The objective is to develop
and demonstrate a family of ultra-low loss nanoplasmonic structures
and materials via the recently proposed new hydrodynamics-inspired
approach to plasmonic nanocircuit engineering based on the
manipulation of the fine structure of the optical energy flow on the
nanoscale. Differently from the traditional design methods based on
antenna and waveguide concepts, the new approach seeks to steer the
optical powerflow through sequences of coupled nanoscale optical
vortices around a landscape of local phase singularities
strategically ‘pinned’ to plasmonic nanostructures. Powerflow
patterns in nanostructures engineered by using the proposed
methodology resemble multiple-gear transmissions, and, to reflect
this fact, are termed ‘vortex nanogear transmissions’ (VNTs). VNTs
are composed of coupled vortex nanogears made of light, with each
nanogear rotating around an axis created by a field phase
singularity.
It has already been shown
that dramatic field enhancements can be realized by arranging vortex
nanogears into pre-designed transmission sequences. These effects
have been explained by invoking a hydrodynamic analogy of the
‘photon fluid,’ whose kinetic energy can be locally increased via
convective acceleration in VNTs and then converted into pressure
energy to generate localized areas of high field intensity. Just as
mechanical and hydrodynamic transmissions form a basis of complex
machinery, rationally designed vortex nanogear transmissions can be
combined into complex plasmonic networks enabling nanoscale light
routing and switching.
New hydrodynamics-inspired approach to plasmonic nanocircuit
engineering seeks to steer optical powerflow through ‘vortex
nanogear transmissions’ pinned to plasmonic nanostructures.
Plasmonics with a
twist: taming optical tornadoes on the nanoscale," in
preparation for Plasmonics in metal
nanostructures: theory and applications (T.V. Shahbazyan
and M.I. Stockman Eds.), Springer 2012
Novel sensing principles & platforms
Hybrid photonic-plasmonic sensors
Whispering-gallery-mode (WGM)
biosensors use high-Q optical resonances to directly detect binding
of molecules and nanoparticles from a frequency shift signal. The
WGM biosensing approach is highly sensitive, down to the single
virus and nanoparticle level. However, rapid sample analysis, varied
surface functionalization of monolithic sensor arrays, and specific
detection against a complex background are some of the challenges in
the field. In turn, nanoparticle (NP) biosensing platforms such as
those based on gold (Au) NPs provide high sensitivity, large sensing
area, and rapid mixing of the analyte yet lack high spectral
resolution of WGM-based sensors.
To address these challenges, we
proposed and demonstrated a hybrid optoplasmonic biosensing platform
that combines the advantages of a NP-based assay with the high
spectral resolution of a WGM biosensor and used it to detect BSA
protein binding to NPs by analyzing wavelength shifts of hybrid
photonic-plasmonic modes. Theoretical investigation of the novel
sensing scheme reveals that orders-of-magnitude larger Q-factors of
hybrid photonic-plasmonic modes in the WGM-NP structure over that of
the single NP plasmon resonance increase the hybrid sensor spectral
resolution and greatly improve the detection limit as compared to
individual, WGM-based or NP-based sensors.
High-Q optical
microcavities emerged as ultra-sensitive label-free biochemical
sensors capable of detecting the shift of the optical mode
wavelength due to the presence of analyte or molecules bound to
resonator surface. Coupled-cavity photonic molecules provide
additional degrees of freedom over individual microcavities for
detecting environmental changes and the presence of biological nano-objects
in their nano-environment. E.g., my research has shown that
collective multicavity resonances in photonic molecules provide
better overlap of the modal fields with the analyte without
sacrificing high modes Q-factors, which results in higher detection
sensitivity.
Furthermore, a new
type of optical biosensor capable of differentiating between bulk
and surface perturbations of the ambient refractive index as well as
between specific and nonspecific binding of molecules on the sensor
surface has been proposed. The new detection scheme is based on
tracking the shifts of hybridized bonding and antibonding optical
modes in photonic molecules.
By using two measurements of spectral shifts it is possible to
discriminate between surface and volume index perturbation, to
detect specific target molecules in a complex environment and to
estimate the thickness of thin layers of adsorbed molecules.
Multiple light
scattering from nano-patterned deterministic aperiodic surfaces,
which occurs over a broad spectral-angular range, leads to the
formation of colorimetric fingerprints in their near and far-field
zones, which can be captured with conventional dark-field
microscopy.
We have recently
proposed to use these colorimetric fingerprints as transduction
signals in a novel type of highly sensitive label-free multiplexed
sensors. In particular, we engineered aperiodic arrays of Cr nano-particles
on quartz substrates, and showed that the information encoded in
both spectral and spatial distributions of structural colors can
be simultaneously utilized.
The potential of
the proposed approach for rapid, label-free detection of
biomolecular analytes in the visible spectral range was
experimentally demonstrated by showing a distinct variation in the
spectral and spatial colorimetric fingerprints in response to
monolayer increments of protein layers sequentially deposited on the
surface of aperiodic arrays of nanoparticles.
The sensitivity of the aperiodic-order-based sensor
(fabricated in Dal Negro group at BU) to different thicknesses of
silk monolayers is quantified by the spectral shift of the scattered
radiation peaks and by monitoring the spatial changes of patterns
quantified by the variances of their spectral auto-correlation
functions (measured in Omenetto group at Tufts University).
Controllable
light-matter
interactions in artificial structures and materials with the
wavelength- and sub-wavelength-scale internal structure leads to
important technological innovations in bio(chemical) sensing and
spectroscopy. I have applied multi-scale electromagnetic modeling to
design and optimize advanced platforms for trace explosives and
biological pathogens detection that combine high field enhancement,
broadband frequency response and multiplexing capabilities.
Aperiodic plasmonic ‘nanogalaxy’, a nano-structured multi-scale
material, used as a versatile SERS platform (fabricated in
Dal Negro group in BU and shown with the Staphylococcus
bacteria on the surface)
Optimization of nanoparticle clusters arrays (fabricated in Reinhard
group in BU) leads to the development of ultra-sensitive
SERS platform for trace explosives & pathogens detection
Array-assisted plasmonic nano-antennas provide multi-color
electromagnetic hot-spots with dramatic light localization &
enhancement and offer new solutions for multispectral
single-molecule detection & imaging (shown is a device
fabricated by Capasso group at Harvard University)
Long- & short-range structural order for Q- factor & radiative rate
engineering
Aperiodic order for
bandstructure engineering
Multiple light scattering in
2D structures with controlled aperiodic order offers an opportunity
to generate unique optical modes with a broad spectrum of
localization properties. This creates opportunities for the
engineering of various lasing states and conditions in
optimally-designed structures with different types of long- and
short-range order. In particular, we have investigated the photonic
bandstructure and mode localization properties of aperiodic
structures ranging from quasicrystals with varying order of
rotational symmetries to pseudo-random lattices.
Furthermore, laser
action from multiple localized and extended modes has been
demonstrated in deterministic aperiodic structures with Rudin-Shapiro
and Thue-Morse morphologies. The nanofabricated aperiodic and
pseudo-random lasers provide a novel approach, alternative to
traditional random media and photonic crystals, for the engineering
of multi-frequency coherent light sources and complex cavities
amenable to predictive theories and device integration.
Lasing in the pseudo-random medium: an engineered photonic structure
with a flat Fourier spectrum (fabricated by Hui Cao group in Yale
University) provides multiple spectrally- & spatially-separated
quasi-localized lasing modes within a narrow frequency range.
Optical microcavities,
which are characterized by discrete spectra of optical modes, can
serve as useful tools for manipulating emission spectra of embedded
atoms, molecules and quantum dots via frequency-dependent selective
coupling of the emitter to the available cavity modes. The
efficiency of such coupling is a function of the quality factor
(Q-factor) of the microcavity mode, and the number of competing
modes within the material emission spectrum. Therefore, to lower
thresholds of microcavity lasers, cavities supporting high-Q modes
with wide spectral range (FSR) are required; however, the demands
for the high Q-factor and a wide FSR are contradictory.
I have demonstrated
that properly configured coupled-cavity structures offer ways to
overcome these design constrains. For example, it was shown that by
arranging microdisks into engineered high-symmetry structures it is
possible to dramatically (up to 2 orders of magnitude) enhance a
single WG-mode while suppressing all the neighboring modes.
Shift and splitting
of wavelengths (a) and change of Q-factors (b) of the TE-polarized
WGE6,1 modes in a square four-disk PM (disk radii 0.9μm,
n=2.63) as a function of the inter-cavity gap width. The inset shows
the magnetic field distribution of two PM super-modes whose
Q-factors are boosted at certain values of the gap width
(symmetry-enhanced super-modes).
Robust schemes for
active nanoscale field modulation, frequency switching and
reversible energy transfer between photons, surface plasmons and
nanoscale emitters are still largely missing in conventional
nanoplasmonic circuitry due to the inherent weakness of the
available material effects and the small propagating distances of
surface plasmons in metals.
We propose a new way
to effectively overcome this problem by integrating high-Q photonic
elements (microcavities) into plasmonic nanocircuits. The resulting
hybrid structures and networks combine superior light
nanoconcentration properties of plasmonic nanostructures with the
capability of photonic atoms to insulate emitter-photon systems from
decohering environmental effects. We show that the proposed
optoplasmonic structures provide significant enhancement of the
emitter radiative rate and efficient long-range transfer of emitted
photons followed by subsequent re-focusing into nanoscale volumes
accessible to near- and far-field detection. They also offer the
opportunity of cascaded signal amplification via interaction of
trapped photons with the gain medium inside the cavities.
Configurable optoplasmonic nano-circuits with multiple spectral and
spatial channels enable long-range on-chip signal transfer and
multiplexing capabilities without sacrificing the extreme light
localization crucial for achieving tailored light interaction with
quantum emitters.
Complex interaction of noble-metal plasmonic structures
with light plays an important role in the manipulation of the
emission rates of embedded molecules and ions. The role of the nano-structure
in the emission rate control two-fold: (i) enhancement of excitation
rates by high-intensity localized electromagnetic fields and (ii)
modification (enhancement or quenching) of the radiative properties
of emitting dipoles due to the local density of states manipulation
at the emission wavelength.
We have designed the
geometry of the plasmonic nanostructured arrays to facilitate
maximum non-radiative transfer of energy to the SP modes in the
metal, followed by its efficient out-coupling into the far-field as
photons. We have explored various types of aperiodic plasmonic
nanostructures with pre-designed broadband frequency spectra and
have demonstrated their potential for the enhancement of the
efficiency of light emission from low-quantum yield systems (such as
Erbium).
Demonstration of
light emission enhancement from Erbium atoms coupled to plasmonic
arrays of Au nnanoparticles (a,b) on top of light emitting
Er:SiNx substrates (fabricated in Dal Negro group at BU). (c) PL spectra
excited at 488nm, (d) PL time decay of Er atoms through unpatterned
substrate (black) and Fibonacci (shown in (b)) arrays with varying
interparticle separations.
Analytical methods & CAD tools for computational electromagnetics
Boundary integral
equations methods
Highly accurate, fast and
economical analytical techniques and algorithms based on 2D surface
integral equations (SIE) were developed and implemented to simulate
and optimize novel-shape wavelength-scale resonator structures for
the next-generation optoelectronic, THz and milimetre-wave systems.
Such specially designed components increase the functionality and
improve the performance of electromagnetic devices. The method has
been applied to simulate and optimize the characteristics of laser
microcavities, wavelength-selective microdisk filters and resonant
lenses. We demonstrated manipulation of resonant frequencies and
Q-factors of natural modes of optical microcavities due to various
deformations of their contours and proposed several spectrally
engineered designs with improved stability of the lasing mode and/or
directional emission pattern. The application of the SIE method
together with the Green’s function technique and high-order
integration results in dramatic reductions in required computational
resources, i.e., memory and CPU time, and thus opens the ways to
model practical complex problems accurately and efficiently.
Highly directional in-plane light output from a notched microdisk
laser can be achieved without serious degrading of the
whispering-gallery-mode mode Q-factor
Test
of the FDTD accuracy in the analysis of the scattering resonances
associated with high-Q whispering-gallery modes of a circular
cylinder,
JOSA A. 25(5) 1169-1173,
2008.
Lens
or resonator? Electromagnetic behavior of an extended hemielliptic
lens for a sub-mm wave receiver,
Microwave Opt. Technol. Lett.
43(6) 515-518, 2004.
Multiple-scattering methods for photonics design
2D
multiple-scattering spectral methods based either on discretization
of surface integral equations in piece-wise inhomogeneous domains or
on direct expansion of partial scattered fields into a series of
functions that form a complete basis have been developed for the
design and optimization of photonic and plasmonic nanostructures.
The developed techniques reduce the problem space to the surfaces of
individual scatterers comprising the nanostructure, which
drastically lowers the numerical effort. They also automatically
impose the radiation condition at infinity, and enable treatment of
both high and low index-contrast materials with material losses and
gain. The developed algorithms account for all electromagnetic
interactions within complex structures and thus provide superior
accuracy of the numerical solutions.
The rich spectrum of morphology-dependent modes in photonic
molecules and lattices makes them very attractive platforms for the
manipulation of spatial emission patterns of embedded emitters. I
have optimized complex photonic structures to single out a preferred
direction of emission and obtain directional light output.
Coupled-optical-microcavity structures can be pre-designed
such that their optical spectra feature points of avoided frequency
crossing of two (or more) optical modes. At such points, modes
interchange their identities, and this interchange offers exciting
prospects for adding new functionalities such as signal modulation,
switching, and routing (as in the branched coupled-resonator
waveguide shown above).
Optical fibers and
waveguides are essential building blocks of most optical devices and
systems related to communications, sensing, and optical computing.
To reduce the cost of waveguide analysis and optimization, efficient
CAD simulation techniques are highly desirable. I have developed
highly efficient full-vectorial contour integral equation analysis
of the natural modes of dielectric waveguides of arbitrary
cross-sections and applied it to study, design and optimize
non-canonical-shape waveguides. The algorithms are formulated in the
complex domain and so immediately allow calculation of leaky modes
and treatment of lossy (e.g. noble metals) and amplifying media.
Both fundamental and higher order mode propagation characteristics
can be investigated in bound, leaky and complex regimes. The method
is very versatile and with some modifications may be applied to
waveguides of arbitrary geometrical shapes located in the layered
dielectric media, multi-core & multi-cladding fibers, waveguides
with significant gain-guiding effects, and hybrid photonic-plasmonic
waveguides.
Reflectors are among
the oldest and most popular antenna configurations used in radar and
communication applications. When a reflector is located a
complicated near-zone environment, conventional approximate
techniques can fail to predict an effect of the surroundings on
antenna properties. Semi-analytical
techniques and design tools based on the contour integral equation
(converted to the dual series equations regularized by analytical
inversion of the static part) have been developed to simulate and
optimize circular cylindrical reflector antennas in the presence of
imperfect flat earth. The feed directivity was included in the
analysis by using the complex source point method.
Radiation features not predicted by approximate methods have been
observed. E.g., the sidelobe level and, hence, the directivity can
be severely affected by the antenna aiming angle, elevation, and the
type of the soil underneath.
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