The aim of plasmonics is to exploit the strong coupling between photons and collective
electron oscillations in metals, so-called surface plasmon polaritons, which enable
a strong confinement of the electromagnetic field to metal-dielectric interfaces. The
interaction of confined optical states with electronic transitions within matter accelerates
these otherwise slow light-matter interactions. This work’s purpose is to
investigate accelerated light-matter interactions within plasmonic lasers, which arise
due to optical confinement, and how these influence laser dynamics. In particular,
this work focuses on the fabrication, demonstration and characterisation of plasmonic
lasers.
The devices investigated in this work consist of semiconductor nanowires made from
zinc oxide (ZnO) placed in the proximity of a silver substrate. In this geometry the
metal allows for strong optical confinement, whereas the semiconductor delivers the
necessary gain to achieve lasing. Operating at room temperature, the emission from
ZnO lies near the surface plasmon frequency, where confinement and loss become
maximal, leading to accelerated spontaneous recombination, gain switching and gain
recovery compared with conventional - photonic - ZnO nanowire lasers.
To assess the lasing dynamics, in this work a novel double-pump spectroscopy technique
is used, which exploits the non-linearity of the laser process to allow the investigation
of accelerated light-matter interactions. This novel technique is necessary, as
the speed of plasmonic devices is too fast for electrical detection, and the emission of
single devices is too weak for non-linear spectroscopic techniques.
Comparing photonic and plasmonic devices reveals contrasting dynamics between
both, highlighting the benefits of plasmonic confinement, but also exposing an important
limitation. Plasmonic devices could potentially be faster, but are ultimately
limited by internal relaxation processes of the chosen gain medium. The findings of
this work will improve the understanding of plasmonic lasers and their limitations,
but also lead to improved knowledge of internal semiconductor processes.