This Letter presents a solution for locating hot spots in active integrated circuits (IC) and devices. This method is based on sensing the phase lag between the power periodically dissipated by a device integrated in an IC (hot spot) and its corresponding thermal gradient into the chip substrate by monitoring the heat-induced refractive index gradient with a laser beam. The experimental results show a high accuracy and prove the suitability of this technique to locate and characterize devices behaving as hot spots in current IC technologies.