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On the steady-state temperature rise during laser heating of multilayer thin films in optical pump-probe techniques

[+] Author and Article Information
Jeffrey Braun

Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA
jb3gb@virginia.edu

Chester Szwejkowski

Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA
cjs5bx@virginia.edu

Ashutosh Giri

Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA
ag4ar@virginia.edu

Patrick Hopkins

Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA; Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, USA; Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA
phopkins@virginia.edu

1Corresponding author.

ASME doi:10.1115/1.4038713 History: Received June 22, 2017; Revised October 26, 2017

Abstract

In this study, we calculate the steady-state temperature rise that results from laser heating for multilayer thin films using the heat diffusion equation. For time- and frequency-domain thermoreflectance (TDTR and FDTR) relying on frequency modulated laser sources, we decouple the modulated and steady-state temperature profiles to understand the conditions needed to achieve a single temperature approximation throughout the experimental volume, allowing for the estimation of spatially invariant thermal parameters within this volume. We consider low thermal conductivity materials, including amorphous silicon dioxide (a-SiO2), polymers, and disordered C60, to demonstrate that often-used analytical expressions fail to capture this temperature rise under realistic experimental conditions, such as when a thin-film metal transducer is used or when pump and probe spot sizes are significantly different. To validate these findings and demonstrate a practical approach to simultaneously calculate the steady-state temperature and extract thermal parameters in TDTR, we present an iterative algorithm for obtaining the steady-state temperature rise and measure the thermal conductivity and thermal boundary conductance of a-SiO2 with a 65 nm gold thin film transducer. Furthermore, we discuss methods of heat dissipation to include the use of conductive substrates as well as the use of bidirectional heat flow geometries. Finally, we discuss the influence of the optical penetration depth (OPD) on the steady-state temperature rise to reveal that only when the OPD approaches the characteristic length of the temperature decay does it alter the temperature profile relative to the surface heating condition.

Copyright (c) 2017 by ASME
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