Long-Lived Phonon Polaritons in Hyperbolic Materials

Abstract

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 -MoO 3 , among other materials. However, HPP dissipation at ambient conditions is substantial and its fundamental limits remain unexplored 1,2 . Here, we exploit cryogenic nano-infrared imaging to investigate propagating HPP in isotopically pure hBN and naturally abundant -MoO 3 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.

High-Q Phonon-polaritons in Spatially Confined Freestanding α-MoO3

“…This is clear evidence of geometric confinement, considering that, in RB3, numerically calculated Q factors from freestanding area (Figure a, red line) are slightly smaller than those from the supported area (Figure a, black line). The large Q factors of up to 40 in spatially confined freestanding α-MoO 3 is 2–4 times higher than previously reported values ,,, (see Table S1) in α-MoO 3 , even surpassing those obtained at the temperature as low as 50 K, where the temperature-dependent PhP damping is largely suppressed. Compared to the reduced temperature, submicron freestanding channels of hyperbolic materials presented in this work can be more conveniently realized and thus promising for real applications.…”

“…Within the x – y plane, hyperbolic PhPs propagate along the [001] and [100] directions in RB1 and RB2, respectively, and elliptic PhPs propagate in the x – y plane in RB3, resulting in anisotropic PhPs . The remarkably low loss of PhPs in α-MoO 3 , compared to counterparts such as boron nitride and graphene , can potentially lead to practical applications in IR signal processing and heat transfer . Due to strong crystallinity of each layer of α-MoO 3 , twistronics can also be readily experienced in this material. − Twisted double layer α-MoO 3 has been shown to be able to manipulate light at the nanoscale with topological transitions that bring huge potential to nanophotonics and polaritonics. − In addition to twisting the angle between adjacent α-MoO 3 layers, PhPs can also be engineered by other approaches, including patterning their microstructures, , changing the material composition, , and controlling the surrounding dielectric environment. , …”

Tunable Planar Focusing Based on Hyperbolic Phonon Polaritons in α‐MoO3

“…[ 10 ] Also, confined electromagnetic waves coupled to phonons in hyperbolic dielectrics are referred to as hyperbolic phonon polaritons (HPhPs). [ 14 ] In particular, orthorhombic molybdenum trioxide (α‐MoO 3 ) [ 10b ] and vanadium pentoxide (α‐V 2 O 5 ) [ 10c ] have been demonstrated to support biaxial HPhPs with extreme anisotropy in the mid‐IR to terahertz spectral region (corresponding to energies ranging from molecular vibrations to thermal emission and absorption), which stems from their highly anisotropic lattice vibrations along different principal axes. In contrast to artificial surfaces, these natural hyperbolic two‐dimensional (2D) materials support higher wave vectors and electromagnetic confinement in the hyperbolic regime and, thus, offer potential advantages in planar focusing.…”

Graphene as a source of entangled plasmons

“…Extension of these nonlinear effects to polaritonic modes [94][95][96] beyond those in graphene is a meaningful future direction. For example, monolayer hexagonal boron nitride [97,98] should exhibit strong second order optical nonlinearity due to broken inversion symmetry of the crystal lattice, and would be a natural platform for generating entanglement between the long lived hyperbolic phonon polaritons [99][100][101][102][103][104][105][106][107][108]. Similar nonlinear processes exist for optical phonons in SiC [109], Josephson plasmons in layered superconductors [110][111][112][113] and the collective modes in excitonic insulators [114][115][116][117][118].…”