Strong dual band metamater ial absorber in terahertz regime

WEN Qi-ye,XIE Yun-song,ZHANG Huai-wu,YANG Qing-hui,L IU Bao-yuan

(State Key Laboratory of Electronic Film s and Integrated Devices,University of Electronic Science and Technology of China,Chengdu610054,China)

1 Introduction

Very recently,there has been considerable interest in the design and fabrication of a resonantmetamaterial(MT M)absorber in the microwave and terahertz band[1-5].By adjusting then(w)andz(w)of the metamaterial,a near unity absorption can be expected to be achieved[1,2]. Such“perfect”absorbers exhibit novel characteristics[6]and have potential applications as detectors for imaging,nondestructive detection,or as thermal emitters for spectroscopic and thermophotovoltaic applications.This concept is especially attractive at terahertz frequencies where a strong absorber is difficult to find in nature.Until now,most of the recent designs for perfect absorbers have only a single-band response.Accordingly the design and fabrication of multi-absorptive metamaterials are potentially useful and attractive.It is also a step to realize broadband THz absorber for microbolometers.

We present here the design and simulation of a THz absorber which has two near-unity absorption bands.The novel MT M absorber is composed of a bi-layer particle:an electric-field-coupled Split Resonance Ring(eSRR)layer and a metal plate layer which is separated by a thick polyimide spacer.This structure is similar to that reported in[3]except that a dual band eSRR resonator was incorporated.It is known that both the trans mission(T) and reflection(R) should be minimized to obtain the maximum absorption(A)whereA=1-T-R.The bottom metal plate is designed to be as thick as 800 nm,which is larger than the skin depth of the terahertz wave so that no trans mission will occur across the entire frequency range.Although the two resonances of eSRR have very different resonance strengths,they give rise to two near-unity absorption bands at about 0.5 THz and 0.94 THz.The mechanis m for the dual-band strong absorption is investigated in detail by studying the power flow and power loss in the absorber.

2 Simulation

The designed single unit of the absorber consists of two metallic elements: an electric-field-coupled eSRR structure and a metal plate bottom layer.The eSRR is composed of two symmetrical single-resonantmetamaterials(inner SRR and outer SRR)with two shared inter mediate girders(the width ist2),as shown in Fig.1(a).The two single-resonant parts have different split gaps and different inductive loops.The total structure is constructed on the GaAs substrate withε=12.9+0.077i,μ=1,and using polyimide withε=3.5+0.011i,μ=1 to separate the bottom plate and the eSRR from each other,as shown in Fig.1(b).Both the metal plate and the eSRR aregold,whoseconductance is 4.09×107S/m. The absorbance of the metamaterials is highly related to the parameters of the structures.Utilizing CST,a commercial electromagnetic simulating software based on the Finite Integration Theory(FIT)method,the structure of the absorberwas optimized to realize a dual-band strong absorption(simulating radiation atnor malincidence with the electric field perpendicular to the split gaps).The parameters of the structure in Fig.1(a)are optimized asa=60,b=136,d=4,h=26,l=28.6,t1=6,t2=4,t3=14 andt4=13. The number denotes the size in unit of micrometers.The period for the eSRR unit is 140μm ×120μm.In the simulations,the thickness of the two metal layers,polyimide spacer and the GaAs slab are 0.8μm,8μm and 10μm,respectively.

Fig.1 (a)Designed eSRR(b)Designed dual-band absorber with 800 nm gold SRR and ground plate,8μm polymide spacer and the substrate.

3 Results and discussion

Since the tran smission is zero across the entire frequency range due to the existence of the ground plate,we calculated the frequency dependence of the reflection parameter(S11) and consequently obtained the frequency characteristic of the absorption fromA=1-S211,as shown in Fig.2. It can be seen that there are two distinct absorptive peaks located at 0.502 THz and 0.942 THz,each with absorption over 99.99%.The fullwidth half absorption bandwidths are 20 GHz and 60 GHz for the lowfrequency and high-frequency peak,respectively.The absorption is near-zero in the frequency range below the first resonance and it is about 5% in the frequency band beyond the second peak. The absorption curve between the two peaks is flat with absorption of about 2%.All these results indicate a very good frequency agile or frequency selectivity of the dual-band perfect terahertz absorber.

Fig.2 Reflection(solid)and absorption(dash)curves of the dual-band absorber and the trans mission curve(short dash)of the eSRR.

A previous investigation of a terahertz absorber by Taoet alreported that in their designed structure,there is a secondary absorption in addition to a lower-frequency strong absorption[3]. It is revealed that the absorption peak at lower-frequency is associated with theLCresonance of the eSRR,which takes advantage of the resonant in-plane frequency response and exhibits a very strong absorption.However,the higher-frequency one is mainly related to the magnetic response between the resonator and the ground plate,giving rise to aweak absorption.Their results hint that the strong absorption of the absorber is ascribed to theLCresonance of the eSRR layer rather than the dipole resonance or magnetic resonance.

In order to clarify the origination of the dualband absorption,we also present the transmission characteristic of the eSRR in Fig.2.It is found that for the eSRR resonator,there are two resonances occurring at 0.525 THz and 0.95 THz,respectively.The low-frequency resonance is very strongwhile thehigh-frequency resonance is much weaker.Nevertheless,when the eSRR is integrated into an absorber with the polyimide spacer and the ground plate,both absorptions are strong and near unity.We note that the two resonances of our eSRR are derived from theLCresonance dueto thetwo different kinds of split gap in the structure,similar to the dual-resonant terahertz resonator reported previously[7,8].The two absorption peaks exhibit a slight“ red shift”from the resonance point of the eSRR,which probably due to the coupling between the eSRR layer and the ground metal plate.

Itmight be interesting to explore the power flow in the THz absorber since it could provide the infor mation about how and where the absorption occurs.For the first time,we present in Fig.3 the power flow of the THz absorber devices at the absorption frequency.Fig.3(a)and(c)show the powder flow streams from a view alongxdirection(inyzplane)andzdirection(inxyplane)for the lower-frequency absorption,while(b)and(d)are for the higher-frequency absorption.The long-dash line in Fig.3(a)represents the eSRR layer.We firstly discuss the low-frequency case. In the space far away from the absorber,the power flows are parallel streams.However,when the streams are close to the absorber,most of them flow“across”the eSRR layer,curl in the poly imide spacer and finally concentrate in the vicinity of the split gaps.Only the“central”streams with aspect to the eSRR plain go straight into the gap.From Fig.3(c),it is found that power flowsmainly concentrate on the two outer split gaps.However,the situation is much different for the high frequency absorption.The streams outside the eSRR area flow directly into the polyimide spacer,then curl and converge into the gaps,while those inside the eSRR area curl into a spiral path and then converge into the gaps instead of flowing across the eSRR layer.The detailed distribution of power flow,as shown in Fig.3(d),indicates that almost power flows mainly concentrate on the inner split gap for the high frequency case.These results hint that the terahertz absorber is an excellent power flow concentrator,which converges the incident power on the poly imide spacer and the eSRR layer,especially on the vicinity of the split gaps.

Fig.3 Distribution of the power flow in the absorber.(a)and(b)areyzplane for the low and high frequency absorption,(c)and(d)arexyplane for low and high frequency absorption.

Fig.4 Absorption distribution of the absorber.A birds eye view for(a)low-frequency(b)high-frequency absorptions,and a side view for(c)low-frequency and(d)high-frequency absorptions.

To further understand the nature of the dualband absorber,the distribution of the power-loss(absorption)of the THz wave were calculated and compared for thetwo absorptions,as shown in Fig.4.Fig.4(a)and(b)show the projection of the power-loss on thex-yplane.A side view of the typical distribution of power loss density,as plotted in Fig.4(c)and(d),provides more detailed infor mation about the absorption in different layers.Obviously,the power is completely limited in the eSRR layer and the polyimide spacer. In the lowfrequency absorption,the power loss mainly takes place in the outer split rings and the polyimide slab with the strongest absorption at the outer split gap.For the high-frequency absorption,mostof the power loss occurs at the fringe of the inner SRR with only a s mall partial at the outer split gap.Since neither the eSRR layer nor the polyimide spacer can induce such a large absorption,it is probably the coupling between the eSRR layer and the metal plate that enhances the power loss to a large degree[6].However,we can conclude that the absorber acts as a strong powerconcentrator. The electromagnetic waves are firstly converged into the spacer and the eSRR and then are significantly absorbed.The split gaps of the eSRR play a very important role in the absorption behaviour including the frequency,place and strength where the absorption occurs.

4 Conclusions

We have presented a design of dual-band perfect terahertz absorber composed of electric Split-Resonance-Ring(eSRR) layer,polyimide spacer and a metal plate layer.The absorber has two nearunity absorptions near 0.502 THz and 0.942 THz,both are related to the LC resonance of the eSRR structure.The absorptive mechanism is theoretically investigated by simulating the power flow and power loss in the absorber. The results show that the designed terahertz absorber concentrates the incident terahertzwave on the polyimide spacer and the eSRR layer,and the coupling between the eSRR layer and the metal ground plate induces a strong absorption.

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