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Two-Photon Ionization of the Calcium 4S3D 1D2 Level in an Optical Dipole Trap

J. E. Daily

Published Jan 1, 2005• 0 citations
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Abstract

This thesis reports an optical dipole trap for atomic calcium. The dipole trap is loaded from a magneto-optical trap (MOT) of calcium atoms cooled near the Doppler limit (~1 mK). The dipole trap is formed by a large-frame argon ion laser focused to 20 microns into the center of the MOT. This laser runs single-line at 488 nm with a maximum power of 10.6 watts. These parameters result in a trap of 125 mK for calcium atoms in the 4s3d 1D2 state. The 488 nm light also photo-ionizes the trapped atoms due to a near-resonant transition to the 4s4f 1F3 level. These ions leave the trap and are detected to determine the trap decay rate. By measuring this decay rate as a function of 488 nm intensity, we determine the 1F3 photo-ionization cross section at this wavelength to be approximately 230 Mb.

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Journal of the Optical Society of America B•May 1, 2003•12 citations
This thesis reports an optical dipole trap for atomic calcium. The dipole trap is loaded from a magneto-optical trap (MOT) of calcium atoms cooled near the Doppler limit (~1 mK). The dipole trap is formed by a large-frame argon ion laser focused to 20 microns into the center of the MOT. This laser runs single-line at 488 nm with a maximum power of 10.6 watts. These parameters result in a trap of 125 mK for calcium atoms in the 4s3d 1D2 state. The 488 nm light also photo-ionizes the trapped atoms due to a near-resonant transition to the 4s4f 1F3 level. These ions leave the trap and are detected to determine the trap decay rate. By measuring this decay rate as a function of 488 nm intensity, we determine the 1F3 photo-ionization cross section at this wavelength to be approximately 230 Mb.
Two-photon photo-ionization of the Ca 4s3d  1D24s3d \; ^1D_24s3d1D2​ level in an optical dipole trap
We report an optical dipole trap for calcium. The trap is created by focusing a $488\text{\ensuremath{-}}\mathrm{nm}$ argon-ion laser beam into a calcium magneto-optical trap. The argon-ion laser photoionizes atoms in the trap because of a near-resonance with the 4s4f1F34s4f\phantom{\rule{0.2em}{0ex}}^{1}F_{3}4s4f1F3​ level. By measuring the dipole-trap decay rate as a function of argon-ion laser intensity, we determine the 1F3^{1}F_{3}1F3​ photoionization cross section at our wavelength to be approximately 230Mb230\phantom{\rule{0.3em}{0ex}}\mathrm{Mb}230Mb.
Laser excitation of the 4s4p3P2−−4s3d1D24s4p {}^{3}{P}_{2} -- 4s3d {}^{1}{D}_{2}4s4p3P2​−−4s3d1D2​ transition in atomic calcium has been observed and the wavelength determined to 1530.5298(6) nm. The metastable 4s4p3P24s4p {}^{3}{P}_{2}4s4p3P2​ atoms were magnetically trapped in the quadrupole magnetic field of a magneto-optical trap. This state represents the only ``loss'' channel for the calcium atoms when laser cooled on the 4s21S0−−4s4p1P14{s}^{2} {}^{1}{S}_{0} -- 4s4p {}^{1}{P}_{1}4s21S0​−−4s4p1P1​ transition. A rate equation model shows that an order of magnitude more atoms are trapped in this state compared with those taking part in the main cooling cycle. Excitation of the 3P2{}^{3}{P}_{2}3P2​ atoms back up to the 4s3d1D24s3d {}^{1}{D}_{2}4s3d1D2​ state provides a means of accessing these atoms. Efficient repumping is achieved if the 1530-nm laser is used in conjunction with a 672-nm laser driving the 4s3d1D2−−4s5p1P14s3d {}^{1}{D}_{2} -- 4s5p {}^{1}{P}_{1}4s3d1D2​−−4s5p1P1​ transition. In the present experiment, we detected about $4.5\ifmmode\times\else\texttimes\fi{}{10}^{4}$ trapped 3P2{}^{3}{P}_{2}3P2​ atoms, a relatively low atom density, and measured a lifetime of approximately 1 s, which is limited by background collisions.
We investigate a repumping scheme for magneto-optically trapped calcium atoms. It is based on excitation of the 4s3d1D24s3{d}^{1}{D}_{2}4s3d1D2​--4s5p1P14s5{p}^{1}{P}_{1}4s5p1P1​ transition at 672672672 nm with an extended cavity diode laser. The effect of the repumping is approximately a factor of three increase in trap lifetime and a doubling of the trapping efficiency from a Zeeman slowed thermal beam. Added to this, the 672-nm laser repumps atoms from an otherwise dark state to yield an overall increase in detected fluorescence signal from the magneto-optic trap (MOT) of more than an order of magnitude. Furthermore, we report isotope shift measurements of the 672672672-nm transition, for the first time, for four naturally occurring even isotopes. Using available charge radii data, the observed shifts, extending up to 4.34.34.3 GHz, display the expected linear dependence in a King plot analysis. The measured shifts are used to determine the isotope shifts of the remaining 41,43,46Ca^{41,43,46}\mathrm{Ca}41,43,46Ca isotopes. These might be of interest where less abundant isotopes are used enabling isotope selective repumping, resulting in enhanced trapping and detection efficiencies.
We describe a new system for laser cooling and trapping of neutral Calcium atoms employing the ¹S0 -¹ P1 resonant transition at 423 nm. An on-axis magneto-optical trap (MOT) is loaded from a Zeeman decelerated atomic beam. When a single laser is used, in order to avoid perturbation of the trap by the deceleration laser beam, this one has been tightly focused near the MOT center, with a waist size much smaller than the atomic cloud. In order to test the efficiency of this novel technique, we have then employed a second, independent decelerating laser, with a profile mode matched to the atomic beam. For an oven temperature of 580±C this system can load 1.2 (2) x 10(7) atoms in 16 (1) ms. By the spatial extension of the atomic cloud the one dimension rms velocity was estimated to be 136 (12) cm/s, corresponding to a temperature of 9 (2) mK. The variation of the number of trapped atoms as a function of laser detuning and intensity, trap magnetic field gradient and oven temperature is analyzed. Spatial structures of the trapped atoms, like stable rings created by vortex forces, have been observed. This is the first time that these structures, already observed in alkali-metal elements, are reported in MOTs of alkaline-earth elements.
We report on a system for atomic beam deceleration and magneto-optical trapping of calcium atoms that uses the 1S0–1P1 transition, in which a single laser is used to trap and slow the atoms. The slower laser beam is focused near the magneto-optical trap’s center, which has a waist size much smaller than the atomic cloud such that its influence on the trapped atoms is greatly reduced. We also investigate the theoretical possibility of cooling by use of a two-photon (4s2)1S0–(4s5s)1S0 transition. Excitation near resonance with the 1P1 level results in an equilibrium temperature seven times smaller than the Doppler limit of the 1S0–1P1 transition.