Spontaneous parametric down-conversion induced by non-degenerate three-wave mixing in a scanning MEMS micro mirror

TL;DR

This paper reports the discovery of spontaneous three-wave mixing in a MEMS scanning micro-mirror, revealing nonlinear phenomena that can affect device performance and reliability, with implications for design and operation.

Contribution

It demonstrates the occurrence of spontaneous parametric down-conversion in MEMS devices due to geometric nonlinearities, a phenomenon previously known mainly in nonlinear optics.

Findings

Observation of three-mode excitation via spontaneous parametric down-conversion

Identification of geometric nonlinearities as the cause of three-wave coupling

Modeling of nonlinear dynamical behaviors including bifurcations and instabilities

Abstract

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.