A. Fioretti

Observation of radiation trapping in a dense Cs magneto-optical trap

Abstract We present an experimental and theoretical investigation of the resonant radiation trapping phenomenon in a magneto-optical trap (MOT) for cesium atoms. We have resonantly excited the 62S1/2(F=4)−62P3/2(F′=5) transition with a probe laser and measured directly its effective radiative lifetime. We compare our experimental data with a model based on an approximate solution of the Holstein equation and with a Monte Carlo simulation of the process. The inadequacy of the standard theory for thermal atomic samples in reproducing the data is pointed out.

A cesium magneto-optical trap for cold collisions studies

A cesium magneto-optical trap operating at a large atomic density in multiple scattering regime has been investigated. The trap operates with a large Rabi frequency for trapping lasers. The atomic number, the dimension of the atomic cloud, the atomic density, and the atomic temperature have been measured as a function of trap parameters. At different trap conditions, the product of the atomic number and of the atomic density, which determines the occurrence of collisional processes, has been also examined. Our results are interpreted on the basis of laser cooling processes.

Deeply bound cold caesium molecules formed after 0(-)(g) resonant coupling.

Translationally cold caesium molecules are created by photoassociation below the 6s + 6p(1/2) excited state and selectively detected by resonance enhanced two photon ionization (RE2PI). A series of excited vibrational levels belonging to the 0(-)(g) symmetry is identified. The regular progression of the vibrational spacings and of the rotational constants of the 0(-)(g) (6s + 6p(1/2)) levels is strongly altered in two energy domains. These deviations are interpreted in terms of resonant coupling with deeply bound energy levels of two upper 0(-)(g) states dissociating into the 6s + 6p(3/2) and 6s + 5d(3/2) asymptotes. A theoretical model is proposed to explain the coupling and a quantum defect analysis of the perturbed level position is performed. Moreover, the resonant coupling changes dramatically the spontaneous decay products of the photoexcited molecules, strongly enhancing the decay into deeply bound levels of the a(3)Σ(+)(u) triplet state and of the X(1)Σ(+)(g) ground state. These results may be relevant when conceiving population transferring schemes in cold molecule systems.

Photoionization spectroscopy of excited states of cold caesium dimers

Photoionization spectroscopy of cold caesium dimers obtained by photoassociation of cold atoms in a magneto-optical trap is reported here. In particular, we report on the observation and on the spectroscopic analysis of all the excited states that have actually been used for efficient detection of cold molecules stabilized in the triplet ground state. They are: the state connected to the 6s + 6p asymptote, the and (2)3Π g states connected to the 6s + 5d asymptote and finally the state connected to the 6s + 7s asymptote. The detection through these states spans a wide range of laser energies, from 8000 to 16,500 cm−1, obtained with different laser dyes and techniques. Information on the initial distribution of cold molecules among the different vibrational levels of the ground state is also provided. This spectroscopic knowledge is important when conceiving schemes for quantum manipulation, population transfer and optical detection of cold caesium molecules.

Cold cesium molecules: from formation to cooling

Recent experiments on formation of translationally cold ground state molecules, their subsequent broadband vibrational cooling and study of rotations are presented together with the related modeling. We produce cold molecules by photoassociating pairs of cold cesium atoms that can decay into ground state molecules in different vibrational levels. Then we laser cool the vibrational degree of freedom by selecting a single target vibrational level. Our method is based on repeated optical pumping by laser light with a spectrum broad enough to excite all populated vibrational levels but limited in its frequency bandwidth with a spatial light modulator. In such a way we are able to eliminate transitions from the selected level, in which molecules accumulate. Results for vibrational cooling into the v = 0, v = 1, v = 2 and v = 7 level of the singlet X 1Σ g ground electronic state are presented. Depletion spectroscopy is performed to study the rotational distribution of the created molecules. In the theoretical modeling of the process we are able to reproduce our results and investigate the prospects for a ‘complete’ cooling of molecules, including also their rotational degree of freedom.

Formation of cold Cs2 molecules through photoassociation

We report on the formation of translationally cold Cs2 ground-state molecules through photoassociation of laser-cooled cesium atoms in a magneto-optical trap. The cold molecules are obtained after spontaneous decay of photoassociated molecules, and detected after pulsed-laser photoionization into Cs2+ ions. Photoassociation spectra of cesium attractive molecular states below the 6S1/2+6P3/2 dissociation limits are reported, obtained with both ion detection and trap-loss method. A temperature as low as 20−5+15 has been measured for the molecular cloud.We report on the formation of translationally cold Cs2 ground-state molecules through photoassociation of laser-cooled cesium atoms in a magneto-optical trap. The cold molecules are obtained after spontaneous decay of photoassociated molecules, and detected after pulsed-laser photoionization into Cs2+ ions. Photoassociation spectra of cesium attractive molecular states below the 6S1/2+6P3/2 dissociation limits are reported, obtained with both ion detection and trap-loss method. A temperature as low as 20−5+15 has been measured for the molecular cloud.

Experimental study of caesium energy pooling collisions and modelling of the excited atom density in the presence of optical pumping and radiation trapping

An experimental study of caesium energy pooling collisions, , at thermal energies, has been carried out in a capillary cell using diode laser excitation. Use of the capillary cell minimizes the effects of radiation trapping, but nonetheless, such effects still play a significant role in the analysis. Consequently, a rate equation model, which treats simultaneous effects of saturation, optical pumping, and radiation trapping, has been developed and is used to determine the atom density under these experimental conditions. The excited atom densities are combined with measured fluorescence ratios to determine rate coefficients for the caesium energy pooling process. Our values for these rate coefficients are in agreement, within combined error bars, with values we have recently obtained under very different experimental conditions.

Experimental evidence for an isotopic effect in the formation of ultracold ground-state rubidium dimers

We perform a comparative study of the formation of ultracold rubidium dimers in their ground state with both 85Rb and 87Rb isotopes. Ultracold rubidium molecules are created through photoassociation of ultracold atoms into rovibrational levels of the 0+u(5S1/2 + 5P1/2) excited state, which interact through resonant coupling (Dion et al 2001 Phys. Rev. Lett. 86 2253) with rovibrational levels of the 0+u(5S1/2 + 5P3/2) excited state. The photoassociated levels decay by spontaneous emission down to rovibrational levels of the X1Σ+g ground state. The resonant coupling is expected to enhance the formation of ultracold molecules in low-lying ground-state levels. By modelling the ionization spectrum of these ultracold molecules, we show that this coupling is larger for 87Rb2 molecules than for 85Rb2 molecules.

Vibrational cooling of cold molecules with optimised shaped pulses

A review of our recent experiments on broadband vibrational cooling of cold cesium molecules and of the related theory is presented. Our method is based on repetitive optical pumping cycles driven by laser light which is broad enough to excite all populated vibrational levels. Originally, the accumulation of molecular population in a particular, pre-selected vibrational level was achieved by removing from the broadband light all frequencies that could excite that vibrational level and thus making it a ‘dark state’ of the system. Here, we focus onto an additional, more sophisticated shaping method, which consists of selecting only specific frequency components that excite molecules into vibrational levels that favourably decay into the pre-selected level. The population transfer to any desired state can thus be optimised, i.e. the total population transfer to the desired vibrational level is maximised while the number of absorption–emission cycles required for the vibrational cooling is minimised. Finally, we apply this optimised technique to some more complex and still experimentally open cases: the pumping into the ground state for the case of Cs2 homonuclear molecules, the rotational pumping into a pre-selected ro-vibrational level and the NaCs as an example for heteronuclear molecules.

Magnetic or optical molasses loading for a Cs dipole trap

We have studied the loading of a single-beam or a crossed dipole trap made by a Nd:YAG laser. The loading was performed from a magnetic trap or after a molasses phase with cesium atoms. Looking for high atomic density to perform a fast evaporation to reach the Bose-Einstein Condensation, we found that a crossed dipole trap with a waist of 30 &mgr;m and a laser power P=10 W, loaded from a molasses, gives a good starting point to begin evaporation with 5 &|m~; 106 atoms loaded in 50 ms.