CN108593624B - Multi-wavelength metal plasma resonance structure with enhanced selectivity and preparation method thereof - Google Patents
Multi-wavelength metal plasma resonance structure with enhanced selectivity and preparation method thereof Download PDFInfo
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Abstract
The invention relates to a multiple optical field coupling Fano resonance metal plasma resonance structure for selectivity enhancement and a preparation method thereof. The resonance structure can simultaneously realize multiple multi-wavelength resonances with extremely high electromagnetic field enhancement factors, and the resonance modes have the characteristics of narrow line width, equivalent resonance peak intensity, main local resonance light field in a metal round hole and the like. These characteristics allow the structure to simultaneously enhance the excitation field and the emission field of the target molecule, and realize selective detection of the target molecule while ensuring high sensitivity and high accuracy of detection. The preparation method of the resonance structure is simple and convenient in process, only needs the traditional nano-imprinting process and the film evaporation process, is good in repeatability and is convenient to apply.
Description
Technical Field
The invention relates to a plasma resonance structure and a preparation method thereof, in particular to a multiple optical field coupling Fano resonance metal plasma resonance structure for selectivity enhancement.
Technical Field
The metal surface plasmon wave is a physical optical phenomenon in which light irradiates on a metal dielectric interface to excite collective oscillation of electrons on the metal surface. In recent ten years, the development of sensors based on the metal plasma resonance principle and the application research thereof has been greatly developed. Because the detection process is rapid, real-time data can be obtained, the operation is convenient, no mark is needed, the biological activity of molecules can be kept and the like, the metal surface plasma resonance sensor is widely applied in the fields of life science and pharmacy, and the metal surface plasma resonance sensor gradually permeates into the research fields of chemistry, chemical engineering, materials, food, environment, national defense and the like at present, and is particularly applied in the aspects of immunodetection, environmental monitoring, adsorption property of material surfaces and interfaces, electrochemical polymerization and the like. The metal surface plasma resonance device can realize local field enhancement of several orders of magnitude on an optical field near a structure, and the enhancement factor of the metal surface plasma resonance device on a Raman signal can reach 10 most strongly14. Therefore, Raman scattering and fluorescence labeling based on the surface plasmon resonance structure are effective ways for realizing high-sensitivity sensing and detection, and have extremely important application values in the aspects of ultrahigh-sensitivity sensing and detection.
At present, the application research for fluorescence enhancement and Raman enhancement based on the metal surface plasmon resonance principle is mainly developed in two directions. One is to seek higher local field enhancement factors to improve detection sensitivity. The plasma resonance structure commonly used at present mainly comprises a single metal nanoparticle and a metal nanoparticle pair, the resonance characteristic of the metal nanoparticle is shown in fig. 1, and the symmetric structure generally has a resonance peak; and the second is to seek a multi-wavelength plasma resonance structure for simultaneously detecting a plurality of fluorescent probes or improving the signal-to-noise ratio of Raman signals. At present, most of the asymmetric structures are adopted to realize the resonance of multiple wavelengths, and the structures comprise an ellipsoid, a metal rod, a polyhedron and other asymmetric structures, the resonance characteristic of the metal nano ellipsoid structure is given in fig. 2, when the polarization state of exciting light is along different directions of the ellipsoid, the positions of the resonance peaks are different, but the relative positions and the strength of the multiple resonance peaks are not easy to adjust, and the asymmetry of the structure causes the inconsistent distribution of an optical field under different resonance modes, so that the selectivity enhancement of target molecules is not facilitated. And the two types of plasma resonance have the characteristic of ultra-wide enhanced spectrum, the half-height width of the resonance peak is about 100 nanometers, and the background signal is enhanced while the target molecule signal is enhanced.
Disclosure of Invention
The purpose of the invention is as follows: in order to overcome the defects of the prior art, the invention provides a multiple optical field coupling Fano resonance metal plasma resonance structure for selectivity enhancement, which realizes selective detection of target molecules on the basis of sensitivity increase, and simultaneously enables excited formants to have the characteristics of narrow line width (for example, 10 nanometers) and equivalent formant intensity.
On the other hand, the invention also provides a preparation method of the multiple optical field coupling Fano resonance metal plasma resonance structure for selective enhancement.
The technical scheme is as follows: the multiple optical field coupling Fano resonance metal plasma resonance structure for selectively enhancing comprises a metal film, an optical transparent dielectric film and a metal hole array from bottom to top in sequence.
Further, the thickness of the metal film is not less than 100 nanometers, so that the possibility that the lower layer interface of the metal film excites surface plasma resonance to generate interference is eliminated.
Further, the metal film is a gold film or a silver film, and the optically transparent dielectric film is SiO2Film or MgF2And (3) a membrane.
Further, the diameter of the metal hole in the metal hole array is in the micro-nano order.
The preparation method of the multiple optical field coupling Fano resonance metal plasma resonance structure for selective enhancement, provided by the invention, comprises the following steps: (S1) sequentially evaporating a metal film and an optically transparent dielectric film on an insulating substrate; (S2) coating an organic thin film on the optically transparent dielectric film by a spin coating method; (S3) imprinting the organic film by using the prepared template, and engraving a disc array with a column symmetric structure by using the template; (S4) evaporating a metal layer on the array of the pillar symmetric structure disks; (S5) after the evaporation is finished, the organic film is cleaned and removed by using organic solution, and the column symmetric metal hole array is remained.
Has the advantages that: compared with the prior art, the invention has the following remarkable effects:
1. the single metal hole can excite local plasma resonance and is mutually coupled with an optical field excited by the periodic metal hole array, and the introduction of the dielectric film enables the metal hole and a mirror optical field excited by the bottom metal film to be mutually coupled to form Fano resonance, so that a plurality of space mode fields are generated while the field enhancement factor is greatly improved, and a plurality of resonance peaks are generated; meanwhile, the position and the coupling strength of the resonance peak can be conveniently adjusted by adjusting the thickness, the area and the period of the metal round hole and the thickness of the dielectric film.
2. The introduction of the periodic metal hole array can effectively compress the width of a resonance peak while improving the field enhancement factor, and in addition, the local plasma resonance excited around a single metal hole is mutually coupled with the surface plasma resonance excited by the metal hole array and the mirror image light field excited by the bottom metal film to form Fano resonance, so that the width of the resonance peak is further compressed. And the column symmetric pore structure can ensure that the mode field distribution under different resonance modes is basically consistent, thereby being convenient for realizing the selectivity enhancement of target molecules.
3. The selective enhancement of the target molecule signal is achieved by corresponding different resonance modes to a plurality of characteristic signals of the same target molecule or to respective specific signals of a plurality of target molecules. Taking raman signal detection as an example, by respectively corresponding three resonance peaks of the metal plasmon resonance structure of the present invention to an excitation mode of a target molecule and two selected raman resonance modes, simultaneous enhancement of the two selected raman scattering signals can be achieved on the basis of excitation light resonance enhancement. Only molecules satisfying both raman resonances can be identified.
4. The structure is simple, the process is simple and convenient, only the traditional nano-imprinting process and the film evaporation process are needed, the repeatability is good, and the application is convenient.
Drawings
FIG. 1 is a graph of resonance characteristics of a symmetric metal nanosphere;
FIGS. 2(a) and 2(b) are a resonance characteristic diagram and an optical field distribution diagram, respectively, of an asymmetric metal ellipsoid structure;
FIGS. 3(a) and 3(b) are a perspective view and a scanning electron microscope top view, respectively, of the metal plasmon resonance structure of the present invention;
FIGS. 4(a) to 4(c) are the resonance spectra of the metal plasmon resonance structure of the present invention under different metal hole periods, dielectric film thicknesses and metal hole radii, respectively;
FIG. 5(a) is a resonance characteristic diagram of different plasmon resonance structures with metal pore structures designed for the target molecule rhodamine 6G; FIGS. 5(b) to 5(d) are light field distribution diagrams of the corresponding three formants in FIG. 5(a), respectively;
FIG. 6 is a Raman spectrum measured on a plasmon resonance structure with a different structure designed for the target molecule rhodamine 6G.
Detailed Description
The present invention will be further described with reference to the accompanying drawings.
Referring to fig. 3(a), the multiple optical field coupling Fano resonant metal plasmon resonance structure for selectivity enhancement of the present invention comprises a metal film 1, an optically transparent dielectric film 2 and a metal hole array 3 sequentially formed on an insulating substrate from bottom to top, wherein the metal film 1 is a gold film or a silver film, the optically transparent dielectric film 2 is made of silicon dioxide SiO2Or magnesium fluoride MgF2. Preferably, the thickness of the metal film 1 is greater than 100 nm to eliminate the possibility that the lower interface of the metal film 1 excites surface plasmon resonance and thus generates interference. In addition, the diameter of the metal hole in the metal hole array 3 is in the micro-nanometer order.
Correspondingly, the preparation method of the metal plasma resonance structure comprises the following steps:
(S1) evaporating the metal film 1 and the optically transparent dielectric film 2 on the glass or quartz substrate by a thermal evaporation method;
(S2) coating an organic thin film on the glass or quartz substrate by spin coating;
(S3) imprinting the organic film by using the prepared template, and engraving a disc array with a column symmetric structure by using the template;
(S4) then evaporating metal on the array of the pillar symmetric structure disks;
(S5) after the evaporation, the organic thin film is removed by cleaning with an organic solution, leaving the pillar symmetric metal hole array 3.
A scanning electron microscope top view of the metal plasmon resonance structure prepared through the above steps is shown in fig. 3 (b).
Fig. 4(a) to 4(c) show resonance spectra obtained by adjusting the period of the metal hole, the thickness of the optically transparent dielectric film, and the radius of the metal hole in the resonance structure of the present invention, respectively. As can be seen from the figure, the metal plasmon resonance structure of the invention can generate three surface plasmon resonance peaks, and thus has obvious advantages in the field of selectivity enhancement compared with a single resonance peak structure. Meanwhile, with the increase of the thickness of the optical transparent medium film and the grating constant, the resonance peak of the structure respectively has blue shift and red shift, which are similar to the characteristics of a periodic disc; as the radius of the metal pore increases, the resonant peak appears blue-shifted, which is exactly opposite to the property of a periodic disk, which ensures the simplicity of sample preparation on the nanometer scale. Therefore, the coupling strength among the modes can be conveniently changed by adjusting the thickness of the dielectric film, the thickness of the metal film, the diameter of the round hole and the period of the round hole, so that the strength and the position of each resonance peak are changed.
The metal plasma resonance structure can well realize the selective detection of target molecules. For the convenience of comparison, the concentration of 10uL is 10-5And (3) dripping the rhodamine 6(R6G) solution of M on the surfaces of the resonance structures shown in the structure 1 and the structure 2, respectively detecting by adopting a Raman spectrometer LabRAM HR800, wherein the excitation wavelength is 784nm, the focusing objective lens is a 50-time objective lens, and the numerical aperture of the focusing objective lens is 0.5. Wherein, the structure 1 corresponds to the resonance structure of the invention, and the material of the dielectric film is silicon dioxide SiO2A thickness of 57 nm; the metal film is made of silver and has the thickness of 100 nanometers; the metal hole array is made of silver, the diameter of the round hole is 220 nanometers, the thickness of the round hole is 30 nanometers, and the period of the round hole is 830 nanometers. The structure 2 is a resonance structure for comparison, wherein the metal hole array 3 is made of silver, the diameter of the circular hole is 220 nm, the thickness of the circular hole is 30 nm, the period of the circular hole is 975 nm, and only an insulating substrate is arranged below the array without a metal film and an optical transparent dielectric film. The resonance characteristic map measured for structure 1 corresponds to the dotted circle line in fig. 5(a), and the resonance characteristic map measured for structure 2 corresponds to the square line in fig. 5 (a).
As shown in the block line of fig. 5(a), the plasmon resonance structure for comparison has only one single resonance peak with a wide line width, which is about 100 nm or so, and simultaneously enhances all raman peaks of the rhodamine 6(R6G) molecule; as shown by the dotted lines in fig. 5(a), the plasmon resonance structure of the present invention can simultaneously form resonances around wavelengths of 784nm, 818 nm and 876 nm, and these resonances have the characteristics of narrow line width (which is one order of magnitude narrower than the line width of the single resonance peak structure; for example, 10 nm), comparable resonance peak intensity, and the resonance light field is mainly localized at the periphery of the structure, compared with the single resonance peak structure. The narrow linewidth characteristic of the plasma resonance structure is caused by Fano resonance formed by mutual coupling of multiple optical fields, the three resonance peaks respectively correspond to a Raman excitation peak of rhodamine 6(R6G) molecules and two different Raman scattering peaks, when the R6G molecules fall near the round hole of the structure, the two Raman scattering peaks corresponding to the resonance wavelength are enhanced, and theoretically, the Raman scattering signal corresponding to the resonance wavelength can be increased by 10000 times. Specifically, fig. 5(b) is a local field enhancement characteristic diagram at a wavelength of 784nm, fig. 5(c) is a local field enhancement characteristic diagram at a wavelength of 818 nm, and fig. 5(d) is a local field enhancement characteristic diagram at a wavelength of 876 nm. In addition, the plasma resonance structure can simultaneously and respectively enhance the excitation field and the plurality of Raman scattering fields of the detected molecules, thereby increasing the distinguishing degree of Raman scattering of the target molecules and the background molecules, improving the signal-to-noise ratio and realizing selective Raman enhancement.
The upper, middle and lower three curves in the raman spectrum of rhodamine 6(R6G) on the left side in fig. 6 correspond to the structures 1 to 3 on the right side, respectively. Wherein, the structure 1 and the structure 2 on the right side in fig. 6 are completely the same as the structure 1 and the structure 2 on the right side in fig. 5(a), and have the same parameters; the structure 3 on the right side in fig. 6 is a case when the metal hole array on the uppermost layer in the resonance structure shown in the structure 1 is replaced with a metal film. It can be seen that the raman peak measured for structure 1 is significantly stronger than the raman spectra of R6G for structures 2 and 3. In addition, the raman scattering signals corresponding to two surface plasmon resonance peaks (i.e., the peaks corresponding to the dashed lines in fig. 6) are significantly stronger than the other non-corresponding raman scattering signals, thereby realizing the simultaneous selective raman enhancement of the structure to different raman peaks of the R6G raman molecule. When the analyte contains a plurality of target molecules, the analyte can be determined to contain R6G only if the two Raman peaks are enhanced simultaneously.
Claims (3)
1. A multi-wavelength metal plasmon resonance apparatus with enhanced selectivity, comprising: the metal film (1), the optical transparent dielectric film (2) and the metal hole array (3) are sequentially arranged from bottom to top; the excitation wavelength of the multi-wavelength metal plasma resonance device is 784 nm; the dielectric film is made of SiO2A thickness of 57 nm; the metal film (1) is made of silver and has the thickness of 100 nanometers; the diameter of a round hole of the metal hole array (3) is 220 nanometers, the thickness is 30 nanometers, and the period is 830 nanometers; the metal plasma resonance device can generate three surface plasma resonance peaks.
2. A preparation method of a selectivity-enhanced multi-wavelength metal plasma resonance device is provided, wherein the excitation wavelength of the multi-wavelength metal plasma resonance device is 784 nm; the metal plasma resonance device is characterized in that the metal plasma resonance device can generate three surface plasma resonance peaks; the preparation method comprises the following steps:
(S1) evaporating a metal film (1) and an optical transparent dielectric film (2) on an insulating substrate in sequence; the metal film (1) is made of silver and has the thickness of 100 nanometers; the dielectric film is made of SiO2A thickness of 57 nm;
(S2) coating an organic thin film on the optically transparent dielectric film (2) by spin coating;
(S3) imprinting the organic film by using the prepared template, and engraving a disc array with a column symmetric structure by using the template;
(S4) evaporating metal on the array of the pillar symmetric structure disks;
(S5) after the evaporation is finished, cleaning by using an organic solution to remove the organic film, and keeping the column symmetric metal hole array (3); the diameter of a round hole of the metal hole array (3) is 220 nanometers, the thickness of the round hole is 30 nanometers, and the period of the round hole is 830 nanometers.
3. The production method according to claim 2, wherein the insulating substrate is a glass substrate or a quartz substrate.
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