Fabrication, instrumentation and application for subwavelength periodic nanophotonic devices
This dissertation focuses on developing novel and efficient fabrication methodology for periodic nanostructures based nanophotonic devices, especially variable or tunable optical/photonic devices that based on photonic crystal or plasmonic crystal slabs. These nanophotonic devices are optically characterized to demonstrate the effectiveness. This dissertation starts by developing an angular-dispersion detection instrument based on a one-dimensional photonic crystal. This instrument was demonstrated to have applications in chemical sensing and imaging by monitoring the guided-mode resonance (GMR) supported in the PC sensor comprised of a one-dimensional grating structure. Exposed to solutions with different refractive indices or adsorbed with biomaterials, the PC sensor exhibited changes of the optical resonant modes.
In order to fabricate tunable nanophotonic devices with continuously varying resonant wavelengths, two different approaches were explored in this dissertation. The first approach is to introducing graded geometry into the structure of the device, such as a varying period over the device surface. To accomplish this, a strain-tunable soft lithography method is developed using PDMS masters as the replicate molds. The process exploits an elastomeric mold made of PDMS to generate the designed periodic pattern in a UV curable polymer (UVCP) on glass or plastic substrates. During the imprint and curing process, the PDMS mold was mechanically deformed by a uniaxial force, which causes the periodic pattern carried on the PDMS mold to vary as designed. By control the stretching direction and magnitude of the applied force carefully, the lattice constant and arrangement can be determined. For example, by stretching the mold with a 2D array in a square lattice, rectangular and triangular lattice arrangements can be obtained. As a specific application, we have applied this programmable nanoimprint lithography method to create a linear variable photonic crystal (PC) filter with continuously tunable resonant wavelength covering a wide spectral range along its length.
The other approach is incorporating materials with tunable optical properties into the constituent material of the periodic nanophotonic devices. In this dissertation, a thin layer of phase-change material, Ge2Sb2Te5Ã Â¯Ã Â¿Ã Â½ (GST), in nanometers was embedded in the waveguide layer of a photonic crystal (PC) structure. The PC structure is based on a one-dimensional grating with a zinc sulfide waveguide. The GST-incorporated PC (GST-PC) structure supports the guided-mode resonance (GMR) that selectively absorbs light at particular wavelengths. The tuning effects were experimentally demonstrated by the crystallization or re-amorphization of the GST thin film. The GST-PC device opens a new path for tuning optical resonances in the near infrared region. Potential applications include color generation, display, optical storage, optical switches, and optical filters.
At last, a novel fabrication method for an ultrathin freestanding gold plasmonic membrane is proposed. The freestanding plasmonic membrane was characterized using FT-IR, and demonstrated to support extraordinary optical transmission in the mid infrared wavelength range. The effect of the thickness of gold was also investigated. This plasmonic device was utilized as a surface-based optical sensor by measuring the absorption of the stretching modes of chemical bonds in the Mid-IR.