  Highly confined and low-loss hyperbolic phonon polaritons (HPhPs) sustained
in van der Waals crystals exhibit outstanding capabilities of concentrating
long-wave electromagnetic fields deep to the subwavelength region. Precise
tuning on the HPhP propagation characteristics remains a great challenge for
practical applications such as nanophotonic devices and circuits. Here, we show
that by taking advantage of the varying air gaps in a van der Waals
{\alpha}-MoO3 crystal suspended gradiently, it is able to tune the wavelengths
and dampings of the HPhPs propagating inside the {\alpha}-MoO3. The results
indicate that the dependences of polariton wavelength on gap distance for HPhPs
in lower and upper Reststrahlen bands are opposite to each other. Most
interestingly, the tuning range of the polariton wavelengths for HPhPs in the
lower band, which exhibit in-plane hyperbolicities, is wider than that for the
HPhPs in the upper band of out-of-plane hyperbolicities. A polariton wavelength
elongation up to 160% and a reduction of damping rate up to 35% are obtained.
These findings can not only provide fundamental insights into manipulation of
light by polaritonic crystals at nanoscale, but also open up new opportunities
for tunable nanophotonic applications.
