  Controlling the thermal expansion of materials is of great technological
importance. Uncontrolled thermal expansion can lead to failure or irreversible
destruction of structures and devices. In ordinary crystals, thermal expansion
is governed by the asymmetry of the microscopic binding potential, which cannot
be adjusted easily. In artificial crystals called metamaterials, thermal
expansion can be controlled by structure. Here, following previous theoretical
work, we fabricate three-dimensional two-component polymer microlattices by
using gray-tone laser lithography. We perform cross-correlation analysis of
optical microscopy images taken at different sample temperatures. The derived
displacement-vector field reveals that the thermal expansion and resulting
bending of the bi-material beams leads to a rotation of the 3D chiral crosses
arranged onto a 3D checkerboard pattern within one metamaterial unit cell.
These rotations can over-compensate the expansion and lead to an effectively
negative thermal length-expansion coefficient for all positive constituents
evidencing a striking level of thermal-expansion control.
