Capturing the Cumulative Effect in the Pump Probe Transient Thermoreflectance Technique using Network Identification by
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Capturing the Cumulative Effect in the Pump Probe Transient Thermoreflectance Technique using Network Identification by Deconvolution Method Y. Ezzahri,1* G. Pernot2, K. Joulain1 and A. Shakouri2 1
Institut Pprime, CNRS-Université de Poitiers-ENSMA, Département Fluides, Thermique, Combustion, ENSIP-Bâtiment de mécanique, 2 rue Pierre Brousse, F 86022 Poitiers, Cedex, France. 2
Department of Electrical Engineering, University of California at Santa Cruz, 1156 High street, Santa Cruz, California, 95064, USA. *[email protected]
ABSTRACT Network Identification by Deconvolution (NID) method is used to capture the heat cumulative effect in the homodyne configuration of the Pump-Probe Transient Thermoreflectance (PPTTR) experiment. This cumulative effect is very important in the interpretation of the PPTTR which is becoming widely used for the extraction of thin film thermal conductivity. We show that this intrinsic behavior can be introduced as a cumulative effect weight function in the time constant spectrum of the structure under study. We show how the main features of this weight function change when we change the laser repetition rate and/or the laser pump beam modulation frequency, and how these changes may affect the extraction of the thermal properties of the sample under study, particularly the thermal conductivity and the interface thermal resistance. Numerical simulations of the PPTTR experiment are used to validate the application of NID method. Limitations of the method will also be discussed. INTRODUCTION Pump-Probe Transient Thermoreflectance (PPTTR) is an interesting technique whose first utilization to study thermal transport experimentally was reported by Paddock and Eesley [1]. For nearly two decades, PPTTR technique has been an effective tool for studying thermal transport in thin films, low dimensional structures (multilayers and superlattices) [2] and recently liquids [3]. In contrast to the 3ω method [4], PPTTR allows for the distinction between the thermal conductivity of thin film and the metal transducer-semiconductor interface thermal resistance [2]. In this technique, an intense short laser pulse “pump” is used to heat the film, and a delayed weak (soft) short laser pulse “probe” is used to monitor the top free surface reflectivity change induced by the cooling of the thin film after absorption of the pump pulse. The pump and probe can come from the same primary laser oscillator source, a configuration called homodyne PPTTR [5], or they can be issued from two locked laser oscillator sources, a configuration called heterodyne PPTTR [6]. Because in a PPTTR technique, high laser repetition rates are generally used, in many situations, the system under study does not have sufficient time to return to its equilibrium state between two successive laser pulses. The effect of the multiple pulses accumulates and the measured thermal decay can significantly differ from the response to as single pulse. Many authors have tackled the question of the accumulation phenomenon in PPTTR experimen
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