University of Toronto, 2/12/2009
Tunable Terahertz Metamaterials
Toni Taylor
Center for Integrated Nanotechnologies
Los Alamos National Laboratory
Collaborators
Los Alamos National Laboratory: John O’Hara, 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
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
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
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 < 0 and m < 0  n < 0
• Negative refraction
• Focusing and superlens
• Cloaking
Tunable μ: the Split Ring Resonator (SRR)
J.B. Pendry et al., IEEE Trans. Microwave Tech. 47, 2075 (1999).
Tunable Permittivity
Metals:
 < 0 when  <  p
 p: UV or visible
Metallic wires:
 p: THz or GHz
Cut wires:
Pendry et al., Phys. Rev. Lett. 76, 4773 (1996).
Tunable e: eSSR:
E
k
H
e < 0 when w0 < w < wp
W. J. Padilla et al, Phys. Rev B 75 (2007).
The first negative refractive index demonstrated for microwaves
Composite metamaterials:
Negative permittivity and negative permeability
composites
 negative refractive index
H
D. R. Smith, W. J. Padilla, et al, Phys. Rev. Lett. 14, 234 (2000)
R. A. Shelby, D. R. Smith, S. Schultz, Science 84, 4184 (2001)
Near infrared metamaterials have been demonstrated
C. Enkrich, et al., Phys. Rev. Lett. 95, 203901 (2005)
THz: A promising region of the EM spectrum
Spectroscopy
(110) HMX single crystal
b -HMX
C4H8N8O8
PBX9501
Imaging
13 mm
Transmission through materials which
are opaque at other wavelengths
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.
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).
Terahertz Time Domain Spectroscopy (THz-TDS)
ESam (w )
~
t (w ) 
ERef (w )
FFT
Full extraction of complex optical properties from
amplitude t(w) and phase j(w)
Novel THz Electric Metamaterials
Symmetric sample designs:
Original
Complementary
Complementary transmission in
accordance with Babinet’s principle.
Thin metal
H.-T. Chen, et al., Opt. Express 15, 1084 (2007).
Complementary THz Electric Metamaterials
E
E
Rectangular eSRR Designs
All eSRRs has same area
Black-Measured data
Red- Simulated data
Azad et al., Appl. Phys. Lett. 92, 011119 (2008)
High-Temperature Superconductor-based Metamaterial
Optimally-doped YBCO
(Tc~90K) used instead of metal
for fabrication of structure
Polarization Control
CSSR
ESSR
Peralta et al., Optics Express 17, 773 (2009)
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).
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
O’Hara et al., Opt. Express 16, 1786 (2008)
Towards Quasi-Three-Dimensional THz Metamaterials
40  m
E
H
Kapton Substrate 84  m
Quartz
THz Switch Experiments: Optical-Pump Terahertz-Probe
THz Pump
Laser
Dynamic Response of THz Switch
E
photo-excitation
The photoexcited Drude
carriers in the GaAs
short the SRR gaps
thereby turning off the
resonance!
W.J. Padilla, A J. Taylor, C. Highstrete, M. Lee, and R.D. Averitt, Phys. Rev. Lett. 96, 107401 (2006)
Ultrafast Recombination in ErAs:GaAs Nanoisland Superlattices
Fabricate MM on
ErAs:GaAs superlattice
(100 nm repeat  20 ps lifetime)
Metamaterial
1 Kadow
2
et al, APL (1999)
Griebel et al, Nat. Mater. (2003)
R. P. Prasankumar, et al., APL (2005)
Ultrafast THz Optical Switch
Switching recovery ~20 ps
H.-T. Chen et al., Opt. Lett. 32, 1620 (2007).
Frequency Tunable Metamaterials: Concept
Tunabilty via optical excitation
Design and Fabrication
1.
2.
3.
4.
5.
6.
7.
8.
9.
Substrate: 0.6 mm thick silicon on sapphire (SOS)
Spin photoresist
Photolithography to define the split-ring resonator array
Metallization
Lift-off
Spin photoresist
Photolithography to define silicon capacitor regions
Reactive ion etch to remove the unwanted silicon regions
Remove the photoresist
Performance
Experimental
Simulation
Frequency tunability ~20%
Chen, et al., Nature Photonics (2008).
Alternative Designs
~30% tunability
THz Electrical Switch/Modulator: Principle
Idea: shunt on or off the capacitive split gaps.
H.-T. Chen et al., Nature. 444, 597 (2006).
THz Electrical Switch/Modulator: Fabrication
Ohmic
Metamaterial
Substrate: 1 mm thick n-GaAs on SI-GaAs wafer; n = 1.9×1016 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).
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).
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
For a single pixel:
• Blue curve: resonance switched ON
• Red curve: resonance switched OFF
• 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.
In collaboration with D. Mittleman, Rice University
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).
Collective Dipolar Resonance – Surface Plasma
a
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
w  kc
e
k  G  2 /P
b
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.
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
Broadband Modulation in THz TDS
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
Center for Integrated Nanotechnologies
Sandia National Laboratories • Los Alamos National Laboratory
“One scientific community focused on nanoscience integration”
http://CINT.lanl.gov