Hyperfine and Zeeman Optical Pumping and Transverse Laser Cooling of a Thermal Atomic Beam of Dysprosium Using a Single 421 nm Laser

We demonstrate the effect of Zeeman and hyperfine optical pumping and transverse laser cooling of a dysprosium (Dy) atomic beam on the $4f^{10}6s^2(J = 8) \rightarrow 4f^{10}6s6p(J = 9)$ transition at 421.291 nm. For $^{163}$Dy, an electro-optic modulator is used to generate five frequency sidebands required to pump the atoms to the $F = 10.5$ ground state hyperfine level and the light polarization is chosen to pump the atoms to the $m_F = 10.5$ Zeeman sublevel. The atoms are simultaneously laser-cooled using a standing wave orthogonal to the atomic beam. The resulting polarized and cooled atomic beam will be used in fundamental physics experiments taking advantage of the accidental degeneracy of excited states in Dy including the ongoing measurement of parity violation in this system.

💡 Research Summary

In this work the authors demonstrate simultaneous Zeeman and hyperfine optical pumping together with transverse laser cooling of a thermal dysprosium (Dy) atomic beam using only a single 421 nm laser source. Dysprosium is an attractive system for precision‑measurement physics because it possesses the largest ground‑state magnetic moment of any atom, several stable isotopes, and a pair of opposite‑parity excited states that become nearly degenerate (the “accidental” degeneracy) under modest magnetic fields (~1.4 G). These features have already enabled searches for variations of the fine‑structure constant, tests of Lorentz invariance, and ongoing parity‑nonconservation (PNC) experiments, particularly with the fermionic isotope ^163Dy.

The experimental apparatus consists of a high‑temperature (≈1500 °C) Dy oven feeding a weakly collimated atomic beam with transverse velocities around 20 m s⁻¹. Inside an ultra‑high‑vacuum chamber a pair of coils creates a bias magnetic field of ≈1.5 G that defines the quantization axis. The strong 421 nm transition (J = 8 → J′ = 9, natural linewidth Γ/2π = 33 MHz, lifetime 4.8 ns) is used for both optical pumping and cooling, while a weaker 599 nm transition (J = 8 → J′ = 7, Γ/2π ≈ 12 kHz) serves as a probe to monitor population redistribution.

A custom broadband electro‑optic modulator (EOM) based on LiNbO₃ generates five radio‑frequency sidebands at detunings of 1190, 830, 503, 235 and 57 MHz relative to the carrier. The carrier is locked to the ^163Dy F = 10.5 → F′ = 11.5 hyperfine component of the 421 nm line, and the sidebands address the remaining hyperfine ground‑state levels (F = 5.5, 6.5, 7.5, 8.5, 9.5). RF powers of 31–33 dBm are applied to the four higher‑frequency sidebands; the lowest‑frequency sideband (57 MHz) is driven at low power to avoid off‑resonant excitation of the nearby F = 9.5 → F′ = 10.5 transition (separation < 2Γ). This multi‑tone scheme enables efficient pumping of the entire hyperfine manifold into the stretched ground state |F = 10.5, m_F = +10.5⟩ with a single laser beam.

Optical pumping is performed with σ⁺ polarization, driving Δm = +1 transitions that funnel population into the highest Zeeman sublevel (m_J = +8 for the bosonic ^164Dy, m_F = +10.5 for ^163Dy). The effectiveness of the Zeeman pumping is verified by probing the 599 nm transition with σ⁻ light: when the pump is correctly polarized, the ^164Dy signal disappears because the atoms are transferred to a dark state for σ⁺ probing. Rotating the probe polarization back to σ⁻ reveals the repopulated m_J = +8 level and allows quantitative assessment of both pumping and cooling.

Transverse cooling is achieved by overlapping a standing‑wave 421 nm beam orthogonal to the atomic beam (optical molasses). By fitting the probe spectra before and after cooling with Gaussian and Voigt profiles, the Doppler width is reduced from 56.9 MHz (uncooled) to 24.6 MHz (cooled), corresponding to a reduction of the transverse velocity spread from ≈13 m s⁻¹ to ≈5 m s⁻¹. The fluorescence signal is amplified by a factor of 1.7, consistent with the increased phase‑space density due to cooling.

When the multi‑tone pump is tuned to the ^163Dy hyperfine transition, the amplitude of the probe line |F = 10.5⟩ → |F′ = 9.5⟩ grows by a factor of 4.2 relative to the unpumped case. The simple Zeeman pumping model predicts only a 1.2‑fold increase; the larger observed gain is attributed to off‑resonant hyperfine pumping from the neighboring F = 9.5 ground state (the transition is only ≈2Γ away) and to the concurrent transverse cooling, both of which funnel additional atoms into the stretched state.

Overall, the study shows that a single, moderately powered (≈230 mW in the interaction region) 421 nm laser, together with a custom EOM, can simultaneously achieve (i) complete hyperfine pumping of fermionic Dy isotopes, (ii) Zeeman pumping to the stretched magnetic sublevel, and (iii) substantial transverse cooling of a thermal atomic beam. This preparation dramatically improves the signal‑to‑noise ratio for downstream precision measurements, notably the ongoing parity‑violation experiment that exploits the near‑degeneracy of opposite‑parity excited states in Dy. The authors suggest that the same technique can be extended to other complex atoms with dense hyperfine structure, eliminating the need for multiple repump lasers or separate AOM/EOM chains.

In summary, the paper provides a practical, compact solution for state preparation in dysprosium, paving the way for higher‑precision tests of fundamental symmetries and for advanced quantum‑simulation experiments with highly magnetic atoms.