An ellipsometer with nanosecond time resolution is developed. The ellipsometer is a passive type single wavelength ellipsometer adopting a DOAP (division-of-amplitude photopolarimeter) configuration. Using the ellipsometer, the phase transformation of the Ge-Sb-Te (GST) alloy caused by a high power nanosecond pulsed laser was monitored in real time. The result indicates that the phase transformation is a two-step process-the fast nucleation-dominant stage followed by the slow anomalous grain growth stage, similar to the isothermal phase transition of GST in the temperature range of 120-145 degreesC. (C) 2004 Elsevier B.V. All rights reserved.