ABSTRACT A thin beam of wave usually diverges due to diffraction, which is a limitation of any device using such waves. However, a surface acoustic wave (SAW) on a sphere with an appropriate aperture does not diverge but is naturally collimated, realizing ultramultiple roundtrips along an equator of the sphere. This effect is caused by the balance between diffraction and focusing on a spherical surface, and it enables realization of high-performance ball SAW sensors. The advantage of ball SAW is most fully appreciated when applied to a very thin sensitive film for which the multiple-roundtrip enhances the sensitivity, but the attenuation loss is not very large. It is exemplified in a hydrogen gas sensor that realizes a wide sensing range of 10 ppm to 100% for the first time, and realizes relatively fast response time of 20 s without heating the sensitive film.
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