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Guan, WY, Chen YC, Wei JYT, Xu YH, Wu MK.  1993.  ION-SIZE EFFECT ON T(M) AND T(C) IN (R1-XPR(X))BA2CU3O7 SYSTEMS (R = YB, TM, ER, HO, DY, GD, EU, SM, ND AND Y), Apr. Physica C-Superconductivity and Its Applications. 209:19-22., Number 1-3 AbstractWebsite

The magnetic ordering temperatures T(m) of Pr ions in (R1-xPrx)Ba2Cu3O7 systems (R = Yb, Tm, Er, Ho, Dy, Gd, Eu, Sm, Nd and Y) with x = 0.5 - 1.0 were measured. We observe that T(m) decreases monotonically with increasing R concentration. At constant x, T(m) is R ion-size dependent. The slope in the T(m) vs. x is steeper for ion with smaller ionic radius. In comparison with the ion-size effect on the superconducting transition temperatures T(c) in these systems, the observed results can be qualitatively interpreted in terms of the hybridization between the local states of Pr ion and the conduction band states of the CuO2 planes.

Guan, WY, Xu YH, Sheen SR, Chen YC, Wei JYT, Lai HF, Wu MK, Ho JC.  1994.  ION-SIZE EFFECT ON TN IN (R1-XPRX)BA2CU3O7-Y SYSTEMS (R=LU, YB, TM, ER, Y, HO, DY, GD, EU, SM, AND ND), Jun 1. Physical Review B. 49:15993-15999. AbstractWebsite

We conducted a detailed study of the structure and magnetic properties of (R1-xPrx)Ba2Cu3O7 sintered samples, where R = Lu, Yb, Tm, Er, Y, Ho, Dy, Gd, Eu, Sm, and Nd for x = 0.5-1.0. We found that the temperature dependence of the dc susceptibility follows the Curie-Weiss law in the temperature range 20-300 K and the paramagnetism of the Pr and R sublattices exist independently of one another. The antiferromagnetic ordering temperature T(N) of Pr ions decreases monotonically with increasing R concentration (1-x). At a given x, T(N) is R-ion-size dependent. The slope in the T(N) vs x curve is steeper for ions with smaller ionic radii. The observed results are interpreted in terms of the hybridization between the local states of the Pr ion and the valence-band states of the CuO2 planes.

Tung, SK, Chen YC, Lin CW, Hsu L, Yu IA.  2000.  Cooling atoms below 100 mu K, Apr. Chinese Journal of Physics. 38:395-399., Number 2 AbstractWebsite

We capture Rb-87 atoms from room-temperature background vapor with a magneto-optical trap (MOT). The temperature of the atoms in the MOT is 320 mu K as the result of Doppler cooling. We further employ polarization gradient cooling to lower atom temperature. The factors that can affect the performance of polarization gradient cooling have been systematically studied. An atom temperature of 75 mu K has been reached with the optimized conditions. Temperatures are measured by the release and recapture method and the time of flight method. Such cold atoms are ready for the evaporative cooling which will finally realize the Bose-Einstein condensation.

Chen, YC, Lin CW, Yu IA.  2000.  Role of degenerate Zeeman levels in electromagnetically induced transparency, May. Physical Review A. 61:6., Number 5 AbstractWebsite

We have observed various Lambda-type electromagnetically induced transparency (EIT) spectra in laser-cooled Rb-87 atoms of different laser polarization configurations. Unexpected profiles occur in the EIT spectra. We have found the degenerate Zeeman sublevels are responsible for these profiles. The experimental data are in good agreement with the results from the theoretical calculation which takes into account all the 13 Zeeman levels in the Lambda system. Our study demonstrates that Zeeman sublevels play important roles in quantum interference phenomena such as EIT and amplification without population inversion (AWI), and should be taken into account in the analysis of these phenomena.

Chen, YC, Lin WB, Hsue HC, Hsu L, Yu IA.  2000.  Effect of the trapping laser linewidth on the atom number in a magneto-optical trap, Oct. Chinese Journal of Physics. 38:920-926., Number 5 AbstractWebsite

We have experimentally studied the effect of the trapping laser linewidth on the number of capped atoms in a magneto-optical trap (MOT). Our data show that a significant number of the atoms can still be trapped in the MOT, even when the trapping laser linewidth is larger than the natural linewidth of the excited state of the driving transition.

Chen, YC, Chen YW, Su JJ, Huang JY, Yu IA.  2001.  Pump-probe spectroscopy of cold Rb-87 atoms in various polarization configurations, Apr. Physical Review A. 63:11., Number 4 AbstractWebsite

We investigate systematically pump-probe spectroscopy of cold Rb-87 atoms produced by a magneto-optical trap. The pump-probe spectra are measured without the presence of the trapping beams or any optical molasses. Various polarization configurations of the probe and pump fields result in very different spectra of probe absorption. The observed spectra exhibit a dispersive profile, a dispersionlike profile, a Lorentzian profile, or a dispersive profile plus a Lorentzian profile. The widths of all the spectral profiles are narrower than the natural linewidth of the excited state. Our work clarifies the mechanisms behind these different spectral profiles and provides essential information for the pump-probe spectroscopy of cold atoms.

Chen, YC, Liao YA, Chiu HY, Su JJ, Yu IA.  2001.  Observation of the quantum interference phenomenon induced by interacting dark resonances, Nov. Physical Review A. 64:5., Number 5 AbstractWebsite

We report an experimental observation of narrow and high-contrast spectra. which are induced by interacting dark resonances and have been predicted in Phys. Rev. A 60, 3225 (1999). Spectra are measured with cold (87)Rb atoms produced by a magneto-optical trap. In this experimental system, a coupling laser and a weak probe laser form a three-level Lambda -type configuration of electromagnetically induced transparency (EIT); a microwave drives a magnetic-dipole transition between the fourth level and the ground state that is coupled with the excited state by the coupling laser. The observed spectral profile of probe absorption exhibits a very sharp peak emerging inside a narrow EIT dip. Such spectral feature provides more opportunities in manipulating atomic-optical response.

Chen, YC, Liao YA, Hsu L, Yu IA.  2001.  Simple technique for directly and accurately measuring the number of atoms in a magneto-optical trap, Sep. Physical Review A. 64 AbstractWebsite

We have systematically studied a simple technique that accurately determines number of atoms in a magneto-optical trap, Absorption energy of a laser field that interacts with cold atoms is a direct measurement of atom number. The measured energy neither depends on the detuning, intensity, and polarization of the laser field nor is affected by other system parameters. Our work also demonstrates that such technique can be applied to study the phenomenon of coherent population trapping.

Chen, YW, Lin CW, Chen YC, Yu IA.  2002.  Quantization axes in coherent two-field spectroscopy, Aug. Journal of the Optical Society of America B-Optical Physics. 19:1917-1921., Number 8 AbstractWebsite

Coherent two-field spectroscopy calculations with different choices of the quantization (z) axis are compared. In a system driven by a strong coupling field and a weak probe field, alignment of the z axis along either the polarization direction of a linearly polarized coupling field or the propagation direction of a circularly polarized coupling field is shown to simplify the calculations and facilitate interpretation of the results. The advantages of a suitable choice of the z axis are highlighted with an example of a degenerate three-level system of electromagnetically induced transparency. (C) 2002 Optical Society of America.

Simien, CE, Chen YC, Gupta P, Laha S, Martinez YN, Mickelson PG, Nagel SB, Killian TC.  2004.  Using absorption imaging to study ion dynamics in an ultracold neutral plasma, Apr. Physical Review Letters. 92:4., Number 14 AbstractWebsite

We report optical absorption imaging of ultracold neutral strontium plasmas. The ion absorption spectrum determined from the images is Doppler broadened and thus provides a quantitative measure of the ion kinetic energy. For the particular plasma conditions studied, ions heat rapidly as they equilibrate during the first 250 ns after plasma formation. Equilibration leaves ions on the border between the weakly coupled gaseous and strongly coupled liquid states. On a longer time scale of microseconds, pressure exerted by the trapped electron gas accelerates the ions radially.

Chen, YC, Simien CE, Laha S, Gupta P, Martinez YN, Mickelson PG, Nagel SB, Killian TC.  2004.  Electron screening and kinetic-energy oscillations in a strongly coupled plasma, Dec 31. Physical Review Letters. 93 AbstractWebsite

We study equilibration of strongly coupled ions in an ultracold neutral plasma produced by photoionizing laser-cooled and trapped atoms. By varying the electron temperature, we show that electron screening modifies the equilibrium ion temperature. Even with few electrons in a Debye sphere, the screening is well described by a model using a Yukawa ion-ion potential. We also observe damped oscillations of the ion kinetic energy that are a unique feature of equilibration of a strongly coupled plasma.

Simien, CE, Chen YC, Gupta P, Laha S, Martinez YN, Mickelson PG, Nagel SB, Killian TC.  2005.  Absorption imaging of ultracold neutral plasmas, Apr. Ieee Transactions on Plasma Science. 33:540-541. AbstractWebsite

We report optical absorption imaging of ultracold neutral plasmas. Imaging allows direct observation of the ion density profile and expansion of the plasma. The frequency dependence of the plasma's optical depth gives the ion absorption spectrum, which is broadened by the ion motion. We use the spectral width to monitor ion equilibration in the first 250 ns after plasma formation. On a microsecond time scale, we observe the radial acceleration of ions resulting from pressure exerted by the trapped electron gas.

Killian, TC, Chen YC, Gupta P, Laha S, Martinez YN, Mickelson PG, Nagel SB, Saenz AD, Simien CE.  2005.  Absorption imaging and spectroscopy of ultracold neutral plasmas, Jan 28. Journal of Physics B-Atomic Molecular and Optical Physics. 38:S351-S362. AbstractWebsite

Absorption imaging and spectroscopy can probe the dynamics of an ultracold neutral plasma during the first few microseconds after its creation. Quantitative analysis of the data, however, is complicated by the inhomogeneous density distribution, expansion of the plasma and possible lack of global thermal equilibrium for the ions. In this paper, we describe methods for addressing these issues. Using simple assumptions about the underlying temperature distribution and ion motion, the Doppler-broadened absorption spectrum obtained from plasma images can be related to the average temperature in the plasma.

Nagel, SB, Mickelson PG, Saenz AD, Martinez YN, Chen YC, Killian TC, Pellegrini P, Cote R.  2005.  Photoassociative spectroscopy at long range in ultracold strontium, Mar 4. Physical Review Letters. 94 AbstractWebsite

We report photoassociative spectroscopy of Sr-88(2) in a magneto-optical trap operating on the S-1(0)-->P-3(1) intercombination line at 689 nm. Photoassociative transitions are driven with a laser red detuned by 600-2400 MHz from the S-1(0)-->P-1(1) atomic resonance at 461 nm. Photoassociation takes place at extremely large internuclear separation, and the photoassociative spectrum is strongly affected by relativistic retardation. A fit of the transition frequencies determines the P-1(1) atomic lifetime (tau=5.22+/-0.03 ns) and resolves a discrepancy between experiment and recent theoretical calculations.

Killian, TC, Chen YC, Gupta P, Laha S, Martinez YN, Mickelson PG, Nagel SB, Saenz AD, Simien CE.  2005.  Ultracold neutral plasmas, May. Plasma Physics and Controlled Fusion. 47:A297-A306. AbstractWebsite

Ultracold neutral plasmas are formed by photo-ionizing laser-cooled atoms near the ionization threshold. Through the application of atomic physics techniques and diagnostics, these experiments stretch the boundaries of traditional neutral plasma physics. The electron temperature in these plasmas ranges from 1 to 1000 K and the ion temperature is around 1 K. The density can approach 10(11) cm(-3). Fundamental interest stems from the possibility of creating strongly coupled plasmas, but recombination, collective modes, and thermalization in these systems have also been studied. Optical absorption images of a strontium plasma, using the Sr+ S-2(1/2) -> P-2(1/2) transition at 422 mn, depict the density profile of the plasma, and probe kinetics on a 50 ns time-scale. The Doppler-broadened ion absorption spectrum measures the ion velocity distribution, which gives an accurate measure of the ion dynamics in the first microsecond after photo-ionization.

Mickelson, PG, Martinez YN, Saenz AD, Nagel SB, Chen YC, Killian TC, Pellegrini P, Cote R.  2005.  Spectroscopic determination of the s-wave scattering lengths of Sr-86 and Sr-88, Nov 25. Physical Review Letters. 95 AbstractWebsite

We report the use of photoassociative spectroscopy to determine the ground-state s-wave scattering lengths for the main bosonic isotopes of strontium, Sr-86 and Sr-88. Photoassociative transitions are driven with a laser red detuned by up to 1400 GHz from the S-1(0)-P-1(1) atomic resonance at 461 nm. A minimum in the transition amplitude for Sr-86 at -494 +/- 5 GHz allows us to determine the scattering lengths 610a(0)< a(86)< 2300a(0) for Sr-86 and a much smaller value of -1a(0)< a(88)< 13a(0) for Sr-88.

Laha, S, Chen YC, Gupta P, Simien CE, Martinez YN, Mickelson PG, Nagel SB, Killian TC.  2006.  Kinetic energy oscillations in annular regions of ultracold neutral plasmas, Oct. European Physical Journal D. 40:51-56., Number 1 AbstractWebsite

A study of ion equilibration in annular regions of ultracold strontium plasmas is reported. Plasmas are formed by photoionizing laser-cooled atoms with a pulsed dye laser. The experimental probe is spatially-resolved absorption spectroscopy using the S-2(1/2)-P-2(1/2) transition of the Sr+ ion. The kinetic energy of the ions is calculated from the Doppler broadening of the spectrum, and it displays clear oscillations during the first microsecond after plasma formation. The oscillations, which are a characteristic of strong coulomb coupling, are fit with a simple phenomenological model incorporating damping and density variation in the plasma.

Lin, YW, Chou HC, Dwivedi PP, Chen YC, Yu IA.  2008.  Using a pair of rectangular coils in the MOT for the production of cold atom clouds with large optical density, Mar. Optics Express. 16:3753-3761., Number 6 AbstractWebsite

We demonstrate a simple method to increase the optical density (OD) of cold atom clouds produced by a magneto-optical trap (MOT). A pair of rectangular anti-Helmholtz coils is used in the MOT to generate the magnetic field that produces the cigar-shaped atom cloud. With 7.2 x 10(8) Rb-87 atoms in the cigar-type MOT, we achieve an OD of 32 as determined by the slow light measurement and this OD is large enough such that the atom cloud can almost contain the entire Gaussian light pulse. Compared to the conventional MOT under the same trapping conditions, the OD is increased by about 2.7 folds by this simple method. In another MOT setup of the cigar-shaped Cs atom cloud, we achieve an OD of 105 as determined by the absorption spectrum of the |6S(1/2), F = 4 > ->| 6P(3/2), F ' = 5 > transition. (C) 2008 Optical Society of America

Tu, MF, Ho JJ, Hsieh CC, Chen YC.  2009.  Intense SrF radical beam for molecular cooling experiments, Nov. Review of Scientific Instruments. 80:5., Number 11 AbstractWebsite

We have developed a continuous SrF radical beam for the loading of helium buffer gas cooling. The SrF molecules are efficiently generated by high-temperature chemical reaction of the solid precursor SrF(2) with boron in a graphite oven. The beam properties are characterized with laser-induced fluorescence spectroscopic method. We obtain a molecular flux of up to 2.1 x 10(15) sr(-1) s(-1) at the detection region for all rotational states. The dependence of the flux on oven temperature suggests that even higher flux is possible if a higher temperature in the oven is achieved. (C) 2009 American Institute of Physics. [doi:10.1063/1.3262631]

Lo, HY, Chen YC, Su PC, Chen HC, Chen JX, Yu IA, Chen YF.  2011.  Electromagnetically-induced-transparency-based cross-phase-modulation at attojoule levels, Apr. Physical Review A. 83:4., Number 4 AbstractWebsite

We report the experimental demonstration of electromagnetically-induced-transparency-based cross-phase-modulation at attojoule or, equivalently, few-hundred-photon levels. A phase shift of 0.005 rad of a probe pulse modulated by a signal pulse with an energy of similar to 100 aJ, equivalent to similar to 400 photons, was observed in a four-level system of cold (87)Rb atoms.

Lin, PY, Shiau BW, Hsiao YF, Chen YC.  2011.  Creation of arbitrary spectra with an acousto-optic modulator and an injection-locked diode laser, Aug. Review of Scientific Instruments. 82:6., Number 8 AbstractWebsite

We use a double-passed acousto-optic modulator (AOM), driven by an arbitrary waveform generator to produce multiple frequency components for a laser with arbitrary frequency spacings. A programmed sequence containing various sections of radio-frequency sinusoidal signal at different frequency is applied to drive the AOM. The diffracted light is used to injection-lock a diode laser. The combined techniques allow us to generate the multi-line spectra for the diode laser with arbitrary frequency spacings in the range of 100 MHz at a relatively high output power of 80 mW and a small power variation of 2%. Such a light source can be used in the application for laser cooling of molecules. (C) 2011 American Institute of Physics. [doi:10.1063/1.3626903]

Shiau, BW, Wu MC, Lin CC, Chen YC.  2011.  Low-Light-Level Cross-Phase Modulation with Double Slow Light Pulses, May. Physical Review Letters. 106:4., Number 19 AbstractWebsite

We report on the first experimental demonstration of low-light-level cross-phase modulation (XPM) with double slow light pulses based on the double electromagnetically induced transparency (EIT) in cold cesium atoms. The double EIT is implemented with two control fields and two weak fields that drive populations prepared in the two doubly spin-polarized states. Group velocity matching can be obtained by tuning the intensity of either of the control fields. The XPM is based on the asymmetric M-type five-level system formed by the two sets of EIT. Enhancement in the XPM by group velocity matching is observed. Our work advances studies of low-light-level nonlinear optics based on double slow light pulses.

Chen, Y-H, Lee M-J, Hung W, Chen Y-C, Chen Y-F, Yu IA.  2012.  Demonstration of the Interaction between Two Stopped Light Pulses. Physical Review Letters. 108:173603. AbstractWebsite
Lin, C-C, Wu M-C, Shiau B-W, Chen Y-H, Yu IA, Chen Y-F, Chen Y-C.  2012.  Enhanced all-optical switching with double slow light pulses, Dec 28. Physical Review A. 86 AbstractWebsite
Huang, SJ, Hsu YT, Lee H, Chen YC, Volosniev AG, Zinner NT, Wang DW.  2012.  Field-induced long-lived supermolecules, May 7. Physical Review A. 85 AbstractWebsite

We demonstrate that the long-lived bound states (supermolecules) can exist in the dilute limit when we tune the shape of the effective potential between polar molecules by an external microwave field. Binding energies, average sizes, and phase diagrams for both s-orbital (bosons) and p-orbital (fermions) dimers are studied, together with bosonic trimer states. We explicitly show that the nonadiabatic transition rate can be easily tuned small for such ground-state supermolecules, so that the system can be stable from collapse even near the associated potential resonance. Our results, therefore, suggest a feasible cold molecule system to investigate novel few-body and many-body physics (for example, the p-wave BCS-Bose-Einstein-condensate crossover for fermions and the paired condensate for bosons) that cannot be easily accessed in single species atomic gases.