State-of-the-art concentrators use free-space optics to concentrate sunlight onto photovoltaic cells. To achieve high concentration factors it is necessary to track the sun's position. In current systems, mechanical actuators keep the focal spot in the solar cell. Planar concentrators have recently emerged, which use a waveguide slab to concentrate the sunlight. Here we demonstrate the development of a phase-change actuator (PCA) for self-adaptive tracking. The demonstrated mechanism is light-responsive and provides self-adaptive light concentration in a planar waveguide while maintaining efficient concentration over an angular range of +/- 16 degrees. The proposed system consists of a lens array to focus the sunlight, a waveguide slab acting as a concentrator, a dichroic prism membrane, splitting the solar spectrum into a visible (VIS) and infrared (IR) part, and the phase-change actuator. The actuator undergoes a phase change upon absorption of the IR light and vertically expands, creating a coupling feature upon contact with the waveguide. Visible light is then reflected off the prism membrane and efficiently coupled into the waveguide. As the focus spot moves, so does the coupling feature due to the light responsiveness of the actuator. We show an experimental proof-of concept prototype, highlighting the desired features of the concept. This is then further expanded by simulations of a full system achieving high effective concentrations (>100X) and first experimental results expanding the prototype to a full system.