Molecular Dynamic Polarizability
We theoretically investigated the effects of AC external fields on ultracold molecules. These external fields are used to create, trap, manipulate, and control molecules. The relevant property for study of the interaction of a molecule with light fields is the molecular dynamic polarizability as a function of frequency and polarization of light. The molecular polarizability depends on the quantum state of the molecule as well.
Trapping ultracold molecules in optical lattices via AC Stark shifts provides benefits for optimized control and precision measurements. First, we investigated the effects of an optical trap or lattice on polar KRb and RbCs molecules. Our goal was to determine lattice parameters that ensure strong trapping forces and small decoherence rates. The depth of the lattice potential is determined by the real part of the polarizability, while the laser-induced decoherence is proportional to the imaginary part of the polarizability. Our calculations show that the polarizability of KRb and RbCs ground state molecules in the optical domain has frequency windows in which resonant excitation is unlikely. We have focused on the dynamic polarizability of the initial and final levels relevant for current experiments. Using these results we proposed frequency intervals, which are most easy to work with experimentally (PRA 73, 041405(R) (2006)).
Polar molecules have a permanent dipole moment and their levels can be shifted and mixed with one another by applying an external electric field. This opens up a new way to create magic trapping conditions for two rotational levels of the molecule. In the presence of an external electric field, J is not a good quantum number and all states, even the “rotationless” ground state, have an anisotropic polarizability. The anisotropy of the dynamic polarizability of these levels mani-fests itself as a dependence on the relative orientation of the polarization of the trapping laser and the dc electric field. The combined action of these two fields can be a powerful tool to manipulate and control ultracold molecules trapped in an optical potential. In PRA 82, 063421 (2010) we studied the dynamic polariza-bility of the states |J=0⟩ and |J=1, M = 0,±1⟩ as a function of the angle θ of the linear polarization of the optical field relative to the static field (such that ε = cos θ z + sin θ x), for several static electric field strengths within the range from 0 to 6 kV/cm for KRb and 0 to 3 kV/cm for RbCs. Figure 2 shows dependence of the dynamic polarizability on the angle between the polarizaton ε of the trapping light at a wave number 9174 cm-1 and the direction of the exter-nal electric field for both KRb and RbCs molecules.
Furthermore, we analyzed the dynamic polarizability of Sr2 ground-state vibrational levels in order to identify Stark-shift-cancelling optical lattice frequencies, where for a pair of vibrational levels the Stark shift is identical. These pairs of levels are relevant for high precision tests of possible time-dependent fundamental constants. (PRL 100, 043201 (2008) and PRA 79, 012504 (2009)).