Working on this experiment: Marvin Proske, Ishan Varma, Rhuthwik Sriranga, Dimitra Cristea
Introduction
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.
Dysprosium
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.
Experimental setup
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) that confines the atoms and enables us to transport them over a distance of 41cm into a home-built science cell by moving the focus with optics mounted on an air-bearing translation stage. After the atoms are optically transported into the science cell, they will be retrapped utilizing a microscopic ODT. The resulting dense cloud will then be probed for signatures of collectivity in the optical response by light-scattering processes. 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 narrow orange 626 nm transition. The ODT for transport and retrapping is generated by a high power 1064 nm laser system.
Publications
Nico Baßler, Ishan Varma, Marvin Proske, Patrick Windpassinger, Kai-Phillip Schmidt, Claudiu Genes
Cooperative effects in dense cold atomic gases including magnetic dipole interactions
Phys. Rev. Research 6 023147 (2024), arXiv:2306.11486
Niels Petersen, Marcel Trümper, Patrick Windpassinger
Spectroscopy of the 1001 nm transition in atomic dysprosium
Phys. Rev. A 101 042502 (2020), arXiv:1907.05754
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
Dominik Studer, Lena Maske, Patrick Windpassinger and Klaus Wendt
Laser spectroscopy of the 1001nm ground state transition in dysprosium
Phys. Rev. A 98 042504 (2018), arXiv:1807.07903
Florian Mühlbauer, Niels Petersen, Carina Baumgärtner, Lena Maske, Patrick Windpassinger
Systematic optimization of laser cooling of dysprosium
Applied Physics B 124:120 (2018), arXiv:1804.01938
Bachelor / Master Theses
We always offer projects both for bachelor and master theses. At the moment the following projects are available in our lab:
- Setup and characterization of a racetrack coil pair to optimize the optical transport of cold atoms
- In our efforts to study light-scattering phenomena we optically transport Dysprosium atoms over a distance of 41cm to a separate science chamber. For this transport an offset magnetic field is required over the full distance to prevent trap losses due to spin-exchanging collisions. Your task is to setup and characterize the necessary racetrack coil and optimize the atom transport efficiency.
- Characterization and stabilization of a frequency doubling cavity (842nm -> 421nm)
- To generate the 421nm light for our Zeeman slower and transversal cooling we use second-harmonic generation. Your task is to characterize a commercial doubling cavity and frequency stabilize the output on the 421nm atomic transition in Dysprosium. To confirm the correct frequency setting, spectroscopy measurements can be taken with the Dysprosium atoms.
- Investigation of the atomic state distribution using the Stern-Gerlach method
- For our experimental endeavours we need to reduce our atomic system to an effective two-level system. For this you will characterize an optical pumping scheme to pump the Dy-atoms into the mJ=-8 state. To test the effectiveness of the pumping, Stern-Gerlach measurements can be taken utilizing strong magnetic field pulses.
If you are interested in joining our team as a bachelor or master student, please contact Prof. Patrick Windpassinger or PhD students Marvin Proske, Ishan Varma and Rhuthwik Sriranga.
Finished Theses
Systematic Investigation of the Role of Light Polarization in Optically Trapped Dipolar gases, master thesis by Chung-Ming Hung (2025)
Aufbau und Charakterisierung einer Hochleistungslaserquelle für optische Dipolfallen, bachelor thesis by Jakob Seckinger (2024)
Gezielte Kontrolle der Schaltzeit von Laserlicht, bachelor thesis by Stefanie Kirschner (2023)
An Optical Transport System for Ultracold Dysprosium, master thesis by Nivedith K. Anil (2023)
Performance characterization of a microtrap objective for cold atom experiments, bachelor thesis by Dimitra Cristea (2023)
Aufbau und Charakterisierung einer optischen Dipolfalle für Dysprosium, bachelor thesis by Hannah Jost (2021)
Generation of Higher Order Laser Beams Using a Spatial Light Modulator, bachelor thesis by Natalija Sheth (2021)
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)