The boundaries between traditional scientific disciplines are disappearing.
Physics and engineering are merging with unlike academic areas such as biology and biochemistry
in an effort to answer fundamental questions in life sciences and to advance tomorrow's frontiers
in biomedicine. The interdisciplinary exchange of concepts and technologies has an immediate
impact on experimental design and interpretation. More importantly, a holistic approach to
scientific endeavor is emerging. Novel biological questions are addressed with newly gained
physical insight, and engineering of new scientific machinery is becoming an integral part of
Label-free Biosensing with Micro- and Nanoscale Optical Resonators. We are interested in design and fabrication of photonic structures and circuits that interface, probe and
manipulate biological systems on the molecular level. To reach this objective, light-matter
interaction can be sufficiently enhanced by photon recirculation in micro- and nano-scale cavities (ring resonators, photonic crystals, etc.) that
offer ultimate Q and record-low modal volume. Once established, the technique can help elucidate recognition,
interaction and transformation of label-free biomolecules, the interplay of which give rise to various
complex functions and networks that have evolved in the cell. Furthermore, access to a vast repertoire of
functionality by self-assembly of purified or genetically altered biological components provides exciting
opportunity for engineering of molecular-photonic device architecture. Read the
Label-free Single Virus Nanoparticle Detection
Influenza A virus particles are detected from discrete changes in resonance frequency of a microsphere cavity. The mass (5.2 x 10(-16) g and size (47 nm) of single virions is determined directly from the magnitude of the wavelength shift, demonstrating a label-free approach towards identification of an unknown virus without the need for antibodies.
Resonator Light Force: Nanoparticle Trapping, Propulsion and Detection
Light force can significantly enhance binding rates and propel nanoparticles. A toroidal resonator is immersed in an aqueous solution of nanoparticles. Within the reach of the evanescent field, the particles are drawn toward the surface by optical gradient forces. In the absence of binding sites at low ionic strength the nanoparticles propel around the resonator.
Journal of Applied Physics
Merging Biosensing & all-optical Computation: Low Power Computing Circuits
combined advantages of high Q-factor label-free biosensors, all-optical tunability of photochromic microcavities, compactness, and low power control signals, with
the flexibility of cascading switches to form circuits, and reversibility and reconfigurability to realize
arithmetic and logic functions, makes the design of sensors merged with all-optical computational circuits promising for practical applications.
This is a collaboration with Dr. Sukhdev Roy, Dayalabagh Educational Institute, India.
Anderson Localization & "Photonic Quasicrystals"
Symposium: 50 Years of Anderson Localization
Strong Photon Localization in Disordered Photonic Crystal Waveguides
Fabrication disorder is perceived as impediment to ideal performance of optical resonators, and disorder ultimately limits the performance of any engineered device. However, when superimposed on a photonic band gap material, disorder itself can induce light localization. We show such strong photon localization by disorder in photonic crystal waveguides. In this design concept, disorder and variability does not limit device performance, but instead is the basis for high-Q resonance. This new approach to engineering circumvents limitations posed by disorder, and illustrates a bioinspired design principle found throughout nature.
Periodic high-index-contrast photonic crystal structures such as two-dimensional arrays of air holes in dielectric slabs confine light in defects where the lattice periodicity is broken. Localized optical modes are formed in cavities that are defined by removing, shifting or changing the size of the lattice components. Optimized introduction of such local structural perturbations has produced optical nanocavities with ultra-small modal volumes and record-high quality (Q) factors of over a million. Sensitivity of such photonic crystal nano-cavities is expected to surpass the single molecule level.
We are interested in a conceptually different approach to photon localization in photonic crystal structures. Our design concept introduces structural perturbations uniformly throughout the fabricated crystal by deliberately changing the shape of elements that form the lattice. Such nanometer-scale disorder effectively represents randomly-distributed strong scatterers that affect propagation of Bloch-waves through the otherwise periodic lattice. We show that the guided modes in line-defect waveguides defined in such disordered photonic crystals experience coherent backscattering that leads to Anderson localization. The effect is observed in a narrow frequency band close to the guided mode's cutoff where the light propagates with a slow group velocity (slow light regime). Optical cavities with Qs of ~2 x 105 and micron-scale modal volumes are observed along disordered waveguides (picture). Preliminary 2D FDTD calculations performed on random structures yield modal volumes that are comparable to photonic-crystal-heterostructure cavities studied by Noda et al. which suggest that we are indeed dealing with nanocavities. We believe our experiments can find various applications e.g. in optical sensing systems and random nano-lasers.
Read the PRL paper
Read the Applied Physics Letters paper
Read the 2008 SPIE Proceedings
Random Microcavities for Active Devices
The experimental observation of enhanced photoluminescence from high-Q silicon-based random photonic crystal microcavities embedded with PbSe colloidal quantum dots is being reported. The emission is optically excited at room temperature by a continuous-wave Ti-Sapphire laser and exhibits randomly-distributed localized modes with a minimum spectral linewidth of 4 nm at 1550 nm wavelength (collaboration with Bhattacharya's Group, Michigan, and Xu's Group, Penn State).
Read the Applied Physics Letters paper
Whispering Gallery Mode Biosensing
Label-Free Detection down to Single Particles
Optical resonance is created by localizing coherent light within a micro- or nanoscale structure so that it interferes constructively. Examples for such miniature optical resonators are silica microspheres and silicon photonic crystals. Because these optical resonators are almost immune to damping in a liquid, they are ultra sensitive biosensors: for example, single virus particles can be detected from discrete resonance frequency-shifts without requiring any chemical or fluorescent labeling of the particles. Sensitivity on the single particle level is possible due to the high quality (Q-) factor and the small size of the resonator.
Probing Molecular Conformation by Q-Enhanced Pump-Probe Spectroscopy
Optical microresonators with small modal volumes and high quality (Q)
factors significantly enhance interaction of the optical field with the material through recirculation,
which makes them exceptionally sensitive to the optical properties of the resonator and the surrounding
medium. We use this attribute for biosensing, where binding of only a few molecules on the microcavity
surface shifts the frequencies of the resonant modes that evanescently interact with the adsorbed
material. Furthermore, a pump-probe spectroscopy is implemented where a visible optical pump
centered at a molecular absorption band induces changes of molecular structure which are dynamically
monitored with an infrared probe. Changes in molecular conformation and orientation can be determined
from measurements with transverse-electric (TE) and -magnetic (TM)
polarized resonant modes 0.5 Å3 resolution. We demonstrate this approach by measuring
orientation and polarizability changes of retinal in bacteriorhodopsin membranes. Our technique
promises novel insights in biological signal transduction (e.g. by G-Protein coupled
receptors) and in energy conversion by photosynthetic pigments.
Read the Biophysical Journal paper
Light Manipulation with Molecularly Functionalized Microcavities
Dynamic, photoinduced molecular transitions in a self-assembled bacteriorhodpsin
(bR) monolayer are used to reversibly configure a micron-scale high-Q photonic circuit
element in which the optical response is resonantly enhanced. The all-optical resonant coupler operates
at the telecom frequencies (1,311/1,550 nm) and represents a unique bottom-up paradigm for
fabrication of hybrid molecular-photonic architectures that employ ordered molecules for optical
manipulation at small scales.
Light manipulation on the molecular level relies on changing the phase or the intensity
of probing light as a result of interactions with molecular dipoles. Although the perturbation to an optical
mode caused by a single molecule is negligible, the effect becomes more pronounced when the mode interacts
with highly-organized molecular assemblies. In such anisotropic systems, the simplest of which is a molecular
monolayer, the optical response is not obscured by local clustering and bulk averaging, and can therefore
be tailored polarization and frequency specific. However, due to the extremely low optical density of these
self-assembled monolayers (SAMs), the possibility of using them to effectively manipulate light at small
scales has been uniformly ignored. Our experiments show that a biological SAM can perform a basic photonic
switching function on a scale of a few hundreds of microns. All-optical signal routing between two optical
fibers was achieved by using a high-Q optical microcavity to resonantly enhance interaction of the
evanescent field of an optical mode with a bR monolayer. The demonstrated all-optical coupler operates
in the frequency-domain, far from the bR absorption bands, which allows it to modulate intense near-IR
probe beams with low-intensity visible pumps (< 200 µW).
Read the .
Bacteria Detection and Biofilm Analysis
Microcavity sensors provide a useful tool to study biologically important surface coatings and biofilms which are formed from larger particles such as micrometer-sized bacteria. The established theory is then no longer valid since a form-factor is necessary to account for well-defined size, shape, refractive index profile and orientation of micron-sized bacterial particles; and one can no longer disregard scattering in the analysis. We develop theory that analyzes perturbations to optical microcavity sensors induced by random adsorption of bacteria. Theoretical results are confirmed in measurements taken with E.coli bacteria as model system, establishing the whispering gallery mode biosensor as sensitive technique for detection and analysis of micro-organisms.
Read the .
Molecular Analysis by Micro-Optical Resonances
Read the review article: ,
B.I.F. Futura, Vol. 20, 239-244 (2005)
Ever since the pioneering work on high quality (Q) factor whispering gallery resonances in
spherical micrometer sized resonators by Ashkin and Dziedzkic there have been a plethora of articles
which have extended the measured ultimate Q to 1010. The interest in optical
resonators has been fueled by a diversity of areas from studies of strong coupling in quantum
electrodynamics to ultra-sensitve biosensing [F.Vollmer et al, Appl. Phys. Lett.,
80, 4057 (2002)]. The latter interest, which has produced a record sensitivity,
depends on the shift in resonance frequency due to the perturbation by adsorbed nanoparticles
(i.e. protein molecules, DNA, etc.).
We present an optical biosensor with unprecedented sensitivity for detection of unlabeled
molecules. Our device uses optical resonances in a dielectric microparticle (whispering gallery modes) as
the physical transducing mechanism. The resonances are excited by evanescent coupling to an eroded optical
fiber and detected as dips in the light intensity transmitted through the fiber at different wavelengths.
Binding of proteins on the microparticle surface is measured from a shift in resonance wavelength. We
demonstrate the sensitivity of our device by measuring adsorption of bovine serum albumin and we show its
use as a biosensor by detecting streptavidin binding to biotin.
The original work was done in collaboration with
Polytechnic University, Brooklyn.
Tuning through whispering gallery modes.
A tapered optical fiber is running vertically through the image. A microsphere
cavity is evanescently coupled to the fiber. Figure-8
shaped resonant modes are excited in the spheroidal cavity at different
Varying the coupling between the fiber and
microsphere leads to the excitation of different resonant modes.
Integration of Fiber Optics, Microcavities and Microfluidics'
There is great interest in the miniaturization of (bio)-chemical analysis systems for
laboratory use as well as for biomedical point-of-care testing, which is driven by availability of simple,
low-cost fabrication methods for microfluidic structures. The soft-lithography-based, microfluidic
technology has already found numerous applications in lab-on-chips and in opto-fluidic devices. We are
interested in controlling aqueous environment of microcavity sensors and photonic crystal devices by
integration with microfluidic structure that allows for controlled sample delivery, multiplexing,
high-throughput, as well as tuning of optical device property. To ensure accurate laminar flow
operation and to implement closed-loop control it is often necessary to integrate flow sensors.
Therefore, there is a need for low-cost, micron-sized flow sensors that can be fabricated using standard
laboratory techniques. We present a robust optical flow sensor with wide dynamic range built from an
optical fiber that can be integrated in a closed microfluidic channel of variable width. The sensing
mechanism is based on displacement of a fiber-tip by microfluidic drag force which reduces the intensity
of transmitted light. The dynamic range is adjustable by thinning the silica fiber-tip in a one-step
chemical etch. Assembly is simple since alignment is guided by preformed, polymer-molded channels.
We find that thin tapers are even sensitive to acoustic induced flow. A direction worth exploring
would be the design of a microfluidic channel that allows for frequency separation, similar in
design to the cochlea.
Read the .
Ring-Resonator for Chiral Discrimination
We investigate the use of optical resonators to observe chirality in molecules.
This project is a collaboration with Peer Fischer. A chiral substance can occur in two mirror-image
forms (enantiomers). Most biochemical reactions involve chiral molecules (e.g. aminoacids,
nucleotides, carbohydrates) and interestingly only one of the two enantiomers is biologically
active. We present an instrument based on an optical ring resonator that can discriminate chiral molecules.
Different from any other optical technique, our approach analyses the chiral phenomenon in the frequency
domain. The sensitivity of the ring resonator is independent of its size which makes it an ideal analytic
component for a lab-on-a chip.
Read the .
Frequency-domain Microscopy and Precision Metrology using Ring Resonators
Ring resonators are in general not amenable to strain-free (non-contact)
displacement measurements. We show that this limitation may be overcome if the ring resonator, here a fiber-
loop, is designed to contain a gap, such that the light traverses a free-space part between two aligned
waveguide ends. Displacements are determined with nanometer sensitivity by measuring the
associated changes in
the resonance frequencies. Miniaturization should increase the sensitivity of the ring resonator interferometer.
Ring geometries that contain an optical circulator can be used to profile reflective samples or as components
in optical data storage devices. With
Read the .