AXT Inc. (AXTI) stock gained momentum following its addition to the S&P SmallCap 600 index on September 21, 2026, driven by passive fund buying and a broader risk-on market for AI and semiconductor stocks. The company's fundamental strength stems from record indium phosphide revenue and growing demand for InP substrates used in AI datacenter optical connectivity infrastructure. While index inclusion is the immediate catalyst, the underlying narrative of InP wafers supplying high-speed optical transceivers positions AXTI as a materials-level play on AI optics buildout.