Institut des
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Dubin François

© INSP - Cécile Duflot
Chargé de recherche au CNRS

Membre de l’équipe Nanostructures et systèmes quantique

Institut des NanoSciences de Paris
Université Pierre et Marie Curie
4 place Jussieu - Case 840 - 75005 Paris
Mél : francois.dubin

Barre 22-32, étage 2, pièce 08 - Tél. : 33(0)1 44 27 46 32

Barre 22-32, étage R-de-C, pièce 15 - Tél. : 33(0)1 44 27 63 80

Research activity
My research activity emphasizes the collective quantum phenomena that can be realized by excitons. Excitons are composite bosons that result from the Coulomb attraction between electrons and holes. We perform experiments focusing onto spatially indirect excitons that are engineered by enforcing a spatial separation between electrons and holes constituting excitons. In our experiments this spatial separation is established in a double quantum well, by applying an electric field perpendicular to the heterostructure such that minimum energy states for electrons and holes lie in a distinct quantum well (Figure 1). Thus, indirect excitons exhibit unique properties to study collective quantum phenomena, notably a long radiative lifetime (≈100 ns for our sample structures) combined to a rapid thermalisation to low temperatures. Also, the electrical polarization applied onto the double quantum well ensures that indirect excitons have a well oriented electric dipole, which prevents the formation of excitonic complexes or an electron-hole plasma, excitonic dipoles being parallel the exciton-exciton interaction is repulsive.
JPEG Figure 1 : A double quantum well (DQW) is embedded in a field-effect device to apply an electric field perpendicular to the plane of the QWs. Energy bands become tilted such that minimum energy states for electrons and holes lie in a different quantum well (bottom). In our experiments indirect excitons are engineered in this way.

Experimental techniques
At the heart of our experiments lies an He3-He4 optical cryostat allowing us to study exciton gases down to 300 mK. The cryostat has been designed to ensure micrometric mechanical stability. It hosts a custom designed sample holders providing all the electrical wiring necessary to control the strength of the electric field applied onto a DQW. As shown in Figure 2, in our experiments indirect excitons are optically created and studied using a microscope objective positioned in front of the sample using piezo-electric transducers. In general, our studies rely on a pulsed laser excitation (≈100 ns) to limit the heat induced onto the sample. We then analyze the dynamics of the reemitted photoluminescence, with ns time resolution.

JPEG Figure 2 : Electrically connected field-effect device mounted in front of a high-numerical aperture microscope objective. All these elements are located inside our cryogenic apparatus.

Exciton traps
We are developing an integrated technology to control the spatial confinement of indirect excitons, and thus establish a thorough control over cold gases. To this aim we use a set of gate electrodes deposited at the surface of a field-effect device embedding a DQW. The electrodes allow applying a spatially inhomogeneous electric field in the plane of the DQW. Indirect excitons being high-field seekers, i.e. attracted towards the regions where the electric field is the strongest, we thus create electrostatic traps for excitons. Figure 3 provides an example of such a box-like trap, constituted by 2 surface electrodes, a larger bias being applied to the central gate-electrode. The trap depth then lies in the range of a few meV which suffices to confine dense (>10^10 cm^-2) gases which a priori undergo Bose-Einstein condensation below a few Kelvin.

JPEG Figure 3 : (a) Surface image of a two-electrode exciton trap. These electrodes are deposited at the surface of a field-effect device embedding a DQW. The substrate of the device serves as ground. Biasing the electrodes individually allows creating an electrostatic trap for indirect excitons, as shown in (b). In this example the trap depth is about 5 meV which is sufficient to confine a dense gas, i.e. with a density at about 5 1010 cm-2.
JPEG Experimental setup
JPEG Photoluminescence image of a macroscopic and fragmented ring of indirect excitons at 330 mK. In the outer rim of the ring we have shown that indirect excitons condense in lowest energy dark states thus leading to macroscopic spatial coherence (see EPL 107, 10012 (2014))
People in this research project
Suzanne Dang, PhD student
Mussie Beian, PhD student
Romain Anankine, PhD student
François Dubin, P.I.
Recent publications
Long-lived spin coherence of indirect excitons in GaAs coupled quantum wells
M. Beian, M. Alloing, E. Cambril, C. Gomez Carbonell, J. Osmond, A. Lemaître, F. Dubin, Europhys. Lett. 110, 27001 (2015)

Effects of fermion exchange on the polarization of exciton condensates
M. Combescot, R. Combescot, M. Alloing, F. Dubin, Phys. Rev. Lett. 114, 090401 (2015)

Evidence for a Bose-Einstein condensate of excitons
M. Alloing, M. Beian, M. Lewenstein, D. Fuster, Y. González, L. González, R. Combescot, M. Combescot, F. Dubin, Europhys. Lett. 107, 10012 (2014)

Optical signatures of a fully dark exciton condensate
M. Combescot, R. Combescot, M. Alloing, F. Dubin, Europhys. Lett. 105, 47011 (2014)

Optically programmable excitonic traps
M. Alloing, A. Lemaître, E. Galopin, F. Dubin, Sci. Rep. 3, 1578 (2013)

Nonlinear dynamics and inner-ring photoluminescence pattern of indirect excitons
M. Alloing, A. Lemaître, E. Galopin, F. Dubin, Phys. Rev. B 85, 245106 (2012)

Quantum signature blurred by disorder in indirect exciton gases
M. Alloing, A. Lemaître, F. Dubin, Europhys. Lett. 93, 17007 (2011)