[Paper Review] Narrow Line Photoassociation in an Optical Lattice

Tanya Zelevinsky, Martin M. Boyd+6 more|CERN Bulletin|Feb 20, 2006

Photonic and Optical Devices18 citations

TL;DR

This study demonstrates narrow-line photoassociation spectroscopy in ultracold 88Sr atoms confined in a 1D magic-wavelength optical lattice, resolving nine least-bound vibrational levels near the 1S₀–³P₁ intercombination transition. The narrow natural linewidth (7.5 kHz) enables high-resolution probing of long-range molecular states and reveals strong optical Feshbach resonance potential with low loss, enabling tunable scattering lengths for evaporative cooling.

ABSTRACT

With ultracold $^{88}$Sr in a 1D magic wavelength optical lattice, we performed narrow line photoassociation spectroscopy near the $^1$S$_0 - ^3$P$_1$ intercombination transition. Nine least-bound vibrational molecular levels associated with the long-range $0_u$ and $1_u$ potential energy surfaces were measured and identified. A simple theoretical model accurately describes the level positions and treats the effects of the lattice confinement on the line shapes. The measured resonance strengths show that optical tuning of the ground state scattering length should be possible without significant atom loss.

Motivation & Objective

• To resolve the least-bound vibrational levels of 88Sr2 near the 1S₀–³P₁ intercombination transition using narrow-line photoassociation.
• To characterize the optical response and line shapes of these molecular states in a 1D optical lattice to enable precise control of atomic interactions.
• To assess the feasibility of optical Feshbach resonances for tuning the ground state scattering length in 88Sr with minimal atom loss.
• To evaluate the potential for efficient production of cold ground-state Sr2 molecules via two-color photoassociation.
• To determine the conditions under which evaporative cooling could be enabled through controlled scattering length tuning.

Proposed method

• Ultracold 88Sr atoms were trapped in a 1D magic-wavelength optical lattice to minimize state-dependent shifts and suppress Doppler broadening and recoil effects.
• Photoassociation spectroscopy was performed using a laser tuned near the 1S₀–³P₁ intercombination transition (689 nm), exploiting the narrow natural linewidth of 7.5 kHz.
• Theoretical modeling used coupled-channel calculations including Coriolis mixing between 0ᵤ and 1ᵤ states to fit observed bound state energies and line shapes.
• Optical length (l_opt) was extracted from line shape analysis and used to estimate effective scattering cross-sections and optical Feshbach resonance strength.
• Franck-Condon factors were calculated to assess the efficiency of two-color photoassociation for producing ground-state Sr2 molecules.
• Theoretical expressions for optical Feshbach resonance strength and loss rates were applied using measured l_opt and detuning-dependent line strengths.

Experimental results

Research questions

• RQ1 Can narrow-line photoassociation spectroscopy resolve the least-bound vibrational levels of 88Sr2 near the 1S₀–³P₁ transition in an optical lattice?
• RQ2 What is the strength and line shape of the photoassociation resonances, and how do they depend on lattice confinement and temperature?
• RQ3 Can the measured photoassociation resonances enable optical Feshbach resonance tuning of the ground state scattering length with low atom loss?
• RQ4 What is the efficiency of two-color photoassociation for producing cold ground-state Sr2 molecules?
• RQ5 Is the favorable ratio of elastic to inelastic collision rates sufficient to support evaporative cooling in 88Sr?

Key findings

• Nine least-bound vibrational levels of the 88Sr2 dimer were identified and measured on the 0ᵤ and 1ᵤ potential energy surfaces near the 1S₀–³P₁ dissociation limit.
• The measured optical length (l_opt) for the -0.4 MHz resonance reaches ~5×10⁵ a₀ cm²/W, enabling a predicted optical scattering length tuning of ±300 a₀ with low loss.
• The loss rate for the -0.4 MHz resonance is estimated at ~2×10⁻¹⁴ cm³/s, over 5 orders of magnitude lower than in comparable 87Rb experiments.
• The elastic collision rate (Γ_el ~ 600/s) and inelastic rate (Γ_inel ~ 0.1/s) for the -0.4 MHz line yield a favorable Γ_el/Γ_inel ratio of ~6000, indicating potential for evaporative cooling.
• Over 50% of molecules excited to the -222 MHz level decay to the last vibrational level of the ground state, suggesting high efficiency for two-color photoassociation of cold Sr2 molecules.
• The theoretical model including Coriolis mixing accurately describes the observed level positions and line shapes, validating the treatment of long-range molecular interactions in the lattice.