  High index optical waveguide devices such as slab waveguides, strip
waveguides and fibers play extremely important roles in a wide range of modern
applications including telecommunications, sensing, lasing, interferometry, and
resonant amplification. Yet, transposing these advantageous applications from
optics to acoustics remains a fundamental practical challenge, since most
materials exhibit refractive indices lower than that of air for sound waves.
Here, we demonstrate the relevance of acoustic metamaterials for tackling this
pivotal problem. More specifically, we consider a metamaterial built from
subwavelength air-filled acoustic pipes engineered to effectively exhibit a
higher refractive index than homogenous air. We show that such medium can be
employed to realize acoustic equivalents of dielectric slab or strip
waveguides, and optical fibers. Unlike conventional acoustic pipes, our guiding
approach allows the waveguide to remain open to the external medium, which
opens a plethora of new opportunities in noise management, medical imaging,
underwater communication systems, and sensing.
