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Excitation of dark multipolar plasmonic resonances at terahertz frequencies
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Scientific RepoRts | 6:22027 | DOI: 10.1038/srep22027
www.nature.com/scientificreports
Excitation of dark multipolar
plasmonic resonances at terahertz
frequencies
Lin Chen
†
, YuMing Wei, XiaoFei Zang, YiMing Zhu & SongLin Zhuang
We experimentally observe the excitation of dark multipolar spoof localized surface plasmon
resonances in a hybrid structure consisting of a corrugated metallic disk coupled with a C-shaped
dipole resonator. The uncoupled corrugated metallic disk only supports a dipolar resonance in the
transmission spectrum due to perfect symmetry of the structure. However, the dark multipolar spoof
localized surface plasmon resonances emerge when coupled with a bright C-shaped resonator which
is placed in the vicinity of the corrugated metallic disk. These excited multipolar resonances show
minimum inuence on the coupling distance between the C-shaped resonator and corrugated metallic
disk. The resonance frequencies of the radiative modes are controlled by varying the angle of the
C-shaped resonator and the inner disk radius, both of which play dominant roles in the excitation of
the spoof localized surface plasmons. Observation of such a transition from the dark to radiative nature
of multipolar spoof localized plasmon resonances would nd potential applications in terahertz based
resonant plasmonic and metamaterial devices.
Surface plasmon polaritons (SPPs) are the electromagnetic waves propagating at the at interface between a
conductor and a dielectric. ey are coupled to an electric charge density uctuation on the metallic surface
1
.
e propagation constant is greater than the wave vector in the dielectric, so that the wave is conned leading
to evanescent decay on both sides of the interface. Because the SPPs are usually limited to the sub-wavelength
range, they could achieve subwavelength spatial resolution and the strong eld enhancement eect, which have
widespread applications in near-eld optics
2
, Plasmon antennas
3
, and surface-enhanced phase shi
4
. Besides
SPPs on the at metal surface, localized surface plasmons (LSPs) arise around the metallic nanoparticles or nano-
shells
5–7
. LSPs originate from the scattering eect of a sub-wavelength conductive nanoparticle in an oscillating
electromagnetic eld. e closed curved surface of the metallic particle exerts an eective restoring force on the
driven electrons, leading to eld amplication both inside and in the near-eld zone outside the particle. us,
dierent from SPPs, LSPs are non-propagating excitations of the conduction electrons in metallic nanostructures
coupled to the electromagnetic eld. In the visible and near-infrared regions, the frequency is much greater than
the collision frequency of the metal (on the order of 10
14
Hz), resulting in the coupling of the electromagnetic eld
to the free charges in the metal and the strong interaction strength between the wave and electron plasma with
small damped inuence. At the very low frequencies of the microwave and terahertz regions, however, the fre-
quency is much lower than the collision frequency. e driving eld comes from bound charges and dissipation
decreases clearly due to the fact that the depth of the wave penetration into the metal is much shorter than the
corresponding wavelength (the skin depth is on the order of 100 nm), leading to the weak interaction between the
wave and electron plasma at these frequencies
8–11
. en, instead of metallic particle, a periodic textured metallic
disk structure was theoretically proposed to support spoof LSPs
12
. Such a designed metallic curved surface with
subwavelength grooves can achieve the connement of longer wavelength electromagnetic waves. us, localized
surface plasmon-like modes are supported with the electric eld localized more strongly on the metallic particle
surface. ese novel spoof LSPs had been experimentally veried in the microwave regime
13–17
. At terahertz
frequencies, only the dipole LSP was successfully excited and studied
9,18
. However, the experimental verica-
tion of multipolar spoof LSPs has not been reported in the terahertz range because the excitation of the coaxial
Shanghai Key Lab of Modern Optical System, and Engineering Research Center of Optical Instrument and
System(Ministry of Education), University of Shanghai for Science and Technology, No. 516 JunGong Road, Shanghai
200093, China.
†
Present address: Oklahoma State University, Stillwater, Oklahoma, USA. Correspondence and
requests for materials should be addressed to Y.Z. (email: ymzhu@usst.edu.cn)
Received: 09 December 2015
Accepted: 03 February 2016
Published: 23 February 2016
OPEN
www.nature.com/scientificreports/
2
Scientific RepoRts | 6:22027 | DOI: 10.1038/srep22027
antenna (or monopole) source cannot extend its band into the terahertz range in the case of grazing incidence.
erefore, novel designs and structures that could excite terahertz multipolar spoof LSPs still require a thorough
investigation.
In this contribution, we theoretically and experimentally demonstrate the excitation of terahertz multipo-
lar spoof LSP resonances in the transmission spectra at normal incidence in a hybrid structure consisting of a
corrugated metallic disk (CMD) coupled with a C-shaped resonator (CSR). One may ask why multipolar spoof
LSPs could not be observed experimentally in terahertz transmission spectra until now. e bottleneck is that
this multipolar spoof LSPs in CMD can be seen as dark modes which cannot be directly excited by normal inci-
dent wave in symmetric structures. Here we added a “bright” CSR in the vicinity of CMD and experimentally
observed the evident transmission dips corresponding to resonant modes of high azimuthal order (dipole to
decapole modes). e physical origin of this phenomenon is due to strong coupling between the bright reso-
nant mode of parallel CSR (the tangent at the center of CSR is parallel to the polarized direction of the incident
wave) and the dark multipolar modes of CMD
19,20
. In addition, the unique features of this near-eld coupling are
discussed by changing three parameters: the gap between CSR and CMD, the angle of CSR, and the inner disk
radius. e property of a hybrid structure with asymmetric CMD is also investigated. Moreover, we found that
the quadrupole and octupole modes does not exist if such hybrid structure is changed by placing second identi-
cal CSR symmetrically with respect to the rst one. Both hybrid structures show polarization dependence. e
observed results in this work provide a new perspective to engineer terahertz multipolar spoof LSPs which could
be extremely useful in the next generation active and passive components, such as sensors, lters and modulators.
Results
Localized surface plasmons in corrugated metallic disk. First, we analyzed the transmission spectra
at the normal incident on a CMD at terahertz frequencies. In Fig.1(a), we schematically illustrate the geometry of
CMD, which is composed of outer disk radius R = 150 μm and inner metallic disk of radius r = 60 μm surrounded
by total of N = 36 grooves with the periodicity d = 2π R/N. e parameter α = a/d = 0.4 is the air-lled ratio in the
single periodic structure (a: groove width) and thickness of the metallic lm (aluminum, σ
Al
= 3.56 × 10
7
S.m
−1
)
disk t = 200 nm.e metallic disk is based on a 25 μm-thick polyimide substrate with dielectric constant of 3.5
and loss tangent of 0.05
21
. e period of the unit cell is p = 420 μm. From the experiment point of view, it is tech-
nically dicult to verify these spoof LSP modes in grazing incidence at terahertz frequencies due to the lack of
suitable terahertz sources and lower side coupling coecient. Here, we fabricated the sample using conventional
photolithography (the geometric parameters are chosen to be same as in Fig.1(a)) and measured the transmission
spectrum of this sample by using fast and slow scan-based terahertz time-domain spectroscopy (THz-TDS) sys-
tems
21–24
, as shown in Fig.1(b) (the simulated results are also shown for comparison). e resonance frequency
Figure 1. (a) e geometric parameters of the CMD. (b) Transmission spectra of the symmetric CMD. e
inset shows electric eld (E
z
) distribution at 0.35 THz resonance, which can be veried as dipole mode.
www.nature.com/scientificreports/
3
Scientific RepoRts | 6:22027 | DOI: 10.1038/srep22027
for simulation (experiment) is 0.35 THz (0.357 THz) and the Q-value of resonance for simulation (experiment) is
14 (4.6).ere is only one resonance observed experimentally. e electric vertical (E
z
) distribution on the plane
2 μm above the conguration at this resonance frequency (0.35 THz) in the inset of Fig.1(b) veried that it is
indeed a dipole mode. As a result, in this symmetric CMD, the normal incident wave cannot propagate along the
groove and excite higher order azimuthal standing surface waves.
Spoof localized surface plasmons in corrugated metallic disk coupled to a C shaped dipole resonator.
Figure1 indicates that only the main dipole mode can be directly excited by normal incidence in CMD. To excite
multipolar resonances at normal incidence by using THz-TDS, we introduce asymmetry in the CMD structure.
In this section, we proposed the hybrid spoof LSP structure consisting of one CMD and one CSR structure. e
inset of Fig.2(a) depicts the schematic diagram of the CMD and CSR hybrid structure. e angle of CSR θ (60
o
),
the CSR inner radius R
c
(170 μm) and width w (20 μm) of CSR are also marked in the inset of Fig.2(a). e other
parameters are the same as Fig.1(a). en the gap between CMD and CSR is g = R
c
– R = 20 μm. e terahertz
wave with electric eld parallel to the CSR arc illuminates the sample at normal incidence.
is hybrid structure can excite spoof LSP modes. To see this clearly, Fig.2(a) shows the simulated and the
experimental transmission spectra of the proposed structure that consists of one CMD and one CSR. We focused
on the spoof LSPs spectrum band (0.1–0.7 THz). Apparent multipolar resonances (marked by C1–C5) could be
found theoretically and most of these resonances (C1–C4) were observed experimentally. e higher resonance
(marked by M) arises from the bright LSPs excited by the CSR. To investigate the underlying physics of the
multipolar resonances, the electric eld E
z
corresponding to dips C1–C5 and M are shown in Fig.2(b). e CSR
is resonant as a LSP mode M, while the disk shows multipolar modes, which are in accordance with the spoof LSP
modes of a CMD. ese ve resonant modes correspond to di- (C1), quadru- (C2), hexa- (C3), octu- (C4) and
decapolar (C5) resonance modes. is phenomenon is similar to that observed at the grazing incidence in the
CMD structure
13
. By investigating the eld response (Fig.2(b)), we can nd that the CSR is directly dipole excited
by the incident parallel electric eld along the arm. ere is direct electrical dipole coupling with the radiation
Figure 2. (a) eoretical (top) and experimental (bottom) transmission spectra of the proposed hybrid
structure. e inset shows the schematic diagram of the CMD and CSR hybrid structure. e angle of CSR θ,
the CSR inner radius R
c
and width w of CSR are also marked (b) E
z
eld distribution of multipolar resonance
frequencies at 0.329 THz (C1, dipole), 0.382 THz (C2, quadrupole), 0.422 THz (C3, hexapole), 0.458 THz (C4,
octupole), 0.477 THz (C5, decapole), and 0.528 THz (M). e resonance (M) comes from the bright LSP mode
supported by the single parallel CSR structure.
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