  Conventional optical components are limited to size-scales much larger than
the wavelength of light, as changes in the amplitude, phase and polarization of
the electromagnetic fields are accrued gradually along an optical path.
However, advances in nanophotonics have produced ultra-thin, co-called "flat"
optical components that beget abrupt changes in these properties over distances
significantly shorter than the free space wavelength. While high optical losses
still plague many approaches, phonon polariton (PhP) materials have
demonstrated long lifetimes for sub-diffractional modes in comparison to
plasmon-polariton-based nanophotonics. We experimentally observe a three-fold
improvement in polariton lifetime through isotopic enrichment of hexagonal
boron nitride (hBN). Commensurate increases in the polariton propagation length
are demonstrated via direct imaging of polaritonic standing waves by means of
infrared nano-optics. Our results provide the foundation for a
materials-growth-directed approach towards realizing the loss control necessary
for the development of PhP-based nanophotonic devices.
