Abstract
We analyzed bichromatic and polychromatic stimulated forces for laser cooling and trapping of Yb atoms using only the narrow ${}^{1}{S}_{0}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}{\phantom{\rule{4pt}{0ex}}}^{3}{P}_{1}$ transition. Our model is based on numerical solutions of optical Bloch equations for two-level atoms driven by multiple time-dependent fields combined with Monte Carlo simulations, which account for realistic experimental conditions such as atomic beam divergence, geometry, and Gaussian laser modes. Using 1 W of laser power, we predict a loading rate of $\ensuremath{\approx}\phantom{\rule{4pt}{0ex}}{10}^{8}$ atoms/s into a 556-nm magneto-optical trap (MOT) with a slowing force of $\ensuremath{\approx}60{F}_{\text{rad}}$. We show that a square-wave modulation can produce similar stimulated forces with almost twice the velocity range and improve the MOT loading rate of Yb atoms by up to 70%.
Highlights
Cold-atom systems and quantum sensors rely upon sources of low-velocity atoms with well-controlled motional degrees of freedom
Inspired by the four-color stimulated forces presented in [37], we demonstrate that adding higher odd harmonics of the bichromatic light via a square-wave amplitude modulation roughly doubles the velocity range of the force and improves the magneto-optical trap (MOT) loading rate by 70% compared to the chirped bichromatic force (BCF) method with the same total laser power
Sweeping the detuning from 240 to 10 m/s in 2 ms substantially broadened the slowing velocity range and increased the population of atoms with velocities below the capture velocity of 5 m/s by at least four orders of magnitude. This result demonstrates that the laser frequency chirp method is an effective method in loading the MOT
Summary
Cold-atom systems and quantum sensors rely upon sources of low-velocity atoms with well-controlled motional degrees of freedom. Magneto-optical traps (MOTs) are the starting point of many experiments including atomic clocks, atom interferometry, optical lattices, tweezers, fountains, and quantum degenerate gases. Many approaches have been developed to create better cold-atom sources with goals of increasing cold-atom flux, MOT loading rates, atom densities, and/or decreasing temperatures. Laser cooling is often done in two stages for closed-shell, alkaline-earth-metal-like atoms (e.g., Ca, Sr, Yb), which are of current interest for optical atomic clocks [1], quantum gases [2,3], quantum measurements [4], and atom interferometry [5,6]. The second stage cools on a narrow transition to reach lower temperatures (in microkelvin for Yb and Sr).
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