THz polarization control with chiral metamaterials

Transcription

1 THz polarization control with chiral etaaterials M. Kafesaki, G. Kenanakis,. N. conoou and C. M. Soukoulis Foundation for Research & Technology, Hellas (FORTH), Crete, Greece, & University of Crete, Greece Aes Lab & Iowa State University (ISU), USA Artificial, structured (in subwavelength scale) aterials with unique electroagnetic properties (not encountered in natural aterials) lectroagnetic properties derive fro shape and distribution of building blocks (usually etallic) Giant to negative electrical perittivity Giant to negative pereability Unusual anisotropy Giant chirality, Possibility to engineer electroagnetic properties 1

2 Why THz? Unique for security and sensing Security Drug detection Picture fro Qinetiq Kodo Kawase group, Japan Molecular rotations Hydrogen Bonds, Torsions, collective vibrational odes Plastic, paper, clothes are transparent in the THz region Many aterials exhibit spectral fingerprints in the THz region Slide by S. Tzortzakis Passive and dynaic THz polarization control? Polarization THz optics (filters, polarizers, waveplates, ) Polarization dynaical odulators Polarization THz switches Zheludev s group Possible with chiral etaaterials! For dynaic coponents: Chiral etaaterials cobined with photoconducting seiconductors 2

3 Outline Introduction to chiral etaaterials Our flexible THz chiral structures Towards tunable/switchable chiral etaaterials for dynaic polarization control Structures of reduced syetry: additional polarization control capabilities (asyetric transission) Chiral etaaterials Chiral structure: not-identical to its irror iage + D 0 i( / c) H B H i( / c) 0 Magnetoelectric coupling - p igenodes: circularly polarized waves ( xˆ iyˆ) n Different index for left- and right-handed circularly polarized waves Alternative path to achieve negative index (Pendry, Tretyakov) 3

4 Chiral etaaterial interesting effects Optical activity (polarization rotation) Rotation angle 1 = [arg( T+ ) arg( T- )] 2 T + : Transission of right-handed circularly polarized waves (T - : of left-handed) Circular dichrois (different abosrption for left- and right-handed circularly polarized waves) Degree of ellipticity η= 1 sin -1 2 T T +T T o η = 0: linear, η = 45 : circular Bilayer planar chiral structures Pair of conductors utually twisted etal etal etal Chiral response: due to electroagnetic coupling of back and front conductor Advantages: asy fabrication with lithographic techniques Multiple low frequency resonances associated with strong chiral response Southapton, Bilkent, KIT, ISU, FORTH, In GHz Negative index Large polarization rotation Large circular dichrois 4

5 Our designs (unit cells) Yellow: etal (a) (b) (c) (d) (e) µ scale fabricated by UV lithography (MRG-FORTH) Pair of conductors encapsulated in polyiide ebrane (thickness ~12 µ) G. Kenanakis et. al, Opt. Mat. xpr. 2, 1702 (2012) Basic principle of bilayer chiral structures p j H j Magnetic dipole oent p lectric dipole oent p j p 5

6 Designs and linear transission results Siulation xperient Total saple area 15 x 15 Unit cell ~20 µ. Thickness ~12 µ Solid lines: co-polarized T++ T 1 T +T +it -T T -T -it +T + = Dashed lines: cross-polarized T T T 2 T -T +it +T T +T -it -T xx yy xy yx xx yy xy yx + xx yy xy yx xx yy xy yx Characterized by FTIR spectroscopy sending linearly polarized waves, i.e. obtain T xx, T yx Kenanakis et al, Opt. Mat. xpress 2, 1702 (2012) Cross-pair transission for linear polarization Siulation xperient,x Co-polarized Cross-polarized Co-polarized Cross-polarized H, y Large cross-polarization 6

7 Cross-pair chiral features Optical activity llipticity Very large optical activity (70 o ) with negligible ellipticity Cross-pair refractive indices n +, n_ Negative index for both circular polarizations Index close to zero with high transittance 7

8 Inverse gaadion odified: Transission Unit cell 26 x 26 µ Metal thickness=spacer thickness= 0.5 µ Total thickness =12 µ Siulation Co-polarized Cross-polarized xperient Co-polarized Cross-polarized Inverse gaadion propagation features ~90 o optical activity with negligible ellipticity and high transittance Optical activity llipticity 8

9 U-SRR circular transission features Narrow-band circular polarization filter Optical activity llipticity Towards tunable/switchable chiral etaaterials Approach: Introducing a photoconducting seiconductor layer (here Si) between the two conductors of the planar chiral structures Tuning/switching by photoexcitation Metal +SiO 2 Silicon (500 n) Metal +SiO 2 & Si 3 N 4 Si Vertical profile Sapphire Fabrication by MRG (G. Deligeorgis), FORTH 9

10 Cross-pair chiral features vs Si conductivity Optical activity Optical activity llipticity llipticity Switchable ellipticity Switchable circular polarizer possibility Significant change of ellipticity at the structure resonances The structures S. He group, APL 2010 Ar length = 16 icrons Ar width = 3 icrons Unit cell (a X =a Y ) = 17 μ Silicon thickness= 0.5μ Metal thickness= 1.5μ Fabrication by MRG (G. Deligeorgis), FORTH 10

11 Bianisotropic structures Beyond chirality lectric field induces agnetic polarization of arbitrary direction Additional polarization control capabilities D 0i( / c) H B H i( / c) Our 3D structure (split-cube-resonator-pair) Asyetric transission (AT) (diode-like response) for linearly polarized waves Magnetic resonance originated AT k H Unit cell G. Kenanakis et. al., ACS Photonics 2, 287 (2015) 0 Structure fabrication 620 n Fabrication by direct laser writing (nonlinear lithography) and electroless silver coating (M. Farsari s group, FORTH) 11

12 Structure characterization: Reflection Co-polarized x Co-polarized y Cross-polarized, x k H, y Good agreeent between siulations and experients Split-Cube-Resonator asyetric transission Forward direct. Backward direct. incident T forw Transitted T back Transitted incident Close to 30% transission asyetry Polarization isolator functionality! (zero vs higher T) 12

13 Asyetric transission? Transission asyetry f T T 2 b 2 T T T x y inc Tx txx txy x inc Ty tyx tyy y y x z k, z incident T b Transitted T f Transitted k, z incident Difference in polarization conversion between Reciprocity dictates the two perpendicular incident polarizations Af Bb f b txx t xx, t A C yy t yy f 2 f 2 C D t f b B D f b txy tyx, txy t xy tyx 2 2 yx C B Menzel et al, PRL, 2010 Split-Cube-Resonator asyetric transission (2) Close to 30% transission asyetry t xx =t yy asyetric transission for linearly polarized waves only Sall co-polarized T+large cross-polarized T90 o one-way optical activity with negligible ellipticity (1-way polarization rotator) 13

14 Mechanis of cross-polarized transission Coupling of agnetic resonances of the two resonators Advantages Ipedance atch possibility high transittance Backward wave possibility Magnetic field intensity H k k H Unit cell H We deonstrated five different flexible THz chiral structures showing very large optical activity and negative refractive index The designs can give switchable ellipticity response (dynaically switchable polarization filters) A bulk anisotropic structure in THz has been deonstrated, showing significant asyetric transission for linearly polarized waves PhotoMeta SolarNano Thank you 14