We are currently looking for new PhD students. For further info, see here: PhD position
A common approach to describe light scattering in atomic samples is to assume that atoms are scattering independently from each other. However, light propagation behaves distinctively different in dense atomic samples, where the interatomic distance is smaller than the wavelength of the interacting light. In these cases, recurrent scattering of photons induces electric dipole-dipole correlations in the sample, influencing the spatial correlations of the scatterers. On the other hand, spatial correlations, which may be induced by other interaction processes, can alter the light propagation properties of the ensemble.
We are interested in the interplay between light-induced and magnetic dipole-dipole interactions of atoms with large magnetic moments. Since both interaction processes result in spatial correlations, we expect to find regimes in which both processes become competing. In this project we are working towards experimental studies of light-induced correlations in such dipolar media made from dense samples of laser cooled and trapped dysprosium atoms, the most magnetic atoms on the periodic table.
Light induced correlations
The scattering of light from atoms, for which the separation is smaller than the wavelength of the interacting light, can no longer be assumed independent for each atom. Instead, the emission of an atom is influenced by every other atom in the dense sample. An interesting phenomenon emerging from this situation is superradiance. The figure beneath summarizes essential properties of superradiance and compares those to conventional fluorescence. In fluorescence, the emission of light from a dilute sample of excited atoms can be described by an exponential decay law. The emission is isotropic, which is a fundamental property required for laser cooling. In superradiance, on the other hand, the emission from the dense atomic sample occurs in a short burst of light. Furthermore, one can observe directionality in the emission.
Collective light-scattering phenomena, like superradiance, can be understood by the coherent synchronization of the radiating atoms. This can be caused by the recurrent scattering of photons, leading to a build-up of spatial order in the sample. In the course of this project, we want to study these light-induced correlations in samples, in which spatial correlations may be governed by a strong magnetic dipole-dipole interaction.
The atomic species for our experiments is dysprosium (Dy) which, with an atomic number of Z = 66, belongs to the group of rare-earth lanthanides. It is a silvery grey metal with large atomic mass (162.5 u) and a high melting point of ~1400 °C. Dysprosium features several stable isotopes, the most abundant being the two bosonic isotopes 162Dy and 164Dy and the two fermionic ones 161Dy and 163Dy with nearly equally high natural abundances. Both bosonic isotopes possess a nuclear spin equal to zero (I=0) whereas the fermions have a nuclear spin of I = 5/2. The almost equally high relative abundances of these four isotopes allow for the generation of Bose-Einstein-Condensates, ultra-cold Fermi Gases and Bose-Fermi-mixtures.
The electronic ground-state configuration of Dy is [Xe]4f106s2, with spin S = 2, orbital angular momentum L = 6 and total angular momentum J = 8. This open f-shell configuration leads to a very rich and complicated energy spectrum with a variety of atomic transitions that are suitable for laser cooling and performing light-matter interaction experiments. More importantly, it is the origin of Dy’s large ground-state magnetic moment which, with a value of about 10 Bohr-magnetons (µ~10μB), is the largest magnetic moment of all elements in the periodic table. Compared to alkali atoms (µ~1μB) the dipole-dipole interaction for Dy atoms is about 100 times stronger.
Cooling and trapping of dysprosium is realized in several sequential steps. Initially the Dy atoms are evaporated in a high temperature effusion cell (EC). Due to its high melting point, temperatures of about 1200°C are needed to evaporate a sufficient number of dysprosium atoms for our experiments. As a result of these high temperatures, Dy atoms are leaving the effusion cell with a very high velocity of about 500 m/s. The atomic beam is then collimated by a set of apertures and transversally cooled (TC) to reduce the beam divergence. The next cooling step is done inside a Zeeman slower in which the atoms are slowed down to mean velocities of about 15 m/s. In a Zeeman slower a spatially varying magnetic field is combined with a laser beam to realize one-dimensional laser cooling. After being precooled in the Zeeman slower, the atoms will be trapped inside a magneto-optical trap (MOT), where they are further cooled to temperatures in the micro Kelvin range. This allows for the transfer into an optical dipole trap (ODT) for a strong confinement of the atoms. In the future, quantum degeneracy can be achieved by evaporative cooling of the atoms in the ODT.
Our experiment utilizes the blue transition at 421 nm for transverse cooling, the Zeeman slower and for absorption imaging, while the MOT is operated on the orange 626 nm transition. The ODT is generated by a high power 1064 nm laser system. An ultra-high vacuum chamber is required to cool, trap and investigate ensembles of Dy atoms.
In order to achieve high density samples needed to study the interplay between magnetic and light-induced correlation processes, we are currently optimizing our optical dipole trap. A lens system is designed to operate on the diffraction limit, allowing a very small focus of the ODT.
Niels Petersen, Marcel Trümper, Patrick Windpassinger
Spectroscopy of the 1001 nm transition in atomic dysprosium
Niels Petersen, Florian Mühlbauer, Lykourgos Bougas, Arijit Sharma, Dmitry Budker and Patrick Windpassinger
Sawtooth wave adiabatic passage slowing of dysprosium
Phys. Rev A 99 063414 (2019), arXiv:1809.06423
Bachelor / Master Theses
We always offer projects both for bachelor and master theses. At the moment, following projects are available in our lab:
- Characterization of a microscopic optical dipole trap
We optimized a lens system for the optical dipole trap in order to achieve microscopic beam waists at the focus. Your task would be to evaluate and characterize the optical system and the performance of the microscopic optical dipole trap.
- Creation of a Bose-Einstein-Condensate from dysprosium
Your task would be to perform and optimize the evaporative cooling of dysprosium atoms on our setup, in order to create a BEC.
- Initial studies on light-induced correlations
Assist in designing and performing our very first experiments to study light-induced correlations in dipolar media from dysprosium!
Laserintensitätsstabilisierung mit dem Red Pitaya, bachelor thesis by Jonas Lehnen (2018)
Aufbau und Charakterisierung einer optischen Dipolfalle für Dysprosium, bachelor thesis by Gunther Türk (2018)
Spektroskopie des 1001nm Übergangs von Dysprosium, master thesis by Lena Maske (2018)
Laserkühlen von Dysprosium, master thesis by Carina Baumgärtner (2017)
Summenfrequenzerzeugung von 626 nm Laserlicht, bachelor thesis by Lena Maske (2016)
Aufbau und Stabilisierung eines ECD-Lasers, bachelor thesis by Ansgar Schaefer (2015)
During extensive spectroscopic efforts we finally found and characterized the ultra-narrow 1001 nm ground state transition in dysprosium. We determined lifetime and polarizability of the excited state and measured the isotope shifts for the isotopes 160Dy, 162Dy and 164Dy.
A preprint of our paper can be found here: https://arxiv.org/abs/1907.05754.
Recently, we measured the spectrum of dysprosium on the 421 nm cooling transition with our homebuilt laser system. After amplifying the master laser seed in a homebuilt tapered amplifier stage, it is frequency doubled in a homebuilt bow tie cavity which delivers up to 460 mW of blue laser power. The five most abundant isotopes of dysprosium can clearly be identified in the spectrum shown below. The inset shows a picture of our main vacuum chamber where a beam of dysprosium atoms crosses the spectroscopy beam