electronics

Article Dynamic Modulation Band Rejection Filter Based on Spoof Surface

Weigao Yang 1,2 , Lixiang Liu 2, Xiaoqiang Su 2, Lijuan Dong 2, Yanhong Liu 2 and Zhanfeng Li 1,3,*

1 College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China; [email protected] 2 Shanxi Provincial Key Laboratory of Microstructure Electromagnetic Functional Materials, Shanxi Datong University, Datong 037009, China; [email protected] (L.L.); [email protected] (X.S.); [email protected] (L.D.); [email protected] (Y.L.) 3 Key Laboratory of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 03002, China * Correspondence: [email protected]

 Received: 3 May 2020; Accepted: 10 June 2020; Published: 13 June 2020 

Abstract: In this paper, we proposed a dynamic modulation band rejection filter based on the spoof polaritons (SSPPs) . The dynamic adjusting mainly derives from changing the capacitance between the U-shape and the waveguide configuration. The capacitance can modulate the cut-off frequency of fundamental mode. The rejection band is formed by the high order propagation mode and the cut-off frequency of fundamental mode. We analyzed the curve and transmission performance of the band rejection filter with different capacitances. Compared with the previous scheme, the design we proposed here has a simpler and more delicate configuration to process and decreases the mechanical error. We experimentally demonstrated the excellent performance of the device by changing the direct voltage loaded on the varactor diode and achieved real time modulation around 2 GHz.

Keywords: spoof surface plasmon polaritons; filter; dynamic modulation

1. Introduction As a kind of special , surface plasmon polaritons (SPPs) exist at the interface of metal and , which propagate along the interface and attenuate exponentially in the direction of normal interface [1–3]. At high frequency electromagnetic range, SPPs can be excited naturally because the metal behaves as plasma, with negative . It had been proven that the SPPs have the ability to confine and enhance the near electromagnetic field, so they will bring a lot of innovative technologies and applications, such as subwavelength focusing [4–6], subwavelength imaging [4,5,7], biosensing [8–11], harmonics generation [12,13], photonic integrated circuit and waveguide [6,14–17]. At a lower frequency, for example, the and terahertz frequency range, SPPs cannot exist because the metal behaves like the perfect electrical conductor (PEC), rather than the plasma with negative permittivity. By means of , the so-called spoof SPPs (SSPPs) or designer SPPs have been proposed, which can also produce a highly localized surface wave and inherit other properties at the high frequency [18,19]. The metal surface, decorated with a one-dimensional array of groove, two-dimensional wedges and holes, can excite the SSPPs [16,20,21]. Furthermore, the dispersion and field confinement of SSPPs can be designed and controlled by changing the geometrical parameters of the unit cell. However, most traditional waveguide devices are made of three-dimensional (3D) structures, which are complicated to produce and difficult to integrate with microwave circuits.

Electronics 2020, 9, 993; doi:10.3390/electronics9060993 www.mdpi.com/journal/electronics Electronics 2020, 9, 993 2 of 9

To apply SSPPs devices in practice, conformal surface plasmon constructions on planar flexible films have been proposed, which can propagate the spoof SPPs in the form of a bent and spiral shape [22]. Therefore, it is a promising solution for spoof SPPs to apply in future microwave integrated circuit regions [23]. Some functional devices based on SSPPs have been proposed in the last few years [24–26]. However, traditional devices based on SSPPsconsist of a passive unit, such as waveguide formed by patch [21], which lacks dynamic modulation. Considering that the function of a passive device is fixed once the dimension and configuration confirmed, the devices which can be dynamically modulated are required in modern microwave systems. Recently, many interesting works about active devices have been reported, for example, in microwave frequency, programmable plasmonic devices [27–29], tunable power dividers [30], reconfigurable metalenses [31,32], polarization converters [33] and other reconfigurable devices [34–38]. Tracing previous researchers’ thoughts [27,35,36], here we proposed an active modulation band rejection filter through controlling the cut-off frequency between the fundamental mode and the high order mode, which arises from changing capacitance generated between the waveguide and U-shape ring. The filter we present here has a simpler configuration that makes the sample easier to produce and decreases the mechanical error to some extent, compared with the former design [27].

2. SSPPs Waveguide Design In this work, we proposed an active filter based on spoof SPPs waveguide in the microwave frequency. A single layer F4B is selected as the substrate, whose thickness is 0.508 mm with the relative dielectric constant and loss tangent at 2.56 and 0.001. Annealed copper (with electric conductivity σ = 5.8e + 007 S/m) is printed on the substrate and the metal with a thickness of 0.018 mm. The details of the proposed waveguide unit cell are plotted in Figure1a, where p = 5.3 mm, h = 2 mm, wa = 0.2 mm, px = 3.5 mm, w2 = 3.5 mm, w3 = 0.5 mm and l4 = 1.6 mm. As demonstrated in ref [21], when decreasing the height of the patch, the dispersion curve gradually deviated from the light line, which proved that the configuration would decrease the speed of the electromagnetic wave and transform the conventional guided wave to spoof SPPs, smoothly. The dispersion curve of the unit cell is simulated by means of the Eigenmode solver of the commercial software CST Microwave Studio. In simulation, periodic boundaries are set along x direction, while the other directions are set to PEC boundaries, 50 mm away from the unit. The corresponding dispersion curve of the waveguide unit cell is shown in Figure1b. From this figure, we can clearly see that the dispersion curves are blowing and deviating gradually from the light line and finally reach different cut-off frequencies. Furthermore, with decreasing the height of unit w2, the cut-off frequency increases correspondingly, which demonstrated that the geometric parameters can be used to design the dispersion characteristics of spoof SPPs. Hence, we gradually decreased the height of w2 to achieve momentum matching and high-efficiency transform between the guided waves and the spoof SPPs waves. The spoof SPPs waveguide is composed of three parts: (I) the (CPW) as a feeding part to match the 50 Ω input impedance, (II) the momentum matching part from the CPW to spoof SPPs waveguide, and (III) the SSPPs waveguide , as illustrated in Figure1d. The dimensions of the SSPPs waveguide are l1 = 10 mm, l2 = 45 mm, l3 = 26.5 mm, w1 = 15 mm. The gap between the ground and the inner metallic strip is set to s = 0.16 mm, which is convenient to connect with 50 Ω Sub-miniature A (SMA) connectors. To achieve the momentum matching, six gradual increases in the height of the units are introduced into our design, with the step of 0.5 mm in the conversion part. We used the time domain solver of the CST Microwave Studio to validate our design and the results are illustrated in Figure1c. It is clear that a high-e fficiency spoof SPPs waveguide is constructed, where the transmission coefficient of the device is around 2 dB and the − reflection coefficient is less than 10 dB from 6 GHz to 11 GHz. − Electronics 2020, 9, 993 3 of 9 Electronics 2020, 9, x FOR PEER REVIEW 3 of 9

FigureFigure 1. 1. (a)( aThe) The unit unit structure structure of the of waveguide, the waveguide, whose whose dimensions dimensions are p are= 5.3p mm,= 5.3 h mm = 2 ,mm,h = 2wa mm, = 0.2wa mm,= 0.2 px mm, = 3.5 px mm,= 3.5 w2 mm =, w23.5 =mm,3.5 mm,w3 = w3 0.5= mm,0.5 mm, l4 = l4 1.6= 1.6mm. mm. (b) (bDispersion) Dispersion relations relations of of SSPPs SSPPs waveguidewaveguide with with different different heights. ( c) The SS parameterparameter ofof thethe waveguide.waveguide. ( d(d)) The The schematic schematic diagram diagram of ofSSPPs SSPPs waveguide, waveguide, where where l1 l1= =10 10 mm, mm, l2 l2= =45 45 mm, mm, l3 l3= =26.5 26.5 mm,mm, w1w1= = 1515 mm,mm, ss == 0.16 mm.

3. DynamicThe spoof Modulation SPPs waveguide Filter Designis composed and Study of three parts: (I) the coplanar waveguide (CPW) as a feedingTo part achieve to match the dynamic the 50 Ω modulation input impedance, of the transmission(II) the momentum and the matching rejection part band, from we the combine CPW theto spoofU-shape SPPs and waveguide, waveguide and unit (III to) the construct SSPPs waveguide the active unittransmission cell, as shown line, as in illustrated Figure2a. in The Figure varactor 1d. Thediodes dimensions are loaded of the at aSSPPs central waveguide position betweenare l1 = 10 the mm, U-shape l2 = 45 and mm, the l3 = waveguide, 26.5 mm, w1 and = 15 we mm. design The a gapthrough-hole between the on theground U-shape and tothe connect inner metallic the bottom strip side is metallicset to s strip= 0.16 structure mm, which in order is convenient to load direct to connectcurrent withvoltage 50 inΩ the Sub experiment-miniature with A (SMA) a top connectors.structure. This To design achieve enables the momentum the dynamic matching, modulation six Electronicsgradualfunction increas20 via20, 9 changing, xes FOR in PEERthe the height REVIEW capacitance of the units loaded are introduced between the into U-shape our design and the, with waveguide the step of unit. 0.5 mm4 of in 9 the conversion part. We used the time domain solver of the CST Microwave Studio to validate our design and the results are illustrated in Figure 1c. It is clear that a high-efficiency spoof SPPs waveguide is constructed, where the transmission coefficient of the device is around −2 dB and the reflection coefficient is less than –10 dB from 6 GHz to 11 GHz.

3. Dynamic Modulation Filter Design and Study To achieve the dynamic modulation of the transmission and the rejection band, we combine the U-shape and waveguide unit to construct the active unit cell, as shown in Figure 2a. The varactor diodes are loaded at a central position between the U-shape and the waveguide, and we design a through-hole on the U-shape to connect the bottom side metallic strip structure in order to load direct current voltage in the experiment with a top structure. This design enables the dynamic modulation function via changing the capacitance loaded between the U-shape and the waveguide unit.

FigureFigure 2. 2. (a()a )The The configuration configuration of of the the dynamic dynamic modulation modulation unit unit cell, cell, where where g g == 0.20.2 m mm,m, wa wa == 0.20.2 mm, mm, w3w3 == 0.50.5 mm. mm. (b ()b Dispersion) Dispersion diagrams diagrams of of the the proposed proposed unit unit cell cell at at different different capacitances capacitances..

We can modulate the cut-off frequency of the unit cell by changing the capacitance, while maintaining the same structure and parameter. Figure 2b illustrates the dispersion curves of the proposed active unit cell with regard to different modes and capacitances, which is conducted by the Eigenmode solver in the commercial software CST. We can observe that the dispersion curves deviate from the light line and different modes arrive at different cutoff frequencies, under the same capacitance. From Figure 2b, we found that the cut-off frequency of mode 1 increases along with the decreasing in capacitances. There is a forbidden band between mode 1 and mode 3 under the same capacitance, whose variates correspond to the alteration of the capacitance from about 7.8 GHz to 9.8 GHz. All of this indicates that the configuration can be used to build a dynamic modulation band rejection filter via changing the capacitance. Moreover, in such a symmetrical CPW feeding system, only even mode can be excited, odd mode 2 cannot be excited since the electric field line is asymmetric with respect to the propagation direction. Based on the above analysis, an active modulation band rejection filter based on spoof SPPs waveguide was realized via changing the capacitance between the U shape and waveguide unit and the configuration of design, as shown in Figure 3. On the bottom side, we designed a metallic strip to connect the direct current voltage via the through-hole in the experiment and used the inductor with 30 μH to connect each independent structure.

Figure 3. The schematic of the proposed SSPPs dynamic modulation filter. (a) The top view. (b) The bottom view. Electronics 2020, 9, x FOR PEER REVIEW 4 of 9

ElectronicsFigure2020 2. ,(9a,) 993The configuration of the dynamic modulation unit cell, where g = 0.2 mm, wa = 0.2 mm, 4 of 9 w3 = 0.5 mm. (b) Dispersion diagrams of the proposed unit cell at different capacitances.

WeWe can can modulate modulate the the cut cut-o-off frequencyff frequency of the of unit the unitcell by cell changing by changing the capacitance the capacitance,, while maintainwhile maintaininging the same the structure same structure and parameter. and parameter. Figure 2b Figure illustrate2b illustratess the dispersion the dispersion curves curves of the proposedof the proposed active unit active cell unit with cell regard with to regard different to di modesfferent and modes capacitances, and capacitances, which is which conducted is conducted by the Eigenmodeby the Eigenmode solver in solver the commercial in the commercial software software CST. We CST.can observe We can that observe the dispersion that the dispersion curves deviate curves fromdeviate the from light the line light and line different and diff erent modes modes arrive arrive at different at different cutoff cuto frequenciesff frequencies,, under under the the same same capacitance.capacitance. From From Figure Figure 2b,2b, we we found found that that the the cut cut-o-offff frequencyfrequency of of mode mode 1 1increases increases along along with with the the decreasingdecreasing in in capacitances. capacitances. There There is is a aforbidden forbidden band band between between mode mode 1 1and and mod modee 3 3under under the the same same capacitancecapacitance,, wh whoseose variates variates correspond correspond to tothe the alteration alteration of the of the capacitance capacitance from from about about 7.8 GHz 7.8 GHz to 9.8 to GHz.9.8 GHz. All of All th ofis thisindicates indicates that that the theconfiguration configuration can can be beused used to tobuild build a adynamic dynamic modulation modulation band band rejectionrejection filter filter via via changing changing the the capa capacitance.citance. Moreover, Moreover, in in such such a asymmetrical symmetrical CPW CPW feeding feeding system system,, onlyonly even even mode mode can can be be excited, excited, odd odd mode mode 2 2cannot cannot be be excited excited since since the the electric electric field field line line is is asymmetric asymmetric withwith respect respect to to the the propagation propagation direction. direction. BasedBased on on the the above above analysis, analysis, an an active active modulati modulationon band band rejection rejection filter filter based based on on spoof spoof SPPs SPPs waveguidewaveguide was was realized realized via via changing changing the the capacitance capacitance between between the the U U shape shape and and waveguide waveguide unit unit and and thethe configuration configuration of of design design,, as as shown shown in in Figure Figure 3.3. On On the the bottom bottom side, side, we we design designeded a a metallic metallic strip strip to to connectconnect the the direct direct current current voltage voltage via via the the through through-hole-hole in in the the experiment experiment and and use usedd the the inductor inductor with with 3030 μHµH to to connect connect each each independent independent structure. structure.

FigureFigure 3. 3. TheThe schematic schematic of of the the proposed proposed SSPPs SSPPs dynamic dynamic modulation modulation filter. filter. (a (a) )The The top top view. view. (b (b) )The The bottombottom view. view.

Toinvestigate the transmission performance, we calculated the S-parameter of dynamic modulation spoof SPPs filter with different capacitances and the corresponding results are shown in Figure4. The transmission coefficients (S12) proves that around a 2 GHz bandwidth band rejection spoof SPPs filter can be achieved and the forbidden band can be accurately controlled by designing the corresponding capacitance. The transmission coefficients of the filter are around 2 dB under different capacitances in the − pass band, meanwhile, in the forbidden band the coefficients can achieve below 35 dB. As depicted − in Figure4, with increasing the capacitance, the forbidden band will shift to a lower frequency. When capacitances are set to 0.3 pF, 1 pF, 3 pF, the forbidden bands (transmission coefficients below 10 dB) are 7.8–8.3 GHz, 8.2–8.7 GHz, 9.2–9.7 GHz, respectively, which coincide with the dispersion − curve shown in Figure2b. Electronics 2020, 9, x FOR PEER REVIEW 5 of 9

To investigate the transmission performance, we calculated the S-parameter of dynamic modulation spoof SPPs filter with different capacitances and the corresponding results are shown in Figure 4. The transmission coefficients (S12) proves that around a 2 GHz bandwidth band rejection spoofElectronics SPPs2020 filter, 9, 993 can be achieved and the forbidden band can be accurately controlled by designing5 of 9 the corresponding capacitance.

Figure 4. The simulation resultsresults ofof transmissiontransmission coe coefficientsfficients under under capacitances capacitances of of 0.3 0.3 pF, pF, 1 pF 1 pF and and 3 pF. 3 pF. To have a visual impression for the filter, we simulated the near-electric field distribution. We selectedThe transmission three different coefficients rejection of frequencies the filter are at diaroundfferent − capacitances2 dB under anddifferent one pass capacitances band frequency. in the passFigure band,5 demonstrates meanwhile, thein the results forbidden of near-electric band the coefficients field distribution can achieve at 7.8 below GHz, − 8.335 dB. GHz, As 9.5depicted GHz, in11 Figure GHz with 4, with capacitances increasing ofthe 3 capacitance pF, 1 pF, 0.3, the pF, forbidden respectively. band Itwill can shift be seento a lower from frequency. Figure5a–c When that capacitancesthe electromagnetic are set to wave 0.3 pF, is reflected1 pF, 3 pF and, the cannot forbidden propagate bands (transmission from one port coefficients to another portbelow when −10 dB) the Electronicsarefrequency 7.8– 208.320 is,GHz, 9 located, x FOR 8.2 PEER– at8.7 the REVIEWGHz, forbidden 9.2 –9.7 band. GHz, respectively, which coincide with the dispersion c6urve of 9 shown in Figure 2b. To have a visual impression for the filter, we simulated the near-electric field distribution. We selected three different rejection frequencies at different capacitances and one pass band frequency. Figure 5 demonstrates the results of near-electric field distribution at 7.8 GHz, 8.3 GHz, 9.5 GHz, 11 GHz with capacitances of 3 pF, 1 pF, 0.3 pF, respectively. It can be seen from Figure 5a–c that the electromagnetic wave is reflected and cannot propagate from one port to another port when the frequency is located at the forbidden band.

Figure 5. The simulated near-field distribution of the device at (a) 7.8 GHz with capacitance 3 pF, Figure(b) 8.3 5. GHz Thewith simulated capacitance near-field 1 pF, distribution (c) 9.5 GHz of with the capacitancedevice at (a) 0.37.8 pF,GHz (d with) 11 GHz capacitance with capacitance 3 pF, (b) 8.30.3 GHz pF. with capacitance 1 pF, (c) 9.5 GHz with capacitance 0.3 pF, (d) 11 GHz with capacitance 0.3 pF. The electromagnetic wave can be efficiently transmitted at the same capacitance when the frequencyThe electromagnetic located pass band wave is can between be efficiently the forbidden transmitted band and at the the same cuto ff capacitancefrequency ofwhen mode the 3, frequencyas shown located in Figure pass5d. band From is the between near-field the electricforbidden distribution, band and wethe cancutoff observe frequency that theof mode filter has3, as a shownbetter in cut-o Figureff function 5d. From at thethe rejectionnear-field band electric and distribution, passes the other we can frequencies observe that efficiently, the filter while has a keeping better cutthe-off highly function field at confined. the rejection band and passes the other frequencies efficiently, while keeping the highly field confined.

4. Fabrication and Measurement To demonstrate the performance of the proposed filter, we fabricated the sample based on the standard printed circuit board process, as shown in Figure 6a. We used SMA and coaxial cables, whose insertion loss is less than about 1 dB, to connect the sample and the vector network analyzer (VNA). In the experiment, we used varactor-diodes (SMV2202-040LF), which were purchased from Skyworks, to realize the dynamic modulation function.

Figure 6. (a)The picture of fabricated sample (b) Measured S-parameter under different capacitances.

The measured S-parameter results with different bias voltages loaded on the varactor diodes are displayed in Figure 6b. We selected three different loading bias voltages, 0 V, 7 V, 20 V, and the corresponding capacitances are 3 pF, 1 pF, 0.3 pF, to measure the transmission coefficients S12. The position of the rejection band changes from 7.8 GHz to 9.9 GHz, along with the variation of voltage loaded on the varactors. From Figure 6b, we can clearly observe that there are three band rejection bands with isolation values below −30 dB, that maintain a high transmission efficiency in the pass band. Namely, the measured S12 results have a good agreement with the simulations, as listed in Figure 2b. We used the homemade near-field mapper to plot the near electric distribution above the substrate plane at 1 mm. From obtained results displayed in Figure 7a–d, we clearly found that the Electronics 2020, 9, x FOR PEER REVIEW 6 of 9

Figure 5. The simulated near-field distribution of the device at (a) 7.8 GHz with capacitance 3 pF, (b) 8.3 GHz with capacitance 1 pF, (c) 9.5 GHz with capacitance 0.3 pF, (d) 11 GHz with capacitance 0.3 pF.

The electromagnetic wave can be efficiently transmitted at the same capacitance when the frequency located pass band is between the forbidden band and the cutoff frequency of mode 3, as shown in Figure 5d. From the near-field electric distribution, we can observe that the filter has a better cut-off function at the rejection band and passes the other frequencies efficiently, while keeping the Electronics 2020, 9, 993 6 of 9 highly field confined.

4.4. Fabrication Fabrication an andd Measurement Measurement ToTo demonstrate demonstrate the the performance performance of of the the proposed proposed filter, filter, we we fabricated fabricated the the sample sample based based on on the the standardstandard printed printed circuit circuit board board process, process, as as shown shown in in Figure Figure 6 6a.a. We used SMA and coaxial cablescables,, whosewhose insertion insertion loss loss is is less less than than about about 1 1 dB dB,, to to conne connectct the the sample sample and and the the vector vector network network analyzer analyzer (VNA).(VNA). In In the the experiment, experiment, we we used used varactor varactor-diodes-diodes ( (SMV2202-040LF),SMV2202-040LF), which which were were purchased purchased from from Skyworks,Skyworks, to to realize realize the the dynamic dynamic modulation modulation function. function.

FigureFigure 6. 6. (a(a)The)The picture picture of of fabricated fabricated sample sample (b (b) )Measured Measured S S-parameter-parameter under under different different capacitances. capacitances.

TheThe measured measured S- S-parameterparameter results results with with different different bias bias voltages voltages loaded loaded on the on thevaractor varactor diodes diodes are displayedare displayed in Figure in Figure 6b. 6 Web. We selected selected three three different di fferent loading loading bias bias voltage voltages,s, 0 V, 0 V, 7 7V, V, 20 20 V V,, and and the the correspondingcorresponding capac capacitancesitances are are 3 pF, 3 pF, 1 pF, 1 pF, 0.3 0.3pF, pF,to measure to measure the transmission the transmission coefficients coefficients S12. The S12. positionThe position of the of rejection the rejection band band changes changes from from 7.8 7.8GHz GHz to to9.9 9.9 GHz GHz,, along along with with the the variation variation of of voltage voltage loadedloaded on on the the varactors. varactors. From From F Figureigure 6b,6b, we we can can clearly observe that that there there are three band rejection bandbandss with with isolation isolation value valuess below below −3030 dB dB,, that that maintain maintain a a high high transmission transmission efficiency efficiency in in the the pass pass − band.band. Namely, Namely, the the measured measured S12 S12 results results have have a agood good agreement agreement with with the the simulations simulations,, as as listed listed in in FigureElectronicsFigure 2b.2b. 20 We20 We, 9 used, x FOR the PEER homemade REVIEW near near-field-field mapper to plot the near electric distribution above 7the of 9 substratesubstrate plane plane at at 1 1 mm. mm. From From obtained obtained results results displayed displayed in in Figure Figure 7a7a–d,–d, we clearly found that the surface electromagnetic wave is rejected at the forbidden band frequenciesfrequencies 7.8 GHz, 8.3 GHz, 9.5 GHz,GHz, corresponding to different different capacitances. Furthermore, the electromagnetic wave can propagate propagate along the device in the pass band 11 GHz, which coincides with simulated results.

Figure 7. The measured near-field distribution of the device at (a) 7.8 GHz with capacitance 3 pF, Figure 7. The measured near-field distribution of the device at (a) 7.8 GHz with capacitance 3 pF, (b) (b) 8.3 GHz with capacitance 1 pF, (c) 9.5 GHz with capacitance 0.3 pF, (d) 11 GHz with capacitance 8.3 GHz with capacitance 1 pF, (c) 9.5 GHz with capacitance 0.3 pF, (d) 11 GHz with capacitance 0.3 0.3 pF. pF.

There are some discrepancies between simulations and measurements which may be caused by insertion loss, inhomogeneity of the substrate, loss of coaxial cables and SMA. Moreover, the shift of frequency between simulation and experiment would be caused by machine error and the imprecise voltage loaded on the varactors.

5. Conclusions In this paper, a simpler method to fabricate a dynamic modulation band rejection filter device loaded with varactor-diodes has been proposed and manufactured. Based on this method, we can modulate the rejection band in real time, at high speed, by changing the bias voltage. Both the simulation and measurement results have demonstrated a good performance of the filter in tuning the rejection of spoof SPPs, which provides high isolation in the rejection band and maintains high efficiency transmission at the passband. The simpler configuration makes our dynamic modulation filter more compact and gives a method for future dynamic modulation device. By designing the bias voltage loaded on capacitance, it is expected that the proposed method will achieve other multiple function devices such as a logic gate device and a multiple-band rejection filter. The proposed method and structure may pave a new way for the application of SSPPs-based devices and integrated circuit systems in the microwave and THz frequencies.

Author Contributions: X.S. and Y.L. methodology; W.Y. and L.L. design; W.Y. and L.L. writing; Z.L. and L.D. review. All authors have read and agreed to the published version of the manuscript.

Funding: This work is supported by the National Natural Science Foundation of China (NSFC) under Grant 11504210, and the Shanxi Provincial Foundation for Returned Scholars, China (Grant No.2015-096).

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Barnes, W.L.; Dereux, A.; Ebbesen, T.W. Surface plasmon subwavelength optics. Nature 2003, 424, 824–830, doi:10.1038/nature01937. 2. Foley, J.J.; McMahon, J.M.; Schatz, G.C.; Harutyunyan, H.; Wiederrecht, G.P.; Gray, S.K. Inhomogeneous Surface Plasmon Polaritons. ACS Photonics 2014, 1, 739–745, doi:10.1021/ph500172f. 3. Xu, X.G.; Ghamsari, B.G.; Jiang, J.H.; Gilburd, L.; Andreev, G.O.; Zhi, C.; Bando, Y.; Golberg, D.; Berini, P.; Walker, G.C. One-dimensional surface phonon polaritons in boron nitride nanotubes. Nat. Commun. 2014, 5, 4782, doi:10.1038/ncomms5782. 4. Fang, N.; Lee, H.; Sun, C.; Zhang, X. Sub-diffraction-limited optical imaging with a silver . Science 2005, 308, 534–537, doi:10.1126/science.1108759. Electronics 2020, 9, 993 7 of 9

There are some discrepancies between simulations and measurements which may be caused by insertion loss, inhomogeneity of the substrate, loss of coaxial cables and SMA. Moreover, the shift of frequency between simulation and experiment would be caused by machine error and the imprecise voltage loaded on the varactors.

5. Conclusions In this paper, a simpler method to fabricate a dynamic modulation band rejection filter device loaded with varactor-diodes has been proposed and manufactured. Based on this method, we can modulate the rejection band in real time, at high speed, by changing the bias voltage. Both the simulation and measurement results have demonstrated a good performance of the filter in tuning the rejection of spoof SPPs, which provides high isolation in the rejection band and maintains high efficiency transmission at the passband. The simpler configuration makes our dynamic modulation filter more compact and gives a method for future dynamic modulation device. By designing the bias voltage loaded on capacitance, it is expected that the proposed method will achieve other multiple function devices such as a logic gate device and a multiple-band rejection filter. The proposed method and structure may pave a new way for the application of SSPPs-based devices and integrated circuit systems in the microwave and THz frequencies.

Author Contributions: X.S. and Y.L. methodology; W.Y. and L.L. design; W.Y. and L.L. writing; Z.L. and L.D. review. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the National Natural Science Foundation of China (NSFC) under Grant 11504210, and the Shanxi Provincial Foundation for Returned Scholars, China (Grant No.2015-096). Conflicts of Interest: The authors declare no conflict of interest.

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