  Heat management is crucial in the design of nanoscale devices as the
operating temperature determines their efficiency and lifetime. Past
experimental and theoretical works exploring nanoscale heat transport in
semiconductors addressed known deviations from Fourier's law modeling by
including effective parameters, such as a size-dependent thermal conductivity.
However, recent experiments have qualitatively shown new behavior that cannot
be modeled in this way. Here, we combine advanced experiment and theory to show
that the cooling of 1D- and 2D-confined nanoscale hot spots on silicon can be
described using a general hydrodynamic heat transport model, contrary to
previous understanding of heat flow in bulk silicon. We use a comprehensive set
of extreme ultraviolet scatterometry measurements of non-diffusive transport
from transiently heated nanolines and nanodots to validate and generalize our
ab initio model, that does not need any geometry-dependent fitting parameters.
This allows us to uncover the existence of two distinct timescales and heat
transport mechanisms: an interface resistance regime that dominates on short
time scales, and a hydrodynamic-like phonon transport regime that dominates on
longer timescales. Moreover, our model can predict the full thermomechanical
response on nanometer length scales and picosecond time scales for arbitrary
geometries, providing an advanced practical tool for thermal management of
nanoscale technologies. Furthermore, we derive analytical expressions for the
transport timescales, valid for a subset of geometries, supplying a route for
optimizing heat dissipation.
