Scientific projects

Project: Strontium ultracold gas. 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. Our experimental projects are developed on two setups:
-SrI: Cooperative effect in light propagation in optical thick medium and  ”Superflash” effect. Generation of artificial non-abelian gauge fields and geometrical qubits. Long-range interaction.

-SrII: Atomic interferometry using the optical clock transition

Figure: Strontium magneto-optical trap on the 461 nm line

Project: Atoms and nanophotonics. We use metamaterials to create unusual optical fields that we couple to atoms. Our experimental projects are developed with Cesium atoms on two setups:
-Hot atoms: Engineering the atom/surface Casimir-Polder interaction, Enhancement of Dipole forbidden transition.

Cold atoms: Superoscillatory field for trapping and manipulating cold atoms. Anisotropic vacuum.

Figure: Artistic view of the atom/metamaterial hybrid system. 10 metamaterials of size 200 μm × 200 μm with different plasmon resonance are engraved on a windows (see real image picture on the right upper corner). The metamaterial is a periodic arrangement (period ~400 nm) of nano-slits (see SEM image on the right).



2D MOT of strontiumFirst evidence of cooling and trapping of Strontium atoms in 2D MOT in our SrII setup. The bright spot on the figure above corresponds to the cold atoms within a transverse view. The vacuum tubes in the form of a cross host the four laser beams. The oven is enclosed in a grey thermal shielding, shown on the left part of the figure. The bottom-left inset shown the cold atoms in an axial view. The weak distortion is due to imperfect local balance of the laser intensity.

Fermionic strontium isotope in  optical dipole trap

Fluorescence imaging system of 10^6 atoms in an optical dipole trap

Setting the vacuum apparatus of SrII project

We are developing atomic interferometry based on optical clock transition (see vacuum setup on the figure). In contrast with more commune atomic interferometer based on two photons Raman transition; our interferometer involves only one optical photon with an energy difference of six order of magnitude larger. The potential applications of this new type of interferometry are:

  • Combining ultraprecise time measurement with local gravity measurement
  • Test of general relativity using quantum coherence
  • Long baseline gradiometer for gravitational wave detection and search of dark matter.

Science Communication Writing competition outcomeFigure: Mehedi Receiving the certificate from Prof. Simon Redfern (Dean, College of Science). The Certificate.

Our Ph.D. student Mehedi Hasan won the merit prize in Science Communication Writing competition, organized by College of Science of NTU. Mehedi explained artificial gauge for non-expert audience. Here is the essay. Congratulation Mehedi!

First Fermi Sea in SrI

We obtained our first Fermi sea on a strontium gas. We load a crossed dipole trap with ~4·106 atoms at ~3 microK. After 3 sec of forced evaporative cooling we get ~105 atoms at T = 0.2 TF.Figure: (left) T=3 TF. (right) T = 0.2 TF. The image is taken after a 13 ms time-of-flight.

Congratulation to Dr Aik for his graduation!

Coupling of atomic quadrupole transitions with resonant surface plasmons

We couple of an electric quadrupole transition in atomic vapor with plasmonic excitation in a nanostructured metallic metamaterial. The quadrupole transition at 685 nm in the gas of cesium atoms is optically pumped, while the induced ground-state population depletion is probed with light tuned on the strong electric dipole transition at 852 nm. We use selective reflection to resolve the Doppler-free hyperfine structure of cesium atoms. We observe a strong modification of the reflection spectra at the presence of metamaterial and discuss the role of the spatial variation of the surface plasmon polariton on the quadrupole coupling.Figure: (left) laser configurations for selective reflection (SR) spectroscopy. The cesium vapor interface is pumped on the weak quadrupole transition at 685 nm, and probed on the strong dipole transition at 852 nm. (right) SR signals on a dielectric/vapor interface and on a dielectric/metamaterial/vapor interface. The expected surface plasmon enhancement is spread on a Doppler broaden contribution and absent on the Doppler-free SR signal.

For more details see: Phys. Rev. A 99, 063801 (2019)

Non-Abelian and adiabatic geometric transformation in a cold atomic gas

We study a laser-cooled gas of strontium atoms coupled to laser fields through a 4-level resonant tripod scheme. By cycling the relative phases of the tripod beams, we realize non-Abelian SU(2) geometrical transformations acting on the dark-states of the system and demonstrate their non-Abelian character. We also reveal how the gauge field imprinted on the atoms impact their internal state dynamics. It leads to a new thermometry method based on the interferometric displacement of atoms in the tripod beams.

Figure: Reconstruction of the geometric unitary operator for two different path ordering of the close loop (see close loop of the tripod laser phase). Green: experiment, red: cold gas at finite temperature and blue: pinned atom. The difference between the two close loop is due to the non-Abelian character of the transformation.

For more info see:  Nature Communications 9, 3580 (2018).

Tailoring optical metamaterials to tune the atom-surface Casimir-Polder interaction

The long-range atom-surface interaction, known as Casimir-Polder interaction, is of fundamental importance in quantum electrodynamics but also attracts a significant interest for platforms that interface atoms with nanophotonic devices. We perform a spectroscopic selective reflection measurement of the Casimir-Polder interaction between a Cs(6P_{3/2}) atom and a nanostructured metallic planar metamaterial. We show that by engineering the near-field plasmonic resonances of the metamaterial, we can successfully tune the Casimir-Polder interaction, demonstrating both a strong enhancement and reduction with respect to its non-resonant value. Engineering Casimir-Polder interactions represents a significant step towards atom trapping in the extreme near field, possibly without the use of external fields.

Figure: (a) experimental setup. (b) Real and imaginary part of the Van der Waal coefficient obtained for 10 different metamaterials. Each of them have a different surface plasmon resonance λp

For more info see: Science Advances 4 (2), eaao4223 (2018)

Photon Hall Scattering from Alkaline-earth-like atoms and Alkali-like ions

We investigate the possibility of observing a magneto-transverse scattering of photons from alkaline-earth-like atoms as well as alkali-like ions and provide orders of magnitude. The transverse magneto-scattering is physically induced by the interference between two possible quantum transitions of an outer electron in a S-state, one dispersive electric-dipole transition to a P orbital state and a second resonant electric-quadrupole transition to a D orbital state. In contrast with previous mechanisms proposed for such an atomic photonic Hall effect, no real photons are scattered by the electric-dipole allowed transition, which increases the ratio of Hall current to background photons significantly.

see: EPJ Special Topics 226, 1515 (2017)

Linear and nonlinear magneto-optical rotation on the narrow strontium intercombination line

In the presence of an external static magnetic field, an atomic gas becomes optically active, showing magneto-optical rotation. In the saturated regime, the coherences among the excited substates give a nonlinear contribution to the rotation of the light polarization. In contrast with the linear magneto-optical rotation, the nonlinear counterpart is insensitive to Doppler broadening. By varying the temperature of a cold strontium gas, we observe both regimes by driving the $J=0\rightarrow J=1$ transition on the intercombination line. For this narrow transition, the sensitivity to the static magnetic field is typically three orders of magnitude larger than for a standard broad alkali transition.

figure: Faraday rotation on the Strontium intercombination line. Increasing the laser intensity we observe a Doppler-free anomalous rotation at the line center. The width of this structure increases with the square-root of the intensity.

Atomic Response in the Near-Field of Nanostructured Plasmonic

We study the reflection spectra of cesium atoms in close vicinity of a nanostructured metallic meta-surface. We show that the  Cesium D2 resonance transition at 852 nm is strongly affected by the coupling to the plasmonic resonance of the nanostructure and shows a Fano-like behavior. Fine tuning of dispersion and positions of the atomic lines in the nearfield of plasmonic metamaterials could have uses and implications for atom-based metrology, sensing, and the development of atom-on-a-chip devices.

Figure: Schematic view of the set-up. A Cesium thermal vapor is interacting with localized surface plasmon modes generated by a periodic array of nanoslit.

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.


February 2015

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.


January 2015

Kick-off meeting of the UMI Majulab

We celebrate the opening of the new UMI Majulab ( 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.


November 2014

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.


June 2014

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.


March 2014

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.


December 2013

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.


September 2013

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.


May 2013

Moving to the ground level lab

Fig: View of the lab from the main door with the three optical tables.

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.

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