中文摘要：

文章从理论和实验上研究了二维三角晶格声子晶体第二能带具有回波（backward-wave）效应的负折射现象。Bragg散射将第二能带等频线（equifrequency surface）分成两类，它们分别凹向简约布里渊区（BZ）的K点和Г点，并由此导致了两种不同的回波负折射现象。理论和实验证明，这两类负折射的角度与声波的频率以及入射角的变化关系截然不同，同时在第二能带中，不仅存在回波负折射，还存在回波正折射，这两种折射所具有的回波效应都可以用于通过位相补偿效应来突破衍射极限。上述现象和左手材料中以及声子晶体第一能带中的折射现象是不相同的，揭示了声子晶体，第二能带具有更丰富的物理现象．

英文摘要：

Acoustic negative refraction with backward-wave （BW） effects have been both theoretically and experimentally established in the second band of a two-dimensional （2D） triangular sonic crystal. Intense Bragg scattering results in extreme deformation of the second band equifrequency surface （EFS） into two classes： one around the K point and the other around the Г point of the reduced Brillouin zone. The two classes can both lead to BW negative refraction but with reverse negative refraction dependence on frequency and incident angle. Not only BW negative refraction but also BW positive refraction can be present at equifrequency surfaces around the K point, so it is possible to enhance the resolution of acoustic waves beyond the diffraction limit regardless of refraction. This has no analogy in either left-handed materials or the first band of sonic crystals. These characteristics make refraction in the second band quite unique.