The two broad approaches of creating cold molecules are direct cooling of ground state molecules (using buffer gas cooling, Stark deceleration, laser cooling etc.) or associating ultracold atoms to create ultracold molecules. The latter approach has been extensively used to create heteronuclear bi-alkali molecules. In particular, two approaches of associating alkali atoms stand out. The first is photoassociation (PA), and the second is magneto-association (MA) followed by Stimulated Raman Adiabatic Passage (STIRAP). The second approach, although elegant, is technically more challenging and works only for species with favorable Feshbach resonances. The first approach is simpler and more general, but the population of molecules is distributed over many rovibronic levels.

Ultracold polar LiRb molecules:

In our typical PA experiments at Purdue University, LiRb molecules are created by shining a PA laser on ultracold Li and Rb atoms that are trapped in a dual species magneto-optical trap (MOT). The molecules created typically leave the trap and molecule formation reduces the number of trapped atoms. Therefore one way to know that molecules are formed by PA is to monitor the number of trapped atoms and look for reduction in the number of atoms as the PA laser frequency is scanned - such a "trap loss PA spectrum" near the Li (2s) + Rb (5p) asymptote is shown below. We have used the PA approach (PA followed by spontaneous emission) to create ultracold LiRb molecules in their electronic ground state. Such ground state molecules can be detected by resonance enhanced multi photon ionization (REMPI) of the molecules and detecting the resulting ions - a PA spectrum using REMPI detection is also shown below. By choosing the PA level appropriately we have been able to produce deeply bound LiRb molecules continuously. We also demonstrated two-photon photoassociation spectroscopy of LiRb molecules and used the spectroscopic information to refine molecular potentials and to predict alternative ways to produce ultracold LiRb molecules.

Spectroscopy and tunable lasers:

When I started my doctoral research in 2007, no spectroscopic data on the LiRb molecule was available. However, knowledge of molecular spectroscopy is essential for the production and detection of ultracold LiRb molecules. My first project was to construct an apparatus called the “heat-pipe oven” where hot vapor phase LiRb molecules were created from Li and Rb atoms. I then performed high resolution laser induced fluorescence (LIF) spectroscopy and excitation spectroscopy of these LiRb molecules. From the observed vibrational and rotational energy levels, we deduced the spectroscopic constants for the ground X 1Σ+ and the excited B 1Π states of LiRb. We also observed rotational perturbations in the molecular spectra and used it to estimate the spectroscopic constants for the C 1Σ+ state. In our LIF spectra, we also observed the nodal pattern in the wave function for different vibrational levels (vʹ) of the B 1Π state (see figure below). These results allowed us to predict efficient pathways for the creation and detection of ultracold ground state LiRb molecules.

I am also interested in making simple external cavity diode lasers (ECDLs) which are ubiquitous in atomic physics labs around the world. We demonstrated the highest mode-hop-free tuning range (135 GHz) of ECDLs by small adjustments to existing designs. The figure at the end shows the absorption spectra of Rb and molecular iodine taken with the ECDL.