Exploring novel properties of atomically smooth metal films as an ideal platform for nano photonics

Su, Ping-Hsiang
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Plasmonics is the science of manipulating light at the metal and dielectric interface. Therefore, a high-quality metal film plays a critical role in this exciting frontier of research. Among available plasmonic metals, silver (Ag) and aluminum (Al) is of particular interest. Ag is a widely used plasmonic material because of its intrinsic low loss. However, conventional thermal Ag films usually show rough surfaces (root-mean-square roughness is as large as several nanometers) and polycrystalline structures with grain boundaries. These features lead to significant plasmonic loss. Preparing an atomically smooth epitaxial Ag film is thus a very critical step in the developement of plasmonics. However, due to the pinning of grain boundaries by contaminants and the propensity to dewetting from commonly used semiconductor substrates (such as Si and GaAs), Ag is a very unforgiving material in the perspective of film growth. In this dissertation we are going to report novel methods to overcome these difficulties and realize the epitaxial growth of atomically smooth Ag films on Si(111). We will show that our films possess excellent optical properties with extraordinarily low loss. The significantly narrow distribution of the lifetime of giant quantum dots placed on our Ag film is another demonstration that our films perfectly apply to, and thus facilitate, the research in plasmonics and quantum photonics. In recent years Al is demonstrated to be an excellent platform for ultraviolet (UV) plasmonics. However, Al is highly reactive with oxygen and can be rapidly oxidized once exposed to even a low partial pressure of oxygen (10⁻⁸ Torr). Therefore it will be very challenging to prepare high-purity Al films. In this dissertation we are going to demonstrate the epitaxial growth of high-purity, atomically smooth Al films on Si(111) with different growth methods accompanied by an optimally controlled, ultraclean method of oxidation. Our epitaxial Al films demonstrate considerable loss reduction in the UV spectral range, in comparison to the polycrystalline Al films. These high-quality epitaxial Al films provide an ideal platform for UV plasmonics. Moreover, the availability of newly-obtained intrinsic optical constants on our single-crystalline Al films from the spectroscopic ellipsometry measurement enables a more accurate theoretical prediction in the design of nano-plasmonic devices.