Planar Chiral Metamaterials for Polarization Control Yuri Svirko - - PowerPoint PPT Presentation

Planar Chiral Metamaterials for Polarization Control

Yuri Svirko

Department of Physics and Mathematics, University of Joensuu, Joensuu, Finland Jari Turunen Markku Kuittinen Benfeng Bai Makoto Gonokami Kuniaki Konishi

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Solut lutio ion-Orien riented d Research arch for Science ce and Technology Core re Research arch of Evoluti lutional l Science ce & Technolo logy gy

CREST SORST

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• 1. Introduction: planar chiral metamaterials
• 2. Metal nanogratings
• 3. All-dielectric nanogratings
• 4. Magneto-optic effects
• 5. Concluding remarks

OUTLINE

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What is Chirality?

 “Handedness”: right glove doesn’t fit the left

hand.

 Mirror-image object is different from the original

• bject.

chiral achiral

Chirality

• 1. Introduction

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 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

chirality Wikipedia: Handedness is an attribute of human beings defined by their unequal distribution of fine motor skill between the left and right hands.

• r handedness

 Two not super-imposable forms of a chiral

• bject are called ENANTIOMERS
• 1. Introduction

Chirality

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C

Mirror r plane

C

• 1. Introduction

Chirality A pair of enantiomeric chiral molecules

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Lightning Whelk Almost always “left handed” Knobbed Whelk Almost always “right handed”

• 1. Introduction

Chirality in nature

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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

• rtage

e of theo eori ries es on the subje ject. ct. Jon Cohen, Science, 1995, 267 (5202), 1265-6

• 1. Introduction

Homochirality

Right circular polarization produces a right threaded screw. Left circular polarization produces a left threaded screw.

Enantiomers

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• 1. Introduction

Circularly Polarized Light

Superposition of left- and right circularly polarized waves of the same amplitude gives an achiral linear polarized light wave

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• 1. Introduction

Linearly Polarized Light

Chirality and Light-Matter Interaction

RCP LCP

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 and after interaction.

• 1. Introduction

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Fundamental Symmetry and Chirality

RCP

Direct scenario: RCP wave interacts with D-molecule

D

C

LCP

PT-reversed scenario: LCP wave interacts with L-molecule

L

C

Light-matter interaction is PT-invariant: n+(D) = n-(L) α+(D) = α -(L) n+(L) = n-(D) , α+(L) = α -(D) Natural optical activity n+- n- (L)= - (D) Circular dichroism α+ - α - (L)= - (D)

• 1. Introduction

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• 1. Introduction

Polarization rotation and circular dichroism are induced by molecular chirality

Chiral medium Molecular chirality Optical activity

A pair of enantiomeric chiral molecules

Reciprocal effect

Natural optical activity Natural optical activity n+- n- (L)= - (D) Circular dichroism α+ - α - (L)= - (D)

Constitutive equation

i D E k×E

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Chirality in Two Dimensions

Nonsuperimposable mirror line reflection

En Enan antiomer tiomers

Can interaction of light with 2D chiral object lead to the

• ptical activity of circular dichroism?

The answer is NO. 2D object in 3D space has a plane of symmetry NO CHIRALITY A film with a set of holes of arbitrary shape have the same transmission coefficients for left- and right- circular polarized waves

• 1. Introduction

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Planar Chiral Object X Y Z X-Z mirror plane Y-Z mirror plane C4

Planar achiral object

X Y Z C4

Planar chiral object

In a planar structure, in-plane mirror symmetry is broken by the substrate

• 1. Introduction

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• 1. Introduction

Artificial optically active nanostructured material

Optical inactive materials

 Array of gammadion nanoparticles with C4 symmetry  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

Planar Chiral Gratings

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Basic properties

(1) Reciprocity (2) Handedness vs. rotation

• 1. Introduction

Gammadion Gratings

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(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 C4 symmetry)

• 1. Introduction

Gammadion Gratings

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Effective parameters

2 2 effective ij

n n

2 2 2 2

1 Im 1 sin 2 sin4 2 2 2 L n nc

1 2 n

2

sin

2 2

1 1 cos 1 2 n

Polarization rotation angle Δ

Ｚ Ｘ Ｙ

Light

• 1. Introduction

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• 2. Metal grating

Experiment

Im sin2 2 L nc

Polarization rotation at normal incidence

2 2 ij

n n

Left (chiral) Right (chiral) Cross (achiral)

Cr：23nm Au ：95nm Cr：3nm Silica substrate

Sample side view Large polarization rotation (~104 º/mm ) enhances by the surface plasmon resonance

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• 2. Metal grating

Plasmon enhancement

2 4 6 8 10 12 600 700 800 900

• 8
• 6
• 4
• 2

0 2 4 6 8

Transmittance (%)

Wavelength (nm) Incident angle (deg.)

• 4
• 2

2 600 700 800 900

• 8
• 6
• 4
• 2

0 2 4 6 8

Polarization azimuth rotatin (deg.) Wavelength (nm) Incident angle (deg.)

Transmission Polarization rotation

p-polarization p-polarization

kx (1/nm)

• 0.0015
• 0.0010
• 0.0005

0.0000 0.0005 0.0010 0.0015

Energy (eV) 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 kx (1/nm)

• 0.0015
• 0.0010
• 0.0005

0.0000 0.0005 0.0010 0.0015

Energy (eV) 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2

• 2. Metal grating

Transmission vs rotation spectra

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Re

air sub air sub air sub x y y x

E E E E n E E

Non-parallel electric field at both interface

Y-polarization 630nm

Metal-Substrate interface

500nm

Air-Metal interface

X Y

• 2. Metal grating

Local electric filed

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Re

air sub air sub x y y x

E E E E

Cross@630nm Left@630nm

Polarization

2 unit cell

1 Re

air sub air sub x y y x

E E E E dxdy A E 630nm 2.62×10-2

• 2.62×10-2

～ 0

(5.04×10-19)

Left Cross Right Left, Right = 0 Cross =0

• 0.06
• 0.04
• 0.02

0.00 0.02 0.04 0.06 Chirality factor (a.u.) 800 700 600 Wavelength (nm)

Right Cross Left

• 2. Metal grating

Chirality factor

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Can we achieve enhanced transmission & enhanced polarization rotation simultaneously? Numerical simulation results d = 800 nm, L = 120 nm

• 2. Metal grating

Complimentary structure

SPP excitation at air-Au interface SPP excitation at SiO2-Au interface

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The optical characterization is in progress

Au gammadion-hole sample

• 2. Metal grating

Complimentary structure

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d = 600 nm, w = l = 100 nm

• 3. Dielectric grating

TiO2 SiO2 Left-twisted (LT) Right-twisted(RT)

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.

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Incident polarization direction

• 3. Dielectric grating

Transmission and rotation spectra

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• 3. Dielectric grating

Guided-mode resonance

Waveguide grating Guided modes manifest themselves as transmission dips on a smooth Fabry-Pérot background

2 2

, 2 / i j G G d

Phase matching at normal incidence

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Measured and calculated spectra

• 3. Dielectric grating

Guided-mode resonance

Different coupling of RCP and LCP waves

λ = 955 nm

Similar RCP and LCP coupling  small CD Coupled field affected slightly by structural

chirality

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λ = 622.5 nm

Different RCP and LCP coupling  large CD Coupled field affected more drastically by the

structural chirality

• 3. Dielectric grating

Local filed pattern

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Kerr effect Bulk MO material B Faraday effect nanograting

?

2D MO resonant nanograting

• 3. Magnetic grating

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Kerr effect: Numerical analysis

• 3. Magnetic grating

Kerr effect

1 ( ) 2

tan a a a a

1 ( ) 2 R R R RCP LCP LP

a a tan

The lifting of the RCP/LCP degeneracy produces strong Kerr rotation

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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%.

• 3. Magnetic grating

LCP/RCP beam splitter

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• d = 420 nm, D = 150 nm, h = 160 nm
• BIG: bismuth iron garnet (Bi3Fe5O12)
• GGG: gadolinium gallium garnet (Gd3Ga5O12)

Square holes array film

• 3. Magnetic grating

Experiment

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• 4. Concluding remarks

 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

Novel metamaterials for polarization control?

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• 4. Concluding remarks

 With planar nanogratings we can achieve a gyratory power several orders larger than that of natural or magneto-

• ptically active media.

 The developed approach allows us to develop novel planar devices for polarization control  THz

• ptics:

light-induced

• ptical

activity at terahertz frequencies (CLEO’09)  THz optics: sub-wavelength devices for polarization control can be created by using conventional ink-jet printing technology  Chiral gratings based on novel nanomaterials e.g. graphene