by Sarah Embacher
Abstract:
This thesis studies optical setups to shape laser light in order to optically trap ultracold atoms. In particular, we will report on two different techniques. The first one is based on the periodic displacement of a laser beam in order to create a time-averaged potentials, whereas the second focuses on the spatial modulation of the polarization of the light. For the time-averaged potential, we develop a new scanning optical dipole trap to replace the old one which has been implemented in the early stages of the ERBIUM experiment [Bai12]. The scanning optical dipole trap realizes a tunable aspect ratio which improves the loading efficiency from the magneto-optical trap and allows to control the trap geometry, which is crucial for observing many-body effects such as supersolidity in a dipolar quantum gas. The tunable aspect ratio is achieved by periodically displacing the 1st diffraction order of an acousto-optical modulator. Because the scanning of the beam happens at a frequency much faster than the frequency of the harmonic trap, the atoms experience a time-averaged potential shaped by the elliptical beam profile. The optical setup is first tested offline, where we find that the waist in the scanning direction can be tuned from 22.38(2) μm to 169.3(2) μm. The setup was then implemented into the experiment. Relying on the excitation of the dipole mode, we measure a trap frequency of 2π × 203.3(6) Hz without scanning and 2π × 64(3) Hz at the maximum scanning amplitude. The second part of this thesis investigates the possibility of using q-plates to generate optical trapping potentials for ultracold erbium. Q-plates are optical elements that contain a liquid crystal layer where a specific pattern is imprinted to generate a helical phase front as light passes through. This creates an optical vortex, meaning that the intensity is zero in the beam’s center. In addition, the polarization profile of the beam varies around the radial direction. The idea is now to use such a spatially non-uniform polarization for erbium, which has a polarization-dependent atom-light interaction. As a lanthiande atom, it features both a strong vectorial and tensorial part of the polarizability in addition to the isotropic scalar part. To enter regimes where the vectorial and tensorial parts become comparable the isotropic part, we aim to use light which is detuned by a few GHz from the narrow 841 nm transition. Indeed, we calculate that q-plate beams can generate a ring lattice potential for erbium atoms. We identify and discuss several challenges for the experimental implementation, for example regarding the heating due to photon scattering or aberrations and imperfections in the optical system that could introduce unwanted effects on the trapping potential.
Reference:
A new scanning optical dipole trap and towards optical potentials for erbium using q-plates,
Sarah Embacher,
Master’s Thesis, 2024.
Sarah Embacher,
Master’s Thesis, 2024.
Bibtex Entry:
@article{EmbacherMSc,
title = {A new scanning optical dipole trap and towards optical potentials for erbium using q-plates},
author = {Embacher, Sarah},
journal = {Master's Thesis},
year = {2024},
month = {Nov},
abstract = {This thesis studies optical setups to shape laser light in order to optically trap ultracold
atoms. In particular, we will report on two different techniques. The first one is
based on the periodic displacement of a laser beam in order to create a time-averaged
potentials, whereas the second focuses on the spatial modulation of the polarization
of the light.
For the time-averaged potential, we develop a new scanning optical dipole trap to
replace the old one which has been implemented in the early stages of the ERBIUM
experiment [Bai12]. The scanning optical dipole trap realizes a tunable aspect ratio
which improves the loading efficiency from the magneto-optical trap and allows to
control the trap geometry, which is crucial for observing many-body effects such as
supersolidity in a dipolar quantum gas. The tunable aspect ratio is achieved by periodically
displacing the 1st diffraction order of an acousto-optical modulator. Because
the scanning of the beam happens at a frequency much faster than the frequency of the
harmonic trap, the atoms experience a time-averaged potential shaped by the elliptical
beam profile. The optical setup is first tested offline, where we find that the waist in
the scanning direction can be tuned from 22.38(2) μm to 169.3(2) μm. The setup was
then implemented into the experiment. Relying on the excitation of the dipole mode,
we measure a trap frequency of 2π × 203.3(6) Hz without scanning and 2π × 64(3) Hz
at the maximum scanning amplitude.
The second part of this thesis investigates the possibility of using q-plates to generate
optical trapping potentials for ultracold erbium. Q-plates are optical elements that
contain a liquid crystal layer where a specific pattern is imprinted to generate a helical
phase front as light passes through. This creates an optical vortex, meaning that the
intensity is zero in the beam’s center. In addition, the polarization profile of the beam
varies around the radial direction. The idea is now to use such a spatially non-uniform
polarization for erbium, which has a polarization-dependent atom-light interaction. As
a lanthiande atom, it features both a strong vectorial and tensorial part of the polarizability
in addition to the isotropic scalar part. To enter regimes where the vectorial
and tensorial parts become comparable the isotropic part, we aim to use light which is
detuned by a few GHz from the narrow 841 nm transition. Indeed, we calculate that
q-plate beams can generate a ring lattice potential for erbium atoms. We identify and
discuss several challenges for the experimental implementation, for example regarding
the heating due to photon scattering or aberrations and imperfections in the optical
system that could introduce unwanted effects on the trapping potential.},
url = {https://www.erbium.at/FF/wp-content/uploads/2025/02/11806934_Embacher_MA.pdf},
}