The overall objective of this research is to understand how light interacts with topological insulator (TI) films and layered structures.  Unlike normal materials, the electrons in TI films are trapped at the top and bottom surfaces of the film.  These electrons have unusual properties, including low mass and high velocity.  Light shining on these trapped electrons will excite electron density waves, called plasmons, which inherit the unusual properties of the electrons.  This project aims to understand how these plasmons interact with each other and how the plasmon properties change as the film dimensions change.  By controlling the physical properties of the films, the optical response of the film can also be controlled.  In addition to single TI films, the project will also investigate the properties of stacks of TI films layered with normal insulating films.  Stacking these materials results in multiple layers of trapped electrons whose plasmons can interact in ever more complex ways.  After these interactions are understood,  we can begin to engineer complex TI structures to obtain designer optical phenomena in the far-infrared and THz, wavelength ranges of interest for environmental monitoring and chemical sensing.  This research directly addresses DOE Grand Challenges, including understanding how properties of matter emerge from complex electronic correlations and learning how to control these properties as well as the mission of the Basic Energy Sciences program to understand and control matter at the electronic/atomic level.