Photothermal microscopy

Photothermal microscopy is a noncontacting and nondestructive technique that employs laser light to excite and probe a diffusive thermal wave and uses the propagation of that wave to measure the material properties or dimensions of the sample on a microscopic scale.; We have developed a photothermal microscope to measure the directional thermal diffusivity as a function of temperature on a sample area of {dollar}sim{dollar}50 x 50 {dollar}mu{dollar}m{dollar}sp2{dollar}. The thermal diffusivity of YBa{dollar}sb2{dollar}Cu{dollar}sb3{dollar}O{dollar}sb{lcub}7-delta{rcub}{dollar} and Bi{dollar}sb2{dollar}Sr{dollar}sb2{dollar}CaCu{dollar}sb2{dollar}O{dollar}sb8{dollar} high-T{dollar}sb{lcub}rm c{rcub}{dollar} superconductors yields information on the electron-phonon scattering process, which plays an important role in the high-T{dollar}sb{lcub}rm c{rcub}{dollar} superconductivity mechanism. We have observed a sharp increase in diffusivity along the ab-plane below the superconducting transition, which indicates the decoupling of electrons and phonons.; The amplitude of the photothermal signal is proportional to the rate of change of reflectivity with temperature, a quantity closely related to the specific heat of the material. We have observed a sharp divergence in the amplitude signal at the phase transition of YBa{dollar}sb2{dollar}Cu{dollar}sb3{dollar}O{dollar}sb{lcub}7-delta{rcub}{dollar} and Bi{dollar}sb2{dollar}Sr{dollar}sb2{dollar}CaCu{dollar}sb2{dollar}O{dollar}sb8{dollar}. This phenomenon is the result of thermodynamic fluctuations near the phase transition. The unique coupling of the optical reflectivity to the electrons makes the amplitude signal an effective tool to investigate the mechanism of electronic phase transition.; We have also developed a high-frequency photothermal microscope that uses phase information of thermal waves at 3 to 500 MHz to measure metal film thicknesses in IC devices. With the microscope, we were able to measure various types of metal films down to a thickness of 150 A with a sensitivity of 30 A.; This dissertation presents our study on the theory of photothermal microscopy, the design and construction of two different microscopes, and the results of measurements on high-T{dollar}sb{lcub}rm c{rcub}{dollar} superconductors, a charge density wave sample, and various metal films. The wealth of new information supplied by the techniques we have developed suggests that photothermal microscopy is a highly effective technique to either probe structures in an opaque sample or to characterize the thermal properties as well as the thermodynamic fluctuation effects near an electronic phase transition of novel materials.