Theories that seek to explain the outstanding puzzles of the Standard Model of particle physics often predict new, light, feebly-interacting particles whose discovery requires novel search strategies. The QCD axion is a unique beyond-the-standard-model particle: it elegantly solves the outstanding strong-CP problem and is a natural dark matter candidate. More general theories include light spin-0 axions and spin-1 dark photons. Their ultraweak interactions and macroscopic wavelengths call for a range of theoretical and experimental approaches to explore the open parameter space. In this talk, I will discuss our experimental proposal based on thin films, in which axion dark matter can efficiently convert to detectable optical photons. I will present new results from the LAMPOST prototype experiment searching for dark photon dark matter.