Adibi, Ali

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Now showing 1 - 10 of 72
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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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    On chip complex signal processing devices using coupled phononic crystal slab resonators and waveguides
    (Georgia Institute of Technology, 2012-03) Mohammadi, Saeed ; Adibi, Ali
    In this paper, we report the evidence for the possibility of achieving complex signal processing functionalities such as multiplexing/demultiplexing at high frequencies using phononic crystal (PnC) slabs. It is shown that such functionalities can be obtained by appropriate cross-coupling of PnC resonators and waveguides. PnC waveguides and waveguide-based resonators are realized and cross-coupled through two different methods of mechanical coupling (i.e., direct coupling and side coupling). Waveguide-based PnC resonators are employed because of their high-Q, compactness, large spurious-free spectral ranges, and the possibility of better control over coupling to PnC waveguides. It is shown that by modifying the defects in the formation of the resonators, the frequency of the resonance can be tuned.
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    Hybrid nanoplasmonic-photonic resonators for efficient coupling of light to single plasmonic nanoresonators
    (Georgia Institute of Technology, 2011-10) Chamanzar, Maysamreza ; Adibi, Ali
    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.
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    Passive all-optical polarization switch, binary logic gates, and digital processor
    (Georgia Institute of Technology, 2011-10) Zaghloul, Yasser A. ; Zaghloul, A. R. M. ; Adibi, Ali
    We introduce the passive all-optical polarization switch, which modulates light with light. That switch is used to construct all the binary logic gates of two or more inputs. We discuss the design concepts and the operation of the AND, OR, NAND, and NOR gates as examples. The rest of the 16 logic gates are similarly designed. Cascading of such gates is straightforward as we show and discuss. Cascading in itself does not require a power source, but feedback at this stage of development does. The design and operation of an SR Latch is presented as one of the popular basic sequential devices used for memory cells. That completes the essential components of an all-optical polarization digital processor. The speed of such devices is well above 10 GHz for bulk implementations and is much higher for chip-size implementations. In addition, the presented devices do have the four essential characteristics previously thought unique to the microelectronic ones.
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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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    Absorbing boundary conditions for low group velocity electromagnetic waves in photonic crystals
    (Georgia Institute of Technology, 2011-03) Askari, Murtaza ; Momeni, Babak ; Reinke, Charles M. ; Adibi, Ali
    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.
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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.