Resonant spectroscopy of one-dimensional excitons in carbon nanotubes.
Carbon nanotubes are unique prototypes of one-dimensional nanostructures leading to unprecedented Coulomb correlations between photo-excited carriers. Therefore, the optical properties of both the semi-conducting and the metallic nanotubes are driven by strong excitonic effects, even at room temperature. Regular luminescence spectroscopy performed in several labs across the world has proved to be extremely powerful for the investigation of these new 1-D excitations. However, key features of the ground excitonic state predicted by theoretical calculations such as dark states, bi-excitonic states, trapped states... could not be observed by means of conventional techniques. In order to investigate these fine effects, the Optics group of LPA aims at developing a new experiment combining high spectral resolution and high spatial resolution (single nano-object spectroscopy) in the resonant spectroscopy configuration (the detected signal is at the same wavelength as the incoming laser). Despite some practical issues such as a large background signal, this kind of spectroscopy (that we have already successfully implemented in the case of semi-conducting quantum dots) allows both a highly improved resolution and the investigation of non emitting states, providing a unique insight into the excitonic fine structure. Practically, the measurement consists in Rayleigh scattering spectroscopy or absorption spectroscopy of single nanotubes either suspended over a trench or deposited on a substrate. The experiment involves the techniques of low flux spectroscopy (photon counting), cryogenic optical measurements and nano-fabrication (clean room techniques). A new setup will be developed during the internship, building on the know-how of the team for such techniques. The student must have a strong background in solid state physics and experimental optics and a strong motivation for experimental work.
Methods and techniques : Micro-photoluminescence spectroscopy, lasers, cryogeny, clean room, vacuum techniques...
Contact : Christophe Voisin
Low temperature spectroscopy of WS2 monolayers.
Graphene, the first truly two-dimensional material available at low cost has brought considerable breakthroughs in a number of domains ranging from high speed electronics to metrology and many others are to come. However, for some applications such as logical electronics or opto-electronics the lack of a band gap is a real issue. The recent discovery of new layered materials such as BN or metal dichalchogenides (such as WS2) allow to consider very attractive alternatives. Such materials bring the advantage of optical bandgaps that become direct gaps for monolayers and allow a spin control of the excited stated through the polarization of the excitation beam, opening the way to optospintronics. In addition, heterostructures combining monolayers of graphene and monolayers of WS2 should allow to build original opto-electronic devices. Since the very recent discovery of this new material, only a few experimental reports established its basic properties but theoretical studies promise much more. The goal of this internship is to explore for the first time the low temperature properties of WS2 monolayers, where the excitonic and spin related properties should be prominent. The study will be carried out in collaboration with a team of Columbia University (NY, USA) that will provide state of the art samples for this study. The student must have a strong background in solid state physics and experimental optics and a strong motivation for experimental work.
Methods and techniques : Micro-photoluminescence spectroscopy, lasers, cryogeny, clean room, vacuum techniques,...
Contact : Carole Diederichs