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Homepage of Svetlana V. Boriskina |
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Department
of Electrical and Computer Engineering,
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H Last updated: 19 Jun 2010 |
Research
projects
Micro and
nano-scale integrated biosensors that combine multiple sensing modalities on
the same platform, including label-free refractive-index-change sensing,
fluorescence sensing and Surface Enhanced Raman Scattering can provide
exciting opportunities for a variety of high throughput lab-on-a-chip
applications, including medical diagnostics, biohazard detection,
environmental sensing, and rapid genome sequencing. I have recently proposed
a way to engineer multiplexed sensing platforms by exploiting a combination
of narrow-linewidth photonic modes and a broadband plasmonic near-field
enhancement in photonic-plasmonic nano-structures with a controlled level of
disorder. One possible realization of such structures is a 2D array of
noble-metal nanoparticles arranged according to a quasi-periodic sequence
such as Fibonacci, Gaussian Prime, etc. We have exploited the ability of
plasmonic arrays to generate localized intense hot-spots in the design of
robust and efficient SERS substrates, and measured spatially-averaged
reproducible 108 SERS enhancement factors using pMA and live
bacteria as Raman markers. Remarkably, the same structures can produce the
narrow-linewidth scattering resonances suitable for label-free detection by
monitoring their spectral shifts. Photonic-plasmonic sensing platforms can
also be engineered to operate in the IR part of the electromagnetic spectrum,
providing an overlap with the spectral fingerprints of many important
biological molecules. They can also be easily integrated with microfluidics
for a controllable delivery of targets to the sensor area.
High-Q optical
microcavities have also 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., typical sharp
asymmetric Fano scattering resonances of photonic molecules translate into
higher sensor sensitivity. My research also showed that collective
multicavity resonances in microdisk and photonic-crystal photonic molecules
provide better overlap of the modal fields with the analyte without
sacrificing high modes Q-factors, which results in higher detection
sensitivity of PM-based biosensors. Furthermore, better field overlap with
biological molecules attached to or surrounding the PM structure provides a
mechanism of enhancing their fluorescence intensity and/or Raman signal
intensity, paving the road for designing chip-scale integrated biosensing
platforms with multiple sensing modalities.
Optical
nano-antennas that couple propagating light into localized surface plasmons
(SPs) have been demonstrated for a variety of applications including optical
manipulation, fluorescence enhancement, and nonlinear spectroscopy. Typical
dimer plasmonic gap nano-antennas consist of two noble-metal nanoparticles
coupled through a nanometer-scale gap. They provide strong resonant
enhancement of the electric field confined to a sub-wavelength gap if the
wavelength of the incident field is matched to the antenna SP wavelength. SP
resonances of nano-antennas can be tuned across the visible and IR by proper
choice of material, by engineering the particles shapes, and by introducing
dielectric nano-loads in the gap. However, the possibility of light focusing
into a single sub-wavelength spot at multiple frequencies is expected to
provide a remarkable range of new functionalities, including resonant
enhancement of pumping and emission efficiency, background-free sensing of
optically-trapped objects, broadband near-field imaging, etc. I have recently
proposed that by enclosing gap nano-antennas into multiple-periodic gratings
of metal nanoparticles with optimally tuned periodicities and particle sizes,
the near-field intensity spectra of nano-antennas can be tailored to feature
several peaks of dramatic field enhancement. These novel antenna designs can
form a basis of new platforms for linear and non-linear spectroscopy with a
potential for single-molecule sensitivity. The spectral positions of antenna
resonant peaks can be tuned across the visible and IR bands to align them
with the absorption bands of detected particles or labels for fluorescence
detection or with molecule vibrational modes for collectively enhanced IR
absorption (CEIRA) or SERS experiments. For example, it was demonstrated that
the multiple-wavelength grating-assisted gap nano-antennas can be configured
to provide simultaneous SP-mediated increase of the excitation rate at the
pumping wavelength and the radiative decay rate at the emission wavelength.
Strong and controllable
modification of the local density of optical states in optical microcavities,
photonic-bandgap dielectric nano-structures, and noble-metal plasmonic
structures (Purcell effect) 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. In the vicinity of a metal
nano-particle, emitting dipole can decay non-radiatively by coupling into the
localized particle surface plasmons (LSP), with subsequent outcoupling into
photons mediated by the scattering properties of the metal nano-structure.
For optimum enhancement, the geometry should be designed to facilitate
maximum non-radiative transfer of energy to the SP modes in the metal,
simultaneously the optical cross-sections have to be optimized for coupling
the energy from the LSP into the far-field as photons. We have explored
various types of aperiodic photonic and plasmonic nanostructures as broadband
platforms for radiative engineering and have demonstrated their potential for
the enhancement of the efficiency of light emission from low-quantum yield
systems (such as Erbium). 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, described by the Purcell
factor, 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 contradiction. 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 neighboring modes.
Furthermore, the rich spectrum of
morphology-dependent modes in photonic and plasmonic nano-structures makes
them very attractive platforms for the manipulation of spatial emission
patterns of embedded emitters. Achieving highly directional (preferably
unidirectional) in-plane light output from conventional microlasers (such as
microdisks and microspheres) without seriously degrading the mode Q-factor
challenges designers of low-threshold microlasers. To address this problem, I
have formulated basic design rules to tune the spectral and emission
characteristics of micro-scale optical microcavities by introducing specially
designed local and global cavity deformations. Furthermore, I demonstrated
that clustering several symmetrical microcavities into configurations that
break their symmetry makes possible singling out a preferred direction of
emission and obtaining directional light output (translating into high
collection efficiency) from microlasers based on coupled microresonators and
photonic-crystal defect cavities.
A unique capability of optical
micro-cavities to trap and delay light pulses can be harnessed in a variety of
on-chip high-bit-rate signal processing devices including multiplexers,
modulators, wavelength-selective add/drop filters, optical buffers, optical
waveguides, switches and routers. High-index-contrast semiconductor
microresonators characterized by strong lateral field confinement provide
advantages for high-density integration with other semiconductor components.
However, a very important and costly issue in highly confined monolithic
systems is optical coupling. The strong optical field confinement and small
coupling interaction lengths between circular microresonators and planar
waveguides require using small air gaps, which are difficult to fabricate. I
have proposed and demonstrated several design strategies to reduce the
dependence of the coupling efficiency on the width of the air gap, including
tailoring the resonator shape and exploring various coupling configurations. Waveguides composed of
electromagnetically-coupled optical microcavities (coupled resonator optical
waveguides or CROWs) can be used for light guiding, slowing and storage. I
have studied mechanisms of the coupling of whispering gallery modes and
guiding light around bends in CROWs composed of both identical and
size-mismatched microdisk resonators. Accurate numerical analysis revealed
differences in WG modes coupling in the vicinity of bends in CROWs composed
of optically-large and wavelength-scale microcavities, and possible ways to
design low-loss CROW bends and to reduce bend losses have been proposed. Modern and future WDM optical
networks require development of dynamically tunable components.
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 memory functions. Tight field confinement
and long photon lifetimes in high-Q cavities enable realization of such
functions in very compact structures with relatively low power. My research
efforts have been focused on exploring tunable coupled-cavity-based optical
components, and yielded several useful designs including optical flip-flops
and CROW routers.
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. One of the attractive features of the approach is that it is formulated in the complex domain and so immediately allows calculation of leaky modes and treatment of lossy and amplifying media. Both fundamental and higher order mode propagation characteristics have been investigated in bound, leaky and complex regimes for several practical dielectric fibers and waveguides. Furthermore, fused fiber couplers have been studied and engineered. The method is very versatile and with some modifications may be applied to waveguides of arbitrary geometrical shapes located in the layered dielectric media, such as rib waveguides of various profiles, multi-cladding fibers, dielectric image guides, and waveguides with significant gain-guiding effects.
Planar slot or strip elements combined
with dielectric lenses are attractive building blocks for mm and sub-mm wave
receivers due to their capability for compact integration with other
electronic components. Furthermore, they provide better efficiency than other
types of antennas printed on homogeneous substrates. Ray-tracing techniques
commonly used to simulate dielectric lenses neglect the lens size and
curvature, and hence fail to characterize the internal resonances in the lens
material. To simulate accurately the electromagnetic
behavior of elliptic and hemielliptic lenses, we apply the Muller boundary integral equation technique. Our
numerical results demonstrate effects that cannot be predicted with GO or
physical optics approximations. The most important
feature revealed by our analysis is that resonances may play a dominant role
in the lens behavior. We have
proposed a narrow-band
receiver based on a hemielliptic lens tuned
to a resonance. Possible features of such a lens-coupled receiver are
stability of the resonance field with respect to the angle of arrival of
incident wave and several times greater values of the peak field intensity
that may potentially lead to higher sensitivity and better scanning
performance.
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. The
radiation of a circular cylindrical reflector antenna in the presence of
imperfect flat earth was simulated with a contour integral equation,
converted to the dual series equations regularized by analytical inversion of
the static part. The feed directivity was included in the analysis by using
the complex source point method.
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