Long-Lived Phonon Polaritons in Hyperbolic Materials
Long-Lived Phonon Polaritons in Hyperbolic Materials
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DOI：
10.1021/acs.nanolett.1c01562
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发表时间：
2021-06-18
期刊：
影响因子：
10.8
通讯作者：
Basov, D. N.
中科院分区：
文献类型：
--
作者：
Ni, Guangxin;McLeod, Alexander S.;Basov, D. N.
自然双曲线材料的介电常数沿沿着不同的主轴相反的符号可以限制长波长的电磁波下降到纳米级，远低于衍射极限。在包括六方氮化硼（hBN）和α-MoO 3的双曲晶系中耦合到声子的受限电磁波被称为双曲声子极化激元（HPP）。HPP在环境条件下的耗散是相当大的，其基本限制仍然未被探索。在这里，我们利用低温nanoinfrared成像研究传播HPPs在同位素纯hBN和自然丰富的α-MoO 3晶体。接近液氮温度时，同位素hBN中HPP的损失显著下降，导致传播长度超过8 μ m，寿命超过5 ps，从而超过了先前关于这种高度受限的极化激元模式的报道。我们的纳米级温度依赖性成像揭示了声学声子在HPP阻尼中的相关性，并将有助于减轻在液氮温度下运行的小型化中红外技术的此类损失。
Natural hyperbolic materials with dielectric permittivities of opposite signs along different principal axes can confine long-wavelength electromagnetic waves down to the nanoscale, well below the diffraction limit. Confined electromagnetic waves coupled to phonons in hyperbolic dielectrics including hexagonal boron nitride (hBN) and alpha-MoO3 are referred to as hyperbolic phonon polaritons (HPPs). HPP dissipation at ambient conditions is substantial, and its fundamental limits remain unexplored. Here, we exploit cryogenic nanoinfrared imaging to investigate propagating HPPs in isotopically pure hBN and naturally abundant alpha-MoO3 crystals. Close to liquid-nitrogen temperatures, losses for HPPs in isotopic hBN drop significantly, resulting in propagation lengths in excess of 8 mu m, with lifetimes exceeding 5 ps, thereby surpassing prior reports on such highly confined polaritonic modes. Our nanoscale, temperature-dependent imaging reveals the relevance of acoustic phonons in HPP damping and will be instrumental in mitigating such losses for miniaturized mid-infrared technologies operating at liquid-nitrogen temperatures.