Frequency combs (FC) are coherent light sources that emit a broad spectrum consisting of discrete, perfectly spaced modes, each having an absolute frequency measurable within the accuracy of an atomic clock. After 20 years of development in the near-infrared and visible domains, FCs have revolutionized frequency metrology with strong impacts in other fields ranging from astronomy to communications. From these developments, mid-infrared (MIR 3-12 µm) FCs have recently become of significant interest for molecular spectroscopy. This is a result of the extraordinary large absorption of molecular vibrational and rotational modes – the spectroscopic ‘finger-print’ region - and the ability to implement superior comb-based spectroscopy with dramatically increased speed, resolution, spectral bandwidth, sensitivity and precision, compared with classic approaches. Broadband FCs are of particular interest since they remove the need for slow tuning or use of several lasers, permitting simultaneous use of multiple absorption features. A variety of approaches for MIR FCs are currently being investigated, each promising to bring new discoveries to MIR spectroscopy.
FC operation in the crucial mid-wave infrared (MWIR) 3 µm to 5 µm region, nonetheless, remains considerably underdeveloped compared to other parts of the MIR. The MWIR is critical as chemicals based on hydrocarbons as well as oxygen- or nitrogen-containing organic compounds have strong spectroscopic signatures. This MWIR region, however, is the sweet-spot operation for Interband Cascade Lasers (ICLs). These semiconductor sources combine the interband transition of a conventional diode laser with the voltage-efficient cascading scheme introduced by the quantum cascade laser (QCL). Furthermore the relatively ‘slow’ gain dynamics of ICLs make them simultaneously adapted for both passive and active modelocking, facilitating the possibility of ICL FCs. These three major points of i) emission in the range 3 – 5 µm, ii) low electrical power requirements and iii) inherent dynamics make them ideal for miniature MWIR FC operation.
The aim of this thesis is to investigate the potential of ICLs for the realization of novel FCs. The candidate will study the optical and ultrafast properties of ICLs using new time resolved experimental techniques, combining both photonic and electronic concepts. The candidate will also study the influence of the bandstructure on the optical transitions and lifetimes, and their influence of the generation of FCs. The candidate will also contribute to Maxwell-Bloch simulations with our collaborators (TUM, Munich) to interpret the experimental results. The objective is to design and realize ICLs with appropriate carrier dynamics and waveguide confinement to induce a frequency comb state that currently does not exist.
The thesis is funded by an ANR project and will work in collaboration with the University of Montpellier, LAAS (Toulouse) and C2N (Orsay).
Applicant profile required: Background in condensed matter physics would be appreciated.
For more informations and application, contact : Sukhdeep DHILLON, Laboratoire de Physique de l'ENS, 24 rue Lhomond, 75005 Paris, tel: 01 44 32 35 07, email :
sukhdeep.dhillon@ens.fr