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arXiv:1510.04054 · Journal of Modern Optics

Bichromatic state-insensitive trapping of cold133Cs–87Rb atomic mixtures

M.M. Metbulut, Ferruccio Renzoni

Published Oct 12, 2015• 0 citations
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Abstract

We investigate simultaneous state-insensitive trapping of a mixture of two different atomic species, Caesium and Rubidium. The magic wavelengths of the Caesium and Rubidium atoms are different, 935.6 and 789.9 nm respectively, thus single-frequency simultaneous state-insensitive trapping is not possible. We thus identify bichromatic trapping as a viable approach to tune the two magic wavelengths to a common value. Correspondingly, we present several common magic wavelength combinations appropriate for simultaneous state-insensitive trapping of the two atomic species.

DOI
10.1080/09500340.2015.1086032

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We investigate simultaneous state-insensitive trapping of a mixture of two different atomic species, Caesium and Rubidium. The magic wavelengths of the Caesium and Rubidium atoms are different, 935.6 and 789.9 nm respectively, thus single-frequency simultaneous state-insensitive trapping is not possible. We thus identify bichromatic trapping as a viable approach to tune the two magic wavelengths to a common value. Correspondingly, we present several common magic wavelength combinations appropriate for simultaneous state-insensitive trapping of the two atomic species.
State-insensitive trapping plays a significant role in many cold atom experiments. In this technique, trapping at a specific "magic" wavelength leads to identical shifts of the ground and excited states of a particular transition. Hence, the internal state of the trapped atoms remains essentially unaffected by the trapping light. This leads to increased coherence times and loading efficiency, and allows for the use of free-space manipulation techniques. However, the applicability of state-insensitive trapping is limited in terms of light sources available, with restrictions coming from either the required frequency or power. In this thesis, state-insensitive trapping of caesium atoms is considered. It can be achieved at the wavelength of 935.6 nm. However, the present work shows that this wavelength is impractical for experimental realizations of atom trapping in free space due to lack of laser sources with sufficiently high power. We therefore propose bichromatic trapping, as produced by two independent laser fields with different frequencies, to overcome the limitations related to monochromatic trapping. We show that bichromatic trapping extends the range of possible magic wavelengths compared to the specific magic wavelength associated with monochromatic trapping. Specifically, we present results for the magic wavelength combinations for caesium atoms with particular attention to the frequency range of currently available laser sources. Simultaneous state-independent trapping of atomic mixtures of caesium with rubidium and lithium atoms was also investigated, and appropriate magic wavelength combinations were identified. Additionally, species-selective state-independent trapping of atomic mixtures of caesium and lithium was explored and common wavelength pairs for tune-out and magic wavelengths of these atoms were determined.
State-insensitive dipole trapping of multilevel atoms can be achieved by an appropriate choice of the wavelength of the trapping laser, so that the interaction with the different transitions results in equal AC Stark shifts for the ground and excited states of interest. However this approach is severely limited by the availability of coherent sources at the required wavelength and of appropriate power. This work investigates state-insensitive trapping of caesium atoms for which the required wavelength of 935.6 nm is inconvenient in terms of experimental realization. Bichromatic state-insensitive trapping is proposed to overcome the lack of suitable laser sources. We first consider pairs of laser wavelengths in the ratio 1:2 and 1:3, as obtained via second- and third- harmonic generation. We found that the wavelength combinations 931.8-1863.6 nm and 927.5-2782.5 nm are suitable for state-insensitive trapping of caesium atoms. In addition, we examine bichromatic state-insensitive trapping produced by pairs of laser wavelengths corresponding to currently available high power lasers. These wavelength pairs were found to be in the range of 585-588 nm and 623-629 for one laser and 1064-1080 nm for the other.
State-insensitive dipole trapping of multilevel atoms can be achieved by an appropriate choice of the wavelength of the trapping laser, so that the interaction with the different transitions results in equal AC Stark shifts for the ground and excited states of interest. However, this approach is severely limited by the availability of coherent sources at the required wavelength and of appropriate power. This work investigates state-insensitive trapping of caesium atoms for which the required wavelength of 935.6 nm is inconvenient in terms of experimental realization. Bichromatic state-insensitive trapping is proposed to overcome the lack of suitable laser sources. We first consider pairs of laser wavelengths in the ratio 1:2 and 1:3, as obtained via second- and third-harmonic generation. We found that the wavelength combinations 931.8–1863.6 nm and 927.5–2782.5 nm are suitable for state-insensitive trapping of caesium atoms. In addition, we examine bichromatic state-insensitive trapping produced by pairs of laser wavelengths corresponding to currently available high-power lasers. These wavelength pairs were found to be in the range of 585–588 nm and 623–629 for one laser and 1064–1080 nm for the other.
State-insensitive traps are an important tool for precision spectroscopy. \nIn these traps both the ground and excited state of the relevant atomic \ntransition are shifted by the same amount. To obtain state-insensitive \ntrapping, a specific trapping wavelength - called the "magic wavelength" - \nmust be used. This thesis describes state-insensitive trapping of caesium \natoms, as realised by using a trapping laser beam at the magic wavelength \nof 935.6 nm. Two different experimental setups were realised and \ncharacterised. \nThe first set of experiments provided the characterisation of a singlewell \nstate-insensitive trap, produced by using the laser beam from a Tisapphire \nlaser. The trap lifetime was determined as a function of the trap \ndepth, with the largest lifetime of 203 ms measured for a trap depth of 2.4 \nmK. Further improvement in the trap lifetime was obtained by applying \na depumper laser beam, which prepared the atoms in the lower ground \nstate. This suppresses hyperfine changing collisions, and the lifetime was \nincreased to 3.6 s as a result. Ultimately, the lifetime was limited by \nthe pointing instability of the dipole trap beam and the background gas \ncollisions. \nA second experimental setup was then realised, to reduce the background \ngas collisions, which is the limitation of lifetime in the first setup. \nFurthermore, the imaging system was upgraded to reduce the background \nnoise, and a MOPA system was used to produce the state-insensitive trap. \nIn a second set of experiments, a single-well trap and a 1D optical lattice \nwere compared to evaluate the suppression of two-body collisions in the \n1D lattice case.
We investigate oscillation frequencies for simultaneous trapping of more than one type of alkali atoms in a common optical lattice. For this purpose, we present numerical results for magic trapping conditions, where the oscillation frequencies for two different kind of alkali atoms using laser lights in the wavelength range 500-1200 nm are same. These wavelengths will be of immense interest for studying static and dynamic properties of boson-boson, boson-fermion, fermion-fermion, and boson-boson-boson mixtures involving different isotopes of Li, Na, K, Rb, Cs and Fr alkali atoms. In addition to this, we were also able to locate a magic wavelength around 808.1 nm where all the three Li, K, and Rb atoms are found to be suitable for oscillating at the same frequency in a common optical trap.
We realize a combined trap for bosonic chromium (52Cr) and rubidium (87Rb) atoms. Initial experiments focus on exploring a suitable loading scheme for the combined trap and on studies of new trap loss mechanisms originating from simultaneous trapping of two species. By comparing the trap loss from the 87Rb magneto-optical trap (MOT) in the absence and presence of magnetically trapped ground state 52Cr atoms we determine the scattering cross-section of σ inel,RbCr = 5.0 ± 4.0 × 10−18 m2 for light-induced inelastic collisions between the two species. Studying the trap loss from the Rb magneto-optical trap induced by the Cr cooling-laser light, the photoionization cross-section of the excited 5P3/1 state at an ionizing wavelength of 426 nm is measured to be σ P = 1.1 ± 0.3 × 10−21 m2.