Phononic crystals (PC) are artificial periodic elastic media made for controlling the prop- agation of acoustic/elastic waves. The propagating modes of the elastic waves in PC are Bloch waves, and their dispersion relations form the phononic band structures, containing passbands and bandgaps. Various structures for reflecting, trapping, and guiding acous- tic/elastic waves relying on the existence of PC bandgaps have been proposed and studied [1–9]. Besides the bandgaps, the phononic passbands are also useful and have interesting properties. When operating in the linear region well below the top of the first band (i.e., the long-wavelength region), a PC behaves just like a homogeneous medium, and lens-like devices can be constructed from it [10, 11]. Recently, the study of wave propagation behav- iors beyond the long-wavelength region has attracted a great deal of interest. It has been shown that a slab of PC operating at an appropriate frequency can focus the acoustic waves emanating from a point source to form a subwavelength image, overcoming the diffraction limit [12, 13]. 

To achieve the slab imaging ability, either negative refraction or anomalous refraction is required [14–17], which means the direction of the averaged Pointying vector 〈 S 〉 must have a component antiparallel to the Bloch wavevector k or some effective Bloch wavevector k$_{ef}$ = k − k$_{0}$ , i,e., k$_{ef}$ · 〈 S 〉 < 0. Here k$_{0}$ stands for certain symmetry point in the first Brillouin zone or on the zone boundary, such as the M point for the square-lattice structure of PC. Recently, it has been pointed out that the PC band structure can also provide other wave guiding mechanisms such as self-collimation [18–21] and canalization [17, 22]. Using these mechanisms, waves can be guided along certain specific directions inside the slab and thus some desired devices such as hyperlens can be realized [23]. 

Up to now most of the studies on the negative refraction or subwavelength imaging phenomena adopted the PC that sustain pressure waves, and the discussions on similar phenomena in solid elastic media are relatively few [15–17]. In this letter, we employ the self-collimation effect of PC to design a flat lens for focusing off-plane pure shear wave. The lens consists of a triangular lattice hole-array embedded in a solid matrix. When the lens is illuminated by the off-plane shear waves emanating from a point source, a subwavelength image appears in the far-field zone. The imaging characteristics for a finite lens are inves- tigated by calculating the displacement fields using the Multiple Scattering (MS) method [11, 24]. The PC band structure is obtained with the Dirichlet-to-Neumann Maps (DtN) method [25, 26]. Effective negative refractive index of the PC can be extracted from the 

