Natural hyperbolic materials with dielectric permittivities of opposite sign along different principal axes can confine long-wavelength electromagnetic waves down to the nanoscale, well below the diffraction limit. This has been demonstrated using hyperbolic phonon polaritons (HPP) in hexagonal boron nitride (hBN) and -MoO3, among other materials. However, HPP dissipation at ambient conditions is substantial and its fundamental limits remain unexplored1,2. Here, we exploit cryogenic nano-infrared imaging to investigate propagating HPP in isotopically pure hBN and naturally abundant -MoO3 crystals. Close to liquid-nitrogen temperatures, the losses for HPP in isotopic hBN drop significantly, resulting in propagation lengths in excess of 25 micrometers, with lifetimes exceeding 5 picoseconds, thereby surpassing prior reports on such highly-confined polaritonic modes. Our nanoscale, temperature-dependent imaging reveals the relevance of acoustic phonons in hyperbolic polariton damping and will be instrumental in mitigating such losses for miniaturized middle infrared technologies operating at the liquid-nitrogen temperatures.
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Posted 10 Mar, 2021
Posted 10 Mar, 2021
Natural hyperbolic materials with dielectric permittivities of opposite sign along different principal axes can confine long-wavelength electromagnetic waves down to the nanoscale, well below the diffraction limit. This has been demonstrated using hyperbolic phonon polaritons (HPP) in hexagonal boron nitride (hBN) and -MoO3, among other materials. However, HPP dissipation at ambient conditions is substantial and its fundamental limits remain unexplored1,2. Here, we exploit cryogenic nano-infrared imaging to investigate propagating HPP in isotopically pure hBN and naturally abundant -MoO3 crystals. Close to liquid-nitrogen temperatures, the losses for HPP in isotopic hBN drop significantly, resulting in propagation lengths in excess of 25 micrometers, with lifetimes exceeding 5 picoseconds, thereby surpassing prior reports on such highly-confined polaritonic modes. Our nanoscale, temperature-dependent imaging reveals the relevance of acoustic phonons in hyperbolic polariton damping and will be instrumental in mitigating such losses for miniaturized middle infrared technologies operating at the liquid-nitrogen temperatures.
Figure 1
Figure 2
Figure 3
Figure 4
This preprint is available for download as a PDF.
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