  We demonstrate nanoscale optical bistability at an exceptionally low power
threshold of 1 mW by leveraging Kerr-type hydrodynamic nonlinearities due to
the heavily doped semiconductor's free carriers. This high nonlinearity is
enabled by a strong coupling between metallic nanopatch surface plasmons and
longitudinal bulk plasmons (LBP) that arise in heavily doped semiconductors due
to the nonlocality. Through the coupling, an efficient, near-unity conversion
of far-field energy into LBP states could be achieved. These findings offer a
viable approach to experimentally probe LBPs and lay the groundwork for
developing efficient and ultrafast all-optical nonlinear devices.
