  We present microfabricated thermal actuators to engineer the biaxial strain
in two-dimensional (2D) materials. These actuators are based on microheater
circuits patterned onto the surface of a polymer with a high thermal expansion
coefficient. By running current through the microheater one can vary the
temperature of the polymer and induce a controlled biaxial expansion of its
surface. This controlled biaxial expansion can be transduced to biaxial strain
to 2D materials, placed onto the polymer surface, which in turn induces a shift
of the optical spectrum. Our thermal strain actuators can reach a maximum
biaxial strain of 0.64 % and they can be modulated at frequencies up to 8 Hz.
The compact geometry of these actuators results in a negligible spatial drift
of 0.03 um/deg, which facilitates their integration in optical spectroscopy
measurements. We illustrate the potential of this strain engineering platform
to fabricate a strain-actuated optical modulator with single-layer MoS2.
