Our research interest focuses on Group II ultracold gas (G2UG). These two valence electrons atoms show electronic spinless ground state and weakly allowed singlet-triplet transitions, offering interesting alternatives and opening new fields of research for cold and ultracold gases with respect to the more commonly used alkaline atoms.
Since 2013, we have a cold gas experimental apparatus running on strontium atoms. On this setup our research activity is mainly around:
-Cooperative effect in light propagation in optical thick medium and ”Superflash” effect
-Generation of artificial non-abelian gauge fields and geometrical qubits
-Nonlinear Faraday rotation
In 2015, we start a new experimental activity on strontium beam aiming to explore relativistic correction on resonant photon scattering.
In parallel to the ultracold gas activity, we have a research activity on hybrid systems made of hot atomic vapor and two-dimensional metamaterials. This activity is located at the Centre for Disruptive Photonic Technologies (CDPT).
Cooperative Emission of a Pulse Train in an Optically Thick Scattering Medium
When driven by a coherent probe, an ensemble of diluted light scatterers displays interesting similarities with Dicke superradiance such as large and fast response to excitation.
Taking advantage of the extremely slow response time of the atomic strontium intercombination line, we found in [C.C. Kwong et al, PRL 113, 223601 (2014)] that the cooperative field intensity can be up to 4 times the incident intensity (“superflash effect”). In our present paper [C.C. Kwong et al, PRL 115, 223601 (2015)], by changing periodically and abruptly the phase of the probe beam, we generate a cooperative pulse train (see figure inset). Because cooperativity has a faster response time than a single emitter, single fluorescence events can be quenched (see figure) and the dynamics of our atomic sample is governed by cooperative processes. This phenomenon is common in the strong coupling regime (atoms in a cavity for example) or with a dense sample, but unusual for a dilute medium in free space.
A high flux source of cold strontium atoms
Our new design for a high loading rate of a Strontium MOT in an UHV environment is in EPJD October issue cover page.
A network of magnetic field sensors for an active control of the magnetic field
Fig: Comparison of the magnetic field in the lab with and without active lock. We note an increase of the human activity from 8am to 6pm.
We design a network of eight magnetic field sensors located at the vertices of a cube surrounding the cold atoms gas. The data are digitalized and transfer to a PC at a 2ms rate. We extrapolate the value of the magnetic field at the atoms and perform an active feedback control of the DC fluctuations as well as the magnetic field induced by the 50Hz line. The accuracy of the measurement is tested on the cold gas using Faraday rotation.
Kick-off meeting of the UMI Majulab
We celebrate the opening of the new UMI Majulab (http://majulab.cnrs.fr/). It is a CNRS/UNS/NUS/NTU joint lab aiming to develop Franco-Singaporean scientific collaboration in different area of physics and chemistry. The G2UG operation is part of it.
Nuclear spin imaging system
Fig: Spectroscopy of different transitions of the strontium intercombination line. Two situations are considered. First, atoms are optically pumped in the mF = 9/2 Zeeman substate (blue dots) and second after a STIRAP, where atoms are transferred in the mF = 5/2, 7/2 states (red points). The amplitude of each line is proportional to the respective Zeeman substate populations.
Laser cooling of fermionic isotope on the intercombination line.
From a blue magneto-optical trap, we transfer typically 108 atoms into, first, a broadband and, then, a single band magneto-optical trap. The transfer efficiency is around 50% and the final temperature is 5 microK. The next step is to transfer the cold cloud into a dipole trap.
Superflash of light with transmittance greater than one
One way of transmitting light through an opaque medium is by abruptly switching off the source. This rather counterintuitive method leads to an emission of a short flash of light. Using resonant atomic scatterers, and bringing the source out-of-resonance, we observe a surprising “superflash” effect, namely a transmittance that is greater than one. We show that the occurrence of the superflash is due to the strong phase rotation of the forward scattered field emitted cooperatively by the atoms. Such a direct observation is impossible to achieve in the stationary regime, where the forward scattered field is masked by interference with the incident field. Moreover, we take advantage of the extraordinary slow response of the intercombination line of strontium atoms to separate those two fields in a time resolved experiment on a laser cooled gas.
Selective Injection Locking of a Multi-mode Semiconductor Laser to a Multi-frequency Reference Beam.
Fig: Fabry-Perot transmission spectra of slave laser injected by three lines separated by 1.2GHz for a seeding power of 50microW. The blue, dark and red curves correspond respectively to injection locking of the red detuned, central, and blue detuned line.
One laser beam passes through an EOM at a fixed frequency of 1.2 GHz generating mainly the +1 and -1 lateral bands. We use this beam with multiple frequency lines to seed slave lasers. We tune the parameters (temperature and current) of the slave lasers to selectively lock on to the +1 or -1 order of the frequency spectrum. Each slave laser will address the F = 9/2 to F = 7/2 or F = 9/2 to F = 11/2 transition of the hyperfine structure of the intercombination line, the seeding beam is tuned on the F = 9/2 to F = 7/2 transition.
The magneto-optical trap on the intercombination line operating for the 88Sr.
Fig: (a) Time sequence to load the 88Sr Red MOT from the blue 3D MOT. (b) Image of the cold cloud in the Red MOT after a ballistic expansion of 10ms.
Moving to the ground level lab
After 18 months of construction, the ground level lab is finally ready. We moved the two laser tables from level 3 to level 1 without dismounting the optics. The vacuum apparatus has been dismounted and reconstructed on a new non-magnetic optical table.