We demonstrate the use of optical pumping of kinetically ultracold NaCs to cool an initial vibrational distribution of electronic ground state molecules X1Σ+(v ≥ 4) into the vibrational ground state X1Σ+(v=0). Our approach is based on the use of simple, commercially available multimode diode lasers selected to optically pump population into X1Σ+(v=0). We investigate the impact of the cooling process on the rotational state distribution of the vibrational ground state, and observe that an initial distribution, Jinitial=0–2 is only moderately affected resulting in Jfinal=0–4. This method provides an inexpensive approach to creation of vibrational ground state ultracold polar molecules.
© 2012 OSA
Lumino-frigoriques is a term originally used by A. Kastler to describe optical pumping (OP) of sodium atoms between hyperfine states to accumulate population in a dark state . The effects of optical pumping “cool” the sample when the final (dark) state is at a lower energy than the initial state. OP is ubiquitous in the laser cooling of atoms [2,3], and has recently been adopted to produce samples of ground state molecules: it has been demonstrated for Cs2 by pumping vibrational levels with a femtosecond laser  or with low power broad band cw light , and for molecular ions by pumping rotational states [6, 7].
Producing a sample of polar molecules in the rovibrational ground state is of significant current interest as a starting point for the creation of dipolar crystals [8, 9], quantum computation  and quantum chemistry . So far, the approaches for accumulating heteronuclear molecules in the (ro)vibrational ground state either involve highly efficient yet technically challenging experimental techniques, or are relatively simple but inefficient. The earliest demonstration is a “pump-dump” procedure in RbCs  that transferred population in select vibrational levels to X1Σ+(v=0) with an experimentally unconfirmed rotational distribution. Then, there is the efficient but technically challenging demonstration of magneto-association and coherent transfer of KRb  to the rovibrational ground state using laser system referenced to a frequency comb. Also, direct photoassociation yields rovibrational ground state LiCs  and vibrational ground state NaCs  but with the majority of the sample distributed across a range of excited rovibrational states.
In this work, we utilize the simple and effective process of luminorefrigeration to vibrationally cool a sample of highly polar NaCs molecules created by photoassociation. Specifically, we optically pump an initial distribution of rovibrational levels to the vibrational ground state, X1Σ+(v=0, J=0–4). We describe a series of experiments that explore the pumping dynamics and discuss the applicability of this approach to other bialkali systems.
2. Experimental setup: molecule formation
During the experiment, we first photoassociate (PA) NaCs from dark-SPOT magneto-optical traps. Initially, we choose a PA resonance that is part of the (4)Ω=1 progression , detuned 32 GHz from the Cs 62S1/2 → 62P3/2 transition, and we lock the PA laser to within a few MHz. This channel has a high formation rate, estimated to be ∼107 molecules/s determined from spectroscopic results , the wavefunction overlap of the PA resonance with the singlet ground state calculated using LEVEL 8.0 , and by considering the volume of the ionizer beam intersecting with the region of molecule formation. The ultracold molecules are not trapped and drift away from the formation/manipulation/detection region over a period of ∼10ms estimated from the measured kinetic temperature of ∼250 μK. To achieve vibrational cooling, the photoassociated NaCs sample is continuously exposed to OP light, driving the rovibrational distribution of molecules through the A1Σ+ and b3Π electronic states (A–b complex). We then probe the populated vibrational states of the NaCs sample using Resonance Enhanced Multi-photon Ionization (REMPI) where the neutral molecules are ionized and subsequently detected by a channel electron multiplier (CEM) using time of flight mass spectroscopy, and these experiments are run at a 10 Hz repetition rate. Other spectroscopic techniques are employed to unambiguously determine the rovibrational state populations, such as pulsed depletion spectroscopy (PDS)  and cw depletion spectroscopy .
The initial population distribution created using the 32 GHz PA resonance has been shown to include X1Σ+(v=4–6,8,9,11,13,15,17,19,21,23,25–27,31), determined using PDS . This range of vibrational states exhibits an electric dipole moment of ∼4.6 Debye . We expect that there is also population in higher singlet ground state vibrational levels from the Franck-Condon (FC) overlap of the potential energy curves (PECs) calculated using LEVEL 8.0 , though we have not yet directly confirmed population of these states using REMPI.
3. Experimental setup: molecule manipulation
The A–b complex is illustrated in Fig. 1. We choose it for OP because it minimizes the number of electronic states involved in the cycling transition, and because it is a well studied system . From transition moment and transition energy calculations provided by A. Stolyarov et al. , we know that the singlet ground state and A–b complex PECs are well overlapped and similar in shape and depth. Therefore for the appropriate choice of OP light, we may drive a maximal number of ground state transitions with minimal spectral width. We note that the A–b complex has singlet and triplet spin components, therefore optical pumping can involve both the singlet and triplet ground states.
Consider the effect of OP light with a broad spectral distribution to the red of 978.62nm. This wavelength couples X1Σ+(v=0) → A–b complex (N=1), N=1 being the lowest vibrational level of the Ω=0 component of the b3Π, and consequently the lowest vibrational level in the excited state manifold. The A–b complex is a heavily mixed system of electronic states, therefore the effective vibrational levels are perturbed and numbered using index N. For this spectral distribution, the OP light would not be energetic enough to drive population from X1Σ+(v=0) forming a dark state, yet it will pump population out of X1Σ+(v ≥1).
To check the viability of this approach and to determine a suitable spectral bandwidth, we estimate the transition rates from X1Σ+ → Ω=0 components of the A–b complex to provide insight into the expected OP behavior. Note, there are 87 singlet ground state vibrational levels; however, transition dipole moments were available only for X1Σ+(v=0–65) from . We choose a 985nm spectral component based on the known initial vibrational distribution, and this wavelength corresponds to the strongest calculated transition moments from deeply bound singlet ground states. We note that the 985nm component addresses population throughout the singlet and triplet ground states. However, in order to more efficiently drive the higher lying vibrational levels, we include a 1206nm spectral component. This wavelength selection is a compromise based on the availability of laser diodes and wavelengths closer to 1255nm would access more efficient pumping pathways. We calculate transition rates based on the readily available diode lasers assuming a flat-top spectral intensity profile spanning 979–991nm at 300 mW and 1203–1209nm at 1.5 W focused to 1mm in diameter. We conclude that a sufficient range of singlet ground state vibrational levels may be coupled via optical pumping by this experimentally feasible laser system. Of course, increasing the spectral width would couple even more levels thereby increasing the transfer efficiency to X1Σ+(v=0).
We create the broadband OP light by overlapping the output of four 2W 980nm multi-mode diode lasers , each tuned to a slightly different wavelength, and a 2.5W 1206nm broad area diode laser . These free-running diodes are temperature stabilized, however there is no active wavelength stabilization. Also, there is no 4-f line shaping with a mask as in  and these diodes are simply temperature tuned for wavelength selection. As shown in Figs. 2(a) and 2(b), the OP light has a spectral component centered at 985nm with ∼10nm width and another component centered at 1206nm with a ∼5nm width, respectively. The solid vertical line in Fig. 2(a) indicates the wavelength coupling X1Σ+(v=0) → A–b complex (N=1). The OP light is to the red of this transition satisfying the requirement for the formation of a dark state.
4. Results and discussion
With this choice of OP light, we observe significant transfer to X1Σ+(v=0), determined by comparing REMPI spectra [15, 21], assigned with PDS, taken with and without OP. To locate detection lines for measuring the populated vibrational levels we utilize both a 2 and a 3 photon REMPI procedure which cover a range of 800–812nm, 840–870nm, and 535–545nm. The 3 photon REMPI utilizes the A–b complex for the first transition and these wavelengths are calibrated to select atomic lines. Figure 3(a) shows the detected vibrational states in the initial distribution with labels for the deepest states, X1Σ+(v=4–6,19). The REMPI range here is selected because it contains one of the detection lines for the vibrational ground state. The state assignments in Fig. 3 are as described in  and known with high certainty due to the well known excited state assignments also from experimental data . After vibrational OP, Fig. 3(b) shows the final distribution. Only the lowest three states, X1Σ+(v=0–2), are populated while other vibrational levels are completely or mostly depopulated. From prior work  we know that another X1Σ+(v=0) REMPI detection line exists in the 535–545nm range and our OP results were confirmed using such a 535–545nm REMPI scan.
OP experiments are conducted using combinations of the 985nm and 1206nm spectral components to investigate how initial vibrational states are being pumped to (v=0). The 1206nm spectral component alone does not significantly populate X1Σ+(v=0); however, it does increase population in other deeply bound singlet ground states. The 985nm spectral component saturates population driven into X1Σ+(v=0) as a function of OP intensity, and when the two spectral components are combined, the transfer rate to X1Σ+(v=0) is nearly doubled. This increase arises from the 1206nm spectral component transferring population into vibrational states that are accessible with the 985nm component and driven to X1Σ+(v=0). These results confirm the presence of higher lying vibrational levels in the initial distribution.
When the OP is optimized, we observe 50 NaCs ions the X1Σ+(v=0) detection channel per REMPI pulse. At this rate, however, we approach saturation of the standard CEM, meaning that the rate of ion bombardment is nearing the max output pulse rate, and actual production rates may be higher than those detected. A conservative estimate for the transfer rate into X1Σ+(v=0) is ∼1×105 molecules/s from the initial rovibrational distribution created by the 32 GHz PA resonance. This is estimated by considering the detected ions per REMPI pulse, the detector efficiency, a geometric factor describing the ratio of detected molecules to those outside of the ionizer pulse region, and the 10 Hz experimental repetition rate. We also find accumulation in X1Σ+(v=1, 2) with 45 ions per REMPI pulse, consistent with the relative spectral intensities of the diodes illustrated in Fig. 2 and the calculated transition rates.
Due to the non-uniformity of the OP spectral intensity in Fig. 2, one might infer that there would be inaccessible ground vibrational states creating pockets of dark and quasi-dark vibrational states where population would persist in the singlet ground state. A dark state is a state that does not couple to the excited state and a quasi-dark state is one that is coupled but still retains population. Quasi-dark states might be the result of a lack of intensity to drive transitions or the rate of excitation is less than the decay rate into that state. In the REMPI spectra, however, we find only a few quasi-dark states with population not completely depleted starting from X1Σ+(v=1,2,8,13,15,19), and we see no evidence of a true dark state other than X1Σ+(v=0) when the two spectral component OP light is used. We observe depletion in the triplet ground state for a3Σ+(v=12), verifying that here the triplet ground state is not a dark state. The formation and assignment of this vibrational state is discussed in . From calculated transition energies provided by , we know that the OP light will drive transitions throughout the triplet ground state to the Ω=0–2 components of the b3Π.
Here, we measure the initial rotational distribution by performing cw depletion spectroscopy on the sample. Figure 4(a) presents the initial rotational distribution created by the 960 GHz PA resonance that is part of the (4)Ω=1 progression; it creates the same ground state rotational distribution as the 32 GHz PA resonance. This PA line is used here because the scan has the best signal to noise ratio. By using the X1Σ+(v=4) → A–b complex (N=70) transition, we find that PA through J=1 in the excited state, populates J=0,1,2 in the singlet ground state as shown by the solid line in Fig. 4(a). The assignments were made by comparing line positions to calculated values from . Selecting the PA state is a crucial step when constructing the initial distribution and higher rotational states may be populated depending on the PA channel selected. The dotted line in Fig. 4(a) illustrates J=1,3 being populated via PA through J=2 in the excited state.
For each transition during OP, the rotational state selection rule is ΔJ=±1 and we expect a spread in rotational states as OP drives population into X1Σ+(v=0). Figure 4(b) illustrates the rotational states that are populated within X1Σ+(v=0) by driving transitions between X1Σ+(v=0) → A–b complex(N=77). From this depletion scan, we determine that the final rotational distribution populates Jfinal=0–4 and a significant fraction is in X1Σ+(v=0, J=2).
The ΔΛ=0 Hönl-London (HL) factors describing the branching ratios for the X1Σ+ → A1Σ+ rotational transitions are used in a monte carlo simulation to model the cw depletion spectroscopy results. We use an initial rotational state distribution of Jinitial=0–2 and alter the rotational state weighted by the HL factors for two thousand molecules and a specified number of cycles. Each cycle consists of the excitation and decay transitions. This model predicts the final population distribution for 1–6 cycles is centered at Jfinal=2 with population ranging from Jfinal=0–6 as depicted in Fig. 5. When modeling 3–8 cycles, we find the rotational population disperses over ΔJfinal=0–10 with a majority in states Jfinal=2,4,6 which is contrary to the observed spectrum. Therefore, we infer the molecules do not undergo many transitions before populating X1Σ+(v=0, J=2) due to the FC overlap of the ground and excited states.
The rotational state modeling predicts a small number of cycles in the experiment; therefore, we estimate the number of vibrational cooling cycles using a separate monte carlo simulation. It models the optical pumping assuming a flat-top OP spectral profile covering 979–991nm and 1203–1209nm. The simulation utilizes the transitions accessible with the OP spectrum, and the excitation and decay transitions are determined using the vibrational state branching ratios between the A–b complex and the singlet and triplet ground states provided by . From this simulation, we predict that 20% of a uniform initial population distribution covering X1Σ+(v=4–65) is driven to X1Σ+(v=0) in 6 cycles (12 transitions) or less. These results are consistent with the observed rotational state dispersion, as well as with our estimated formation rates, highlighting the optical pumping efficiency obtained using the A–b complex.
5. Applications to other heteronuclear species
Luminorefrigeration may be generalized to any system where the lowest excited electronic states are well matched with the ground electronic state. We note that KCs, LiCs, and RbCs have PECs similar to NaCs for the ground state and A–b complex. For example, we calculate FC factors for the experimentally determined singlet ground state  and deperturbed A1Σ+ and b3Π states  using LEVEL  for the KCs molecule. The calculated FC map indicates reasonable overlap, and favorable transition moments  suggest similar OP results may be obtained with appropriate laser diode selection for ∼988–1000 nm coverage.
To conclude, we demonstrate a simple method to enhance production of ultracold, rovibrational singlet ground state molecules. This is the first application of vibrational optical pumping in a heteronuclear, highly polar sample with an experimentally verified rotational distribution. Broadband cw light drives the initial population distribution from the singlet ground state to the A1Σ+–b3Π complex in a series of absorption and spontaneous emission cycles and population accumulates in X1Σ+(v=0) at an estimated rate of ∼1×105 molecules/s, populating X1Σ+(v=0, J=0–4). A cw approach has low risk of multiphoton excitation and minimal rotational state dispersion and the efficiency is limited only by the bandwidth of the OP laser system.
In the future, we plan to increase population in X1Σ+(v=0, J=0) via rotational optical pumping. We are currently exploring the unique properties of the lowest vibrational levels in the A1Σ+–b3Π complex which will facilitate the transfer of X1Σ+(v=0, J=2) → X1Σ+(v=0, J=0).
This project was funded by the NSF and ARO.
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