Tunable Terahertz Metamaterials

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University of Toronto, 2/12/2009
Tunable Terahertz Metamaterials
Toni Taylor Center for Integrated
Nanotechnologies Los Alamos National Laboratory
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Collaborators
Los Alamos National Laboratory John OHara,
Hou-Tong Chen, Abul Azad, Stuart Trugman, Evgenya
Smirnova, Nina Weisse-Bernstein, Quanxi
Jia Boston College Willie Padilla, David
Shrekenhamer Boston University Richard
Averitt Duke University David Smith, Nan
Jokerst, Sabarni Palit Oklahoma State University
Weili Zhang, Jiaguang Han, Ranjan Singh Sandia
National Laboratories Igal Brener, Xomalin
Peralta, Darren Branch, Clark Highstrete, Mark
Lee, Michael Cich UCSB Josh Zide, Seth Bank, Art
Gossard, Lu Hong NIST Chris Holloway University
of Munich Roland Kersting, G. Acuna, S. Heucke,
F. Kuchler Rice University Dan Mittleman, Wai
Chan
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Outline

• Electromagnetic Metamaterials
• Terahertz Gap
• Metamaterials as a Solution to the THz Gap
• novel metamaterials
• modulator/switch
• active frequency tuning
• spatial modulator
• broadband modulation
• Summary

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Why Metamaterials?
It is frequently said that any advanced
technology is indistinguishable from
magic. -Directing Matter and Energy Five
Challenges for Science and the Imagination
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Electromagnetic metamaterials
Metamaterials Artificially constructed
materials with properties derived from their
sub-wavelength structures, not from the materials
from which they are made. e.g.,
simultaneously e lt 0 and m lt 0 ? n lt 0

• Negative refraction
• Focusing and superlens
• Cloaking

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Tunable µ the Split Ring Resonator (SRR)
J.B. Pendry et al., IEEE Trans. Microwave Tech.
47, 2075 (1999).
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Tunable Permittivity
Metals
e lt 0 when ? lt ?p ?p UV or visible
Metallic wires
?p THz or GHz
Cut wires
Pendry et al., Phys. Rev. Lett. 76, 4773 (1996).
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Tunable e
eSSR
e lt 0 when w0 lt w lt wp
W. J. Padilla et al, Phys. Rev B 75 (2007).
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The first negative refractive index demonstrated
for microwaves
Composite metamaterials Negative permittivity
and negative permeability composites ? negative
refractive index
H
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Near infrared metamaterials have been demonstrated
C. Enkrich, et al., Phys. Rev. Lett. 95, 203901
(2005)
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THz A promising region of the EM spectrum
Spectroscopy
b -HMX C4H8N8O8
Transmission through materials which are opaque
at other wavelengths
Imaging
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Terahertz Gap

• THz region is located at the interface of
  electronics and photonics where technologies
  directly translated from microwave and optical
  regimes generally fail to operate.
• THz gap is caused by weak/nonexistant materials
  response at THz frequencies
• Results in a lack of sources, detectors,
  modulators, filters, polarizers, sensors, etc. in
  the THz regime.

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Metamaterials A solution to the THz Gap
w (THz)
w (THz)
Typical parameters Unit cell 50 mm Outer
dimension 36 mm Line width 4 mm Split gap 2 mm

• Resonance enhances THz interactions
• Modulators (AM, FM, PM)
• Filters
• Sensors

First THz MM T. J. Yen, W. J. Padilla, et al.,
Science 393, 1494 (2004).
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Terahertz Time Domain Spectroscopy (THz-TDS)
FFT
Full extraction of complex optical properties
from amplitude t(w) and phase j(w)
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New Electric Metamaterials
W. J. Padilla et al, Phys. Rev B 75 (2007).
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Novel THz Electric Metamaterials
Symmetric sample designs
Original
Complementary
Complementary transmission in accordance with
Babinets principle. Thin metal
H.-T. Chen, et al., Opt. Express 15, 1084 (2007).
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Complementary THz Electric Metamaterials
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Terahertz Reflection and Transmission Measurements

• Coherent
• Collect phase
• Collect amplitude
• Variable bistatic angle
• Variable polarization
• Broadband, time - selective

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Transmission, Reflection, and Absorption
Simulations
Experimental
A1-(TR)
J. OHara, et al., J. Nanoelectro. Optoelectron.
2, 90 (2007)
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Rectangular eSRR Designs
All eSRRs has same area
Black-Measured data Red- Simulated data
Azad et al., Appl. Phys. Lett. 92, 011119 (2008)
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High-Temperature Superconductor-based Metamaterial
Optimally-doped YBCO (Tc90K) used instead of
metal for fabrication of structure
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Polarization Control
CSSR ESSR
Peralta et al., Optics Express 17, 773 (2009)
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Design of a THz Quarter Wave Plate using ESSR
0.65, 1.06, 1.83 THz
ESSR
Also see R. Averitt et al, Optics Express 17, 137
(2009).
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Metamaterials for THz Sensing Concepts
Limitations
Frequency-dependent amplitude transmission of a
double SRR metamaterial without (solid curves)
and with (dotted curves) a 16 µm thick
photoresist overlayer.

• Minimize substrate thickness (10-20 mm)
• Minimum detectable layer 20 nm

Resonance positions vs. dielectric loading Top
LC resonance Bottom dipole resonance
OHara et al., Opt. Express 16, 1786 (2008)
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Towards Quasi-Three-Dimensional THz Metamaterials
40 µm
Quartz
E
H
Kapton Substrate 84 µm
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THz-TDS Response of Split Ring Resonator (SRR)
E
36 mm X 36 mm X 3 mm
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SRR THz Magnetic Response
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THz Switch Experiments Optical-Pump
Terahertz-Probe
THz
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Dynamic Response of THz Switch
E
W.J. Padilla, A J. Taylor, C. Highstrete, M. Lee,
and R.D. Averitt, Phys. Rev. Lett. 96, 107401
(2006)
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Ultrafast Recombination in ErAsGaAs Nanoisland
Superlattices
Fabricate MM on ErAsGaAs superlattice (100 nm
repeat ? 20 ps lifetime)
1 Kadow et al, APL (1999) 2 Griebel et al, Nat.
Mater. (2003)
R. P. Prasankumar, et al., APL (2005)
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Ultrafast THz Optical Switch
Switching recovery 20 ps
H.-T. Chen et al., Opt. Lett. 32, 1620 (2007).
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Frequency Tunable Metamaterials Concept
Tunabilty via optical excitation
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Design and Fabrication

1. Substrate 0.6 mm thick silicon on sapphire (SOS)
2. Spin photoresist
3. Photolithography to define the split-ring
   resonator array
4. Metallization
5. Lift-off
6. Spin photoresist
7. Photolithography to define silicon capacitor
   regions
8. Reactive ion etch to remove the unwanted silicon
   regions
9. Remove the photoresist

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Performance
Simulation
Experimental
Frequency tunability 20
Chen, et al., Nature Photonics (2008).
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Alternative Designs
30 tunability
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THz Electrical Switch/Modulator Principle
Idea shunt on or off the capacitive split gaps.
H.-T. Chen et al., Nature. 444, 597 (2006).
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THz Electrical Switch/Modulator Fabrication
Ohmic
Metamaterial
Substrate 1 mm thick n-GaAs on SI-GaAs wafer n
1.91016 cm-3
Ohmic contact 20 nm Ni, 20 nm Ge, and 150 nm Au,
RTA at 350 oC
Metamaterial 10 nm adhesive Ti, 200 nm Au, as
deposited to form Schottky
H.-T. Chen et al., Nature. 444, 597 (2006).
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THz Electrical Switch/Modulator Results

• Results
• THz switching efficiency 50
• Switching e between and -
• 2 MHz modulation frequency

H.-T. Chen et al., Nature. 444, 597 (2006).
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Polarization Dependence
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Electrical Modulation of THz Waves
_at_ 0 volt
Problem Our measurements can only go 100
kHz Solution Connected with external series
resistor
External series resistor 0 kW 1 kW 2 kW
Roll off frequency (Experimental) 100200 kHz 24 kHz 14 kHz
Roll off frequency (Calculation) 101 kHz 15 KHz 8 kHz
External Series Resistor
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Megahertz Modulation Speed
External series resistor 0 kW 2.16 kW
Roll off frequency (Experimental) 2 MHz 100 kHz
Roll off frequency (Calculation) 2.1 MHz 135 kHz
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Active Metamaterials as THz Spatial Modulator

• 4x4 pixels metamaterial spatial modulator
• Each pixel (4 mm x 4 mm) is comprised of an SRR
  array
• Each pixel is individually controllable

wo 0.36 THz

• Solid blue curves Measured THz fringe patterns
  produced by the transmission of THz beam through
  the spatial THz modulator in a double-slit
  configuration
• Dashed red curves analytical calculations
• Grey pixels zero bias
• White pixels modulated with a 3-kHz square
  voltage 0-14V.

• For a single pixel
• Blue curve resonance switched ON
• Red curve resonance switched OFF

In collaboration with D. Mittleman, Rice
University
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THz Near-Field Study of Metamaterial Resonances
Near-field imaging of metamaterial structure
Differential near-field imaging of metamaterial
excitation
Spectrally-resolved near-field Excitation of
metamaterial
The resonances are most effectively excited when
the tungsten tip located at the center metal
stripe with split gap
G. Acuna et al, Optics Express 16, 101535 (2008).
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Collective Dipolar Resonance Surface Plasma
a
b

• Assignment of resonances
• a Inductive-capacitive resonance, does not
  depend on the periodicity
• b Collective dipolar resonance (or surface
  plasma), strongly depends on the periodicity

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New Design of Electrically Switchable THz
Metamaterials

• In the previous design, the split gap is located
  at the center and enclosed by a ring, where
  depletion underneath the surrounding ring may
  prevent the electrical connection between the
  split gap and the ohmic contact.
• In this new design, split gaps are directly
  exposed to the ohmic contact.

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THz Amplitude Switching and Phase Shifting
Max AM 50 (intensity 76) Max PM 0.57
rad Phase shift linear with applied voltage
Chen et al, to appear in Nature Photonics
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Broadband Modulation in THz TDS
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Modulation Frequency Response
Metamaterial modulation
THz emitter modulation

• When increasing the modulation frequency, the
  modulation signal decreases
• The decreasing modulation signal is partially due
  to the THz-TDS system response

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Summary

• Properties of THz metamaterials
• Planar THz metamaterials and their resonances
• Polarization control
• Thin film sensing
• Quasi-three-dimensional THz metamaterials
• Optically switchable, frequency tunable THz
  metamaterials
• Low optical fluence, high efficiency
• Ultrafast switching
• 20 frequency tunability
• Electrically switchable THz metamaterials
• High modulation (intensity and phase) depth
• Integration into spatial modulator
• Broadband modulation in THz TDS
• These results show that metamaterials will play
  an increasingly important role in THz science and
  technology

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Acknowledgments
This work was performed, in part, at the Center
for Integrated Nanotechnologies, a U.S.
Department of Energy, Office of Basic Energy
Sciences nanoscale science research center
operated jointly by Los Alamos and Sandia
National Laboratories. Los Alamos National
Laboratory is a multiprogram laboratory operated
by the University of California, for the U.S.
Department of Energy under Contract No.
W-7405-ENG-36. We gratefully acknowledge the
support of the U.S. Department of Energy through
the LANL/LDRD Program for this work.