The ability to control, with atomic precision, heterostructures comprising two dissimilar electronic materials has provided us with an unprecedented opportunity: to tailor novel low-dimensional electronic systems. In this it has been responsible for the discovery of many novel physical phenomena and has facilitated new technologies. While tremendous success has been achieved using semiconductor heterostructures or metallic heterostructures, the same cannot be said for metal/semiconductor heterostructures, due to the fact that most metals do not wet on semiconductors. Over the last decade, however, significant progress has been made in epitaxial growth of metastable metallic thin films on semiconductor substrates. Most interesting is the discovery of an intriguing interplay between the quantum confinement of electronic systems and the surface free energy, leading to the so-called “quantum growth” phenomenon [1, 2]. In the last few years, tremendous research effort has been focused on the exploration of the novel physical properties enabled by the capability to tailor metallic thin films on semiconductor substrates. This talk will focus on two dramatically different applications pursued in our group: (a) atomic layer superconductivity [3-5], and (b) deep sub-diffraction limit plasmonic nanolasers [6].