High-Resolution Imaging of Ultrasound in Dielectric Materials using Near-Field Scanning Optical Microscopy

In this work, models for scanning near-field optical probes based on dipole radiators are developed for representing the behaviors of these probes for high resolution detection of ultrasound in dielectric materials. The simplest case considered uses a vertically-oriented electric dipole radiator that is located a distance above a surface being displaced by ultrasound. The relatively high symmetry of this model geometry permits analytical representation of the fields radiated by the dipole including those associated with interactions with the material surface. The amplitude and the phase of the directly-reflected and the lateral wave fields depend on the material properties and on the distance of the dipole above the surface. When combined with the direct field, these fields coherently interfere to produce a radiation pattern above the surface that includes information about surface displacements associated with ultrasonic arrivals. In particular, the optical power radiated into the far-field can be monitored and used for ultrasound detection. Expressions for the radiated power are developed that include the dependence on material properties and explicitly show the contributions of the directly-reflected and lateral wavefields. These expressions are particularly simple when the material has limiting values of either electrical conductivity or dielectric permittivity, but the focus of the current work is on materials that cannot be described by these extreme property limits. Consideration of the general case permits a broader exploration of the ultrasonic signals that would be produced in these types of systems. This work examines the sensitivity of near-field probes to ultrasonic displacements and provides guidance on approaches to optimization of ultrasound detection using these types of probes.