Narrowline Laser Cooling and Spectroscopy of Molecules via Stark States

TL;DR

This work demonstrates how the metastable A′^2Δ electronic state in YO can be Stark-polarized with modest electric fields to isolate pure-parity, field-insensitive transitions, enabling quasi-closed photon cycling in a diatomic molecule. Using high-resolution Stark spectroscopy, the authors determine absolute transition frequencies with a fractional uncertainty of 9×10^-12 and constrain the ΔΛ-doublet splitting to below 7 kHz, establishing a robust footing for narrowline cooling. They achieve the first narrowline laser cooling of a molecule, cooling YO in two dimensions by 0.73 μK using an all-narrowline scheme that minimizes repump-induced heating and suggests scalable paths to recoil-limit cooling in optical lattices or tweezers. The study also lays out a roadmap for extending these techniques to the longer-lived A′^2Δ5/2 state, which could enable low-field dipolar quantum simulation and enhanced sensitivity to fundamental symmetry tests, including nuclear Schiff moments in heavy MO molecules such as AcO. Overall, the approach broadens molecular laser cooling beyond traditional strong transitions and opens opportunities for precision measurements and quantum simulations with MO molecules.

Abstract

The electronic energy level structure of yttrium monoxide (YO) provides a long-lived, low-lying $^{2}Δ$ state ideal for high-precision molecular spectroscopy, narrowline laser cooling at the single photon-recoil limit, and studying dipolar physics with unprecedented interaction strength. High-resolution laser spectroscopy of ultracold laser-cooled YO molecules is used to study the Stark effect in the A$^{\prime}\,^{2}Δ_{3/2}\,J=3/2$ state. An immediate onset of the linear Stark effect is observed in the presence of weak applied electric fields due to the near degenerate $Λ$-doublet and the large electric dipole moment. By applying a small electric field the Stark insensitive state is spectroscopically isolated and the absolute transition frequency to the X$\,^2Σ^+$ electronic ground state is determined with a fractional frequency uncertainty of 9 $\times$ 10$^{-12}$. This electric field control is necessary to implement a quasi-closed photon cycling scheme that preserves parity. With this scheme the first narrowline laser cooling of a molecules is demonstrated, reducing the temperature of sub-Doppler cooled YO in two dimensions.

Figures (4)

• Figure 1: Energy level diagram of the lowest three electronic states in the diatomic $M$O molecules. The large Fermi contact interaction dominates the fine structure in the X$\,^2\Sigma^+$ ground electronic state, whereas, the spin-orbit interaction ($A_{\mathrm{SO}}$) is dominant in the $^2\Pi$ and $^2\Delta$ excited states. The lifetime and the $\Lambda$-doublet splitting in the $^2\Delta$ state are primarily inherited from the $^2\Pi_{3/2}$ state through homogeneous $\mathcal{H}_{\text{hom}}$ and heterogeneous $\mathcal{H}_{\text{het}}$ interactions depicted by the double arrows. The parity of each state is depicted within the parentheses.
• Figure 2: (a) The relevant electronic states in YO with their transition wavelengths and radiative decay rates, along with the Franck-Condon factors q$_{v,v^{\prime}}$ between the first two vibrational states of X$\,^2\Sigma^{+}$ and the ground vibrational state of A$^{\prime}\,^2\Delta_{3/2}$. (b) The allowed decay channels for the A$^{\prime}\,^2\Delta_{3/2}[3/2,1,|m_F|^{\mathrm{p}}]\rightarrow$ X$\,^2\Sigma^{+}$ transitions. The branching ratios are provided and represented by the relative darkness of the double-arrow to each ground state. A weak electric field mixes the parity of $|m_F| =1$ states, leading to a breakdown of the parity selection rule for electric dipole transitions. Dashed lines represent the entirety of allowed decay pathways when the excited state has mixed parity.
• Figure 3: Laser spectroscopy to the A$^{\prime}\,^{2}\Delta_{3/2}\,[3/2,1,|m_F|^{\mathrm{p}}]$ states in the presence of a weak applied electric field $E_{\mathrm{appl}}$. Panel (a) depicts the detuning-dependent depletion (solid markers) of the initial (1,1,0) ground-state population for three different values of $E_{\mathrm{appl}}$. Panel (b) shows the effect of the Stark interaction in the small field limit for the four field-sensitive and the two field-insensitive states. Panel (c) presents spectra from the $(1,1,0)$ (solid dots) and $(0,1,1)$ (solid diamonds) ground states to the two opposite pure-parity states of the A$^{\prime}\,^{2}\Delta_{3/2}\,[3/2,1,0]$ state. A Gaussian lineshape model (orange lines) is used to obtain each center frequency.
• Figure 4: Radial temperatures after applying a 4-ms long narrowline laser cooling sequence involving (a) repump lasers connecting the $^2\Pi_{1/2}$ state and (b) only narrowline transitions. A least-squares fit to the time-integrated solution to Eq.\ref{['Diffusion']} is provided (black line) with the $95\%$ confidence interval (gray shaded region) to model the detuning-dependent cooling for both scans. Each laser cooling scheme is depicted on the right-hand side.