D. Dimitropoulos

University of California, Los Angeles, Los Ángeles, California, United States

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Publications (30)35.61 Total impact

  • Dimitris Dimitropoulos · Bahram Jalali
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    ABSTRACT: Modern computing and data storage systems increasingly rely on parallel architectures. The necessity for high-bandwidth data links has made optical communication a critical constituent of modern information systems and silicon the leading platform for creating the necessary optical components. While silicon is arguably the most extensively studied material in history, one of its most important attributes, i.e., an analysis of its capacity to carry optical information, has not been reported. The calculation of the information capacity of silicon is complicated by nonlinear losses, which are phenomena that emerge in optical nanowires as a result of the concentration of optical power in a small geometry. While nonlinear loss in silicon is well known, noise and fluctuations that arise from it have never been considered. Here, we report fluctuations that arise from two-photon absorption, plasma effect, cross-phase modulation, and four-wave mixing and investigate their role in limiting the information capacity of silicon. We show that these fluctuations become significant and limit the capacity well before nonlinear processes affect optical transmission. We present closed-form analytical expressions that quantify the capacity and provide an intuitive understanding of the underlying physics.
    No preview · Article · Jun 2015 · IEEE Photonics Journal
  • D. Dimitropoulos · B. Jalali
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    ABSTRACT: Non-linear optical losses result from convergence of wafer scale economics and information theory in silicon nanophotonics. We discuss new sources of noise and fluctuations that arise from twophoton and free carrier plasma effects.
    No preview · Article · May 2015
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    Dimitris Dimitropoulos · Bahram Jalali
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    ABSTRACT: Modern computing and data storage systems increasingly rely on parallel architectures where processing and storage load is distributed within a cluster of nodes. The necessity for high-bandwidth data links has made optical communication a critical constituent of modern information systems and silicon the leading platform for creating the necessary optical components. While silicon is arguably the most extensively studied material in history, one of its most important attributes, an analysis of its capacity to carry optical information, has not been reported. The calculation of the information capacity of silicon is complicated by nonlinear losses, phenomena that emerge in optical nanowires as a result of the concentration of optical power in a small geometry. Nonlinear losses are absent in silica glass optical fiber and other common communication channels. While nonlinear loss in silicon is well known, noise and fluctuations that arise from it have never been considered. Here we report sources of fluctuations that arise from two-photon absorption and free-carrier plasma effects and use these results to investigate the theoretical limit of the information capacity of silicon. Our results show that noise and fluctuations due to nonlinear absorption become significant and limit the information capacity well before nonlinear loss itself becomes dominant. We present closed-form analytical expressions that quantify the capacity and provide an intuitive understanding of the underlying interactions. Our results provide the capacity limit and its origin, and suggest solutions for extending it via coding and coherent signaling. The amount of information that can be transmitted by light through silicon is the key element in future information systems. Results presented here are not only applicable to silicon but also to other semiconductor optical channels.
    Full-text · Article · Aug 2014
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    Dimitris Dimitropoulos · Bahram Jalali
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    ABSTRACT: We present a mathematical approach that simplifies the theoretical treatment of electromagnetic localization in random media and leads to closed-form analytical solutions. Starting with the assumption that the dielectric permittivity of the medium has delta-correlated spatial fluctuations, and using Ito's lemma, we derive a linear stochastic differential equation for a one-dimensional random medium. The equation leads to localized wave solutions. The localized wave solutions have a localization length that scales as L approximately omega(-2) for low frequencies whereas in the high-frequency regime this length behaves as L approximately omega(-2/3) .
    Preview · Article · Apr 2009 · Physical Review E
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    Dimitris Dimitropoulos · Bahram Jalali
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    ABSTRACT: We propose a new method to effect heat removal from an object by using a laser beam. The proposed method is based on inelastic scattering of a laser beam from the object and in particular by making the anti-Stokes emission more efficient than the Stokes emission. In that manner more energy is removed from the body per unit time than deposited. Various ways are outlined in order to achieve this result ranging from careful selection of the laser frequency with respect to the resonant frequencies of the medium, use of the frequency dependence of the density of electromagnetic modes in a three-dimensional system, use of photonic crystals and the polarization dependence of electromagnetic modes in cavities. The proposed methods could find use for example in the cooling of devices of nanoscale dimensions.
    Preview · Article · Dec 2008
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    Bahram Jalali · Dimitris Dimitropoulos · Varun Raghunathan · Sasan Fathpour

    Full-text · Chapter · Jun 2008
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    D Dimitropoulos · D R Solli · R Claps · B Jalali
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    ABSTRACT: Coherent anti-Stokes Raman scattering (CARS) is a well-known Raman scattering process that occurs when Stokes, anti-Stokes and pump waves are properly phase-matched. Using a quantum-field approach with Langevin noise sources, we calculate the noise figure for wavelength conversion between the Stokes and anti-Stokes waves in CARS and show its dependence on phase mismatch. Under phase matched conditions, the minimum noise figure is approximately 3 dB, with a correction that depends on the pump frequency, Stokes shift, refractive indices, and nonlinear susceptibilities. We calculate the photon statistics of CARS and show that the photon number distribution is non-Gaussian. Our findings may be significant for currently pursued applications of CARS including wavelength conversion in data transmission and spectroscopic detection of dilute biochemical species.
    Full-text · Article · Dec 2006 · Optics Express
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    B. Jalali · V. Raghunathan · D. Dimitropoulos · Ö. Boyraz
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    ABSTRACT: This paper reviews recent progress in a new branch of silicon photonics that exploits Raman scattering as a practical and elegant approach for realizing active photonic devices in pure silicon. The large Raman gain in the material, enhanced by the tight optical confinement in Si/SiO2 heterostructures, has enabled the demonstration of the first optical amplifiers and lasers in silicon. Wavelength conversion, between the technologically important wavelength bands of 1300 and 1500 nm, has also been demonstrated through Raman four wave mixing. Since carrier generation through two photon absorption is omnipresent in semiconductors, carrier lifetime is the single most important parameter affecting the performance of silicon Raman devices. A desired reduction in lifetime is attained by reducing the lateral dimensions of the optical waveguide, and by actively removing the carriers with a reverse biased diode. An integrated diode also offers the ability to electrically modulate the optical gain, a unique property not available in fiber Raman devices. Germanium-silicon alloys and superlattices offer the possibility of engineering the otherwise rigid spectrum of Raman in silicon.
    Full-text · Article · Jun 2006 · IEEE Journal of Selected Topics in Quantum Electronics
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    ABSTRACT: The maximum achievable total gain of silicon Raman amplifiers can be significantly increased by injecting the pump power into a surrounding cladding instead of directly into the silicon core.
    No preview · Article · Jan 2006
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    D. Dimitropoulos · S. Fathpour · B. Jalali
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    ABSTRACT: The lifetime of two-photon generated carriers has been established as the critical parameter that determines the performance of silicon Raman lasers and amplifiers since it determines the optical loss. Here, we investigate the intensity dependence of the carrier lifetime in the case where the carriers are swept out by means of a p-n junction. Numerical simulations show that at sufficiently high pump intensities, the generated carriers screen the applied electric field and therefore result in a higher lifetime and hence a lower net Raman gain. We also quantify the electrical power dissipation necessary to maintain low optical losses.
    Full-text · Article · Dec 2005 · Applied Physics Letters
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    B. Jalali · O. Boyraz · P. Koonath · V. Raghunathan · D. Dimitropoulos · T. Indukuri
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    ABSTRACT: Nonlinear optical devices, with silicon-on-insulator material system as platform for their fabrication are discussed. Third order nonlinearity in silicon offers active functionalities in silicon by taking advantage of the high index contrast and tight beam confinement. Among the third order effects; Raman, Kerr nonlinearity, two-photon absorption are particularly strong
    Full-text · Conference Paper · Nov 2005
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    V. Raghunathan · R. Claps · O. Boyraz · P. Koonath · D. Dimitropoulos · B. Jalali
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    ABSTRACT: Stimulated Raman scattering in SOI waveguides has received significant attention recently with the demonstration of pulsed, continuous wave Raman lasers and high gain Raman amplification. However, the limited bandwidth of the Raman signal in silicon (∼105GHz) renders this scheme unsuitable for broadband WDM amplification unless multi-pumping scheme is employed. Large pulsed gain and lasing have been reported in GeSi waveguides. The SiGe on SOI platform represents a Raman medium with a flexible gain spectrum.
    Full-text · Conference Paper · Nov 2005
  • D. Dimitropoulos · R. Jhaveri · R. Claps · V. Raghunathan · J.C.S. Woo · B. Jalali
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    ABSTRACT: A model describing dimensional scaling of carrier lifetime and hence nonlinear optical absorption is presented. It is shown that nonlinear absorption, at a given optical intensity, can be mitigated with proper design of waveguide dimensions.
    No preview · Conference Paper · Jun 2005
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    D. Dimitropoulos · R. Jhaveri · R. Claps · J. C. S. Woo · B. Jalali
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    ABSTRACT: The lifetime of photogenerated carriers in silicon-on-insulator rib waveguides is studied in connection with the optical loss they produce via nonlinear absorption. We present an analytical model as well as two-dimensional numerical simulation of carrier transport to elucidate the dependence of the carrier density on the geometrical features of the waveguide. The results suggest that effective carrier lifetimes of ⩽ 1 ns can be obtained in submicron waveguides resulting in negligible nonlinear absorption. It is also shown that the lifetime and, hence, carrier density can be further reduced by application of a reverse bias pn junction.
    Full-text · Article · Feb 2005 · Applied Physics Letters
  • B Jalali · O Boyraz · V Raghunathan · D Dimitropoulos · P Koonath · N Sugimoto · A Bilenca · D Hadass · R Alizon · H Dery · others

    No preview · Conference Paper · Jan 2005
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    O. Boyraz · V. Raghunathan · D. Dimitropoulos · P. Koonath · T. Indukuri · B. Jalali
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    ABSTRACT: This paper presents the design and operation of Si and GeSi Raman lasers and amplifiers. Different laser applications in various practical fields are also discussed.
    Full-text · Article · Jan 2005
  • V. Raghunathan · R. Claps · D. Dimitropoulos · B. Jalali
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    ABSTRACT: In this paper, we study the dispersion properties of silicon waveguides as the waveguide dimensions are varied, from the point of view of achieving phase matching and hence enhancing the CARS process
    No preview · Article · Jan 2005
  • V. Raghunathan · D. Dimitropoulos · R. Claps · B. Jalali
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    ABSTRACT: Raman induced four-wave mixing is used to demonstrate wavelength conversion in silicon waveguides with sub-micron modal area. The properties of submicron waveguides that lend themselves to high efficiency conversion are also discussed.
    No preview · Conference Paper · Dec 2004
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    B Jalali · V Raghunathan · R Claps · D Dimitropoulos
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    ABSTRACT: We demonstrated conversion of optical signals from 1550nm band to the 1300nm band in silicon waveguides. The conversion is based on parametric Stokes to anti-Stokes coupling using the Raman susceptibility of silicon. Achieving high conversion efficiency requires phase matching in the waveguides as well as means to reduce waveguide losses including the free carrier loss due to two photon absorption.
    Full-text · Article · Aug 2004 · Proceedings of SPIE - The International Society for Optical Engineering
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    V. Raghunathan · R. Claps · D. Dimitropoulos · B. Jalali
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    ABSTRACT: Coarse wavelength conversion in silicon waveguides based on parametric Stokes to anti-Stokes coupling was analyzed using the Raman susceptibility. Conversion efficiency of 1.37×10 -5 was achieved and conversion of analog- and digital-modulated data from a 1500 band to 1300 nm band was demonstrated. The conversion efficiency and hence the SNR and bit rate supported by the conversion process are limited by the phase difference between the optical waves and the magnitude of the Raman nonlinearity. The results show that quasi matching can be achieved by proper rib-waveguide design to cancel out material dispersion using the birefringence in the waveguides.
    Full-text · Article · Jul 2004 · Applied Physics Letters