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Photoassociation and vibrational cooling of Rb2 molecules with a high-power laser

Paulo Cesar Ventura

Published Apr 4, 2017• 0 citations
Publisher

Abstract

We have developed a technique to produce, manipulate and trap Rb2 molecules with a single optical beam. This beam is generated by a high-power fiber amplifier (50 W of total output power) in the 1060 to 1070 nm range, which is seeded by two light sources: a sharp-band laser and a broadband superluminescent diode. The laser source is tuned to produce Rb2 molecules from an ultracold 85Rb sample via photoassociation. The broadband spectrum vibrationally cool the molecules by optical pumping. This source is spectrally shaped in order to populate the molecules at the fundamental vibrational state ν = 0. The molecular sample is probed by two-photon ionization, promoted by a pulsed dye laser in the 475 to 480 nm range. By scanning the photoassociation laser frequency, we have obtained a photoassociation spectrum of the ν\\' = 138 state of the 0+u potential, confirming previous observations. We have also obtained two vibrational spectra of the molecules by varying the ionization laser frequency, in the presence and absence of the broadband source. The comparison between the two spectra, along with ab-initio data, provides evidences that the molecules are optically pumped to the fundamental vibrational state.

DOI
10.11606/d.76.2017.tde-14032017-145533

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We have developed a technique to produce, manipulate and trap Rb2 molecules with a single optical beam. This beam is generated by a high-power fiber amplifier (50 W of total output power) in the 1060 to 1070 nm range, which is seeded by two light sources: a sharp-band laser and a broadband superluminescent diode. The laser source is tuned to produce Rb2 molecules from an ultracold 85Rb sample via photoassociation. The broadband spectrum vibrationally cool the molecules by optical pumping. This source is spectrally shaped in order to populate the molecules at the fundamental vibrational state ν = 0. The molecular sample is probed by two-photon ionization, promoted by a pulsed dye laser in the 475 to 480 nm range. By scanning the photoassociation laser frequency, we have obtained a photoassociation spectrum of the ν\' = 138 state of the 0+u potential, confirming previous observations. We have also obtained two vibrational spectra of the molecules by varying the ionization laser frequency, in the presence and absence of the broadband source. The comparison between the two spectra, along with ab-initio data, provides evidences that the molecules are optically pumped to the fundamental vibrational state.
We have developed a technique to produce, manipulate and trap Rb2 molecules with a single optical beam. This beam is generated by a high-power fiber amplifier (50 W of total output power) in the 1060 to 1070 nm range, which is seeded by two light sources: a sharp-band laser and a broadband superluminescent diode. The laser source is tuned to produce Rb2 molecules from an ultracold 85Rb sample via photoassociation. The broadband spectrum vibrationally cool the molecules by optical pumping. This source is spectrally shaped in order to populate the molecules at the fundamental vibrational state ν = 0. The molecular sample is probed by two-photon ionization, promoted by a pulsed dye laser in the 475 to 480 nm range. By scanning the photoassociation laser frequency, we have obtained a photoassociation spectrum of the ν\' = 138 state of the 0+u potential, confirming previous observations. We have also obtained two vibrational spectra of the molecules by varying the ionization laser frequency, in the presence and absence of the broadband source. The comparison between the two spectra, along with ab-initio data, provides evidences that the molecules are optically pumped to the fundamental vibrational state.
The development of the field of the science of ultra-cold matter has opened some exciting possibilities in exploring the quantum-mechanical processes which dominate matter interactions at the sub-microscopic scale. Although methods of cooling atoms are well established, molecular cooling is made difficult by molecules’ additional vibrational and rotational degrees of freedom. It was the goal of the research in this work to approach molecular cooling indirectly, by using broadband shaped-pulse photoassociation for the generation of tightly bound ultracold Rb2 molecules. The experiments towards this goal conducted by our group included a pumpdecay experiment to observe the generation of ground state singlet or triplet molecules. However, attempts to observe an increase in ground state population have been unsuccessful. A pump-probe study of wavepacket dynamics in the 5s+5p electronic state was conducted in order to determine the appropriate timing for the application of an additional pulse to dump population into the ground state. Although the attempt to observe wavepacket oscillations has been unsuccessful, pump-probe studies have yielded the observation of loosely bound excited state molecules as a result of the photoassociation pulse. These results are promising as a first stage in a fully coherent pump-dump approach to stabilisation into the lowest vibrational ground state. This thesis will provide an introduction and overview to the concerns involved in addressing the problem of molecular cooling and generation. Experimental techniques will be discussed including pulsed laser systems, optical parametric amplifi- cation, and the presentation of an original design for pulse shaping with an acoustooptic modulator. The emphasis of these discussions will be on the principles and operating procedures required for the use of these devices as home-built systems. The thesis will conclude with the results of pump-probe experiments utilising the pulse shaper as a spectral cutting device.
We form ultracold ground-state Rb 2 molecules by photoassociating pairs of atoms in a magneto-optical trap into the 0 u + state, which decays radiatively into high vibrational levels of the X 1 Σ g + state. Sensitive and vibrationally state-selective detection is achieved by means of resonantly-enhanced two-photon ionization with a pulsed laser. Frequency scans of the detection laser reveal a long vibrational progression to a previously unobserved electronic excited state, which we identify as the 2 1 Σ u + state. Most of its vibrational spectrum is in excellent agreement with predictions based on ab initio potentials, although the lowest vibrational levels exhibit strong perturbative mixing with the triplet 2 3 Π u state. The detection method reported here, with minor variations, should be effective for the entire potential well of the X state. In this work no transitions are observed from vibrational levels above v = 118, but this turns out to be a limitation not of the detection method but rather of the photoassociative formation scheme, due to re-excitation of the highest- v levels by the same photoassociation laser that produces them.
The methods producing cold molecules from cold atoms tend to leave molecular ensembles with substantial residual internal energy. For instance, cesium molecules initially formed via photoassociation of cold cesium atoms are in several vibrational levels nu of the electronic ground state. We applied a broadband femtosecond laser that redistributes the vibrational population in the ground state via a few electronic excitation/spontaneous emission cycles. The laser pulses are shaped to remove the excitation frequency band of the nu = 0 level, preventing re-excitation from that state. We observed a fast and efficient accumulation ( approximately 70% of the initially detected molecules) in the lowest vibrational level, nu = 0, of the singlet electronic state. The validity of this incoherent depopulation pumping method is very general and opens exciting prospects for laser cooling and manipulation of molecules.
By using broadband lasers, we demonstrate the possibilities for control of cold molecules formed via photoassociation. Firstly, we present a detection REMPI scheme (M. Viteau et al., Phys. Rev. A, 2009, 79, 021402) to systematically investigate the mechanisms of formation of ultracold Cs2 molecules in deeply bound levels of their electronic ground state X1sigma(g)+. This broadband detection scheme could be generalized to other molecular species. Then we report a vibrational cooling technique (M. Viteau et al., Science, 2008, 321, 232) through optical pumping obtained by using a shaped mode locked femtosecond laser. The broadband femtosecond laser excites the molecules electronically, leading to a redistribution of the vibrational population in the ground state via a few absorption-spontaneous emission cycles. By removing the laser frequencies corresponding to the excitation of the v = 0 level, we realize a dark state for the so-shaped femtosecond laser, leading, with the successive laser pulses, to an accumulation of the molecules in the v = 0 level, ie. a laser cooling of the vibration. The simulation of the vibrational laser cooling allows us to characterize the criteria to extend the mechanism to other molecular species (R. V. Krems, Int. Rev. Phys. Chem., 2005, 24, 99). We finally discuss the generalization of the technique to laser cooling of the rotation of the molecule.
The expansion of laser cooling capabilities from the atomic to the molecular regime has proven challenging due to the lack of availability of an equivalent closed-loop cooling cycle within the rich molecular internal energy level structure. An enticing solution is the application of ultrafast coherent control techniques to ultracold molecule generation. Unlike continuous-wave photoassociation techniques, the use of broadband 'pump' and 'dump' pulses offers the opportunity to access deeply bound vibrational states. Unlike alternative vibrational cooling strategies such as stimulated Raman adiabatic passage or incoherent optical cycling, this approach preserves coherence and does not require a detailed knowledge of the system spectroscopy. We report on pump-probe experiments investigating the dynamical evolution of bound excited-state Rb2dimers after photoassociation. Learning about these dynamics is an important step towards efficient pump-dump formation of deeply bound ground-state molecules. Based on a spectroscopic measurement of the initial state occupied by the interacting atom pairs, we provide evidence that background triplet molecules preassociated by the trapping lasers, additionally to unbound scattering atom pairs, may play an important role in photoassociation dynamics.