Theoretical Study for Detection of Defects in Weakly Absorbing Samples by Crossed-Beam Photothermal Technique

Abstract

The photothermal techniques have been successfully applied as tools for nondestructive evaluation (NDE) of inhomogeneous materials. For optically opaque samples, photothermal techniques are widely used for detection and characterization of subsurface defects and interfaces within the samples [1,2]. For optically transparent samples, photothermal techniques are used for absorption mapping of thin films deposited on optical substrates [3], biological and medical samples [4 ]. Due to nondestructive micro-analyses is very important for studies of thin films and interfaces, as well as biological, and medical samples. Photothermal techniques with two crossed-beams are developed for highly spatially resolved detection, in which the excitation and probe beams interact only in the intersection volume, therefore very high spatial resolution in three dimensions can be achieved by tightly focusing both beams. Highly resolved depth profiling of weakly absorbing samples can be achieved with the photothermal crossed-beam techniques by scanning either the samples or the intersection point of both beams along the depth direction. The optical absorbency or thermal conductivity inhomogeneities of small, localized regions within larger samples can be detected by the technique. In this paper the theoretical investigation on the depthprofiling capability of the crossed-beam photothermal deflection (PTD) is described in detail. The theoretical results provide quantitative evaluation for depth-profiling capability of weakly absorbing samples with crossed-beam photothermal techniques.