CN105703041A - Artificial surface plasmon-based miniaturized low-pass filter - Google Patents

Abstract

The invention discloses a miniaturized low-pass filter based on artificial surface plasmons, which is characterized in that it is mainly composed of a coplanar waveguide, and two grounding strips of the coplanar waveguide are respectively provided with periodic Distributed slot unit arrays, the two slot unit arrays are symmetrical up and down with respect to the central conductor strip, the depth of the slot units increases uniformly from both sides to the middle in the length direction of the coplanar waveguide, and the depth of the slot units in the middle remains unchanged. Based on the coplanar waveguide, the present invention uses the coplanar waveguide to etch periodic groove units on the grounding strip to construct an artificial surface plasma structure to realize the filter function. The T-shaped groove is introduced to increase the equivalent depth of the groove in the width direction, thereby realizing the miniaturization of the device. The invention has a series of advantages such as simple structure, compact size, good filtering performance, easy processing, and especially suitable for matching with traditional microwave transmission lines and devices.

Description

Miniaturized low-pass filter based on artificial surface plasmons

technical field

The invention relates to a filter, in particular to a miniaturized low-pass filter based on artificial surface plasmons.

Background technique

Surface Plasmon Polaritons (SPPs for short) are special electromagnetic waves transmitted at the interface between metal and medium (usually air) at a light frequency. SPPs are a kind of surface wave with two distinctive features: first, along the perpendicular to the interface, it is an evanescent wave, and the intensity of the wave decreases exponentially, so the wave is bound near the interface; second, the SPPs The transmission wavelength is shorter, so it can break the diffraction limit. The research on SPPs was once limited to the optical band or higher frequencies. The plasma frequency of metals is generally in the ultraviolet band, and the dielectric constant of metals in the low frequency band is very large, making the skin depth of electromagnetic waves very small. Therefore, in the low frequency band, metals are close to ideal conductors (Perfect Electric Conductor, PEC for short). As a result, the field confinement of SPPs on the metal surface is poor, and effective propagation on the metal surface cannot be achieved, which greatly limits the application of SPPs in the low frequency band. Due to the excellent characteristics of SPPs, if the concept of surface plasmons is extended to the low frequency band (microwave or terahertz band), it will help to obtain highly constrained microwave or terahertz signal waveguide technology, and the device of low frequency band Dimensions are reduced to sub-wavelength levels for high integration. In order to realize the efficient confinement of waves in the low frequency band, a method of etching grooves or digging holes in metals is proposed, because it inherits the related properties of SPPs, it is called artificial surface plasmon polaritons (SpoofSurfacePlasmonPolaritons, referred to as SSPPs) ). The basic idea is to dig periodically distributed holes on the metal surface. The size and interval of the holes are much smaller than the wavelength to enhance the penetration of electromagnetic waves, thereby reducing the plasma frequency of the metal surface by means of an equivalent medium. This approach, which can control the wave transmission through the geometry of the structure, has great potential in microwave terahertz design. In 2005, Hibbins et al. confirmed the phenomenon of SSPPs in the microwave band. Subsequently, Williams et al. also verified the existence of SSPPs in the terahertz band, which opened a new page for the development and application of low-frequency SPPs. Since then, SSPPs have aroused great interest of researchers.

Recently, some studies on the transmission of SpoofSPPs have been published in major journals. In these studies, most of them are passive structures or devices, and these devices cannot complete the work alone, because it has certain difficulties in feeding efficiency and signal transmission. Usually, these designs need to be combined with microwave terahertz circuits or devices. Therefore, the conversion and transmission efficiency of guided wave and spp wave is particularly important. Committed to the efficiency of transmission and conversion, the research on prisms, gratings, domino arrays, etc. has successively obtained some results. However, the mismatch of the wave vector leads to the failure of the transmission conversion efficiency to meet the ideal requirements. In view of this, a tapered structure is proposed to solve the problem of mismatch between wave vector and impedance. Secondly, the miniaturization of microwave devices is an unavoidable topic and plays an important role in the design of microwave circuit systems. British scholars proposed a structure of square curved convoluted metal strips to increase the equivalent length of metal branches. Later, Zhou Yongjin of Shanghai University further applied miniaturization.

As a microwave planar transmission line with superior performance and convenient processing, coplanar waveguide is playing an increasingly important role in monolithic microwave integrated circuits. Especially in the millimeter wave frequency band, coplanar waveguide has incomparable performance compared to microstrip lines. Advantage. Compared with conventional microstrip transmission lines, coplanar waveguides have the advantages of easy fabrication, easy realization of serial and parallel connections of passive and active devices in microwave circuits (no need to perforate the substrate), and easy increase of circuit density.

Contents of the invention

The purpose of the present invention is to combine the artificial surface plasmon waveguide supporting SSPPs with the coplanar waveguide supporting space wave, and design a miniaturized low-pass filter based on artificial surface plasmon polaritons.

Technical scheme of the present invention is realized like this:

A miniaturized low-pass filter based on artificial surface plasmons, which is characterized in that it is mainly composed of a coplanar waveguide, and the two grounding strips of the coplanar waveguide are respectively provided with grooves periodically distributed along the length direction of the coplanar waveguide In the unit array, the two slot unit arrays are symmetrical up and down with respect to the central conductor strip. The depth of the slot unit increases uniformly from both sides to the middle in the length direction of the coplanar waveguide, and the depth of the slot unit in the middle remains unchanged.

Based on the coplanar waveguide, the invention uses the coplanar waveguide to etch periodic groove units on the grounding strip to construct an artificial surface plasma structure to realize the filtering function.

Furthermore, the present invention also designs a T-shaped groove unit, which is used to increase the equivalent depth of the groove unit in the width direction of the coplanar waveguide, thereby further realizing the miniaturization of the device.

For SSPPs propagating on coplanar waveguides with periodic sub-wavelength rectangular groove arrays, different groove depths have different dispersion curves, and the wave vectors of grooves with different depths are different. The wave vector and impedance can be realized through the gradual change of groove depth Furthermore, since the plasma frequency of SSPPs is controlled by the surface geometry, the artificial surface plasmon waveguide has tunable dispersion characteristics; the present invention can control the final cut-off frequency by several units in the constant depth part of the slot unit array , realizing the function of controllable filtering. The design of the T-shaped slot increases the equivalent depth of the slot unit in the width direction of the coplanar waveguide, and realizes a smaller size of the filter geometry.

The invention is suitable for matching with traditional microwave transmission lines, and provides a brand-new idea and solution for the design and application of filter devices.

The present invention has following beneficial effect:

1. The present invention mainly proposes a miniaturized low-pass filter based on artificial surface plasmons. On the one hand, it utilizes the superior transmission performance of the coplanar waveguide, and on the other hand, utilizes the tunable dispersion characteristics of the artificial surface plasmon waveguide to realize Low-pass filtering effect for signal transmission. This structure can be matched with microwave devices or circuits, and thus has greater flexibility in the design of microwave devices and integrated circuit structures.

2. Simple structure: The structure is derived from the coplanar waveguide. The wave vector and impedance matching section and the transmission of SSPPs are formed by etching periodic slot arrays on the ground with the coplanar waveguide. The structure is simple and compact, and it is easy to process.

3. Strong innovation and good technical foresight: the present invention uses the combination of artificial surface plasmon waveguide and coplanar waveguide in the microwave frequency band to realize low-pass filtering characteristics and strong innovation; it can be well used in conjunction with traditional microwave transmission lines , easy to integrate into microwave circuits, expand the application range of artificial surface plasmon devices, and have good technical foresight.

4. Strong filtering performance: The filtering performance is derived from the transmission characteristics of artificial surface plasmons, with significant out-of-band suppression and good in-band transmission performance.

5. The miniaturization design of the device is realized: the equivalent depth of the slot is increased by introducing a T-shaped structural slot, and the miniaturization of the length perpendicular to the transmission direction is realized.

Description of drawings:

Fig. 1 is a three-dimensional schematic diagram of the dispersion unit structure of Embodiment 1;

Fig. 2 is a dispersion curve diagram of different groove depths of the dispersion unit structure shown in Fig. 1;

Fig. 3 is a three-dimensional schematic diagram of the overall structure of Embodiment 1;

Fig. 4 is a detailed structure diagram of the slot array of Embodiment 1;

Fig. 5 is the S parameter corresponding to embodiment one;

6 is a three-dimensional schematic diagram of the overall structure of Embodiment 2;

Fig. 7 is a detailed structure diagram of the groove array of the second embodiment;

Fig. 8 is the S parameter corresponding to the second embodiment.

detailed description:

Below in conjunction with accompanying drawing, the implementation of technical scheme is described in further detail:

Embodiment one

As shown in Figure 3, the structure of the present invention is based on the coplanar waveguide, and its two metal grounding strips are etched with a rectangular structure slot unit array periodically distributed along the coplanar waveguide length direction (x direction), and the slot unit array is about The width direction of the coplanar waveguide (y direction) is left-right symmetrical, and the two groove unit arrays are vertically symmetrical with respect to the central conductor strip. The depth of the slot unit (that is, the height in the y direction) increases uniformly from both sides to the middle in the x direction to form a pattern matching section, and the depth of the slot unit in the middle remains unchanged to form the SSPPs transmission section, and the number of slot units with gradually changing depths on both sides and the depth in the middle remain unchanged The number of slot units is set according to the transmission requirements of the guided wave signal.

The dielectric substrate of filter is Rogers 6010 (dielectric constant is 10.2), and Fig. 1 is the periodic dispersion unit structure that comprises rectangular groove unit among the embodiment one, and length (being groove unit distribution period) is p=4mm, and width is W= 20mm, the thickness of the dielectric substrate a=1.0mm, the thickness of the metal layer t=0.018mm, the width of the rectangular groove b=0.5mm, and the depth variable is h. Using electromagnetic simulation software to simulate the periodic dispersion unit structure in Figure 1, the dispersion results can be obtained when the variable h increases uniformly from 1.0mm to 5.0mm, as shown in Figure 2. It can be seen from the distribution of the curve that the asymptotic frequency is related to the depth of the groove, and the asymptotic frequency decreases with the increase of the groove depth.

The overall structure of the low-pass filter is shown in Figure 3. The length of the structure is L=100mm, the width is W=20mm, the thickness of the dielectric substrate is a=1.0mm, and the thickness of the metal layer is t=0.018mm.

As shown in Fig. 4, the coplanar waveguide signal line width w1=3mm, and the distance between the signal line and the ground g=0.8mm is used to construct the port impedance of 50ohm. In this embodiment, the width of the rectangular slot units is b=0.5 mm, and they are distributed along the x direction at a period of p=4 mm. The groove depth increases uniformly from both ends to the middle from h1=0.8mm to h6=4.8mm, and the depth of the middle 7 groove units is 4.8mm.

According to Embodiment 1, the S-parameter results shown in FIG. 5 can be obtained by using electromagnetic simulation software. Electromagnetic waves can be efficiently transmitted around 0 to 5.5GHz, and an effective cutoff is formed at 5.5GHz. The out-of-band S21 is below -40dB, and the in-band S11 is basically below -10dB.

Embodiment two

As shown in Figure 6, it is consistent with the basic structure of Embodiment 1, the difference is that the groove unit of this embodiment adopts a T-shaped structure, the length of the overall structure is L=100mm, the width is W=20mm, and the thickness of the dielectric substrate a=1.0mm, Metal layer thickness t=0.018mm.

As shown in FIG. 7 , the coplanar waveguide signal line width w1 = 3 mm, and the distance between the signal line and the ground g = 0.8 mm is used to construct an impedance of 50 ohm at the port. In this embodiment, the width of the transverse branch and the vertical branch of the T-shaped slot unit is equal to b=0.5mm, and the width of the transverse branch and the vertical branch can also be unequal. It is set according to the transmission requirements of the guided wave signal, and the period along the x direction is p= 4mm distribution. The length of the branches of the T-shaped groove unit in the x direction remains constant at q=2.5mm, the depth increases uniformly from h1=0.5mm to h6=3.0mm, and the depth of the middle 7 groove units is 3.0mm.

According to the second embodiment, the S-parameter results shown in FIG. 8 can be obtained by using electromagnetic simulation software. Electromagnetic waves can be efficiently transmitted around 0 to 5.5GHz, and an effective cutoff is formed at 5.5GHz. The out-of-band S21 is below -35dB, and the in-band S11 is below -15dB.

In the above embodiments, the slot unit array is left-right symmetric, but in practical applications, it can also be set to be asymmetrical. In the above embodiments, any one end of the filter can be used as an input end of the guided wave signal, and the other end can be correspondingly used as an output end.

For the T-slot structure filter designed in the present invention, to achieve the 5.5GH cut-off in the result, the y-direction branch length of the T-slot needs to be 3 mm, and the transmission in the passband meets the requirement of high efficiency. From the dispersion curve in Figure 2, it can be seen that to achieve the same cutoff frequency (about the asymptotic frequency of the dispersion curve), the depth of the T-shaped groove must be about 4.8 mm if it is replaced by a rectangle. The reason is that the transverse structure of the T-shaped groove is equivalent to increasing the equivalent depth of the groove in the y direction, and the asymptotic frequency of the dispersion curve of the SSPP structure is controlled by the depth of the groove, and the deeper the groove, the smaller the asymptotic frequency. Therefore, miniaturization in the y direction is realized.

When the material of the dielectric plate changes, if the dielectric constant becomes smaller, the confinement performance of the dielectric plate to electromagnetic energy is weakened, resulting in an increase in the cut-off frequency of the filter. In this case, the T-shaped groove structure is adopted, which can also achieve the purpose of reducing the frequency and miniaturizing the device.

Claims (5)

1. A miniaturized low-pass filter based on artificial surface plasmons, characterized in that it is mainly composed of coplanar waveguides, and two grounding strips of the coplanar waveguide are respectively provided with periodic distribution along the length direction of the coplanar waveguide. The slot unit arrays are vertically symmetrical about the central conductor strip between the two slot unit arrays, the depth of the slot units increases uniformly from both sides to the middle in the length direction of the coplanar waveguide, and the depth of the slot units in the middle remains unchanged. 2 . The artificial surface plasmon-based miniaturized low-pass filter according to claim 1 , wherein the groove unit has a rectangular structure. 3 . 3. A miniaturized low-pass filter based on artificial surface plasmons according to claim 1, characterized in that: the groove unit is in a T-shaped structure. 4. A low-pass filter based on artificial surface plasmon miniaturization according to claim 1, characterized in that: the slot unit array is left-right symmetrical with respect to the width direction of the coplanar waveguide. 5. A low-pass filter based on artificial surface plasmon miniaturization according to claim 1, characterized in that: any end of the filter is used as the input end of the guided wave signal, and the other end is used as the output end.