  We present high quality factor optical nanoresonators operating in the mid-IR
to far-IR based on phonon polaritons in van der Waals materials. The
nanoresonators are disks patterned from isotopically pure hexagonal boron
nitride (isotopes 10B and 11B) and {\alpha}-molybdenum trioxide. We
experimentally achieved quality factors of nearly 400, the highest ever
observed in nano-resonators at these wavelengths. The excited modes are deeply
subwavelength, and the resonators are 10 to 30 times smaller than the exciting
wavelength. These results are very promising for the realization of
nano-photonics devices such as optical bio-sensors and miniature optical
components such as polarizers and filters.
