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IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 17, NO. 1, JANUARY/FEBRUARY 2011
Recent Progress in Electromagnetic Metamaterial
Devices for Terahertz Applications
Hu Tao, Willie J. Padilla, Xin Zhang, and Richard D. Averitt
(Invited Paper)
Abstract—Recent advances in metamaterials (MMs) research
have highlighted the possibility to create novel devices with unique
electromagnetic (EM) functionality. Indeed, the power of MMs lies
in the fact that it is possible to construct materials with a userdesigned EM response at a precisely controlled target frequency.
This is especially important for the technologically relevant terahertz (THz) frequency regime with a view toward creating new
component technologies to manipulate radiation in this hard to access wavelength range. Considerable progress has been made in the
design, fabrication, and characterization of MMs at THz frequencies. This article reviews the latest trends in THz MM research.
Index Terms—Metamaterials (MMs), terahertz (THz).
I. INTRODUCTION
ECENTLY, artificially structured electromagnetic (EM)
materials have become an extremely active research area
because of the possibility of creating materials which exhibit
novel EM responses not available in natural materials. This includes negative refractive index [1]–[6], superlensing [7]–[9],
cloaking [10]–[12], and more generally, coordinating transformation materials [13]–[16]. This has generated tremendous
worldwide interdisciplinary efforts including physicists, material scientists and engineers. For the most part, these composites,
often called metamaterials (MMs), are subwavelength composites, where the EM response originates from oscillating electrons
in highly conducting metals such as gold or copper allowing for
a designed specific resonant response of the electrical permittivity (ε = ε1 + iε2 ) or magnetic permeability (μ = μ1 + iμ2 ).
Continuous media with negative parameters, namely, with
negative εr or μr have been known in EM theory for a long
time. The Drude–Lorentz model, applicable to many materials
in nature, predicts that above resonance there exists a region,
where ε1 or μ1 is negative. If the losses are sufficiently low, it
becomes possible to take advantage of this negative response.
Media with negative ε1 over a broad frequency range are found
R
Manuscript received February 11, 2010; revised March 22, 2010; accepted
March 23, 2010. Date of publication June 1, 2010; date of current version
February 4, 2011.
H. Tao and X. Zhang are with the Department of Mechanical Engineering, Boston University, Boston, MA 02215 USA (e-mail: hutao@bu.edu;
Xinz@bu.edu).
W. J. Padilla is with the Department of Physics, Boston College, Chestnut
Hill, MA 02467 USA (e-mail: willie.padilla@bc.edu).
R. D. Averitt is with the Department of Physics, Boston University, Boston,
MA 02215 USA (e-mail: raveritt@physics.bu.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JSTQE.2010.2047847
Fig. 1. Negative refractive index materials with a periodic array of SRRs
interleaved with metallic thin wires for the magnetic responses and electric
responses, respectively.
in nature [17], [18]. The best-known examples are metals or
doped semiconductors, where ε1 is negative below the plasma
frequency [19]–[21]. However, media with negative μ1 are less
common in nature, which is partly due to the weak magnetic interactions in most of the naturally existing materials. At terahertz
(THz) frequencies, it has long been known that antiferromagnets
such as FeF2 or MnF2 exhibit a resonant magnetic response. In
artificial materials, negative μ1 was first realized with split-ring
resonators (SRRs).
SRRs are composed of metallic rings with gaps as theoretically introduced by Pendry et al. [22] and experimentally verified by Smith et al. in 2000 [23]. As shown in Fig. 1, an
appropriately designed combination of two sets of resonators,
namely, metallic wires and SRRs, can yield negative ε1 and μ1
in the same frequency band resulting in a negative refractive
index. Furthermore, these MM structures are scalable to operate
over most of EM spectrum spanning from microwaves to optical
frequencies [24].
The THz regime of the EM spectrum extends from 100 GHz
to 10 THz (1 THz = 1012 Hz, where 1 THz corresponds to a
wavelength of 300 μm and photon energy of 4.1 meV) [25], [26].
This region, alternatively called the far-IR, lies below the visible
and IR frequencies and above the microwave frequencies, as
shown in Fig. 2. The response of natural materials, arising from
interactions of the EM field with the electron, forms the basis for
the construction of most modern devices. However, the nature of
the EM response of materials changes as a function of frequency.
At frequencies of a few hundred gigahertz and lower, the motion
of free electrons forms the basis of most EM devices. On the
other hand, at IR through optical/UV wavelengths, photon-based
1077-260X/$26.00 © 2010 IEEE
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93
Fig. 2. THz regime of the EM spectrum extends from 100 GHz to 10 THz,
which lies below visible and infrared (IR) wavelengths and above microwave
wavelengths.
devices are dominant. In between these two regions, there exists
the so-called “THz gap,” where the efficiency of electronic and
photonics responses tend to taper off. As such, the THz regime
is arguably the least developed and least understood portion of
the EM spectrum scientifically and technologically [27], [28].
Recently, there have been important advances using electronic and optical techniques to generate and detect THz radiation. During the past two decades, significant progress has been
achieved in THz science and technology. As examples, the emergence of THz time-domain spectroscopy (THz-TDS) [29]–[32]
and THz quantum cascade lasers [33]–[37] have been spectacular in advancing the state of the art.
Since the EM response of MMs can be designed over a large
portion of the EM spectrum by, to first order, simply scaling
the dimensions of the structures, MMs have played an increasingly important role particularly in the construction of functional
THz devices. For THz MMs, the unit cell is few tens of microns
with critical feature sizes of a few microns. For these length
scales, conventional microfabrication techniques offer considerable flexibility to experimentally realize novel MM structures
and devices. This paper focuses on MMs that are designed to operate in the THz regime with the emphasis on the recent progress
on developing THz MM devices, which may have real-world
applicability.
II. FUNDAMENTAL STUDIES OF THZ MMS
Various types of subwavelength resonators for building MMs
have been theoretically designed and experimentally demonstrated during the last decade, as for example, thin metallic
wires [38]–[40], Swiss rolls [41], [42], pairs of rods and crosses
[43]–[47], fishnet structures [48]–[52], and SRRs [53]–[57].
Among these resonator structures, SRRs are the canonical
subwavelength particle used in the majority of THz MMs to
date [58]–[63]. SRRs were originally designed and utilized for
magnetic responses [64]–[68]. When a time-varying magnetic
field is polarized normal to the plane of the SRRs, circulating currents will be induced within the ring, resulting in an
out-of-phase or negative magnetic response above the resonant
frequency [1]. A potential limitation of SRRs to be used for
the magnetic response at THz and higher frequencies is that
the magnetic field needs to be perpendicular to the SRR plane
for full magnetic field coupling. However, the EM waves are
usually incident normal to the planar SRR structure with the
magnetic field lying in the SRRs plane, which does not excite
the magnetic resonance directly.
Fig. 3. Magnetically coupled SRRs with different geometries excited by a 30◦
off-normal incident wave with an ellipsometer [69].
Fig. 4. Magnetic MMs at (a) ∼100 THz (mid-IR) [70] and (b) ∼200 THz
(near-IR) [71].
Nonetheless, the first THz MM was experimentally demonstrated by Yen et al. in 2004, showing a strong magnetic response around 1 THz, using a single planar layer of double
SRRs array [69]. The MM sample was put in an ellipsometer
measured in a transverse electric (TE) configuration at an angle
of incidence of 30◦ off-normal for the excitation of the magnetic response as shown in Fig. 3. The electric field was aligned
parallel to the side with the SRR gap to eliminate electric field
coupling. By scaling down the SRR size and slightly simplifying
the structure for the ease of fabrication, similar measurements
have been performed with single SRRs showing a magnetic
resonance at mid-IR (100 THz) [70] and near-IR (200 THz)
regime [71], as shown in Fig. 4. The scaling rule breaks down
beyond ∼200 THz due to the fact that metals cannot be treated
as perfect conductors anymore.
In principle, SRRs can also be used as electrically resonant
particles, as they exhibit a strong resonant permittivity at the
same frequency as the magnetic resonance by rotating the incident THz radiation 90◦ with the electric field perpendicular
to the gap and the magnetic field lying completely in the SRRs
plane. This enables using SRRs as electric MMs with the same
SRR structures for constructing magnetic MMs [72], [73]. Of
course, the full EM response is complicated, and care must be
taken to determine the nature of the response for a given incident field orientation. The SRR can be though of as an LC
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Fig. 5. (a) Single SRR excited purely by magnetic field normal to SRR plane
with electric field parallel to the gap. (b) Same SRR excited purely by in-plane
electric field perpendicular to the SRR gap with magnetic field lying in plane of
the SRR. (c) Equivalent circuit of SRR with the gap as the capacitance and the
current path as the inductance. (d) Two resonances are present, namely, (e) LC
resonance and (f) dipole resonance.
Fig. 7. Coupling between the LC and dipole resonances can be tuned by
changing the shape of the SRRs [75].
Fig. 8. Designs of novel planar electric MMs with (a) two distinct electric
resonances [76], (b) three electric resonances [77], and (c) multiple electrical
resonances depending on the fractal level [78].
Fig. 6. THz electric MMs with symmetry geometries along the electric field
that suppress the magnetic response in favor of a pure electric response [74].
resonator
in a simple representation with a resonance frequency
of ω0 ∼ 1/LC, where the inductance results from the current path of the SRR and capacitance is mainly determined
by the split gap and the dielectric properties of the substrate.
With the polarization of electric field perpendicular to the SRR
gap, two resonances are present, namely, an LC resonance (ω0 )
at lower frequency associated with circulating currents and a
dipole resonance (ω1 ) at higher frequency, as shown in Fig. 5.
The dielectric constant of the substrate affects both resonances,
showing a redshift for higher values of the dielectric constant.
Furthermore, though SRRs can exhibit either a purely negative electric or a magnetic response by being exposed to different
orientations of incident radiation and polarization of electric or
magnetic fields, the electric and magnetic resonant responses
are coupled. This results in a complicated bianisotropic EM behavior, leading to considerable complexity in characterizing the
comprehensive EM response of MMs. A number of alternative
SRR structures have been designed to suppress the magnetic
response in favor of a pure electric response [74], as shown in
Fig. 6.
The number and position of the capacitive gaps affect the position and linewidth of the LC resonance. The dipole resonance
can be tuned by changing the distance between SRR sidebars,
which are parallel to the electric field [75]. For example, provided that the current path of the SRR is fixed, resulting in a
constant LC resonance frequency, the dipole resonance can be
shifted to a lower frequency by enlarging the SRR sidebar distance, as shown in Fig. 7. Tuning the LC and dipole resonances
is very important in many applications, where the resonance
reshaping due to the LC–dipole coupling is undesired.
So far, the operation of ordinary MMs is confined with a
narrow spectral range due to the nature of the resonator, which
is a major impediment to most broadband applications. Efforts
have been made on broadening the bandwidth of THz MMs by
packing two or more resonators with different geometries into a
single unit cell for multiresonant responses, as shown in Fig. 8.
III. FABRICATION AND CHARACTERIZATION
TECHNIQUES OF THZ MMS
A great deal of MMs research has focused on the microwave
frequencies due in part to the ease of fabrication and characterization of subwavelength structures at these frequencies. While
MMs can theoretically operate from submicrowave through the
visible portion of the EM spectrum, some limitations may affect their performance [79], [80]. Currently available microand nanofabrication technologies are the preferred methods for
fabricating planar THz MMs, also called metafilms or metasurfaces, with the smallest geometric features down to tens of
nanometers.
To date, the majority of the reported THz MMs has been on
single planar of resonator arrays on semiconductor substrates
such as gallium arsenide (GaAs) [58], [74], and high resistance
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95
Fig. 9. (a) Free-standing THz MMs fabricated on polyimide substrates [84].
(b) Out-of-plane THz chiral MMs fabricated using electro-plating technique
[85].
silicon [81]. Recently, directly patterning of the MM resonator
structures on soft polymer substrates [82]–[84], which are
highly mechanically flexible and transparent to THz radiation,
have been demonstrated providing a promising path to creating
nonplanar MMs at THz frequencies, as shown in Fig. 9(a).
However, single layer THz MMs raise the issue of lack of
magnetic coupling for normal incidence THz waves. The magnetic resonance is difficult to access, which, as mentioned earlier, requires a component of the magnetic field to thread the
loop-like metallic resonators. Though this issue may be partially
solved by using an oblique angle of incidence, full coupling of
the magnetic field required the resonators to “stand up” out of the
supporting substrates, and complicates the fabrication process.
Furthermore, most of these THz MMs are highly anisotropic,
which can, for example, complicate the designing transformation optics devices such as perfect lenses and invisibility cloaks.
Recently, researchers have demonstrated THz negative index
MMs using out-of-plane chiral structures, as shown in Fig. 9(b).
However, in order to facilitate the progress in those advanced
MM applications, new manufacturing methodologies are indeed
needed.
THz-TDS is one of the most successful and thus most commonly used technologies to characterize the THz MMs performance, which relies on the generation of broadband EM transients using ultrafast laser pulses. The generation and detection
scheme is sensitive to the changes in the amplitude and the phase
of the THz radiation [29]. The transmission of the THz electric
field is measured for a sample and a reference. The electric field
spectral amplitude and phase are calculated through Fourier
transformation of the time-domain pulses. Dividing the sample
by the reference yields the complex spectral transmission for
the far-field characterization of the THz MMs [73], [74].
While THz-TDS measurements provide important macroscopic far-field information of MM samples and enables extraction of the effective permeability and permittivity, understanding the local fields and current distribution of the subwavelength
elements is also interesting and challenging. This necessitates
the development of new technologies such as near-field optical
imaging as shown schematically in Fig. 10(a) and (b). Fig. 10(c)
displays the in-plane electric field distribution, shown as black
arrows, near the SRR surface, measured using an aperture-based
technique. This can be processed to determine the change of the
out-of-plane magnetic field as shown in color. The information
of the EM fields distribution and variation helps in understand-
Fig. 10. (a) Overview of the THz near-field microscope setup and (b) the
cross section of the detector chip and the mounted sample. (c) Measured EM
near-field distribution and (d) the simulated current density [86].
ing the origin of the far-field resonances [86], [87], especially in
combination with simulations of the surface current density as
shown in Fig. 10(d). Therefore, combination of THz-TDS and
near-field imaging for tracing the localized near-field electric
and magnetic responses provides the opportunity to fully characterize the functional origin and response of THz MMs. Such
measurements will be important in gaining a better understanding of interactions between nearest neighbor SRRs, and will in
turn, facilitate the design of novel THz MM devices.
IV. RECENT PROGRESS IN THZ MM DEVICES
MMs play a potentially important role in creating necessary
functional devices for THz applications. A variety of THz components/devices based on THz MMs have been presented. This
includes perfect absorbers, THz amplitude and phase modulators, structurally reconfigurable THz MMs, and memory THz
MMs. In this section, we present a brief review of some recent
progress regarding novel THz MM devices for manipulating
THz radiation.
A. THz MM Absorbers
As described earlier, MMs can be regarded as an effective medium characterized by a complex electric permittivity
ε = ε1 + iε2 and complex magnetic permeability μ = μ1 + μ2 .
Considerable effort has focused on the real parts of permittivity
(ε1 ) and permeability (μ1 ) to create a negative refractive material. To create such structures, it is important to minimize
losses (over the operating frequency range) associated with
the imaginary portions (ε2 and μ2 ) of the effective response
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Fig. 11. (a) Schematic of the THz MM absorber with the electric resonator on
the top of a polyimide spacer and a cut wire on the GaAs substrate for magnetic
coupling. (b) Experimental results (blue) showing the absorber reaches a value
of 70% at 1.3 THz, which is in good agreement with the simulation results (red),
considering the material loss and fabrication imperfections [88].
functions. Conversely, for many applications, it would be desirable to maximize the loss, which is an aspect of MM research
that, to date, has received less attention. Such an absorber would
be of particular importance at THz frequencies, where it is difficult to find naturally occurring materials with strong absorption
coefficients that, further, would be compatible with standard
microfabrication techniques.
Tao et al. experimentally demonstrated a MM-based absorber
with an absorptivity of 0.70 at 1.3 THz [88]. A single unit of the
absorber consists of two distinct metallic elements: an electrical
ring resonator and a magnetic resonator, as shown in Fig. 11.
The electrical ring resonator consists of two single split rings
sitting back to back, which couple strongly to the electric field,
and negligibly to the magnetic field. The magnetic coupling is
realized by combining the center wire of the electric resonator
on the top layer with a cut wire on the bottom layer in a parallel
plane separated by an 8-μm-thick polyimide spacer. The magnetic response can be tuned by changing the geometry of the cut
wire and the thickness of the polymer spacer. The EM responses
are tuned to match the impedance to the free space and minimize
the transmission at specific frequency. Given the 8 μm thickness
of the MM, this corresponds to a power absorption coefficient of
2000 cm−1 , which is significant at THz frequencies, making this
low-volume structure a promising candidate for the realization
of enhanced, spectrally selective, thermal detectors.
This study has been extended to a polarization insensitive
design with a demonstrated absorptivity of 0.65 at 1.15 THz
[90]. Tao et al. reported a resonant MM absorber fabricated
on a metallic ground plane showing an absorptivity of 0.97
at 1.6 THz [89]. The absorber design is on a highly flexible
polyimide substrate, which enables its use in nonplanar applications. In addition, the absorber can operate over a very wide
range of incident angles for both TE and transverse magnetic
(TM) configurations (see Fig. 12). Those MM absorbers may
find numerous applications ranging from the active element in
a thermal detector to THz stealth technology.
B. THz Quarter Waveplates
Birefringent crystals have long been used as quarter waveplates (QWPs) in optics for converting linear polarization to
Fig. 12. (a) Schematic of flexible wide angle THz MM absorber. (b) Absorber
can be applied to nonplanar surfaces. (c) and (d) Numerical calculations show
that the absorber has high absorption over a wide range of angles of incidence
for both TE and TM radiation [89].
Fig. 13. Individual unit cells for (a) MM QWP and (b) meanderline QWP.
Simulated (blue) and measured (purple) circular polarization percentage for
(c) MM QWP and (d) meanderline QWP [98].
circular and vice versa. The meanderline polarizer is an artificial alternative to the crystal-based QWPs for use at millimeter
wave and near-IR frequencies partly due to its low cost and ease
of fabrication [91]–[94]. Recently, MMs and meanderlines have
been investigated at microwave and THz frequencies [95]–[97].
Strikwerda et al. reported a meanderline QWP and an electricsplit-ring resonator (ELC)-based QWP with a center frequency
of 639 GHz [98]. As shown in Fig. 13, the 70-μm-thick meanderline QWP with double layer meanderline structures achieved
99.6% circular polarization, while ELC-based QWP achieved
99.9% with much thinner structure of 20 μm. The meanderlinebased QWP showed a larger bandwidth of operation with over
99% circular polarization from 615 to 743 GHz, while the
ELC-based QWP displayed 99% from 626 to 660 GHz, both
TAO et al.: RECENT PROGRESS IN EM METAMATERIAL DEVICES
Fig. 14. (a) THz MM modulator with electrically controlled modulation.
(b) MM elements are connected together with metal wires to serve as Schottky
gate. (c) Electrons in the n-GaAs layer are driven away from the interface by a
reverse-bias voltage, which forms a depletion region near the split gap resulting
in a restoration of the resonance [99].
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Fig. 15. (a) Schematic and (b) optical microscopy photograph of the electrically driven THz MM phase modulator. (c) Measured transmission phase
spectra of the devices at 0 V and reverse-bias voltages of 4 and 16 V. (d) Phase
shift (red) and amplitude modulation depth (blue) show a linear dependence of
the applied voltage [100].
of which are broad enough for use with continuous wave (CW)
sources.
C. THz MM Switches and Modulators
As existing THz switches and modulators prove to be insufficient for practical applications, MMs offer a promising
alternative for the dynamic control and manipulation of THz
radiation. Various THz MM switches and modulators have been
proposed through various modifications to the existing MMs.
Functionalizing these MM modulators is mainly achieved via
dynamic modification of the surrounding environment using
external electrical, thermal, or optical stimuli, leading to a modulation of the resonance strength in the transmission amplitude.
Padilla et al. reported the first THz MM switch fabricated on
GaAs substrates with the potential for creating dynamic MM
resonance responses [73]. The electric resonance could be turn
ON/OFF by photoexcitation of the free carriers in the GaAs substrate to short or open the resonator gap, which leads to a modulation on the THz radiation transmission.
Another type of THz modulator via an external electric bias
was reported by Chen et al. [99]. The SRR structures were fabricated on a thin n-type GaAs layer, where the conductivity can
be externally modified by applying a voltage bias through a
group of metallic wires connecting those resonators to a voltage
source. No resonance is observed, as the n-type GaAs substrate
electrically shorts out the resonator gaps. The resonators serve
as a Schottky contact with the substrate. A reverse voltage bias
depletes carriers in the capacitive regions, thereby, isolating the
metal from the doped substrate, resulting in the restoration of
the resonance. A schematic of the device is shown in Fig. 14.
This device could modulate the THz transmission by 50% at a
few KHz. An improved version with a similar modulation mechanism but having a reduced RC time constant through judicious
layout design enabled modulation at over 2 MHz [101].
It has also been demonstrated that the phase of THz radiation
can be modulated by a THz MM modulator with a similar design
Fig. 16. (a) SEM photograph of frequency-agile THz MM. (b) Experimentally
measured transmission spectrum as a function of photoexcitation power [102].
to [99]. The device achieves a voltage-controlled linear phase
shift at 0.89 THz, with a modulation of ∼π/6 rad at 16 V [100],
as shown in Fig. 15.
As mentioned earlier, the resonant frequency of MMs is fixed
by the geometry, dielectric properties, and dimensions of the
resonators. Chen et al. reported a frequency-agile MM device,
which is able to shift the center resonance frequency by 20%
with external optical pumping [102]. The MMs were fabricated
on a silicon on sapphire wafer. The resistive silicon layer is
∼600 nm thick and is patterned as the capacitor plates of the
SRRs. Fig. 16 shows that as the conductivity of the silicon capacitor plates is increased through photoexcitation, the resonance
redshifts from 1.06 THz to 850 GHz.
As these examples highlight, this new class of THz MM
switches/modulators demonstrate a promising approach toward
promoting real-world applications such as THz communication
and THz wave detections.
D. Structurally Reconfigurable THz MMs
It is well known that the overall properties of a material are
not only determined by the nature of the constituent atoms, but
also depend dramatically on the lattice structure. The same rule
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Fig. 17. (a) Schematic and (b) optical microscopy photograph of the structurally reconfigurable THz MM fabricated on bimaterial cantilevers. Measured
transmission phase spectra of the tunable (c) magnetic response and (d) electric
response as a function of frequency for (e) various orientations of the SRRs set by
rapid thermal annealing process at successive temperatures up to 550 ◦ C [104].
applies to MMs, and even better to some extent. Compared with
natural materials, where the tunability of their properties through
tuning the crystal lattice is determined by the chemical bonding
and nature of the atoms themselves, the range of tunability for
MMs is more accessible, as the “lattice effects” can be made
much stronger by an appropriate design [103].
Tao et al. reported a novel structurally reconfigurable THz
MM with tunable electrically and magnetically resonant responses through mechanically reorienting the microfabricated
resonators within their unit cells [104]. The SRR structures
were fabricated on bimaterial cantilevers designed to bend out
of plane in response to a thermal stimulus, as shown in Fig. 17.
A 30% and 50% tunability of the electric and magnetic resonance in the transmission spectrum are experimentally observed
at ∼0.5 THz, respectively, which can be potentially used for reconfigurable filters, cloaks, and concentrators.
E. THz MMs With Memory Effects
Though most MMs are designed to operate at a single resonant frequency, decent progress has been made on developing
frequency-agile MMs at THz frequencies that allow their resonant frequency to be tuned with certain stimulus, as mentioned
earlier. However, the tuning is lost when the stimulus is taken
away.
Driscoll et al. reported a new THz MM device that can remember the new frequency of operation by incorporating vanadium dioxide (VO2 ) into the conventional SRR structures [105].
It is known that the metal-to-insulator phase transition of VO2
can be controlled with external optical or electrical stimulus
and exhibiting hysteresis enables programming of the MMs
response [106], [107]. The resonant frequency of the SRRs depends on the VO2 ’s capacitive properties and is set until the
phase changes back. In their device, the resonant frequency was
shifted from 1.65 THz by as much as 20% and persisted for at
least 20 min, as shown in Fig. 18.
Fig. 18. (a) Schematic of the memory metamaterial device at THz frequencies
with a gold array of SRRs patterned on a VO2 film. (b) Measured transmission spectrum of the device as a function of frequency under various ambient
temperatures, showing that the resonance frequency was shifted to the lower
frequencies with increased temperature. (c) Resonance frequency can be also
modified by applying electric pulse (top), and the tuned resonance persisted for
at least 20 min until the device was thermally reset (bottom) [105].
V. SUMMARY
The utilization of and implementation of MMs at THz frequencies holds great promise for advancing applications in this
technologically relevant region of the EM spectrum. THz EM
MMs have attracted enormous attention and intensive research
efforts, and a number of practical MM-based THz devices have
been developed, including filters, absorbers, QWPs, switches,
and modulators. In many cases, the MM-based THz devices outperform their conventional counterparts, though in many cases
such conventional counterparts do not exist at THz frequencies.
Though most MM devices operate over a narrow spectral band
due to their resonant nature, efforts have been put in making
devices with frequency tunability and multiple/broadband functionality, which are favored for applications employing CW
THz sources/detectors. New fabrication techniques, advanced
near-field characterization and novel MM designs have lead to
dramatic advances during the past five years. There will be certain additional fundamental advances during the next decade
coupled with the implementation of MMs into real-world THz
applications.
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Hu Tao received the B.S. degree in mechanical and automation from the University of Science and Technology of China, Hefei, China, in 2003, and the
M.S. degree in physical electronics from the Institute of Electronics of Chinese
Academy of Sciences, Beijing, China, in 2006.
He is currently involved in manufacturing engineering at Boston University,
Boston, MA. His research interests include design and realization of microthermal detectors and microelectromechanical systems-enhanced active metamaterial devices.
Willie J. Padilla received the B.S. degree in physics from San Diego State
University, San Diego, CA, in 1996, and the M.S. and Ph.D. degrees in physics
from the University of California, San Diego, in 2002 and 2004 respectively.
He was a Director’s Postdoctoral Fellow at Los Alamos National Laboratory from 2004 to 2006. In 2006, he joined the Department of Physics, Boston
College, Chestnut Hill, MA, as an Assistant Professor, where he is currently
involved in the investigation of the infrared, optical, and magneto-optical properties of novel materials.
101
Xin Zhang received the Ph.D. degree in mechanical engineering from the Hong
Kong University of Science and Technology, Hong Kong, China, in 1998.
From 1998 to 2001, she was a Postdoctoral Researcher and then a Research
Scientist with the Massachusetts Institute of Technology (MIT), Cambridge.
She joined the Department of Manufacturing Engineering and the Department
of Aerospace and Mechanical Engineering, Boston University, Boston, MA, in
January 2002, where she is currently an Associate Professor. She has authored
or coauthored more than 50 journal publications and more than 100 conference
proceeding publications and presentations in the field of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS). Her research interests include applying materials science, micro-/nanomechanics, and
micro-/nanomanufacturing technologies to solve various engineering problems
that are motivated by practical applications in MEMS/NEMS and emerging
nanobiotechnologies, which has been sponsored by the National Science Foundation, the Air Force Office of Scientific Research, the Air Force Research
Laboratory, the Army Research Laboratory, and a host of industries.
Dr. Zhang was in the Technical Program Committee for both the IEEE
International Conference on MEMS and the IEEE Solid State Sensors and Actuators Workshop. She is the recipient of the Boston University SPRInG Award
(2002), the National Science Foundation Faculty CAREER Award (2003), and
the Boston University Technology Development Award (2004), and was an Invitee of the National Academy of Engineering, Frontiers of Engineering (2007).
Richard D. Averitt received the Ph.D. degree in applied physics from Rice
University, Houston, TX, in 1998.
He was a Director’s Postdoctoral Fellow at Los Alamos National Laboratory
from 1999 to 2001, and then became a Staff Scientist in 2001. In 2006, he joined
the Department of Physics, Boston University, Boston, MA, as an Assistant
Professor. His research interests include the optical and electronic properties of
artificial and quantum-based multifunctional materials.