  We present a proof of concept demonstration of a novel optical element: a
light-responsive aperture that can track a moving light beam. The element is
created using a thermally-activated transparency-switching material composed of
paraffin wax and polydimethylsiloxane (PDMS). Illumination of the material with
a focused beam causes the formation of a localized transparency at the focal
spot location, due to local heating caused by absorption of a portion of the
incident light. An application is proposed in a new design for a self-tracking
solar collector.
