Aerospace & Defense Technology - April 2021 - 32

Tech Briefs

Frequency Agile Plasmonic Antennas and Sensors
Developing an integrated method for assembly and characterization of individual nanostructures will
facilitate the exploration of new design principles for plasmonic photonic devices.
Army Research Office, Research Triangle Park, North Carolina
s demand for faster and smaller electronic and photonic devices increases, plasmonic technology, which
shows promise for controlling light at
length scales well below the optical diffraction limit, has emerged. The small
length scale of plamsmonic devices, however, brings serious challenges in assembling, designing, and characterizing. The
objective of this research is to develop an
integrated method for assembly and characterization of individual nanostructures,
to explore new design principles for plasmonic photonic devices, and to demonstrate prototypical devices to verify the effectiveness of both theoretical simulation
and experimental approaches.
The nanomanipulation method was
used, which is based on atomic force microscopy (AFM), to assemble plasmonic
nanophotonic device in a reconfigurable
manner and to characterize these devices
using optical dark-field scattering spectra.
In addition to the development of the
methodology, the expected outcome of
the project included the demonstration of
two novel photonic devices. The first device is a frequency agile nanoantenna.
Different from plasmonic waveguides,
which have short propagation lengths
limited by material loss, antennas may
serve as an alternative approach to transfer energy/optical signals in plasmonic
circuits via free-space radiation.
The second device is a novel sensor
based on optical dark modes in nanorods.
Dark modes can effectively enhance fields
and store energy; therefore, they may find
applications in sensing as well as lasing
and switching. The design of these novel
plasmonic devices take advantage of optical circuit element concepts and explore
unique near-field couplings between
metallic nanostructures.
As a result of this research, it was experimentally demonstrated for the first time
that a single semiconductor quantum dot
placed in nanometer-scale proximity of a
plasmonic cavity can be used to control
the scattering spectrum and anisotropy of
the latter. Many quantum network and

Scattering spectra (a.u.)

A

a

b

ϕA= π / 2

Z

y

ϕA
x

ϕA=π / 4

ϕA=0

E0

c

4
3
2
1
0

480

500

520
540
560
Wavelength (nm)

580

600

E0

Calculation demonstrating how the near-field coupling modifies the far-field scattering spectra of a NP-QD
hybrid structure placed on a glass substrate. (a) The scattering spectra of a NP-QD hybrid excited by the
unpolarized evanescent wave coming from the glass substrate side in all azimuthal angles. The angle A
indicates the orientation of the analyzer in the path of the scattered light. Fano feature is the most (least)
prominent when the orientation of the analyzer is parallel (perpendicular) with the in-plane component of
the Fano axis, which connects the QD and MNP centers. (b) and (c) shows the field near the NP-QD hybrid
at 500 nm and 552 nm, respectively, as indicated by the black arrows in the scattering spectrum at A =
0 in (a). The scattering signal at these two wavelengths is the same (indicated by the dotted line on the
blue curve) while the MNP is excited much more strongly in Fig 1c. This wavelength dependence proves that
the presence of the QD indeed controls the MNP's scattering and anisotropy resonantly.

information processing schemes require
the enhanced light- matter interaction between a single quantum emitter and a
cavity, enabling the effective conversion
between photonic and matter-based
quantum states. Those cavity-quantum
electrodynamics (QED) effects require a
high Purcell factor FP㲍Q/V, where Q is the
quality factor, and V is the volume of the
cavity mode.
Prior experiments exploring cavity
QED effects associated with single emitters coupled to plasmonic cavities or
waveguides focused almost exclusively
on the observations of reducing the emitter's lifetimes. The possibility of controlling the scattering of a plasmonic
nanocavity by a single (and inherently
quantum and nonlinear) two-level system has also been proposed but never experimentally observed.
The strongly coupled MNP-QD hybrid
structure was assembled into a well-controlled geometry using the technique of

AFM nanomanipulation. The strong coupling between the MNP and QD is experimentally confirmed by measuring the exciton lifetime. Analyzing the polarization
and spectral properties of light scattered
by the MNP-QD hybrid, it was observed
that the overall plasmonic cavity scattering is significantly modified over a broad
spectral range. A Fano resonance spectrally aligned with the QD's quantized exciton resonance is clearly identified when
the polarization of the scattered photon is
along the Fano axis connecting the MNP's
center with the QD. The anisotropic scattering spectrum observed in the experiments suggests that a polarization-controlled, versatile quantum light source
may be realized in this simple QD-MNP
cavity system.
The calculated polarization-resolved
scattering spectra by the QD-MNP (diameters: 2rQD = 6nm and 2rMNP = 30nm) hybrid are shown in Fig.1(a) for three polarization angles cpA of the analyzer placed

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Aerospace & Defense Technology - April 2021

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Aerospace & Defense Technology - April 2021 - Intro
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