The Micromechanical Measurement of Photothermal Expansion †

The measurement of photothermal expansion via the deflection of a cantilevered probe has attracted attention as a tool for the investigation of condensed-phase materials. In the present work, I provide a simple theoretical analysis of signal generation by this micromechanical detection mechanism using the classical description of constrained thermal expansion. I relate signal intensity to the thermomechanical properties of the sample and the probe, and their contact geometry for the case of gradual excitation. The results support the general applicability of this configuration for the measurement of photothermal expansion and provide guidelines for optimization of the signal. I also provide a conceptual framework to analyze the measurement by describing it as a thermodynamic engine cycle, comprising a series of quasi-static processes. The engine is intrinsically inefficient when assessed in its capacity to convert energy into work, and the energy derived from light absorption is dispersed into the environment as heat. However, the thermodynamic efficiency is not a critical performance parameter when considering the main purpose of this mechanism, which is analytical.