Symmetrical broken micro-nano structure for realizing optical directional propagation

Abstract

The invention discloses a symmetrical broken micro-nano structure for realizing light directional propagation. The symmetrical broken micro-nano structure comprises a metal notch nano disc and an excitation source, wherein the metal notch nanometer disc is in the shape of a metal disc with a notch, the notch is formed by cutting off a non-concentric disc with the radius smaller than that of the metal disc on one side of the metal disc, and the non-concentric disc is overlapped with the metal disc. Meanwhile, due to the constraint of the disc-shaped structure, light emitting is limited in the plane where the disc is located, directional propagation of in-plane light is finally obtained, and the disc-shaped directional light source has the advantages of being simple in structure, convenient to operate and the like, is easy to machine and integrate and has considerable prospects in many neighborhoods of the scientific research field, the medical field, the industrial field and the like.

Description

Technical Field

[0001] The present invention relates to the field of surface plasmon photonics, and particularly to a symmetry-breaking micro-nano structure for realizing the directional propagation of light. Background Art

[0002] The directional propagation of light was initially achieved by lasers based on Fabry-Perot optical cavities. However, with the increasing requirement for device integration, the diffraction limit problem of traditional lasers cannot be ignored. Although microdisk optical cavities, photonic crystal optical cavities, etc. that can reduce the size of the optical cavity to the micron level have emerged successively with the reduction of the laser size, the diffraction limit problem has still not been solved. In recent years, the discovery of surface plasmons in noble metal micro-nano structures has made new progress in further reducing the optical cavity below the diffraction limit. Surface plasmons, which originate from the collective oscillation of free electrons in metals under the excitation of incident light, can form evanescent waves that are confined to the metal-dielectric interface and propagate, thereby generating a huge near-field in the vicinity of the interface, and thus triggering enhanced light scattering and absorption. Localized surface plasmons compress the optical field mode into the nanoscale region, enabling the interaction between light and matter to reach a level unattainable by previous technologies. However, when an optical resonance cavity is scaled down to the micron level using a specific design, the system itself can provide optical feedback without any external cavity. And its laser emission is either directional or omnidirectional, depending on the geometry of the resonance cavity and the main resonance cavity mode. So far, cavity structures of many shapes have been studied by people, such as core-shell sphere structures, nanorod structures, sandwich structures, trench structures, array structures, etc. Among them, for some structures, due to the symmetry of the cavity, the emission direction is still isotropic; for some other structures, because of the existence of symmetry breaking, they can exhibit a certain directionality, but some have a large divergence angle, and some can only emit unidirectional light in three dimensions; for some array structures, although they can achieve lateral light emission in the plane, the light emission in this plane is isotropic.

[0003] When light interacts with nanostructures, due to the isotropy of light scattering and the dissipation effect of metal materials, it is usually difficult to achieve the directional propagation of light, and the scattered energy is often dispersed in multiple directions, thus reducing its efficiency in practical applications. To solve this problem, R. Alaee proposed a method to precisely control the direction of light propagation by satisfying the "generalized Kerker condition". This condition optimizes the design of the light scattering pattern by adjusting the interference effect between the electric dipole moment and the electric quadrupole moment in the nanostructure. In the forward scattering direction, the radiation fields of the electric dipole moment and the electric quadrupole moment have the same phase, forming constructive interference, which significantly enhances the forward scattering; while in the backward scattering direction, the phases of their radiation fields are opposite, resulting in destructive interference, which greatly suppresses the backward scattering. This control method based on the interference effect provides technical support for the development of photonic chips and shows broad application prospects in fields such as high-density optical path integration, low-loss optical communication, and high-resolution optical imaging. Summary of the Invention

[0004] The object of the present invention is to provide a symmetry-breaking micro-nano structure for realizing the directional propagation of light, which uses the symmetry breaking of the structure to regulate the coupling between modes to achieve the directional emission of light. At the same time, due to the constraint of the disk-shaped structure, the light emission is limited to the plane where the disk is located, and finally the in-plane directional propagation of light is obtained.

[0005] To achieve the above functions, the present invention designs a symmetry-breaking micro-nano structure for realizing the directional propagation of light, including a metal-notch nano-disk and an excitation source; wherein the metal-notch nano-disk has the shape of a metal disk with a notch, and the notch is the overlapping part of a non-concentric disk with a radius smaller than it cut off from one side of the metal disk and the metal disk. The combination of the metal disk and the notch forms an asymmetric nano-cavity structure; the excitation source is incident from the direction perpendicular to the disk surface of the metal disk, and the polarization direction is along the direction perpendicular to the symmetry plane of the metal disk, and the symmetry plane of the metal disk is the plane passing through the center of the metal disk and perpendicular to the disk surface.

[0006] As a preferred technical solution of the present invention: the material of the metal disk and the notch is gold material that can generate surface plasmons.

[0007] As a preferred technical solution of the present invention: the radius of the metal disk is 90 nm, the radius of the notch circle is 40 nm, the center distance between the two is 103 nm, and the thickness of the metal disk is 20 nm.

[0008] As a preferred technical solution of the present invention: the media above and below the metal-notch nano-disk are air.

[0009] Advantageous Effects: Compared with the prior art, the advantages of the present invention include:

[0010] The present invention designs a symmetry-breaking micro-nano structure for realizing the directional propagation of light. By regulating the geometric shape of metal nanoparticles, the plasmon modes in the cavity are regulated. The symmetry breaking of the structure is created by making a notch, guiding the coherent hybridization between surface plasmon modes, and finally realizing the in-plane light directional propagation in the direction opposite to the notch direction. Compared with other current single-particle structures, due to its in-plane emission property, it is easier to be coupled into other integrated optical circuits.

[0011] The present invention realizes the directional scattering of light through a symmetry-breaking nanostructure, demonstrating excellent directivity control. By regulating the geometric parameters of the metal nanodisk and the notch, the present invention can enhance the scattering in a specific direction while weakening or eliminating the scattering in other directions. Especially in the reverse direction, a highly concentrated scattering emission is achieved through the phase interference effect, with a small divergence angle and strong directivity.

[0012] The present invention also has the advantages of simple structure, convenient operation, etc., and is easy to process and integrate, having promising prospects in many fields such as the scientific research community, the medical field, and the industrial field. Brief Description of the Drawings

[0013] Figure 1 is a schematic diagram of a symmetry-breaking micro-nano structure for realizing the directional propagation of light provided according to an embodiment of the present invention;

[0014] Figure 1 In the figure: 1. Metal-notch nanodisk; 2. Excitation source;

[0015] Figure 2 is a spectral diagram of the extinction, scattering, and absorption cross-sections of the metal-notch nanodisk as a passive cavity provided according to an embodiment of the present invention;

[0016] Figure 3(a) is a scattering energy flow diagram of the bonding mode of the metal-notch nanodisk as a passive cavity provided according to an embodiment of the present invention;

[0017] Figure 3(b) is a scattering energy flow diagram of the anti-bonding mode of the metal-notch nanodisk as a passive cavity provided according to an embodiment of the present invention;

[0018] Figure 4 is a diagram showing the influence of the center position of the metal notch of the metal-notch nanodisk on the scattering directivity of the anti-bonding mode provided according to an embodiment of the present invention. Detailed Embodiments

[0019] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and should not be used to limit the protection scope of the present invention.

[0020] A symmetry-breaking micro-nano structure for realizing the directional propagation of light provided by an embodiment of the present invention includes a metal-notch nanodisk and an excitation source; referring to Figure 1 , wherein the metal-notch nanodisk has the shape of a metal disk with a notch, and the notch is the overlapping part of a non-concentric disk with a radius smaller than it cut off from one side of the metal disk and the metal disk, and the metal disk and the notch form an asymmetric nano-cavity structure; the asymmetric structure can excite the antibonding mode by breaking the symmetry, thereby realizing the directional propagation of light; the excitation source is incident from the direction perpendicular to the disk surface of the metal disk, and the polarization direction is along the direction perpendicular to the symmetry plane of the metal disk. The symmetry plane of the metal disk is the plane passing through the center of the metal disk and perpendicular to the disk surface. The polarization direction of the excitation source is along the direction perpendicular to this symmetry plane to ensure the excitation of an asymmetric plasmon mode, thereby realizing the directional propagation of light.

[0021] Figure 1 In O 1 is the center of the metal disk, O 2 is the center of the notch, R is the radius of the metal disk, r is the radius of the notch, t is the thickness of the metal disk, k is the excitation source, E inc is the polarization direction of the excitation source;

[0022] The materials of the metal disk and the notch are gold materials that can generate surface plasmons. The radius of the metal disk is 90 nm, the radius of the notch circle is 40 nm, the center distance between the two is 103 nm, and the thickness of the metal disk is 20 nm. The geometric parameters of the metal disk and the notch can be optimized according to the working wavelength and application scenarios. By controlling the geometric parameters of the metal disk and the notch, the phase modulation of the surface plasmon mode is carried out, and finally the purpose of the directional propagation of light is achieved.

[0023] The media above and below the metal-notch nanodisk are air.

[0024] The preparation method of the metal-notch nanodisk is as follows:

[0025] On the SiO2 substrate, a nanodisk pattern with the above structure size is obtained by electron beam lithography; on the obtained nanodisk pattern, the required gold-notch nanodisk is obtained by electron beam evaporation and lift-off.

[0026] Figure 2 Shows the extinction, scattering, and absorption cross-section spectra of the metal-notch nanodisk structure as a passive cavity. For the passive cavity, the optical cross-section spectrum of the entire system can be obtained at this time. In this spectrum, the extinction cross-section is the sum of the scattering and absorption cross-sections. The horizontal axis represents the wavelength of light (unit: nm), and the vertical axis is the optical cross-section (unit: ×10 -14 m 2)。It can be seen from the figure that there are two peaks in the extinction spectrum, located at 832 nm and 648 nm respectively. The one at 832 nm represents the bonding mode, which is manifested as the free electrons of the nanoparticles vibrating in coordination in the same direction, forming a low-energy state, similar to the dipole mode. The overlapping of the electron clouds reduces the overall energy of the system; the one at 648 nm is the antibonding mode, where the electrons vibrate in opposite directions, forming a higher energy state, similar to the quadrupole mode, and the interference of the electron clouds leads to an increase in energy. According to the plasmon hybridization theory, the appearance of these two modes stems from the plasmon hybridization coupling phenomenon between the nanodisk and the notch. For the formation of the bonding mode and the antibonding mode, it can be described by the following formula:

[0027] When the plasmon modes are in-phase coupled, a bonding mode is formed, manifested as a lower energy state (832 nm), and the total radiation field can be expressed as:

[0028]

[0029] where, E total is the total radiation field, E dipole and E quadrupole respectively represent the radiation fields of the electric dipole moment and the electric quadrupole moment, A dipole and A quadrupole respectively represent the amplitudes of the electric dipole moment and the electric quadrupole moment, ω is the angular frequency of the light wave, r is the distance of the radiation field, and k dipole is the wave number of the electric dipole mode.

[0030] When the plasmon modes are out-of-phase coupled, an antibonding mode is formed, manifested as a higher energy state (648 nm), and the total radiation field can be expressed as:

[0031]

[0032] This indicates that the geometric structure of the nanodisk has a significant regulatory effect on the resonance wavelength of the plasmon. In the present invention, the antibonding mode is adopted to obtain directional light propagation.

[0033] Figure 3(a) - Figure 3(b)The scattering energy flux diagrams of the bonding and antibonding modes are given when the metal notched nanodisk acts as a passive cavity. Figure 3(a) shows the scattering energy flux diagram of the bonding mode. The energy distribution presents a dipole mode, with the energy evenly distributed in the upper and lower directions of the disk, and the directivity is poor. In contrast, Figure 3(b) shows the scattering energy flux diagram of the antibonding mode. The energy is mainly concentrated in the negative y-axis direction (i.e., the reverse direction of the notch), forming an obvious unidirectional scattering. This indicates that strong directional scattering can be achieved through the anti-phase coupling of nanostructures. Through calculation, the half-power beamwidth (HPBW) of the antibonding mode is about 70.6°, which is smaller than the divergence angles of other common nanoantenna structures (such as nanodisks, nanocups, etc.), indicating that this structure has stronger directivity control. For its near-field distribution, when the threshold is not reached, the influence of the plasmon tip effect is greater, so the directivity is poor; when the threshold is reached, the mode is amplified so that the influence of the tip effect can be ignored; when exceeding the threshold, due to the anti-phase resonance of the antibonding mode, the coherent hybridization between modes is affected.

[0034] Figure 4 Shows the influence of different notch positions on the scattering directivity of the antibonding mode. The scattering energy flux distribution diagrams are shown in the figure when the notch positions are 110nm, 103nm, 90nm, and 70nm respectively. It can be seen that when the center of the notch is 103nm away from the center of the nanodisk, the directivity of the scattering mode is the strongest, the energy is concentrated in a single direction in the reverse direction of the notch, and the divergence angle is the smallest.

[0035] In summary, the present invention provides a symmetry-breaking micro-nano structure for realizing the directional propagation of light. Through the design of a symmetry-breaking metal nanodisk, efficient light directional scattering is achieved. By adjusting the geometric parameters of the nanostructure, the coherent coupling of surface plasmon modes can be effectively controlled, and finally an excellent directional scattering effect is realized. The structure of the present invention is simple, easy to operate, and easy to process and integrate, and has promising prospects in many fields such as the scientific community, the medical field, and the industrial field.

[0036] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention within the knowledge scope of those of ordinary skill in the art.

Claims

1. A symmetry-breaking micro-nanostructure for achieving directional light propagation, characterized in that: The invention comprises a metal notch nano disk and an excitation source; wherein the metal notch nano disk is in the shape of a metal disk with a notch, wherein the notch is a portion of an overlap between a non-concentric disk with a radius smaller than the notch and the metal disk cut off from one side of the metal disk, and the metal disk and the notch are combined to form an asymmetric nano cavity structure; the excitation source is incident from a direction perpendicular to the disk surface of the metal disk, and the polarization direction is along a direction perpendicular to the symmetry plane of the metal disk, and the symmetry plane of the metal disk is a plane passing through the center of the metal disk and perpendicular to the disk surface.

2. The symmetry-breaking micro-nanostructure for achieving directional light propagation according to claim 1, characterized in that: The material of the metal disk and the gap is gold material which can generate surface plasmons.

3. The symmetry-breaking micro-nanostructure for achieving directional light propagation according to claim 1, characterized in that: The radius of the metal disk is 90nm, the radius of the notch circle is 40nm, the distance between the centers of the two circles is 103nm, and the thickness of the metal disk is 20nm.

4. The symmetry-breaking micro-nanostructure for achieving directional light propagation according to claim 1, characterized in that: The medium in the upper and lower parts of the metal notch nanodisk is air.

Images