Ultra-broadband high-resolution photoacoustic / photothermal microscopy system for material characterization

Abstract

A non-contact ultra-broadband photoacoustic (PA) / photothermal (PT) microscopy system has been developed to characterize material properties of specimens using optical transduction techniques. PT microscopy exploits optical changes induced by heat to highlight the presence of inhomogeneities such as defects, contaminants, inclusions, and impurities in materials. A monochromatic light source (e.g., a pulsed and amplitude-modulated laser) typically is used to create the PA effect. Heating the material produces a stress distribution that launches broadband ultrasonic emissions. Measurement of the ultrasonic emissions can be used to compute material properties like density, elastic modulus, anisotropy, etc. Sub-surface features can be detected using time-reversal and back-propagation techniques. In this work, PT-induced refractive index changes as well as the PA effect are detected optically on a microscopic scale using a Michelson-interferometer configuration. The system has a spatial resolution of ~600 nm with a detection bandwidth of 1 GHz and a displacement sensitivity of 1 pm per root Hz. Experimental results from thin films, coatings, nanoelectromechanical systems (NEMS) and biological samples demonstrate the versatility of the system as a nondestructive tool for material characterization.