Making Optical MetaMaterials for Fun and Applications

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Making Optical Meta-Materials for Fun and
Applications
Gennady Shvets, The University of Texas at Austin
Saturday Physics Workshop, April 22, 2006
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What is a MetaMaterial?
Originates from a Greek word meta
"after/beyond" Example metaphysics ("beyond
nature") a branch of philosophy concerned with
giving a general and fundamental account of the
way the world is. (Wilkepidia)

• Metamaterials are artificially engineered
  materials possessing properties (e.g.,
  mechanical, optical, electrical) that are not
  encountered in naturally occurring materials.
• The emphasis of this talk in on unusual
  electromagnetic properties such as dielectric
  permittivity e, magnetic permeability m, and
  refractive index n.

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What is a MetaMaterial?

• Material properties are determined by the
  properties of the sub-units plus their spatial
  distribution.
  
• For a ltlt l ? effective medium theory.
• For a l ? photonic effects.

What about meso-scale materials bigger than atom
but smaller than the wavelength??
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MetaMaterials for fun fundamental physics on
small scale
The physics of "small-scale" lies at the heart of
the metamaterial advantage. The physics at small
scale is different than bulk physics and, from a
performance standpoint, often significantly
better. Quantum confinement, exchange-biased
ferromagnetism, and effective media responses are
all examples of how the physics at small-scale
can result in enhanced electromagnetic
properties.

• Example unit cell of a microwave metamaterial
  consisting of a split-ring resonator and metal
  wires

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MetaMaterials for profit applications
Endoscope for MRI using the power of
metamaterials for medical imaging (Wiltshire et.
al., Science'01)
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MetaMaterials for profit applications
Can one make a "perfect magnetic conductor"? Yes!
Shown is the "High Impedance" (ZE/H) surface
that suppresses magnetic field Application
low-lying patch antennas that would not work on a
conducting substrate.
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Elecromagnetic Spectrum
We may think that radio waves are completely
different physical objects or events than
gamma-rays. They are produced in very different
ways, and we detect them in different ways. Radio
waves, visible light, X-rays, and all the other
parts of the electromagnetic spectrum are
fundamentally the same thing. They are all
electromagnetic radiation!
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Microscopes the Engines of Discovery
Van Leeuwenhooke (1676) discovered bacteria,
blood cells
What if the imaged specimen is a
sub-wavelength grating?
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Difficult to resolve sub-l features
L gtgt d/2p
Small features (or large wavenumbers) of the
object are lost because of the exponential
evanescence of short-wavelength waves
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Getting up close from far to near field
E.H. Synge, "A suggested method for extending the
microscopic resolution into the ultramicroscopic
region" Phil. Mag. 6, 356 (1928) U. Durig, D. W.
Pohl, and Rohrer, Near-field Optical Scanning
Microscopy (1986) E. Betzig et. al., Near-field
scanning optical microscopy (NSOM) (1986)
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Getting too close may not be possible!
Buried (sub-surface) features
Amplify evanescent waves?
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What is a dielectric permittivity?

• External field polarizes dielectric ? field
  inside dielectric is smaller that outside ? ratio
  is called dielectric permittivity e

-

-

-

• In most materials e gt 1 (e.g., e 12 for Si, e
  2.25 for glass)
• Permittivity depends on frequency long lookup
  tables!
• Not without exceptions e lt 0 in metals (visible,
  IR,)

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What is a magnetic permeability?

• External B-field magnetizes material
• More complex mechanisms electron and nuclear
  spins
• Field inside can be smaller, or larger, or much
  larger

• In most materials m gt 0
• There are exceptions (ferrites), but only at
  microwave frequencies

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How waves propagate (or not)?

• Propagation of electromagnetic waves in medium is
  determined by e and m of the medium (J. C.
  Maxwell)

• In most natural materials m gt 0, e gt 0 ?waves
  propagate
• Sometimes either e lt 0, or m lt 0 ? no propagation

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Basic properties of Negative Index Waves
E
H

• In vacuum right-hand rule relates E, H, and k.
  Note
  normally m gt 0 and e gt 0

k

• Consequence phase velocity (along k) and group
  velocity (along the Poynting ExH vector) are in
  the same direction
• In NIMs group and phase velocity are in opposite
  directions

Negative Index Medium
Positive Index Medium
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Positive/Negative Index Interface
Positive Index Medium
Negative Index Medium
What happens for the oblique incidence?
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Unusual refractive properties of NIMs
Light enters n gt 0 material ? deflection
Light enters n lt 0 material ? focusing (Veselago
Lens)
Surface waves make Veselagos lens a super-lens!
(Pendry, 2000)
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Straw in a negative index water
empty glass
regular water, n 1.3
negative water, n -1.3
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Magnification in a NIM dropping ball
negative index material, n -1
regular material, n 1
n 1
n -1
From Dolling et. al., Opt. Lett06
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Negative Index Materials to the Rescue m
-1, e -1 ? n -1
L/2 L L/2

• Super-lens prevents image degradation ? beats the
  diffraction limit established by Abbe

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How to Make a Negative-Index Material

• In microwave range use perfectly conducting
  components to simulate e lt 0 and m lt 0, Smith
  et.al., (2000)

Metal poles e 1 wp2/w2 lt 0 Split-ring
resonators, Pendry99 geometric resonance at wM

• Challenges
  
  (a) moving to optical frequencies (infrared,
  visible, UV)
  (b) simplifying the structure (e lt 0
  and m lt 0 from same element)

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Another Example a m-wave NIM

• Basic Elements of a NIM
  
  (a) Split ring resonator just a well
  designed inductor resonating at w ltlt c/L ? gives
  m lt 0
  (b) Metal wires (continuous or
  cut) r ltlt L to ensure that e lt 0 for w ltlt
  c/L

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Applications of Negative-Index Materials
Miniaturizing Everything!
Artists rendition of a sub-wavelength antenna
embedded in a negative index shell
Nano-cavities, nano-waveguides,if you can make
optical NIM
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Making a better short focus lens
To make a short-focus lens, one needs a
positive-index material with a much larger index
n-1
n3
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Optical magnetism from SRRs to nanorods
Simplify the structure (a) easy fabrication
(b) e lt 0, m lt 0 from same element
NIMs in 2005
Zhang et.al., PRL 05
Grigorenko et.al., Nature05
Dolling et. al., Opt.Lett.05
Shalaev et. al., Opt.Lett.05
Many interesting designs but nothing works so
far unit size comparable to the wavelength ?
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The high cost of simplification from
meta-material to antennas
Meta-material size ltlt l/2n
Photonic crystals size l/2n
Cannot use SRRs or other microwave
tricks how to miniaturize?? Plasmonics!

• Plasmonic Nano-rings

• Plasmonic strips and rods

Shvets, PRB03 ShvetsUrzhumov, PRL04
Alu,Salandrino, Engheta, archive05
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Engineering m resonantly-induced magnetic dipole
moments in a nanoparticle

• Use proximity effect in a lattice electric
  octupole resonance has finite magnetic dipole
  moment for finite size particles!

magnetic field resonance at e -5.3
electrostatic potential
Goal Use radiation in doubly-negative band for
sub-wavelength imaging ? plasmonic superlens
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Sub-wavelength imaging with SPC
Nanostructured super-lens
Hot spots at the super-lens
Electric field lineouts
Blue ? w/wp 0.6, X -0.2l Red ?
w/wp 0.6, X 0.8l no damping
Black ? same as red, but with damping Dotted ?
w/wp 0.606 (outside of the left-handed band)
Magnetic field behind plane wave illuminated
double-slit D l/5, separation 2D
Shvets, Urzhumov, PRL 93, 243902 (2004)
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Poor Mans Super-Lens e lt 0, m gt 0

• Inserting a slab of matched material with
  negative e (and, one day, m) can prevent image
  degradation
• Super-lensing is a highly resonant phenomenon
  frequency-dependent permittivities must match

Recent UV results Fang et.al, Science 05,
Melville and Blaikie, Opt. Expr. 05 We have
demonstrated super-lensing in IR and (a) proved
its resonant nature, (b) demonstrated a new
application sub-surface imaging
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Superlens in mid-IR sub-surface imaging
pattern on bottom
NSOM image from top
with Taubner and Hillenbrandt (MPQ/Munich)

• SiO2/SiC/SiO2 superlens with a metallic pattern
  (0.5 mm slits in Ag film separated by 3 mm on the
  bottom side) was imaged from the top using NSOM
• Sub-surface imaging of sub-l features at 800 nm
  depth accomplished at 10.85 mm (CO2 laser) using
  a superlens ? opens the way to applications of
  super-lensing to sub-surface imaging of
  integrated circuits

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Summary of 1-D periodic array imaging through a
super-lens

• Resonant phenomenon ? in narrow frequency range

• Hi-Fi image ? multiple diffraction orders
  amplified

regular imaging at 9.47 mm
Fang et. al., Science 05
super-imaging at 10.85 mm
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Sub-surface imaging of isolated holes
small hole 500nm
SEM image
l10.62 mm
l11.03 mm
l10.85 mm
l9.27 mm

• Resolution of l/20
• Increased range of frequencies for imaging ?
  amplitude or phase
• Higher resolution with phase imaging ? less
  sensitive to topography

phase images
amplitude images
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Conclusions

• Optical meta-materials have been shown to have
  remarkable applications
• Can be used to engineer exotic meta-media
  Negative Index Materials ? plasmonic approach to
  making a sub-l NIM
• NIMs and negative e materials can be used to
  overcome diffraction limit and construct a
  super-lens
• A super-lens enables ultra-deep sub-surface
  imaging using NSOM probe
• Very new field ? lots of work to do (theory and
  experiments)