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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, 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.

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, 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, 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, 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, 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.

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.

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.

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.