The "Coherent and Non Linear Optics" group is interested in the optical properties of nano-objects, which can be either semiconductor based heterostructures or less classical materials like carbon nanotubes. These systems have at least one characteristic length of the order of the nanometer, their electronic and optical properties being governed by quantum confinement effects. These nano-objects can be considered as the building blocks necessary to the implementation of a quantum treatment of the information in solid state physics. We are particularly involved in the development of new integrated non classical light sources (Optical Parametric Oscillators, twin photons and single photon sources) and in the conception of new devices based on quantum optics effects.
Contact: Philippe Roussignol
- Photon pair generation in multiple microcavities
In a semiconductor microcavity the active medium is a 2D quantum well. This well is inserted in a Fabry-Pérot cacity. Under well-defined conditions a strong coupling regime between the quantum well electronic excitations and the cavity confined photons can be reached. The system normal modes become mixed light-matter states the socalled microcavity polaritons. These polaritons show exciting properties either for basic physics (specific dynamics, non-linear properties, condensation, etc.) or for applications (optical parametric oscillation, twin photons generation). In 2005, we designed a micro-OPO (Optical Parametric Oscillator) based on a triple microcavity. This kind of laser, which was up to now restricted to basic research labs due to its size and complexity, could lead to appreciable new developments in quantum opticsi (quantum cryptography). We have demonstrated the feasibility of such an OPO at cryogenic temperatures under optical pumping. A noise reduction under the laser shot noise has also been observed (indicating quantum correlations). Future works involve electrical injection and room temperature operation.
(More informations are available in the "semiconductor microcavities" section.)
- Single carbon nanotube spectroscopy
The geometrical structure of a carbon nanotube is very simple: a graphene sheet is enrolled to form a tube with typical length 1 µm. The tube diameter is about one nanometer. This kind of quasi-onedimensional nano-object shows numerous unique mechanical or electronic propertiesnales.
The study of carbon nanotubes optical properties (apart from Raman) really started after the publication in 2002 of a paper by the Rice University group reporting on the observation of photoluminescence signal at room temperature arising from aqueous suspensions of nanotubes. In fact carbon nanotubes self organize in bundles and have to be isolated for light emission. We have developped optical techniques of investigation and shown how to address a specific class of carbon nanotubes within an ensemble by combining selective optical excitation and appropriate detection. By time-resolved measurements we have also demonstrated the key role played by nanotube-nanotube and nanotube-environment interactions in the light emission process. Moreover low temperature measurements allowed to obtain important results on the excitonic structure of these nano-objects.
Up to now all measurements were performed on large ensembles of nanotubes and our current interst deals with single nanotube spectroscopy.
(For more informations see the section "Carbone Nanotubes")
- Fluctuating electrostatic environment effect in InAs/GaAs quantum dots.
A quantum dot is a zero-dimensionnal system, where quantum confinment occurs in all three space directions. Quantum dots, although sharing several commun characteristics with atoms (they are often called "macro-atoms), remain solid state objects. They interact especially with their environnment and this interaction leads to a degradation of their coherent properties.
The spectral broadening of optical transitions in quantum dots is due to the Stark effect induced by trapped carriers in the vicinity of the dot and the trapping and detrapping of these carriers gives rise to fluctuations of the electric field experienced by the dot.
In october 2006, we have oserved a non conventional "motional narrowing" phenomenon leading to a partial inhibition of the decoherence mechanisms induced by the electrostatic environment of a quantum dot. The motional narrowing is a rather surprising phenomenon : the resonance of a system coupled to a fluctuating reservoir narrows when the reservoir fluctuations increase. One of the most famous example is the Nuclear Magnetic Resonance, where the the effects of fluctuating local magnetic fields created by the nuclear spins orientated randomly are averaged for incresing temperature, due to the acceleration of the nuclei motion. On the contrary, in a quantum dot the motional narrowing occurs when the temperature decreases. This is due to the asymmetry between the capture and escape processes for the carriers trapped at the vicinity of the quantum dot.
This observation open new prospects for the decoherence dynamics control in quantum dots and thus increasing their potentiality in quantum information. At moment we are studying the effect of an applied transverse electric field in order to monitor the capture and escape of the carriers from the traps.
(for more informations see the section "Quantum dots")
- Hole-burning spectroscopy in GaN based quantum dots.
Intra-band spectrocopy is at moment performed only in large ensemble of quantum dots. The unipolar excitation optical spectroscopy of a single quantum dot still remains a challenge. The experimental difficulties are closely related to the spectral domain associated to these transitions, lying in the far infra-red for usual III-V semiconductors like InAs/GaAs. In the GaN/AlN system, the large conduction band offset (1.75 eV) leads to transitions in the near infra-red, corresponding to the optical telecommunication domain. This study of intra-band transitions in nitride base quantum dots is thus motivated by potential applications in unipolar devices such as photodetectors, unipolar lasers or saturable absorbers.
(This activity has moved with Guillaume Cassabois to University of Montpellier)