CN109543220B - Metal nanoparticle micro-nanostructure and its method for enhancing spontaneous emission in the gap - Google Patents

Abstract

The invention discloses a metal nanoparticle micro-nano structure and a method for enhancing spontaneous radiation in a gap of the metal nanoparticle micro-nano structure, which comprises gold nanoparticles (1), a PMMA layer (2) and a metal substrate (3), wherein the gold nanoparticles (1) are arranged on the PMMA layer (2) at equal intervals, and a molecular or quantum dot radiation source (4) is positioned under the central gold nanoparticles; step 1, calculating the total radiation rate and the far-field radiation rate of single gold nanoparticles on a gold substrate by using a finite element method, and selecting a proper gold nanoparticle radius; and 2, selecting the radius of the gold nanoparticles in the step 1, calculating the change of the total radiation rate and the far-field radiation rate when the distance between every two gold nanoparticles is gradually increased by using a finite element method, and calculating the change of the radiation pattern along with the distance between each gold nanoparticle in the MATLAB-COMSOL based on the Lorentz reciprocity theorem. Compared with the prior art, the invention can enhance the spontaneous radiation rate of the molecular or quantum dot radiation source and change the radiation direction.

Description

Metal nanoparticle micro-nanostructure and its method for enhancing spontaneous emission in the gap

technical field

The invention relates to the technical field of enhanced spontaneous emission, in particular to a novel micro-nano design structure of metal nanoparticles on a metal substrate and a method for enhancing spontaneous emission in a metal nanogap structure based on metal nanoparticles.

Background technique

Spontaneous emission refers to the process in which an excited state atom spontaneously moves from a high energy level to a low energy level without any external action, and at the same time radiates a photon. Currently, metal nanogap structures based on metal nanoparticles have received extensive attention. A spacer layer is coated between the nanoparticles and the metal substrate. This structure can enhance radiation efficiency, shorten fluorescence lifetime, enhance photoluminescence, and control the direction of far-field radiation, etc. The radiation pattern shows the radiation direction of the nanostructure and also reflects the magnitude of the radiation intensity. Commonly used structures to adjust the radiation direction of nanostructures include Yagi-Uda antennas, periodic metal grooves, gold nanorods, triangular gold nanoparticles, core-shell nanostructures, and V-shaped antennas. It is of great practical value to enhance the radiation rate of point sources with nanostructures. In the field of molecular fluorescence sensing, it can improve the quantum yield of fluorescent signals. The design of some micro-nano structures can realize the directional emission of the single photon source, or even the central emission. At this time, all the light can be collected without an objective lens with a large numerical aperture, and the conversion of the spherical wave to the plane wave will be realized.

Contents of the invention

In view of the above-mentioned prior art and its existing defects, the present invention proposes a metal nanoparticle micro-nano structure and a method for enhancing spontaneous emission in the gap, not only designing five kinds of metal nano-particle micro-nano structures, but also changing the metal substrate The nano-gap of the upper metal nano-particles realizes the enhancement of the emission of radiation from molecules or quantum dots and the control of the radiation direction.

A kind of metal nano particle micro-nano structure of the present invention, this micro-nano structure comprises gold nano particle 1, PMMA layer 2 and metal substrate 3, and described metal nano particle 1 is placed on PMMA layer 2, and described PMMA is coated on the On the metal substrate 3, the molecular or quantum dot radiation source 4 is placed in the middle of the PMMA layer 2, the gold nanoparticles 1 are arranged at equal intervals and placed on the PMMA layer 2, and the molecular or quantum dot radiation source 4 located directly below the central gold nanoparticle.

Design different numbers of gold nanoparticles 1 to act on the molecular or quantum dot radiation source structure, including single, two, three, five, nine gold nanoparticles.

By controlling the spacing of two, three, five or nine gold nanoparticles, the change of radiation direction and the enhancement of total radiation rate and far-field radiation rate are realized.

The metal base 3 is a gold base.

The gold nanoparticle 1 is a nanosphere, and the radius of the nanosphere is determined by calculating the spontaneous emission rate by the finite element method, and the first resonance radius is 45 nm.

The selected PMMA layer 2 has a thickness of 10 nm.

A method for enhancing spontaneous emission in the metal nanoparticle gap in the metal nanoparticle micro-nano structure of the present invention, the method comprises the following steps:

Step 1, use the finite element method to calculate the total radiation rate and far-field radiation rate of a single gold nanoparticle on the gold substrate, select a suitable gold nanoparticle radius, and the calculation formula for the total radiation rate is:

in, is the real part of the electric field component of the point current source along the polarization direction;

The far-field radiation rate is expressed as:

Among them, A is a closed surface containing a point current source, S is the time-averaged energy-flux density vector, n is the outer normal vector of surface A, and a is a closed surface element;

The radiation rate in free space is expressed as:

Among them, η _vac is the wave impedance in vacuum, k ₀ =2π/λ, λ is the wavelength, k ₀ is the wave number, and n _a is the refractive index in air;

Step 2. Select the radius of the gold nanoparticles in step 1, and use the finite element method to calculate the normalized total radiation rate Γ _total /Γ _air and the far-field radiation rate Γ _rad when the distance between every two gold nanoparticles increases gradually. The change of / _Γair is based on the Lorentz reciprocity theorem in MATLAB-COMSOL to calculate the variation of the radiation pattern with the spacing of each gold nanosphere.

Compared with the prior art, the invention can not only enhance the spontaneous radiation rate of the molecular or quantum dot radiation source, but also change the radiation direction of the molecular or quantum dot radiation source.

Description of drawings

Fig. 1 is a structural diagram of an embodiment of the present invention. (a), (c), (e), (g), (i) are the side views of single, two, three, five, and nine gold nanoparticles structures, respectively; (b), (d), ( f), (h), (j) are top views of single, two, three, five, and nine gold nanoparticle structures respectively; molecular or quantum dot radiation sources are located directly below the central gold nanoparticle;

Fig. 2 is the change curve of the total radiation rate and the far-field radiation rate with the radius R of a single gold nanoparticle of the embodiment of the present invention;

Reference signs: 1. Gold nanoparticles, 2. PMMA layer, 3. Gold substrate, 4. Molecular or quantum dot radiation source.

Detailed ways

The technical solutions of the present invention will be further described in detail below in conjunction with examples.

As shown in FIG. 1 , it is a schematic diagram of a metal nanoparticle micro-nano structure of the present invention. The embodiment of the present invention has five structural designs. Each of the five designed micro-nano structures is gold nanoparticles, PMMA layer, and gold substrate from top to bottom. The difference between the two structures lies in the number of gold nanoparticles. Molecular or quantum dot radiation sources 4 are placed in the middle of PMMA, and gold nanoparticles 1 are placed on the PMMA layer 2 in an array at equal intervals. Design different numbers of gold nanoparticles to act on the structure of molecules or quantum dot radiation sources, including single, two, three, five, nine gold nanoparticles, molecules or quantum dot radiation sources are located in the center of gold nanoparticles lower position. This structural design can enhance the spontaneous emission rate of molecular or quantum dot radiation sources, while also changing the radiation direction. The incident wavelength is 632.8nm, the refractive index of air is 1, the refractive index of PMMA is 1.5, and the thickness of the PMMA layer is 10nm (the thinner PMMA glue can ensure that the gold nanoparticles and quantum dots are fully coupled and play a role in fixing the structure). The gold nanoparticles have a radius R and a spacing d.

The gold nanoparticles are nanospheres, and the radius R of the nanospheres is calculated by the finite element method (COMSOL Multiphysics software) to calculate the total radiation rate and the far-field radiation rate. When the gold nanoparticle radius satisfies the plasmon resonance condition, the total radiation rate and far-field radiation rate peaks will appear. At this time, the corresponding gold nanoparticle radius is the resonance radius. The present invention selects the gold nanoparticle as 45nm for research.

Assuming that the current density direction is the z direction, the expression of the point radiation source is J=δ(x,y,z)z, where δ is the Dirac function, and z is the unit length vector of the point current source along the polarization direction.

The total radiation rate of this radiation point current source is expressed as:

in, is the real part of the electric field component of the point current source along the polarization direction.

The far-field radiation rate is expressed as:

Among them, A is a closed surface containing a point current source, S is the time-averaged energy-flux density vector, n is the external normal vector of surface A, and a is a closed surface element.

The radiation rate in free space is expressed as:

Wherein, η _vac is the wave impedance in vacuum, k ₀ =2π/λ, λ is the wavelength, k ₀ is the wave number, and n _a is the refractive index in air.

Define the normalized total radiation rate as:

Γ _total /Γ _air

The normalized far-field radiation rate is expressed as:

Γ _rad /Γ _air

First, the finite element method (COMSOLMultiphysics software) is used to calculate the spontaneous emission rate of a single point radiation source in (a) and (b) in Figure 1, and the variation curves of Γ _total /Γ _air and Γ _rad /Γ _air with the radius of gold nanoparticles are respectively As shown in Figure 2(a) and (b). When the gold nanoparticle radius satisfies the plasmon resonance condition, the total radiation rate and the peak value of the far-field radiation rate will appear. At this time, the corresponding gold nanoparticle radius is the resonance radius. It can be seen from Figure 2 that there are four resonance radii, namely R = 45nm, 120nm, 180nm, 260nm, and Γ _total /Γ _air and Γ _rad /Γ _air decrease with the increase of the resonance radius of gold nanoparticles.

The total radiation rate and the far-field radiation rate under different gold nanoparticle structures in Figure 1 can then be calculated, and the variation of the radiation pattern with the gold nanoparticle spacing d can also be calculated.

The invention discloses a method for enhancing spontaneous emission in the nano-gap of metal nanoparticles on a metal substrate. A PMMA layer is arranged between the gold nanoparticles and the gold substrate, and a point source is placed in the PMMA layer to study the number of gold nanoparticles, The influence of layout and size on the radiation rate and radiation direction of point sources, and based on the Lorentz reciprocity theorem, the method of deriving the far field from the near field is used to reproduce the far field radiation pattern. This structure can enhance the efficiency of spontaneous radiation and change the direction of radiation. Compared with the existing structure, the method for regulating spontaneous radiation in the nanogap of metal nanoparticles on the metal substrate of the present invention can change the radiation direction of molecular or quantum dot radiation sources, and has a simple structure and is convenient for experimental testing.

Claims (4)

1. a metal nanoparticle micro-nano structure, this micro-nano structure comprises gold nanoparticle (1), PMMA layer (2) and metal substrate (3), and described metal nanoparticle (1) is placed on PMMA layer (2) On, the PMMA is coated on the metal substrate (3), the molecular or quantum dot radiation source (4) is placed in the middle of the PMMA layer (2), it is characterized in that the gold nanoparticles (1) Equidistant arrangement is placed on the PMMA layer (2), and the molecule or quantum dot radiation source (4) is located directly below the central gold nanoparticle; different numbers of gold nanoparticles (1) are designed to act on the molecule or quantum dot The structure of the radiation source, including single, two, three, five, nine gold nanoparticles, through the control of the distance between two, three, five or nine gold nanoparticles, the radiation direction can be changed and Enhancements to Total Radiation Rate and Far Field Radiation Rate: Step 1. Calculate the total radiation rate and the far-field radiation rate of a single gold nanoparticle on the gold substrate using the finite element method, and select the radius of the gold nanoparticle, where: The formula for calculating the total radiation rate is: in, is the real part of the electric field component of the point current source along the polarization direction; The far-field radiation rate is expressed as: Among them, A is a closed surface containing a point current source, S is the time-averaged energy flow density vector, n is the external normal vector of surface A, and a is a closed surface element; the radiation rate in free space is expressed as: Among them, η _vac is the wave impedance in vacuum, k ₀ =2π/λ, λ is the wavelength, k ₀ is the wave number, and n _a is the refractive index in air; Step 2. Select the radius of the gold nanoparticles in step 1, and use the finite element method to calculate the normalized total radiation rate Γ _total /Γ _air and the far-field radiation rate Γ _rad when the distance between every two gold nanoparticles increases gradually. The change of / _Γair is based on the Lorentz reciprocity theorem in MATLAB-COMSOL to calculate the variation of the radiation pattern with the spacing of each gold nanosphere. 2. A metal nanoparticle micro-nano structure as claimed in claim 1, characterized in that the metal substrate (3) is a gold substrate. 3. a kind of metal nanoparticle micro-nano structure as claimed in claim 1, is characterized in that, described gold nanoparticle (1) is nanosphere, and nanosphere radius chooses to calculate spontaneous emission rate according to finite element method and decides, Choose the first resonance radius which is 45nm. 4. metal nanoparticle micro-nano structure as claimed in claim 1, is characterized in that, described PMMA layer (2) thickness is 10nm.