A microwave metamaterial with integrated power harvesting functionality

We present the design and experimental implementation of a power harvesting metamaterial. A maximum of 36.8% of the incident power from a 900 MHz signal is experimentally rectified by an array of metamaterial unit cells. We demonstrate that the maximum harvested power occurs for a resistive load close to 70 Ω in both simulation and experiment. The power harvesting metamaterial is an example of a functional metamaterial that may be suitable for a wide variety of applications that require power delivery to any active components integrated into the metamaterial. © 2013 AIP Publishing LLC

An invisibility cloak using silver nanowires

In this paper, we use the parameter retrieval method together with an analytical effective medium approach to design a well-performed invisible cloak, which is based on an empirical revised version of the reduced cloak. The designed cloak can be implemented by silver nanowires with elliptical cross-sections embedded in a polymethyl methacrylate host. This cloak is numerically proved to be robust for both the inner hidden object as well as incoming detecting waves, and is much simpler thus easier to manufacture when compared with the earlier proposed one [Nat. Photon. 1, 224 (2007)].Comment: 7 pages, 4 figures, 2 table

Multilayer metamaterial absorbers inspired by perfectly matched layers

We derive periodic multilayer absorbers with effective uniaxial properties similar to perfectly matched layers (PML). This approximate representation of PML is based on the effective medium theory and we call it an effective medium PML (EM-PML). We compare the spatial reflection spectrum of the layered absorbers to that of a PML material and demonstrate that after neglecting gain and magnetic properties, the absorber remains functional. This opens a route to create electromagnetic absorbers for real and not only numerical applications and as an example we introduce a layered absorber for the wavelength of $8$~$\mu$m made of SiO$_2$ and NaCl. We also show that similar cylindrical core-shell nanostructures derived from flat multilayers also exhibit very good absorptive and reflective properties despite the different geometry

Metamaterial bricks and quantization of meta-surfaces

Controlling acoustic fields is crucial in diverse applications such as loudspeaker design, ultrasound imaging and therapy or acoustic particle manipulation. The current approaches use fixed lenses or expensive phased arrays. Here, using a process of analogue-to-digital conversion and wavelet decomposition, we develop the notion of quantal meta-surfaces. The quanta here are small, pre-manufactured three-dimensional units—which we call metamaterial bricks—each encoding a specific phase delay. These bricks can be assembled into meta-surfaces to generate any diffraction-limited acoustic field. We apply this methodology to show experimental examples of acoustic focusing, steering and, after stacking single meta-surfaces into layers, the more complex field of an acoustic tractor beam. We demonstrate experimentally single-sided air-borne acoustic levitation using meta-layers at various bit-rates: from a 4-bit uniform to 3-bit non-uniform quantization in phase. This powerful methodology dramatically simplifies the design of acoustic devices and provides a key-step towards realizing spatial sound modulators

Acoustic cloak based on Bézier scatterers

[EN] Among the different approaches proposed to design acoustic cloaks, the one consisting on the use of an optimum distribution of discrete scatters surrounding the concealing object has been successfully tested. The feasibility of acoustic cloaks mainly depends on the number and shape of the scatterers surrounding the object to be cloaked. This work presents a method allowing the reduction of the number of discrete scatterers by optimizing their external shape, which is here defined by a combination of cubic Bézier curves. Based on scattering cancellation, a two-dimensional directional cloak consisting of just 20 Bézier scatters has been designed, fabricated and experimentally characterized. The method of fundamental solutions has been implemented to calculate the interaction of an incident plane wave with scatterers of arbitrary shape. The acoustic cloak here proposed shows a performance, in terms of averaged visibility, similar to that consisting of 120 scatterers with equal circular cross sections. The operational frequency of the proposed cloak is 5940 Hz with a bandwidth of about 110 Hz.J. Sanchez-Dehesa acknowledges the financial support by the Spanish Ministerio de Economia y Competitividad and the European Union Fondo Europeo para el Desarrollo Regional (FEDER) under Grant with Ref. TEC2014-53088-C3-1-R. Lu Zhimiao acknowledges the financial support from the program of China Scholarships Council (No. 201503170282), Wen Jihong, Cai Li and Lu Zhimiao acknowledge the support by National Natural Science Foundation of China (Grant Nos 51275519 and 11372346)Lu, Z.; Sanchis Martínez, L.; Wen, J.; Cai, L.; Bi, Y.; Sánchez-Dehesa Moreno-Cid, J. (2018). Acoustic cloak based on Bézier scatterers. Scientific Reports. 8. https://doi.org/10.1038/s41598-018-30888-7S8Cummer, S. A. & Schurig, D. One path to acoustic cloaking. New J. Phys. 9(3), 45 (2007).Cai, L.-W. & Sánchez-Dehesa Analysis of Cummer–Schurig acoustic cloaking. J. New J. Phys. 9(12), 450 (2007).Chen, H. & Chan, C. Acoustic cloaking in three dimensions using acoustic metamaterials. Appl. Phys. Lett. 91(18), 183518 (2007).Norris, A. N. Acoustic cloaking theory. Proc. R. Soc. A 464(2097), 2411–2434 (2008).Torrent, D. & Sánchez-Dehesa, J. Acoustic cloaking in two dimensions: a feasible approach. New J. Phys. 10(6), 063015 (2008).Zhang, S., Xia, C. & Fang, N. Broadband acoustic cloak for ultrasound waves. Phys. Rev. Lett. 106, 024301 Jan (2011).Popa, B.-I., Zigoneanu, L. & Cummer, S. A. Experimental acoustic ground cloak in air. Phys. Rev. Lett. 106, 253901 Jun (2011).Zigoneanu, L., Popa, B.-I. & Cummer, S. A. Design and measurements of a broadband two-dimensional acoustic lens. Nat. Mat 13, 352 (2014).Kan, W. et al. Broadband acoustic cloaking within an arbitrary hard cavity. Phys. Rev. Applied 3, 064019 Jun (2015).Scandrett, C. L., Boisvert, J. E. & Howarth, T. R. Acoustic cloaking using layered pentamode materials. J. Acoust. Soc. Am. 127(5), 2856–2864 (2010).Chen, Y. et al. Broadband solid cloak for underwater acoustics. Phys. Rev. B 95, 180104 May (2017).Alù, A. & Engheta, N. Achieving transparency with plasmonic and metamaterial coatings. Phys. Rev. E 72(1), 016623 (2005).Guild, M. D., Alu, A. & Haberman, M. R. Cancellation of acoustic scattering from an elastic sphere. J. Acoust. Soc. Am. 129(3), 1355–1365 (2011).García-Chocano, V. M. et al. Acoustic cloak for airborne sound by inverse design. Appl. Phys. Lett. 99(7), 074102 (2011).Sanchis, L. et al. Three-Dimensional Axisymmetric Cloak Based on the Cancellation of Acoustic Scattering from a Sphere. Phys. Rev. Lett. 110, 124301 Mar (2013).Andkjær, J. & Sigmund, O. Topology optimized for Airborne sound. ASME J. Vib. Acoust. 135(2), 041011 (2013).Guild, M. D. Acoustic Cloaking of Spherical Objects Unsing Thin Elastic Coatings. Univ. of Texas at Austin (2012).Guild, M. D., Haberman, M. R. & Alú, A. Plasmonic-type Acoustic cloak made of a bilaminate shell. Phys. Rev. B 86(10), 104302 (2012).Rohde, C. A. et al. Experimental demonstration of underwater acoustic scattering cancellation. Sci. Rep. 5, 13175 (2015).Popa, B.-I. & Cummer, S. A. Cloaking with optimized homogeneous anisotropic layers. Phys. Rev. A 79, 023806 Feb (2009).Urzhumov, Y., Landy, N., Driscoll, T., Basov, D. & Smith, D. R. Thin low-loss dielectric coatings for freespace cloaking. Opt. Lett. 38(10), 1606–1608 (2013).Andkjaer, J. & Sigmund, O. Topology optimized low-contrast all-dielectric optical cloak. Appl. Phys. Lett. 98(2), 021112 (2011).Climente, A., Torrent, D. & Sánchez-Dehesa, J. Sound focusing by gradient index sonic lenses. Applied Physics Letters 97(10), 104103 (2010).Håkansson, A., Sánchez-Dehesa, J. & Sanchis, L. Acoustic lens design by genetic algorithms Phys. Rev. B 70, 214302 Dec (2004).Håkansson, A., Cervera, F. & Sánchez-Dehesa, J. Sound focusing by flat acoustic lenses without negative refraction. Applied Physics Letters 86(5), 054102 (2005).Li, D., Zigoneanu, L., Popa, B.-I. & Cummer, S. A. Design of an acoustic metamaterial lens using genetic algorithms. The Journal of the Acoustical Society of America 132(4), 2823–2833 (2012).Prautzsch, H., Wolfgang Boehm, W. & Paluszny, M. Bézier and B-Spline Techniques. Springer Science & Business Media (2002).Andersen, P. R., Cutanda-Henríquez, V., Aage, N. & Sánchez-Dehesa, J. Viscothermal effects on an acoustic cloak based on scattering cancellation. Proceedings of the 6th International Conference on Noise and Vibration Emerging methods (NOVEM 2018 ), 171971, June (2018).Golberg, D. Genetic Algorithms in Search, Optimization and Learning. Addison Wesley, Reading, MA (1989).Kirkpatrick, S., Gelatt, C. D. & Vecchi, M. P. Optimization by simulated annealing. Science 220(4598), 671–680 (1983).Sanchis, L., Cryan, M. J., Pozo, J., Craddock, I. J. & Rarity, J. G. Ultrahigh Purcell factor in photonic crystal slab microcavities Phys. Rev. B 76, 045118 Jul (2007).Karageorghis, A. & Fairweather, G. J. The method of fundamental solutions for axisymmetric acoustic scattering and radiation problems. J. Acoust. Soc. Am. 104(6), 3212–3218 (1998).Fairweather, G., Karageorghis, A. & Martin, P. The method of fundamental solutions for scattering and radiation problems. Engineering Analysis with Boundary Elements 27(7), 759–769 (2003).Seybert, A. F., Soenarko, B., Rizzo, F. J. & Shippy, D. J. A special integral equation formulation for acoustic radiation and scattering for axisymmetric bodies and boundary conditions. J. Acoust. Soc. Am. 80(4), 1241–1247 (1986)

Optical Cloaking with Non-Magnetic Metamaterials

Artificially structured metamaterials have enabled unprecedented flexibility in manipulating electromagnetic waves and producing new functionalities, including the cloak of invisibility based on coordinate transformation. Here we present the design of a non-magnetic cloak operating at optical frequencies. The principle and structure of the proposed cylindrical cloak are analyzed, and the general recipe for the implementation of such a device is provided. The cloaking performance is verified using full-wave finite-element simulations.Comment: 10 pages, 4 figure

Experimental evidence of rainbow trapping and Bloch oscillations of torsional waves in chirped metallic beams

[EN] The Bloch oscillations (BO) and the rainbow trapping (RT) are two apparently unrelated phenomena, the former arising in solid state physics and the latter in metamaterials. A Bloch oscillation, on the one hand, is a counter-intuitive effect in which electrons start to oscillate in a crystalline structure when a static electric field is applied. This effect has been observed not only in solid state physics but also in optical and acoustical structured systems since a static electric field can be mimicked by a chirped structure. The RT, on the other hand, is a phenomenon in which the speed of a wave packet is slowed down in a dielectric structure; different colors then arrive to different depths within the structure thus separating the colors also in time. Here we show experimentally the emergence of both phenomena studying the propagation of torsional waves in chirped metallic beams. Experiments are performed in three aluminum beams in which different structures were machined: one periodic and two chirped. For the smaller value of the chirping parameter the wave packets, with different central frequencies, are back-scattered at different positions inside the corrugated beam; the packets with higher central frequencies being the ones with larger penetration depths. This behavior represents the mechanical analogue of the rainbow trapping effect. This phenomenon is the precursor of the mechanical Bloch oscillations, which are here demonstrated for a larger value of the chirping parameter. It is observed that the oscillatory behavior observed at small values of the chirp parameter is rectified according to the penetration length of the wave packet.Work partially supported by DGAPA-UNAM under projects PAPIIT IN103115 and IN109318 and by CONACYT project 284096. A.A.L. acknowledges CONACYT for the support granted to pursue his Ph.D. studies. G. Baez received CONACYT's financial support. RAMS received support from DGAPA-UNAM under program PASPA. We thank M. Martinez, A. Martinez, V. Dominguez-Rocha, E. Flores and E. Sadurni for invaluable comments. F.C., A.C. and J.S-D. acknowledge the support by the Ministerio de Economa y Competitividad of the Spanish government, and the European Union FEDER through project TEC2014-53088-C3-1-R.Arreola-Lucas, A.; Baez, G.; Cervera Moreno, FS.; Climente Alarcón, A.; Mendez-Sanchez, R.; Sánchez-Dehesa Moreno-Cid, J. (2019). Experimental evidence of rainbow trapping and Bloch oscillations of torsional waves in chirped metallic beams. Scientific Reports. 9:1860-1872. https://doi.org/10.1038/s41598-018-37842-7S186018729Ascroft, N. W. & Mermin, N. D. Solid State Physics (Hold, Reinhart & Winston, 1972).Kadic, M., Buckmann, T., Schittny, R. & Wegener, M. Metamaterials beyond electromagnetism. Rep. Prog. Phys. 76, 126501 (2013).Cummer, S. A., Christensen, J. & Alù, A. Controlling sound with acoustic metamaterials. Nat. Rev. Mat. 1, 16001 (2016).Tsakmakidis, K. L., Boarman, A. D. & Hess, O. Trapped rainbow storage of light in metamaterials. Nature 450, 397–401 (2007).Kathryn, H. et al. Designing perturbative metamaterials from discrete models. Nat. Mat. 17, 323–328 (2018).de Lima, M. M. Jr., Kosevich, Y. A., Santos, P. V. & Cantarero, A. Surface acoustic Bloch oscillations and Wannier-Stark ladders and Landau-Zenner tunneling in a solid. Phys. Rev. Lett. 104, 165502, https://doi.org/10.1103/PhysRevLett.104.165502 (2010).Tian, Z. & Yu, L. Rainbow trapping of ultrasonic guided waves in chirped phononic crystal plates. Sci. Rep. 7, 40004, https://doi.org/10.1038/srep40004 (2017).Waschke, C. et al. Coherent submillimeter-wave emission from bloch oscillations in a semiconductor superlattice. Phys. Rev. Lett. 70, 3319–3322, https://doi.org/10.1103/PhysRevLett.70.3319 (1993).Sapienza, R. et al. Optical analogue of electronic Bloch oscillations. Phys. Rev. Lett. 91, 263902 (2014).Morandotti, R., Peschel, U., Aitchison, J. S., S., E. H. & Silberberg, Y. Experimental observation of linear and nonlinear optical Bloch oscillations. Phys. Rev. Lett. 83, 4756 (1999).Battestti, R. et al. Bloch oscillations of ultracould atoms: a tool for a metrological determination of h / mRb. Phys. Rev. Lett. 92, 253001, https://doi.org/10.1103/PhysRevLett.92.253001 (2007).Sanchis-Alepuz, H., Kosevich, Y. & Sánchez-Dehesa, J. Acoustic analogue of electronic Bloch oscillations. Phys. Rev. Lett. 98, 134301, https://doi.org/10.1103/PhysRevLett.104.197402 (2007).Lanzilotti-Kimura, N. D. et al. Bloch oscillations of THz acoustic phonons in coupled nanocavity structures. Phys. Rev. Lett. 104, 197402, https://doi.org/10.1103/PhysRevLett.104.197402 (2010).Floß, J., Kamalov, A., Averbukh, I. S. & H., B. P. Observation of Bloch oscillations in molecular rotation. Phys. Rev. Lett. 115, 203002, https://doi.org/10.1103/PhysRevLett.115.203002 (2015).Gan, Q., Ding, Y. J. & Bartoli, F. Trapping and releasing at telecommunication wavelengths. Phys. Rev. Lett. 102, 056801, https://doi.org/10.1103/PhysRevLett.102.056801 (2009).Park, J., Boarman, A. D. & Hess, O. Trapping light in plasmonic waveguides. Opt. Express 18, 598–623, https://doi.org/10.1364/OE.18.000598 (2010).Zhao, D., Li, Y. & Zhu, X. Trapped rainbow effect in visible light left-handed heterostructures. Appl. Phys. Lett. 95, 071111, https://doi.org/10.1063/1.3211867 (2009).Smolyaninova, V. N., Smolyaninov, I. I., Kildishev, A. V. & Shalaev, V. Experimental observation of the trapped rainbow. Appl. Phys. Lett. 96, 211121, https://doi.org/10.1063/1.3442501 (2010).Ni, X. et al. Acoustic rainbow trapping by coiling up space. Sci. Rep. 4, 7038, https://doi.org/10.1038/srep07038 (2014).Zhu, J. et al. Acoustic rainbow trapping. Sci. Rep. 3, 1728, https://doi.org/10.1038/srep01728 (2013).Romero-García, V., Picó, R., Cebrecos, A., Sánchez-Morcillo, V. J. & Staliunas, K. Enhancement of sound in chirped sonic cristals. Appl. Phys. Lett. 102, 091906, https://doi.org/10.1063/1.4793575 (2013).Cebrecos, A. et al. Enhancement of sound by soft reflections in exponentially chirped cristals. AIP Adv. 4, 124402, https://doi.org/10.1063/1.4902508 (2014).Zhao, D., Li, Y. & Zhu, X. Broadband lamb wave trapping in cellular metamaterial plates with multiple local resonances. Sci. Rep. 5, 9376, https://doi.org/10.1038/srep09376 (2015).Gutierrez, L. et al. Wannier-stark ladders in one-dimensional elastic systems. Phys. Rev. Lett. 97, 114301, https://doi.org/10.1103/PhysRevLett.97.114301 (2006).Morales, A., Flores, J., Gutierrez, L. & Méndez-Sánchez, R. A. Compressional and torsional wave amplitudes in rods with periodic structures. J. Acoust. Soc. Am. 112, 1961, https://doi.org/10.1121/1.1509431 (2002).Arreola-Lucas, A. et al. Bloch oscillations in mechanical vibrations. PIERS proceedings. (to appear).Graff, K. F. Wave Motion in Elastic Solids (Dover, 1991)

Analogue Transformations in Physics and their Application to Acoustics

Transformation optics has shaped up a revolutionary electromagnetic design paradigm, enabling scientists to build astonishing devices such as invisibility cloaks. Unfortunately, the application of transformation techniques to other branches of physics is often constrained by the structure of the field equations. We develop here a complete transformation method using the idea of analogue spacetimes. The method is general and could be considered as a new paradigm for controlling waves in different branches of physics, from acoustics in quantum fluids to graphene electronics. As an application, we derive an analogue transformation acoustics formalism that naturally allows the use of transformations mixing space and time or involving moving fluids, both of which were impossible with the standard approach. To demonstrate the power of our method, we give explicit designs of a dynamic compressor, a spacetime cloak for acoustic waves and a carpet cloak for a moving aircraft.This work was developed under the framework of the ARIADNA contract 4000104572/11/NL/KML of the European Space Agency. A. M. and J. S.-D. also acknowledge support from Consolider EMET project (CSD2008-00066), A. M. from project TEC2011-28664-C02-02, J.S.-D. from US Office of Naval Research, and C. B. and G. J. from the project FIS2008-06078-C03-01. We thank Reme Miralles for her help with Fig. 2.García Meca, C.; Carloni, S.; Barcelo, C.; Jannes, G.; Sánchez-Dehesa Moreno-Cid, J.; Martínez Abietar, AJ. (2013). Analogue Transformations in Physics and their Application to Acoustics. Scientific Reports. 3(2009):1-5. https://doi.org/10.1038/srep02009S1532009Pendry, J. B., Schurig, D. & Smith, D. R. Controlling electromagnetic fields. Science 312, 1780–1782 (2006).Leonhardt, U. Optical conformal mapping. Science 312, 1777–1780 (2006).Schurig, D. et al. Metamaterial Electromagnetic Cloak at Microwave Frequencies. Science 314, 977–980 (2006).Shalaev, V. M. Transforming Light. Science 322, 384–386 (2008).Greenleaf, A., Kurylev, Y., Lassas, M. & Uhlmann, G. Invisibility and inverse problems. B. Am. Math. Soc. 46, 55–97 (2009).Genov, D. A., Zhang, S. & Zhang, X. Mimicking celestial mechanics in metamaterials. Nat. Phys. 5, 687–692 (2009).Chen, H., Chan, C. T. & Sheng, P. Transformation optics and metamaterials. Nat. Mater. 9, 387–396 (2010).Leonhardt, U. & Philbin, T. Geometry and light. The science of invisibility (Dover Publications, 2010).Pendry, J. B., Aubry, A., Smith, D. R. & Maier, S. A. Transformation Optics and Subwavelength Control of Light. Science 337, 549 (2012).Post, E. G. Formal Structure of Electromagnetics: General Covariance and Electromagnetics (Interscience Publishers, New York, 1962).Cummer, S. A. & Schurig, D. One path to acoustic cloaking. New J. Phys. 9, 45 (2007).Chen, H. & Chan, C. T. Acoustic cloaking in three dimensions using acoustic metamaterials. Appl. Phys. Lett. 91, 183518 (2007).Norris, A. N. Acoustic metafluids. J. Acoust. Soc. Am. 125, 839 (2009).Chen, H. & Chan, C. T. Acoustic cloaking and transformation acoustics. J. Phys. D: Appl. Phys. 43, 113001 (2010).Zhang, S., Genov, D. A., Sun, C. & Zhang, X. Cloaking of Matter Waves. Phys. Rev. Lett. 100, 123002 (2008).McCall, M. W., Favaro, A., Kinsler, P. & Boardman, A. A spacetime cloak, or a history editor. J. Opt. 13, 024003 (2011).Fridman, M., Farsi, A., Okawachi, Y. & Gaeta, A. L. Demonstration of temporal cloaking. Nature 481, 62–65 (2012).Cummer, S. A. & Thompson, R. T. Frequency conversion by exploiting time in transformation optics. J. Opt. 13, 024007 (2011).Barceló, C., Liberati, S. & Visser, M. Analogue Gravity. Living Rev. Relativity 14, 3 (2011).Visser, M. Acoustic black holes: Horizons, ergospheres and Hawking radiation. Class. Quant. Grav. 15, 1767 (1998).Barceló, C. & Jannes, G. A Real Lorentz-FitzGerald contraction. Found. Phys. 38, 191 (2008).Bergmann, P. G. The Wave Equation in a Medium with a Variable Index of Refraction. J. Acoust. Soc. Am. 17, 329 (1946).Torrent, D., Håkansson, A., Cervera, F. & Sánchez-Dehesa, J. Homogenization of two-dimensional clusters of rigid rods in air. Phys. Rev. Lett. 96, 204302 (2006).Torrent, D. & Sánchez-Dehesa, J. Effective parameters of clusters of cylinders embedded in a nonviscous fluid or gas. Phys. Rev. B 74, 224305 (2006).Unruh, W. G. Experimental black hole evaporation? Phys. Rev. Lett. 46, 1351 (1981).Li, J. & Pendry, J. B. Hiding under the Carpet: A New Strategy for Cloaking. Phys. Rev. Lett. 101, 203901 (2008).Popa, B. I., Zigoneanu, L. & Cummer, S. A. Experimental acoustic ground cloak in air. Phys. Rev. Lett. 106, 253901 (2011).Garay, L. J., Anglin, J. R., Cirac, J. I. & Zoller, P. Black holes in Bose-Einstein condensates. Phys. Rev. Lett. 85, 4643 (2000).Lahav, O., Itah, A., Blumkin, A., Gordon, C. & Steinhauer, J. Realization of a sonic black hole analogue in a Bose-Einstein condensate. Phys. Rev. Lett. 105, 240401 (2010).Castro Neto, A. H., Guinea, F., Peres, N. M. R., Novoselov, K. S. & Geim, A. K. The electronic properties of graphene. Rev. Mod. Phys. 81, 109 (2009).Cortijo, A. & Vozmediano, M. A. H. Electronic properties of curved graphene sheets. Europhys. Lett. 77, 47002 (2007).Vakil, A. & Engheta, N. Transformation optics using graphene. Science 332, 1291 (2011)

Broadband polygonal invisibility cloak for visible light

Invisibility cloaks have recently become a topic of considerable interest thanks to the theoretical works of transformation optics and conformal mapping. The design of the cloak involves extreme values of material properties and spatially dependent parameter tensors, which are very difficult to implement. The realization of an isolated invisibility cloak in the visible light, which is an important step towards achieving a fully movable invisibility cloak, has remained elusive. Here, we report the design and experimental demonstration of an isolated polygonal cloak for visible light. The cloak is made of several elements, whose electromagnetic parameters are designed by a linear homogeneous transformation method. Theoretical analysis shows the proposed cloak can be rendered invisible to the rays incident from all the directions. Using natural anisotropic materials, a simplified hexagonal cloak which works for six incident directions is fabricated for experimental demonstration. The performance is validated in a broadband visible spectrum