Development of photothermal imaging technique

Abstract

When an infrared laser beam is absorbed by the sample, a part or all of the light energy is converted into heat. If the exciting laser beam intensity is modulated, the heat is generated repeatedly at the modulation frequency. The absorption of optical energy leads to the generation and propagation of energy-carrying thermal waves as well as energy-carrying elastic waves. It therefore, appears that thermal and elastic waves based subsurface images can be recorded. In this thesis, we have developed an infrared optical technique that takes advantage of these rapidly damped waves to image structures beneath the surface of the measured sample inaccessible by conventional microscopy techniques. We have tested the developed technique on silicon carbide substrates irradiated with fluxes of H+ ions of different doses. The energy with which these ions were sent allows creating damages at a depth of approximately 60 microns beneath the surface. The developed technique was able to detect these subsurface defects with high precision. Furthermore, we were able to study the thermal transports in intact and bombarded regions using the same technique. We found that the damages created by ion-bombardment cause an important thermal resistance in the volume of the sample, which forces the heat flux to remain localized in the surface layers of the sample.