Active Terahertz Metamaterials
A.J. Taylor, H.-T. Chen, A.K. Azad, N.K. Grady, Y. Zeng,
D.A.R Dalvit, S. Trugman, Q. Jia. M. Reiten, S. Ramani
ECOC 2013
UNCLASSIFIED
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Center for Integrated Nanotechnologies
Sandia National Laboratories • Los Alamos National Laboratory“One scientific community focused on nanoscience integration”• State-of-the-art facilities• Leverages LANL and Sandiacapabilities• Develop innovative approaches
to nanoscale integration	

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http://CINT.lanl.gov

Collaborators/Acknowledgements
Boston University: Rick Averitt
Boston College: Willie Padilla
Oklahoma State University: Weili Zhang, John O’Hara
Rice University: Dan Mittleman
Sandia National Laboratories: Igal Brener
UCSB: Art Gossard
University of Delaware: Josh Zide
University of South Florida: Jiangfeng Zhou
Nantang Technical University: Ranjan Singh, Dibakar Chowdhury
We gratefully acknowledge the support of the U.S. Department ofEnergy through the Center for Integrated Nanotechnologies andthe LANL/LDRD Program for this work.

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Outline• Metamaterials as a solution to the Terahertz (THz) Gap• THz functionalities achieved in few-layered ultrathin
planar metamaterials
oooo

Antireflection
Perfect absorption
Linear polarization conversion
Anomalous reflection/refraction (generalized Snell’s law)

• Active THz Metamaterials
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modulator/switch
active frequency tuning
spatial modulator
thermal and optical tuning using complex oxides• SummaryOperated by Los Alamos National Security, LLC for NNSA

Emergent Phenomena in Metamaterialsq Metamaterials are a class ofman-made effective media
with properties attained fromtheir subwavelength
structures rather than thechemical compositionsq Negative index of refractionq Cloaking andtransformation optics
Operated by Los Alamos National Security, LLC for NNSAq Superlens – breaking
the diffraction limits

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Terahertz Gap•

THz region is located at theinterface of electronics andphotonics where technologies
directly translated frommicrowave and optical regimes
generally fail to operate.

•THz gap is caused by weakmaterials response at THz
frequencies•Results in a lack of sources,
detectors, modulators, filters,polarizers, sensors, etc. in theTHz regime.

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(110) HMX single crystal

Spectroscopy

Imaging

Metamaterials: A solution to the THz Gap

ω (THz)	

ω (THz)	

Typical parameters:
Unit cell: 50 µm
Outer dimension: 36 µm
Line width: 4 µm
Split gap: 2 µm

Resonance enhances THz interactions•Modulators (AM, FM, PM)•Filters•Sensors
Operated by Los Alamos National Security, LLC for NNSAFirst THz MM: T. J. Yen, W. J. Padilla, et al., Science 393, 1494 (2004).

Bulk Metamaterials vs Planar Meta-Surfaces
Bulk metamaterials

Meta-surfacesq Effective material parameters: ε, µ, Z, n q Effective surface propertiesq Feasible mostly at microwave frequencies q Any relevant frequencyq Difficult to fabricate in the optical regimeq Planar structures: ease in fabrication

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Tailored Transmission and Reflectionq The resonator array modifies thecomplex reflection and transmission
coefficients at the interface

r12 =| S11 | t12 =n1n2| S21 |r21 =| S22 | t21 =n2n1| S21 |φ12 = arctan( S11 ) θ12 = arctan( S21 )φ21 = arctan( S22 ) θ12 = arctan( S12 )9Operated by Los Alamos National Security, LLC for NNSA

Ultrathin Metamaterial Anti-Reflection CoatingChen et al., Phys. Rev. Lett. 105, 073901 (2010).
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Ultrathin Metamaterial ARC: Experimentsq Reflectance: 32% à ~0q Transmittance: 68% à 90%q Transmittance does not reach 100% due to lossesq Operate over a wide incident angle rangeq Brewster angle behavior for TM polarization
Chen et al., Phys. Rev. Lett. 105, 073901 (2010).
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Metamaterial Perfect Absorbers: Trapped Lightq Zero transmission due to the metal
ground planeq Minimizing reflection due to destructive
interference of multi-reflection
Chen, Opt. Express 20, 7165 (2012).
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Bandwidth Broadening for Metamaterial Absorbers
Experiments

1.0

1.0

0.8

0.8

0.6

0.4

IIIIII

Absorption

Absorption

Simulations

l2 = 16.5 µm

0.60.4l2 l1 l2

0.20.20.0
0.6 0.7 0.8 0.9 1.0 1.1 1.2
Frequency (THz)

0.0
0.6 0.7 0.8 0.9 1.0 1.1 1.2
Frequency (THz)q Broadening the absorption bandwidth by multiplexing
Huang et al., Opt. Lett. 37, 154 (2012).
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High Absorption over a Wide Incident Angle Range
EHkTE

TMAbsorption

0.8

0.6
0.4

1.0

Incidenceangle:

TM0.8
Absorption

1.030oo40o5060o

Incidenceangle:

0.60.4TEo30o40o50
60o0.20.2

0.00.6 0.7 0.8 0.9 1.0 1.1 1.2
Frequency (THz)

0.0
0.6 0.7 0.8 0.9 1.0 1.1 1.2
Frequency (THz)

Huang et al., Opt. Lett. 37, 154 (2012).
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Polarization Conversion in Anisotropic MMs

q Incident field E0 along x-direction(α=45°): the excited dipole p has
x- and y-componentsq px: co-polarized reflection andtransmissionq py: cross-polarized reflection and transmissionq Single-layered metamaterial: polarization conversion is lowq Output waves are elliptically polarized
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Metamaterial Polarization Converter: Reflection

αq Backed with a metal ground plane with appropriate thickdielectric spacerq Multiple reflections contain both x and y componentsq The co-polarized multiple reflections interfere
destructively, leading to minimal co-polarized reflectionq The cross-polarized multiple reflections interfere
constructively, leading to high cross-polarized reflection
cross-polarized
co-polarized

Grady et al., Science 340, 1304 (2013).
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Metamaterial Polarization Converter: Reflectionq Demonstrated in both simulations
and experimentsq Device thickness 33 µm
q Co-polarized reflection is minimalq Cross-polarized reflectance is morethan 80%
q Broadband: destructive interference
at multiple frequencies

Grady et al., Science 340, 1304 (2013).
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Polarization Converter: Transmissionq The gratings are equivalent to groundplane for electric field polarized along
the grating lines.
q The back grating prevents co-polarized
light from transmitting throughq The front grating prevents theconverted cross-polarized light fromreflecting back
Grady et al., Science 340, 1304 (2013).
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Anomalous Refraction: Generalized Snell’s Law

q Each individual element can be usedin polarization converterq Conversion efficiency is designed tobe largely constantq 8 elements form the unit cell, with alinear phase shift spanning a 2π range
Grady et al., Science 340, 1304 (2013).
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Near-Perfect Anomalous Refraction: Exp.

q At a specific frequency, the “refraction angle” is determined byperiodicityq At 1.4 THz, the anomalous refraction carries 60% of the incident powerq Measure the cross-polarized transmission vs. scanning angleq Operate over a broad bandwidth
Grady et al., Science 340, 1304 (2013).
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Flat Opticsq Flat prism for beam steeringq Wave front shapingq Flat lens for beam focusing
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Optically Switchable Terahertz Metamaterials

photo-excitation

Amplitude transmission

1.0n Switching speed is only limited bycarrier lifetime (~ns for GaAs)

0.8
0.60.0 mW
0.5 mW
1.0 mW
5.0 mW
10 mW

0.4

0.2
0.0
0.0

0.5

1.0
1.5
Frequency (THz)n ~ps by reducing the carrier lifetime (GaAs:ErAs nano superlattice)
Padilla et al., Phys. Rev. Lett. 96, 107401 (2006).
Chen et al., Opt. Lett. 32, 1620 (2007).
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2.0

SiAu
Sapphire

Amplitude transmission

Optically Frequency Tunable Terahertz Metamaterials

1.00.8Pump (mW)00.610200.450100
0.22005000.00.25 0.50 0.75 1.00 1.25 1.50
Frequency (THz)n ~20% frequency tuning rangen Could be red or blue shifting
Chen et al., Nature Photon. 2, 295 (2008).
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Electrically Switchable Terahertz Metamaterials
Power transmission

1.0
Voltage bias:
0V-1 V
-2 V
-4 V
-8 V
-16 V

0.8
0.6

0.4
0.2
0.0

0.5

1.0
1.5
Frequency (THz)2.0n Amplitude modulation depth 50%n Phase modulation of Δφ = π/6n Operate at room temperature
Chen et al., Nature 444, 597 (2006).
Chen et al., Nat. Photon. 3, 148 (2009).
Chen et al., Opt. Express 16, 7641 (2008).
Chen et al., APL 93, 091117 (2008).
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Terahertz Spatial Light Modulatorn Each pixel consisting of hundreds of SRRsn Resonance can be independently switched at each pixeln For high-speed terahertz imaging
Chan et al., Appl. Phys. Lett. 94, 213511 (2009).
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Thermally Tunable Metamaterials Using
Ferroelectric Substrate (Strontium Titanate)••Substrate refractive index (dielectric constant)decreases with decreasing temperature
Resonance frequency red-shifts with decreasing
temperature
1.03025200 K

20295 K409 K

15100.10

0.8150 K

0.15
0.20
0.25
Frequency (THz)

0.30Transmission

Refractive index n35

0.6
0.4
0.2

150 K 200 K
295 K 409 K

0.0
0.10

0.15
0.20
0.25
Frequency (THz)

Singh et al., Opt. Lett. 36, 1230 (2011).
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0.30

High-Temperature Superconducting (HTS) Metamaterials

Transmission

1.00.80.60.4
50 nm

0.20.0100 nm0.4

180 nm

0.5
0.6
0.7Frequency (THz)n Strong resonant response at low temperaturesn Resonance frequency strongly depend on the nano-scale thicknessn Need to take into account the kinetic inductance in YBCO metamaterials
Singh et al., Nanophotonics 1, 117 (2012).
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Thermal & Optical Resonance Tuning in HTS Metamaterials

Transmission

0.8

••
••

82K
80K
78K
75K
70K
60K
50K
40K
20K

0.6
0.4
0.2
0.00.21.0

0.3

0.40.50.6
Frequency (THz)

0.7Transmission

1.0

Thermal tuning

Optical tuning

0.8
0.6
0.4
0.2

0.00.2Power (mW)
1006030
1000.4
0.6
Frequency (THz)

0.850 nm thick YBCO metamaterial
Resonance strength and frequency are both tuned by temperature
Imaginary conductivity plays a key role
The resonance tuning is more significant in thinner YBCO metamaterials
Chen et al., PRL. 105, 247402 (2010).
Singh et al., Nanophotonics 1, 117 (2012).

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Metamaterials will play an increasingly important role in THz S&T
q Few-layered ultrathin planar metamaterials can accomplish many
optical functionalities of bulk materialsØ Ultrathin metamaterial perfect absorbers
Ø Ultrathin metamaterial antireflection coatingsØ Thin film like metamaterial linear polarization convertersØ Near perfect anomalous reflection/refraction: generalized Snell’s lawq Electrically switchable THz metamaterials
Ø High modulation (intensity and phase) depthØ Integration into spatial modulatorØ Broadband modulationq Optically switchable, frequency tunable THz metamaterialsØ Low optical fluence, high efficiencyØ Ultrafast switchingØ 20% frequency tunabilityØ Temperature & optical tunability with complex oxide metamaterials
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