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arXiv:1807.07655 · Physical Review A

Characteristics of unconventional Rb magneto-optical traps

K. N. Jarvis, B. E. Sauer, Michael R. Tarbutt

Published Oct 25, 2018• 6 citations
arXivHTMLPDF

Abstract

We study several magneto-optical trapping configurations in $^{87}\mathrm{Rb}$. These unconventional MOTs all use type-II transitions, where the angular momentum of the ground state is greater than or equal to that of the excited state. Some use red-detuned light, and others blue-detuned light. The properties of these MOTs are strongly influenced by the balance between opposing Doppler and Sisyphus forces, and vary widely from one configuration to another. In the blue-detuned MOT, Sisyphus cooling dominates over Doppler heating for all relevant speeds and magnetic fields. We measure the capture velocity of this MOT as a function of intensity and detuning, finding a maximum of $3.8\ifmmode\pm\else\textpm\fi{}0.1$ m/s. Atomic densities are particularly high in the blue-detuned MOT, and its lifetime is limited by collisions between the trapped atoms. We present measurements of the loss rate due to these ultracold collisions as a function of laser intensity and detuning. In the red-detuned MOTs, Sisyphus heating dominates at low speeds and Doppler cooling at higher speeds. Consequently, temperatures in the red-detuned MOTs are up to a thousand times higher than in the blue-detuned MOTs. One MOT forms large ring structures, with no density at the center, showing how atoms driven towards a non-zero equilibrium speed remain trapped by orbiting around the center. Another MOT demonstrates that magnetic mixing of the excited-state hyperfine levels can be an important mechanism in type-II MOTs.

DOI
10.1103/physreva.98.043432

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We study several magneto-optical trapping configurations in 87Rb^{87}\mathrm{Rb}87Rb. These unconventional MOTs all use type-II transitions, where the angular momentum of the ground state is greater than or equal to that of the excited state. Some use red-detuned light, and others blue-detuned light. The properties of these MOTs are strongly influenced by the balance between opposing Doppler and Sisyphus forces, and vary widely from one configuration to another. In the blue-detuned MOT, Sisyphus cooling dominates over Doppler heating for all relevant speeds and magnetic fields. We measure the capture velocity of this MOT as a function of intensity and detuning, finding a maximum of 3.8\ifmmode±\else\textpm\fi0.13.8\ifmmode\pm\else\textpm\fi{}0.13.8\ifmmode±\else\textpm\fi0.1 m/s. Atomic densities are particularly high in the blue-detuned MOT, and its lifetime is limited by collisions between the trapped atoms. We present measurements of the loss rate due to these ultracold collisions as a function of laser intensity and detuning. In the red-detuned MOTs, Sisyphus heating dominates at low speeds and Doppler cooling at higher speeds. Consequently, temperatures in the red-detuned MOTs are up to a thousand times higher than in the blue-detuned MOTs. One MOT forms large ring structures, with no density at the center, showing how atoms driven towards a non-zero equilibrium speed remain trapped by orbiting around the center. Another MOT demonstrates that magnetic mixing of the excited-state hyperfine levels can be an important mechanism in type-II MOTs.
In red-detuned magneto-optical traps (MOTs) of molecules, sub-Doppler heating competes with Doppler cooling, resulting in high temperature and low density. A solution is offered by the blue-detuned MOT where sub- Doppler cooling dominates and the cloud is compressed. Several blue-detuned molecular MOTs have been implemented. A recent implementation relies on a pair of orthogonally polarized components whose frequency separation is smaller than the transition linewidth. We identify the trapping force in these MOTs. At a certain magnetic field, there is a state that is dark to the laser propagating in one direction, but not to the counterpropagating one. This Zeeman-induced dark state (ZIDS) sets up an imbalance in the photon scattering rate, leading to a restoring force. We also study the role of the moving lattices generated by the closely spaced frequency components of the light. We show that there is a velocity-dependent force that drives the molecules toward the speeds of these moving lattices, and that over a relevant range of magnetic fields this combines with the ZIDS force to transport molecules toward the center of the MOT. Here, gray molasses cooling, assisted by nonadiabatic transitions driven by the time-varying polarization of the light field, cools the molecules toward zero velocity. We study these mechanisms for model systems with simple level structures and then extend them to molecules with ground-state hyperfine structure.
We present the properties and advantages of a new magneto-optical trap (MOT) where blue-detuned light drives "type-II" transitions that have dark ground states. Using ^{87}Rb, we reach a radiation-pressure-limited density exceeding 10^{11} cm^{-3} and a temperature below 30 μK. The phase-space density is higher than in normal atomic MOTs and a million times higher than comparable red-detuned type-II MOTs, making the blue-detuned MOT particularly attractive for molecular MOTs, which rely on type-II transitions. The loss of atoms from the trap is dominated by ultracold collisions between Rb atoms. For typical trapping conditions, we measure a loss rate of 1.8(4)×10^{-10} cm^{3} s^{-1}.
It has been more than 30 years since the first demonstration of a magneto-optical trap (MOT) using sodium atoms. Since then the MOT has revolutionised the field of atomic physics by facilitating the emergence of a broad range of productive avenues of research using atoms prepared at low temperatures and high densities. This thesis describes the development of a novel kind of magneto-optical trap: the blue-detuned MOT. Unlike in all previous MOTs the light is blue detuned from atomic resonances and drives "type-II" transitions that have dark ground-state sub-levels. A discussion of the position-dependent and velocity-dependent forces experienced by an atom or molecule in a MOT is first used to consolidate recent theoretical work and, in particular, to introduce the concept of a blue-detuned MOT. The design and construction of an experiment that has been built to demonstrate a blue-detuned MOT using \Rb{87} is described. A thorough characterisation of this novel MOT has been performed. At high magnetic field gradients, radiation-pressure-limited densities exceeding 101110^{11}1011~cm−3^{-3}−3 have been reached whilst temperatures are cooled below 30~\si{\micro K} by the efficient and robust sub-Doppler cooling mechanisms. The maximum phase-space density measured is 6×10−66\times10^{-6}6×10−6, which is higher than in most normal atomic MOTs, comparable to the best dark SPOTs, and a million times higher than that reported for red-detuned type-II MOTs. This makes the blue-detuned MOT particularly attractive for molecules where laser cooling and trapping always uses type-II transitions. For the first time, a study of trap loss due to ultra-cold collisions between atoms occurring in the presence of near-resonant blue-detuned light is undertaken. Finally, the experiment is used to demonstrate many new and unreported configurations of MOT for \Rb{87}, showing that a comprehensive understanding of complicated MOTs is now possible, and presenting a clear direction for further research.
Abstract Laser cooling is used to produce ultracold atoms and molecules for quantum science and precision measurement applications. Molecules are more challenging to cool than atoms due to their vibrational and rotational internal degrees of freedom. Molecular rotations lead to the use of type-II transitions ( F ⩾ F ′ ) for magneto-optical trapping (MOT). When typical red detuned light frequencies are applied to these transitions, sub-Doppler heating is induced, resulting in higher temperatures and larger molecular cloud sizes than realized with the type-I MOTs most often used with atoms. To improve type-II MOTs, Jarvis et al (2018 Phys. Rev. Lett. 120 083201) proposed a blue-detuned MOT to be applied after initial cooling and capture with a red-detuned MOT. This was successfully implemented (Burau et al 2023 Phys. Rev. Lett. 130 193401; Jorapur et al 2024 Phys. Rev. Lett. 132 163403; Li et al 2024 Phys. Rev. Lett. 132 233402), realizing colder and denser molecular samples. Very recently, Hallas et al (2024 arXiv:2404.03636) demonstrated a blue-detuned MOT with a ‘1+2’ configuration that resulted in even stronger compression of the molecular cloud. Here, we describe and characterize theoretically the conveyor-belt mechanism that underlies this observed enhanced compression. We perform numerical simulations of the conveyor-belt mechanism using both stochastic Schrödinger equation and optical Bloch equation approaches. We investigate the conveyor-belt MOT characteristics in relation to laser parameters, g-factors and the structure of the molecule, and find that conveyor-belt trapping should be applicable to a wide range of laser-coolable molecules.
We exploit red- and blue-detuned magneto-optical trapping (MOT) of 87Rb benefitting from a simplified setup and a novel approach based on liquid crystal variable retarders (LCVR). To maintain the trapping forces when switching from a red- to a blue-detuned MOT, the handedness of the circular polarization of the cooling beams needs to be reversed. LCVRs allow fast polarization control and represent compact, simple, and cost-efficient components, which can easily be implemented in existing laser systems. This way, we achieve a blue-detuned type-II MOT for 8.7 × 108 atoms of 87Rb with sub-Doppler temperatures of 44 μK well below the temperatures reached in a conventional 87Rb type-I MOT. The phase space density is increased by more than two orders of magnitude compared to the standard red-detuned type-I MOT. The setup can readily be transferred to any other systems working with 87Rb.
Alkaline-earth-metal atoms present an ideal platform for exploring magneto-optic trap (MOT) dynamics, enabling unique and definitive tests of laser cooling and trapping mechanisms. We have measured the trapping beam intensity, detuning, magnetic-field gradient, trap density, and lifetime dependence of the spring constant $\ensuremath{\kappa}$ and damping coefficient $\ensuremath{\alpha}$ for a ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ 88Sr{}^{88}\mathrm{Sr}88Sr MOT by fitting the oscillatory response of the atom cloud to a step-function force. We find that the observed behavior of $\ensuremath{\kappa}$ and $\ensuremath{\alpha}$ provide a unified and consistent picture of trap dynamics that agrees with Doppler cooling theory at the level of 1010%.10 Additionally, we demonstrate that the trapped atom temperature can be determined directly from measured value of $\ensuremath{\kappa}$ and the trap size, in excellent agreement with free-expansion temperature measurements. However, the experimentally determined temperature is much higher than Doppler cooling theory, implying significant additional heating mechanisms.