Light is fast — but not infinitely fast. When electronic circuits become as small as atoms and information travels in the picosecond scale (10^-12 s), detectors must react to ultrafast changes reliably and return to equilibrium just as quickly. Solid-state chemistry offers a solution in the form of topological semimetals: highly symmetric quantum materials with high electron mobility. As part of this class of compounds, GdSbTe has a square-net structure whose vibrations are tunable according to Sb content. This offers a way to relate the light-induced behavior of this material to its chemical structure. In my research, I synthesize GdSb(x)Te(2-x-δ) single crystals using chemical vapor transport, and characterize them using X-ray crystallography. I then quantify the ultrafast response of these crystals to light-excitation in the mid-infrared using pump-probe spectroscopy. By analyzing data collected by Dr. Robert Kirby, I found an interplay between the amount of light (excitation fluency) and the formation of coherent vibrations (phonons) in this material. The relation indicates a possible photo-induced phase transition, which might hold new physics and applications in photovoltaic cells. Moving forward, I will investigate this transition for different Sb contents 0<x<1. In the long-term, I seek a path to refine the search for light-tunable (photoswitch-like) materials in the square-net topological semimetals.