The possibility to measure the glass transition temperature in poly(methyl methacrylate) (PMMA) films by picosecond ultrasonics with thicknesses ranging from 458 nm to 32 nm is demonstrated. A shift of the longitudinal acoustic eigenmodes towards lower frequencies with temperature is observed accompanied by a change in the temperature-frequency slopes at the glass transition temperature. The contributions to the frequency shift from changes in film thickness and sound velocity are discussed and the latter is extracted below the glass transition temperature. Finally, the advantages and disadvantages of the current approach in a comparison to other methods based on acoustic measurements in the GHz regime are reviewed.

证明了通过皮秒超声波测量聚（甲基丙烯酸甲酯）（PMMA）薄膜玻璃化转变温度的可能性，厚度范围为 458 nm 到 32 nm。观察到纵向声学本征模式随温度向较低频率的转变，伴随着玻璃化转变温度下温度-频率斜率的变化。讨论了薄膜厚度和声速变化对频移的贡献，后者在玻璃化转变温度以下被提取。最后，回顾了当前方法与基于 GHz 范围内声学测量的其他方法相比的优缺点。