Research On Active Modulation Methods Of Metamaterials And Spoof Surface Plasmon Polaritons Based On Thin-Film Materials

Metamaterials（MTMs）are artificial periodic subwavelength structures with unnatural electromagnetic（EM）characteristics,which can achieve the modulation of the EM waves.Among them,an artificial metal surface structure can mimic the EM properties of optical surface plasmon polaritons（SPPs）at infrared frequencies and below,which are typically referred to as spoof surface plasmon polaritons（SSPPs）.However,the EM properties（such as resonant frequency,amplitude,polarization direction and phase）of traditional MTMs are not tunable due to their fixed structures,which prevents their development in real-time controllable devices.To dynamically manipulate EM waves,various active MTMs have been reported by incorporating passive MTMs with tunable components.Recently,many active MTMs have been reported at microwave and THz frequencies with varactor diodes,phase change materials,liquid crystal materials,superconductors,III-V semiconductors and graphene.However,these laboratory studies ignore the cost and yield that need to be considered in actual production,and the process compatibility with very large scale integration（VLSI）is rarely mentioned.For example,the device size and integration tolerance have so far limited the appliaction of soldering varactor in compact high frequency integrated circuits.Several electro-thermal materials with extreme physical tunability have been studied for active control,such as phase-change materials,liquid crystals,and superconductors,however,whose modulation speed is typically less than several kilohertz.For improved modulation speed,Ⅲ-Ⅴ semiconductors with high electron mobility have been proposed to control MTMs with the compensation of high cost in material growth and device fabrication.In addition,graphene can provide high modulation speed and relatively low cost for active MTMs.However,it is challenging to obtain inch-scale uniform film for MTM arrays.Complementary metal oxide semiconductors（CMOS）have also been investigated to actively modulate THz MTMs with high uniformity in inch-scale size.However,their modulation depth is limited to several percent,and the CMOS fabrication is expensive.In sum,novel approaches with high uniformity,reasonable modulation depth,and low cost need to be developed for several inch-scale and flexible active MTMs and SSPP MTMs at microwave and THz frequencies.According to the research status of active MTMs and SSPP MTMs,novel active MTMs and active SSPP MTMs have been designed and verified in this thesis based on finite element method,micro/nano-fabrication technology and thin-film growth technology.First,a novel SSPP MTMs with single-loop split-ring resonators（SRRs）is proposed at millimeter wave frequencies,which can be used as a sensitive and damagefree biosensor for sucrose solutions and ovarian cancers.Next,an active modulation approach for SSPP MTMs based on IGZO SBDs is proposed to achieve transmission,reflection,and absorption for SSPP MTMs at millimeter wave frequency.Then,the multi-frequency multi-bit amplitude modulation of SSPP MTMs at millimeter wave frequency and active THz electric-field-coupled inductor-capacitor metamaterials（ELC MTMs）are further realizer based on the IGZO SBDs.Finally,a compact graphene-based SSPP MTMs with ultra-high Q-factor is proposed to achieve significant modulation of transmission and reflection.The main research contents of this thesis are as follows:1.The novel SSPP MTMs based on SRRs and its application in biosensor（1）A novel SSPP MTMs with single-loop SRRs is proposed at millimeter wave frequencies,achieving a miniaturized size of 0.052λ0×0.278λ0 at its resonant frequency.The electric size of the design is the smallest of the reported SSPP MTMs.In the proposed SSPP MTMs,the SRRs provide both a low-pass response as the conventional rectangular corrugations and an additional band-stop response induced by the resonance of SRRs.To verify this design,a back-to-back device with two co-planar waveguides as the input and output feeding was fabricated and characterized,the measured Sparameters of which agree well with the simulation.The measured stop band is centered at 49 GHz with a-10 dB bandwidth of 4.1 GHz and a high Q-factor of 93,in which the maximum attenuation is 31 dB.The filter has a low insertion loss of less than 1.6 dB in the pass band.band.The proposed SSPP MTMs may find many applications to achieve compact microwave and THz circuits due to its compact size,high Q-factor and low pass-band insertion loss.（2）The application of the proposed SRR-based SSPP MTMs in the field of biosensor is studies.The band stop response of the proposed SSPP MTMs comes from SRR resonance,and its frequency changes with the equivalent dielectric constant in the gap of the SRR capacitor.Meanwhile,the resonance enhances the localized field by a factor of 250 so that the proposed device is highly sensitive for detecting permittivity difference.In the detection for sucrose solutions with different concentrations,the resonant frequency is 48.2 GHz for the water,and becomes 49.6,50.75,and 51.9 GHz for the sucrose solutions with a concentration of 0.25,0.43,and 1 mg/mL,respectively.The frequency shift of the 0.25 mg/mL sucrose solution is 1.4 GHz with respect to the water,which is around 18 times larger than the data of the reported SRR-based biosensor.In the detection for ovarian cancers,the resonant frequency of the biosensor is 53.980 GHz originally,becomes 53.800 GHz with paraffin,and shifts to 53.85253.915,53.722-53.798,and 53.750-53.800 GHz with the normal,serous ovarian cancer,and ovarian clear cell carcinoma tissues,respectively.The average resonant frequencies are 53.8904 GHz,53.7680 GHz and 53.7770 GHz respectively.The corresponding frequency shift with respect to the original frequency is 90,212,and 203 MHz,respectively.According to the statistical results,the red shift of resonance frequency of cancerous ovarian tissue is about twice that of normal ovarian tissue.In addition,serous ovarian cancer tissues show greater resonance amplitude than clear cell carcinoma tissues.The proposed planar SSPP MTM biosensor reveals a fast,sensitive,label-free,and damage-free detection approach for sucrose solutions and ovarian cancers.2.The active modulation approach of SSPP MTMs based on IGZO SBDs（1）An active modulation approach based on monolithic IGZO SBDs is proposed,which enables a significant modulation of transmission,reflection,and absorption for SSPP MTMs at millimeter wave frequencies.The proposed device consists of single SRRs on an SSPP waveguide and IGZO SBDs fabricated to bridge the SRR split gaps.The IGZO SBDs can actively tune the film conductivity within the split gap so that the resonance magnitude of SRRs and the corresponding attenuation of SSPP MTMs are modulated.As verification,the characterized continuous modulation depth of transmission and absorption is 40%and 19%at 49 GHz,respectively,which is,to the best of our knowledge,the record for SPPs.Based on the averaged conductivity derived from the current-voltage measurement of the IGZO SBDs,finite element method（FEM）simulation reveals an ideal modulation range of 67%for SSPP transmission.（2）Multi-frequency multi-bit programmable amplitude modulation（AM）of SSPP MTMs is realized at millimeter wave frequencies with interdigital SRRs and IGZO SBDs.In the proposed device,interdigital capacitor substitutes for parallel plate capacitor to get larger AM spectral range without sacrificing the AM depth.To verify our design,a dual-frequency AM device was simulated,fabricated,and characterized.The whole device has a total length of 11.13 mm（about 1.38 wavelength at 37.4 GHz）,which is much smaller than the reported multi-frequency SSPP MTMs.By using two separate bias,the transmission of SSPPs can be programmably tuned from-12.5 to-6.2 dB at 34.7 GHz and from-26 to-8.5 dB at 50 GHz independently,showing significant AM at millimeter wave frequencies.The proposed method provides a novel insight into multi-frequency multi-bit programmable modulation of SSPP MTMs at microwave and THz regime.3.The active modulation approach of ELC MTMs based on IGZO SBDs（1）IGZO SBDs are proposed to reconfigure ELC MTMs and actively control THz waves for the first time.The SBDs are designed to bridge the capacitors of the ELC resonators so that the average conductivity within the capacitor gap can be modulated by bias voltage while keeping the capacitance value almost unchanged.The measured transmission shows a continuous modulation from-14.2 to-9.4 dB at 0.39 THz,which corresponds to a modulation depth of 14.3%.This work paves a new way for active THz MTMs using industrial compatible thin-film technology.（2）To precisely simulate the mechanism of active ELC MTMs electrically modulated by IGZO SBDs,two U-shaped resistive sheet models beside the gap are built for IGZO SBD in 3-D simulation for maintaining the same capacitance and resonant frequency.When the average conductivity changes from 7.9×10-6 to 0.23 S/m,the transmission can be modulated from-14.5 to-9.9 dB at 0.388 THz with little change of the resonant frequency,which is in accordance to the experimental results.4.The active modulation approach of SSPP MTMs based on grapheneA compact graphene-based SSPP MTMs with ultra-high Q-factor is proposed,achieving significant amplitude modulation of transmission and reflection by bias voltage.In the proposed device,two graphene sheets are designed in each SRR as load and connected to the SRR capacitor respectively.The square resistance of the graphene sheets can be actively tuned by the spin-coating LiClO4 ionic gel,and thus the amplitude modulation is accomplished.As the square resistance of graphene decreases from 2400 Ω/□ to 500 Ω/□,350 Ω/□ and 250 Ω/□,the transmission of the proposed graphene-based active SSPP MTMs at the resonant frequency also gradually decreases from-52 dB to-35.3 dB,-26.5 dB and-21.7 dB.As verification,the monolayer CVD graphene-based SSPP MTMs was fabricated and characterized.Moreover,LiClO4 ionic gel shows no effect on the transmission of the passive SSPP MTMs under bias voltage.The experimental results show an ultra-high Q-factor of 1471 near the Dirac point of graphene.Upon varying the bias from-1 to 2 V,the transmission and reflection are in good agreement with the simulation results.When V=5 V,a remarkable modulation of 15%and 44%without spectral shifting were observed for transmission and reflection,respectively.