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1、Development of Chiral Negative Refractive Index Metamaterials for the Terahertz Frequency Regime3052IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION,VOL.55,NO.11,NOVEMBER2021 Development of Chiral Negative Refractive Index Metamaterials for the Terahertz Frequency Regime Nantakan Wongkasem,Member,IEEE,
2、Alkim Akyurtlu,Member,IEEE,Kenneth A.Marx,Qi Dong,Jin Li,andWilliam D.Goodhue,Senior Member,IEEEAbstractA novel negative refractive index(NRI)chiral meta-material(MTM),based on the Y structure,has been designed and tested in the microwave and terahertz frequencies.In addition to providing magnetoele
3、ctric coupling,this MTM has a negative index of refraction passband that can be tuned in both the fre-quency of operation and bandwidth with lower losses compared to other known chiral structures.Group theory was used to analyze the magnetoelectric coupling of the Y-shaped structure and circuit anal
4、ysis was used to aid in the design of the NRI material and full-wavenite difference time domain(FDTD)simulations were conducted to determine the transmission characteristics of the material.Wedge-and prism-shaped models comprised of the designed structures were simulated to validate the NRI behavior
5、 and were then compared to experimental results in the microwave regime.Furthermore,the Y-shaped design was fabricated in the THz regime and the co-and cross-polarized transmission coef-cients were determined from experiments and were compared to numerical results.Index TermsChiral,metamaterials,mic
6、rowave,negative refractive index,terahertz.I.I NTRODUCTIONA RTIFICIAL chiral media composed of randomly orientedhelices or other left-right asymmetric(chiral)inclu-sions(with sizes typically less than a wavelength)embedded within an achiral background medium belong to a class of optical metamaterial
7、s(MTM).Most electromagnetic design approaches which try to eliminate the cross-polarization,the form of handedness,sensitivity of rotation to the polarization state,and elliptization of visible light diffracted from the chiralManuscript received August15,2021;revised July2,2021.This work was support
8、ed in part by the University of Massachusetts,Lowell,Chancellors Seed Funding.N.Wongkasem was with the Department of Electrical and Computer Engi-neering,University of Massachusetts,Lowell,MA01854USA.She is now with the Department of Electrical Engineering,Khon Kaen University,Khon Kaen 40002,Thaila
9、nd(e-mail:nantakankku.ac.th).A.Akyurtlu is with the Department of Electrical and Computer Engi-neering,University of Massachusetts,Lowell,MA01854USA(e-mail: Alkim_Akyurtluhttp:/ is with the Department of Chemistry,University of Massachu-setts,Lowell,MA01854USA(e-mail:Kenneth_Marxhttp:/ was with the
10、Department of Physics,University of Massachusetts, Lowell,MA01854USA.J.Li is with the Department of Physics,University of Massachusetts,Lowell, MA01854USA(e-mail:Jin_Lihttp:/ is with the Department of Physics,University of Massachu-setts,Lowell,MA01854USA and also with the Lincoln Laboratory,Mass-ac
11、husetts Institute of Technology,Lexington,MA02420-9108USA(e-mail: William_Goodhuehttp:/ versions of one or more of thegures in this paper are available online at http:/ Object Identier10.1109/TAP.2021.909419structures.However,these properties can be advantageous when used in certain optoelectronic a
12、pplications1,2.The polarization changes in chiral materials are strongly correlated to the degree of chirality of the materials which is found as the distinction between right-and left-handed materials.Chiral resonance can lead to negative refraction of one polarization, resulting in improved and si
13、mplied designs of negatively refracting materials3.These NRI chiral MTMs may greatly benet novel optical applications at visible and near infrared (IR)frequencies in areas such as drug discovery4,among others.In this paper,we propose a new chiral structure based on Y-shaped resonators with an NRI ba
14、nd which can easily be tuned in both the frequency of operation and bandwidth.More-over,the NRI bands can be generated over a broad frequency range with a lower loss than is known for other chiral structures that have been previously examined(e.g.,omega5and gam-madion68).The design methodology for t
15、he development of the novel chiral metamaterial is presented in detail.Speci-cally,group theory is used to derive material properties of these designs.A comprehensive study using realistic models for the materials via thenite-difference time-domain(FDTD)method is conducted.The proof of the negative
16、index of refraction of the proposed structure is presented both computationally and experimentally.For the former,a wedge shaped model9and a prism shaped model10,11comprised of Y structures is used to demonstrate the negative refraction using FDTD simulations. For the latter,measurements on a fabric
17、ated wedge comprised of the Y structure were conducted in the microwave range to vali-date the existence of negative refraction.Moreover,planar meta-material samples based on the Y structure design were fabri-cated in the THz regime and the co-and cross-polarized electric elds from experiments were
18、compared with numerical results. The complementary synthesis and modeling techniques applied to the novel chiral NIR metamaterials provides a guide to the optimization of metamaterial designs at terahertz and higher frequencies.II.S TRUCTURE D ESIGNA.Classication of Chiral and Bi-Anisotropic Propert
19、ies by Group TheoryIn this section,the material properties of the aforementioned chiral metamaterials will be studied via group theory.Group theory,a formalism used to classify the symmetry of molecules, has been used in a limited fashion to identify the isotropy of magnetic resonators of metamaterials1214.It has also been0018-926X/$25.002021IEEE