Surface plasma resonance holographic imaging resolution improvement method based on metal grating structure

Abstract

The invention discloses a surface plasma resonance holographic imaging resolution improvement method based on a metal grating structure. A grating excitation structure model is constructed according to propagation characteristics of surface plasma waves, a plane structure of a grating is estimated by using Bragg conditions and an effective medium theory, and the intensity and phase response of reflected light waves in an excitation structure are calculated by using a strict coupling wave analysis method. Therefore, surface plasma resonance response curves corresponding to different grating structures are obtained. The sensing performance and the imaging resolution improvement condition are evaluated integrally, and balanced grating structure parameters are selected to ensure the sensing performance and improve the imaging resolution. And finally, designing a surface plasma resonance chip based on a grating structure, and improving the resolution of the surface plasma resonance holographic microscopic imaging system.

Description

Technical Field

[0001] The present invention relates to the field of optical imaging, and in particular to a method for improving surface plasma resonance intensity and phase imaging resolution by utilizing a metal grating structure. Background Art

[0002] In near-field optical detection technology, surface plasmon resonance (SPR) technology has attracted widespread attention due to its high sensitivity, non-invasiveness, and label-free characteristics. Its detection mode relies on the interaction between the surface plasmon wave (SPW) and the sample in the near-field region above the metal. By detecting the complex amplitude information of the reflected light wave when SPR occurs, the physical parameters of the sample can be demodulated. SPR technology is usually used for the detection and characterization of biological and chemical molecules. Most SPR measurement devices are equipped with an objective lens to achieve SPR microscopic imaging. The SPR microscopic imaging device based on prism coupling introduces imaging distortion, and the magnification is very limited, and is only suitable for samples with a size of more than tens of microns; the SPR measurement device based on objective lens coupling achieves an increase in magnification while avoiding imaging distortion, and is suitable for SPR imaging of micron or nanometer samples. However, the SPR image of a tiny sample shows a "tailing" pattern, which seriously reduces the lateral resolution. According to the cause of the formation of "tailing", researchers have proposed a "hybrid" SPR technology based on a metal grating structure (Opt. Express 26, 10616-10630 (2018), J. Opt. Soc. Am. B 27, 1252-1259 (2010)), which uses the grating structure on the metal surface to suppress the propagation of SPW, thereby reducing the impact of "tailing" to improve the resolution of SPR imaging. At the same time, this technology supports the adjustment of structural parameters to suit different application scenarios, but is limited to SPR imaging and sensing based on the detection of reflected light wave intensity. However, the SPR technology based on phase detection has a higher response sensitivity. The surface plasmon resonance holographic microscope (SPRHM) can detect the complex amplitude information of the reflected light wave and realize the simultaneous detection and imaging of SPR intensity and phase. The introduction of metal grating structure into the SPR excitation device is expected to bring more possibilities in improving the imaging resolution of SPRHM and broadening the scope of application. Summary of the invention

[0003] Technical issues to be solved

[0004] In order to overcome the shortcomings of existing methods and technologies and to suppress the lateral propagation of SPW while maintaining excellent sensing performance of SPR intensity and phase, the present invention proposes a method for improving the imaging resolution of SPRHM using a metal grating structure.

[0005] Technical Solution

[0006] The idea of ​​the present invention is that the lateral resolution of SPR imaging is mainly limited by the "tailing" effect, which is caused by the lateral propagation of SPW. Replacing the flat metal layer in the SPR excitation structure with a metal grating structure can achieve the propagation suppression of SPW. The propagation direction of SPW depends on the wave vector of the excitation light wave, and the one-dimensional metal grating consistent with the wave vector of the excitation light wave can be adapted to the detection and imaging of SPRHM. According to the propagation properties of SPW and the properties of the dielectric above the metal, the influence of the grating structure parameters is evaluated in combination with relevant algorithms, and the appropriate parameters are selected to meet the balance between the excellent sensing performance of simultaneous detection of SPR intensity and phase and high imaging resolution.

[0007] The technical solution adopted by the present invention is characterized by the following steps:

[0008] Step 1: Construct a surface plasmon resonance excitation structure consisting of a cover glass, a metal layer, and a dielectric layer, namely a Kretschmann structure, in which the metal layer is replaced by a one-dimensional metal grating. The specific structural parameters are divided into grating period p, duty cycle f, metal substrate thickness h1, and grating structure thickness h2. According to the wavelength of the incident light wave of the system, the dielectric constant of the metal and the dielectric above it, the grating period p0 that meets the Bragg condition is estimated, so that the propagation of the surface plasmon wave is suppressed;

[0009] Step 2: Use the effective medium theory to estimate the equivalent dielectric constant of the metal-dielectric mixed state at the grating structure, and calculate the "tail" length in combination with the wave vector of the surface plasma wave, and obtain the influence of different grating periods and duty cycles on the "tail" length near the period p0. Based on this, select the period p1 and duty cycle f1 to ensure that the "tail" is effectively suppressed;

[0010] The effective medium theory is a theory that approximates a multiphase medium to a single-phase medium and can be replaced by a similar method;

[0011] Step 3: Fix the two parameters p1 and f1, set different thickness parameters h1 and h2 as two-dimensional variable parameters, and use the independent variable of the resonance curve as the variable parameter of the third dimension. Use the rigorous coupled wave analysis method to simulate the intensity reflectivity and reflection phase shift difference of the reflected light wave, that is, the intensity and phase information, and obtain the three-dimensional matrix of intensity and phase respectively. Extract the intensity and phase resonance curve clusters corresponding to different combinations of thickness parameters along the third dimension of the matrix;

[0012] The resonance curve can be divided into angle, wavelength or refractive index resonance curve according to different application scenarios, that is, the intensity or phase response curve of the reflected light wave that changes with the incident angle, the wavelength of the incident light or the refractive index of the dielectric;

[0013] The rigorous coupled wave analysis method is an electromagnetic field calculation method suitable for periodic structures and can be replaced by similar methods;

[0014] Step 4: Extract the variation range of each curve in the intensity and phase curve cluster as the difference, and the maximum slope of the curve as the response sensitivity. Then use the Fresnel formula to fit the intensity resonance curve one by one to obtain a more accurate dielectric constant of the grating structure layer, and then calculate the "tail" length; compare each index with the calculation results of the surface plasma resonance structure containing an ordinary metal layer; based on the comparison results, set the comprehensive performance factor according to the sensing or imaging requirements, and select the thickness parameter h 1-1 With h 2-1 ;

[0015] Step 5: Fix h 1-1 With h 2-1 Two parameters, again use the rigorous coupled wave analysis method to accurately scan the planar structure parameters p and f, and obtain a three-dimensional matrix similar to step 3, and use the method of step 4 to evaluate the parameters p2 and f2;

[0016] Step 6: Based on the calculated grating period p2, duty cycle f2, and thickness parameter h 1-1 With h 2-1 A "hybrid" surface plasmon resonance chip based on the Kretschmann structure is manufactured and ultimately applied to the measurement and imaging of a surface plasmon resonance holographic microscopy system.

[0017] Beneficial Effects

[0018] The present invention proposes a method for improving the intensity and phase imaging resolution of surface plasmon resonance using a metal grating structure. The method is aimed at the mode of simultaneously detecting the intensity and phase information of the reflected light wave in the surface plasmon resonance holographic microscopy system. According to the reasons for the limited imaging resolution, the Bragg condition, effective medium theory and strict coupled wave analysis method are used to design a grating type Kretschmann excitation structure that meets the imaging and sensing requirements. The method allows the "tailing" effect of imaging to be suppressed while maintaining the surface plasmon resonance intensity and phase sensing performance, thereby making up for the shortcomings of the current technology and effectively improving the resolution of surface plasmon resonance imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 : It is an optical path diagram of a surface plasmon resonance holographic microscopy system according to the present invention.

[0020] In the figure: 1-He-Ne laser, 2-fiber coupling device, 3-collimating lens, 4-half-wave plate, 5-converging lens (1-5 are connected to the electric-controlled translation stage), 6-depolarizing beam splitter prism, 7-oil immersion objective lens, 8-surface plasmon resonance excitation structure, 9-imaging lens, 10-Wollaston prism, 11-polarizer, 12-photoelectric imaging device.

[0021] Figure 2 : This is an illustration of the "hybrid" type surface plasmon resonance excitation structure based on the Kretschmann structure.

[0022] In the figure: 1-cover glass, 2-metal adhesion layer, 3-metal grating layer, 4-dielectric layer.

[0023] Figure 3 : Schematic diagram of the improvement of “tailing” in surface plasmon resonance imaging.

[0024] Figure 4 : This is an example of the comparison of the sensing performance of "hybrid" type and ordinary type surface plasmon resonance. DETAILED DESCRIPTION

[0025] The present invention will now be further described with reference to the embodiments and the accompanying drawings:

[0026] The surface plasmon resonance holographic microscopy imaging system of the present invention is as follows Figure 1 As shown, it includes a He-Ne laser 1, a fiber coupling device 2, a collimating lens 3, a half-wave plate 4, a converging lens 5, a depolarizing beam splitter prism 6, an oil immersion objective lens 7, a surface plasmon resonance excitation structure 8, an imaging lens 9, a Wollaston prism 10, a polarizer 11, and a photoelectric imaging device 12, wherein the devices 1-5 are connected to an electrically controlled displacement stage to control the incident angle.

[0027] The specific working process of the method for improving the resolution of surface plasmon resonance imaging based on a grating structure is as follows:

[0028] Designed for the "tailing" characteristics of SPR imaging as shown in the attached Figure 2The grating type Kretschmann excitation structure shown in the figure has structural parameters mainly including grating period p, duty cycle f, metal substrate thickness h1 and grating structure thickness h2. According to the system parameters of SPRHM, namely the wavelength is 632.8nm, the metal type is gold, and the dielectric environment is air, the grating period p0=302nm that meets the Bragg condition is estimated. The effective medium theory is used to estimate the influence of grating structures with different periods and duty cycles on the propagation length of SPW, and the parameters p1=320nm and f1=0.57nm are selected in combination with the required Bragg period. In order to improve the SPR imaging resolution and evaluate the sensing characteristics at the same time, the intensity reflectivity and reflection phase shift difference of the reflected light wave when the grating type Kretschmann structure excites the "hybrid" SPR are simulated by the rigorous coupled wave analysis method, and the parameters p1 and f1 are fixed to scan the two thickness parameters. Taking the SPR sensing method based on incident angle scanning as an example, the response curve of the reflected light wave corresponding to different thickness parameters changing with the incident angle is simulated. Directly extract the difference and response sensitivity from the intensity and phase curves respectively, and then use the Fresnel formula to fit the intensity curve to obtain the dielectric constant of the grating structure layer, and then calculate the SPW propagation length. The specific fitting method is: extract the simulated intensity curve, regard the grating structure layer as an unknown dielectric constant layer, and use the Fresnel formula to calculate the intensity curve corresponding to different dielectric constants, so that the error between the corresponding intensity resonance curve and the extracted curve is minimized to obtain a more accurate equivalent dielectric constant of the grating structure layer. Compare the obtained intensity and phase difference, response sensitivity and SPW propagation length with the relevant calculation results of traditional SPR, and select two thickness parameters. In this embodiment, all calculation indicators are considered without preference, and the sum of the ratios of each indicator of the "hybrid" type and the traditional SPR (that is, the sum of the resolution improvement ratio and the intensity and phase difference, sensitivity ratio) is set as the comprehensive performance factor, where the resolution improvement ratio is the inverse ratio of the SPW propagation length. Select the thickness parameter h according to the comprehensive performance factor 1-1 =22.5nm and h 2-1 =20nm. On this basis, the thickness parameter is fixed, and the grating period and duty cycle are accurately scanned again. Similar to the above steps, the parameters p2 = 314nm and f2 = 0.58nm are selected in combination with the comprehensive performance factor. In summary, the structural parameters are p2 = 314nm, f2 = 0.58nm, h 1-1 =22.5nm and h 2-1 = 20nm "hybrid" SPR chip can be directly used for SPRHM, and its imaging resolution improvement (i.e. "tailing" improvement) is shown in the attached figure. Figure 3 As shown, the specific sensing performance and comparison are shown in the attached Figure 4 .

Claims

1. A method for improving the resolution of surface plasmon resonance holographic imaging based on a metal grating structure, characterized in that Here are the steps: Step 1: Construct a surface plasmon resonance excitation structure consisting of a cover glass, a metal layer, and a dielectric layer, namely a Kretschmann structure, in which the metal layer is replaced by a one-dimensional metal grating. The specific structural parameters are divided into grating period p, duty cycle f, metal substrate thickness h1, and grating structure thickness h2. According to the wavelength of the incident light wave of the system, the dielectric constant of the metal and the dielectric above it, the grating period p0 that meets the Bragg condition is estimated, so that the propagation of the surface plasmon wave is suppressed; Step 2: Use the effective medium theory to estimate the equivalent dielectric constant of the metal-dielectric mixed state at the grating structure, and calculate the "tail" length in combination with the wave vector of the surface plasma wave, and obtain the influence of different grating periods and duty cycles on the "tail" length near the period p0. Based on this, select the period p1 and duty cycle f1 to ensure that the "tail" is effectively suppressed; The effective medium theory is a theory that approximates a multiphase medium to a single-phase medium and can be replaced by a similar method; Step 3: Fix the two parameters p1 and f1, set different thickness parameters h1 and h2 as two-dimensional variable parameters, and use the independent variable of the resonance curve as the variable parameter of the third dimension. Use the rigorous coupled wave analysis method to simulate the intensity reflectivity and reflection phase shift difference of the reflected light wave, that is, the intensity and phase information, and obtain the three-dimensional matrix of intensity and phase respectively. Extract the intensity and phase resonance curve clusters corresponding to different combinations of thickness parameters along the third dimension of the matrix; The resonance curve can be divided into angle, wavelength or refractive index resonance curve according to different application scenarios, that is, the intensity or phase response curve of the reflected light wave that changes with the incident angle, the wavelength of the incident light or the refractive index of the dielectric; The rigorous coupled wave analysis method is an electromagnetic field calculation method suitable for periodic structures and can be replaced by similar methods; Step 4: Extract the variation range of each curve in the intensity and phase curve cluster as the difference, and the maximum slope of the curve as the response sensitivity, and then use the Fresnel formula to fit the intensity resonance curves one by one to obtain a more accurate dielectric constant of the grating structure layer, and then calculate the "tail" length; compare each index with the calculation results of the surface plasma resonance structure containing an ordinary metal layer; Based on the comparison results, the comprehensive performance factor is set according to the sensing or imaging requirements, and the thickness parameter h is selected. 1-1 With h 2-1 ; Step 5: Fix h 1-1 With h 2-1 Two parameters, again use the rigorous coupled wave analysis method to accurately scan the planar structure parameters p and f, and obtain a three-dimensional matrix similar to step 3, and use the method of step 4 to evaluate the parameters p2 and f2; Step 6: Based on the calculated grating period p2, duty cycle f2, and thickness parameter h 1-1 With h 2-1 A "hybrid" surface plasmon resonance chip based on the Kretschmann structure is manufactured and ultimately applied to the measurement and imaging of a surface plasmon resonance holographic microscopy system.

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