Terahertz Metamaterial Sensor Based on Electromagnetically Induced Transparency Effect

Shao-Xian Li， Hong-Wei Zhao， and Jia-Guang Han

Terahertz Metamaterial Sensor Based on Electromagnetically Induced Transparency Effect

Shao-Xian Li， Hong-Wei Zhao， and Jia-Guang Han

—A terahertz metamaterial sensor adopting the metamaterial-based electromagnetically induced transparency （EIT） effect is presented for determining the 1，4-dioxane concentration in its aqueous solution. The metamaterial sensor， which consists of an EIT element unit with a cut-wire metallic resonator and two split-ring metallic resonators fabricated on a 490-μm thick silicon substrate， operates in a transmission geometry. The EIT peak was red-shifted and decreased with the increase of the water volume. A maximum redshift about 54 GHz of the EIT peak was detected between the 1，4-dioxane and water. The presented linear behavior and high sensitivity of the EIT peak depending on the water concentration pave a novel avenue for sensor applications.

Index Terms—Chemical and biological sensors，metamaterials， mode coupling， terahertz.

1. Introduction

Metamaterials are artificially structured media with unit cells much smaller than the operating wavelengths and exhibit exotic properties that are difficult to achieve with natural materials［1］-［4］. With the rapid expansion of research into metamaterials， enormous interest has been attracted to their practical applications from microwave， infrared to optical frequency regions. In the terahertz （THz） regime，developing novel metamaterial-based practical devices is also of great interest. Terahertz filters［5］，［6］， amplitude modulator［7］，［8］， phase controller［9］，［10］， switch［11］，［12］，lens［13］，［14］， waveplates［15］， and polarization rotators［16］have been proposed and demonstrated well. Electromagnetically induced transparency （EIT） is an important phenomenon in atomic physics and has many interesting properties， such as high dispersion in the narrow transparency window［17］，［18］. In 2008， a plasmonic metamaterial analogue of EIT was theoretically suggested by using coupled optical resonators［19］. Since then， many different structures exhibiting similar behavior with EIT have been designed［20］-［26］. In fact， the metamaterial-based EIT effect is the coupling results of bright modes and dark modes， and those modes are sensitive to the surrounding dielectric condition［27］. Here we propose a terahertz metamaterial liquids sensor based on EIT effect. Unlike many solid chemical compounds that possess characteristic THz fingerprint spectra， liquids always show broad and smooth feature in THz range， which makes it difficult for identification and quantitative analysis. Our approach can convert the characterless dielectric information of liquid into the frequency shift and amplitude change of the transmission peak and dips， which is easy for probing and tracking the difference among liquids. The proposed method is easy-to-fabrication， high efficient， fast， direct，and non-destructive.

2. Experiment

The unit cell of EIT sensor consists of a pair of split-ring metallic resonators （SRRs） symmetrically placed on the left and right sides of a cut wire （CW） and its geometry parameters are shown in Fig. 1. The 200-nm-thick aluminum metamaterial samples were fabricated on a 490-μm-thick N-type silicon substrate by conventional optical lithography. Taking a widely used solvent 1，4-dioxane as one example， we could show that how the proposed sensor worked at terahertz frequencies.

The 1，4-dioxane is a heterocyclic organic compound classified as an ether. It is used as a solvent in a wide range of industrial organic products （e.g.， paints， varnishes， inks，and dyes） and is also present as a byproduct in many consumer products （e.g.， cleaning products， cosmetics，shampoos， and laundry detergents）. The 1，4-dioxane is a weak genotoxin［28］. It is miscible with water in all proportions， moderately volatile， and also resistant to hydrolysis and microbial degradation［29］. Those properties make 1，4-dioxane easily release into environments andcause harm to humans and other creatures. Hence， efficient detection of 1，4-dioxane in water is necessary and important.

The 1，4-dioxane （99.5% and super dry） was purchased from J&K and used without further purified. The 1，4-dioxane was mixed with distilled water in different water volume fraction （0%， 20%， 40%， 60%， 80%， and 100%） thoroughly before measurement. A home-built photoconductive switch-based 8-F confocal terahertz time-domain spectroscopy （THz-TDS） system was employed to study the performance of the EIT sensor. A mode-locked Ti:sapphire laser （800 nm， 100 MHz， ＜35 fs，Mantis， Coherent， Inc.） was used as the light source to generate and detect the THz waves. The THz-TDS system was equipped with a GaAs photoconductive transmitter and a silicon-on-sapphire photoconductive receiver， covering the spectral range from 0.2 THz to 3.0 THz. Four parabolic mirrors were aligned in an 8-F confocal geometry， enabling excellent THz beam coupling between the transmitter and the receiver with a THz beam waist diameter less than 6 mm and a signal to noise ratio （SNR） about 10000:1. Samples were placed in the THz beam waist. The entire THz beam pathway was purged with dry air to keep a relative humidity below 2.0%. All experiments were performed at about 297.6 K.

Fig. 1. Schematic diagram of the EIT metamaterial senor with geometrical parameters: Px=106 μm， Py=125 μm， L=110 μm， w=10 μm， R=19.5 μm， r=14.5 μm， g=5 μm， δx=27.5 μm， and δy=28 μm.

3. Discussions

3.1 Sensor Characterization

Fig. 2 shows the measured and simulated THz transmission spectra of the EIT sensor， the sole pair of SRRs， and the CW. The simulation was carried on the CST microwave studio. The CW can be excited by the polarized electric field along the y axis （Ey）. It shows a resonance at 0.52 THz， serving as a bright mode. The SRRs cannot directly be excited by Eydue to its structural symmetry with respect to the y axis. While interacting with Ex， the SRRs are resonant at the same frequency as the CW， thus acting as a dark mode under Eyexcitation. Upon the excitation by Ey， the coupling between the bright modes and dark modes results in a “W” shape spectrum with a transmission peak at 0.52 THz. The experiments and simulations agree well as shown in Fig. 2. The amplitude differences can be attributed to the deviation in fabrication of the sample and short scans of THz time signal to avoid including reflective echoes.

Fig. 2. Experimental and simulated THz transmission spectra of EIT sensor and its two components: the CW and a pair of SRRs.

3.2 Sensing Performance

The sensor was inserted into a quartz cell filled with water/dioxane mixture. The water volume fractions were 100%， 80%， 60%， 40%， 20%， and 0%， respectively. The light path of the cell was 1 mm. An identical silicon inserted into the cell filled with the same solution was set as a reference. The experimentally measured transmission spectra are shown in Figs. 3 （a）， （b）， and （c）. The red-shift of the EIT peak occurred when the volume fraction of water increased. At the same time， the amplitude of the transmission peak decreased. The two transmission dips of EIT became less absorptive， which weakened the EIT effect. The CST simulation results in Figs. 3 （a’）， （b’）， and （c’）exhibit a similar trend. It should be noted that the difference between the experiment and the simulation probably came from the deviation of fabrication of the sensor and the clearness of the sensor.

The red-shift of the transmission peak was due to the increase of the refractive index around the metamaterial. As shown in Fig. 4， the refractive index and absorption coefficient of the mixture increase when the water volume fraction increases.

The EIT phenomenon was induced by the coupling of bright modes and dark modes. To explain the weakening of EIT effect， the transmission spectra of bright modes （CW）and dark modes （a pair of SRRs） in different water/dioxane mixtures were measured， respectively， as shown in Fig. 5.

Equally， the red-shift of the absorption was due to the increase of the ambient refractive index. The increased amplitude of the transmission dip showed a detuning of the original resonance and the diminution of Q value which resulted from the absorption of the water. The red-shift of the original resonance of the bright mode and dark mode led to the red-shift of the EIT transmission peak. The weakening of both modes resulted in the weakening of the EIT phenomenon.As shown in Fig. 6， the frequencies at the EIT peak and two transmission dips decrease almost linearly with the increase of water concentration. Accordingly， the water concentration of water/dioxane mixture can be deduced from the frequency shift of the EIT peak and the two transmission dips with the following derived formulas:

Fig. 3. Experimental （a）， （b）， and （c） and simulated （a’）， （b’）， and（c’） results of the EIT sensor in different water/dioxane mixtures.

Fig. 4. Experimentally measured refractive index and absorption coefficients of different water/dioxane mixtures.

Fig. 5. Experimentally measured THz transmission spectra in different water/dioxane mixtures: （a） CW and （b） a pair of SRRs.

Experiment: Simulation:

EIT peak: f=0.524-0.054c EIT peak: f=0.498-0.046c

Left foot: f=0.455-0.050c Left foot: f=0.426-0.041c

Right foot: f=0.566-0.059c Right foot f=0.569-0.043c，

where f is the frequency in unit THz and c is the volume fraction of water. A maximum red-shift about 54 GHz of the EIT peak was detected between 1，4-dioxane and water.

Fig. 6. Experimentally measured （full symbols） and simulated（half full symbols） frequencies at the EIT peak and two transmission dips in different concentration of water/dioxane mixtures.

4. Conclusions

In summary， a THz metamaterial sensor based on the EIT peak shift was presented. A maximum red-shift about 54 GHz of the EIT peak was detected between 1，4-dioxane and water. The linear dependence of frequency shift of the EIT peak on the water concentration makes the EIT sensor be a useful tool to fast determine the water concentration of unknown water/dioxane mixtures.

［1］ K. Fan and W. J. Padilla， “Dynamic electromagnetic metamaterials，” Materials Today， vol. 18， no. 1， pp. 39-50，2015.

［2］ K. Yao and Y.-M. Liu， “Plasmonic metamaterials，”Nanotechnology Reviews， vol. 3， no. 2， pp. 177-210， 2014.

［3］ N. Meinzer， W. L. Barnes， and I. R. Hooper， “Plasmonic meta-atoms and metasurfaces，” Nature Photonics， vol. 8， no. 12， pp. 889-898， 2014.

［4］ N. I. Zheludev and Y. S. Kivshar， “From metamaterials to metadevices，” Nature Materials， vol. 11， no. 11， pp. 917-924， 2012.

［5］ Z.-H. Zhu， X.-Q. Zhang， J.-Q. Gu， R. Singh， Z. Tian， J.-G. Han， et al.， “A metamaterial-based terahertz low-pass filter with low insertion loss and sharp rejection，” IEEE Trans. Terahertz Science and Technology， vol. 3， no. 6， pp. 832-837， 2013.

［6］ Q. Li， X.-Q. Zhang， W. Cao， A. Lakhtakia， J. F. O'Hara， J.-G. Han， et al.， “An approach for mechanically tunable， dynamic terahertz bandstop filters，” Applied Physics， vol. 107， no. 2，pp. 285-291， 2012.

［7］ Y.-M. Yang， R. Huang， L.-Q. Cong， Z.-H. Zhu， J.-Q. Gu， Z. Tian， et al.， “Modulating the fundamental inductivecapacitive resonance in asymmetric double-split ring terahertz metamaterials，” Applied Physics Letters， vol. 98，no. 12， pp. 121114， 2011.

［8］ Q. Li， Z. Tian， X.-Q. Zhang， N.-N. Xu， R. Singh， J.-Q. Gu，et al.， “Dual control of active graphene-silicon hybrid metamaterial devices，” Carbon， vol. 90， pp. 146-153， Aug. 2015.

［9］ L.-X. Liu， X.-Q. Zhang， M. Kenney， X.-Q. Su， N.-N. Xu，C.-M. Ouyang， et al.， “Broadband metasurfaces with simultaneous control of phase and amplitude，” Advanced Materials， vol. 26， no. 29， pp. 5031-5036， 2014.

［10］ X.-Q. Zhang， Z. Tian， W.-S. Yue， J.-Q. Gu， S. Zhang， J.-G. Han， et al.， “Broadband terahertz wave deflection based on c-shape complex metamaterials with phase discontinuities，”Advanced Materials， vol. 25， no. 33， pp. 4567-4572， 2013.

［11］ J.-Q. Gu， R. Singh， X.-J. Liu， X.-Q. Zhang， Y.-F. Ma， S. Zhang， et al.， “Active control of electromagnetically induced transparency analogue in terahertz metamaterials，” Nature Communications， vol. 3， pp. 1151， Oct. 2012.

［12］ X.-Q. Su， C.-M. Ouyang， N.-N. Xu， S.-Y. Tan， J.-Q. Gu， Z. Tian， et al.， “Broadband terahertz transparency in a switchable metasurface，” IEEE Photonics Journal， vol. 7， no. 1， pp. 5900108， 2015.

［13］ Q.-L. Yang， J.-Q. Gu， D.-Y. Wang， X.-Q. Zhang， Z. Tian，C.-M. Ouyang， et al.， “Efficient flat metasurface lens for terahertz imaging，” Optics Express， vol. 22， no. 21， pp. 25931-25939， 2014.

［14］ Q. Wang， X.-Q. Zhang， Y.-H. Xu， Z. Tian， J.-Q. Gu， W.-S. Yue， et al.， “A broadband metasurface-based terahertz flat-lens array，” Advanced Optical Materials， 2015， doi: 10.1002/adom.201400557

［15］ L.-Q. Cong， N.-N. Xu， J.-Q. Gu， R. Singh， J.-G. Han， and W.-L. Zhang， “Highly flexible broadband terahertz metamaterial quarter-wave plate，” Laser & Photonics Reviews， vol. 8， no. 4， pp. 626-632， 2014.

［16］ L.-Q. Cong， W. Cao， X.-Q. Zhang， Z. Tian， J.-Q. Gu， R. Singh， et al.， “A perfect metamaterial polarization rotator，”Applied Physics Letters， vol. 103， no. 17， pp. 171107， 2013.

［17］ K. J. Boller， A. Imamo?lu， and S. E. Harris， “Observation of electromagnetically induced transparency，” Physical Review Letters， vol. 66， no. 20， pp. 2593-2596， 1991.

［18］ S. E. Harris， “Electromagnetically induced transparency，”Physics Today， vol. 50， no. 7， pp. 36-42， 1997.

［19］ S. Zhang， D. A. Genov， Y. Wang， M. Liu， and X. Zhang，“Plasmon-induced transparency in metamaterials，” Physical Review Letters， vol. 101， no. 4， pp. 047401， 2008.

［20］ Z.-Y. Li， Y.-F. Ma， R. Huang， R. J. Singh， J.-Q. Gu， Z. Tian，et al.， “Manipulating the plasmon-induced transparency in terahertz metamaterials，” Optics Express， vol. 19， no. 9， pp. 8912-8919， 2011.

［21］ X.-J. Liu， J.-Q. Gu， R. Singh， Y.-F. Ma， J. Zhu， Z. Tian， et al.，“Electromagnetically induced transparency in terahertz plasmonic metamaterials via dual excitation pathways of the dark mode，” Physical Review Letters， vol. 100， no. 13， pp. 131101， 2012.

［22］ Y.-R. He， H. Zhou， Y. Jin， and S.-L. He， “Plasmon induced transparency in a dielectric waveguide，” Applied Physics Letters， vol. 99， no. 4， pp. 043113， 2011.

［23］ Z.-G. Dong， H. Liu， J.-X. Cao， T. Li， S.-M. Wang， S.-N. Zhu，et al.， “Enhanced sensing performance by the plasmonic analog of electromagnetically induced transparency in active metamaterials，” Applied Physics Letters， vol. 97， no. 11， pp. 114101， 2010.

［24］ T. Zentgraf， S. Zhang， R. F. Oulton， and X. Zhang，“Ultranarrow coupling-induced transparency bands in hybrid plasmonic systems，” Physical Review B， vol. 80， no. 19， pp. 195415， 2009.

［25］ R. D. Kekatpure， E. S. Barnard， W.-S. Cai， and M. L. Brongersma， “Phase-coupled plasmon-induced transparency，” Physical Review Letters， vol. 104， no. 24， pp. 243902， 2010.

［26］ D.-J. Meng， S.-Y. Wang， X.-L. Sun， R.-Z. Gong， and C.-H. Chen， “Actively bias-controlled metamaterial to mimic and modulate electromagnetically induced transparency，”Applied Physics Letters， vol. 104， no. 26， pp. 261902， 2014.

［27］ Y.-M. Yang， I. I. Kravchenko， D. P. Briggs， and J. Valentine，“All-dielectric metasurface analogue of electromagnetically induced transparency，” Nature Communications， vol. 5， pp. 5753， Dec. 2014.

［28］ J. A. Stickney， S. L. Sager， J. R. Clarkson， L. A. Smith， B. J. Locey， M. J. Bock， et al.， “An updated evaluation of the carcinogenic potential of 1，4-dioxane，” Regulatory Toxicology and Pharmacology， vol. 38， no. 2， pp. 183-195，2003.

［29］ J. H. Suh and M. Mohseni， “A study on the relationship between biodegradability enhancement and oxidation of 1，4-dioxane using ozone and hydrogen peroxide，” Water Research， vol. 38， no. 10， pp. 2596-2604， 2004.

Shao-Xian Li was born in Zhejiang Province， China in 1990. He received the B.S. degree from Tianjin University， Tianjin in 2012 in electronic science and technology（optoelectronics） and the M.S. degree from Tianjin University in 2015， in optical engineering. He is currently pursuing the Ph.D. degree with the College of Precision Instrument and Opto-electronics Engineering， Tianjin University. His research interests include terahertz applications in biochemistry and terahertz metamaterial sensor.

Hong-Wei Zhao received her M.S. and Ph.D. degrees in chemistry from Tongji University and Shanghai Institute of Applied Physics，Chinese Academy of Sciences （CAS） in 1998 and 2003， respectively. Since 2003 she has been with Shanghai Institute of Applied Physics， CAS as an associate professor. She was a visiting scholar at Rensselaer Polytechnic Institute Terahertz Center， USA during 2007 to 2008. Her current research interests include radiation biochemistry，terahertz technique and its applications.

Jia-Guang Han received the B.S. degree in material physics from Beijing Normal University， Beijing in 2000 and the Ph.D. degree in applied physics from the Shanghai Institute of Applied Physics， CAS in 2006. respectively. He was a visiting researcher at the Japanese High Energy Accelerator Research Organization during 2004 to 2005. From 2006 to 2007， he was a postdoctoral researcher at the School of Electrical and Computer Engineering， Oklahoma State University， Stillwater. In 2007， he joined the Department of Physics， National University of Singapore， Singapore， where he was a Lee Kuan Yew Research Fellow. He is currently a full professor with the College of Precision Instruments and Optoelectronics Engineering， and a member of the Center for Terahertz Waves， Tianjin University. His current research interests include surface plasmon polaritons， metamaterials， and material studies in the terahertz regime.

Manuscript received May 29， 2015； revised June 13， 2015. This work was supported by the National Basic Research Program of China under Grant No. 2014CB339800.

S.-X. Li and J.-G. Han are with the Center for Terahertz wave， Key laboratory of Opto-electronic Information Science and Technology，Ministry of Education， College of Precision Instrument and Optoelectronics Engineering， Tianjin University， Tianjin 300072， China（e-mail: lishaoxian31415926@aliyun.com and jiaghan@tju.edu.cn）.

H.-W. Zhao is with the Key Laboratory of Interfacial Physics and Technology， Shanghai Institute of Applied Physics， Chinese Academy of Sciences， Shanghai 201800， China （Corresponding author e-mail: zhaohongwei@sinap.ac.cn）.

Digital Object Identifier: 10.3969/j.issn.1674-862X.2015.02.006