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.
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.
The establishment of robust laser-cooling techniques for the formation of ultracold atoms has provided a test-bed for low-temperature science, with scattering events changing character from incoherent thermal interactions to coherent quantum mechanical events. A natural extension is the pursuit of ultracold molecules in prescribed low-energy internal states. Atomic cooling techniques, however, do not generalize to the molecular regime due to the complex energy-level structure afforded by its extra degrees of motion. An indirect approach to ultracold molecule formation - photoassociation using ultrafast laser pulses - is the focus of this thesis. A broadband field associates atom pairs into a localized molecular wavepacket that evolves within the attractive excited-state potential. A suitably timed dump pulse may thus be applied to stabilize population into deeply bound ground vibrational states. This strategy may be generalized to any species whose spectroscopy matches the pulse spectrum, and offers a coherent population transfer scheme that does not require precise knowledge of the system. This thesis presents experiments using high-energy photoassociation pulses applied to ultracold rubidium atoms. The pulses quench the background ground-state molecular population but form bound dimers within the excited state. A pump-probe experiment was designed to chart the excited-state dynamics; however, the oscillations predicted by theoretical calculations were not evident in the molecular signal. The nature of the dynamics is expected to be strongly dependent on the initial state of the atom pairs addressed by the ultrafast pulse: a bound molecular population provides an additional candidate to free atoms. A spectroscopic measurement characterizes these bound molecules and identifies their formation mechanism. A subsequent experiment provides evidence that the predominant contributor to the pump-probe signal is the unbound initial population. The consequences with regard to both the observation of excited-state dynamics and the subsequent application of a dump pulse are discussed.
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.
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.
In the ultracold regime, where the interactions between atoms become quantum mechanical in nature, we can investigate the fundamental properties of matter. A natural progression from the catalogue of pioneering experiments using ultracold atoms is to extend the size of our quantum system by producing ultracold molecules in prescribed low-energy internal states. Techniques for cold molecule production are split into two methods: direct and indirect cooling. While direct cooling methods have yet to realize ultracold temperatures, collisional relaxation in the molecules leads to low internal energy states. By contrast, indirect cooling — the association of molecules from pre-cooled atoms—has produced a range of molecules at ultracold temperatures; the challenge with this technique is to control the internal state.This thesis concentrates on a technique that is complementary to those already in existence: ultrafast photoassociation. Key to this technique is the formation of time non-stationary wavepackets in the excited-state in order to improve FranckCondon overlap of the excited state with deeply bound ground-state vibrational levels. A pump-probe experiment was designed and built to demonstrate the formation of bound excited-state dimers. In this work we show that the initial state from which the wavepacket originates is of critical importance to the evolution of excited-state population. We find that the internuclear separation of the wavepacket produced in a rubidium magneto-optical trap is too large to observe coherent oscillations in the excited state. The implications of this are discussed along with recommendations for future ultrafast photoassociation experiments. Consequently, a new ultracold atom apparatus was built utilizing magnetic and dipole-force trapping to increase the density of the atomic sample; this apparatus will enable future experiments combining the exciting fields of ultracold matter and ultrafast light.
Some of us have recently reported (Viteau et al 2008 Science 321 232–4) vibrational cooling of translationally cold Cs 2 molecules into the lowest vibrational level v =0 of the singlet X 1 Σ g ground electronic state. Starting from a sample of cold molecules produced in a collection of vibrational levels of the ground state, our method was based on repeated optical pumping by laser light with a spectrum broad enough to excite all populated vibrational levels but frequency-limited in such a way to eliminate transitions from v =0 level, in which molecules accumulate. In this paper, this method is generalized to accumulate molecules into an arbitrary selected 'target' vibrational level. It is implemented by using ultrashort pulse shaping techniques based on liquid crystal spatial light modulator. In particular, a large fraction of the initially present molecules is transferred into a selected vibrational level such as v =1, 2 and 7. Limitations of the method as well as the possible extension to rotational cooling are also discussed.
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.