CN203616532U - Sub-wavelength plasmon polarization converter - Google Patents

Abstract

The utility model provides a subwavelength plasmon polarization converter, including incident polaroid and outgoing polaroid, by medium or air bed interval between incident polaroid and the outgoing polaroid. The incident polaroid and the emergent polaroid are made of plasmon materials; small holes with sub-wavelength sizes are engraved on the two polarizing plates, an included angle is formed between each small hole on the incident polarizing plate and the corresponding small hole on the emergent polarizing plate, and the end points of the two small holes forming the included angle are crossed. When the included angle between the two small holes is 90 degrees, the incident polaroid only allows the electromagnetic wave with one polarization to be coupled in, and the emergent polaroid only allows the electromagnetic wave orthogonal to the polarization direction of the incident polaroid to be emitted. By utilizing the near-field coupling effect between the orthogonal rectangular small holes, the system can break through the limitation of the Malus law, so that the electromagnetic waves can generate effective transmission, and the polarization direction can be rotated by 90 degrees. The converter can be used as a unidirectional transmitter, a sub-wavelength switch and a modulator.

Description

A Subwavelength Plasmon Polarization Converter

technical field

The utility model relates to a sub-wavelength plasmon polarization converter, which is composed of a pair of metal polarizers engraved with sub-wavelength small holes, which can rotate the linear polarization direction of electromagnetic waves by 90 degrees. The converter can also be used as a unidirectional transmitter, subwavelength switch or modulator.

Background technique

Traditional polarizers and polarization converters are mainly based on the dichroism, birefringence effect or optical activity of natural materials, which can generate linearly polarized light or change the polarization state of light waves. It is well known that two linear polarizers can be used to generate linearly polarized light and to rotate the polarization direction of the light. However, according to Marius' law, the light transmission efficiency will decrease with the increase of the rotation angle θ of the polarizer. When θ=90°, the light transmission efficiency becomes 0. This means that light cannot pass through two polarizers with orthogonal polarization directions, nor can a 90-degree rotation of the polarization direction be obtained with orthogonal polarizers. In contrast, by using the birefringence and phase delay effects of the wave plate, the polarization direction or polarization state of the incident light wave can be effectively controlled. In particular, when the incident polarization forms an angle of 45 degrees with the optical axis of the half-wave plate, the vibration direction of linearly polarized light can be rotated by 90 degrees. In addition, due to the natural optical rotation or Faraday effect of certain substances, the polarization plane of light waves gradually rotates with the propagation distance. Because the rotation angle is proportional to the distance through which the light passes, the polarization direction of the light can be freely adjusted. However, due to weak birefringence or small optical rotation coefficient, the implementation of the above effects requires a strong external magnetic field or a large light transmission distance (the light transmission distance is much longer than the electromagnetic wavelength). This is a very unfavorable factor for the development and integration of micro-nano photonics components.

Recently, microstructured surface plasmonic materials (i.e., metallic materials) have provided new avenues for the development of subwavelength electromagnetic wave polarizers and converters. Due to the unique dielectric response of metals, the electromagnetic field can couple with free electron oscillations on the metal surface to form surface plasmon or localized plasmon resonance. Through the microstructure design and research of surface plasmon materials, people have discovered many interesting physical effects, such as enhanced transmission effect, beam collimation effect, negative refraction effect, etc. In terms of polarization characteristics, the transmission of one-dimensional metal slit gratings and two-dimensional elliptical or rectangular aperture arrays has strong polarization dependence, which can be used to develop subwavelength polarizers. Based on the enhanced transmission effect, single-layer or double-layer metal films engraved with subwavelength small holes (such as L or S-shaped small hole arrays) can also be used for polarization conversion, such as realizing 90-degree polarization rotation. However, these polarization converters usually have low conversion efficiency or narrow operating bandwidth (or apply to specific operating wavelengths). On the other hand, based on particles such as subwavelength metal split rings and metal rods, people have constructed composite metamaterials. These materials can possess properties such as anisotropy (similar to a wave plate) or optical activity (chiral materials), which offer the possibility of switching between polarization states. However, subwavelength metamaterials usually have the disadvantages of complex preparation and high loss, which limits their application in short wavelengths, especially in the visible and near-infrared bands.

Contents of the invention

In order to solve the shortcomings of low efficiency, narrow bandwidth or complicated preparation of the current 90-degree polarization converter, the utility model provides a sub-wavelength polarization converter. The bandwidth is large.

The technical scheme that the utility model solves its technical problem adopts is:

A sub-wavelength plasmon polarization converter includes an incident polarizer and an outgoing polarizer, and the incident polarizer and the outgoing polarizer are separated by a medium or an air layer. Wherein, both the incident polarizer and the outgoing polarizer use plasmonic materials; the incident polarizer and the outgoing polarizer are engraved with small holes of sub-wavelength size, and each small hole on the incident polarizer is connected to the corresponding small hole on the outgoing polarizer. An included angle is formed between the holes, and the ends of the two small holes forming the included angle intersect.

The included angle formed by the two small holes is 0-90 degrees.

The shape of the polarizer is rectangular or circular; the small holes on the incident polarizer and the outgoing polarizer are rectangular, elliptical or trapezoidal, etc., and the shapes of the polarizer and the small holes are not limited.

The pitch between the entrance and exit polarizers is sub-wavelength. The small holes on the incident polarizer and the outgoing polarizer are arranged periodically or aperiodically.

The utility model utilizes the near-field coupling effect between two surface plasmon polarizers with orthogonal polarization directions to realize the 90-degree rotation of linear polarization. The incident polarizer only allows electromagnetic waves of one polarization to be coupled in, while the outgoing polarizer only allows electromagnetic waves orthogonal to the former polarization direction to exit. Here, the incident and outgoing metal polarizers are engraved with rectangular sub-wavelength small holes; viewed from the direction of light transmission, the corresponding small holes of the two polarizers cross each other in a vertical L shape. This configuration enhances the coupling effect between the two orthogonal polarization states, thereby increasing the transmission/conversion efficiency and expanding the operating bandwidth.

The scientific value of the utility model lies in revealing an abnormal transmission effect: electromagnetic waves can pass through two surface plasmon polarizers with orthogonal polarization directions. This effect breaks through the limitation of the traditional Marius' law. The application value of the utility model lies in that the converter can achieve 90-degree polarization conversion, and has high conversion efficiency and wide working bandwidth; besides the orthogonal direction prohibited by Marius' law, the polarization of incident rays can also be switched to any polarization Direction, to achieve "universal" deflection conversion. In the infrared band, the transmission or conversion efficiency can reach 52%, and the working bandwidth can reach 12%. In the microwave segment, the transmission efficiency can reach 100%, and the working bandwidth can reach 15%. The thickness of the polarization converter is sub-wavelength, its structure is simple, it is easy to prepare and integrate, and it can be applied to optical frequency, terahertz or microwave bands. Additionally, the converter can be used as a unidirectional transmitter, subwavelength switch, and modulator.

Description of drawings

Figure 1 (a) is a schematic structural view of the utility model, (b) is a side view of a single cell formed by two small holes, and (c) is a front view of a single cell.

Figure 2 is a partial scanning electron microscope (SEM) image of orthogonal outgoing (panel a) and incident (panel b) polarizers fabricated on Au/SiN/Au using a focused ion beam (FIB) system.

Figure 3 shows the transmission curves of experimental measurements (panel a) and theoretical simulations (panel b).

Figure 4 is the current distribution of the input polarizer (figure a) and the output polarizer (figure b) calculated by simulation.

Figure 5(a) is the transmission efficiency Txy curve of different hole array periods, and (b) is the transmission efficiency Txy curve of different SiN thicknesses.

Figure 6(a) and (b) are the physical pictures of a single polarizer processed in the microwave section and the polarization converter in an orthogonal configuration, (c) is the transmission curve of a single polarizer measured by theoretical calculation and experiment, (d ) are the theoretically calculated and experimentally measured transmission curves of the polarization converter.

Figure 7(a) is the theoretical and experimental transmission efficiency Txy curves of the polarization converter at different pitches, and (b) is the theoretical transmission efficiency Txy curve of the polarization converter at different rectangular hole side lengths.

Figure 8(a) is the physical picture of the output polarizer with a tilt angle of 30 degrees, and (b) is the transmission curve of different L-shaped "crossing" angles in experiment (top) and theory (bottom).

Figure 9(a) is a schematic diagram of the structure of the incident or exit polarizer of the "universal" polarization converter, (b) is a schematic design of a local small hole, and (c) is the incident or exit polarization of the "universal" polarization converter (d) is the transmission curve of the experimentally measured "universal" polarization converter at different rotation angles.

Detailed ways

Figure 1 shows the schematic diagram of the structure of the polarization converter and the side and front views of a single cell. The converter consists of two metallic films/sheets M engraved with subwavelength rectangular apertures, separated by a dielectric (or air) layer I. The period of the rectangular hole array on the metal film is d, and the other structural parameters are shown in Fig. 1 . Viewed from the direction of light transmission, the two layers of rectangular holes form a vertical L-shaped "intersection", that is, the intersection angle is 90 degrees. Here, the long side of the small hole on the incident surface is set to be along the horizontal y direction, and the x-polarized electromagnetic wave is vertically incident on the metal surface. This system ensures that only the x-polarized component of the electromagnetic wave can be coupled in, and only the y-polarized component can exit. Thus, if transmission occurs, a 90-degree rotation of the linear polarization must result. The utility model will be further described below in conjunction with the accompanying drawings by taking the infrared (Fig. 2-5) and the microwave segment (Fig. 6-9) as three embodiments.

Example 1

As a first embodiment, the working frequency of the polarization converter is in the optical frequency band. The converter is composed of a gold film/silicon nitride/gold film sandwich structure, and pre-designed microstructures are respectively prepared on the gold film. During the experiment, a 100-nm-thick gold film was coated on both sides of a suspended 50-nm-thick silicon nitride film by magnetron sputtering, and then a focused ion beam (FIB) system was used to coat the gold films on both sides. Subwavelength pinhole arrays were fabricated separately. The period of the prepared small hole array is 600 nanometers, the length of the rectangular small holes is 400 nanometers, and the width is 150 nanometers, and the size of the entire array is 50 micrometers*50 micrometers. In order to align the rectangular holes on both sides into an L-shaped "cross", four through-marks were made with FIB at the four corners of the sample during processing to assist in positioning and processing. Figure 2(a) and Figure 2(b) are partial SEM images of the outgoing and incoming polarizers, respectively. It can be seen from Figure 2(b) that the rectangular small holes processed on both sides correspond to each other, basically forming an L-shaped vertical "cross".

其中ω_p=1.37×10¹⁶rad/s，γ=8.5×10¹³Hz。计算结果显示，理论和实验能够很好地吻合。这些结果一致表明，在特定波段，两个正交的表面等离激元偏振片不仅能产生有效的透射，而且还能使光的偏振方向旋转90度。Figure 3(a) presents the experimental measurement results of this polarization converter. The solid and hollow marks in the figure represent the transmission efficiencies of the outgoing waves with y and x polarizations, respectively. It can be seen from the test results that there is no effective x-polarized component in the outgoing light within the entire test wavelength band; this also reflects the good polarization characteristics of the rectangular hole on the outgoing surface. However, the outgoing y-polarized component, that is, the component perpendicular to the incident polarization, obtains the maximum value of the transmission efficiency around 800 nm, 1000 nm and 1340 nm, respectively. Especially around 1000nm, the transmission efficiency reaches 40%, and the FWHM or operating bandwidth is about 80nm (relative bandwidth is 8%). In order to verify the above experimental results, Fig. 3(b) shows the transmission spectrum of the structure simulated and calculated based on the finite difference time domain (FDTD) method. In the calculation process, the refractive index of silicon nitride is set to n _I =2.0, and the dielectric constant of gold adopts the Drude model:

Where ω _p =1.37×10 ¹⁶ rad/s, γ=8.5×10 ¹³ Hz. The calculation results show that the theory and experiment can be in good agreement. These results consistently show that two orthogonal surface plasmon polarizers can not only produce efficient transmission but also rotate the polarization direction of light by 90 degrees at specific wavelength bands.

（δ=22nm为金属的趋肤深度，m和n为两个整数）。因而ISPP的共振波长近似为

据此，800纳米、1000纳米和1340纳米处的透射峰可分别归功于对应(2,1)、(2,0)、(1,1)倒格矢的ISPP激发[(1,0)倒格矢激发的透射峰位于测量波长范围之外]。由于1000纳米处的透射峰非常靠近矩形小孔的截止波长（矩形孔的截止波长为

因而其透射效率也远高于其它的透射峰。但是，什么原因导致电磁偏振的转化呢？实际上，在双层穿孔金属膜内，ISPP的激发必将和矩形小孔的波导模产生耦合（波导模又和小孔周围的电子振荡相关联）。不仅如此，两层金属膜上的矩形小孔也会发生相互作用。这些互作用使得系统的共振波长偏离理想的ISPP共振，也使得系统的电磁偏振发生改变。为了简要说明这个问题，本实用新型用FDTD方法模拟了波长为1000纳米处的金属偏振片内部的电流分布图。图4（a）显示的是入射偏振片xy面内的电流分布（金属的一半厚度处）。在x偏振的入射电磁场和ISPP的共同作用下，入射端小孔内的电磁场或波导模获得增强。波导模的激发在小孔周围伴随着环绕电流并同时聚集正负电荷。这些环绕电流和聚集的正负电荷将和出射偏振片的矩形小孔产生耦合作用，即在出射小孔附近感应出新的正负电荷和环绕电流（图4（b））。后者在出射小孔内激发电磁场并产生y偏振的电磁辐射。Theoretical analysis shows that the generation of these transmission peaks is related to the excitation of internal surface plasmon polaritons (ISPPs) between the two metal films. The excitation condition of ISPP can be approximately expressed as k _ISPP =G _mn , where

(δ=22nm is the skin depth of the metal, m and n are two integers). Therefore, the resonance wavelength of ISPP is approximately

Accordingly, the transmission peaks at 800 nm, 1000 nm and 1340 nm can be attributed to the ISPP excitations corresponding to (2,1), (2,0), (1,1) reciprocal vectors [(1,0) inverse The transmission peak excited by the lattice vector lies outside the measured wavelength range]. Since the transmission peak at 1000 nm is very close to the cut-off wavelength of the rectangular hole (the cut-off wavelength of the rectangular hole is

Therefore, its transmission efficiency is much higher than other transmission peaks. But what causes the transformation of electromagnetic polarization? In fact, in the double-layer perforated metal film, the excitation of ISPP must be coupled with the waveguide mode of the rectangular hole (the waveguide mode is associated with the electronic oscillation around the hole). Not only that, but the small rectangular holes in the two metal films also interact. These interactions make the resonance wavelength of the system deviate from the ideal ISPP resonance, and also change the electromagnetic polarization of the system. In order to illustrate this problem briefly, the utility model uses the FDTD method to simulate the current distribution diagram inside the metal polarizer at a wavelength of 1000 nanometers. Figure 4(a) shows the current distribution in the xy plane of the incident polarizer (at half the thickness of the metal). Under the joint action of x-polarized incident electromagnetic field and ISPP, the electromagnetic field or waveguide mode in the small hole at the incident end is enhanced. The excitation of the waveguide mode is accompanied by a surrounding current around the hole and simultaneously accumulates positive and negative charges. These surrounding currents and accumulated positive and negative charges will have a coupling effect with the rectangular aperture of the exit polarizer, that is, new positive and negative charges and surrounding currents will be induced near the exit aperture (Figure 4(b)). The latter excites an electromagnetic field in the exit aperture and produces y-polarized electromagnetic radiation.

The above analysis indicates that the enhanced transmission and polarization conversion effects originate from the near-field coupling effect enhanced by the ISPP resonance. This effect will depend on the lattice period d and the spacing h between the two polarizers (ie the thickness of the silicon nitride layer). Fig. 5(a) shows the simulated transmission spectra T _xy (d=550, 600, 650 nm; h=50 nm) under different periods. With the increase of the period, the transmission peak has a significant red shift; the magnitude of the red shift is basically the same as the result predicted by the ISPP resonance. Figure 5(b) shows the theoretically calculated transmission efficiency curves (T _xy ) of the main peak at different distances h. When h=30nm, the transmission peak is located at 1080nm, and the transmission efficiency is 12.5%. As the spacing increases, the ISPP resonance wavelength will gradually decrease, and the transmission peak will shift blue (however, when h>>δ, this change trend will slow down and tend to disappear). At the same time, the transmission efficiency increases due to the blue-shifted transmission peak gradually approaching the cut-off wavelength. Near the cut-off wavelength, the ISPP mode and the waveguide mode of the incident and exit polarizers have a strong coupling effect, resulting in the splitting of the transmission peak. The two split transmission peaks gradually separate with the increase of h, and the transmission efficiency reaches the maximum when h=80nm. The simulations revealed that the generation of doublets is associated with symmetric and antisymmetric modes of diporosity formation. That is to say, under the charge and current distribution of the incident pinhole in Figure 4(a), the exit pinhole can produce the distribution pattern as shown in Figure 4(b) or the completely opposite charge and current pattern as shown in Figure 4(b). The splitting effect of the transmission peak can be used to construct broadband polarization converters. For example, when h=80nm, the transmission or conversion efficiency reaches 52%, the full width at half maximum can reach 122nm, and the relative bandwidth is about 12%. However, with the further increase of h, the coupling effect between the two layers of small holes is weakened, and the transmission efficiency of the doublet will be significantly reduced.

Example 2

As a second embodiment, the working frequency of the polarization converter is in the microwave range. Figure 6(a) and Figure 6(b) are physical pictures of a single polarizer and a polarization converter processed by water cutting (water jet), respectively. The perforated metal plate is made of aluminum, its thickness is t=1.5 mm, the period of the hole array is d=60 mm, the side length of the rectangular hole is l=40 mm, and the width w=10 mm. The side length of the entire sample is 660 mm, and contains 11*11 rectangular holes in total. The two metal plates of the converter are separated by an air layer, and the small holes form an L-shaped vertical "cross", and the distance between the plates can be adjusted freely. Figure 6(c) presents the experimentally measured transmission spectra (circles and squares) of a single polarizer. Two transmission peaks (T _xx , squares) were detected in the incident polarization x direction: the main peak is at 3.53GHz, and the second peak is at 5.92GHz. The transmission efficiency of the main peak reaches 98%; if the area duty cycle of the small holes is normalized, the normalized transmission efficiency reaches 880%. This effect is precisely the enhanced transmission effect of the perforated metal film. Additionally, the transmission efficiency (T _xy , circle) for the y-polarization direction orthogonal to the incident polarization was measured experimentally. The efficiency of this polarization is negligibly small over the entire test band 2-7GHz. This shows that the polarizer has good polarization characteristics in the tested microwave band (especially near the transmission peak). The test results and FDTD calculations (see solid and dashed lines) are in good agreement. Figure 6(d) shows the experimental and theoretical transmission efficiency curves of the polarization converter when the distance between two polarizers is h = 10 mm. Experiments have found that when the incident polarization is in the x direction, the outgoing signal with x polarization is almost 0 (T _xx , square) in the entire test band. On the contrary, there are two transmission maxima (T _xy , circle) around 3.6GHz and 6.0GHz for the y-polarized outgoing signal. At 3.6GHz, the efficiency of the transmission peak reaches 98%. This shows that incident electromagnetic waves in the microwave segment can also effectively pass through this type of polarization converter: not only the polarization direction of electromagnetic waves is rotated by 90 degrees, but also the transmission or conversion efficiency is close to 100%. It is worth noting that the operating bandwidth (full width at half maximum) of the above-mentioned effects is found to be about 400MHz through experiments and theory, and the relative bandwidth reaches 11%. This is roughly comparable to the results in the optical band. In the microwave segment, although the real SPP mode on the metal surface does not exist, it is generally believed that the modulation of the metal surface structure can generate a "pseudo" surface plasmon polariton (spoof SPP) mode. The latter will act like an SPP and result in enhanced transmission. It can be understood that the occurrence of the above-mentioned polarization conversion effect is closely related to the "pseudo" surface plasmon resonance and the near-field coupling effect between double-layer pinholes.

The polarization conversion effect in the microwave segment can be freely regulated by changing the structural parameters. On the one hand, the near-field coupling effect strongly depends on the distance between two crossed polarizers. This coupling effect must decrease as the distance between the two polarizers increases. Figure 7(a) depicts the experimentally measured and theoretically calculated transmission spectra at different polarizer spacings (here only the transmission efficiency T _xy of the y component is given, and T _xx is negligible). It is easy to see that theory (solid line) and experiment (solid markers) agree well. As the polarizer spacing increases (h = 10–30 mm), the position of the transmission peak is only slightly red-shifted (showing a decrease in coupled mode energy), but the transmission efficiency decreases significantly. When h=30 mm, the transmission efficiency is less than 2%. This shows that when the polarizer distance is large, the transmission of electromagnetic waves is cut off; and when the distance is small and near-field coupling is allowed, the limitation of Marius' law will be broken. This phenomenon can be used to construct subwavelength modulators or switches, that is, to adjust microwave transmission efficiency or realize microwave on and off by controlling the distance of polarizers. It was also found that when h is smaller than 10 mm, the transmission peak is split. This is also related to the symmetric and antisymmetric modes that result from strong interactions. This effect can also be used to develop broadband microwave polarization converters. For example, when h=5 mm, the obtained half maximum width can reach 540MHz, and the relative half maximum width is about 15%. On the other hand, the operating frequency of the effect can be adjusted by adjusting the structural parameters, so that it can operate under different frequency requirements. One approach is to scale down the size of the system based on frequency requirements, or simply change the size of the aperture. Figure 7(b) shows the theoretically calculated transmission spectra (T _xy ) under three different hole lengths (l=40, 35, 30 mm, h=10 mm; other parameters remain unchanged). The results show that the transmission peak moves from 3.6 GHz to 3.9 and 4.2 GHz with the decrease of the side length of the small hole (and the cut-off wavelength), and the polarization conversion efficiency at the transmission peak is close to 100%.

Example 3

Based on the above results, the utility model further proposes a design method of a "universal" polarization converter. The converter can switch the polarization of the incoming line to any polarization direction, including orthogonal directions prohibited by Malius' law. To achieve this goal, two schemes are proposed here. The first scheme is used as a transitional scheme, which uses the polarizer in Fig. 6(a) as the input polarizer, while the output polarization direction is controlled by different output polarizers. The output polarizer is still made of metal aluminum plate, engraved with periodic oblique rectangular holes (the angle θ between the long side of the rectangular hole and the horizontal direction can be set according to needs; the corresponding rectangular holes of the incident and outgoing polarizers form ∠-shaped "cross"). As an example, Figure 8(a) shows a real picture of the exit polarizer when θ = 30 degrees. Fig. 8(b) shows the experimentally measured (top) and theoretically calculated (bottom) transmission spectra at different tilt angles θ (the output electric field component is perpendicular to the long side of the exit hole). It can be seen that around 3.6GHz, the converter is able to produce a valid output. When θ = 0 degrees, the maximum transmission efficiency is 55%; when θ = 30, 45, 60 and 90 degrees, the peak transmission or conversion efficiency is greater than 90% or even close to 100%. Typical relative operating bandwidths are between 10% and 17%, except at 0 degrees.

The second approach uses a single polarization converter to perform all of the above functions, thereby achieving "gimbal" polarization conversion. This "gimbal" polarization converter consists of two identical circular polarizers, where the input polarizer is fixed and the output polarizer is free to rotate about a central axis. Figure 9(a) shows the structural design of the incident or exit polarizer. Each polarizer is "uniformly" engraved with sub-wavelength rectangular holes on concentric circles with radii d, 2d, 3d and 4d, and all the rectangular holes are arranged in parallel. Here, "uniform" means that the circumference is evenly divided by a radial straight line, and a rectangular small hole is carved at the intersection of the radial straight line and the circumference: the distance between the intersection point and the upper, lower and left sides of the small hole is w/2 respectively [see Figure 9(b)]. In order to avoid adjacent small holes overlapping each other, the small holes have quasi-8-fold rotational symmetry on a circle with a radius of d; on a circle with a radius of 2d, small holes have a quasi-12-fold rotational symmetry; on a circle with a radius of 3d and On the circumference of 4d, the small hole has quasi 24-fold rotational symmetry. In this way, by rotating the outgoing polarizer at a certain angle (such as 15, 30, 45, 60, 75, 90 degrees, etc.), the corresponding rectangular holes of the two polarizers can form a ∠ or L shape with the same angle" cross". Thus, the incident polarization can be switched to other directions based on the near-field coupling effect. Figure 9(c) shows the actual picture of the polarizer prepared in the experiment. Here, the diameter of the entire sample is 600 mm, the distance between the concentric circles is d=60 mm, the size of the small holes, and the thickness of the aluminum plate are the same as before. Figure 9(d) is the transmission spectrum measured experimentally at different rotation angles (0, 30, 45, 60, 90 degrees) (the output electric field component is perpendicular to the long side of the exit hole). It can be seen from the figure that under different rotation angles, the converter can produce effective transmission around 3.5 GHz (compared with the periodic structure, the transmission efficiency is weakened to a certain extent due to the lack of periodicity). These results also show that by rotating the output polarizer, the converter can rotate the polarization direction of microwaves (including 90 degrees), and the conversion efficiency can be maintained at a high level. In contrast, the transmission or conversion efficiency of two conventional polarizers decreases with increasing rotation angle, and the transmission of light is cut off when the polarization directions are orthogonal. In addition, it should be pointed out that at other rotation angles, although the rectangular holes of the two polarizers deviate from the ∠ or L-shaped "crossing", the near-field coupling effect can still produce effective polarization conversion. It can be seen that the "universal" polarization converter provides a powerful tool for adjusting the direction of linear polarization.

In practical applications, material structure parameters can be properly adjusted or changed as required. For example, the thickness of the metal, the period of the hole array, the size of the rectangular hole, and the refractive index of the dielectric can all be adjusted freely. The rectangular apertures of the two polarizers can also be changed from the same size to different sizes to improve the bandwidth. Moreover, the rectangular small hole can also be replaced by narrow and long small holes such as ellipse or trapezoid. In addition, in addition to the optical frequency and microwave bands, the utility model can be naturally extended to the terahertz band. These changes do not violate the spirit of the present utility model. It is worth noting that in addition to polarization converters, sub-wavelength modulators or switches, the present invention can also be used as a unidirectional transmitter. That is, the x-polarized electromagnetic wave can effectively propagate along the z direction, while the reverse propagation of the same polarization is strictly prohibited. These have potential applications in the construction of novel microwave devices.

Claims (5)

1. a sub-wavelength phasmon polarization converter, comprises incident polaroid and outgoing polaroid, between incident polarization sheet and outgoing polaroid, by medium or air layer interval, it is characterized in that, incident polarization sheet and outgoing polaroid all adopt phasmon material; On described incident polarization sheet and outgoing polaroid, be carved with the aperture of sub-wavelength dimensions, on optical direction, on the each aperture on incident polarization sheet and outgoing polaroid, between corresponding aperture, shape has angle, and the end points that forms two apertures of angle intersects.

2. a kind of sub-wavelength phasmon polarization converter according to claim 1, is characterized in that, described angle is 0～90 degree.

3. a kind of sub-wavelength phasmon polarization converter according to claim 1 and 2, is characterized in that, described polaroid be shaped as rectangle or circle; Aperture on described incident polarization sheet and outgoing polaroid is rectangle, ellipse or trapezoidal.

4. a kind of sub-wavelength phasmon polarization converter according to claim 3, is characterized in that, the spacing of described incident polarization sheet and outgoing polaroid is sub-wavelength.

5. a kind of sub-wavelength phasmon polarization converter according to claim 1, is characterized in that, the aperture on described incident polarization sheet and outgoing polaroid is periodic arrangement or no periodic array.

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