure 6 As shown in (b), PMMA photoresist 5 is uniformly coated on top of alumina using a spin coater, with a thickness of approximately 220 nm.

[0076] (3) such as Figure 6 As shown in (c), a nano-strip periodic array of photoresist 5 is formed by electron beam exposure patterning of photoresist 5, development for 35 seconds, and fixing for 35 seconds.

[0077] (4) such as Figure 6 As shown in (d), the entire structure was heated at 180 degrees Celsius for 90 seconds on a heating plate, and then etched for 40 seconds by inductively coupled plasma (ICP) to prepare a periodic array of nanostrips on alumina with an etching depth of about 250 nm. Subsequently, the photoresist 5 was removed to form an alumina nanostrip periodic array.

[0078] (5) such as Figure 6 As shown in (e), a single-crystal gold wafer 6 with a thickness of 20-30 nm on a substrate, prepared by chemical growth method, is transferred and bonded to polydimethylsiloxane PDMS7 by a semi-dry method.

[0079] (6) For example Figure 6 As shown in (f), the single crystal gold sheet 6 on the PDMS7 is aligned with the alumina nano-strip periodic array under a microscope. It is slowly lowered until the single crystal gold sheet 6 adheres to the alumina nano-strip periodic array, and then slowly lifted. This allows the single crystal gold sheet 6 to be peeled off from the PDMS7 and transferred to the surface of the alumina nano-strip periodic array.

[0080] (7) For example Figure 6 As shown in (g), the structure obtained in (6) was placed in cysteamine chloroform solution 8 (concentration of 20 mg / mL) for etching at a rate of about 8 nm / h. The etching time was controlled to reduce the thickness of the gold sheet 6 to about 5 nm, forming an ultrathin gold film covering the alumina nanoribbon periodic array.

[0081] (8) such as Figure 6 As shown in (h), the structure was removed from the etching solution, washed with deionized water, dried with nitrogen, and stored.

[0082] The above are only two specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. The scope of protection of the present invention should be determined by the claims.

[0083] The above embodiments are used to explain and illustrate the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

[0084] National Key Research and Development Program

[0085] Project Number: 2023YFB3405600

[0086] Project Name: Atomic-Precision Surface Fabrication and Atomic-Precision Feature-Dimensional Structures and Devices.

Claims

1. A surface plasmon composite structure based on an ultrathin metal film, characterized in that, Including from bottom to top: High-refractive-index dielectric nanostrip periodic array; An ultrathin continuous metal film covering a periodic array of nanostrips in a high refractive index medium; The nanostrips are rectangular blocks arranged at intervals, with grooves formed between adjacent rectangular blocks; The structure forms two regions: region 1, corresponding to the air / metal film / air of the nanostrip grooves, and region 2, corresponding to the air / metal film / / high refractive index medium of the nanostrip rectangular blocks. The two regions respectively form surface plasmon modes with different effective refractive indices.

2. The surface plasmon composite structure based on an ultrathin metal film according to claim 1, characterized in that, The high-refractive-index dielectric nanostrip periodic array and the ultrathin continuous metal film also include: A low-refractive-index medium interstitial layer is placed over the high-refractive-index medium nanostrips to form a periodic array of low-refractive-index nanostrips; When a periodic array of low-refractive-index nanostrips is included, region 2 is air / metal film / low-refractive-index medium / high-refractive-index medium.

3. The structure according to claim 1 or 2, characterized in that, The ultrathin metal film is any one of the following metal materials: gold, silver, platinum, and copper.

4. The structure according to claim 3, characterized in that, The thickness of the ultrathin metal film is less than 10 nm.

5. The structure according to claim 4, characterized in that, The high refractive index dielectric nanostrip periodic array is silicon nitride, silicon, aluminum oxide, titanium dioxide, or other dielectric materials with a refractive index greater than 1.8, and the thickness of the ultrathin metal film is preferably 1-5 nm.

6. The structure according to claim 5, characterized in that, The low-refractive-index interstitial layer is any one of silicon oxide, aluminum oxide, or polystyrene, and the thickness of the low-refractive-index interstitial layer is less than 5 nm.

7. The structure according to claim 6, characterized in that, Nanostrips excite surface plasmon modes in two regions through boundary scattering. Simultaneously, the two modes can interconvert at the boundary. To achieve efficient excitation, the following resonance condition must be satisfied: ； Where k1 and k2 are the wave vectors of the surface plasmon modes in region 1 and region 2, respectively, and w and s are the width of the strip rectangular block and the strip spacing groove, respectively. and These are the phase transitions during mode conversion in Region 1 and Region 2, respectively, where m is an integer, w is 50-800 nm, and s is 50-500 nm.

8. A method for preparing the structure as described in claim 1, 2, 4, 5, 6, or 7, characterized in that... Includes the following steps: 1) Prepare periodic nanostrips on a high-refractive-index medium layered film or substrate to form a high-refractive-index medium nanostrip periodic array; or prepare periodic nanostrips on a high-refractive-index medium layered film or substrate to form a high-refractive-index medium nanostrip periodic array, and deposit or form a low-refractive-index medium interstitial layer on the high-refractive-index periodic nanostrips through natural oxidation to form a low-refractive-index nanostrip periodic array with a thickness of less than 5 nm; 2) A single-crystal metal sheet with a thickness of 20–30 nm is applied to the surface of a high-refractive-index medium nanostrip periodic array or a low-refractive-index nanostrip periodic array by dry transfer, so that the metal sheet covers the rectangular blocks and groove regions of the nanostrips. 4) Using a chemical etching solution, the etching time, temperature and solution concentration are controlled to chemically thin the metal sheet to the required thickness, usually less than 10 nm, preferably 1-5 nm, to form an ultrathin continuous metal film; 5) Clean and dry with inert gas to obtain the final structure.

9. A surface plasmon composite structure based on an ultrathin metal film is applied to high-sensitivity biochemical sensing, near-infrared photoelectric detection, and field-enhanced second and third frequency-harmonic nonlinear optics applications.

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