Two-photon photoionization of the Ca $4s3d{}^{1}D_2$ level in an optical dipole trap

We report an optical dipole trap for calcium. The trap is created by focusing a $488\text{−}\mathrm{nm}$ 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 $4s4f{}^{1}F_3$ level. By measuring the dipole-trap decay rate as a function of argon-ion laser intensity, we determine the ${}^{1}F_3$ photoionization cross section at our wavelength to be approximately $230\mathrm{Mb}$.

Figure 1

A schematic drawing of the MOT laser system and frequency-stabilization electronics used in these experiments.ACM: acousto-optical modulator. EOM: electro-optical modulator.

Figure 2

A partial energy-level diagram for calcium (not to scale).

Figure 3

Ion count rate versus $488\text{−}\mathrm{nm}$ laser power. (a) plots the raw count rate for a (measured) Gaussian waist of $w=20\mu \mathrm{m}$. The laser power was changed by varying the argon-ion laser tube current. The different symbols show measurements made with different intracavity laser apertures. (b) plots the ion count rate divided by the square of the laser power, which is proportional to the number of ${}^{1}D_2$ atoms in the dipole trap. The solid line is a fit of our Monte Carlo simulation of trap loading to the data.

Figure 4

Dipole-trap decay versus power squared. The decay signal from the dipole trap. (a) shows the ion signal versus time with $1\mathrm{W}$ focused to a $90\text{−}\mu \mathrm{m}$ waist. The effective decay rate for these data is $1.02\mathrm{kHz}$. (b) shows the apparent decay rate for a range of powers, all focused to a $90\text{−}\mu \mathrm{m}$ waist. The decay rate is fitted to a line proportional to the square of the power, as suggested by Eq. (3).