Current Thermal Emission from Photonic Nanostructures Composed of TA, W, GE, and HFO2 Thin Films
AIR FORCE INSTITUTE OF TECHNOLOGY WRIGHT-PATTERSON AFB OH GRADUATE SCHOOL OF ENGINEERING AND MANAGEMENT
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Combining an understanding of thin films and structureproperty relationships, photonic nano-structures were developed in order to affect the spectral and directional radiative properties of coherent thermal emission. The targeted emission range was 2-20 micrometer. Structures with appropriate materials in order to achieve thermal stability were designed and tested. Implementing a Finite Difference Time Domain numerical method, four truncated multilayer resonators were designed to selectively emit at certain transmissive wavelength bands of the atmosphere in 2-20 micrometer. Ellipsometric measurements and models were used in order to extract the optical constants of thin layers of materials chosen for the resonator designs. These values showed significant disagreement with bulk values found in literature and were used to make more accurate theoretical predictions. A Direct Current Magnetron Sputtering technique was used to fabricate the four resonators, W-Ge-W, W-HfO2-W, Ta-Ge-Ta, and Ta-HfO2-Ta, with layer thicknesses of 10-760-145 nm, 10-701-145 nm, 9.6-728-169 nm, and 9.6-301-169 nm, respectively. Reflectance measurements were taken at room temperature and various high temperatures to investigate the thermal stability of the spectral reflectance of the structures. From these reflectance measurements, the W-Ge-W and Ta-Ge-Ta designs proved to hold up the best at higher temperatures.