Infrared Physics & Technology 53 (2010) 61–64

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Infrared Physics & Technology

journal homepage: www.elsevier .com/locate / infrared

Micro-fabrication and properties of the meta materials for the terahertz regime

Li Sheng, Zhang Huai-Wu *, Wen Qi-Ye, Song Yuan-Qiang, Xie Yun-Song, Ling Wei-Wei, Li Yuan-Xun, Zha JieState Key Laboratory of Electronic Thin Film and Integrated Devices, Chengdu 610054, Sichuan, PR ChinaUniversity of Electronic Science and Technology of China, Chengdu 610054, Sichuan, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 June 2009Available online 1 September 2009

Keywords:Micro-fabricationMeta materialTerahertz time domain spectroscopyCoupling the LC and dipole resonanceTransmission-line RLC circuit model

1350-4495/$ - see front matter � 2009 Published bydoi:10.1016/j.infrared.2009.08.009

* Corresponding author. Address: State Key Laboraand Integrated Devices, No. 4, Section 2, North JianshChina. Tel./fax: +86 28 83207063.

E-mail address: hwzhang@uestc.edu.cn (H.-W. Zha

Utilizing a series of micro-fabrication processes for terahertz meta materials, we fabricated three geom-etries of split-ring resonators: a circular geometry and two elliptical geometries. The samples were mea-sured by the transmission spectroscopy in the terahertz time domain. Comparing these spectra, we foundthat these plots clearly reveal the lower frequency LC resonances and the higher frequency dipole reso-nances, meanwhile, the coupling of the LC and dipole resonance has an important influence on the overallresponse. It is shown that we can design the LC and dipole resonance frequency, and regulate the cou-pling of the LC and dipole resonance to adjust the overall response. A simple transmission-line RLC circuitmodel is used to help us understand this coupling behavior and the extent of its effects.

� 2009 Published by Elsevier B.V.

1. Introduction

In the 1960s, the left-handed material (LHM) with simultaneousnegative permeability and negative permittivity was theoreticallypredicted by Veselago [1]. Latter, Pendry et al. [2] proposed twoartificial structures (meta materials), split-ring resonators (SRR)which exhibits a band of negative permeability, and wires whichprovide the negative permittivity in the vicinity of its resonant fre-quency. Following its initial theoretical [2] and experimental [3]introductions, meta materials research has attracted more andmore interests. While meta materials promise novel devices andinteresting science over very broad frequency bands [4,5], the ter-ahertz (THz) spectrum (0.1–10 THz, 1 THz = 1012 Hz, k = 30 lm to3 mm) represents a particularly interesting region [6]. Terahertzfrequencies cannot clearly be attributed to be either on the ‘‘elec-tronic” side or on the ‘‘optics” side. The THz frequency radiationhas been proven to be a fertile region in the electromagnetic spec-trum and a powerful tool in scientific research and many applica-tions [7,8]. Meta materials have the feature that is a suitablematerial from which to form the basic elements crucial to THztechnology implementation on a large scale. Additionally, theTHz regime also serves as a scale model to investigate the dynamicnature and limitations of higher-frequency meta material designs.Continuing THz meta materials research will become particularlyrelevant as new fabrication techniques and nanotechnology solu-tions continue to enable ever smaller resonator structures [6].

Elsevier B.V.

tory of Electronic Thin Filme Road, Chengdu 610054, PR

ng).

In this paper, we designed and fabricated three kinds of split-ring resonators including a circular geometry and two ellipticalgeometries, where the radius is stretched in the x-direction, andthe y-direction for the two sets, respectively. The samples weremeasured by terahertz time domain spectroscopy (THz-TDS). TheTHz transmission spectra of these samples were calculated fromthe terahertz time domain spectra. Comparing these spectra, wefound that these plots clearly reveal the lower frequency LC reso-nances and the higher frequency dipole resonances, and that thecoupling of the LC and dipole resonance has important influenceon the overall response. It shown that we can design the LC and di-pole resonances frequency, and regulate the coupling of the LC anddipole resonance for adjusting the overall response. A simple mod-el is used to help us understand this coupling behavior and the ex-tent of its effects.

2. Fabrication processes and measurement

All of the meta films studied in this work are based on theelectric-resonator design [6,14,15] in which the symmetry of thesplit-ring resonator is used to eliminate or minimize the magneticresponse. The meta material samples we studied are shown inFig. 1. All samples are comprised of plane periodic resonator arraysfabricated on intrinsic silicon substrates. We characterized threedifferent samples MM1, MM2, and MM3, all with the same line-width d = 4 lm, gap spacing g = 2 lm, and gap width w = 6 lm.The circle sample MM1 had an outer radius of 20 lm with latticeparameters Lx = Ly = 50 lm. The short ellipse sample MM2 haddimensions outer x-radius of 24 lm and y-radius of 11.5 lm withlattice parameters Lx = 60 lm, Ly = 26 lm, and the tall ellipse

Fig. 1. Split-ring resonator unit cells for planar meta material samples. Shadedregions indicate metallized areas.

0 500 1000 1500 2000 2500 30000.0

0.2

0.4

0.6

0.8

1.0

Nor

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ized

tran

smis

sion

Freq. (GHz)

Short Ellipse Circle Tall Ellipse

Fig. 3. Normalized transmission through meta film samples MM1, MM2, and MM3.

62 S. Li et al. / Infrared Physics & Technology 53 (2010) 61–64

sample MM3 had dimensions x-radius of 11.5 lm and y-radius of24 lm with lattice parameters Lx = 26 lm, Ly = 60 lm.

At first, we transferred MM1, MM2, and MM3 pattern to mask.Aluminum with a 210 nm-thickness was deposited on a 500 lm-thickness intrinsic silicon wafer by a radio frequency magnetronsputtering method. About 1 lm AZ-601 photoresist was coatedon the wafer by GKF-411 gluing purifying machine. To evaporatethe solvent and densify the AZ-601 film, put the wafers in hotplateat 70 �C persistence 5 min, and cool the wafer for 5 min. Mountthe mask and the wafer onto the mask aligner, the AZ-601 filmwas exposed by URE-2000S deep ultraviolet lithography exposureplane. Bake the AZ-601 on a hotplate at 70 �C persistence 5 min foracid-initiated, thermally driven epoxy cross-linking. Develop withRZX-3038 positive photoresists developer with agitation until thepattern is clear, rinse with deionized water. To evaporate deion-ized water and densify the AZ-601 film, put the wafers in hotplateat 70 �C persistence 5 min, following with 120 �C oven 10 min, andcool the wafer for 15 min. The wafer is etched by MNL/DIII reac-tive ion etcher, rinse with deionized water and bake with 120 �Coven 10 min. Strip photoresist with ORNIII 5532 plasma strippingphotoresist system. The wafer is sawed to 1 cm � 1 cm samplesby HP-603 automatic precision dicing saw. Those samples wereevaluated by OLYMPUS BX51M optical microscope and shown inFig. 2.

Experimental characterization was performed with terahertztime domain spectroscopy (THz-TDS) operating in a confocal trans-mission geometry. A detailed description of this system can befound in Refs. [9,10]. A linearly polarized THz beam was focusedto a spot approximately 3 mm in diameter and propagated nor-mally through the samples. The total sample area of the meta filmwas (1 � 1) cm2 to prevent beam clipping. Measurements wereconducted in a dry-air environment to mitigate the effects of water

Fig. 2. Photograph of me

vapor absorption. Transmission measurements were performed onthe meta film samples and, for reference purpose, a bare intrinsicsilicon substrate with the same thickness. Since the THz measure-ment is coherent, we directly record the time-varying electric fieldof the transmitted THz radiation following passage through thesample. With a fast Fourier transformation of these time-domainwaves, the frequency dependent amplitude of THz wave was ex-tracted from the time waves, which permits the extraction of thefrequency dependent complex transmission coefficient, ~tðxÞ ¼tðxÞe�j/ðxÞ.

3. Results and discussion

Fig. 3 shows the normalized frequency dependent amplitudetransmission coefficients, tmeas (x), obtained from our measure-ments. This data is normalized by dividing the measured transmis-sion spectra of the meta films, EMF (x), by the measuredtransmission spectrum of the reference substrate, ER (x), i.e.tmeasðxÞ ¼ j~tMeasðxÞj ¼ jEMFðxÞ=ERðxÞj. Two main resonances aretypically observed in terahertz SRR-based planar meta materials.The first is the low frequency inductive–capacitive (LC) resonancecaused by oscillating currents in the SRR loops and charge accumu-lation at the gap. The second is the higher frequency dipole (or cutwire) type electric resonance caused by interactions between theSRR sides or components parallel to the incident electric field[11–14]. The lengths of these sides determine the frequency ofthe dipole resonance. Typically, the LC and dipole resonances arenot widely separated in frequency, to the extent that they partiallyoverlap [14].

Fig. 3 shows that the LC resonances are actually displaced fromtheir apparent positions of 0.690, 0.855 and 0.867 THz for samplesMM1, MM2 and MM3. It is generally agreed that the gap providesthe majority of the capacitance C for the LC resonance, thoughother factors, such as SRR spacing, can also contribute to the capac-itance C. The inductive response is related to the area of the SRR

ta material samples.

Fig. 4. The transmission-line RLC model similar to Fig. 6b in Ref. [6] and Fig. 3 inRef. [14].

Table 1Circuit parameters for matching the TL-RLC model to test data.

Structure Circuit component

R1

(X)L1

(pH)C1

(fF)R2

(X)L2

(pH)C2

(fF)M(pH)

Shortellipse

15 98 0.352 37.5 16.8 0.2585 �12

Circle 12 131 0.398 12.5 24.05 0.395 �17Tall ellipse 7.4 69.9 0.452 89 63.8 0.2085 �20

Resistor values are given in Ohms, capacitor values in femto farads, and inductorsvalues and coupling coefficients in picohenries.

0 500 1000 1500 2000 2500 3000

0 500 1000 150

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

tran

smis

sion

Frequency(GHz)

TL-RLC Without R1 Without R2 Measured Data

Short Ellipse

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

tran

smis

sion

Frequen

Tall Ellipse

Fig. 5. The TL-RLC model results a

S. Li et al. / Infrared Physics & Technology 53 (2010) 61–64 63

[11–14]. Therefore, in an effort to minimize variances in the LCresonances caused by the drastic SRR reshaping, we designed allof the rings to have the same gap width and gap spacing. Becausethe gap width, the gap spacing and the area of the SRR of MM2 andMM3 are same, the LC resonances of MM2 and MM3 are similar.

Fig. 3 also shows that, the dipole resonances are actually dis-placed from their apparent positions of 1.73, 2.53 and 1.45 THzfor samples MM1, MM2 and MM3. It is generally agreed that thelengths of these sides determine the frequency of the dipole reso-nance [11–14]. The resonant frequency is approximately deter-mined by x � 2� Llm

ffiffiffiffiffiffiffiffieavgp� ��1 [11], where L is the lengths of

these sides and eavg is the average dielectric constant near thegap of the SRR. Because the lengths of the samples MM1, MM2and MM3 are changed distinguished, the dipole resonances aredistinguishing.

From Fig. 3, it is evident that the ellipse and circle resonatorsbehave quite differently. The most obvious difference is the separa-tion between the LC and dipole resonances. In sample MM2, thisseparation has increased to about 1.6 THz and the off-resonancetransmission stays above 80% (in amplitude) all the way out to1.5 THz. This is an improvement over the eSRR design, MM1, whichimmediately begins to lose transmission amplitude following theLC resonance. Sample MM3 shows the other extreme; the long sidearms of the eSRR push the dipole resonance even closer to the LCresonance frequency. This results in a sharp decay in transmissionand a strong reshaping due to resonance coupling [14], the trans-mission between LC resonances and the dipole resonances no morethan 75% (in amplitude), the transmission decrease sharply from1.0 THz and reach the least point at dipole resonances. From

0 500 1000 1500 2000 2500 3000

0 2000 2500 3000

0.0

0.2

0.4

0.6

0.8

1.0

Frequency(GHz)

Nor

mal

ized

tran

smis

sion

Measured Data TL-RLC Without R1 Without R2

Circle

cy(GHz)

Measured Data TL-RLC Without R1 Without R2

gainst the experimental data.

64 S. Li et al. / Infrared Physics & Technology 53 (2010) 61–64

Fig. 3, it shows that the coupling of the LC and dipole resonance hasimportant influence on the overall response.

To better understand the shifting and reshaping of the reso-nances due to coupling, we used the transmission-line and RLC cir-cuit (TL-RLC) model in Refs. [8,14] that describes the wavepropagation through our samples. Fig. 4 shows the meta materialarrays by two coupled lumped-element resonant circuit. R1, L1

and C1 represent the LC resonance, R2, L2 and C2 represent the di-pole resonance, M represent the coupling between the LC reso-nance and dipole resonance, ZSi = 109 X is the impedance of theintrinsic silicon substrate and Z0 = 377 X is the impedance of freespace. To match this model with the data, the values of R1, R2, L1,L2, C1, C2 and M shown in Table 1 were obtained by manual adjust-ment and observation of the result match to the test data. Themodel can also be used to decouple the LC and dipole resonancesby setting the coupling parameter M equal to zero, or to turn offa single resonance by setting either R1 ? 1 (without R1) or R2 ?1 (without R2) [14].

Fig. 5 compares the TL-RLC model results with the experimentaldata. The short ellipse is good over most of the data range and be-come inaccurate with the onset of high frequency resonances insample circle and tall ellipse. The reason is that high frequencieshave significant high-order modes [6]. By decoupling the LC and di-pole resonances it is clear that the individual resonance positionand strength are not equivalent to their apparent values in themeasured data. This is because LC-dipole resonance coupling dis-torts the overall response [14]. Table 1 shows the coupling param-eter M of short ellipse, circle and tall ellipse is �12 pH, �17 pH and�20 pH, respectively. Fig. 5 shows that decoupling the LC and di-pole resonance make the dipole resonance frequency large change,because the high frequency is more sensitive of coupling coeffi-cients. Comparing the short ellipse to the tall ellipse in Fig. 5, wehad seen that the more coupling coefficients the more change of di-pole resonance relatively to the separation between the LC and di-pole resonance while decoupling the resonances. It is clear that theLC-dipole resonance coupling plays an important role in the overallresponse.

4. Conclusion

A series of micro-fabrication processes for terahertz meta mate-rials are reported. These meta materials are measured using atransmission spectroscopy in terahertz time domain. By comparingthese spectra, it is found that these plots clearly reveal the lowerfrequency LC resonances and the higher frequency dipole reso-nances, and the coupling of the LC and dipole resonance has impor-tant influence on the overall response. It is showed that we can

designed the LC and dipole resonances frequency, and regulatethe coupling of the LC and dipole resonance for adjusting the over-all response.

Acknowledgments

This work was supported by the National Basic Research Pro-gram (973) of China, under Grant No. 2007CB310407, Foundationfor Innovative Research Group of the NSFC under Grant No.60721001, the International S&T Cooperation Program of China un-der Grant No. 2006DFA53410 and 2007DFR10250.

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