M.Tech Thesis Abstract: Theory and Applications of Optically Actuated MEMS Structures
Photons possess momentum, and hence, a beam of light can exert a force when reflected by a surface. Higher the reflection, higher is the radiation pressure. Radiation pressure is nearly insignificant for most macro scale applications, but it can be quite significant for microelectromechanical devices. In this thesis we investigate the possibility of using optical pressure for actuating microelectromechanical devices.
We study the mechanics of laser actuated singly and doubly clamped polysilicon micro-beams under steady state and transient conditions. We show, through analysis and simulations, that there exists an optimum point of incidence of the laser beam that produces the maximum steady-state deflection of a singly clamped cantilever. An expression for the point of incidence corresponding to maximum deflection is derived.
When a microstructure is actuated by radiation pressure, a fraction of the incident power gets absorbed and heats up the device. Overheating may be detrimental to device performance and may even cause melting. In order to estimate the maximum laser power that will not cause melting, we solved the heat equation numerically. We also present an analytical solution of the steady state heat equation. We find that the temperature dependence of material properties has negligible effect on the steady state deflection. However, when the surrounding medium is air, the damping coefficient increases by about 16% from 300 K to 400 K.
Based on our calculations, we propose three possible applications of optical actuation---a photodetector cum beam profiler, an optical switch and a diffraction grating based switch.
The photodetector cum beam profiler consists of a micro-beam structure. An incident laser beam causes the micro-beam to get strained. The laser beam can be characterized by measuring this strain.
The optical switch consists of a singly clamped micro-cantilever structure. On actuation, the cantilever bends and obstructs the path of light between two optical fibers.
The diffraction grating based switch is made of a doubly clamped micro-beam with diffraction gratings at the center. An incident laser beam causes the micro-beam to get strained. Consequently, the grating period changes with incident power. Different diffraction angles may be achieved by varying the laser power.