阅读说明：本技术 一种基于硅球三聚体的高方向性横向单向散射实现方法 (High-directivity transverse one-way scattering implementation method based on silicon sphere tripolymer ) 是由 王湘晖 王建鑫 于 2020-06-16 设计创作，主要内容包括：本发明涉及一种产生高方向性横向单向散射的纳米光学方法,属于新型纳米光学技术领域。本发明基于聚焦径向偏振光与硅球三聚体的相互作用,通过调节硅球三聚体的三个球半径以及在聚焦平面上的横向位置,使得三个硅球激发的总电偶极距轴向分量之和与磁偶极距横向分量之和满足横向Kerker散射条件,实现高方向性的横向单向散射。本发明提出的实现高方向性横向单向散射的方法可以将横向单向散射的散射角降低至43°,显著提高了横向单向散射的方向性。(The invention relates to a nano-optical method for generating high-directivity transverse unidirectional scattering, belonging to the technical field of novel nano-optics. Based on the interaction of the focused radial polarized light and the silicon sphere trimer, the method ensures that the sum of the axial components of the total electric dipole moment excited by the three silicon spheres and the sum of the transverse components of the magnetic dipole moment to meet the transverse Kerker scattering condition by adjusting the three sphere radiuses of the silicon sphere trimer and the transverse position on a focusing plane, thereby realizing the high-directivity transverse unidirectional scattering. The method for realizing high-directivity transverse unidirectional scattering can reduce the scattering angle of the transverse unidirectional scattering to 43 degrees, and remarkably improves the directivity of the transverse unidirectional scattering.)

1. A method for realizing high-directivity transverse one-way scattering based on silicon sphere tripolymer comprises the following steps:

1) a beam of radial polarized light (1) passes through a microscope objective (2) to generate a focused light field, three silicon spheres are placed on a focal plane along the y axis, the transverse offsets of the three silicon spheres along the x axis are equal, and the intervals between the silicon spheres are equal;

2) designing the radius and position distribution of three full dielectric spheres in the silicon sphere trimer, so that the sum of axial components of total electric dipole moments excited inside the three silicon spheres and the sum of transverse components of magnetic dipole moments meet a transverse kerker scattering condition;

firstly, calculating an electric field and a magnetic field of focused radial polarized light by utilizing Richard-Wolf diffraction integral;

secondly, introducing the electric field and the magnetic field into a finite difference time domain algorithm, and calculating the near-field electromagnetic field distribution of the silicon hollow nano disc;

thirdly, based on the near-field electromagnetic field distribution, respectively calculating total electric dipole moment, magnetic quadrupole distance and electric quadrupole distance excited in three silicon spheres in the silicon sphere tripolymer by adopting a multi-polar moment expansion method, and analyzing the relative contribution of the polar moments in far-field scattering;

and fourthly, repeatedly adjusting the radius of the three silicon spheres so that the contribution of the total electric dipole moment and the magnetic dipole moment excited by each silicon sphere plays a main role in the scattering spectrum, and the contribution of the electric quadrupole moment and the magnetic quadrupole moment can be ignored.

And fifthly, repeatedly adjusting the intervals among the silicon spheres in the trimer, and moving the transverse offset of the silicon sphere trimer along the x axis so that the sum of the axial components of the total electric dipole moment excited inside the three silicon spheres and the sum of the transverse components of the magnetic dipole moment meet the transverse Kerker scattering condition at a certain wavelength.

3) Excitation of the silicon sphere trimer with focused radially polarized light will produce highly directional transverse unidirectional scattering.

2. The method for realizing high-directivity transverse one-way scattering based on silicon sphere trimer according to claim 1, characterized in that: the light source is a laser light source or a common light source.

3. The method for realizing high-directivity transverse one-way scattering based on silicon sphere trimer according to claim 1, characterized in that: the specific method of producing the radially polarized light is not limited and may be any of the corresponding techniques and methods.

Technical Field

The invention relates to the technical field of nano-optics, in particular to a nano-optical method capable of realizing high-directivity transverse unidirectional scattering.

Background

The scattering property of the nano structure has important research significance and application value for many fields, such as optical communication, sensing, laser, biomedicine and the like. Different from the loss problem of a plasma metal nano structure, the high-refractive-index all-dielectric nano structure has very low energy loss in visible light and near infrared light bands, and is favorable for developing various powerful low-energy-loss photonic devices [1 ].

The high-refractive-index all-dielectric nano structure can simultaneously excite the electric field and magnetic field response of an optical waveband, presents anisotropic scattering characteristics and can enable electromagnetic wave radiation to propagate along a certain specific or appointed direction [2 ]. In general, the directionality of directional scattering [3, 4, 5] can be characterized by the scattering angle corresponding to the full width at half maximum (the scattering intensity decreases to half the peak) of the main lobe of the far-field scattering intensity, with smaller scattering angles indicating higher directionality of directional scattering. By utilizing the high-directivity directional scattering of the high-refractive-index all-dielectric nano structure, various small radiation sources, sensors and the like with high directivity can be designed, and the method has important application value in the fields of nano antennas, nano lasers, integrated optics, chip signal processing, solar cells, biosensing, medical treatment and the like. Currently, a number of techniques and methods have been proposed in succession to increase the directionality of directional scattering by high index all-dielectric nanostructures. For example, under the excitation of plane waves, the interference between dipole moment excited by a single core-shell nano structure and high-order polar distance is utilized [3], or the directivity of directional scattering is improved based on a high-refractive-index full-dielectric hollow nano dish array structure [5 ]. However, these techniques and methods can only improve the directionality of forward (along the incident light propagation direction) directional scattering, and many practical applications have a strong need to improve the directionality of lateral unidirectional scattering (i.e., directional scattering perpendicular to the incident light propagation direction) of high refractive index all-dielectric nanostructures.

Under the action of a focused light field, the axial electric (magnetic) dipole component and the transverse magnetic (electric) dipole component excited by the single silicon nanosphere [6], the single core-shell nano structure [7] and the single nanowire [8] interfere with each other to generate transverse unidirectional scattering. However, the scattering angle of the far-field scattering main lobe is about 142 °, and the directivity thereof is far from the preparation requirement of high-directivity small radiation sources, sensors and other devices. For a single nanostructure, too little dipole response can be utilized and highly directional transverse unidirectional scattering is difficult to achieve. The high-refractive-index full-dielectric trimer composed of three nanostructures can excite electric dipole and magnetic dipole responses on each particle, mutual interference exists in the dipole responses among the particles, and after more dipole responses interact, more regulation and control degrees of freedom can be provided to improve and enhance the directivity of directional scattering. Silicon is a relatively common high refractive index dielectric material, is economical, and has mature semiconductor processing technology as a technical support. The radial polarized light with axisymmetric distribution has its electric vector direction (i.e. polarization direction) along the radial direction all the time in the beam cross section. After the compact focusing is realized by the microscope objective, the radial polarized light can generate strong axial electric field component in the focus area, act on the nano structure, can excite axial electric dipole response, and is favorable for generating transverse directional scattering after further interfering with transverse magnetic dipole response. At present, many methods and techniques for generating radially polarized light have been proposed, and some of them have been commercialized. Therefore, the invention provides a method for realizing high-directivity transverse unidirectional scattering based on the interaction of the focused radial polarized light and the silicon sphere trimer.

Reference documents:

[1]Yang Z J,Jiang R,Zhuo X,et al.Dielectric nanoresonators for light manipulation[J].Physics Reports,2017,701:1-50。

[2]Fu Y H,Kuznetsov A I,Miroshnichenko A E,et al.Directional visible light scattering by silicon nanoparticles[J].Nature Communications,2013,4:1527。

[3]Liu W,Zhang J,Lei B,et al.Ultra-directional forward scattering by individual core-shell nanoparticles[J].Optics Express,2014,22(13):16178-16187。

[4]Ziolkowski R W.Using Huygens multipole arrays to realize unidirectional needle-like radiation[J].Physical Review X,2017,7(3):031017。

[5]Zhang X M,Zhang Q,Zeng S J,et al.Dual-band unidirectional forward scattering with all-dielectric hollow nanodisk in the visible[J].Optics Letters,2018,43(6):1275-1278。

[6]Bag A,Neugebauer M,P,et al.Transverse Kerker Scattering forLocalization of Nanoparticles[J].Physical Review Letters,2018,121(19):193902-193902。

[7]Shang W,Xiao F,Zhu W,et al.Unidirectional scattering exploited transverse displacement sensor with tunable measuring range[J].Optics Express,2019,27(4):4944-4955。

[8]Xi Z,Urbach H P.Magnetic dipole scattering from metallic nanowire for ultrasensitive deflection sensing[J].Physical Review Letters,2017,119(5):053902。

disclosure of Invention

Aiming at the urgent need of high-directivity transverse unidirectional scattering, the method is based on the interaction of focused radial polarized light and silicon sphere tripolymers, and the sum of the axial components of the total electric dipole moment excited by the three silicon spheres and the sum of the transverse components of the magnetic dipole moment meet the transverse Kerker scattering condition by reasonably designing the three spherical radii of the tripolymers and the transverse position on a focal plane, so that the high-directivity transverse unidirectional scattering is realized. The technical scheme of the invention is as follows:

a method for realizing high-directivity transverse one-way scattering based on silicon sphere tripolymer comprises the following steps:

1) a beam of radial polarized light (1) passes through a microscope objective (2) to generate a focused light field, three silicon spheres are placed on a focal plane along the y axis, the transverse offsets of the three silicon spheres along the x axis are equal, and the intervals between the silicon spheres are equal;

2) the radii and the transverse positions of the three silicon spheres are designed so that the sum of the axial components of the total electric dipole moment excited by the three silicon spheres and the sum of the transverse components of the magnetic dipole moment satisfy the transverse Kerker scattering condition:

firstly, calculating an electric field and a magnetic field of focused radial polarized light by utilizing Richard-Wolf diffraction integral;

secondly, introducing the electric field and the magnetic field into a finite difference time domain algorithm, and calculating the near-field electromagnetic field distribution of the silicon sphere tripolymer;

thirdly, based on the near-field electromagnetic field distribution, respectively calculating total electric dipole moment, magnetic quadrupole distance and electric quadrupole distance excited in three silicon spheres in the silicon sphere tripolymer by adopting a multi-polar moment expansion method, and analyzing the relative contribution of the polar moments in far-field scattering;

and fourthly, repeatedly adjusting the radius of the three silicon spheres so that the contribution of the total electric dipole moment and the magnetic dipole moment excited by each silicon sphere plays a main role in the scattering spectrum, and the contribution of the electric quadrupole moment and the magnetic quadrupole moment can be ignored.

And fifthly, repeatedly adjusting the intervals among the silicon spheres in the trimer, and moving the transverse offset of the silicon sphere trimer along the x axis so that the sum of the axial components of the total electric dipole moment excited inside the three silicon spheres and the sum of the transverse components of the magnetic dipole moment meet the transverse Kerker scattering condition at a certain wavelength.

3) Excitation of the silicon sphere trimer with focused radially polarized light will produce highly directional transverse unidirectional scattering.

The invention has the beneficial effects that: high directional transverse unidirectional scattering is achieved and the scattering angle can be reduced to 43 deg..

Drawings

FIG. 1 is a light path diagram according to an embodiment of the present invention.

FIG. 2 is a two-dimensional cross-sectional view of a silicon sphere trimer in the xy plane according to an embodiment of the present invention.

FIG. 3 illustrates the intensity distribution of the components of the focused light field at the focal plane;

(a) electric field longitudinal component (b) electric field transverse component (c) magnetic field transverse component.

FIG. 4 the multipole moment development of each silicon sphere in the silicon sphere trimer of the embodiment of the present invention;

(a) no. 1 silicon ball (R)₁＝100nm,x₁＝235nm,y₁＝375nm,z₁＝0nm)；

(b) 2 nd silicon ball (R)₂＝125nm,x₂＝235nm,y₂＝0nm,z₂＝0nm)；

(c) No. 3 silicon ball (R)₃＝100nm,x₃＝235nm,y₃＝-375nm,z₃＝0nm)；

FIG. 5 silicon sphere trimer (R) according to the embodiment of the present invention₁＝R₃＝100nm,R₂＝125nm,x₁＝x₂＝x₃＝235nm；y₁＝-y₃＝375nm，y₂＝0nm；z₁＝z₂＝z₃0nm, d 150nm) total scattered intensity line.

FIG. 6 shows the sum Dz of the axial components of the total electric dipole moment and the sum m of the axial components of the magnetic dipole moment of a silicon sphere trimer excitation according to an embodiment of the present invention_yPhase difference and amplitude ratio of (2).

FIG. 7 shows the three-dimensional far-field radiation of the silicon sphere trimer at 745nm wavelength and the two-dimensional far-field distribution of the transverse unidirectional scattering diagram in the xy plane in accordance with the embodiment of the present invention;

(a) three-dimensional far-field radiation (b) two-dimensional far-field distribution in the xy-plane;

the figure shows that: radial polarized light 1, microscope objective 2 and silicon sphere trimer 3.

Detailed Description

The invention is further described with reference to the following drawings and detailed description.

1) Exciting silicon sphere trimer by using focused radial polarized light, as shown in fig. 1, a beam of radial polarized light (1) propagating along the z direction generates a focused light field after passing through a microscope objective (2), the numerical aperture NA of the objective is 0.3, and the silicon sphere trimer (3) is placed on a focal plane, wherein the lateral offsets of the three silicon spheres along the x axis are equal, and the x axis is equal to the x axis₁＝x₂＝x₃I.e. arranged in the y-direction at the focal area, the spacing d between the particles is equal, and a two-dimensional cross-sectional view of the silicon sphere trimer at the focal plane is shown in fig. 2.

2) Adjusting the radius R of three silicon spheres in the silicon sphere tripolymer₁、R₂And R₃The spacing D between the silicon spheres and the lateral offset along the x-axis are such that the sum D of the axial components of the total electric dipole moment excited by the three silicon spheres_z(D_z＝D_1z+D_2z+D_3z,D_jzAxial component of total electric dipole moment excited by jth silicon sphere) and transverse component of magnetic dipole moment_y(m_y＝m_1y+m_2y+m_3y，m_jyIs the transverse component of the magnetic dipole moment of the jth silicon sphere excitation) satisfies the transverse Kerker scattering condition.

Firstly, a focused light field is calculated by Richard-Wolf diffraction integral, as shown in fig. 3, the focused light field has the characteristic of non-uniform distribution, the distribution of the axial component of the electric field is represented as a rotationally symmetric bright spot with the maximum value at the focal point, the transverse component of the electric field is represented as a hollow circular ring distribution, the value at the focal point is zero, and the magnetic field only has the transverse component and is also distributed in a hollow circular ring shape.

Secondly, setting the structural parameters of three silicon spheres in the silicon sphere tripolymerAre each R₁＝100nm，R₂＝125nm，R₃100nm, position parameter x₁＝x₂＝x₃＝235nm；y₁＝-y₃＝375nm，y₂＝0nm；z₁＝z₂＝z₃0nm and a spacing d of 150 nm. Under the excitation of focusing radial polarized light, a near-field electromagnetic field and far-field scattering of the silicon sphere trimer are calculated by using a finite difference time domain algorithm FDTD.

And thirdly, respectively calculating the relative contributions of the total electric dipole moment (TED), the magnetic dipole Moment (MD), the electric quadrupole moment (EQ) and the magnetic quadrupole Moment (MQ) of the three silicon spheres to far-field scattering by adopting a multi-polar moment expansion method based on the electromagnetic fields inside the three silicon spheres, wherein the TED is the result after the mutual interference of the electric dipole moment (ED) and the annular dipole moment (TD). As shown in FIG. 4(a), the first silicon sphere, radius R₁100nm, position x thereof₁＝235nm，y₁＝375nm，z₁0nm, mainly TED and MD are excited in the wavelength range of 700nm-900nm, the resonance peak of only one MD is located at 765nm, and the contribution of the electric quadrupole EQ and the magnetic quadrupole MQ is still almost zero. The results of considering only the scattering spectra contributed by the total electric dipole moment TED and the magnetic dipole moment MD are in good agreement with the results of the total scattering spectra, which indicate that the interaction of TED and MD is sufficient for analyzing the scattering properties of the first silicon spheres. Second silicon sphere, radius R, as shown in FIG. 4(b)₂125nm, position x thereof₂＝235nm，y₂＝0nm，z₂0nm, mainly TED and MD are excited in the wavelength range of 700nm-900nm, only one resonance peak of TED is located at the wavelength of 760nm, and the contribution of the electric quadrupole distance EQ and the magnetic quadrupole distance MQ is still almost zero. The results of considering only the total electric dipole moment TED and the magnetic dipole moment MD contributions to the scattering spectrum are also very consistent with the results of the total scattering spectrum, with only a certain difference in the short wavelength direction, which indicates that the interaction of TED and MD is sufficient for analyzing the scattering properties of the second silicon spheres. Third silicon sphere, radius R, as shown in FIG. 4(c)₃100nm, position x thereof₃＝235nm，y₃＝-375nm，z₃0nm, in the wavelength range of 700nm-900nm, mainly exciting TED and MD, with only one MD resonatingThe peak is located at 765nm wavelength, the contribution of the electric quadrupole EQ and the magnetic quadrupole MQ is still almost zero. The results of considering only the scattering spectra contributed by the total electric dipole moment TED and the magnetic dipole moment MD are also very consistent with the total scattering spectra, indicating that the interaction of TED and MD is sufficient for analyzing the scattering properties of the third silicon sphere. Then, all the dipoles excited in the three silicon spheres are brought into a dipole array radiation model to calculate a total scattering intensity spectral line of the silicon sphere trimer, as shown in fig. 5, the calculation result of the dipole array model is basically consistent with the calculation result of FDTD, and only a little difference exists in the short wave direction, which is enough to prove that the dipoles excited by the three silicon spheres take a main role in the total scattering intensity spectral line of the silicon sphere trimer, and can be used for analyzing the scattering characteristics of the silicon sphere trimer.

Fourthly, according to the non-uniform distribution characteristic of the focusing light field shown in the figure 3, the sum D of the total electric dipole moment axial components excited in the silicon sphere trimer_z(D_z＝D_1z+D_2z+D_3z,D_jzIs the sum m of the axial component of the total electric dipole moment excited by the jth silicon sphere) and the transverse component of the magnetic dipole_y(m_y＝m_1y+m_2y+m_3y，m_jyIs the transverse component of the magnetic dipole moment excited by the jth silicon sphere), the scattered electric fields in the + x and-x directions can be expressed as:

wherein, θ andpolar and azimuthal angles, k, respectively, of far-field observation points₀、ε₀And c is the wave number, dielectric constant and speed of light, respectively, in vacuum, and r is the far field observation distance. Formula (1) shows that when D_z＝m_yAt/c, the phase difference between the two is arg (m)_y)-arg(cD_z) 0, i.e. parity, and amplitude ratio | m_y|/|cD_zWhen 1, scattering electric field in + x directionTransverse unidirectional scattering in the-x direction will occur; when D is present_z＝-m_yAt/c, the phase difference is arg (m)_y)-arg(cD_z) At 180 °, i.e. in antiphase, and amplitude ratio | m_y|/|cD_zWhen 1, the scattered electric field in the-x directionLateral unidirectional scattering in the + x direction will occur. As shown in FIG. 6(a), the sum D of the axial components of the total electric dipole moment of the silicon sphere trimer at a wavelength of 745nm_zM is summed with the transverse component of the magnetic dipole moment_yAmplitude ratio of | m_y|/|cD_zL is 1; as shown in FIG. 6(b), the sum D of the axial components of the total electric dipole moment of the silicon sphere trimer at a wavelength of 745nm_zM is summed with the transverse component of the magnetic dipole moment_yThe phase difference of (a) is 23 °, it is considered that the condition of the same phase is satisfied; i.e., the silicon sphere trimer, satisfies the lateral Kerker scattering condition at a wavelength of 745 nm.

3) Fig. 7 shows the three-dimensional far-field radiation of the silicon sphere trimer and the two-dimensional far-field distribution of the lateral scattering in the xy plane at an incident light wavelength of 745nm, and it can be seen from fig. 7(b) that the scattering angle of the two-dimensional far-field scattering of the silicon sphere trimer in the xy plane is reduced to 43 °.

The foregoing is a more detailed description of the present invention that is presented in conjunction with specific embodiments, which are not to be construed as limiting the invention to the specific embodiments described above. Numerous other simplifications or substitutions may be made without departing from the spirit of the invention as defined in the claims and the general concept thereof, which shall be construed to be within the scope of the invention.