Thermal Expansion-Recovery Microscopy (ThERM) for microstructural characterization
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Thermal Expansion-Recovery Microscopy (ThERM) for microstructural characterization Esteban A. Domené1, Nélida Mingolo2, and Oscar E. Martínez3 1 Laboratorio de Electrónica Cuántica (LEC), Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires (UBA), Buenos Aires, Argentina. 2 Laboratorio de Haces Dirigidos (LHD), Departamento de Física, Facultad de Ingeniería, Universidad de Buenos Aires (UBA), Buenos Aires, Argentina 3 Tolket SRL, Int. Güiraldes 2160, Pabellón I, Ciudad. Universitaria, CABA C1428EAH, Buenos Aires, Argentina. ABSTRACT In this work we compare two different detection schemes that are sensitive to the focus shift of a probe beam due to induced surface curvature. The technique on which both detection schemes are based is called ThERM (Thermal Expansion-Recovery Microscopy) and allows the retrieval of the thermal diffusivity at microscopic levels, hence mapping such magnitude over a sample surface. The induced thermal expansion defocuses the probe beam due to the surface deformation (curvature). The dependence of the defocusing with the pump modulation frequency yields the thermal diffusivity of the sample at the impinging location. The explored depth is controlled by the pump beam size. By scanning both beams, a complete map of the thermal diffusivity can be retrieved. INTRODUCTION There are in the literature many known photothermal techniques that allow thermal characterization with high spatial resolution, by heating up the sample with a pump beam and measuring the thermal response, either the thermoreflectance (change in the reflectivity with temperature) or photodeflection of a probe beam [1-5]. These techniques have very stringent requirements on pointing stability of the pump and probe beams [6]. Recently two new techniques have been introduced that are sensitive to the curvature of the thermal expansion of the surface [7,8]. The thermal properties are retrieved by performing nonlinear fits of frequency sweeps of the signal, where a cutoff frequency ω0 is obtained that only depends on thermal diffusivity and the pump beam size [6]. Therefore, the thermal diffusivity of the sample can be measured with microscopic resolution. In this work a detailed comparison of these two different detection schemes will be presented. The first technique [7] measures the change in the reinjection due to the defocusing of the probe beam arising from the curvature of the surface using confocal detection. The same optical fiber that delivers the beams to the surface of the sample is used as a pinhole for the detection. The other detection method [8] uses an astigmatic probe beam and a four quadrant detector to determine the change in astigmatism due to defocusing at the sample surface, induced by surface curvature.
DESCRIPTION OF THE TWO TECHNIQUES A schematic of the defocused confocal detection technique scheme can be seen in figure 1 and is discussed in more detail in [7]. Two laser beams of different wavelengths are combined into the same optical fiber, using fiber couplers
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