For most acoustic metamaterials, once they have been fabricated, their operating frequencies and functions cannot be adjusted, which is an intrinsic barrier for development of realistic applications. The study to overcome this limit has become an urgent issue at the heart of acoustic metamaterial engineering. Although with the advance of metamaterials in the past two decades, a series of methods such as electric or magnetic control have been proposed, most of them can only work in the condition of no fluid passage. Some metamaterials with large transmission losses have been proposed, but the sounds are essentially reflected rather than absorbed. Here, to overcome this intrinsic difficulty, we propose a ventilated sound absorber that can be manually tuned in a large range after being manufactured. During the tuning which is 2 achieved through an intricately designed slider, high-performance absorption and ventilation are both ensured. The tunable ventilated sound absorber is demonstrated experimentally and the effective model of coupled lossy oscillators can be employed to understand its mechanism. The manually tunable ventilated metamaterial has the potential application values in various complicated pipe systems that require frequency adjustment and it also establishes the foundation for future development of active tunable ventilated acoustic metamaterials.

Single crystal Zn 3 N 2 films with (100) orientation have been grown by plasma-assisted molecular beam epitaxy on MgO and A-plane sapphire substrates with in situ optical reflectance monitoring of the growth. The optical bandgap was found to be 1.25-1.28 eV and an electron Hall mobility as high as 395 cm 2 V −1 s −1 was measured. The films were n-type with carrier concentrations in the 10 18 -10 19 cm −3 range.

Epitaxial Mg3N2 films with a (100) orientation have been grown by plasma-assisted molecular beam epitaxy on single crystal MgO substrates. The growth was monitored in situ by both reflection high-energy electron diffraction and optical reflectivity. The growth rate was determined from the optical reflectivity during growth. The index of refraction of Mg3N2 was measured by spectroscopic ellipsometry and found to be in good agreement with the in situ reflectivity. The optical bandgap was found to be ∼2.5 eV from transmission measurements.

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