Focus shift photothermal method for thermal diffusivity mapping

Abstract

In this work, we present a photothermal technique that measures at microscopic spatial scales the curvature of the surface of the sample due to the thermal expansion produced by a focused modulated laser beam. A modulated pump beam heats the sample surface and a probe beam delivered through the same optical fiber and slightly defocused probes the curvature, by analyzing the power collected by the same optical fiber. The collected power depends on the defocusing produced by the curvature of the surface induced by the thermal expansion. The phase delay is directly related to the thermal diffusivity of the sample at the impinging location. The scheme is based on the use of photonic technology borrowed from the optical communications field. In this way, the setup results extremely robust, both lasers (pump and probe) are inherently collinear, as they emerge from the same single mode fiber, and the detection is made through the same optical fiber.