arXiv:0804.2942v2 [physics.optics] 12 May 2008
Electronic control of extraordinary
terahertz transmission through
subwavelength metal hole arrays
Hou-Tong Chen1, Hong Lu2, Abul K. Azad1, Richard D. Averitt3,
Arthur C. Gossard2, Stuart A. Trugman1, John F. O’Hara1, and
Antoinette J. Taylor1
1Los Alamos National Laboratory, MPA-CINT, MS K771, Los Alamos, New Mexico 87545,
2Materials Department, University of California, Santa Barbara, California 93106, USA
3Department of Physics, Boston University, 590 Commonwealth Avenue, Boston,
Massachusetts 02215, USA
transmission through subwavelength metal hole arrays fabricated on doped
semiconductor substrates. The hybrid metal-semiconductor forms a Schot-
tky diode structure, where the active depletion region modifies the substrate
conductivity in real-time by applying an external voltage bias. This enables
effective control of the resonance enhanced terahertz transmission. Our
proof of principle device achieves an intensity modulation depth of 52% by
changing the voltage bias between 0 and 16 volts. Further optimization may
result in improvement of device performance and practical applications.
This approach can be also translated to the other optical frequency ranges.
We describe the electronic control of extraordinary terahertz
© 2008 Optical Society of America
OCIS codes: (160.3918) Metamaterials; (240.6680) Surface plasmons; (250.6715) Switching
(260.5740) Resonance; (300.6495) Spectroscopy, terahertz
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Artificially structured composite materials are playing an increasingly important role in over-
comingthe deficiencyof naturalmaterials to obtaina functionalresponsein the terahertz(THz)
frequency range. One excellent example is the successful demonstration of THz quantum cas-
cade lasers (QCLs) using composite semiconductorheterostructures[1–3]. The so-called “THz
gap” has resulted in the general failure to translate technologies at microwave and optical fre-
quencies to the THz frequency range . The recent progress in THz metamaterials [5–9],
photonic crystals [10–14], and subwavelength metallic hole arrays [15–20], also proposed or
demonstrated high performancefunctional THz devices, which may ultimately result in a com-
plete manipulation of THz waves.
Since their experimental demonstration in the optical frequency range , subwavelength
metal hole arrays have attracted considerable attention regarding extraordinary optical trans-
mission at THz frequencies [16–19]. Such phenomena are generally attributed to the resonant
excitation of surface plasmon (SP) modes at a metal-dielectric interface [21,22]. In the THz
frequency range, the dielectric constant of metals exhibits very large values, thus the free-
space wavelength of the fundamental SP mode excited by the normally incident electromag-
netic waves in a square array of metal holes can be approximated as [16,22],
where L is the lattice parameter of the metal hole array and ε1is the dielectric constant of
the interface medium. The enhanced transmission exceeds the geometrical transmission ,
and is orders of magnitude higher than that expected for non-resonant transmission through
sub-wavelength metal holes .
The majority of research has focused on the passive properties of extraordinary THz trans-
mission, e.g., the effects of metal film thickness , hole geometry , periodicity , etc.
It has been shown that the resonance can also be affected by depositing a dielectric layer on the
metal hole arrays  and by doping a semiconductor substrate , both of which result in
significant shifting of the resonance frequency. However, little work has focused on the active
manipulationof the extraordinaryoptical transmission thoughit is essential to realize many ap-
plications. All-optical switching and modulation were demonstrated [20,27–29] by stimulated
optical modification of either the metallic surface or the interface dielectric medium. Liquid
crystal  and thermal-optical [31,32] approaches were also used to switch and modulate op-
tical signals via the application of electric field or electrical heating; however, these inherently
slow operations severely restrict potential applications. Additionally most of these demonstra-
tions are in the visible. In this paper, we demonstrate electronic switching of the extraordinary
THz transmission through subwavelength metal hole arrays fabricated on doped semiconduc-
tor substrates. The passive resonance properties are mainly determined by the geometry and
dimensions of the metal holes as well as the array periodicity. By electronically altering the
substrate conductivity via an external voltage bias, we accomplish switching of the extraordi-
nary THz transmission in real time with an intensity modulation depth as high as 52%, which
is comparable with the electronic THz modulation using metamaterial devices .
2.Sample design and fabrication
In Fig. 1(a) we show the conceptual design of subwavelength metal hole arrays for the elec-
tronically switchable resonance enhanced extraordinary THz transmission. The n-doped semi-
conductor and metal film form a Schottky diode structure, where the depletion region can be
actively controlled with an external voltage bias, thereby switching the damping of the reso-
nance and the extraordinaryTHz transmission. The samples characterized here were fabricated
on gallium arsenide (GaAs) substrates. The substrates have a 2 µm thick n-doped GaAs layer
with a free carrier density of 3.2×1016cm−3on an intrinsic GaAs wafer grown by molecular
beam epitaxy. An ohmic contact surrounding but separated from the hole array of approxi-
mately 1 mm was fabricated by electron beam deposition of 20 nm of germanium, 20 nm of
gold, 20 nm of nickel, and 200 nm of gold in sequence, followed by rapid thermal annealing
at 400◦C for 1 minute. The metal hole arrays were fabricated using standard photolithographic
methods and electron beam deposition of 10 nm of titanium and 200 nm of gold, followed by a
lift-off process. Two samples with hole geometries shown in Figs. 1(c) and (d) were made. The
hole widths are 20 µm and 5 µm, respectively.The same metal hole arrays were also fabricated
Gold hole array
Fig. 1. (a) Schematic design of the metal hole arrays exhibiting an electronically switch-
able extraordinary THz transmission. (b) Cross-sectional view of structures showing the
depletion regions under reverse voltage bias. Two unit cells are shown with (c) wide and
(d) narrow holes. All dimensions are in microns.
on intrinsic GaAs substrates to enable a comparison of the resonance damping caused by the
lossy n-doped GaAs substrates.
3. Experiments and results
We measuredthe frequencydependentTHz transmission using THz time-domainspectroscopy
(THz-TDS) . The electric field of the THz pulses was coherently measured in the time-
domain after propagating through the metal hole array samples and a bare substrate serving
as the reference. Fourier transformation of the time-domain data then yielded the frequency
dependent THz electric field amplitude and phase. Dividing the sample’s complex spectrum by
the referencewe obtainedthe normalizedtransmission amplitudet(ω) andphase φ(ω) through
the metal hole arrays. The intensity (or power) transmission is given by T(ω) = t2(ω). The
THz time-domain data was temporally windowed to eliminate effects of multiple reflections
within the GaAs substrate. This, however,truncates the THz oscillation in the time-domaindata
yielding ringing structures in the transmission spectra, particularly for the metal hole arrays
fabricated on intrinsic GaAs substrates where the damping is small, as shown by the black
curves in Fig. 2.
Without any applied voltage bias, the measured THz transmission is shown in Fig. 2 for the
metal hole arrays fabricated on n-doped GaAs and intrinsic GaAs substrates, respectively. The
intrinsic GaAs substrate can be considered as a lossless dielectric material at THz frequencies.
We observed transmission peaks near 0.75 THz and minima (Wood’s anomalies) at 0.84 THz
for the lowest modes as shown by the black curves in Fig. 2, which are as expected and whose
frequencies are determined by the periodicity of metal hole arrays. The values of the peak
0.250.50 0.751.00 1.251.50
0.250.500.75 1.00 1.251.50
Fig. 2. THz intensity transmission spectra for samples fabricated on an intrinsic GaAs sub-
strate (black curves) and on an n-GaAs substrate (red curves). (a) Hole width 20 µm and
(b) hole width 5 µm.
intensity transmission are 4 and 5 times as large as those of the geometrical transmission (i.e.
the fraction of hole area normalized to the unit cell) for sample geometries in Figs. 1(c) and
(d), respectively. When n-doped GaAs substrates are used, the extraordinary THz transmission
is significantly damped as shown by the red curves in Fig. 2. The values of peak intensity
transmission are only 1 and 0.6 times as large as those of the geometrical transmission for
sample geometries in Figs. 1(c) and (d), respectively. In addition, we observed a small red-
shift of THz transmission peaks and Wood’s anomalies, which is consistent with results in the
mid-infraredfrequencyrange where metal hole arrays were fabricated on GaAs substrates with
variousdopingconcentration.This shift is associated withthe changeof dielectricconstant
in the 2 µm n-doped GaAs layer, and is confirmed by finite-element numerical simulations.
As described above, the substrate charge carrier density and conductivity of the 2 µm n-
doped GaAs layer can be electronically modified by applying a reverse voltage bias to the
Schottky diode structure. Higher reverse voltage bias increases the depletion region thereby
reducing the damping of the resonance. In Figs. 3(a) and (b) we show the experimental THz
transmissionspectra as a functionofthe reversevoltagebias forthe metalholearraysfabricated
on n-doped GaAs substrates and shown in Figs. 1(c) and (d), respectively. In both samples
we observed increasing values of THz transmission peaks as the applied reverse voltage bias
increased, while the transmission dips are much less affected. Furthermore, the results reveal
that the modulation depth of THz transmission is also dependent on the dimensions of the
0.25 0.50 0.751.001.251.50
0.250.500.75 1.001.25 1.50
Fig. 3. The THz intensity transmission spectra as a function of the applied reverse voltage
bias for samples fabricated on n-GaAs substrates. (a) Hole width 20 µm and (b) hole width
rectangular holes. Under reverse voltage biases of 0 and 16 volts, the intensity modulation
depth of the transmitted THz radiation is definedas h=(T16V−T0V)/T16V. For the sample with
the geometry in Fig. 1(c) the modulation depth is hc= 30%, while it is as high as hd= 52% for
the sample having narrower metal holes in Fig. 1(d).
The above experimental results show that the resonance strength is very sensitive to the sub-
strate conductivity and loss. Without the voltage bias, the conductingsubstrate provides a large
loss, as well as a “short-circuit” of the metal holes, thereby significantly damping the reso-
nance and reducing the THz transmission. With the reverse voltage bias, on the other hand, the
increasing depletion reduces the substrate loss, thereby enhancing the resonance and extraordi-
nary THz transmission. Furthermore, the depletion happens not only in the regions underneath
metal, butalso extendslaterallyto theholeregionsas indicatedin Fig. 1(b).Since thelateral ex-
tension of the depletionregioninto the holes is the same forthe wider and narrowerholes under
the same voltage bias, this results in a larger proportionof depletion area in the narrower holes.
On the other hand, the electromagnetic field is significantly enhancedin the hole regions at res-
onance, which has been confirmed by NSOM measurements  and numerical simulations.
This means that the loss in the hole regions plays an important role in switching the extraor-
dinary THz transmission. In the sample with narrower holes, a larger portion of depletion has
0.250.50 0.751.001.25 1.50
Fig. 4. The THz intensity transmission spectra at 0 (black solid curve) and 4 volts (red
dashed curve) forward voltage bias.
more impact on reducing the damping of the resonance yielding a higher modulation depth of
the extraordinary THz transmission, as shown in Fig. 3. Further narrowing of the rectangular
holes should yield even higher modulation depths at the expense of lower total transmission
efficiency. The depletion dependence of the resonance and extraordinary optical transmission
may be even more promising for shorter wavelengths, where the required dimensions of in-
dividual holes are smaller and the hole area could be completely depleted, producing a larger
total transmission through the sample.
In semiconductor substrates, the dielectric constant varies with doping concentration. Ac-
cording to Eq. (1) the resonance frequency will therefore experience a significant shift .
However, with the reverse voltage bias, which should significantly change the dielectric con-
stant of the 2 µm thick n-dopedGaAs layer, we observedonly a small frequencyshift as shown
in Fig. 3. The explanation is that, due to the long wavelength of THz radiation, its decay into
the substrate could be as large as hundreds of micrometers. So modification of the dielectric
constant in the 2 µm thick n-doped GaAs layer by applying the reverse voltage bias only has a
very small effect on the resonance frequency.
Finally, we exclude the possibility of thermal effects from the current flow under the reverse
voltage bias by performing measurements for the samples under forward voltage bias, where
the change of depletion is negligible but the current flow is much larger than under reverse
bias. We observed no perceptible change in the THz transmission as compared to the unbiased
transmission (see Fig. 4), thus the current flow or sample heating has a negligible effect on
device performance. The switching of extraordinary THz transmission is purely due to the
electronic modification of the substrate conductivity and loss in the 2 µm thick n-doped GaAs
In conclusion,we havedemonstratedthe electronicallyswitchable resonanceenhancedextraor-
dinary optical transmission at THz frequencies. The formation of a Schottky diode structure
between the metal film and doped semiconductor substrate enables real-time modification of
the substrate conductivity and loss, resulting in switching of the resonance and extraordinary
THz transmission. The depletion extends into the hole regions, so that the metal hole geom-
etry and dimensions play an important role, because at resonance the electromagnetic field is
significantly enhanced in the metal hole regions. Further, a metal hole array with smaller hole Download full-text
dimensions enables a higher modulation depth. In our first generation proof-of-principle de-
vices, we accomplished an intensity modulation depth as high as 52%. Optimizing the hole
geometry and dimensions as well as the free carrier concentration and thickness of the doped
semiconductor substrate layer, the device performance may be further improved. Due to the
long decay length of the THz field into the substrate, the change of dielectric constant with
increasing depletion in the thin doped semiconductor layer is insufficient to significantly tune
the resonance frequency. These results are scalable to higher optical frequencies, where the
switching performance could be even more promising.
We acknowledgesupportfromtheLosAlamosNationalLaboratoryLDRD program.Thiswork
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, an affir-
mative action/equal opportunity employer, is operated by Los Alamos National Security, LLC
for the National Nuclear Security Administration of US Department of Energy under contract