Optically controlled background-free terahertzswitching in chiral metamaterial

T. T. Lv,1,† Z. Zhu,1,† J. H. Shi,1,2,* C. Y. Guan,1 Z. P. Wang,1 and T. J. Cui2,31Key Laboratory of In-Fiber Integrated Optics of Ministry of Education, College of Science,

Harbin Engineering University, Harbin 150001, China2State Key Laboratory of Millimeter Waves, Southeast University, Nanjing 210096, China

3e-mail: tjcui@seu.edu.cn*Corresponding author: hrbeusjh@gmail.com

Received February 21, 2014; revised April 14, 2014; accepted April 17, 2014;posted April 18, 2014 (Doc. ID 206887); published May 15, 2014

We demonstrate a multiband background-free terahertz (THz) switch in photoactive chiral metamaterialusing polarization conversion. Orthogonal arrangement of two asymmetrical split-ring apertures allows a highpolarization conversion efficiency and low copolarization transmission. The chiral metamaterial embedded withphotoactive silicon promises a dynamic control on cross-polarization transmission and thus enables an efficientbackground-free THz switch. The on/off state of THz metamaterial switching can be efficiently controlled by anoptical pump. The realization of a cross-polarization THz switch provides a new mechanism of mode switchingto control THz wave propagation and will be a promising candidate for polarization devices. © 2014 OpticalSociety of AmericaOCIS codes: (160.3918) Metamaterials; (260.5430) Polarization; (160.1585) Chiral media; (260.5150) Photoconductivity.http://dx.doi.org/10.1364/OL.39.003066

Terahertz (THz) radiation occupies a large portion of theelectromagnetic spectrum between the microwave andIR frequencies. However, most conventional materialshave very weak responses to THz radiation. The adventof novel functional materials and technological innova-tions offers many opportunities to greatly enhance theinteraction between THz waves and materials and easilymanipulate the THz radiations [1,2]. Recently, THz tech-nology has received rapidly growing attention due to nu-merous important applications, such as information andcommunication technology, biological sensing, analyticalchemistry, and imaging. As promising candidates, newlyinvented metamaterials can provide unprecedentedmanipulation of THz radiation [3–6].Metamaterials with artificial subwavelength elements

can achieve unusual electromagnetic properties thatare inaccessible in naturally occurring materials, for in-stance, negative refraction and invisibility. Remarkably,metamaterials can be tailored to operate anywhere frommicrowave to optical frequencies. A series of metamate-rials have been proposed to realize many interestingphenomena in the THz regime, including artificial mag-netism [3], negative refractive indices [7], asymmetrictransmission [8], and anomalous refraction [9,10]. Fillingthe so-called THz gap, metamaterial-based devices havebeen demonstrated to efficiently modulate propagationof THz waves [11–22]. In particular, using active THzmetamaterials, i.e., controlled by external stimuli viaphotoexcitation, electric bias, temperature, and micro-electromechanical systems, is now an intense subject[4,5,14–22], as they are capable of dynamic and flexiblemodulation of THz waves. Such metamaterials weregenerally constructed by being incorporated with photo-active materials. An optical pump source was used toexcite photocarriers and dynamically change the conduc-tivity of photoconductive semiconductor silicon. Mostmetamaterials modulate, filter, or switch THz wavesbased on copolarization transmission in the absence ofpolarization conversion [4,5,11,14–20]; however, they

will suffer from copolarization background noises.There is an alternative route to realize manipulation ofTHz waves in terms of cross-polarization transmission.Obviously, this scheme is background-free, since thedesired output signal can be separated from the opticalbackground. On the other hand, efficiently controllingthe state of polarization is of great importance in manyapplications. Therefore, the cross-polarization manipu-lation of THz waves in metamaterials is promisingand significant for realizing novel and practicalapplications.

In this Letter, we demonstrate a new mechanism ofmode switching to control the THz wave propagation.Here, we create an ultrathin switchable THz metamate-rial constructed of an array of 90°-twisted asymmetricallysplit ring apertures (ASRAs) with incorporated photo-conductive silicon. The orthogonal arrangement of twostacked resonators makes the metamaterial chiral dueto the cross coupling between the magnetic and electricresponses. At normal incidence, the anisotropic chiralmetamaterial enables a broadband polarization conver-sion, with the copolarization transmission being totallysuppressed. The hybridized chiral metamaterial allowsus to effectively tune the cross-polarization transmissionby controlling the conductivity of the active materialusing external optical stimuli. Multiband switching func-tionality based on polarization conversion is achievedby dynamically controlling the optical pump-dependentconductivity of silicon. Such dynamic control of activemetamaterial could be implemented in the THz regimeto achieve diverse functionalities, such as switches,filters, and modulators.

The electromagnetic response of metamaterials is notonly directly determined by the structural design of meta-molecules, but it also strongly depends on their spatial ar-rangement. The bilayered asymmetrically split ring (ASR)metamaterials can be well engineered via the twist angle,exhibiting Fano resonance, the fast roll-off pass band, ormultiband cross-polarization transmission [23,24].

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In this work, we propose a novel (to our knowledge)hybridized chiral metamaterial composed of an array ofsquare stereo ASRA dimers with incorporated silicon,sketched in Fig. 1. The negative ASRA design has one ma-jor advantage over the positive ASR design: the activemedium is easily incorporated into the voids, thus facili-tating the dynamic control of THz devices or the detec-tion of surrounding environment change. Each stereoASRA dimer consists of two spatially separated coaxialASRAs perforated through gold films with a thickness oftm � 200 nm. The period of perforation is d � 100 μm.The two ASRAs are geometrically identical, but the backlayer is twisted by 90° with respect to the front one. Thetwo ASRAs are spatially separated by a thin dielectriclayer of polyimide with a thickness of t � 16 μm EachASRA consists of two different arc slits corresponding toopen angles α � 90° and β � 160°, as shown in Fig. 1(a).The radius of the ASRA is r � 42.5 μm, and the split widthis w � 10 μm. In order to realize an active THz metama-terial, silicon is incorporated into two short apertures ofα � 90°. The thickness of silicon is identical to that ofgold films, i.e., 200 nm. Each ASRA can be regarded asa combination of a metallic layer with silicon and air arcscorresponding different arc angles. For simplicity, westudy the freestanding THz metamaterial, since one canneglect the insignificant impact of the dielectric substrate.For other dielectrics or substrates, good performance canbe obtained by optimizing the geometrical structures ofthe ASRAs. Under an external optical stimulus, siliconincorporated into the chiral metamaterial will exhibitan insulator-to-metal transition due to the excitation ofthe semiconductor carrier, thus modulating the associ-ated cross-polarization transmission. The transmissionproperties of bilayered metamaterials were studiedfor linearly polarized waves at normal incidence in the

frequency range from 0.7 to 1.7 THz. Numerical simula-tions were performed by use of the commercial softwareCST Microwave Studio. Here, gold can be treated as alossy metal with its conductivity of 4.561 × 107 S∕m,and the polyimide spacer is taken as lossy dielectric withεp � 2.4� 0.005i. For the photoactive silicon parts, semi-conductor silicon is modeled with εsi � 11.7, and the cor-responding conductivity σsi is determined by opticalpump levels. For the case without light illumination, σsiis chosen to be 1 S∕m. Practice applications will be takeninto account, and the silicon conductivity can be assumedto be as high as an order of 105 S∕m [15]. The calculatedresults can be presented in terms of transmission ampli-tudes tij in a Jones matrix. The subscripts i and j corre-spond to the polarization states of the transmitted andincident waves, which could be either x or y linearlypolarized waves in our case.

Figures 2(a) and 2(b) show the simulated opticallycontrolled cross-polarization and copolarization trans-mission spectra, respectively, of the proposed THz meta-material switch for the y-polarized wave propagatingalong z direction. The optical control of the THz metama-terial can be implemented using an amplified kilohertzTi:Sa laser system delivering 35 fs pulses at 800 nmcentral wavelength and maximum energy of 2.3 mJ perpulse [18], which can sufficiently excite photocarriers inthe silicon. The photoexcitation occurs at about 5–10 psbefore the arrival of the THz beam. This delay may assurea quasi-steady state for the charge carriers of silicon,since the lifetime (a few hundred nanoseconds) is ordersof magnitude longer than the picoseconds duration of the

Fig. 1. Schematic of an active THz metamaterial switch.(a) The front view of a unit cell in the bilayered chiral metama-terial, (b) the stereogram of a unit cell, and (c) the schematicprinciple of the optically controlled THz switch based on polari-zation conversion. Photoconductive silicon (purple part) isfilled into two short apertures of the gold film (gray part),and the dielectric spacer is polyimide (light blue part).

Fig. 2. Simulated transmission spectra of THz metamaterialswitching. (a) Cross-polarization transmission jtxyj and (b) co-polarization transmission jtyyj as a function of siliconconductivity. (c) Simulated polarization rotation angle θ andellipticity η.

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THz pulse. The different conductivity values can be usedto model the increasing intensity of external opticalstimuli. In Fig. 2(a), σsi may be seen as 1 S∕m lackingof light illumination. For the cross-polarization transmis-sion, three resonant peaks occur around at the frequen-cies of 0.90, 1.16, and 1.52 THz. Resonance III has atransmission maximum jtxyj � 73.7%, while resonancesI and IV have intermediate cross-polarization transmis-sions jtxyj � 55.2% and jtxyj � 61.9%, respectively. As theconductivity of silicon increases, resonances I and IVrapidly weaken and eventually vanish, while resonanceIII slowly weakens. Obviously, both resonances I andIV are in the “OFF” state when the THz metamaterial isilluminated by strong optical pump. When σsi exceeds40; 000 S∕m, corresponding to the level with an energyflux of 155 μJ∕cm2 for optical pump, a pronounced res-onance II emerges at 1.00 THz. As σsi further increases,the cross-polarization transmission is up to 54.2% andresonance II is in the “ON” state. At resonance III, thecross-polarization transmission increases and reaches70.7% for strong pump, equivalent to the case of no pump.All resonant frequencies remain unchanged regardlessof optical pump. In particular, the THz metamaterialexhibits a photoexcited mode-switching effect betweenresonances I and II, and the tunability of the resonantfrequency is about 11%. More importantly, the copolariza-tion transmission is totally suppressed below 1% in thefrequency range of 0.7–1.7 THz due to the orthogonalarrangement of bilayered structures regardless of opticalpump, as shown in Fig. 2(b). Therefore, the transmittedwave in our proposed metamaterial exhibits relatively highpurity of polarization that is different from the input signaland optical background, which is also verified by thepolarization rotation angle and ellipticity in Fig. 2(c) [25].To identify the underlying mechanism of the observed

polarization effect, the Born–Kuhn model can be re-called, in which two charged oscillators couple with eachother [26]. Excitation of one of them by the incidentelectromagnetic wave is then transferred by the elasticcoupling to the other. Similarly, current driven aroundthe ASRAs by the incident wave is electromagneticallycoupled to the current around the ASRA of the secondlayer. The induced current in the second layer is thenre-emitted into the transmitted wave with a differentpolarization state. The orthogonal arrangement of twoASRAs allows a high polarization conversion efficiencyand low copolarization transmission. The chiral metama-terial embedded with photoactive silicon promisesdynamic control on the cross-polarization transmissionand thus enables an efficient background-free THzswitch. To further investigate the physical origin of thephotoexcited mode-switching effect, Fig. 3 presentsresonant modes of the metamaterial at the cross-polarization transmission peaks for the two cases of nopump and strong pump. For a y-polarized wave, thein-plane magnetic dipole can be excited along the ASRA,manifested by instantaneous surface current distribu-tions. The excited magnetic dipole in the back layer isrotated by 90° with respect to the one in the front layer,resulting in polarization conversion. With no pump, sili-con has a low conductivity, and excitations of both shortand long slot antennas contribute to the first transmis-sion peak I, while the other transmission peaks III and IV

are individually dominated by excitations of long andshort slot antennas. With strong optical pump, siliconhas a high conductivity, and no induced surface currentsoccur around silicon parts. Both the two transmissionpeaks II and III are dominated by excitations of longslot antennas; however, after 90° rotation, in-planemagnetic dipoles are antisymmetric at resonance IIand symmetric at resonance III. Therefore, a photoex-cited mode-switching effect can be achieved in theproposed metamaterial.

In addition to the conductivity of silicon, the interlayercoupling is also important for modulating the perfor-mance of the THz switching. When geometrical parame-ters are kept unchanged, we can consider how theinterlayer coupling strength affects the cross-polarizationtransmission by varying the thickness of the dielectricspacer layer. Increasing the thickness of the polyimidespacer layer corresponds to increasing the permittivityof the dielectric layer with a constant thickness. Figure 4presents the simulated results of the optically controlledTHz switching at resonant frequencies of 0.90 (I), 1.00(II), and 1.52 THz (IV) as a function of the thickness

Fig. 3. Distributions of surface current densities at resonances(a) I, (b) III, and (c) IV for the case of no pump (correspondingto σsi � 1 S∕m) and (d) II and (e) III for the case of strong pump(corresponding to σsi � 105 S∕m).

Fig. 4. Engineering optical control efficiency of the THzswitching for resonant modes (a) I, (b) II, and (c) IV by varyingthe dielectric layer thickness.

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of the dielectric layer. Obviously, the triple-band cross-polarization transmissions can be switched on/off throughoptical control of silicon. With an increase in the conduc-tivity of the silicon, the cross-polarization transmissionsare switched off in the first and third bands and switchedon in the second band. The appropriate thickness enablesgood switching performance with a giant modulation ofthe cross-polarization transmission. It is noted that theresonant frequency of resonance I is more sensitive to thespacer layer thickness than those of resonances II and IV.Therefore, the optical control efficiency can be engineeredvia the thickness of the dielectric layer.In summary, we present the first demonstration of a

multiband background-free THz switch in a photoactivechiral metamaterial based on polarization conversion.The chiral metamaterial embedded with photoactivesilicon promises dynamic control on cross-polarizationtransmission and thus enables an efficient background-free THz switch. The THz metamaterial exhibits a photo-excited mode-switching effect, and the tunability of theresonant frequency is about 11%. The transmitted wave inour proposed metamaterial exhibits relatively high purityof polarization that is different from the input signal andoptical background. The physical origin of the photoex-cited mode-switching effect can be well understood bysurface current distributions at resonant modes of thecross-polarization transmission peaks. Further, the opti-cal control efficiency can be engineered via the thicknessof the dielectric layer. The proposed THz metamaterialsare beneficial in designing polarization devices and offerconsiderable flexibility in dynamic control on THz andoptical wave propagation.

This work is supported by the National Natural Sci-ence Foundation of China under Grant Nos. 61201083,61275094, U1231201, and 613111156, and in part by theChina Postdoctoral Science Foundation under GrantNos. 2012M511171 and 2013T60487, the SpecialFoundation for Harbin Young Scientists under GrantNo. 2012RFLXG030, the Fundamental Research Fundsfor the Central Universities, and the 111 Project underGrant No. B13015. H. F. Ma and T. J. Cui acknowledgethe support from the National Natural Science Foundationof China under Grant Nos. 61171024, 61171026, 60990320,and 60990324, the National High Tech (863) Projectsunder Grant Nos. 2011AA010202 and 2012AA030402,and the 111 Project under Grant No. 111-2-05.†T. T. Lv and Z. Zhu contributed equally to this work.

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