The advent of metamaterials, e.g. artificially structured materials with physical properties significantly distinct from their bulk counterparts, has ushered in new perspectives in materials science and photonics, and constitutes today a new research frontier. Tailored order at the micro- or nanoscale has unlocked a host of novel opportunities in fields such as harmonic generation, wavefront shaping, and biochemical detection and analysis. Recent progress in dynamic tuning of metamaterial properties (active metamaterials) further promise to demultiply their impact, radically redefining functionality in modern optical devices. This progress has heavily relied on thin films, which consti-tutes fundamental building blocks for many micro- and nanoscale devices today. As dimensions of devices has shrunk, the question of thin film stability has become critical in numerous systems, given that their intrinsic high surface to volume ratio makes them particularly subject to instabilities. Such instabilities are potentially responsible for loss of percolation and disordered geometries, hence threatening both metamaterial integrity and functionality. In this thesis, we propose to investigate the stability of thin optically functional films for application in passive and active 2D-metamaterials (metasurfaces) along two complementary persepectives. In a first perspective, we investigate the possibility to guide pre-defined (or templated) instabilities in thin chalcogenide films to induce prescribed order at the micro and nanoscale through a diverse set of original processes, effectively providing a novel frame for scalable yet high performance metasurfaces. In a second perspective, thin film instabilities are supressed in highly unstable liquid metal thin films, opening up novel perspectives for electronic and optical nanostructures.