Coherence in Semiconductor Nanostructures
Jacek Kasprzak
CNRS researcher at Néel Institute, Grenoble, Franceand a prospective Visiting Scientist at the University of Tsukuba, Japan
Optical spectra of semiconductors display resonances below the band edge, which are attributed
to excitons, i.e., bound electrons holes pairs, correlated by Coulomb interaction. For decades, epitaxial
growth provides modulated nano-structures, where a smaller bandgap material is surrounded by a higher
bandgap one. Such growth-designed, static potentials restrict the motion of the charge carriers andexcitons in one, two or three dimensions in so-called quantum wells, wires and dots, respectively, andtheir energies consequently become quantized. Engineering the quantum confinement of excitons is a
key to control the light-matter interaction dynamics in semiconductors, where the electron-hole overlap
integral along with the Fermi golden rule govern the radiative lifetime T1. Another relevant opticalobservable is coherence, describing the phase relation within the exciton polarization, vanishing beyond
the dephasing time T2. In particular quantum dots, whose relevance was recognized with the Nobel Prizein Chemistry in 2023, host individual excitons with extended T2. They can serve as optically adressablequbits, which are key ingredients in prospective quantum technologies.
During my research carrier I have been developing a technique of nonlinear spectroscopy
conceived to measure dynamics of quantum coherence of individual excitons, which also turned out tobe a perfect optical tool to investigate coherences in their ensembles. In this talk, I would like to highlightthe relevance of the scientific results obtained with this approach over the last decade.
I will start by presenting my track record and highlighting multiple facets of my professional
activity. I will then introduce the concept of a single exciton coherence and demonstrate that - bycombining coherent nonlinear spectroscopy with nano-photonic devices; microcavities, photonic
waveguide antennas, micro-lenses, etc. – the single exciton coherence retrieval efficiency can beenhanced by several orders of magnitude, redefining the state of the art in this field. To illustrate newphenomena that can be measured in this settings I will discuss two experiments: i) a recent achievementof a controlled coherent coupling in a vertically stacked quantum dot molecule, ii) observation ofphonon-induced dephasing and polaron formation within a single quantum dot. I will elaborate on theintrinsic limits of measuring coherence of single quantum dots, which will bring me to the project that I
wish to develop in the following years in the framework of the CNRS International Research Laboratory
J-FAST, hosted by the University of Tsukuba. The project consists in fabricating photonic structuresin diamond, by developing Atomic Layer Etching and performing back in my lab in Grenoble coherent
spectroscopy of single colour centres in diamond at elevated temperatures, possibly up to the roomtemperature.
In the remaining part of the talk, I would like to highlight my recent prospects in coherent
spectroscopy of two-dimensional materials, with the aim to find further collaboration axes with thepartners from Tsukuba. As a first example, I will show how nonlinear, multi-pulse spectroscopy can beemployed to reveal coherent spatial propagation of excitons in a classical CdTe quantum well. As asecond example, I will discuss results regarding exciton coherence in emerging two-dimensional
magnetic material namely, tri-layers of CrSBr. Finally, I will show how with coherent ultrafast
spectroscopy one can monitor a variable exciton confinement in an advanced transition metal
dichalcogenide heterostructure, namely in a doubly-gated MoSe2 device.