ABSTRACT We developed a picosecond transient thermoreflectance (ps-TTR) system for thermal property characterization, using a low-repetition-rate picosecond pulsed laser (1064 nm) as the heating source and a 532 nm CW laser as the probe. Low-repetition-rate pump eliminates the complication from thermal accumulation effect. Without the need of a mechanical delay stage, this ps-TTR system can measure the thermal decay curve from 500 ps up to 1 ms. Three groups of samples are tested: bulk crystals (glass, Si, GaAs, and sapphire); MoS2 thin films (157 ~ 900 nm thickness); InGaAs random alloy and GaAs/InAs digital alloy (short period superlattices). Analysis of the thermoreflectance signals shows that this ps-TTR system is able to measure both thermal conductivity and interface conductance in nanostructures. The measured thermal conductivity values in bulk crystals, MoS2 thin films, and InGaAs random alloy are all consistent with literature values. Cross-plane thermal conductivity in MoS2 thin films does not show obvious thickness dependence. Thermal conductivities of GaAs/InAs digital alloys are smaller than InGaAs random alloy, due to the efficient scattering at interfaces. We also discuss the advantages and disadvantages of this newly developed ps-TTR system comparing with the popular time-domain thermoreflectance system.

中文翻译：

---

用于热导率表征的皮秒瞬态热反射

摘要 我们开发了一种用于热特性表征的皮秒瞬态热反射 (ps-TTR) 系统，使用低重复率皮秒脉冲激光器 (1064 nm) 作为加热源，使用 532 nm CW 激光器作为探头。低重复率泵消除了热积聚效应的复杂性。无需机械延迟阶段，该 ps-TTR 系统可以测量从 500 ps 到 1 ms 的热衰减曲线。测试了三组样品：块状晶体（玻璃、Si、GaAs 和蓝宝石）；MoS2薄膜（157~900 nm厚度）；InGaAs 随机合金和 GaAs/InAs 数字合金（短周期超晶格）。热反射信号的分析表明，该 ps-TTR 系统能够测量纳米结构中的热导率和界面电导率。在块状晶体、MoS2 薄膜和 InGaAs 随机合金中测得的热导率值均与文献值一致。MoS2 薄膜的跨平面热导率没有表现出明显的厚度依赖性。由于界面处的有效散射，GaAs/InAs 数字合金的热导率小于 InGaAs 随机合金。我们还讨论了这种新开发的 ps-TTR 系统与流行的时域热反射系统相比的优缺点。