  Localized plasmons formed in ultrathin metallic nanogaps can lead to robust
absorption of incident light. Plasmonic metasurfaces based on this effect can
efficiently generate energetic charge carriers, also known as hot electrons,
owing to their ability to squeeze and enhance electromagnetic fields in
confined subwavelength spaces. However, it is very challenging to accurately
identify and quantify the dynamics of hot carriers, mainly due to their
ultrafast time decay. Their non-equilibrium temperature response is one of the
key factors missing to understand the short time decay and overall transient
tunable absorption performance of gap-plasmon metasurfaces. Here, we
systematically study the temperature dynamics of hot electrons and their
transition into thermal carriers at various timescales from femto to
nanoseconds by using the two-temperature model. Additionally, the hot electron
temperature and generation rate threshold values are investigated by using a
hydrodynamic nonlocal model approach that is more accurate when ultrathin gaps
are considered. The derived temperature dependent material properties are used
to study the ultrafast transient nonlinear modification in the absorption
spectrum before plasmon-induced lattice heating is established leading to
efficient tunable nanophotonic absorber designs. We also examine the damage
threshold of these plasmonic absorbers under various pulsed laser
illuminations, an important quantity to derive the ultimate input intensity
limits that can be used in various emerging nonlinear optics and other tunable
nanophotonic applications. The presented results elucidate the role of hot
electrons in the response of gap-plasmon metasurface absorbers which can be
used to design more efficient photocatalysis, photovoltaics, and photodetection
devices.
