**1. Introduction**

330 Microelectromechanical Systems and Devices

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Many microelectromechanical systems (MEMS) applications utilize laser irradiation as an integral part of the system functionality, including projection displays, optical switches, adaptive optics (Andrews et al., 2011; Andrews et al., 2008), optical cross-connects (Knoernschild et al., 2009), and laser powered thermal actuators (Serrano & Phinney, 2008; Serrano et al., 2005). When laser irradiation is incident on small-scale systems, such as these MEMS applications, the propensity for exceeding the thermal handling capability of the devices dramatically increases, often leading to overheating, and subsequent deformation and permanent damage to the devices. In most instances, this damage is a direct consequence of the device geometry and the material thermal properties, which hinder the transport of heat out of any locally heated area. Such thermally-driven failures are common in electrically-powered systems (Baker et al., 2004; Plass et al., 2004). However, for laserirradiated MEMS, particularly those fabricated of surface-micromachined polycrystalline silicon (polysilicon), the optical properties can also affect the thermal response of the devices by altering how the laser energy is deposited within the material. Even more concerning in these types of devices is the fact that the thermal, optical, and mechanical response can be intimately coupled such that predicting device performance becomes difficult. In this chapter, we focus on understanding some of the basics of optical interactions in laserirradiated MEMS. We will first look at how the optical properties of the materials affect the laser energy deposition within a device. We will then expand upon this by looking at the coupling that exists between the optical and thermal properties, paying particular attention to the implications that transient temperature changes have in the optical response, ultimately leading to device failure. Finally, we will look at various cases of laser-induced damage in polysilicon MEMS where the device geometry and design and optical-thermal coupling have led to device failure.
