Asymmetric pyramid optical fiber probe, method and device

Abstract

The invention provides an asymmetric pyramid optical fiber probe, a method and a device, the asymmetric pyramid optical fiber probe comprises an asymmetric pyramid optical fiber core layer, and the surface of the asymmetric pyramid optical fiber core layer is covered with a metal film layer; when the number of pyramid edges of the asymmetric pyramid fiber core layer is an odd number, any pyramid geometrical shape is included; when the number of pyramid edges of the asymmetric pyramid optical fiber core layer is an even number, the geometrical shape of the asymmetric pyramid optical fiber core layer is not axisymmetric about any axis in the space and is not centrally symmetric about any axis. According to the invention, the surface plasmon can be excited on the asymmetric pyramid surface of the probe under the incidence of the linear polarization laser and is propagated to the tip of the probe to form a focused light field, so that near-field optical measurement with high resolution and high signal intensity is realized.

Description

Technical Field

[0001] The invention belongs to the technical field of near-field optical imaging, and relates to an asymmetric pyramid optical fiber probe, a method and a device. Background Art

[0002] With the advent of near-field optical microscopy, people have been able to break through the limitations of the diffraction limit, detect near-field light signals within the near-field range of the sample surface, and achieve optical measurements with nanometer-level resolution.

[0003] In traditional near-field optical microscopy based on fiber probes, people usually use aperture fiber probes with a small hole of hundreds of nanometers in diameter at the tip or conical fiber probes covered with metal film layers for near-field optical detection. Aperture fiber probes collect near-field optical information through a light-passing hole with a diameter of hundreds of nanometers at the tip, which also determines that the intensity of its detection signal is closely related to the size of the aperture. The larger the aperture, the greater the signal intensity, but the excessively large aperture size will restrict its resolution, making it difficult to break through the hundreds of nanometers scale. The conical fiber probe covered with a metal film layer excites surface plasmons on the surface of the conical metal film layer through the incidence of radially polarized light. The surface plasmons propagate along the metal film layer to the tip to form a nanoscale focused light field, which greatly enhances the intensity of the near-field optical signal while improving its resolution. However, the modulation and coupling conditions of radially polarized light are very harsh, requiring a large and complex spatial modulation coupling optical path, and it is very susceptible to interference during transmission in the optical fiber, resulting in mode distortion. Moreover, in the practice of probe preparation, it is difficult to ensure the accurate preparation of the conical geometry of the needle tip. The defects in its geometric symmetry and the defects in the flatness of the needle tip surface greatly reduce the conversion of radially polarized light into surface plasmons and the propagation focusing efficiency, which in turn affects the detection signal intensity. Summary of the invention

[0004] In order to solve the problems existing in the prior art, the present invention provides an asymmetric pyramid fiber probe, method and device, which can excite surface plasmons on the asymmetric pyramid surface of the probe under the incidence of linear polarized laser and propagate them to the tip of the probe to form a focused light field, thereby realizing near-field optical measurement with high resolution and high signal intensity.

[0005] To achieve the above-mentioned purpose, the present invention provides the following technical solutions: an asymmetric pyramidal optical fiber probe, comprising: an asymmetric pyramidal optical fiber core layer, the surface of the asymmetric pyramidal optical fiber core layer is covered with a metal film layer; when the number of pyramidal edges of the asymmetric pyramidal optical fiber core layer is an odd number, it contains any pyramidal geometric shape; when the number of pyramidal edges of the asymmetric pyramidal optical fiber core layer is an even number, the geometric shape of the asymmetric pyramidal optical fiber core layer is not axisymmetric about any axis in space and is not centrosymmetric about any axis.

[0006] Furthermore, the pyramid has a curvature radius of 1 nm to 1000 nm, and the aspect ratio of the pyramid viewed from any viewing angle is 0.5 to 10.

[0007] Furthermore, the thickness of the metal film layer is 1 nm to 300 nm, and the material used for the metal film layer includes but is not limited to one of gold, silver, platinum, copper, aluminum, and chromium.

[0008] The present invention also provides a method for preparing an asymmetric pyramid optical fiber probe, comprising the following steps: After cutting the optical fiber horizontally, the optical fiber is inserted into hydrofluoric acid for etching by chemical etching method, and after the etching is completed, the optical fiber is inserted into an organic solvent to dissolve and soften the organic coating layer on the outer wall of the optical fiber, and after removing the coating layer, the optical fiber core layer with a conical nanometer-scale tip is obtained; Using a focused ion beam to cut and modify the conical fiber core from the side into an asymmetric pyramid fiber core; A metal film layer is prepared on the surface of the asymmetric pyramid optical fiber core layer to obtain an asymmetric pyramid optical fiber probe.

[0009] Furthermore, after cutting the optical fiber horizontally, the end face of the optical fiber is inserted into hydrofluoric acid with a concentration of 30% to 40% for etching for 1h to 4h, forming a conical needle tip on the end face of the optical fiber. The conical needle tip surface of the optical fiber is inserted into an organic solvent for dissolution for 30s to 120s to dissolve and soften the organic coating on the outer wall of the optical fiber. After removing the coating, an optical fiber core layer with a conical nanoscale tip is obtained.

[0010] Furthermore, the organic solvent includes but is not limited to one of acetone, butanone, methanol, ethanol, and carbon tetrachloride.

[0011] Furthermore, the focused ion beam is a gallium ion beam, and the current is 1A~10A.

[0012] Furthermore, a metal film layer is prepared on the surface of the asymmetric pyramid optical fiber core layer by electron beam evaporation or magnetron sputtering. The process condition of the electron beam evaporation method is that the vacuum degree is less than 1×10 -5 Bar, metal evaporation rate 0.1nm / s~0.5nm / s; the process conditions of magnetron sputtering are temperature 25℃~150℃, vacuum degree in the chamber is less than 1×10 -5 Bar, the voltage is 360V~400V, the current of the metal target is controlled to be 0.5A~6A, and the deposition rate of the metal film layer is 0.1nm / s~0.5nm / s.

[0013] The present invention also provides a near-field optical microscope, which adopts the asymmetric pyramid optical fiber probe.

[0014] Furthermore, when the near-field optical microscope is used for sample testing, linearly polarized light is incident from the tail end of the optical fiber core of the asymmetric pyramid optical fiber probe, exciting surface plasmons on the surface of the metal film layer, and the surface plasmons propagate along the metal film layer toward the tip of the asymmetric pyramid optical fiber probe, forming a focused light field at the tip; when the tip of the asymmetric pyramid optical fiber probe interacts with the surface of the sample, the near-field optical information of the surface of the sample being tested excited by the focused light field of the tip of the asymmetric pyramid optical fiber probe returns to the optical fiber in the form of plasmons, and the detected near-field optical signal is collected at the tail of the light.

[0015] Compared with the prior art, the present invention has at least the following beneficial effects: The present invention provides an asymmetric pyramid optical fiber probe, which adopts an asymmetric pyramid geometric shape design, so that the probe can excite surface plasmons under the simplest linear polarized light incidence and focus to form a nanoscale light field, and has extremely high processing accuracy and focusing effect. When exciting and focusing surface plasmons, it has a high polarization direction insensitivity characteristic, can realize the excitation and focusing of surface plasmons in a wide range of polarization directions, and increases the flexibility of use and the scope of application.

[0016] The method for preparing an asymmetric pyramid optical fiber probe of the present invention obtains a conical needle tip by chemical etching, which greatly saves the time and cost of subsequent focused ion beam processing and improves the preparation efficiency. The method of combining chemical etching and focused ion beam cutting and modification is adopted, and the preparation process is simple and controllable, and an optical fiber probe with an asymmetric pyramid geometry can be accurately prepared. During the preparation and processing, as long as the asymmetric characteristics are met, the excitation and focusing of the surface plasmon can be achieved, which has a high fault tolerance rate and reduces the difficulty of preparation.

[0017] The asymmetric pyramid fiber probe of the present invention is used in a near-field optical microscope, which can achieve high-resolution imaging at the nanoscale, providing a more sophisticated means of observation for scientific research. The focused light field at the tip of the probe can interact with the surface of the sample being measured, and the excited near-field optical information is returned to the optical fiber in the form of plasmons, thereby realizing sensitive detection of the near-field optical information on the sample surface. The near-field optical microscope of the present invention has broad application prospects in the fields of materials science, biology, physics, etc. It can be used to study the structure and properties of materials at the nanoscale, providing a powerful tool for scientific research and technological innovation. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings described herein are used to provide a further understanding of the present invention, constitute a part of the present application, and do not constitute an improper limitation of the present invention. In the drawings: Figure 1 Schematic diagram of an asymmetric odd-numbered pyramid optical fiber probe structure represented by an asymmetric quadrangular pyramid; Figure 2 is a schematic diagram of the structure of an asymmetric odd-numbered pyramid optical fiber probe; Figure 3 is the simulation result of the optical fiber probe of Example 1; Figure 4 is the simulation result of the optical fiber probe of Example 2; Figure 5 This is the simulation result of the optical fiber probe of Example 3.

[0019] In the figure: 1. Optical fiber core layer; 2. Metal film layer; 3. Surface plasmon; 4. Focused light field; 5. Nano-focused light field. DETAILED DESCRIPTION

[0020] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0021] The present invention provides an asymmetric pyramid optical fiber probe based on linear polarized light incidence, wherein a chemical etching method is used to etch an optical fiber base material to form a large-angle conical needle tip, and the needle tip is cut and modified into an asymmetric pyramid by focused ion beam etching, and then a metal film layer 2 is covered on the surface of the asymmetric pyramid to obtain an asymmetric pyramid optical fiber probe. The preparation method of the asymmetric pyramid optical fiber probe comprises the following steps: Step 1: After cutting the optical fiber horizontally, use a chemical etching method to insert one end of the optical fiber into a hydrofluoric acid with a concentration of 30% to 40% for etching for 1 hour to 4 hours. After the etching is completed, insert it into an organic solvent to dissolve for 30 seconds to 120 seconds to dissolve and soften the organic coating layer on the outer wall of the optical fiber, and after removing the coating layer, obtain the optical fiber core layer 1 with a conical nanometer-scale tip.

[0022] Preferably, the optical fiber is a standard single-mode optical fiber or a multi-mode optical fiber; Preferably, the organic solvent includes but is not limited to one of acetone, butanone, methanol, ethanol, carbon tetrachloride Step 2: Using a focused ion beam, the conical optical fiber core layer is cut and modified from the side to form an asymmetric pyramid optical fiber core layer 1.

[0023] Preferably, the focused ion beam is a gallium ion beam, and the current is 1A-10A.

[0024] Step 3, using electron beam evaporation or magnetron sputtering to cover the surface of the asymmetric pyramid optical fiber core layer 1 with a 1nm-300nm thick metal film layer 2 to obtain an asymmetric pyramid optical fiber probe.

[0025] Preferably, the electron beam evaporation method: the asymmetric pyramid optical fiber core layer in step 2 is placed in the evaporation chamber, the optical fiber is perpendicular to the target material direction, the door is closed and the vacuum is drawn, and the vacuum degree in the chamber is less than 1×10 -5Bar, set the metal evaporation rate from 0.1nm / s to 0.5nm / s until the target thickness is reached; Magnetron sputtering method: Place the asymmetric pyramid optical fiber core layer in step 2 in the evaporation chamber, adjust the temperature of the magnetron sputtering deposition equipment to 25℃~150℃, and the vacuum degree in the chamber is less than 1×10 -5 Bar, the voltage is 300~400V, the current of the metal target is controlled to be 0.5A~6A, and the deposition rate of the metal film layer 2 is 0.1nm / s~0.5nm / s.

[0026] Preferably, the metal film layer 2 is made of gold, silver, aluminum, chromium or titanium.

[0027] Preferably, the shape of the asymmetric pyramid optical fiber probe is a pyramid with 3 to 15 edges; Preferably, when the number of edges of the asymmetric pyramid optical fiber probe is an even number, the geometric shape of the asymmetric pyramid optical fiber probe is not axisymmetric about any axis in space and is not centrosymmetric about any axis; Preferably, when the number of edges of the asymmetric pyramid optical fiber probe is an odd number, the geometric shape of the asymmetric pyramid optical fiber probe is not limited; Preferably, the curvature radius of the probe pyramid tip is 1 nm to 1000 nm. The smaller the curvature radius of the probe tip is, the smaller the scale of the nano-light field formed by the focus is, and the higher the near-field optical resolution is. Preferably, the aspect ratio of the asymmetric pyramid optical fiber probe at any viewing angle is 0.5 to 10; When the asymmetric pyramid optical fiber probe of the present invention is used, under the irradiation of linearly polarized laser in the optical fiber, the metal film layer 2 can effectively excite surface plasmon resonance, and the surface plasmon propagates along the conical metal film layer 2 toward the tip. Since the pyramid at the tip of the probe of the present invention has geometric asymmetry, the surface plasmons excited by the linearly polarized laser on each side and edge do not meet the interference destructive conditions when reaching the needle tip, so a focusing-enhanced light field can be formed at the needle tip for near-field optical detection.

[0028] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention described and shown in the drawings here can usually be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0029] Example 1 The preparation of asymmetric even-numbered pyramid fiber probe is as follows: Step 1: After cutting the standard single-mode optical fiber horizontally, one end of the optical fiber is inserted into 40% hydrofluoric acid for etching for 3 hours by etching method. After the etching is completed, the end is inserted into acetone to dissolve and soften the organic coating layer on the outer wall of the optical fiber, and the coating layer is removed to obtain the optical fiber core layer with a conical nanoscale tip.

[0030] Step 2, using a focused ion beam to cut and modify the conical optical fiber core layer from the side into an asymmetric even-numbered pyramid optical fiber core layer 1, wherein the number of edges is 4, the radius of curvature of the pyramid tip is 20 nm, and the aspect ratio viewed from any direction is 2; Step 3: Using electron beam evaporation, the vacuum degree in the chamber is less than 1×10 -5 Bar, set the metal evaporation rate to 0.1nm / s, and cover the surface of the asymmetric even-numbered pyramid optical fiber core layer 1 with a 100nm thick silver metal film layer, and obtain the following Figure 1 The asymmetric even-numbered pyramid optical fiber probe shown in the figure has the following asymmetric characteristics: it is not axisymmetric with respect to any axis in space and is not centrally symmetric with respect to any axis.

[0031] Under the incident of linear polarized laser, surface plasmon 3 is excited on the surface of the silver metal film layer, and the surface plasmon 3 propagates along the conical silver metal film layer toward the tip. Since the tip pyramid of the probe of the present invention has geometric asymmetry, the surface plasmon excited by the linear polarized laser on each side and edge does not meet the interference destructive condition when reaching the needle tip, so a focused light field 4 is formed at the needle tip. The simulation results are as follows: Figure 3 As shown, a nano-focused light field 5 is formed at the needle tip for near-field optical detection.

[0032] Example 2 The preparation of asymmetric odd-numbered pyramid fiber probe is as follows: Step 1: After cutting the standard multimode optical fiber horizontally, one end of the optical fiber is inserted into 40% hydrofluoric acid for etching for 4 hours by etching method. After the etching is completed, the end is inserted into acetone to dissolve and soften the organic coating layer on the outer wall of the optical fiber, and the coating layer is removed to obtain the optical fiber core layer with a conical nanoscale tip.

[0033] Step 2, using a focused ion beam to cut and modify the conical optical fiber core layer from the side into an asymmetric odd-numbered pyramid optical fiber probe optical fiber core layer 1, the number of which is 3, the radius of curvature of the pyramid tip is 100 nm, and the aspect ratio at any viewing angle is 3; Step 3: Using magnetron sputtering, adjust the temperature of the magnetron sputtering deposition equipment to 100°C and the vacuum degree in the chamber to less than 1×10 -5Bar, the voltage is 350V, the current of the metal target is controlled to be 6A, the metal film deposition rate is 0.2nm / s, and a 200nm thick aluminum metal film layer is covered on the surface of the asymmetric even-numbered pyramid optical fiber core layer 1 to obtain the following Figure 2 The asymmetric odd-pyramid fiber probe shown.

[0034] Under the incident of linear polarized laser, surface plasmons 3 are excited on the surface of the aluminum metal film layer and propagate toward the tip of the cone, eventually forming a focused light field 4 at the tip of the cone. The simulation results are as follows: Figure 4 As shown, a nano-focused light field 5 is formed at the needle tip.

[0035] Example 3 The preparation of asymmetric odd-numbered pyramid fiber probe is as follows: Step 1: After cutting the standard single-mode optical fiber horizontally, one end of the optical fiber is inserted into a 40% hydrofluoric acid for etching for 1 hour by etching method. After the etching is completed, the end is inserted into acetone to dissolve and soften the organic coating layer on the outer wall of the optical fiber, and after removing the coating layer, the optical fiber core layer with a conical nanoscale tip is obtained.

[0036] Step 2, using a focused ion beam to cut and modify the conical optical fiber core layer from the side into an asymmetric odd-numbered pyramid optical fiber core layer 1, wherein the number of edges is 5, the radius of curvature of the pyramid tip is 20 nm, and the aspect ratio viewed from any direction is 5; Step 3: Using electron beam evaporation, the vacuum degree in the chamber is less than 1×10 -5 Bar, set the metal evaporation rate to 0.5nm / s, and cover the surface of the asymmetric even-numbered pyramid optical fiber core layer 1 with a 100nm thick gold metal film layer to obtain an asymmetric even-numbered pyramid optical fiber probe, whose asymmetric characteristics include not being axially symmetrical with any axis in space and not being centrally symmetrical about any axis.

[0037] Under the incidence of linearly polarized laser, surface plasmons 3 are excited on the surface of the gold metal film layer, and the surface plasmons 3 propagate along the conical silver metal film layer toward the tip. Since the pyramid at the tip of the probe of the present invention has geometric asymmetry, the surface plasmons excited by the linearly polarized laser on each side and edge do not meet the interference destructive conditions when arriving at the tip, and thus a focused light field 4 is formed at the tip for near-field optical detection.

[0038] Under the incident of linear polarized laser, surface plasmons 3 are excited on the surface of the gold metal film layer and propagate toward the tip of the cone, eventually forming a focused light field 4 at the tip of the cone. The simulation results are as follows: Figure 5 As shown, a nano-focused light field 5 is formed at the needle tip.

[0039] Example 4 The difference between this embodiment and embodiment 2 is that in step 2, the asymmetric even-numbered pyramid optical fiber core layer 1 has 3 edges, a radius of curvature of the pyramid tip is 1 nm, and an aspect ratio of 0.5 when viewed from any viewing angle.

[0040] Example 5 The difference between this embodiment and embodiment 2 is that in step 2, the asymmetric even-numbered pyramid optical fiber core layer 1 has 15 edges, a radius of curvature of the pyramid tip is 1000 nm, and an aspect ratio of 10 when viewed from any viewing angle.

[0041] Example 6 The difference between this embodiment and embodiment 1 is that in step 3, an electron beam evaporation method is used, the metal evaporation rate is set to 0.5 nm / s, and a 1 nm thick titanium metal film layer 2 is covered on the surface of the asymmetric even-numbered pyramid optical fiber core layer 1.

[0042] Example 7 The difference between this embodiment and embodiment 1 is that in step 3, electron beam evaporation is used, the metal evaporation rate is set to 0.3 nm / s, and a 300 nm thick chromium metal film layer 2 is covered on the surface of the asymmetric even-numbered pyramid optical fiber core layer 1.

[0043] Example 8 The difference between this embodiment and embodiment 1 is that in step 3, the magnetron sputtering method is used, the temperature of the magnetron sputtering deposition equipment is adjusted to 25°C, and the vacuum degree in the chamber is less than 1×10 -5 Bar, the voltage is 400V, the current of the metal target is controlled to be 0.5A, and the deposition rate of the metal film layer is 0.1nm / s.

[0044] Example 9 The difference between this embodiment and embodiment 1 is that in step 3, the magnetron sputtering method is used, the temperature of the magnetron sputtering deposition equipment is adjusted to 150° C., and the vacuum degree in the chamber is less than 1×10 -5 Bar, the voltage is 300V, the current of the metal target is controlled to be 5A, and the deposition rate of the metal film layer is 0.5nm / s.

[0045] Example 10 The difference between this embodiment and embodiment 1 is that in step 1, one end of the optical fiber is inserted into hydrofluoric acid with a concentration of 30% and etched for 1 hour by a corrosion method.

[0046] Embodiment 11 The difference between this embodiment and embodiment 1 is that in step 1, one end of the optical fiber is inserted into hydrofluoric acid with a concentration of 30% for etching for 3 hours by a corrosion method.

[0047] Example 12 The difference between this embodiment and embodiment 1 is that in step 1, one end of the optical fiber is inserted into hydrofluoric acid with a concentration of 30% for etching for 4 hours by using an etching method.

[0048] Embodiment 13 The present invention provides a working method of the pyramid-shaped tip optical fiber probe for exciting and collecting near-field optical signals, comprising the following process: An asymmetric pyramid optical fiber probe for exciting and collecting near-field optical signals is installed on a near-field optical microscope; When testing the sample, a linearly polarized laser beam is incident on the cone through the optical fiber core to excite surface plasmons, which propagate along the cone to the tip to form a nanoscale localized light spot. When the cone tip interacts with the surface of the sample, the near-field light on the sample surface is scattered to the surrounding far-field. At this time, the scattered light signal can be directly collected by an objective lens at the side or directly below the sample.

[0049] In addition, the near-field optical information on the surface of the sample being tested excited by the focused light field at the tip of the asymmetric pyramidal fiber probe will also return to the optical fiber in the form of plasmons. At this time, the detected near-field optical signal can be collected at the tail of the light.

[0050] The advantage of the present invention over conventional fiber optic probes is that the asymmetric pyramid geometry of the tip can directly excite surface plasmons under the incidence of linearly polarized lasers (laser output light is generally linearly polarized lasers) and form a focused spot at the tip. It is not necessary to modulate the linearly polarized laser into radially polarized light for plasmon excitation and focusing, which greatly reduces the complexity of the system optical path. And because there is no need to modulate radially polarized light in the spatial optical path, the illumination and collection process of the probe can be completed entirely in the optical fiber optical path, greatly improving the signal-to-noise ratio.

[0051] When preparing the asymmetric pyramid optical fiber probe of the present invention, a conical needle tip is obtained by using a chemical etching method, which saves a lot of time and cost for subsequent focused ion beam processing and modification. The focused ion beam etching method is further used with high processing accuracy. The asymmetric pyramid geometry formed can excite surface plasmons and focus to form a nanoscale light field under the simplest linear polarized light incidence, without modulating a special polarization state laser. In addition, the asymmetric pyramid geometry of the optical fiber probe of the present invention has a high polarization direction insensitivity when exciting and focusing surface plasmons, and can excite and focus surface plasmons in a wide range of polarization directions to form a focused light field, thereby realizing near-field optical measurement with high resolution and high signal intensity.

[0052] The present invention is not limited to the above-mentioned embodiments. On the basis of the technical solution disclosed in the present invention, technicians in this field can make some substitutions and deformations to some technical features therein according to the disclosed technical content without creative labor, and these substitutions and deformations are all within the protection scope of the present invention.

Claims

1. An asymmetric pyramid optical fiber probe, characterized in that: include: An asymmetric pyramid optical fiber core layer (1), wherein the surface of the asymmetric pyramid optical fiber core layer is covered with a metal film layer (2); When the number of pyramid edges of the asymmetric pyramid optical fiber core layer (1) is an odd number, any pyramid geometric shape is included; when the number of pyramid edges of the asymmetric pyramid optical fiber core layer (1) is an even number, the geometric shape of the asymmetric pyramid optical fiber core layer (1) is not axisymmetric about any axis in space and is not centrosymmetric about any axis.

2. The asymmetric pyramid optical fiber probe according to claim 1, characterized in that: The pyramid curvature radius is 1nm~1000nm, and the aspect ratio of the pyramid viewed from any viewing angle is 0.5~10.

3. The asymmetric pyramid optical fiber probe according to claim 1, characterized in that: The thickness of the metal film layer (2) is 1 nm to 300 nm, and the material used for the metal film layer (2) includes, but is not limited to, one of gold, silver, platinum, copper, aluminum, and chromium.

4. A method for preparing an asymmetric pyramid optical fiber probe according to any one of claims 1 to 3, characterized in that: The following steps are involved: After cutting the optical fiber horizontally, the optical fiber is inserted into hydrofluoric acid for etching by chemical etching method, and after the etching is completed, the optical fiber is inserted into an organic solvent to dissolve and soften the organic coating layer on the outer wall of the optical fiber, and after removing the coating layer, the optical fiber core layer with a conical nanometer-scale tip is obtained; Using a focused ion beam, a conical optical fiber core is cut and modified from the side to form an asymmetric pyramid optical fiber core (1); A metal film layer (2) is prepared on the surface of an asymmetric pyramid optical fiber core layer (1) to obtain an asymmetric pyramid optical fiber probe.

5. The method for preparing an asymmetric pyramid optical fiber probe according to claim 4, characterized in that: After cutting the optical fiber horizontally, the end face of the optical fiber is inserted into hydrofluoric acid with a concentration of 30% to 40% for etching for 1h to 4h, and a conical needle tip is formed on the end face of the optical fiber. The conical needle tip of the optical fiber is inserted into an organic solvent for dissolution for 30s to 120s to dissolve and soften the organic coating on the outer wall of the optical fiber. After removing the coating, the optical fiber core layer with a conical nanoscale tip is obtained.

6. The method for preparing an asymmetric pyramid optical fiber probe according to claim 5, characterized in that: The organic solvent includes but is not limited to one of acetone, butanone, methanol, ethanol and carbon tetrachloride.

7. The method for preparing an asymmetric pyramid optical fiber probe according to claim 4, characterized in that: The focused ion beam is a gallium ion beam, and the current is 1A-10A.

8. The method for preparing an asymmetric pyramid optical fiber probe according to claim 4, characterized in that: The metal film layer (2) is prepared on the surface of the asymmetric pyramid optical fiber core layer (1) by electron beam evaporation or magnetron sputtering. The process conditions of the electron beam evaporation method are that the vacuum degree is less than 1×10 -5 Bar, metal evaporation rate 0.1nm / s~0.5nm / s; the process conditions of magnetron sputtering are temperature 25℃~150℃, vacuum degree in the chamber is less than 1×10 -5 Bar, the voltage is 360V~400V, the current of the metal target is controlled to be 0.5A~6A, and the deposition rate of the metal film layer is 0.1nm / s~0.5nm / s.

9. A near-field optical microscope, characterized in that: An asymmetric pyramid optical fiber probe as described in any one of claims 1 to 3 is used.

10. A near-field optical microscope according to claim 9, characterized in that: When the near-field optical microscope is used to test a sample, linearly polarized light is incident from the tail end of the optical fiber core (1) of the asymmetric pyramid optical fiber probe, exciting surface plasmons (3) on the surface of the metal film layer (2), and the surface plasmons (3) propagate along the metal film layer (2) toward the tip of the asymmetric pyramid optical fiber probe, forming a focused light field (4) at the tip; when the tip of the asymmetric pyramid optical fiber probe interacts with the surface of the sample, the near-field optical information of the surface of the sample being tested excited by the focused light field (4) at the tip of the asymmetric pyramid optical fiber probe is returned to the optical fiber in the form of plasmons, and the detected near-field optical signal is collected at the tail of the light.

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