CN104852116B - A kind of surface phasmon directional coupler and its control method independent of polarization - Google Patents

Abstract

The invention discloses a polarization-independent surface plasmon directional coupler and a control method thereof. The present invention proposes to process an asymmetric defective small hole structure on a sub-wavelength ridge waveguide to achieve directional coupling of the SPP mode with p-polarization and s-polarization incident light; the ridge waveguide only supports a single mode; due to the influence of defects, Both the p-polarized and s-polarized incident light can directionally couple out the SPP mode propagating along the ridge waveguide; by adjusting the geometric parameters of the defect, the p-polarized and s-polarized incident light can be coupled out of the SPP mode along the same or along the Propagate in the opposite direction; in the case of propagating in the same direction, the s-polarized incident light can be fully utilized to modulate the intensity of the total coupled out SPP mode; in the case of propagating in the opposite direction, the information of the polarization-encoded incident light is obtained is retained, thereby realizing that the coupling process of the SPP mode is independent of polarization.

Description

A polarization-independent surface plasmon directional coupler and its control method

technical field

The invention relates to the field of nanophotonics, in particular to a polarization-independent surface plasmon directional coupler and a control method thereof.

Background technique

Surface plasmon polaritons (SPPs) are transverse magnetic TM electromagnetic wave modes bound at the metal-dielectric interface. Due to their excellent subwavelength field confinement and strong field enhancement effect, SPPs are considered as potential next-generation information carriers. Coupling free-space light into SPPs is crucial for studying the performance of plasmonic devices. This has led to a lot of research on SPP unidirectional coupling devices. Unidirectional coupling devices can efficiently couple free-space light into SPPs propagating in specified or interested directions on metal planar or ridge waveguide structures. However, due to the intrinsic polarization-dependent property of SPPs (TM polarization), only the incident light with p-polarization (magnetic vector parallel to the magnetic field direction of SPPs) can be coupled into SPPs. So the energy and information carried by s-polarization (magnetic vector perpendicular to the magnetic field direction of SPPs) is completely lost. Recently, it was reported that the polarization dependence of SPPs coupling process can be overcome to some extent by using circularly polarized incident light. Since any polarized incident light can be decomposed into two mutually perpendicular linear polarization components, this can be used to explain many optical phenomena and effects (including optical Kerr effect, optical birefringence, electro-optic effect). Therefore, it may be more practical to couple out SPPs with p-polarized or s-polarized incident light. However, due to the p-polarization-dependent nature of SPPs, directional coupling out of SPPs with s-polarized incident light is a serious challenge.

Contents of the invention

In view of the problems existing in the above prior art, the present invention proposes a small hole structure with defects processed on the sub-wavelength ridge waveguide, so that both p-polarized and s-polarized incident light can be directional coupled out of SPPs, thereby solving the problem of SPP orientation Coupled polarization-dependent problems.

An object of the present invention is to provide a polarization-independent surface plasmon directional coupler.

The surface plasmon directional coupler of the present invention includes: a metal film, a small hole, a ridge waveguide, and a protruding nanoparticle; wherein, a small hole is arranged on the metal film; the positions on the metal film and at both ends of the small hole are symmetrical Two ridge waveguides can be arranged in a perfect manner, and the width of the ridge waveguide and the width of the small hole can be inconsistent. By adjusting the width w and height h of the ridge waveguide, the ridge waveguide can only support a single SPP mode; on the metal film and in the small hole Protruding nanoparticles are set on one side of the structure, and the protruding nanoparticles do not completely cover the edge of the small hole, forming a defective small hole structure; the incident light illuminates the defective small hole structure from the back, and the charge is in the sharp corner of the defective small hole structure. The region accumulates and forms hot spots; for p-polarized incident light, the charge accumulates in the sharp corner region of the ridge waveguide and forms a hot spot, and the radiation field of the hot spot is coupled into an SPP mode propagating along the ridge waveguide, thereby p-polarized The incident light is coupled into the SPP mode propagating along the ridge waveguide; for the s-polarized incident light, the charge accumulates in the sharp corner region of the protruding nanoparticle and forms a hot spot, and the radiation field of the hot spot is coupled into the SPP mode propagating along the ridge waveguide , so that the s-polarized incident light can also be coupled into the SPP mode propagating along the ridge waveguide, realizing a polarization-independent surface plasmon coupler.

Further, the center of the protruding nanoparticles is not at the center of the side of the small hole, thus forming an asymmetric defective small hole structure along the direction of the ridge waveguide, the ridge waveguide at one end of the small hole is far away from the protruding nanoparticles, and the small hole A ridge-shaped waveguide at the other end of the hole is adjacent to the protruding nanoparticle. For p-polarized incident light, charges accumulate at the sharp corners of the ridge waveguide and form hot spots. And the number of hot spots formed on the ridge waveguide away from the protruding nanoparticles is more than that formed on the ridge waveguide close to the protruding nanoparticles, so the SPP mode coupled by the p-polarized incident light is mainly along the Ridge waveguide propagation; for s-polarized incident light, charges are accumulated at the sharp corners of the protruding nanoparticles and hot spots are formed, and the geometric structure size of the protruding nanoparticles will affect the direction of the radiation field of the hot spots, so that the s-polarized incident light can be directional Coupled SPP modes propagating along the ridge waveguide. Thus, a polarization-independent surface plasmon directional coupler can be realized in a subwavelength waveguide by utilizing an asymmetric defect pinhole structure.

The geometrical size of the protruding nanoparticles affects the field distribution of the hot spot, and thus the geometrical size of the protruding nanoparticles affects the intensity of the SPP mode. Due to the asymmetry of the small hole structure, the intensity of the SPP mode in the opposite direction is not the same, so the intensity of the SPP mode can be adjusted by adjusting the geometric structure size of the protruding nanoparticles. By adjusting the geometric structure size of the protruding nanoparticles, the specific gravity of the intensity of the p-polarized and s-polarized coupled SPP modes can be tuned.

Further, the geometric parameters of the protruding nanoparticles are adjusted so that the SPP modes coupled with p-polarized and s-polarized incident light propagate along the same direction on the ridge waveguide. Therefore, for the incident linearly polarized light, the intensity of the coupled-out SPP mode can be easily adjusted by rotating the polarization angle of the incident light. The geometric parameters of the protruding nanoparticles include: cross-sectional geometric dimensions and the position from the central axis of the small hole.

The thickness of the metal thin film is greater than 300nm; the width and height of the ridge waveguide are both greater than 50nm; the length of the small hole is between 200nm and 2μm; the size of the protruding nanoparticles is greater than 50nm.

Another object of the present invention is to provide a method for controlling a polarization-independent surface plasmon directional coupler.

The control method of the surface plasmon directional coupler of the present invention comprises the following steps:

1) There is a small hole on the metal film, and two ridge waveguides are symmetrically arranged on the metal film and at the two ends of the small hole. The width of the ridge waveguide does not need to be consistent with the width of the small hole. By adjusting the ridge The width w and height h of the waveguide make the ridge waveguide only support a single SPP mode, and the protruding nanoparticles are arranged on the metal film and on the side of the small hole, and the protruding nanoparticles do not completely cover the edge of the small hole;

2) The incident light irradiates the defective small hole structure from the back, and the charge accumulates in the sharp corner area of the defective small hole structure and forms a hot spot;

3) For the p-polarized incident light, the charge accumulates in the sharp corner region of the ridge waveguide and forms a hot spot, and the radiation field of the hot spot couples into the SPP mode propagating along the ridge waveguide, so that the p-polarized incident light couples into the SPP mode along the ridge waveguide. SPP mode propagating in a shaped waveguide;

4) For the s-polarized incident light, the charge accumulates in the sharp corner region of the protruding nanoparticle and forms a hot spot, and the radiation field of the hot spot is coupled into the SPP mode propagating in the ridge waveguide, so the s-polarized incident light can also be coupled into a Waveguide-propagated SPP modes for polarization-independent surface plasmon coupling.

The control method of the surface plasmon directional coupler of the present invention further includes, by adjusting the geometric parameters of the protruding nanoparticles, adjusting the specific gravity of the intensity of the SPP modes coupled with p-polarization and s-polarization.

Further, the geometric parameters of the protruding nanoparticles are adjusted so that the p-polarized coupled SPP modes propagate along the same direction on the ridge waveguide. Therefore, for the incident linearly polarized light, the intensity of the coupled-out SPP mode can be adjusted by rotating the polarization angle of the incident light.

Advantages of the present invention:

The present invention proposes to process an asymmetric defective small hole structure on a sub-wavelength ridge waveguide to realize directional coupling of the SPP mode with p-polarized and s-polarized incident light; considering the single-mode working conditions in the photon circuit, the sub-wavelength The ridge waveguide has been optimized to support only a single mode; due to the influence of defects, both p-polarized and s-polarized incident light can be directionally coupled out of the SPP mode propagating along the ridge waveguide; by adjusting the geometric parameters of the defect, the p-polarized The SPP mode coupled to the s-polarized incident light can propagate in the same or opposite direction. In the case of propagating in the same direction, the s-polarized incident light can be fully utilized to modulate the intensity of the total outcoupled SPP mode. In the case of propagation in the opposite direction, the polarization-encoded information of the incident light is preserved. Therefore, in this ultra-small defective hole structure, both the p-polarized and s-polarized incident light can be directionally coupled into the SPP mode propagating along the ridge waveguide, so the coupling process of the SPP mode can be independent of the polarization .

Description of drawings

Fig. 1 is a schematic structural diagram of a polarization-independent surface plasmon directional coupler of the present invention, wherein (a) is a schematic diagram of a defective small hole structure, (b) is a cross-sectional view of a ridge waveguide, (c ) is the field distribution diagram of the SPP mode supported by the ridge waveguide, (d) is the scanning electron microscope SEM image of the sample with the defect-free pinhole structure, and (e) is the scanned surface of the sample with the flawed pinhole structure electron micrograph;

Fig. 2 is when the incident light is illuminated from the back, the SPP mode comparison diagram of the coupling of the defect-free small hole structure and the defective small hole structure of the present invention, wherein, (a) and (b) are respectively p-polarized and s-polarized The electric field distribution diagram of the incident light irradiating the defect-free small hole structure from the back, (c) and (d) are the electric field distribution diagrams of the p-polarized and s-polarized incident light irradiating the defective small hole structure from the back in the experiment, ( e) and (f) are theoretically calculated electric field distribution diagrams of p-polarized and s-polarized incident light irradiating the defective small hole structure from the back, respectively;

Fig. 3 is the interferogram of the coupled SPP mode of the p-polarization and s-polarization obtained by the polarization-independent surface plasmon directional coupler of the present invention, wherein (a) is obtained by combining the incident light of p-polarization and s-polarization There is a 45° angle between the linearly polarized light and the ridge waveguide, (b) is the -45° angle between the linearly polarized light and the ridge waveguide, and (c) is 45° The field distribution diagram obtained by the numerical calculation of , (d) is the field distribution diagram obtained by the numerical calculation of the -45° included angle, (e) the relationship between the intensity of the SPP mode coupled by linearly polarized light and the polarization direction, (f) the line The relationship between the intensity of the SPP mode coupled out by polarized light and the polarization direction;

Fig. 4 is the electric field diagram of the SPP mode of p polarization and s polarization two-way coupling of embodiment two, and wherein, (a) is the scanning electron microscope diagram of embodiment two, (b) and (c) distribution is incident light λ= The electric field CCD diagram obtained by coupling p-polarization and s-polarization at 780nm, (d) and (e) are the field distribution diagrams obtained by numerical calculation, respectively;

Figures 5(a) to (c) respectively show the intensities of the p-polarized and s-polarized SPP modes coupled by numerical calculations by changing the structural parameters w _d and L _d of the protruding nanoparticles in Example 3, Figure 5(d) to (f) respectively change the structural parameters w _d and L _d of the protruding nanoparticles when two defect structures are symmetrically placed on both sides of the small hole, and the intensities of the p-polarization and s-polarization coupled SPP modes obtained by numerical calculation;

Figure 6 is an excitation diagram of the SPP mode when the incident light is circularly polarized in Example 1, where (a) and (b) are the numerically calculated right-handed and left-handed circularly polarized incident light when the sample is irradiated from the back, respectively, The energy flow distribution at 100nm above the waveguide, (c) and (d) are the images of right-handed and left-handed elliptical polarization incident images obtained by CCD respectively, (e) and (f) are the corresponding numerical calculations at 100nm on the waveguide energy flow distribution;

Fig. 7 is the extinction spectrum of the SPP mode in embodiment two, wherein, (a) and (b) are the extinction spectrum of the SPP mode when the incident light that experiment obtains is p polarization and s polarization respectively, (c) and (d ) are the calculated extinction spectra of the SPP mode when the incident light is p-polarized and s-polarized, respectively;

Figure 8 is a schematic diagram of the coupling mechanism of p-polarization and s-polarization, wherein (a) is a schematic diagram of a hot spot, (b) is a field strength diagram of p-polarization, and (c) is a field strength diagram of s-polarization;

Figure 9 is the intensity diagram of the SPP mode when two defect structures are symmetrically placed on both sides of the small hole;

Figure 10 is the analysis diagram of polarization analysis, in which (a) and (b) are the images of CCD when the Glan-Taylor prism is p-polarized and the incident light is p-polarized and s-polarized respectively, (c) and (d) are grid The Blue-Taylor prism is s-polarized, and the incident light is the image of the CCD when the p-polarized and s-polarized, respectively.

detailed description

The present invention will be further described through the embodiments below in conjunction with the accompanying drawings.

Embodiment one

As shown in Figure 1, the metal film of the present embodiment, small hole, ridge waveguide and protrusion nanoparticle; Wherein, the thickness of metal film is t, is provided with small hole on metal film; On metal film and is positioned at small hole Two ridge waveguides are arranged symmetrically at both ends, and the width of the ridge waveguide and the width of the small hole can be inconsistent. By adjusting the width w and height h of the ridge waveguide, the ridge waveguide only supports a single SPP mode; in the metal film Protruding nanoparticles are arranged on and on one side of the small hole; the protruding nanoparticles do not completely cover the edge of the small hole, forming a defective small hole structure.

First, a ridge waveguide is set on the metal film, w=290nm, h=300nm, and the metal film is made of gold film. The field distribution of the SPP mode supported by the ridge waveguide is shown in Figure 1(c). At this time, the incident light wavelength λ=780nm, and the corresponding dielectric constant of gold is ε _Au =-22.5+1.4i ³⁰ . It can be seen that the electromagnetic field is well confined, and the electric field is mainly perpendicular to the metal surface, indicating TM polarization. In order to couple out the SPP mode when the incident light is illuminated from the back, a rectangular hole is set on the metal film, and protruding nanoparticles are placed next to the edge of the hole, as shown in Fig. 1(a). The length of the rectangular hole is L, and the width and length of the protruding nanoparticles are w _d and L _d , respectively. For the convenience of sample processing, the protruding nanoparticles and ridge waveguides overlap by a distance of ΔL. Due to the influence of the protruding nanoparticles, both the p-polarized and s-polarized incident light can be directionally coupled out of the SPP mode propagating along the ridge waveguide.

Experimentally, defective pinhole structures were fabricated on a 450 nm thick gold film (deposited on a glass substrate with a 30 nm thick titanium adhesion layer in between) using focused ion beam FIB. Figure 1(d) and (e) are the scanning electron microscope SEM images of the sample, respectively, where Figure 1(d) is a common small hole on the ridge waveguide structure, and Figure 1(e) is a defect on the ridge waveguide small hole structure. At the same time, a defect-free small hole structure without defect structure was also processed as a reference sample. Details of the defective pore structure are shown in the right half of Fig. 1(e). In the process of FIB processing, a ridge-shaped waveguide is processed on the gold film first, and the width of the waveguide is w. To process the protruding nanoparticles, a small rectangular area was left unprocessed during the process of machining the ridge waveguide. Then, a small rectangular hole is machined on the ridge waveguide next to the protruding nanoparticles. According to the measurement results, the structural parameters of the whole structure are roughly as follows: w= _290nm , L=600nm, _wd =300nm, Ld=360nm, and ΔL=60nm, and the cross-section of the ridge waveguide is a rectangle of 290×300nm ² . So the area of the whole structure is about 0.28 μm ² (<λ ² /2). In order to scatter the SPP mode into free space for far-field detection, 6 μm long gratings (period 800 nm, pitch 31.2 μm) were fabricated on both sides of the ridge waveguide.

In order to prove that the coupling of the SPP mode on the ridge waveguide structure depends on the polarization, the ordinary small hole as a reference sample is irradiated from the back with a laser with p polarization (magnetic vector perpendicular to the ridge waveguide) at λ=780nm. The noise introduced by the incident light can be eliminated after being illuminated from the back. The SPP mode coupled out from the back incident light propagates along the ridge waveguide and is scattered by the grating. Scattered light was collected with a long working distance objective lens (Mitutoyo, 100×, NA=0.5) and imaged on a CCD. Since the ordinary pinhole structure is symmetrical, the intensity of light scattered from the left and right gratings is basically the same, as shown in Figure 2(a). Rotate the polarization of the incident light by 90° (s polarization, the magnetic vector is parallel to the ridge waveguide), at this time no scattered light can be seen at the grating, indicating that the incident light of s polarization cannot be coupled out of the SPP mode, as shown in Figure 2(b ) shown. This phenomenon is also consistent with the previously reported dependence of SPP mode coupling on polarization. However, the situation is different for defective small-pore structures. Because the symmetry of the structure is broken, when the incident light λ=780nm, p-polarized incident light is irradiated from the back, the grating on the left is basically dark, while the grating on the right becomes bright, as shown in Figure 2( c) as shown. This indicates that the coupled-out SPP modes mainly propagate to the right (direction away from the protruding nanoparticles) along the ridge waveguide. Moreover, the intensity of light scattered from the right grating in the defective pinhole structure is about 1.8 times that of the normal sample, which indicates that the intensity of the SPP mode increases in the defective pinhole structure. Rotate the polarization of the incident light by 90° (s-polarization), the phenomenon is similar to the p-polarized incident shown in Figure 2(c), as shown in Figure 2(d). This shows that in this structure, the s-polarized incident light can also be coupled out of the SPP mode. In addition, it can be seen that the area of the scattering spot is much smaller than the area of the scattering grating, as shown in the dashed boxes in Fig. 2(a) to (d). This shows that the SPP modes coupled out by the two mutually perpendicular incident light are well bound on the ridge waveguide instead of propagating along the metal surface. Numerical calculation shows that when the p-polarized and s-polarized incident light illuminates the defective small hole structure from the back, the distribution of the electric field is shown in Figure 2(e) and (f). It can be seen from these two figures that both the p-polarized and s-polarized incident light can couple out the SPP mode propagating in a specific direction, which is in good agreement with the experimental results shown in Figure 2(c) and (d) it is good. Therefore, in the defective small hole structure, the SPP mode can be coupled with both p-polarized and s-polarized incident light, and the intensity of the coupled SPP mode can be compared with that obtained by ordinary small holes.

Next, the interference between the p-polarized and s-polarized SPP modes coupled out in the defective ridge waveguide structure is studied. Consider two special cases. As shown by the arrow in Fig. 3(a), when the p-polarized and s-polarized incident light with the same intensity are synthesized to obtain a linearly polarized light with a 45° angle between the magnetic vector and the ridge waveguide, the SPP coupled by the p-polarized incident light mode and the SPP mode coupled out by the s-polarized incident light are coherent and destructive. So there is basically no scattered light at the grating, as shown in the CCD image in Figure 3(a). As shown by the arrow in Figure 3(b), when the magnetic vector of the synthesized linearly polarized light has an angle of -45° with the ridge waveguide, the SPP mode coupled out of the p-polarized incident light and the SPP mode coupled out of the s-polarized incident light The modes are coherent and constructive, so the strength of the SPP mode will increase. This will significantly increase the intensity of scattered light at the grating, as shown in Figure 3(b), at this time the intensity of scattered light is 5.2 times that of ordinary small holes. The corresponding numerically calculated field distributions are shown in Fig. 3(c) and (d), which agree well with the experimental results. Therefore, the outcoupled SPP intensity can be effectively modulated by using the s-polarized component of the incident light. And the relationship between the intensity and polarization direction of the SPP mode coupled out by linearly polarized light of the same intensity was measured and calculated, and the results are shown in Figure 3(e) and (f). It can be seen that the results of experiment [Fig. 3(e)] and numerical calculation [Fig. 3(f)] are in good agreement. From Figure 3(e) and (f), it can be seen that the intensity of the coupled-out SPP mode can be easily tuned by rotating the polarization angle of the incident light. In addition, when circularly polarized light is incident, the one-way coupling of the SPP mode is also confirmed experimentally, and it is in good agreement with the numerical calculation results.

Figure 6 is an excitation diagram of the SPP mode when the incident light is circularly polarized in Example 1, where (a) and (b) are the numerically calculated right-handed and left-handed circularly polarized incident light when the sample is irradiated from the back, respectively, The energy flow distribution at 100nm above the waveguide, (c) and (d) are the images of right-handed and left-handed elliptical polarization incident images obtained by CCD respectively, (e) and (f) are the corresponding numerical calculations at 100nm on the waveguide The energy flow distribution of , the circular arrow represents the electric vector, and the dashed box represents the position of the scattering grating.

Embodiment two

By tuning the structural parameters of the protruding nanoparticles, the p-polarized and s-polarized coupled SPP modes can be separated and propagate in opposite directions. In this example, the width of the protruding nanoparticles was reduced to 60 nm. The corresponding SEM images and detailed images are shown in Fig. 4(a). The other structural parameters of the defective pore structure are as follows: w = 260 nm, L = 560 nm, L _d = 240 nm, h = 350 nm, and ΔL = 60 nm. The electric field CCD images obtained by incident light λ=780nm are shown in Figure 4(b) and (c). It can be seen from the CCD image that the SPP mode coupled by the p-polarized incident light mainly propagates to the right along the ridge waveguide, and the corresponding extinction ratio is about (I _R /I _L ) ^p ≈5.1, so the grating on the right is illuminated , as shown in Figure 4(b). However, the SPP mode coupled by the s-polarized incident light mainly propagates to the left along the ridge waveguide, and the corresponding extinction ratio is about (I _L /I _R ) ^s ≈ 3.3, so the left grating is illuminated, as shown in Figure 4 (c) shown. Here, the extinction ratio (I _R /I _L or I _L /I _R ) is defined as the ratio of the intensities of light scattered from two scattering gratings. The subscripts R and L indicate right or left. The intensity of the right-propagating SPP mode coupled out of the p-polarized incident light is about twice that of the left-propagating SPP mode coupled out of the s-polarized incident light. The corresponding numerically calculated field distributions are shown in Fig. 4(d) and (e), which agree well with the experimental results. Therefore, in the defective pinhole structure, the SPP modes coupled out by the two incident light polarizations perpendicular to each other can be effectively separated, and the information carried by the incident light is preserved. Since the SPP modes coupled out by two mutually perpendicular linearly polarized lights propagate in opposite directions, any incident light with any polarization state can couple out the SPP modes propagating along the ridge waveguide. More importantly, the information carried by the two mutually perpendicular linearly polarized lights can be independently modulated within the chip.

Fig. 7 is the extinction spectrum of the SPP mode in embodiment two, wherein, (a) and (b) are the extinction spectrum of the SPP mode when the incident light that experiment obtains is p polarization and s polarization respectively, (c) and (d ) are the calculated extinction spectra of the SPP mode when the incident light is p-polarized and s-polarized, respectively.

Embodiment Three

In this example, COMSOL Multiphysics is used to calculate the influence of the defective pinhole structure on the SPP mode coupling on the ridge waveguide. The calculation formula, the structural parameters of the ridge waveguide and the small hole are the results obtained from the experimental measurement of the sample shown in Figure 1, and the parameters are w=290nm, L=600nm, h=300nm, ΔL=60nm. The intensities of the SPP modes obtained by varying the structural parameters w _d and L _d of the protruding nanoparticles are shown in Fig. 5(a) to (c). It can be seen that the intensity of the SPP mode coupled by the incident light of p-polarization and s-polarization has different dependence on the structural parameters.

There is a sharp corner in the defective pinhole structure, as shown in Fig. 1(a), when the incident light is illuminated, charges tend to accumulate in these regions and form hot spots. For p-polarized incident light, there are three hot spots at the sharp corners of the ridge waveguide, as shown in Fig. 8(a). Also, the edge of the ridge waveguide on the right is bright. The strong radiating fields of these hot spots at the edges of the sharp and ridge waveguides can be coupled into SPP modes propagating along the ridge waveguide and then interfere. Due to the influence of the protruding nanoparticles, the hotspot area of the ridge waveguide at the right end is much larger than that of the ridge waveguide at the left end, as shown in Fig. 8(b). Therefore, the intensity of the SPP mode propagating to the right is always greater than that of the SPP mode propagating to the left, as shown by the dashed and solid lines in Fig. 5(a) to (c), indicating that the coupled SPP mode is mainly along the ridge waveguide Spread right. For s-polarized incident light, a strong hot spot appears at the sharp corner of the defect, as shown in Fig. 8(c). The intensity of the hot spot at this time is about 10 times that of the p-polarized incidence. Furthermore, the radiating field of the hot spot can be coupled to the ridge waveguide. This is why s-polarized incident light can couple SPP modes. Because the central field of the small hole is the strongest, the hot spot becomes stronger when the sharp corner of the defect structure is closer to the center of the small hole. In addition, the numerical calculation results show that the width of the protruding nanoparticles can affect the field distribution of the hot spot, so both the width and length of the protruding nanoparticles can affect the intensity of the SPP mode, as shown in the dotted line and dotted line in Figure 5(a) to (c) line shown. Due to the asymmetry of the structure, the intensity of the SPP mode in the opposite direction is not the same, and its intensity can be tuned by adjusting the parameters of the protruding nanoparticles. For example, when the parameters of the protruding nanoparticles are w _d =100nm and L _d =260nm, the SPP mode coupled with the incident light of s-polarization mainly propagates to the left along the ridge waveguide, as shown by the vertical dotted line in Fig. 5(a) . When the parameters of the defect structure are w _d =300nm and L _d =360nm, the SPP mode coupled with the s-polarized incident light mainly propagates to the right along the ridge waveguide, as shown by the vertical dotted line in Fig. 5(c). These numerical results are in good agreement with previous experimental results. Further calculations found that the proportion of the SPP intensity coupled out by p-polarization and s-polarization can be adjusted by adjusting the structural parameters of the defect structure. Among them, I _L ^P and I _R ^P represent the left and right SPP modes of the p-polarized incident light, respectively, and I _L ^S and I _R ^S represent the left and right SPP modes of the s-polarized incident light, respectively.

To support these analyzes, the strength of the SPP mode when two defect structures are placed symmetrically on both sides of the small hole is further investigated. For s-polarized incident light, two defect structures can introduce two hot spots, as shown in Figure 9. But these two hotspots are anti-phase to each other, so the SPP mode coupled by this hotspot is coherent and destructive. Therefore, the s-polarized incident light cannot be coupled out of the SPP mode at this time, as shown by the dotted and dashed lines in Figure 5(d) to (f), which is consistent with the above analysis.

Figure 10 is the analysis diagram of polarization analysis, in which (a) and (b) are the images of CCD when the Glan-Taylor prism is p-polarized and the incident light is p-polarized and s-polarized respectively, (c) and (d) are grid The Blue-Taylor prism is s-polarized, and the incident light is the image of the CCD when the p-polarized and s-polarized, respectively.

In summary, by fabricating a defective pinhole structure on a subwavelength ridge waveguide, both p-polarized and s-polarized incident light can be directionally coupled out of a single SPP mode supported by the ridge waveguide. This completely overcomes the polarization dependence of the free-space optical coupling SPP mode process. In the sharp corner region of the pinhole, both p-polarization and s-polarization can generate hot spots. Due to the broken symmetry of the defect structure, the radiation field of the hot spot can be directionally coupled into SPP modes. By adjusting the parameters of the defect, the SPP modes coupled out by the incident light of p-polarization and s-polarization can propagate along the waveguide in the same way or oppositely. As shown, the outcoupling of SPP modes propagating along the ridge waveguide is polarization-independent, which has important applications in future plasmonic circuits and systems.

Finally, it should be noted that the purpose of publishing the implementation is to help further understand the present invention, but those skilled in the art can understand that various replacements and modifications can be made without departing from the spirit and scope of the present invention and the appended claims. It is possible. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the protection scope of the present invention is subject to the scope defined in the claims.

Claims (10)

1. a kind of surface phasmon directional coupler, it is characterised in that the directional coupler includes：Metallic film, aperture, Ridge waveguide and projection nano particle；Wherein, aperture is provided with metallic film；On metallic film and positioned at aperture two ends Positional symmetry two ridge waveguides are set, by the width w and height h that adjust ridge waveguide so that ridge waveguide is only propped up Hold single SPP mode；Projection nano particle is set on metallic film and positioned at the side of aperture, projection nano particle is incomplete The edge of aperture is covered, defective small structure is formed；Incident light is having from the defective small structure of back side illuminaton, electric charge The corner region of the small structure of defect is accumulated and forms focus；For the incident light of p-polarization, wedge angle of the electric charge in ridge waveguide Zone-accumulation simultaneously forms focus, the radiation field of focus is coupled into the SPP mode propagated along ridge waveguide, so that p-polarization enters Penetrate the SPP mode that optocoupler synthesis along ridge shape waveguide is propagated；The incident light polarized for s, wedge angle of the electric charge in projection nano particle Zone-accumulation simultaneously forms focus, and the radiation field of focus is coupled into the SPP mode propagated along ridge waveguide, thus s polarization enter Penetrate optocoupler synthesis along ridge shape waveguide and propagate SPP mode, realize the surface phasmon directional coupler independent of polarization.

2. directional coupler as claimed in claim 1, it is characterised in that further, the center of projection nano particle is not small On the center of hole side, so as to form the asymmetric defective small structure along ridge waveguide direction, aperture The ridge waveguide of one end is away from projection nano particle, and the ridge waveguide of the aperture other end is realized not close to projection nano particle Rely on the surface phasmon directional coupler of polarization.

3. directional coupler as claimed in claim 2, it is characterised in that for the incident light of p-polarization, in the ridge waveguide Corner region stored charge and form focus, and the focus formed on the ridge waveguide away from projection nano particle is more than The focus number formed on the ridge waveguide close to projection nano particle, so that the SPP mode master of the incident optical coupling of p-polarization To be propagated along the ridge waveguide away from projection nano particle.

4. directional coupler as claimed in claim 2, it is characterised in that the incident light polarized for s, in projection nano particle Corner region stored charge and form focus, the geometric structure diamete of projection nano particle influences the radiation field direction of focus, So as to which the s incident lights polarized are directionally coupled into the SPP mode propagated along ridge waveguide.

5. directional coupler as claimed in claim 1, it is characterised in that the geometric structure diamete shadow of the projection nano particle The intensity of SPP mode is rung, by the SPP for adjusting the geometric structure diamete of projection nano particle, regulation p-polarization and s polarization coupleds The proportion of the intensity of pattern.

6. directional coupler as claimed in claim 1, it is characterised in that the geometry chi of the adjustment projection nano particle It is very little so that p-polarization and the SPP mode of s polarized incident optical couplings are propagated along equidirectional on ridge waveguide, for incidence Linearly polarized light, by rotating the polarization angle of incident light, adjusts the intensity for the SPP mode being coupled out.

7. a kind of control method of surface phasmon directional coupler, it is characterised in that the control method includes following step Suddenly：

1) aperture is provided with metallic film, on metallic film and positioned at aperture two ends positional symmetry two ridges are set Shape waveguide, by the width w and height h that adjust ridge waveguide so that ridge waveguide only supports single SPP mode, in metal foil Projection nano particle, the edge of the endless all standing aperture of projection nano particle are set on film and positioned at the position of aperture side；

2) incident light is from the defective small structure of back side illuminaton, and electric charge is accumulated simultaneously in the corner region of defective small structure Form focus；

3) for the incident light of p-polarization, electric charge is accumulated in the corner region of ridge waveguide and forms focus, the radiation field coupling of focus The SPP mode propagated along ridge waveguide is synthesized, so that the incident light of p-polarization is coupled into the SPP moulds propagated along ridge waveguide Formula；

4) incident light polarized for s, electric charge is accumulated in the corner region of projection nano particle and forms focus, the radiation of focus Field is coupled into the SPP mode of ridge waveguide propagation, so that the incident light of s polarizations is coupled into the SPP mode propagated along ridge waveguide, Realize the surface phasmon directional couple independent of polarization.

8. control method as claimed in claim 7, it is characterised in that the center of the projection nano particle is not in aperture side Center on, so as to form the asymmetric defective small structure along ridge waveguide direction, aperture one end Ridge waveguide is away from projection nano particle, and the ridge waveguide of the aperture other end is close to projection nano particle.

9. control method as claimed in claim 8, it is characterised in that by the geometry chi for adjusting projection nano particle It is very little, the proportion of the intensity of the SPP mode of regulation p-polarization and s polarization coupleds.

10. control method as claimed in claim 8, it is characterised in that further, adjusts the geometry of projection nano particle Size so that p-polarization and the SPP mode of s polarization coupleds are propagated along identical direction on ridge waveguide, for incident line Polarised light, by rotating the polarization angle of incident light, adjusts the intensity for the SPP mode being coupled out.