  Uniaxial strain has been widely used as a powerful tool for investigating and
controlling the properties of quantum materials. However, existing strain
techniques have so far mostly been limited to use with bulk crystals. Although
recent progress has been made in extending the application of strain to
two-dimensional van der Waals (vdW) heterostructures, these techniques have
been limited to optical characterization and extremely simple electrical device
geometries. Here, we report a piezoelectric-based \textit{in situ} uniaxial
strain technique enabling simultaneous electrical transport and optical
spectroscopy characterization of dual-gated vdW heterostructure devices.
Critically, our technique remains compatible with vdW heterostructure devices
of arbitrary complexity fabricated on conventional silicon/silicon dioxide
wafer substrates. We demonstrate a large and continuously tunable strain of up
to $-0.15\%$ at millikelvin temperatures, with larger strain values also likely
achievable. We quantify the strain transmission from the silicon wafer to the
vdW heterostructure, and further demonstrate the ability of strain to modify
the electronic properties of twisted bilayer graphene. Our technique provides a
highly versatile new method for exploring the effect of uniaxial strain on both
the electrical and optical properties of vdW heterostructures, and can be
easily extended to include additional characterization techniques.
