Ultrafast dynamics of Terahertz Quantum cascade lasers – Graphene as a Terahertz emitter
Jean Maysonnave (LPA)

Abstract

The terahertz (THz) range is a region of the electromagnetic spectrum
which lies at the limit between the electronic and optical domain.
Currently, THz applications suffer from the lack of sources and detectors.
In this context, this thesis focuses on the fundamental study and the
development of new functionalities of different THz sources, using THz
time-domain spectroscopy (TDS) as a base. This powerful tool enables to
acquire the temporal profile of a THz electric field and is used to
explore the THz emission properties of quantum cascade lasers (QCLs) and
graphene.
In the first part, the ultrafast response of QCLs is investigated. A phase
control of the electric field of THz QCLs via injection seeding is
realised and optimised. This enables the measurement of the amplitude and
temporal profile of the laser emission. Through these experiments and
simulations, a quantitative description of the gain dynamics can be
accessed. This information is critical for modelocking. Finally, a fast
modulation of the gain of QCLs is realized and leads to short pulses
generation (15 ps) in a modelocked regime. These studies open the way for
using QCLs as powerful sources in TDS.
In the second part, THz radiation generation from graphene under optical
excitation is demonstrated by a second order non-linear process. The THz
emission results from the momentum transfer from the photons to the
electrons of graphene (photon drag). As well as broadband THz generation,
novel bandstructure properties of graphene can be explored such as the
different dynamics between the photogenerated electrons and holes.