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Quantum technologies

Dieter Meschede's research group
Home Group members Jae-Ihn Kim
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Dr. rer. nat. Jae-Ihn Kim
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Publications(up to 2009)

  • J. Kim
    Efficient sub-Doppler Transverse Laser Cooling of an Indium Atomic Beam, (2009), PhD thesisBibTeXPDF
    ABSTRACT »
    In this dissertation, I describe transverse laser cooling of an Indium atomic beam. For efficient laser cooling on a cycling transition, I have built a tunable, continuous-wave coherent ultraviolet source at 326 nm based on frequency tripling. For this purpose, two independent high power Yb-doped fiber amplifiers for the generation of the fundamental radiation at λ = 977 nm have been constructed. I have observed sub-Doppler transverse laser cooling of an Indium atomic beam on a cycling transition of In by introducing a polarization gradient in the linear-perpendicular-linear configuration. The transverse velocity spread of a laser-cooled In atomic beam at full width at half maximum was achieved to be 13.5 ± 3.8 cm/s yielding a full divergence of only 0.48 ± 0.13 mrad. In addition, nonlinear spectroscopy of a 3-level, lambda-type level system driven by a pump and a probe beam has been investigated in order to understand the absorption line shapes used as a frequency reference in a previous two-color spectroscopy experiment. For the analysis of this atomic system, I have applied a density matrix theory providing an excellent basis for understanding the observed line shapes.
  • J. Kim, D. Haubrich and D. Meschede
    Efficient sub-Doppler laser cooling of an Indium atomic beam, Opt. Express 17, 21216-21221 (2009)BibTeXPDF
    ABSTRACT »
    We have realized efficient transverse cooling of an Indium atomic beam by combining optical pumping with a closed cycle UV laser cooling transition at 325.6 nm. The transverse velocity of the atomic beam is reduced to 13.5 ±3.8 cm/s, well below the Doppler cooling limit. The fraction of laser-cooled In atoms is enhanced to 12±3 % by optical pumping in the present experiment. It can be scaled up to approach 100% efficiency in cooling, providing high brightness atomic beams for further applications. Our results establish In on the map of elements suitable for applications involving laser cooling.
  • J. Kim, D. Haubrich, B. Klöter and D. Meschede
    Strong effective saturation by optical pumping in three-level systems, Phys. Rev. A 80, 063801 (2009)BibTeXPDF
    ABSTRACT »
    We have studied nonlinear absorption from the In P1/2,3/2 ground-state doublet in a resistively heated high-temperature cell and a hollow cathode lamp. Using probe and pump lasers at 410 and 451 nm, respectively, absorption spectra with nonlinear properties caused by saturated absorption, coherent dark resonances, and optical pumping are observed. A theoretical description in terms of a density-matrix theory agrees very well with the observed spectra and identifies optical pumping as a dominating process of broadening in the stepwise contribution rather than velocity-changing collisions. Our experiments suggest that the theory used here is widely applicable in saturation spectroscopy on three-level Λ systems.
  • J. Kim and D. Meschede
    Continuous-wave coherent ultraviolet source at 326 nm based on frequency trippling of fiber amplifiers, Opt. Express 16, 16803-16808 (2008)BibTeXPDF
    ABSTRACT »
    We have demonstrated a tunable single frequency source of continuous-wave (CW) coherent ultraviolet (UV) radiation at λ_3ω = 326 nm. Laser light of a tunable diode laser at λ_ω = 977 nm was split and injected into two independent fiber amplifiers yielding 1 W and 0.4 W, respectively. The 1 W branch was resonantly frequency doubled, resulting in 120 mW of useful power at λ_2ω = 488 nm. The third harmonic was generated by summation of the second branch of λ_ω and λ_2ω which were enhanced by a doubly resonant cavity. This light source has TEM_00 character and can be continuously tuned over more than 18 GHz. It is of interest for efficient laser cooling of In and potentially other applications.
  • J. Choi, J. M. Kim, Q. Park and D. Cho
    Optically induced Faraday effect in a lambda configuration of spin-polarized cold cesium atoms, Phys. Rev. A 75, 013815 (2007)BibTeX
    ABSTRACT »

    Polarization rotation of weak probe light induced by circularly polarized strong coupling light in a Λ configuration is studied. We use spin-polarized cold cesium atoms trapped in a magneto-optical trap to remove complications from Zeeman distribution, Doppler broadening, and collisional decoherence. By using a very low probe intensity and short illumination period we work in a strictly linear regime. The probe and the coupling fields are optically phase locked to eliminate phase fluctuation and consequent atomic decoherence. Using this idealized situation we clarify the roles of optically induced Faraday rotation, circular dichroism, and electromagnetically induced transparency (EIT) in determining the final state of the probe light. In particular, we identify an experimental situation where the roles of atomic coherence and EIT are important.

  • J. Choi, J. M. Kim and D. Cho
    Faraday rotation assisted by linearly polarized light, Phys. Rev. A 76, 053802 (2007)BibTeX
    ABSTRACT »

    We demonstrate a type of chiral effect of an atomic medium. Polarization rotation of a probe beam is observed only when both a magnetic field and a linearly polarized coupling beam are present. We compare it with other chiral effects like optical activity, the Faraday effect, and the optically induced Faraday effect from the viewpoint of spatial inversion and time reversal transformations. As a theoretical model we consider a five-level configuration involving the cesium D2 transition. We use spin-polarized cold cesium atoms trapped in a magneto-optical trap to measure the polarization rotation versus probe detuning. The result shows reasonable agreement with a calculation from the master equation of the five-level configuration.

  • Y. Choi, S. Yoon, S. Kang, W. Kim, J. Lee and K. An
    Direct measurement of loading and loss rates in a magneto-optical trap with atom-number feedback, Phys. Rev. A 76, 013402 (2007)BibTeX
    ABSTRACT »
    We have measured the loading and loss rates in a magneto-optical trap only with a few atoms by directly counting atom-number changing events. Unambiguous formulas are presented for the calculation of those rates from a step-wise time sequence of the fluorescence of the trapped atoms. With a recently developed atom-number feedback technique we could efficiently measure the loading rate as a function of the magnetic field gradient for the initial number of trapped atoms of zero. We could also measure the one- and two-atom loss rates as functions of the trap laser intensity for a precisely prepared initial number of trapped atoms. Each of these rates has been measured independently by directly counting the corresponding atom-number-changing events, without any influence from or inference to the other rates.
  • H. Chon, G. Park, S. Lee, S. Yoon, J. Kim, J. Lee and K. An
    The dependence of transverse and longitudinal resolutions on incident Gaussian beam widths in the illumination part of optical scanning microscopy, J. Opt. Soc. Am. A 24, 60 (2007)BibTeX
    ABSTRACT »
    We studied both theoretically and experimentally the intensity distribution of a Gaussian laser beam when it was focused by an objective lens with its numerical aperture up to 0.95. Approximate formulas for full width at half-maximum (FWHM) of the intensity distribution at focus were derived for very large and very small initial beam waists with respect to the entrance pupil radius of the objective lens. In experiments, the energy flux through a 0.5μm pinhole was measured for various pinhole positions. It was found in theoretical analysis and confirmed in experiments that the FWHMs at focus in the transverse and longitudinal directions do not increase much from the ultimate FWHMs until the input beam waist is reduced below half of the entrance pupil radius.
  • S. Yoon, Y. Choi, S. Park, J. Kim, J. Lee and K. An
    Definitive number of atoms on demand: controlling the number of atoms in a-few-atom magneto-optical trap, Appl. Phys. Lett. 88, 211104 (2006)BibTeX
    ABSTRACT »
    A few math atoms were trapped in a micron-size magneto-optical trap with a high quadrupole magnetic-field gradient and the number of atoms was precisely controlled by suppressing stochastic loading and loss events via real-time feedback on the magnetic-field gradient. The measured occupation probability of a single atom was as high as 99%. Atoms up to five were also trapped with high occupation probabilities. The present technique could be used to make a deterministic atom source.
  • D. Cho, J. Choi, J. M. Kim and Q. Park
    Optically induced Faraday effect using three-level atoms, Phys. Rev. A 72, 023821 (2005)BibTeX
    ABSTRACT »
    We investigate an all-optical, pump-probe scheme for the polarization rotation of linearly polarized light in an atomic medium. A circularly polarized control light is shown to play the role of a static magnetic field via a Zeeman-like ac Stark interaction and to induce the optical Faraday rotation (OFR) of the probe light. As a model system, we consider a stationary atom with nS-nP-nD level scheme without electronic or nuclear spin. In addition to OFR, we find that electromagnetically induced transparency for the three-level atom and circular dichroism also contribute to the polarization change. We characterize these three effects over different frequency ranges through an explicit calculation of the fractional transmission and the output polarization of the probe light. Our results are compared with the nS-nP-(n+1)S scheme, which has been previously studied both theoretically and experimentally. We also compare the optically induced Faraday effect with the Faraday effect from a static magnetic field. We propose an experimental situation to test the theory and address the possibility of producing an atomic medium that is both optically active and transparent.
  • J. Choi, J. M. Kim, J. H. Lee, Q. Park and D. Cho
    Magneto-optical effect near the D1 resonance of spin-polarized cold cesium atoms, Phys. Rev. A 71, 043409 (2005)BibTeX
    ABSTRACT »
    We report our study of the magneto-optical effect in a strictly linear regime on spin-polarized cold cesium atoms. Due to the low intensity and the short illumination period of the probe beam, less than 7.5% of the sample atoms change their states by absorbing probe photons. We produce a medium of atoms at rest in either the 6S1∕2,F=3,mF=0 or 6S1∕2,F=3,mF=3 state by optically pumping atoms trapped in a magneto-optical trap. We use the D1 resonance with large lower and upper state hyperfine splittings as a probe transition to avoid hyperfine mixing from the Zeeman interaction. Under this idealized situation we measure the Stokes parameters in order to find the polarization rotation and circular dichroism experienced by the probe light. We find that there are qualitative differences between the results for the mF=0 and mF=3 cases. While dispersion and consequent Faraday rotation play a dominant role when the atoms are in the mF=0 state, it is dissipation and circular dichroism that are important when they are in the mF=3 state. Similarly, while the size of the Faraday rotation and the circular dichroism for the mF=0 case scales linearly with the applied magnetic field, for the mF=3 case it is the shift of the probe polarization change versus frequency that is linearly proportional to the magnetic field strength.
  • Y. Lim, S. Yoon, K. Yee, J. Lee, D. Kim and D. Lee
    Coherent phonon oscillations excited by second-order Raman scattering in CdZnTe multiple quantum wells and cubic ZnTe, Phys. Rev. B 68, 153308 (2003)BibTeX
    ABSTRACT »
    We report observation of strong phonon-pair oscillations in CdZnTe multiple quantum wells and cubic ZnTe(001). Using femtosecond laser pulses with photon energies far below the bandgap, the coherent phonon-pair oscillations are generated via the impulsive stimulated Raman scattering of the second order. Polarization- and phase-sensitive measurements allow us to resolve both relative Raman polarizabilities of various symmetry modes and their initial phases, simultaneously.