Fredrik K. Fatemi

United States Naval Research Laboratory, Washington, Washington, D.C., United States

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Publications (64)105.34 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Biocompatible, near-infrared luminescent gold nanoclusters (AuNCs) are synthesized directly in water using poly(ethylene glycol)-dithiolane ligands terminating in either a carboxyl, amine, azide, or methoxy group. The ≈1.5 nm diameter AuNCs fluoresce at ≈820 nm with quantum yields that range from 4–8%, depending on the terminal functional group present, and display average luminescence lifetimes approaching 1.5 μs. The two-photon absorption (TPA) cross-section and two-photon excited fluorescence (TPEF) properties are also measured. Long-term testing shows the poly(ethylene glycol) stabilized AuNCs maintain colloidal stability in a variety of media ranging from saline to tissue culture growth medium along with tolerating storage of up to 2 years. DNA and dye-conjugation reactions confirm that the carboxyl, amine, and azide groups can be utilized on the AuNCs for carbodiimide, succinimidyl ester, and CuI-assisted cycloaddition chemistry, respectively. High signal-to-noise one- and two-photon cellular imaging is demonstrated. The AuNCs exhibit outstanding photophysical stability during continuous-extended imaging. Concomitant cellular viability testing shows that the AuNCs also elicit minimal cytotoxicity. Further biological applications for these luminescent nanoclustered materials are discussed.
    Particle and Particle Systems Characterization 05/2013; 30(5-5):453-466. · 0.54 Impact Factor
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    Joseph A Pechkis, Fredrik K Fatemi
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    ABSTRACT: We demonstrate guiding of cold 85Rb atoms through a 100-micron-diameter hollow core dielectric waveguide using cylindrical hollow modes. We have transported atoms using blue-detuned light in the 1st order, azimuthally-polarized TE01 hollow mode, and the 2nd order hollow modes (HE31, EH11, and HE12), and compared these results with guidance in the red-detuned, fundamental HE11 mode. The blue-detuned hollow modes confine atoms to low intensity along the capillary axis, far from the walls. We determine scattering rates in the guides by directly measuring the effect of recoil on the atoms. We observe higher atom numbers guided using red-detuned light in the HE11 mode, but a 10-fold reduction in scattering rate using the 2nd order modes, which have an r4 radial intensity profile to lowest order. We show that the red-detuned guides can be used to load atoms into the blue-detuned modes when both high atom number and low perturbation are desired.
    Optics Express 06/2012; 20(12):13409-18. · 3.53 Impact Factor
  • Joseph A. Pechkis, Fredrik K. Fatemi
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    ABSTRACT: We have guided cold rubidium atoms in blue-detuned hollow optical modes of a hollow fiber. These higher order modes allow large optical depth, low scattering rates, and efficient use of guide laser power. Atoms are transported through a 3-cm-long hollow fiber with a 100 micron diameter using the first three optical modes of the fiber. We compare guiding properties in the red-detuned, fundamental HE11 mode with the blue-detuned TE01 (first order) and HE12 (second order) modes. Using guide laser powers below 50 mW and detunings below 1.5 nm, we have directly measured recoil scattering rates in the three different guides and found that atoms in the HE12 mode typically have a 10x lower recoil scattering rate compared to the red-detuned HE11 mode for equal guide peak intensity. Furthermore, we have observed optical depths of ˜20 for the blue-detuned guides with recoil scattering rates below 10 Hz. We will discuss our ongoing experiments using the atoms in these guides. This work supported by the Office of Naval Research and the Defense Advanced Research Projects Agency.
    06/2012;
  • Mark Bashkansky, Fredrik K. Fatemi, Igor Vurgaftman
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    ABSTRACT: Quantum memory is regarded as one of the essential components in the fields of quantum computing and quantum communication. Warm atomic vapor cells for quantum memory, as originally described in DLCZ (for Duan, Lukin, Cirac, and Zoller) protocol, are appealing due to the perceived reduction in experimental complexity and commercial availability. However, published studies on quantum memory using warm vapor cells were performed under widely dissimilar experimental conditions and reported ambiguous results. In order for the memory to exhibit non-classical behavior to a high degree of certainty, the cross-correlation value between the Stokes and anti-Stokes photons needs to be greater than two. In this work we demonstrate quantum memory with cross-correlation value between the Stokes and anti-Stokes photons greater than two lasting for 4 μs using warm Rb vapor with buffer gas for nearly co-propagating write and read beams.
    Proceedings of SPIE - The International Society for Optical Engineering 05/2012; · 0.20 Impact Factor
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    Mark Bashkansky, Fredrik K Fatemi, Igor Vurgaftman
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    ABSTRACT: The realization of quantum memory using warm atomic vapor cells is appealing because of their commercial availability and the perceived reduction in experimental complexity. In spite of the ambiguous results reported in the literature, we demonstrate that quantum memory can be implemented in a single cell with buffer gas using the geometry where the write and read beams are nearly copropagating. The emitted Stokes and anti-Stokes photons display cross-correlation values greater than 2, characteristic of quantum states, for delay times up to 4 μs.
    Optics Letters 01/2012; 37(2):142-4. · 3.39 Impact Factor
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    F K Fatemi
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    ABSTRACT: We demonstrate the use of cylindrical vector beams - beams with spatially varying polarization - for detecting and preparing the spin of a warm rubidium vapor in a spatially dependent manner. We show that a modified probe vector beam can serve as an atomic spin analyzer for an optically pumped medium, which spatially modulates absorption of the beam. We also demonstrate space-variant atomic spin by optical pumping with the vector beams. The beams are thus beneficial for making single-shot polarization-dependent measurements, as well as for providing a means of preparing samples with position-dependent spin.
    Optics Express 12/2011; 19(25):25143-50. · 3.53 Impact Factor
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    ABSTRACT: We have observed time-varying spin relaxation of trapped cold atoms due to photon scattering in blue-detuned, crossed, hollow Laguerre-Gaussian beams. These beams are formed by imparting an azimuthal phase of ℓφ to a Gaussian beam, where ℓ is an integer, and have an intensity distribution that scales with r2ℓ to the lowest order. For all degrees of anharmonicity, we observe a time-varying spin-relaxation rate due to energy-dependent photon scattering. For ℓ=8, we directly measure temperature-dependent scattering rates and show that by removing the most energetic atoms from the trap, a more purely spin-polarized sample remains. The results agree well with Monte Carlo simulations, and we present a simple functional form for the spin-relaxation curves.
    Physical Review A 08/2011; 84(2). · 2.99 Impact Factor
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    ABSTRACT: Pulsed coherent population trapping in atomic vapor provides a convenient method for generating frequency narrowed atomic resonance using Ramsey interference. Here, we present our experimental results showing high-contrast and sub-kilohertz Ramsey interference fringes produced by coherently prepared 85Rb atoms using Raman excitation formed in D1 line transitions. A test system for atomic clock has been constructed to measure light shift for both continuous and pulsed Raman excitation cases. Our measurements show that light shift measured using pulsed Raman excitation, is reduced compared to the continuous excitation case. Our ability to measure light shift is currently limited by the resolution of the frequency counter, and performance of the locking electronics used.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2011; · 0.20 Impact Factor
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    ABSTRACT: Parallel code presents a non-trivial problem of load balancing computational workload throughout a system of hardware and software resources. The task of load balancing is further complicated when the number of allowable processors changes through time. This paper presents a two-component load-balancing mechanism using optimal initial workload distribution and dynamic load maintenance. The initial guess is provided by inversion of the workload distribution function. Workload distribution inversion enables efficient domain decomposition for arbitrary workloads and easily compensates for changes in system resources. Dynamic load balancing is provided by process feedback control as used, for example, in control mechanisms of physical processes. Proportional, integral, and differential (PID) feedback readily allows controls to compensate for runtime-changes of the workload distribution function. This paper demonstrates a one-dimensional realization of the ideas presented here. We apply this load-balancing technique to our gridless direct simulation Monte Carlo algorithm. We demonstrate that the method does indeed maintain uniform workload distribution across available resources as the workload and usable system resources undergo change through time.
    Computer Physics Communications 12/2010; 181:2063-2071. · 2.41 Impact Factor
  • Fredrik Fatemi, Mark Bashkansky, Eunkeu Oh
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    ABSTRACT: We demonstrate excitation of the azimuthally-polarized TE01 cylindrical waveguide mode in hollow glass and metal waveguides with 780 nm light. We describe stable and efficient techniques for mode conversion of an incident Gaussian beam to a vectorial vortex beam, and measure attenuation lengths of the TE01 mode in hollow optical fibers with diameters of 50-100 mum. By silver-coating the inner walls of the dielectric fibers, we demonstrate a ˜200% increase in the attenuation length. We discuss progress in implementing these fibers into a cold atom system.
    03/2010;
  • Fredrik K Fatemi, Mark Bashkansky
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    ABSTRACT: We use Faraday spectroscopy of atoms confined to crossed hollow beam tweezers to map magnetic fields over 3 millimeters with 200 micron resolution in a single trap loading cycle. The hollow beams are formed using spatial light modulation, and the trap location is scanned using acousto-optic deflectors. We demonstrate the technique by mapping a linear quadrupole magnetic field with 10 nT sensitivity.
    Optics Express 02/2010; 18(3):2190-6. · 3.53 Impact Factor
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    ABSTRACT: The phenomenon of all-optical Ramsey interference using pulsed coherent population trapping (CPT) beams provides a new avenue for developing frequency standards using atomic vapor. In this study, we show that frequency narrowed Ramsey fringes can be produced in rubidium vapor without the effect of power broadening. We observed fringes of width as narrow as 1 kHz using a buffer-gas filled rubidium cell. A compact injection-locked laser (ILL) system was used to generate CPT beams. Studies also show that ac Stark effect on Ramsey fringes can be reduced, and higher frequency stability can be achieved in a clock application. The results are encouraging to propose an architecture for development of a pulsed CPT Ramsey clock. In this paper, we also provide related discussions on clock frequency stability, and our plans for future experiments.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2010; · 0.20 Impact Factor
  • Mark Bashkansky, Doewon Park, Fredrik K Fatemi
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    ABSTRACT: We demonstrate a technique for generating azimuthally and radially polarized beams using a nematic liquid crystal spatial light modulator and a pi phase step. The technique is similar in concept to prior techniques that interfere TEM(01) and TEM(10) laser modes, but the presented technique removes the requirement of interferometric stability. We calculate an overlap integral of >0.96 with >70% efficiency from an input Gaussian mode. The technique can easily switch between beams with azimuthal and radial polarization.
    Optics Express 01/2010; 18(1):212-7. · 3.53 Impact Factor
  • Fredrik K Fatemi, Mark Bashkansky, Eunkeu Oh, Doewon Park
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    ABSTRACT: We demonstrate excitation of the azimuthally-polarized TE(01) cylindrical waveguide mode in hollow glass and metal waveguides with 780 nm light. Experimentally, we demonstrate formation of the vectorial vortex beams, and measure attenuation lengths of the TE(01) mode in hollow optical fibers with diameters of 50-100 microns. By silver-coating the inner walls of the dielectric fibers, we demonstrate a approximately 200% increase in the attenuation length.
    Optics Express 01/2010; 18(1):323-32. · 3.53 Impact Factor
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    ABSTRACT: We demonstrate techniques for imaging both magnetic fields and magnetic sublevel distributions using velocity-selective Raman transitions in cold 87Rb and 85Rb ensembles. When an expanding atom cloud is exposed to counterpropagating laser beams, resonant atom trajectories undergoing Raman transitions are perturbed and result in distinctive features in images of the expanded cloud. If the counterpropagating laser fields couple magnetic sublevels in different hyperfine manifolds, the images provide a spectrograph of sublevel occupation. The techniques are simple to implement and provide a useful, rapid diagnostic tool for cold atom experiments.
    Journal of Modern Optics 10/2009; 56(Nos. 18-19):2022-2028. · 1.17 Impact Factor
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    ABSTRACT: We have studied velocity-selective resonances in the presence of a uniform magnetic field and shown how they can be used for rapid, single-shot assessment of the ground state magnetic sublevel spectrum in a cold atomic vapor. Cold atoms are released from a magneto-optical trap in the presence of a small bias magnetic field ( approximately 300 mG) and exposed to a laser field comprised of two phase-locked counterpropagating beams connecting the two ground state hyperfine manifolds. An image of the expanded cloud shows the velocity-selected resonances as distinct features, each corresponding to specific magnetic sublevel, in a direct, intuitive manner. We demonstrate the technique with both 87Rb and 85Rb, and show the utility of the technique by optically pumping into particular magnetic sublevels. The results are shown to agree with a theoretical model, and are compared to traditional Raman spectroscopy.
    Optics Express 08/2009; 17(15):12971-80. · 3.53 Impact Factor
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    ABSTRACT: Stokes gain and delay in Stimulated Brillouin scattering in fibers is strongly affected by polarization changes due to unavoidable fiber disturbances. We demonstrate the use of a Faraday rotator mirror to counteract polarization fluctuations.
    07/2009;
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    ABSTRACT: We use Faraday spectroscopy of atoms confined to crossed hollow beam tweezers to map magnetic fields over several millimeters with 100 micron resolution. The traps permit several hundred measurements in a single loading cycle.
    Lasers and Electro-Optics, 2009 and 2009 Conference on Quantum electronics and Laser Science Conference. CLEO/QELS 2009. Conference on; 07/2009
  • Fredrik Fatemi, Mark Bashkansky
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    ABSTRACT: We use Faraday spectroscopy of atoms confined to a movable crossed hollow beam trap to measure the magnetic field in a 200-micron-diameter spot over 5 mm in a single trap loading cycle. We have used blue-detuned, high-charge number hollow beams to create a box-like potential for cold Rb87 atoms. The trap and probe laser beams are scanned dynamically using acousto-optic deflection, and the magneto-optic polarization rotation of the probe is monitored by a polarimeter. The dark trap allows deep optical potentials with low photon scattering rate using near resonant (δ = +100 GHz) light so that multiple magnetic field measurements can be made in a single loading cycle. We demonstrate the technique by mapping quadrupole magnetic fields with 10 μG field sensitivity.
    05/2009;
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    ABSTRACT: We demonstrate a simple technique for single-shot imaging of magnetic sublevel distributions in cold atoms. The technique relies on velocity-selective stimulated Raman transitions in a magnetic field. The Raman transitions couple sublevels between the two hyperfine manifolds so that each possible transition pathway is resonant only for a specific velocity. Cold atoms expanding from a magneto-optical trap are exposed to a brief (˜1ms) counterpropagating pulse in a lin-perp-lin configuration. Ballistic expansion of the atom sample separates the atoms according to their velocities, so that the each sublevel's population is easily observed using fluorescence imaging from cycling transition light. We demonstrate this technique for a variety of optical pumping configurations in both Rb85 and Rb87.
    01/2009;