Adibi, Ali

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Now showing 1 - 10 of 18
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    Azimuthal-order variations of surface-roughness-induced mode splitting and scattering loss in high-Q microdisk resonators
    (Georgia Institute of Technology, 2012-05) Li, Qing ; Eftekhar, Ali Asghar ; Xia, Zhixuan ; Adibi, Ali
    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.
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    Self-sustained gigahertz electronic oscillations in ultrahigh-Q photonic microresonators
    (Georgia Institute of Technology, 2012-05) Soltani, Mohammad ; Yegnanarayanan, Siva ; Li, Qing ; Eftekhar, Ali Asghar ; Adibi, Ali
    We report on theoretical and experimental observations of self-sustained fast [gigahertz (GHz)] electronic oscillations resulting from coupled electron-photon dynamics in ultrahigh-Q Si microdisk resonators with cw pumping. Our theoretical analysis identifies conditions for generating steady-state GHz oscillations while suppressing thermal oscillations [megahertz (MHz)] with submilliwatt input laser power. Such fast oscillations are tunable via changing the free-carrier (FC) lifetime of the resonator. Integrating a p-i-n diode with these high-Q resonators for controlling the FC lifetime promises the realization of an integrated voltage-controlled oscillator (VCO) in a silicon photonics chip.
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    Fully reconfigurable compact RF photonic filters using high-Q silicon microdisk resonators
    (Georgia Institute of Technology, 2011-08) Alipour, Payam ; Eftekhar, Ali Asghar ; Atabaki, Amir Hossein ; Li, Qing ; Yegnanarayanan, Siva ; Madsen, Christi K. ; Adibi, Ali
    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 mm²). 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.
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    Label-free flow cytometry using multiplex coherent anti-Stokes Raman scattering (MCARS) for the analysis of biological specimens
    (Georgia Institute of Technology, 2011-06) Camp, Charles Henry, Jr. ; Yegnanarayanan, Siva ; Eftekhar, Ali Asghar ; Adibi, Ali
    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.
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    Comparison of Cascade, Lattice, and Parallel Filter Architectures
    (Georgia Institute of Technology, 2010-12) Patnaik, Rohit ; Vandrasi, Vivek ; Madsen, Christi K. ; Eftekhar, Ali Asghar ; Adibi, Ali
    We examine the use of different high-level filter architectures (cascade, lattice, and parallel). We discuss their advantages and disadvantages, and we present simulation results and filter-tolerance tests. This information serves as a useful comparative analysis in the selection of a high-level filter architecture for a particular problem. The sensitivity to nonlinearity is also evaluated as resonance-enhanced power in the feedback path. For narrowband band-pass responses, cascade architectures appear to be more tolerant to filter parameter variations than lattice architectures and are substantially more efficient than parallel architectures
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    Acoustic confinement and waveguiding with a line-defect structure in phononic crystal slabs
    (Georgia Institute of Technology, 2010-10) Khelif, Abdelkrim ; Mohammadi, Saeed ; Eftekhar, Ali Asghar ; Adibi, Ali ; Aoubiza, Boujamaa
    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.
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    Simultaneous two-dimensional phononic and photonic band gaps in opto-mechanical crystal slabs
    (Georgia Institute of Technology, 2010-04) Mohammadi, Saeed ; Eftekhar, Ali Asghar ; Khelif, Abdelkrim ; Adibi, Ali
    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.
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    Tuning of resonance-spacing in a traveling-wave resonator device
    (Georgia Institute of Technology, 2010-04) Atabaki, Amir Hossein ; Momeni, Babak ; Eftekhar, Ali Asghar ; Hosseini, Ehsan Shah ; Yegnanarayanan, Siva ; Adibi, Ali
    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.
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    Multiplex coherent anti-Stokes Raman scattering (MCARS) for chemically sensitive, label-free flow cytometry
    (Georgia Institute of Technology, 2009-12) Camp, Charles Henry, Jr. ; Yegnanarayanan, Siva ; Eftekhar, Ali Asghar ; Sridhar, Hamsa ; Adibi, Ali
    Flow cytometry is an ever-advancing high-throughput multivariate analysis tool that natively provides size and morphological information. To obtain molecular information, however, typically requires the addition of fluorophores, which are limited by spectral overlap, nonspecific binding, available conjugation chemistries, and cellular toxicity. A complementary or alternative, label-free approach to molecular information is through multiplex coherent anti-Stokes Raman scattering (MCARS), which is a coherent, nonlinear optical method that provides a wealth of molecular information by probing the Raman energies within a molecule. In this work, we demonstrate the unique capability of our MCARS flow cytometer to distinguish flowing particles and discuss system performance capabilities and possibilities.
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    Large-scale array of small high-Q microdisk resonators for on-chip spectral analysis
    (Georgia Institute of Technology, 2009-10) Soltani, Mohammad ; Li, Qing ; Yegnanarayanan, Siva ; Momeni, Babak ; Eftekhar, Ali Asghar ; Adibi, Ali
    We demonstrate on-chip, large-scale arrays of small high-Q microdisk resonators, suitable for both in-plane coupling and out-of-plane (imaging) spectral analysis devices with high resolution (linewidth < 50pm to 0.5nm), and large FSR (> 50nm).