Abstract: A pump beam is modulated at a first frequency and a modulated pump beam is used to periodically heat the surface of a semiconductor wafer at a location, thereby generating a disturbance at such location. Two probe beams are provided which are coherent with each other having different frequencies or phase. One probe beam is directed towards the location where the disturbance is generated and the other probe beam is directed towards the sample surface at a location away from the disturbance so that it is substantially unaffected by the disturbance but is subject to substantially the same environmental factors as the location where the disturbance is generated. Reflections of the two probe beams are combined and interfere at a detector, The detector output is analyzed to provide the normalized amplitude of the sidebands for determining the physical characteristics or composition of the wafer, including the dose of any ion implants.