Physikalisches Kolloqui­um: Util­iz­ing light-mat­ter in­ter­ac­tion to mon­it­or and con­trol ex­cit­a­tions across in­tern­al in­ter­faces

Prof. Dr. Sangam Chatterjee
Universität Gießen
Utilizing light-matter interaction to monitor and control excitations across
internal interfaces


Low-dimensional quantum structures of compound semiconductors such as type-I quantum
wells are the backbone of the established optoelectronic devices. Their fundamental
properties have been intensely studied for more than half a century. The accompanying
immense development in materials quality are indispensable for reliable long term device
performance and true scalability, presumably the two prime advantages of semiconductor
technology. More recently, heterostructures featuring type-II band offsets offer additional
degrees of freedom in tailoring their optoelectronic response. From a fundamental
standpoint, these type-II heterostructures offer the potential to investigate charge-transfer-
like excitations. These developments have been accompanied by stringent development of
microscopic theoretical description. The interplay of experimental and theoretical research
has successfully fostered design and understanding of this class of quantum structures.
Regardless, even decades old predictions remain under discussion as they have not been
demonstrated experimentally.
This colloquium reviews the potential of light-matter interaction to monitor, clarity, and
eventually control quasiparticle excitations in semiconductor quantum structures. The
virtues and caveats of photon-based techniques are reviewed. Understanding the ultrafast
dynamics of type-II heterostructures is crucial for the design and optimization of next-
generation mid-infrared light sources. By tailoring the band offsets, it is possible to reduce
the emission energies of active devices, potentially eliminating Auger losses. Furthermore,
these structures serve as ideal model systems for investigating the structure and dynamics of
excitations across internal interfaces.
Exemplary experimental observations of the nonlinear dynamics are presented using various
techniques such as polarization resolved four wave mixing, optical-pump optical-probe, and
optical-pump terahertz-probe spectroscopy. In type-II heterostructures, this holistic approach
reveals, e.g., spectroscopic signatures consistent with biexciton states of charge-transfer
excitons. For applications, type-II active devices combine the advantages of a spectrally
broad, temperature stable efficient gain with the potential for electrical injection pumping.
The intrinsic charge-carrier relaxation dynamics limit the feasible repetition rates beyond
constraints of cavity design and heat removal. Here, we investigate the initial buildup of gain
after optical excitation as well as its recovery after a stimulated emission process in the near