The Interferometric Detection of Ultrafast Pulses of Laser Generated Ultrasound.

Abstract

Abstract : A novel technique for nondestructively measuring the thickness and properties of thin coatings has been developed. High frequency pulses of ultrasound are generated in a coating using ultrafast (100 fs) pulses of laser light These ultrasonic pulses are used to inspect thin coatings (30 nm -500 nm) in a pulse-echo mode. Experimental results demonstrating the interferometers capabilities are presented. The thickness of a 25 nm gold coating. a 67 nm aluminum coating. and a 170 nm molybdenum coating were measured using ultrasonic echoes. The frequency spectrum of the echoes from the gold coating extended up to 100 GHz for displacements as small as 4 pm. The piezoreflective detection technique is not suitable for materials which are weakly piezoreflective such as gold and molybdenum. The interferometric detection technique, which is not material dependent however, was able to detect multiple ultrasonic echoes in these two materials and therefore extends the utility of ultrafast techniques. The feasibility of this inspection technique for thin coatings has been demonstrated, and there is great potential for this technique to become an important tool for characterizing the properties and methodologies of thin coatings.