  Scanning micro-mirror actuators are silicon-based oscillatory
micro-electro-mechanical systems (MEMS). They enable laser distance
measurements for automotive LIDAR applications as well as projection modules
for the consumer market. For MEMS applications, the geometric structure is
typically designed to serve a number of functional requirements. Most
importantly, the mode spectrum contains a single high-Q mode, the drive mode,
which per design is expected to yield the only resonantly excited geometric
motion during operation. Yet here, we report on the observation of a resonant
three-mode excitation via a process known as spontaneous parametric
down-conversion. We show that this phenomenon, most extensively studied in the
field of nonlinear optics, originates from three-wave coupling induced by
geometric nonlinearities. In combination with further Duffing-type
nonlinearities, the micro mirror displays a variety of nonlinear dynamical
behaviour ranging from stationary state bifurcations to dynamical instabilities
observable via amplitude modulations. We are able to explain and emulate all
experimental observations using a single fundamental model. In particular, our
analysis allows us to understand the conditions for the onset of three-wave
down-conversion which if not accounted for in the design of the MEMS structure,
can have drastic impact on its functionality even leading to fracture.
