Schultz Jeremy F, Krylyuk Sergiy, Schwartz Jeffrey J, Davydov Albert V, Centrone Andrea
Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
Nanophotonics. 2024;13(9):1581-92. doi: 10.1515/nanoph-2023-0717.
Hyperbolic phonon polaritons (HPhPs), hybrids of light and lattice vibrations in polar dielectric crystals, empower nanophotonic applications by enabling the confinement and manipulation of light at the nanoscale. Molybdenum trioxide (α-MoO) is a naturally hyperbolic material, meaning that its dielectric function deterministically controls the directional propagation of in-plane HPhPs within its reststrahlen bands. Strategies such as substrate engineering, nano- and heterostructuring, and isotopic enrichment are being developed to alter the intrinsic die ectric functions of natural hyperbolic materials and to control the confinement and propagation of HPhPs. Since isotopic disorder can limit phonon-based processes such as HPhPs, here we synthesize isotopically enriched MoO (Mo: 99.93 %) and MoO (Mo: 99.01 %) crystals to tune the properties and dispersion of HPhPs with respect to natural α-MoO, which is composed of seven stable Mo isotopes. Real-space, near-field maps measured with the photothermal induced resonance (PTIR) technique enable comparisons of inplane HPhPs in α-MoO and isotopically enriched analogues within a reststrahlen band (≈820 cm to ≈ 972 cm). Results show that isotopic enrichment (e.g., MoO and MoO) alters the dielectric function, shifting the HPhP dispersion (HPhP angular wavenumber × thickness vs IR frequency) by ≈-7% and ≈ +9 %, respectively, and changes the HPhP group velocities by ≈ ±12 %, while the lifetimes (≈ 3 ps) in MoO were found to be slightly improved (≈ 20 %). The latter improvement is attributed to a decrease in isotopic disorder. Altogether, isotopic enrichment was found to offer fine control over the properties that determine the anisotropic in-plane propagation of HPhPs in α-MoO, which is essential to its implementation in nanophotonic applications.
双曲线声子极化激元（HPhPs）是极性介电晶体中光与晶格振动的混合体，通过在纳米尺度上实现光的限制和操纵，推动了纳米光子学应用的发展。三氧化钼（α-MoO）是一种天然的双曲线材料，这意味着其介电函数决定性地控制了其剩余射线带内平面内HPhPs的定向传播。目前正在开发诸如衬底工程、纳米和异质结构以及同位素富集等策略，以改变天然双曲线材料的本征介电函数，并控制HPhPs的限制和传播。由于同位素无序会限制诸如HPhPs等基于声子的过程，因此我们在此合成了同位素富集的MoO（Mo：99.93%）和MoO（Mo：99.01%）晶体，以相对于由七种稳定Mo同位素组成的天然α-MoO来调节HPhPs的性质和色散。用光热诱导共振（PTIR）技术测量的实空间近场图能够比较α-MoO和同位素富集类似物在剩余射线带（≈820 cm至≈972 cm）内的平面内HPhPs。结果表明，同位素富集（例如，MoO和MoO）改变了介电函数，分别使HPhP色散（HPhP角波数×厚度与红外频率）移动了≈-7%和≈+9%，并使HPhP群速度改变了≈±12%，而发现MoO中的寿命（≈3 ps）略有改善（≈20%）。后者的改善归因于同位素无序的减少。总之，发现同位素富集能够对决定α-MoO中HPhPs各向异性平面内传播的性质进行精细控制，这对其在纳米光子学应用中的实现至关重要。
