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.
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