CN113933282B - Medium probe for near-field optical detection and near-field microscope - Google Patents
Medium probe for near-field optical detection and near-field microscope Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
- G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
- G01N21/65—Raman scattering
- G01N21/658—Raman scattering enhancement Raman, e.g. surface plasmons
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
- G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
- G01N21/65—Raman scattering
- G01N2021/653—Coherent methods [CARS]
- G01N2021/656—Raman microprobe
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Abstract
The invention discloses a dielectric probe for near-field optical detection and a near-field microscope. The probe comprises a probe column prepared by adopting a micron-sized photoconductive medium, one end of the probe column is hemispherical, and metal nano particles or a metal nano particle array is arranged at the top of the hemispherical. The micron-sized optical fiber is used as a light guide medium, one end of the micron-sized optical fiber is prepared into a hemispherical shape to serve as a probe base, then the metal nano particles are prepared to the top end of the probe column to serve as a generation point 'hot spot' of a local enhanced electric field, and the hot spot position can generate extremely strong electric field enhancement in a nanoscale space range. The light guiding medium may be used as an efficient light guide for incident excitation light, or as an efficient collection light guide for scattered and reflected light. The near field microscope constructed from such probes has a combined efficiency of spectral scanning that is improved by at least 2 orders of magnitude over the far field collected tip enhanced raman spectroscopy device due to the simultaneous enhancement of near field excitation and collection.
Description
Technical Field
The invention relates to the technical field of active scanning detection needle tip enhanced Raman spectroscopy, in particular to a multidisciplinary intersection field based on an optical spectroscopy technology, and relates to precision machining, an automation technology and an information processing technology.
Background
As an optical technology with molecular level characteristic recognition capability, the Raman spectrum technology has small damage to a sample and can be operated under normal temperature and normal pressure, but the spatial resolution of the Raman spectrum is limited by the optical diffraction limit, and a conventional Raman microscope (such as a confocal Raman microscope) cannot recognize molecules in a nanoscale (size of 10 nanometers or less).
After 2000 years, due to further development of micro-nano processing technology, regular shape metal nano particles with different materials, shapes and structures can be conveniently prepared on different substrates by utilizing technologies such as chemical synthesis, optical lithography, particle beam direct writing and the like. These regular metal particles will produce an electric field enhancement phenomenon on their surface under laser irradiation. Typically, some "hot spot" locations on the surface of the metal nanoparticles can produce 1-2 orders of magnitude electric field enhancement, and 3 orders of magnitude enhancement when surface plasmons resonate between arrays of metal particles. Since the raman spectrum intensity is proportional to the fourth power of the electric field intensity, a raman scattering enhancement of 13 orders of magnitude is ideally obtained. Therefore, the sample single molecules with extremely small scattering cross sections near the surfaces of the metal particles can generate Raman spectra meeting the signal-to-noise ratio resolution, so that the nano-scale single-molecule Raman spectrum identification is possible.
Current nanoscale molecular raman spectroscopy techniques can be categorized into two categories, surface Enhanced Raman Spectroscopy (SERS) without scanning detection and Tip Enhanced Raman Spectroscopy (TERS) with active scanning detection, depending on whether a probe is used for scanning. TERS requires a scanning action on the sample surface using a probe, and thus requires a more complex spatial position control assistance system, but can also yield more accurate spatial information of the sample molecules than SERS. The metal probes in the TERS system can be multiplexed with the probes of an Atomic Force Microscope (AFM) and a Scanning Tunneling Microscope (STM), so that the TERS can be well combined into the AFM and STM systems which are mature at present. AFM and STM systems are mature detection equipment with atomic scale resolution, and can form TERS equipment based on AFM or STM after being added into a Raman optical system, so that the whole equipment can obtain molecular morphology and molecular Raman characteristic spectrum at the same time, and the molecular level recognition capability of the whole equipment is greatly improved.
The development of TERS greatly enhances the capability of Raman spectrum, so that the Raman technology is expected to play a more outstanding role in the fields of single molecule, nano science and the like.
The currently used tert technique is a spectroscopic technique of active scanning detection, the effective detection distance is far smaller than the wavelength of incident light, and the spectrum technique is a near-field detection mode in the range of about 10 nanometers from the surface of the measured object, including AFM and STM which are also detected in the near-field range, and these microscopes with nanoscale resolution can be collectively called near-field microscope (NFM). Those microscopes whose probes or lenses are farther than the incident wavelength from the object under test may be collectively referred to as Far Field Microscopes (FFM).
The optical nature is a special substance with wave grain bispectivity, and mainly shows the fluctuation characteristic of an electromagnetic field under the far-field propagation condition, and more shows the granularity in a microscopic nanometer scale, namely, in a spatial scale range which is far smaller than the wavelength, the interaction between single photon or few photons and the substance shows more energy exchange of quanta, the energy size is represented by the wavelength, the exchange probability can be determined by the scattering cross section size, and the quanta characteristic is larger than the fluctuation characteristic. When the Raman spectrum is excited, the Stokes and anti-Stokes scattering probability and intensity are mainly determined by the density of light quanta in unit volume and the quantum transition probability of each energy state of the detected molecule, and are in indirect relation with the wavelength of incident light. The photons are limited to the near field range of the nanometer scale, so that diffraction problems caused by the fluctuation effect can be effectively avoided, and therefore, the near field technology is a key technology for obtaining the nanometer scale resolution of a Raman microscope.
The latest TERS technology is generally based on a metal probe used by an AFM or an STM, and the metal nano conical structure probe used by the equipment can meet the use requirement of the AFM (STM) per se, can excite a local enhanced electric field at the position of a probe tip of the AFM (STM), and can well excite the Raman spectrum of a measured object in the near field range of the measured object. By coating the tip of an AFM or STM with a Surface Enhanced Raman Scattering (SERS) active metal or metal nanoparticle to have Surface Plasmon Resonance (SPR) activity, the tert enhancement effect can occur in a small range near the tip. Since the dimensions of the tip are typically smaller than 100nm, the spatial resolution of such a measurement will be correspondingly smaller than 100nm.
The current excitation modes of TERS generally include proximity (Adjacent), on-axis (On-axis), and Off-axis (Off-axis). In the three modes, the precision of the approach is low, and the coaxial sample is required to be transparent, so that the two modes are not good modes, the precision of the detection point position can be improved by off-axis mode, and meanwhile, the sample is not required to be subjected to transparent treatment, so that the method becomes the main stream of TERS design. However, the off-axis excitation design of tert still has an unsolvable problem that the collection of scattered light cannot be accomplished by near-field collection.
All TERS using metal probes have a drawback in that the metal probes cannot be used as long-distance optical waveguide structures, and therefore the efficiency of the TERS is low, both in exciting a localized enhanced electric field and in collecting scattered photons. In particular, the collection of scattered raman photons is a far field collection, so that the current tert is effectively a semi-near field mode of operation, i.e., near field excitation far field collection. Although nanometer-scale spatial resolution can be achieved, it is inefficient, i.e., it takes more time to identify a feature location, and there is still a theoretical limit to spatial resolution.
To sum up, the following disadvantages exist for TERS at present:
1. the failure to collect the reflected and scattered light from the object to be measured in the near field, especially the weak raman scattered light, results in a low collection efficiency in the far field, which results in a large detection time.
The structure of TERS is complex, and as the probe cannot be used as a light guide, the probe detection and the light guide system are separated, namely two paths of light paths are needed, one path is used for detecting the working state of the cantilever probe, and the other path is used for excitation and collection of Raman spectrum.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a dielectric probe for near-field optical detection and a near-field microscope.
The technical scheme adopted by the invention is that the medium probe for near-field optical detection comprises a probe column prepared by adopting a micron-sized light guide medium, one end of the probe column is hemispherical, a metal nanoparticle or a metal nanoparticle array is arranged at the top of the hemispherical, and metal gold (Au) or silver (Ag) is used for preparing the nanoparticle and the array.
Further, the micron-sized optical medium is a micron-sized optical fiber, and the optical medium material is silicon dioxide (SiO 2 ) In addition, according to the application of different wavelengths, sodium oxide (Na 2 O), boron oxide (B) 2 O 3 ) Potassium oxide (K) 2 O), germanium dioxide (GeO) 2 ) The oxides can be made into multicomponent optical fibers, and optical fibers can also be made from Fluoride fibers.
Further, the probe light guide is internally penetrated with a metal nano column.
Further, in the probe column, the path of incidence of the excitation light is the same as the path along which the excitation back-scattered and reflected light is collected.
On the basis of the scheme, the probe column is also provided with a probe cantilever.
In summary, micron-sized optical fibers are used as a photoconductive medium, one end of the micron-sized optical fibers is prepared into a hemispherical shape to serve as a probe base (probe column), then metal nano particles are prepared to the top end of the probe column to serve as a generation point 'hot spot' of a local enhanced electric field, and the hot spot position can generate extremely strong electric field enhancement in a nanoscale space range. The light guiding medium can be used as an effective light guide for incident excitation light and can also be used as an effective light collecting guide for scattered light. The combined efficiency of the spectral scanning is improved by at least 2 orders of magnitude over the tert collected in the far field, thanks to near field excitation and collection.
The invention also comprises a near field microscope, which comprises a light source, a spectrometer, a photoelectric converter, a circuit controller and an XYZ triaxial workbench, wherein the excitation light emitted by the light source enters a medium probe, the excitation light is focused on a medium probe tip, the metal nano particles of the medium probe tip form a local enhanced electric field to excite a sample on the XYZ triaxial workbench, the medium probe collects Rayleigh light and Raman light of the sample, the Rayleigh light and the Raman light are separated by the total reflection beam splitter, the Rayleigh light is received by the photoelectric converter and converted into an electric signal, the electric signal is input into the circuit controller, the circuit controller controls the XYZ triaxial workbench to move, and the Raman light enters the spectrometer and is processed by the spectrometer.
Further, the excitation light emitted by the light source enters the medium probe after being reflected by the half-reflection half-transmission beam splitter; the Rayleigh light and the Raman light collected by the medium probe are reflected by the reflecting mirror to enter the total reflection beam splitter after passing through the half reflection beam splitter. And a lens is arranged on a light path between the half-reflection and half-transmission beam splitter and the medium probe.
The total reflection beam splitter is fixed on an angle-adjustable rotating disc.
On the basis of the scheme, the medium probe further comprises a probe cantilever, one end of the probe cantilever is connected with the circuit controller, and the other end of the probe cantilever is fixed on a probe column of the medium probe.
By adopting the technical scheme, the invention has the following beneficial technical effects:
1. the medium/metal Raman particles are utilized to form the probe with the composite structure, so that the photoconductive structure of the probe can excite and collect photons in the near field, the working efficiency of Raman spectrum scanning is greatly improved, and higher spatial resolution can be obtained to a certain extent.
2. The probe base (probe column) is prepared by using a micron-sized dielectric material capable of being used as a light guide path, so that channels for exciting and collecting Raman light are integrated on the same probe, and simplification of equipment can be realized.
3. Because of the photoconductive structure of the probe, the whole structure of the TERS is simplified, and the same optical path can be used for controlling the probe and the cantilever and transmitting photons.
4. The optical detection device is further integrated, and the near-field optical probe can be directly used as a fluorescence microscope and a Rayleigh light microscope besides being used as Raman spectrum detection, so that one device can complete the detection of all optical signals (Rayleigh scattered light, raman light and fluorescence) and becomes a multifunctional near-field scanning optical microscope (SNOM).
5. The probe is used as a light guide, near-field photon collection is realized, the collection efficiency can be improved by more than two orders of magnitude (according to the distance between the probe tip and a sample, the closer the probe is to the sample, the higher the photon collection efficiency), the direct benefit is that the detection time is reduced, and the indirect benefit can improve the scanning precision and the spatial resolution.
6. The probe can be also suitable for the combination of AFM and STM, can be used singly or combined with fluorescence spectrum and phase spectrum, and can collect the molecular information of almost all nanometer dimensions of a sample.
Drawings
FIG. 1 is a schematic diagram of the structure of the present invention;
FIG. 2 shows the electric field excitation of different composition forms of the probe of the present invention, (a) metal nanoparticles (b) metal nanoparticle arrays (c) penetrating metal nanopillars;
FIG. 3 is a near field scattered light collection diagram of a probe of the present invention;
FIG. 4 is a schematic diagram of a structure of two laser sources implementing SNOM according to an embodiment of the present invention;
fig. 5 is a schematic structural diagram of implementing conventional AFM multiplexing by oblique incidence of laser light in another embodiment of the present invention.
Detailed Description
Referring to fig. 1, the probe of the present invention includes a probe column 6-2 prepared using a micron-sized photoconductive medium, one end of the probe column 6-2 is hemispherical, and metal nanoparticles 6-1 are disposed on the top of the hemispherical. The metal nanoparticles 6-1 may also be replaced with an array of metal nanoparticles.
Referring to fig. 1, a near field microscope comprises a light source 1, a spectrometer 2, a photoelectric converter 3, a circuit controller 4 and an XYZ triaxial workbench 5, wherein excitation light emitted by the light source 1 is reflected by the semi-reflective and semi-transmissive beam splitter 8 and enters a medium probe 6, the excitation light is focused on a tip of the medium probe 6, a local enhanced electric field is formed by metal nano particles 6-1 on the tip of the medium probe 6, a sample on the XYZ triaxial workbench 5 is excited, rayleigh light and raman light collected by the medium probe 6 are reflected by a reflecting mirror 9 and enter a total reflection beam splitter 10 after passing through the semi-reflective and semi-transmissive beam splitter 8, the rayleigh light and raman light are separated by the total reflection beam splitter 10, a light signal is received by the photoelectric converter 3 and converted into an electric signal, the electric signal is input to the circuit controller 4, the circuit controller 4 controls the XYZ triaxial workbench 5 to move, and the raman light enters the spectrometer 2 and is processed by the spectrometer 2. The probe column 6-2 of the medium probe 6 is fixedly suspended by a probe cantilever 11, the probe cantilever 11 is similar to a cantilever in ATM equipment, and the other end of the probe cantilever 11 is connected with the circuit controller 4. The circuit controller 4 controls the cantilever 11 to vibrate in a fixed period, the intensity of the Rayleigh light obtained by the photoelectric converter 3 can change periodically, and the distance between the metal nano particles 6-1 and the sample can be calculated when the periodic change is different like the AFM principle.
In order to facilitate focusing, a lens 7 is disposed on the optical path between the half-reflecting beam splitter 8 and the dielectric probe 6.
The total reflection beam splitter 10 is fixed on an angle-adjustable rotating disc, so that the incidence angle of the light is adjusted to enable the Rayleigh light to be totally reflected by the total reflection beam splitter 10, and the Raman light is incident at an angle smaller than the total internal reflection angle and transmitted through the total reflection beam splitter 10.
In the above structure, the light source 1, the spectrometer 2, the photoelectric converter 3, the circuit controller 4, and the XYZ stage 5 can employ the devices and techniques in the existing tert technique.
By combining the above structures, the specific principle of the invention is as follows: after the laser source is incident and passes through the half-reflection half-transmission beam splitter 8, the laser source passes through an objective lens, the objective lens focuses the collimated laser on the probe cantilever, a passage can be formed to focus the laser on the probe dome tip because the probe is of a micron-sized structure, and the laser can excite the Raman scattered light of the sample on the XYZ workbench through the enhanced electric field due to the locally enhanced electric field of the surface of the metal nano-particles on the excitation medium tip due to the photon injection effect of the tip of the micron-sized medium structure. Meanwhile, since the medium probe is in the near-field range of the sample surface, reflected light, scattered light (collectively, rayleigh light) and raman light incident on the sample are effectively collected. The photons sequentially pass through a medium probe, an objective lens, a semi-reflection semi-transmission beam splitter and a reflecting mirror until reaching the total reflection beam splitter. The total reflection beam splitter separates Rayleigh light from Raman light and is fixed on an angle-adjustable rotating disc. After the light of the reflector is led into the total reflection beam splitter, the refractive index n of the glass material of the reflector determines the total internal reflection angle, the total reflection beam splitter is rotated to enable the Rayleigh light of different excitation light sources to be incident at the angle of just total internal reflection, the Rayleigh light is totally reflected by the beam splitter, and the Raman light is incident at the angle smaller than the total internal reflection and is transmitted through the total reflection beam splitter, so that the separation of the Rayleigh light and the Raman light is realized. The Rayleigh light is totally reflected to a photoelectric converter, an optical signal is converted into voltage, and the voltage is processed by a circuit controller and then controls an XYZ workbench. The Raman light enters a Raman spectrometer after passing through a total reflection beam splitter, and the Raman light is processed by the spectrometer to obtain the Raman spectrum of the sample.
Because the distance between the probe tip and the workbench sample determines the intensity of Rayleigh light obtained by the photoelectric converter, the system can realize the distance control and Raman light collection of the probe by utilizing one path of light path.
The above embodiments are directed to raman light, and the invention may also be used for excitation and collection of fluorescence.
Besides being used for detection, the invention can be used for laser direct writing or photoetching by utilizing an enhanced electric field breaking through diffraction limit resolution.
Conventional TERS is based on an AFM technology, and detection cannot be achieved without AFM, but the invention can be directly used as SNOM alone or combined with AFM.
The optical path can be changed according to different designs of the probe and the cantilever, so that SNOM can be used independently or multiplexed with AFM detection. Independently used as SNOM, two paths of lasers can be used for completing Raman detection, as shown in FIG. 4, the optical path 1 is used as an excitation optical path for completing Raman detection; the optical path 2 detects the phase change of the light reflected by the cantilever 11 to realize the position processing of the probe, and near-field optical scanning can be realized without vibrating the cantilever. The AFM multiplexing can adopt a mode that excitation light is inclined to the incidence of a probe in a far field (similar to the traditional tert), only the probe is used for collecting raman light, the half-reflection and half-transmission beam splitter 8 in fig. 1 can be omitted, and the multiplexing with AFM or STM measurement can be realized more conveniently, as shown in fig. 5.
Based on the above embodiment, if a cantilever-fixed probe like an AFM system is not used, i.e. the association between the circuit controller 4 and the cantilever 11 is canceled, the structure of the probe itself can be used to collect the intensity of the near-field reflection to determine the distance from the sample.
In the invention, the micron-sized medium probe column is combined with the metal nano particles to form the composite structure probe, the probe column and the metal nano particles can be connected by Van der Waals force adsorption in low strength, the medium strength is connected by chemical bond force after carboxylation surface treatment, and the higher strength is physically fixed by a buried growth mode after plasma etching.
The probe formed by the composite structure formed by the medium micrometer structure and the metal nano particles is the core of a microscope, and by using the probe, the optical path symmetry of the photon light guide can be realized, namely, the incident path of photons is the same as the collected path after scattering, and the maximum imaging efficiency can be obtained under the near field condition, so that all optical imaging, especially Raman spectrum imaging with higher requirements on an electric field, can be conveniently realized.
The composite probes with different functions can be obtained by respectively changing the appearance structures of the medium and the metal particles. For example, a metal nanoparticle array can be formed on the surface of a medium by taking Raman imaging as a main purpose, so as to enhance a local electric field and obtain better Raman light intensity; if the optical imaging and AFM multiplexing are used as main purposes, the combination firmness of the dielectric micrometer probe column and the metal nano particles is mainly considered, a groove is processed on the surface of the dielectric light guide by a plasma etching method, and then the metal nano particles are embedded in the hemispherical surface index structure by a vacuum evaporation method, so that the firm combination is realized; if optical imaging and STM multiplexing are used, gold nanowires are embedded into the optical fiber light guide and penetrate through the dielectric micrometer probe column to obtain the structure shown in fig. 2 (c), and multiplexing is realized by combining an STM mechanism.
Claims (9)
1. The utility model provides a near field microscope, includes light source (1), spectrum appearance (2), photoelectric converter (3), circuit controller (4) and XYZ triaxial workstation (5), its characterized in that: the method comprises the steps that excitation light emitted by a light source (1) enters a medium probe (6), the excitation light is focused to a tip of the medium probe (6), a local enhanced electric field is formed by metal nano particles (6-1) at the tip of the medium probe (6), a sample on an XYZ triaxial workbench (5) is excited, the medium probe (6) collects Rayleigh light and Raman light of the sample, the Rayleigh light and the Raman light are separated by a total reflection beam splitter (10), the Rayleigh light is received by a photoelectric converter (3) and converted into an electric signal, the electric signal is input into a circuit controller (4), the circuit controller (4) controls the XYZ triaxial workbench (5) to move, and the Raman light enters a spectrometer (2) and is processed by the spectrometer (2); the medium probe (6) comprises a probe column prepared by adopting a micron-sized light guide medium, one end of the probe column is hemispherical, and a single metal nanoparticle or a metal nanoparticle array is arranged at the top of the hemispherical.
2. The near field microscope of claim 1, wherein: the micron-sized photoconductive medium is a micron-sized optical fiber.
3. The near field microscope of claim 1, wherein: and the probe light guide is internally penetrated with a metal nano column.
4. A near field microscope according to claim 1 or 2 or 3, characterised in that: in the probe column, the path of incidence of the excitation light is the same as the path along which the excitation back-scattered and reflected light is collected.
5. The near field microscope of claim 4, wherein: and the probe column is also provided with a probe cantilever.
6. The near field microscope of claim 1, wherein: excitation light emitted by the light source (1) enters the medium probe (6) after being reflected by the half-reflection and half-transmission beam splitter (8); rayleigh light and Raman light collected by the medium probe (6) are reflected into the total reflection beam splitter (10) through the reflecting mirror (9) after passing through the half reflection beam splitter (8).
7. The near field microscope of claim 6, wherein: a lens (7) is arranged on a light path between the half-reflection and half-transmission beam splitter (8) and the medium probe (6).
8. The near field microscope of claim 1 or 6 or 7, wherein: the total reflection beam splitter (10) is fixed on an angle-adjustable rotating disc.
9. The near field microscope of claim 7, wherein: the probe comprises a medium probe (6), a probe cantilever (11), a circuit controller (4) and a probe column (6-2) fixed on the medium probe (6).
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