A spiral cone metal probe and a method for super-resolution near-field detection using the same

Abstract

The present invention discloses a spiral cone metal probe and a method for super-resolution near-field detection using the same. The spiral cone metal probe is a spiral cone metal probe based on the end face of an optical fiber, which can be excited by linearly polarized light to form a local electromagnetic field enhancement "hot spot" at the tip of the cone, thereby achieving ultra-high near-field resolution below 10 nm. The preparation method of the spiral cone metal probe provided by the present invention is simple and can be mass-produced. The probe can achieve a near-field detection resolution below 10 nm and has broad application prospects in the field of near-field detection.

Description

Technical Field

[0001] The present invention belongs to the technical field of surface plasmon optics, and particularly relates to a spiral cone metal probe and a method for super-resolution near-field detection using the same. Background Art

[0002] The near-field scanning optical microscope (SNOM) is based on a scanning probe to excite or collect near-field signals, and can break through the optical diffraction limit. It is a scanning probe technology for visible light, infrared, and terahertz imaging and spectroscopy, with a spatial resolution at the nanometer scale. It has been proven to have great application potential in fields ranging from material characterization to bioscience. As a type of SNOM, in the aperture-type scanning near-field optical microscope (a-SNOM), when the probe aperture is less than 100 nm, the light transmittance through the aperture will drop sharply. Currently, in practical applications, the probe light-transmitting aperture of a-SNOM is about 100 nm, and the highest optical spatial resolution is about 50 nm. Therefore, it becomes particularly important to improve the light transmittance and optical spatial resolution.

[0003] In recent years, due to the fact that surface plasmon polaritons (SPPs) have a mode volume that can break through the diffraction limit, the successful application of the metal probe plasmon focusing technology in the near-field scanning optical microscope (SNOM) has attracted extensive research. In these applications, conical structure probes are widely used. However, due to the symmetry of this structure, the surface plasmon polaritons (SPPs) excited by linearly polarized light on its surface will interfere destructively at the apex. Therefore, radially polarized light must be used for excitation, which is not convenient for practical applications.

[0004] In 2014, Li Jiafang et al. first proposed a hollow metal tapered fiber probe with spiral corrugations. By fabricating spiral corrugations along the tapered structure, asymmetry was introduced, realizing the focusing of SPPs insensitive to the incident light polarization at the cone tip and eliminating the complexity of beam and polarization alignment. Although this structure can achieve focusing at the cone tip, the surrounding noise field is also relatively large, and the transmission efficiency is only 1.4% (J. Li, J. Mu, B. Wang, W. Ding, J. Liu, H. Guo, W. Li, C. Gu, and Z.-Y. Li, “Direct laser writing of symmetry-broken spiral tapers for polarization-insensitive three-dimensional plasmonic focusing,” Laser Photonics Rev. 8, 602-609 (2014)). Summary of the Invention

[0005] The object of the present invention is to propose a spiral cone metal probe and a method for super-resolution near-field detection using the same.

[0006] The method for preparing the spiral cone metal probe provided by the present invention is simple and can be mass-produced. In addition, the resolution of this probe in near-field detection can be reduced to below 10 nm, showing broad application prospects in the field of near-field detection.

[0007] The present invention adopts the following technical solutions.

[0008] A spiral cone metal probe based on the end face of an optical fiber, comprising a photoresist dielectric layer and a gold film layer, wherein the photoresist dielectric layer includes a conical structure and a semi-elliptical thread structure on the conical surface.

[0009] The method for super-resolution near-field detection using the above spiral cone metal probe includes the following steps:

[0010] (1) Strip the coating layer from a section of single-mode optical fiber, cut the end face of the optical fiber flat with an optical fiber cutter, and fix it on an optical fiber fixture with a single-axis displacement stage. Fix a glass slide at the front end of the fixture, adjust the displacement stage so that the distance between the end face of the optical fiber and the glass slide is less than 0.5 mm, and drop photoresist at the edge of the glass slide to flow to the end face of the optical fiber, completely submerging the end face of the optical fiber in the photoresist;

[0011] (2) Fix the optical fiber fixture in a laser direct writing platform, write a program through a computer system to control the laser direct writing platform. First, print a conical structure on the end face of the optical fiber in the galvanometer mode, then use the piezoelectric mode to print the thread structure along the conical surface. After that, remove the optical fiber from the fixture, first immerse it in a developer to dissolve the unreacted photoresist, then transfer the optical fiber to an isopropyl alcohol solution to dissolve the developer, and the isopropyl alcohol solution on the surface of the optical fiber can naturally volatilize at room temperature after being taken out;

[0012] (3) Transfer the optical fiber to a magnetron sputtering coating machine to coat the surface of the structure on the end face of the optical fiber. The coating target is gold, set the coating power and coating time. After coating, the spiral cone metal probe is obtained on one end face of the optical fiber, and the other end face of the optical fiber is a smooth optical fiber cut surface without coating and without a probe;

[0013] (4) Fix the smooth cut surface end of the optical fiber with a metal probe at one end and a smooth cut surface at the other end on the displacement stage. Couple the laser output by the laser into the optical fiber from the end without a probe on the smooth cut surface through a coupling lens. Adjust the position of the end without a probe on the smooth cut surface of the optical fiber using the displacement stage to make the coupling efficiency reach about 9%. The input laser can excite the SPPs of the spiral cone metal probe to generate a "hot spot" at the tip of the cone, and the spot size at the end face of the optical fiber is about 8 nm;

[0014] (5) Glue the metal probe end of the optical fiber, which has a metal probe at one end and a smooth cut surface at the other end, to the tuning fork, and use it as the probe for the SNOM near-field imaging system of NT-MDT;

[0015] (6) The SNOM operates in the illumination mode. The light source is coupled from the distal end of the optical fiber into the probe, and the local hot spot signal generated inside the probe is used as the near-field illumination source for the sample. The probe is used to scan the sample in the near-field optical imaging scanning mode of the SNOM. Set the scanning speed to 15 um / s - 20 um / s, the acquisition interval to 2 nm - 40 nm. The light passing through the sample passes through a beam splitter and enters a photomultiplier tube (PMT) detector to collect the light intensity after passing through the grating, thereby obtaining the SNOM near-field scanning image.

[0016] Further, in step (2), remove the optical fiber from the fixture and first immerse it in the developer solution to dissolve the unreacted photoresist. The immersion time is not less than 20 minutes.

[0017] Further, in step (2), move the optical fiber to the isopropyl alcohol solution to dissolve the developer solution. The immersion time is not less than 3 minutes.

[0018] Further, in step (3), set the coating power to 50 W - 80 W and set the coating time to 80 seconds - 110 seconds.

[0019] Further, in step (4), use linearly polarized light as the light source for excitation.

[0020] Further, in step (6), extract a one-dimensional SNOM scan line profile across the boundary between two different materials in the sample from the SNOM near-field scanning image. The spatial resolution in the microscope is usually evaluated by measuring the width of the typical point-symmetric line profile across the sharp boundary between two different materials. This symmetric line profile is also called the edge spread function (ERF). The characteristic width w of the edge spread function (ERF) can be determined by its differential function, the line spread function (LSF). The LSF represents the image of a linear object and is a Gaussian function centered on the material boundary. The width of the LSF determines the spatial resolution according to specific criteria (such as Rayleigh). In the SNOM experiment, the spatial resolution is usually determined by the width value directly measured in the line profile (i.e., ERF) recorded across the boundary between two different materials. Among them, the width of the ERF can be determined by the full width at half maximum of the corresponding LSF function. Thus, the sharp change in the one-dimensional SNOM scan line profile is the boundary between two different materials in the sample. This section of the line profile is the ERF, and the resolution of the probe can be obtained from the full width at half maximum of the corresponding LSF function.

[0021] Compared with the conical structure probe in the prior art, the present invention uses a spiral cone structure with threads added to the surface, which can effectively break this symmetry. Using linearly polarized light can excite SPPs to generate "hot spots" at the tip of the cone, and the light spot can reach the order of a few nanometers. It can achieve an ultra-high near-field resolution with the resolution of near-field detection reduced to below 10 nm, and has a good signal-to-noise ratio in the mode field. The transmission efficiency can reach about 9%, which can better achieve high throughput and high resolution. Therefore, the proposal of the present invention makes it promising to have a broad application prospect in the field of high near-field super-resolution detection.

[0022] The present invention has the following advantages and beneficial effects:

[0023] (1) The preparation method of the spiral cone-shaped metal probe provided by the present invention is simple and can be mass-produced.

[0024] (2) The spiral cone metal probe of the present invention can achieve a resolution of near-field detection reduced to below 10 nm.

[0025] (3) The spiral cone metal probe of the present invention can also achieve accurate measurement of the sizes of one-dimensional and two-dimensional objects at the nanometer scale.

[0026] (4) The spiral cone metal probe of the present invention can achieve both a relatively fast scanning speed and accurate scanning results.

[0027] (5) The spiral cone metal probe of the present invention has a higher transmission efficiency, thus achieving a higher optical flux. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the preparation method and structure of the spiral cone probe of the present invention.

[0029] Figure 2 It is an experimental layout diagram of the spiral cone probe of the present invention applied to SNOM imaging and a partial enlarged view of the SNOM probe and the sample area.

[0030] Figure 3 It is an electron microscope image of a Ge film standard grating with a period of 510 nm and a grating line width of 83 nm, as well as an intensity spectrum diagram and a resolution curve of scanning the standard grating using the spiral cone probe of the present invention.

[0031] Figure 4 It is an electron microscope image of a Ge film standard two-dimensional grating with the letters "SCUT" engraved, as well as an intensity spectrum diagram and a resolution curve of scanning the two-dimensional standard sample using the spiral cone probe of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] To make the objectives, technical solutions, and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions of the present invention in conjunction with specific embodiments of the present invention and the corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0033] The following will, in conjunction with the drawings, elaborate on the technical solutions provided by each embodiment of the present invention.

[0034] Embodiment 1

[0035] The preparation steps of a spiral cone metal probe are as follows:

[0036] (1) Strip the coating layer from a section of single-mode optical fiber, use an optical fiber cutter to flatten the end face of the optical fiber, and fix it on an optical fiber fixture with a single-axis displacement stage. Fix a glass slide at the front end of the fixture, adjust the displacement stage so that the distance between the end face of the optical fiber and the glass slide is less than 0.5 mm. Drop photoresist at the edge of the glass slide and let it flow to the end face of the optical fiber, so that the end face of the optical fiber is completely immersed in the photoresist.

[0037] (2) Fix the optical fiber fixture in the laser direct writing platform, write a program through the computer system to control the laser direct writing platform. First, print a conical structure on the end face of the optical fiber in the galvanometer mode, and then use the piezoelectric mode to print a thread structure along the conical surface. Then remove the optical fiber from the fixture, first immerse it in the developer for 25 minutes to dissolve the unreacted photoresist, and then transfer the optical fiber to an isopropyl alcohol solution for 3 minutes to dissolve the developer. After taking it out, the isopropyl alcohol solution on the surface of the optical fiber can naturally volatilize at room temperature.

[0038] (3) Transfer the optical fiber to a magnetron sputtering coating machine, and coat the surface of the structure on the end face of the optical fiber. The coating target material is gold. Set the coating power to 60 W and the coating time to 110 s. After the coating is completed, the spiral cone metal probe is obtained on one end face of the optical fiber, and the other end face of the optical fiber is a smooth optical fiber cut surface without coating and without a probe.

[0039] The preparation method of the above spiral cone metal probe is as shown in Figure 1 a) in, and the probe structure is as shown in Figure 1 b) in. The optical fiber used in this example is a visible light single-mode optical fiber. The structural parameters of the prepared spiral cone metal probe are: the diameter D of the conical structure is 10 μm, the height H is 15 μm, the thread period is 750 nm, and the width L is 580 nm.

[0040] Embodiment 2

[0041] The preparation steps of a spiral cone metal probe are as follows:

[0042] (1) Strip the coating layer from a section of single-mode optical fiber, cut the end face of the optical fiber flat with an optical fiber cutter, and fix it on an optical fiber fixture with a single-axis displacement stage. Fix a glass slide at the front end of the fixture, adjust the displacement stage so that the distance between the end face of the optical fiber and the glass slide is less than 0.5 mm. Drop photoresist at the edge of the glass slide and let it flow to the end face of the optical fiber, so that the end face of the optical fiber is completely immersed in the photoresist.

[0043] (2) Fix the optical fiber fixture in the laser direct writing platform, write a program through the computer system to control the laser direct writing platform. First, print a conical structure on the end face of the optical fiber in the galvanometer mode, then use the piezoelectric mode to print a threaded structure along the conical surface. Then remove the optical fiber from the fixture, first immerse it in the developer for 20 minutes to dissolve the unreacted photoresist, and then transfer the optical fiber to the isopropyl alcohol solution for 5 minutes to dissolve the developer. The isopropyl alcohol solution on the surface of the optical fiber can be naturally volatilized at room temperature after being taken out.

[0044] (3) Transfer the optical fiber to a magnetron sputtering coating machine, and coat the surface of the structure on the end face of the optical fiber. The coating target is gold. Set the coating power to 80 W and the coating time to 100 s. After the coating is completed, the spiral cone metal probe is obtained on one end face of the optical fiber, and the other end face of the optical fiber is a smooth optical fiber section without coating and without a probe.

[0045] The preparation method of the above spiral cone metal probe is as Figure 1 shown in a) of Figure 1 , and the probe structure is as shown in b) of

[0046] . The optical fiber used in this example is a visible light single-mode optical fiber. The structural parameters of the prepared spiral cone metal probe are: the diameter D of the conical structure is 10 μm, the height H is 15 μm, the thread pitch is 750 nm, and the width L is 580 nm.

[0047] A method for one-dimensional super-resolution near-field detection using a spiral cone metal probe is as follows:

[0048] (1) Fix the smooth section end of the optical fiber with a metal probe at one end and a smooth section at the other end prepared in Example 1 on the displacement stage. Couple the laser output by the laser into the optical fiber from the end without the probe on the smooth section through a coupling lens. Use the displacement stage to adjust the position of the end without the probe on the smooth section of the optical fiber so that the coupling efficiency reaches about 9%. The input laser can excite the SPPs of the spiral cone metal probe to generate a "hot spot" at the tip of the cone, and the spot size on the end face of the optical fiber is about 8 nm.

[0049] (2) Glue the metal probe end of the optical fiber with a metal probe at one end and a smooth section at the other end prepared in Example 1 to the tuning fork, and use it as the probe of the SNOM near-field imaging system of NT-MDT. Figure 2a) and b) are schematic diagrams of the experimental setup used and local enlarged views of the probe and sample areas, respectively.

[0050] (3) SNOM works in illumination mode, coupling linearly polarized light from the far end of the optical fiber to the local hot spot signal generated inside the probe as a near-field illumination source to illuminate a one-dimensional standard grating sample with a period of 510nm and a line width of 83nm. The electron microscope image of the standard one-dimensional grating sample is shown in Figure 2. Figure 3 As shown in a), the grating sample has a width of 510nm as metal chromium and a width of 83nm as air, which are arranged periodically in sequence.

[0051] (4) Use the probe to scan the one-dimensional standard grating sample using the SNOM near-field optical imaging scanning mode, with a scanning speed of 15um / s and a collection interval of 2nm. The light passing through the sample passes through a beam splitter and enters a photomultiplier tube (PMT) detector to collect the light intensity after passing through the grating. The resulting SNOM near-field scanning image is shown in Figure 1. Figure 3 As shown in b).

[0052] (5) By Figure 3 b) Near-field scanning image can be obtained Figure 3 c) One-dimensional SNOM scanning curve of the standard grating sample, where the one-dimensional scanning curve position is Figure 3 b) (the white line L spans metal chromium, air, and metal chromium material in sequence). In the one-dimensional scanning curve, the normalized intensity close to 0 represents opacity, that is, the metal chromium film material area, and the normalized intensity close to 1 represents translucency, that is, the grating line area (air). The results show that the width of the transparent area (air) is 80nm, and the width of the opaque area (metal chromium) is 510nm, which is consistent with the line width and period of the sample. Therefore, the period and line width of the one-dimensional grating are accurately obtained by the metal probe.

[0053] (6) Figure 3 d) is Figure 3 c) A magnified image of the black vertical line region in the one-dimensional curve. The one-dimensional curve in the black vertical line region is the line profile (ERF) on the boundary between two materials (from the metallic chromium material to the air material). Since the probe resolution in the SNOM experiment can be determined by the width of the ERF (the half-height width of the corresponding LSF function), Figure 3 From d), we can see that the half-height width of the LSF function is Figure 3 d) The range drawn by the black vertical line is 5.7 nm, that is, the resolution of the probe is about 5.7 nm, which shows that the spiral cone metal probe can greatly reduce the resolution of SNOM to below 10 nm.

[0054] Example 4

[0055] A method for two-dimensional image super-resolution near-field detection using a spiral cone metal probe is as follows:

[0056] (1) Fix the smooth cut end of the optical fiber with a metal probe at one end and a smooth cut surface at the other end, which is prepared in Example 2, on the displacement stage. Couple the laser output from the laser into the optical fiber through a coupling lens from the end without the probe on the smooth cut surface. Use the displacement stage to adjust the position of the end without the probe on the smooth cut surface of the optical fiber so that the coupling efficiency reaches about 9%. The input laser can excite the SPPs of the spiral cone metal probe to generate a "hot spot" at the tip of the cone. The spot size at the end face of the optical fiber is about 8 nm.

[0057] (2) Glue the metal probe end of the optical fiber with a metal probe at one end and a smooth cut surface at the other end, which is prepared in Example 1, to the tuning fork and use it as the probe of the SNOM near-field imaging system of NT-MDT. Figure 2 Figures a) and b) are respectively the schematic diagram of the experimental device used and the partial enlarged view of the probe and the sample area.

[0058] (3) When the SNOM works in the illumination mode, use the local hot spot signal generated by coupling the linearly polarized light from the distal end of the optical fiber into the probe as the near-field illumination source to illuminate a two-dimensional standard sample with a slit width of 198 nm. The electron microscope image of the two-dimensional standard sample is as shown in Figure 4 Figure a). That is, the slit width of this grating sample is 198 nm (the material is air), and the letters SCUT are engraved on the metal chromium film.

[0059] (4) Use the probe to scan the two-dimensional standard grating sample in the near-field optical imaging scanning mode of the SNOM. The scanning speed is 15 μm / s, and the acquisition interval is 2 nm. The light passing through the sample passes through the beam splitter and enters the photomultiplier tube (PMT) detector to collect the light intensity after passing through the grating. The obtained SNOM near-field scanning image is as shown in Figure 4 Figure b).

[0060] (5) From the near-field scanning image in Figure 4 Figure b), the one-dimensional SNOM scanning curves of the standard sample in Figure 4 Figure c) and Figure 4 Figure d) can be obtained. The positions of the one-dimensional scanning curves are respectively the dashed lines L1 and L2 in Figure 4 Figure b) (the dashed lines successively span the metal chromium, air, and metal chromium materials). In the one-dimensional scanning curve, the normalized intensity close to 0 represents non-light transmission, that is, the metal chromium film material area, and the normalized intensity close to 1 represents light transmission, that is, the engraved line area (air) of the grating. The results show that the widths of the light transmission areas (air) are 205 nm and 198 nm respectively, which coincide with the slit width of the sample. Thus, the size of the engraved line width of the two-dimensional sample is obtained through the metal probe.

[0061] (6)Figure 4 c) and Figure 4 The one-dimensional curve in the black vertical line area in (d) is the line profile (ERF) on the boundary between the two materials (from the metallic chromium material to the air material). Since the probe resolution in the SNOM experiment can be determined by the width of the ERF (the half-height width of the corresponding LSF function), Figure 4 c) and Figure 4 From d), we can see that the half-height width of the LSF function is Figure 4 c) and Figure 4 d) The range drawn by the black vertical lines is 4.5 and 4.7 nm respectively, that is, the resolution of the probe is about 4.5 nm, which shows that the spiral cone metal probe can greatly reduce the resolution of SNOM to below 10 nm.

Claims

1. A method for super-resolution near-field detection using a spiral cone metal probe, characterized in that The spiral cone metal probe includes a photoresist dielectric layer and a gold film layer; the photoresist dielectric layer includes a conical structure and a semi-elliptical thread structure on the conical surface, the thread period is 750 nm, and the width L is 580 nm; after the spiral cone metal probe is excited by linearly polarized light, a local electromagnetic field enhancement "hot spot" is formed at the tip of the cone, thereby achieving an ultra-high near-field resolution below 10 nm; The method for super-resolution near-field detection includes the following steps: (1) Strip the coating layer from a section of single-mode optical fiber, use an optical fiber cutting knife to cut the end face of the optical fiber flat, and fix it on an optical fiber fixture with a uniaxial displacement stage. Fix a glass slide at the front end of the fixture, adjust the distance between the end face of the optical fiber and the glass slide, and drop photoresist at the edge of the glass slide to flow to the end face of the optical fiber so that the end face of the optical fiber is completely immersed in the photoresist; (2) Fix the optical fiber fixture in the laser direct writing platform, write a program through the computer system to control the laser direct writing platform. First, print a conical structure on the end face of the optical fiber in the galvanometer mode, and then use the piezoelectric mode to print the thread structure along the conical surface. Then remove the optical fiber from the fixture, first immerse it in the developer to dissolve the unreacted photoresist, and then transfer the optical fiber to the isopropyl alcohol solution to dissolve the developer. The isopropyl alcohol solution on the surface of the optical fiber can be naturally volatilized at room temperature after being taken out; (3) Transfer the optical fiber to a magnetron sputtering coater to coat the surface of the structure on the end face of the optical fiber. Set the coating power to 50 W - 80 W and the coating time to 80 seconds - 110 seconds. After the coating is completed, the spiral cone metal probe is obtained on one end face of the optical fiber, and the other end face of the optical fiber is a smooth optical fiber cut surface without coating and without a probe; (4) Glue the end of the optical fiber with the spiral cone metal probe to the tuning fork of the NT-MDT's SNOM near-field imaging system. The SNOM operates in the illumination mode. The light source is coupled from the distal end of the optical fiber into the probe to generate a local hot spot signal as the near-field illumination source to illuminate the standard sample. The sample is scanned by near-field optical imaging. Set the scanning speed and acquisition time. The light passing through the sample passes through a beam splitter and enters a photomultiplier detector to collect the light intensity after passing through the grating, thereby obtaining the SNOM near-field scanning image; In step (1), adjust the displacement stage so that the distance between the end face of the optical fiber and the glass slide is less than 0.5 mm; In step (2), when removing the optical fiber from the fixture, first immerse it in the developer to dissolve the unreacted photoresist, and the immersion time is not less than 20 minutes; In step (2), transfer the optical fiber to the isopropyl alcohol solution to dissolve the developer, and the immersion time is not less than 3 minutes; In step (3), the coating target is gold; In step (4), use linearly polarized light as the light source for excitation; In step (4), the scanning speed is 15 μm / s - 20 μm / s, and the acquisition interval is 2 nm - 40 nm.

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