This dissertation reports on the design, fabrication, and characterization of optical modulators and filters based upon resonant metallic gratings. The optical and electromechanical theory behind the operation of these devices is also presented with modeling results to confirm experimental results. Optical modulators and filters were made for both the infrared (IR) and visible wavelengths.
A tunable IR filter based on a metal grating patterned with subwavelength holes coupled with a microelectromechanical systems (MEMS) actuator is presented. The optical properties of the filter, including center wavelength and passband width, are dependent on the lithographically defined pattern of the grating. Electrostatic actuation is used to modulate the transmission intensity by moving the metal film into and out of contact with the underlying dielectric layer. A maximum transmission of 72% is achieved, with a 4 to 1 rejection ratio at an actuation voltage of 46 V. This IR device holds promise for gas sensing and other IR sensing environments.
Devices consisting of large-area two-dimensional arrays of nanoholes in Ag and Al films are presented. Fabrication is based on thermal nanoimprint lithography (NIL) and metal evaporation. The center wavelength of the reflectance minimum varies linearly with the refractive index of the fluid with sensitivities over 500 nm per refractive index unit. Applications for this device include biological and chemical detection.
Also, a MEMS optical modulator and filter for visible wavelengths is presented. The MEMS pixel is fabricated on a silicon-on-insulator (SOI) wafer. NIL is used to form an array of 150 nm diameter nanoholes in a 60 nm thick aluminum film. A quartz superstrate with an indium tin oxide (ITO) electrode and photoresist spacers is used to electrostatically actuate the MEMS pixel. Motion of the pixel in relation to the superstrate causes shifts in the wavelengths of optical interference from the periodic nanohole array. The MEMS pixel demonstrates a switching speed of 85 μs with a 23 V driving voltage. An optical modulation depth of over 67 % is demonstrated. Squeeze-film damping was found to be important in the dynamic operation of the pixel. This device shows promise for optical display applications.
December 31, 2007
Skinner, J. L. (2007). Diffractive Optical MEMS Technology for Tunable Filters and Modulators. (n.p.): University of California, Davis.