  Hyperbolic metamaterials (HMMs) are highly anisotropic optical materials that
behave as metals or as dielectrics depending on the direction of propagation of
light. They are becoming essential for a plethora of applications, ranging from
aerospace to automotive, from wireless to medical and IoT. These applications
often work in harsh environments or may sustain remarkable external stresses.
This calls for materials that show enhanced optical properties as well as
tailorable mechanical properties. Depending on their specific use, both hard
and ultrasoft materials could be required, although the combination with
optical hyperbolic response is rarely addressed. Here, we demonstrate the
possibility to combine optical hyperbolicity and tunable mechanical properties
in the same (meta)material, focusing on the case of extreme mechanical
hardness. Using high-throughput calculations from first principles and
effective medium theory, we explored a large class of layered materials with
hyperbolic optical activity in the near-IR and visible range, and we identified
a reduced number of ultrasoft and hard HMMs among more than 1800 combinations
of transition metal rocksalt crystals. Once validated by the experiments, this
new class of metamaterials may foster previously unexplored optical/mechanical
applications.
