D. Dimitropoulos

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

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Publications (30)47.61 Total impact

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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) .
    Physical Review E 04/2009; 79(3 Pt 2):036606. · 2.31 Impact Factor
  • Xinzhu Sang, D. Dimitropoulos, B. Jalali, O. Boyraz
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    ABSTRACT: The effect of the relative intensity noise (RIN), transferred from the pump to the signal, in 1-cm-long chip scale silicon Raman amplifiers is investigated in the presence of nonlinear losses. We show that due to the short waveguide length, the reduction in fluctuations that normally occurs due to ldquowalk-offrdquo between pump and signal waves in fiber amplifiers is inefficient in chip scale Raman amplifiers. In the counterpropagating pump configuration, which leads to minimum frequency RIN transfer, fluctuations up to 1.5 GHz are transferred from the pump to the signal. As a case study, the noise figure degrades by as much as 11 dB in the silicon waveguide with the free carrier life time of 0.1 ns, when it is pumped with a laser with a RIN value of -125 dB/Hz.
    IEEE Photonics Technology Letters 01/2009; · 2.04 Impact Factor
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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.
    12/2008;
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    Bahram Jalali, Dimitris Dimitropoulos, Varun Raghunathan, Sasan Fathpour
    06/2008: pages 147 - 189; , ISBN: 9780470994535
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    D. Dimitropoulos, D. R. Solli, R. Claps, O. Boyraz, B. Jalali
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    ABSTRACT: The noise figure of silicon Raman amplifiers in the presence of nonlinear losses is calculated. The impact of two-photon absorption (TPA) and free-carrier scattering on the noise figure is quantified using the quantum formulation of the Langevin approach. It is found that TPA-induced free-carrier loss degrades the noise figure by an amount that depends on the carrier lifetime. For example, in a 1-cm-long waveguide pumped at 200 MW/cm<sup>2</sup>, the noise figure is 5.2 dB for a lifetime of tau = 1.6 ns and is reduced to 3.7 dB for tau = 0.1 ns. The reduction in the noise figure along with a concomitant increase in Raman gain from 2 to 8 dB suggests that lifetimes on the order of 0.1 ns or less are needed to create a useful silicon Raman amplifier that operates in the continuous-wave mode. It is also shown that in devices that use a p-n junction for carrier sweep-out, the screening of the junction field by generated free carriers results in a sharp increase in the noise figure at high-pump intensities. These results apply to operation in the near-infrared communication wavelengths. For mid-infrared wavelengths above the two photon absorption band-edge (2.3 nm), the absence of TPA and pump-induced free-carrier absorption ensures that the amplifier has a low-noise figure.
    Journal of Lightwave Technology 05/2008; 26(7):847-852. · 2.56 Impact Factor
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    ABSTRACT: This paper presents the case for the silicon Raman laser as a potential source for the technologically important midwave infrared (MWIR) region of the optical spectrum. The mid-IR application space is summarized, and the current practice based on the optical parametric oscillators and solid state Raman lasers is discussed. Relevant properties of silicon are compared with popular Raman crystals, and linear and nonlinear transmission measurements of silicon in the mid-IR are presented. It is shown that the absence of the nonlinear losses, which severely limit the performance of the recently demonstrated silicon lasers in the near IR, combined with unsurpassed crystal quality, high thermal conductivity and excellent optical damage threshold render silicon a very attractive Raman medium, even when compared to the very best Raman crystals. In addition, silicon photonic technology, offering integrated low-loss waveguides and microcavities, offers additional advantages over today's bulk crystal Raman laser technology. Using photonic crystal structures or microring resonators, the integrated cascaded microcavities can be employed to realize higher order Stokes emission, and hence to extend the wavelength coverage of the existing pump lasers. Exploiting these facts, the proposed technology can extend the utility of silicon photonics beyond data communication and into equally important applications in biochemical sensing and laser medicine
    IEEE Journal of Selected Topics in Quantum Electronics 12/2006; · 4.08 Impact Factor
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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.
    Optics Express 12/2006; 14(23):11418-32. · 3.55 Impact Factor
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    B. Jalali, V. Raghunathan, D. Dimitropoulos, O. 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.
    IEEE Journal of Selected Topics in Quantum Electronics 06/2006; · 4.08 Impact Factor
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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
    Lasers and Electro-Optics Society, 2005. LEOS 2005. The 18th Annual Meeting of the IEEE; 11/2005
  • Source
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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.
    SOI Conference, 2005. Proceedings. 2005 IEEE International; 11/2005
  • Source
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    ABSTRACT: This paper presents recent breakthroughs and applications of Raman based silicon photonics such as silicon Raman amplifiers and lasers. These lasers would extend the wavelength range of III-V laser to mid-IR where important applications such as laser medicine, biochemical sensing, and free space optical communication await the emergence of a practical and low cost laser.
    Group IV Photonics, 2005. 2nd IEEE International Conference on; 10/2005
  • Source
    V. Raghunathan, R. Claps, D. Dimitropoulos, B. Jalali
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    ABSTRACT: The benefits of using submicrometer modal-dimension silicon waveguides in realizing high-efficiency parametric Raman wavelength conversion are demonstrated theoretically and experimentally. The combined effects of Raman nonlinearities and free-carrier losses induced by two-photon absorption (TPA) are analyzed using the coupled-mode theory. The analysis indicates that scaling down the lateral dimensions increases the conversion efficiency of the Raman process and reduces the effective lifetime of free carriers and hence ameliorates the free-carrier losses. The feasibility of data conversion is demonstrated by coherent transfer of the analog radio-frequency (RF) signal from Stokes to anti-Stokes channels. The conversion efficiency, and hence signal-to-noise ratio (SNR), and bandwidth of the conversion process are found to be limited by the phase mismatch between the pump, Stokes, and anti-Stokes fields. The dispersion properties of submicrometer waveguides are also studied from the point of view of achieving phase matching and enhancing the conversion efficiency.
    Journal of Lightwave Technology 07/2005; · 2.56 Impact Factor
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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.
    Lasers and Electro-Optics, 2005. (CLEO). Conference on; 06/2005
  • Source
    B Jalali, O Boyraz, D Dimitropoulos, V Raghunathan
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    ABSTRACT: Scaling properties of two photon absorption, free carrier scattering, Raman scattering and Kerr effect in silicon waveguides is reported. It is shown that the dependence of minority carrier lifetime on waveguide dimensions has a profound impact on the performance of nonlinear optical devices built using silicon waveguides.
    Proc SPIE 03/2005;
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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.
    Applied Physics Letters 02/2005; 86(7):071115-071115-3. · 3.79 Impact Factor
  • Active and Passive Optical Components for WDM Communications; 01/2005
  • Source
    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.
    Applied Physics Letters 01/2005; 87. · 3.79 Impact Factor
  • Source
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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.
    01/2005;
  • V. Raghunathan, R. Claps, D. Dimitropoulos, B. Jalali
    [Show abstract] [Hide abstract]
    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
    Information Sciences - ISCI. 01/2005;
  • V. Raghunathan, D. Dimitropoulos, R. Claps, B. Jalali
    [Show abstract] [Hide abstract]
    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.
    Lasers and Electro-Optics Society, 2004. LEOS 2004. The 17th Annual Meeting of the IEEE; 12/2004