# Recent Advances in Non-Traditional Elastic Wave Manipulation by Macroscopic Artificial Structures
## 4 Conclusions  
MTMs present a new way to manipulate waves. However, in EMTMs, the longitudinal and transverse elastic waves are coupled, so they cannot be analyzed using techniques that apply to electromagnetic and acoustic waves. Many important technical challenges must be solved before EMTMS are appropriate for real-world applications. EMTMs manipulate and control the elastic waves by arranging micro-structures to exploit Bragg scattering or local resonance. To understand the current state of EMTMs, important studies that have been conducted are selected and represented. We introduced representative studies of locally resonant EMTMs. We explained how to handle elastic waves by using GRIN PC lenses. We have also described hyperlenses for super-resolution beyond the diffraction limit. Cloaking MTMs that effectively block elastic waves have been introduced. Also, a new budding field on Willis parameters is discussed. Elastic MSs have been introduced to overcome the limitations of existing EMTMs. In addition to these applications, EMTM-related  
Figure 12: **(a)** Schematic of the MS. \(L,l,w_{0}\) are the widths of the supercell, subunit, groove, respectively and \(h\) is the thickness of MS; **(b)** Numerical simulations of the wave field of the MS composed of four subunits. Upper side: vertical incident wave. Below side: oblique incident wave (\(30^{\circ}\)). Black arrows: analytical predictions; **(c)** Numerical simulations of the wave field of the MS composed of two subunits. Upper side: vertical incident wave. Below side: oblique incident wave (\(30^{\circ}\)). Black arrows: analytical predictions; **(d)** Unit cell consisting of two independent substructures (left) and the mass-spring model of the unit cell (right); **(e)** Upper side: simulation of longitudinal wave with \(d\varphi/dy=\pi/6h\) (left), \(d\varphi/dy=\pi/8h\) (center) and simulation of focal lens with its focal length is 0.15 m (right). Lower side: corresponding theoretical and discrete phase profile of upper side. (**a–c**) adapted from [149]. **(d,e)** adapted from [153].  
technologies are expected to advance rapidly due to other kinds of EMTM applications such as mechanical MTMs [154; 155]. This would provide deeper understanding of elastic wave manipulation, energy harvesting, super-resolution imaging, and seismic wave shielding.  
[PERSON], [PERSON] wrote the paper. [PERSON] guided the entire paper. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by the K-Cloud grant (No. 2018-TECH-04) funded by the Korea Hydro & Nuclear Power Co. Ltd., Republic of Korea. This work was also partially supported from the National Research Foundation of Korea (NRF) grants (NRF-2019R1A2C3003129, CAMM-2019M3A6B3030637, NRF-2019R1A5A8080290, NRF-2018M3D1A1058998, NRF-2015R1A5A1037668) funded by the Ministry of Science and ICT (MSIT) of the Korean government. [PERSON] acknowledges support for the Global Ph.D. Fellowship (NRF-2018H1A2A1062053) from NRF-MSIT of the Korean government.  
Special thanks to [PERSON] for helping to check our manuscript.  
The authors declare no conflict of interest.