26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

A Tunable Blue-Detuned Optical Ring Trap for Fast Compression of Ultracold Metastable Helium-4

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral AIP | Atomic and Molecular Physics (ATMOP)

Speaker

Shijie Li (The Australian National University)

Description

Achieving Bose-Einstein condensation (BEC) of metastable helium-4 (⁴He*) requires tight final confinement to raise phase-space density during evaporation. This is commonly supplied by bi-planar quadrupole-Ioffe or quadrupole-Ioffe magnetic traps, but their coils restrict optical access for flexible beam geometries [1-3]. A tunable blue-detuned optical box or semi-box with weak magnetic confinement could provide optical compression while preserving access [4].

We present a size-tunable blue-detuned optical ring/box trap for ultracold ⁴He. Axicon-generated conical wavefronts [5] are relayed by an electronically tunable-lens zoom system. Transfer-matrix and angular-spectrum simulations maintain a sharp ring at the atoms during zooming and evaluate the intensity profile. The design enables about five-fold zooming of a millimetre-scale ring with ~50 μm radial width. We also model dilute-gas compression ramps, including the time-dependent repulsive potential and photon-scattering recoil heating near 1082.7 nm, about +160 GHz blue-detuned from the ⁴He 2³S₁ → 2³P₂ transition.

Experimentally, we demonstrate 10 ms optical zooming, reducing the in-vacuum ring diameter from ~2.5 mm to 0.5 mm while narrowing the fitted radial width from ~100 μm to 40 μm. Residual near-resonant light is the main heating constraint: distributed-feedback-laser amplified spontaneous emission, even ~40 dB below the carrier, can shorten the ⁴He* lifetime in the trap from ~1 s to ~10 ms. This motivates an atomic resonance filter, consistent with blue-detuned dysprosium BEC traps [6].

References
[1] C. Gross and I. Bloch, Science 357, 995-1001 (2017).
[2] S. Inouye et al., Science 285, 571-574 (1999).
[3] N. Navon, R. P. Smith, and Z. Hadzibabic, Nature Physics 17, 1334-1341 (2021).
[4] R. Grimm, M. Weidemuller, and Y. B. Ovchinnikov, Adv. At. Mol. Opt. Phys. 42, 95-170 (2000).
[5] A. P. Hansen, Master's thesis, Aarhus University (2021).
[6] N. Preti et al., Phys. Rev. A 110, 023307 (2024).