This seminar will discuss the design, fabrication, and characterization of optical modulators and filters based on plasmonic interactions with resonant metallic gratings. The optical and electromechanical theory behind the operation of these devices is covered 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 shown. 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 shared. The MEMS pixel is fabricated on a silicon-on-insulator (SOI) wafer. NIL is used to form an array of ~100 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 enable electrostatic action of 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 exhibits promise for optical display applications.
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