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ABSTRACT: Multispectral imaging has shown promise in subcutaneous vein detection and localization in human subjects. While many limitations of single-wavelength methods are addressed in multispectral vein detection methods, their performance is still limited by artifacts arising from background skin reflectance and optimality of postprocessing algorithms. We propose a background removal technique that enhances the contrast and performance of multispectral vein detection. We use images acquired at visible wavelengths as reference for removing skin reflectance background from subcutaneous structures in near-infrared images. Results are validated by experiments on human subjects.
Journal of Biomedical Optics 05/2013; 18(5):50504. · 3.16 Impact Factor
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ABSTRACT: We present a novel approach for achieving tightly concentrated optical field by a hybrid photonic-plasmonic device in an integrated platform, which is a triangle-shaped metal taper mounted on top of a dielectric waveguide. This device, which we call a plasmomic light concentrator (PLC), can achieve vertical coupling of light energy from the dielectric waveguide to the plasmonic region and light focusing into the apex of the metal taper(at the scale ∼ 10nm) at the same time. For a demonstration of the PLCs presented in this paper, we numerically investigate the performance of a gold taper on a Si<sub>3</sub>N<sub>4</sub> waveguide at working wavelengths around 800nm. We show that three major effects (mode beat, nanofocusing, and weak resonance) interplay to generate this light concentration phenomenon and govern the performance of the device. Combining these effects, the PLC can be designed to be super compact while maintaining high efficiency over a wide band. In particular, we demonstrate that under optimized size parameters and wavelength a field concentration factor (FCF), which is the ratio of the norm of the electric field at the apex over the average norm of the electric field in the inputting waveguide, of about 13 can be achieved with the length of the device less than 1μm for a moderate tip radius 20nm. Moreover, we show that a FCF of 5 - 10 is achievable over a wavelength range of 700 - 1100nm with the length of the device further reduced (to about 400nm).
Optics Express 01/2013; 21(2):1898-910. · 3.59 Impact Factor
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ABSTRACT: Fluorescence tomography (FT) is depth-resolved three-dimensional (3D) localization and quantification of fluorescence distribution in biological tissue and entails a highly ill-conditioned problem as depth information must be extracted from boundary measurements. Conventionally, L<sub>2</sub> regularization schemes that penalize the Euclidean norm of the solution and possess smoothing effects are used for FT reconstruction. Oversmooth, continuous reconstructions lack high-frequency edge-type features of the original distribution and yield poor resolution. We propose an alternative regularization method for FT that penalizes the total variation (TV) norm of the solution to preserve sharp transitions in the reconstructed fluorescence map while overcoming ill-posedness. We have developed two iterative methods for fast 3D reconstruction in FT based on TV regularization inspired by Rudin-Osher-Fatemi and split Bregman algorithms. The performance of the proposed method is studied in a phantom-based experiment using a noncontact constant-wave trans-illumination FT system. It is observed that the proposed method performs better in resolving fluorescence inclusions at different depths.
Applied Optics 12/2012; 51(34):8216-27. · 1.41 Impact Factor
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ABSTRACT: Current theoretical treatment of mode splitting and scattering loss resulting
from sub-wavelength scatterers attached to the surface of high-quality-factor
whispering-gallery-mode microresonators is not satisfactory. Different models
have been proposed for two distinct scatterer regimes, i.e., a-few- and
many-scatterers. In addition, many experimental results seem difficult to
understand within the existing theoretical framework. Here we develop a unified
approach that applies to an arbitrary number of scatterers, which reveals the
applicable conditions and the limits of the existing theoretical models.
Moreover, many new understandings on mode splitting and scattering loss have
been achieved, which are supported by numerical and experimental evidences.
Such a unified approach is essential for the fundamental studies as well as the
practical applications of mode splitting and scattering loss in
high-quality-factor whispering-gallery-mode microresonators.
05/2012;
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ABSTRACT: We report an experimental observation of strong variations of quality factor and mode splitting among whispering-gallery modes with the same radial order and different azimuthal orders in a scattering-limited microdisk resonator. A theoretical analysis based on the statistical properties of the surface roughness reveals that mode splittings for different azimuthal orders are uncorrelated, and variations of mode splitting and quality factor among the same radial mode family are possible. Simulation results agree well with the experimental observations.
Optics Letters 05/2012; 37(9):1586-8. · 3.40 Impact Factor
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ABSTRACT: We present a novel approach for achieving tightly concentrated optical field
by a hybrid photonic-plasmonic device in an integrated platform, which is a
triangle-shaped metal taper mounted on top of a dielectric waveguide. This
device, which we call a plasmomic light concentrator (PLC), can achieve
side-coupling of light energy from the dielectric waveguide to the plasmonic
region and light focusing into the apex of the metal taper(at the scale ~10nm)
at the same time. For demonstration, we numerically investigate a PLC, which is
a metal (Au) taper on a dielectric (Si3N4) waveguide at working wavelengths
around 800nm. We show that three major effects (mode beat, nanofocusing, and
weak resonance) interplay to generate this light concentration phenomenon and
govern the performance of the device. By coordinating these effects, the PLC
can be designed to be super compact while maintaining high efficiency over a
wide band. In particular, we demonstrate that under optimized size parameters
and wavelength a field concentration factor (FCF), which is the ratio of the
norm of the electric field at the apex over the average norm of the electric
field in the inputting waveguide, of about 13 can be achieved with the length
of the device less than 1um for a moderate tip radius 20nm. Moreover, we show
that a FCF of 5-10 is achievable over a wavelength range 700-1100nm when the
length of the device is further reduced to about 400nm.
04/2012;
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ABSTRACT: Silver nanocubes (AgNCs), 60 nm, have four extinction surface plasmon resonance (SPR) peaks. The finite difference time domain (FDTD) simulation method is used to assign the absorption and scattering peaks and also to calculate the plasmon field intensity for AgNCs. Because AgNCs have a highly symmetric cubic shape, there is a uniform distribution of the plasmon field around them, and they are thus sensitive to asymmetric dielectric perturbations. When the dielectric medium around a nanoparticle is changed anisotropically, either by placing the particle on a substrate or by coating it asymmetrically with a solvent, the plasmon field is distorted, and the plasmonic absorption and scattering spectra could shift differently. For the 60 nm AgNC, we found that the scattering resonance peak shifted more than the absorption peak. This changes the extinction bandwidth of these overlapping absorption and scattering bands, and consequently the figure of merit of the nanoparticle, as a localized SPR sensor, no longer has a constant value.
Journal of the American Chemical Society 03/2012; 134(14):6434-42. · 9.91 Impact Factor
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ABSTRACT: We show that efficient coupling of lightwave is possible to an individual plasmonic nanoresonator in a hybrid plasmonic-photonic resonator structure. The proposed hybrid structure consists of a photonic microresonator strongly coupled to a plasmonic nanoresonator. The theory and simulation results show that more than 73% of the input power in the waveguide can be coupled to the localized resonance mode of the plasmonic nanoresonator.
Optics Express 10/2011; 19(22):22292-304. · 3.59 Impact Factor
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ABSTRACT: We present a fully reconfigurable fourth-order RF photonic filter on SOI platform with a tunable 3-dB bandwidth of 0.9-5 GHz, more than 38 dB optical out-of-band rejection, FSR up to 650 GHz, and compact size (total area 0.25 mm(2)). The center wavelength of the filter can be tuned over a wide range with a power consumption of 10 mW/nm. The filter architecture uses a unit-cell based approach to realize the desired filter specifications. The use of high-Q resonator-based components enables a dramatic reduction in size, weight and power (SWaP) of each unit cell, with the possibility of cascading a large number of these unit cells on a single chip. Thermal reconfiguration allows for low insertion loss and therefore results in the scalability of these filters. The demonstrated filter can be used in many different applications including RF photonic front-ends and high speed optical A/D conversion.
Optics Express 08/2011; 19(17):15899-907. · 3.59 Impact Factor
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ABSTRACT: We experimentally demonstrate a high resolution integrated spectrometer on silicon on insulator (SOI) substrate using a large-scale array of microdonut resonators. Through top-view imaging and processing, the measured spectral response of the spectrometer shows a linewidth of ~0.6 nm with an operating bandwidth of ~50 nm. This high resolution and bandwidth is achieved in a compact size using miniaturized microdonut resonators (radius ~2 μm) with a high quality factor, single-mode operation, and a large free spectral range. The microspectrometer is realized using silicon process compatible fabrication and has a great potential as a high-resolution, large dynamic range, light-weight, compact, high-speed, and versatile microspectrometer.
Optics Express 06/2011; 19(13):12356-64. · 3.59 Impact Factor
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ABSTRACT: We present the first demonstration, to our knowledge, of a label-free flow cytometer for the analysis of biological specimens using multiplex coherent anti-Stokes Raman scattering (MCARS) and elastic scatter measurements. The MCARS system probes the Raman vibrational energy levels and the elastic scatter provides morphological information. We demonstrate these capabilities by probing a culture of Saccharomyces cerevisiae at 100 spectra/s and constructing a background-free Raman reconstruction using a Kramers-Kronig relation. A theoretical analysis shows that this system could operate at speeds of 10 kHz with appropriate hardware; thus facilitating integration into commercial flow cytometers or use as a high-speed, stand-alone device.
Optics Letters 06/2011; 36(12):2309-11. · 3.40 Impact Factor
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ABSTRACT: We present an efficient method for the absorption of slow group velocity electromagnetic waves in photonic crystal waveguides (PCWs). We show that adiabatically matching the low group velocity waves to high group velocity waves of the PCW and extending the PCW structure into the perfectly matched layer (PML) region results in a 15 dB reduction of spurious reflections from the PML. We also discuss the applicability of this method to structures other than PCWs.
Applied Optics 03/2011; 50(9):1266-71. · 1.41 Impact Factor
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ABSTRACT: Plasmonic nanoresonators can localize light beyond diffraction limit and can provide large field enhancements and thus can be used in sensing and spectroscopy applications. Here, we numerically show that efficient excitation of plasmonic resonances of nanoparticles is possible when they are integrated with Silicon Nitride waveguides in an integrated hybrid photonic-plasmonic platform.
MRS Proceedings. 12/2010; 1294.
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ABSTRACT: We present a new way of forming phononic crystal waveguides by coupling a series of line-defect resonators. The dispersion proprieties of these coupled resonator acoustic waveguides (CRAW) can be engineered by using their geometrical parameters. We show that single-mode guiding over a large bandwidth is possible in CRAW formed in a honeycomb-lattice phononic crystal slab of holes in zinc oxide. In addition, a finite length of CRAW structure acts as an efficient selective acoustic filter for Lamb waves.
Journal of Applied Physics 10/2010; 108(8):084515-084515-5. · 2.17 Impact Factor
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ABSTRACT: We present a method for eliminating the temperature dependence of the resonance wavelength in high-Q silicon-based microdisk resonators by using a polymer cladding with a negative thermo-optic coefficient. Design requirements for athermal performance are derived based on theory and simulation, and their validity is experimentally verified.
Optics Letters 10/2010; 35(20):3462-4. · 3.40 Impact Factor
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ABSTRACT: High Q traveling-wave resonators (TWR)s are one of the key building block components for VLSI Photonics and photonic integrated circuits (PIC). However, dense VLSI integration requires small footprint resonators. While photonic crystal resonators have shown the record in simultaneous high Q (~10(5)-10(6)) and very small mode volumes; the structural simplicity of TWRs has motivated many ongoing researches on miniaturization of these resonators with maintaining Q in the same range. In this paper, we investigate the scaling issues of silicon traveling-wave microresonators down to ultimate miniaturization levels in SOI platforms. Two main constraints that are considered during this down scaling are: 1) Preservation of the intrinsic Q of the resonator at high values, and 2) Compatibility of resonator with passive (active) integration by preserving the SiO(2) BOX layer (plus a thin Si slab layer for P-N junction fabrication). Microdisk and microdonut (an intermediate design between disk and ring shape) are considered for high Q, miniaturization, and single-mode operation over a wide wavelength range (as high as the free-spectral range). Theoretical and experimental results for miniaturized resonators are demonstrated and Q's as high as ~10(5) for resonators as small as 1.5 μm radius are achieved.
Optics Express 09/2010; 18(19):19541-57. · 3.59 Impact Factor
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ABSTRACT: We propose a hybrid scheme in which light is coupled into gold nanodisks from a silicon nitride waveguide or travelling wave resonator. Large field enhancements in the vicinity of the nanodisk resonator can be achieved.
Lasers and Electro-Optics (CLEO) and Quantum Electronics and Laser Science Conference (QELS), 2010 Conference on; 06/2010
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ABSTRACT: We propose the use of titanium dioxide as cladding material to reduce the temperature sensitivity of silicon-based microresonators. The advantages of using titanium dioxide over the conventional alternatives are discussed, and experimental results are presented.
Lasers and Electro-Optics (CLEO) and Quantum Electronics and Laser Science Conference (QELS), 2010 Conference on; 06/2010
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ABSTRACT: We demonstrate planar structures that can provide simultaneous two-dimensional phononic and photonic band gaps in opto-mechanical (or phoxonic) crystal slabs. Different phoxonic crystal (PxC) structures, composed of square, hexagonal (honeycomb), or triangular arrays of void cylindrical holes embedded in silicon (Si) slabs with a finite thickness, are investigated. Photonic band gap (PtBG) maps and the complete phononic band gap (PnBG) maps of PxC slabs with different radii of the holes and thicknesses of the slabs are calculated using a three-dimensional plane wave expansion code. Simultaneous phononic and photonic band gaps with band gap to midgap ratios of more than 10% are shown to be readily obtainable with practical geometries in both square and hexagonal lattices, but not for the triangular lattice.
Optics Express 04/2010; 18(9):9164-72. · 3.59 Impact Factor
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ABSTRACT: In this work a traveling-wave resonator device is proposed and experimentally demonstrated in silicon-on-insulator platform in which the spacing between its adjacent resonance modes can be tuned. This is achieved through the tuning of mutual coupling of two strongly coupled resonators. By incorporating metallic microheaters, tuning of the resonance-spacing in a range of 20% of the free-spectral-range (0.4nm) is experimentally demonstrated with 27mW power dissipation in the microheater. To the best of our knowledge this is the first demonstration of the tuning of resonance-spacing in an integrated traveling-wave-resonator. It is also numerically shown that these modes exhibit high field-enhancements which makes this device extremely useful for nonlinear optics and sensing applications.
Optics Express 04/2010; 18(9):9447-55. · 3.59 Impact Factor
