  Nanoporosity in silicon leads to completely new functionalities of this
mainstream semiconductor. A difficult to assess mechanics has however
significantly limited its application in fields ranging from nanofluidics and
biosensorics to drug delivery, energy storage and photonics. Here, we present a
study on laser-excited elastic guided waves detected contactless and
non-destructively in dry and liquid-infused single-crystalline porous silicon.
These experiments reveal that the self-organised formation of 100 billions of
parallel nanopores per square centimetre cross section results in a nearly
isotropic elasticity perpendicular to the pore axes and an 80% effective
stiffness reduction, altogether leading to significant deviations from the
cubic anisotropy observed in bulk silicon. Our thorough assessment of the
wafer-scale mechanics of nanoporous silicon provides the base for predictive
applications in robust on-chip devices and evidences that recent breakthroughs
in laser ultrasonics open up entirely new frontiers for in-situ,
non-destructive mechanical characterisation of dry and liquid-functionalised
porous materials.
