Surface phonon polaritons (SPhPs) in van der Waals (vdW) materials are of great interest in fundamental and applied research fields. Probing the characteristics of vdW SPhPs at cryogenic temperatures is an essential task for their implementation in low-temperature physics. However, the most commonly used characterization technique of vdW SPhPs—scattering-type scanning near-field optical microscopy (s-SNOM) operating in a tapping mode (an intermittent-contact mode)—can be problematic at low temperatures because the sample being tested may become brittle and fragile. Therefore, high fracture toughness is desired for the samples under intermittent-contact s-SNOM scanning at low temperatures. In this work, by taking α-phase molybdenum trioxide (α-MoO3) as an example, we first confirm the potential surface deterioration induced by tip-sample interactions at low temperatures. Then, we propose to use few-layer hexagonal boron nitride as a mechanically tough yet optically passive cladding layer to enhance the surface stability of α-MoO3. Finally, we demonstrate the validity of our surface reinforcement strategy by probing the previously unexplored temperature dependence of SPhPs within the third Reststrahlen band of α-MoO3. Our method allows a sustained operation of tapping mode s-SNOM at cryogenic temperatures with negligible effect on intrinsic properties of SPhPs.

中文翻译：

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少层六方氮化硼作为脆性材料的屏蔽，用于声子极化子的低温 s-SNOM 探索

范德华 (vdW) 材料中的表面声子极化激元 (SPhP) 在基础和应用研究领域引起了极大的兴趣。在低温下探索 vdW SPhP 的特性是其在低温物理学中实施的一项重要任务。然而，最常用的 vdW SPhPs 表征技术——以轻敲模式（间歇接触模式）运行的散射型扫描近场光学显微镜 (s-SNOM)——在低温下可能会出现问题，因为被测试的样品可能会变得脆弱。因此，在低温间歇接触 s-SNOM 扫描下，样品需要高断裂韧性。在这项工作中，通过取α相三氧化钼（α-MoO3个）作为例子，我们首先确认在低温下由尖端-样品相互作用引起的潜在表面劣化。然后，我们建议使用少层六方氮化硼作为机械坚韧但光学被动的包覆层，以提高 α-MoO 的表面稳定性3个. 最后，我们通过探测 α-MoO 的第三个 Reststrahlen 带内 SPhP 的先前未探索的温度依赖性来证明我们的表面强化策略的有效性3个. 我们的方法允许在低温下持续运行轻敲模式 s-SNOM，而对 SPhP 的固有特性的影响可以忽略不计。