Planar Chiral Metamaterials for Polarization Control Yuri Svirko Department of Physics and Mathematics, University of Joensuu, Joensuu, Finland Jari Turunen Makoto Gonokami Markku Kuittinen Kuniaki Konishi Benfeng Bai CREST SORST Core re Research arch of Evoluti lutional l Science ce & Technolo logy gy Solut lutio ion-Orien riented d Research arch for Science ce and Technology 1
OUTLINE 1. Introduction: planar chiral metamaterials 2. Metal nanogratings 3. All-dielectric nanogratings 4. Magneto-optic effects 5. Concluding remarks 2
1. Introduction Chirality What is Chirality? “Handedness”: right glove doesn’t fit the left hand. Mirror-image object is different from the original object. achiral chiral 3
1. Introduction Chirality An object is “chiral” if and only if it is not super imposable on its mirror image. The lack of a plane of symmetry is called or handedness chirality Wikipedia: Handedness is an attribute of human Two not super-imposable forms of a chiral beings defined by their unequal object are called ENANTIOMERS distribution of fine motor skill between the left and right hands. 4
1. Introduction Chirality A pair of enantiomeric chiral molecules C C Mirror r plane 5
1. Introduction Chirality in nature Knobbed Whelk Almost always “right handed” Lightning Whelk Almost always “left handed” 6
1. Introduction Homochirality HOMOCHIRALITY Amino acids in proteins are always LEFT-HANDED Sugars in DNA and RNA are always RIGHT-HANDED The reaso son n these se mole lecul cules es have ve such a uniform form chiral rality is not known wn, , but there re is no shorta ortage e of theo eori ries es on the subje ject. ct. Jon Cohen, Science, 1995 , 267 (5202), 1265-6 7
1. Introduction Circularly Polarized Light Enantiomers Right circular Left circular polarization produces a polarization produces a right threaded screw. left threaded screw. 8
1. Introduction Linearly Polarized Light Superposition of left- and right circularly polarized waves of the same amplitude gives an achiral linear polarized light wave 9
1. Introduction Chirality and Light-Matter Interaction RCP In a chiral medium, left- and right- circular polarized light interacts with medium differently. The difference is a measure of the chiral influence, which can be visualized by comparing the polarization state of the light before LCP and after interaction.
1. Introduction Fundamental Symmetry and Chirality Direct scenario: RCP wave interacts with D-molecule D RCP Light-matter interaction is PT-invariant: C n + (D) = n - (L) α + (D) = α - (L) n + (L) = n - (D) , α + (L) = α - (D) Natural optical activity n + - n - (L)= - (D) PT-reversed scenario: LCP wave interacts with L-molecule Circular dichroism L α + - α - (L)= - (D) LCP C 11
1. Introduction Natural optical activity Polarization rotation and Natural optical activity circular dichroism are n + - n - (L)= - (D) induced by molecular Circular dichroism chirality α + - α - (L)= - (D) Constitutive equation D E i k×E Chiral medium Reciprocal effect A pair of enantiomeric chiral molecules Molecular chirality Optical activity 12
1. Introduction Chirality in Two Dimensions Can interaction of light with 2D chiral object lead to the En Enan antiomer tiomers optical activity of circular dichroism? The answer is NO. 2D object in 3D space has a plane of symmetry mirror reflection NO CHIRALITY line A film with a set of holes of arbitrary shape have the same transmission coefficients for left- and right- circular polarized waves Nonsuperimposable 13
1. Introduction Planar Chiral Object In a planar structure, in-plane mirror symmetry is broken by the substrate Z X-Z mirror plane C 4 Z C 4 Y-Z mirror plane Y Y X X Planar achiral object Planar chiral object 14
1. Introduction Planar Chiral Gratings Artificial optically active nanostructured material Array of gammadion nanoparticles with C 4 symmetry Optical inactive materials Resembles chiral uniaxial crystal Chirality comes from the pattern Optical activity enhanced by optical resonances (such as SP resonance) Works in the visible and near-IR spectral range 15
1. Introduction Gammadion Gratings Basic properties (1) Reciprocity (2) Handedness vs. rotation 16
1. Introduction Gammadion Gratings (3) θ = 0 in presence of the symmetry plane (4) θ = 0 in reflected light (5) The effect does not depend on the incident polarization direction (due to the C 4 symmetry) 17
1. Introduction Effective parameters Ｙ 2 n 0 Light effective 2 n 0 ij Ｚ 0 0 Ｘ Polarization rotation angle Δ 2 2 L 1 n 2 2 Im 1 sin 2 sin4 2 nc 2 2 2 2 2 2 1 1 n 2 2 2 1 n 2 cos 1 sin 1 2 2 18
2. Metal grating Experiment Polarization rotation at normal incidence 2 n 0 L 2 n 0 Im sin2 ij 2 nc 0 0 19
2. Metal grating Plasmon enhancement Left (chiral) Cross (achiral) Sample side view Right Cr ： 23nm (chiral) Au ： 95nm Cr ： 3nm Silica substrate (~10 4 Large polarization rotation º/mm ) enhances by the surface plasmon resonance 20
2. Metal grating Transmission vs rotation spectra Polarization rotation Transmission p-polarization p-polarization Polarization azimuth rotatin (deg.) 2.2 2.2 Transmittance (%) 2.1 2.1 12 2 10 2.0 2.0 Energy (eV) Energy (eV) 8 0 1.9 1.9 6 Incident angle (deg.) -2 0 2 4 6 8 1.8 1.8 0 2 4 6 8 4 Incident angle (deg.) 2 1.7 1.7 -4 -2 600 -2 -4 600 700 -4 -6 1.6 1.6 -6 -8 700 Wavelength (nm) 800 Wavelength (nm) -8 800 900 900 1.5 1.5 1.4 1.4 -0.0015 -0.0010 -0.0005 0.0000 0.0005 0.0010 0.0015 -0.0015 -0.0010 -0.0005 0.0000 0.0005 0.0010 0.0015 k x (1/nm) kx (1/nm) 21
2. Metal grating Local electric filed Non-parallel electric Air-Metal interface field at both Y-polarization 630nm interface air sub n E E air sub air sub Re E E E E x y y x Metal-Substrate interface Y 500nm X 22
2. Metal grating Chirality factor 1 air sub air sub air sub air sub Re E E E E Re E E E E dxdy x y y x x y y x 2 A E Left @630nm unit cell Right 0.06 Polarization Cross Chirality factor (a.u.) 630nm Left 0.04 0.02 2.62 × 10 -2 Left 0.00 - 0.02 -2.62 × 10 -2 Right - 0.04 ～ 0 Cross - 0.06 Cross @630nm (5.04 × 10 -19) 600 700 800 Wavelength (nm) Left, Right = 0 Cross =0 23
2. Metal grating Complimentary structure Can we achieve enhanced transmission SPP excitation at SPP excitation at & enhanced polarization rotation air-Au interface SiO 2 -Au interface simultaneously? d = 800 nm, L = 120 nm Numerical simulation results 24
2. Metal grating Complimentary structure Au gammadion-hole sample The optical characterization is in progress 25
3. Dielectric grating Left-twisted (LT) TiO 2 SiO 2 Right-twisted(RT) d = 600 nm, w = l = 100 nm 26
3. Dielectric grating Transmission and rotation spectra Spectra for LT and RT samples Giant polarization rotation (26.5 ° @ 634 nm) in direct transmission, 10 times larger than in metal gratings . Optical activity is enhanced by resonances. Incident polarization direction 27
3. Dielectric grating Guided-mode resonance Waveguide grating Phase matching at normal incidence 2 2 i j G , G 2 / d Guided modes manifest themselves as transmission dips on a smooth Fabry-Pérot background 28
3. Dielectric grating Guided-mode resonance Measured and calculated spectra 29
3. Dielectric grating Local filed pattern Different coupling of RCP and LCP waves λ = 955 nm λ = 622.5 nm Similar RCP and LCP coupling small CD Different RCP and LCP coupling large CD Coupled field affected slightly by structural Coupled field affected more drastically by the chirality structural chirality 30
3. Magnetic grating 2D MO resonant nanograting Bulk MO material nanograting Kerr effect B ? Faraday effect 31
3. Magnetic grating Kerr effect Kerr effect: Numerical analysis LCP RCP 1 ( R R R ) LP 2 1 ( ) 2 a a a tan a a a tan The lifting of the RCP/LCP degeneracy produces strong Kerr rotation 32
3. Magnetic grating LCP/RCP beam splitter Reflectance Transmittance An incident linearly polarized wave is split into a reflected LCP (RCP) and a transmitted RCP (LCP) wave, each with an efficiency of 50%. 33
3. Magnetic grating Experiment Square holes array film • d = 420 nm, D = 150 nm, h = 160 nm • BIG: bismuth iron garnet (Bi 3 Fe 5 O 12 ) • GGG: gadolinium gallium garnet (Gd 3 Ga 5 O 12 ) 34
4. Concluding remarks Novel metamaterials for polarization control? Artificial media/structure? The property is not possessed by the composing media? The property does not exist in nature? d << λ , i.e. the structure can be seen as homogeneous medium? x 35
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