We show that a sonic crystal made of periodic distributions of rigid cylinders in air acts as a new material which allows the construction of refractive acoustic devices for airborne sound. It is demonstrated that, in the long-wave regime, the crystal has low impedance and the sound is transmitted at subsonic velocities. Here, the fabrication and characterization of a convergent lens are presented. Also, an example of a Fabry-Perot interferometer based on this crystal is analyzed. It is concluded that refractive devices based on sonic crystals behave in a manner similar to that of optical systems.

This Letter presents the design, fabrication, and experimental characterization of a directional three-dimensional acoustic cloak for airborne sound. The cloak consists of 60 concentric acoustically rigid tori surrounding the cloaked object, a sphere of radius 4 cm. The major radii and positions of the tori along the symmetry axis are determined using the condition of complete cancellation of the acoustic field scattered from the sphere. They are obtained through an optimization technique that combines genetic algorithm and simulated annealing. The scattering cross section of the sphere with the cloak, which is the magnitude that is minimized, is calculated using the method of fundamental solutions. The low-loss fabricated cloak shows a reduction of the 90% of the sphere scattering cross section at the frequency of 8.55 kHz.

In this work we use multiple scattering in conjunction with a genetic algorithm to reliably determine the optimized photonic-crystal-based structure able to perform a specific optical task. The genetic algorithm operates on a population of candidate structures to produce new candidates with better performance in an iterative process. The potential of this approach is illustrated by designing a spot size converter that has a very low F-number (F=0.47) and a conversion ratio of 11:1. Also, we have designed a coupler device that introduces the light from the optical fiber into a photoniccrystal-based waveguide with a coupling efficiency over 87% for a wavelength that can be tuned to 1.5 µm.42. 42.25.Fx;42.82.Bq; A new generation of optical devices is envisaged thanks to the properties of photonic crystals (PC's).1 Though the recent advances in three-dimensional PC's structures, in the last years much attention has been focused on systems based on two-dimensional (2D) PC's because of their easiness in the fabrication process. Thus, very compact optical devices and circuits can be designed by introducing point and/or line defects. In order to use such PC circuits in actual applications it is necessary to establish a connection with an optical fiber. However, the core of the optical fiber is about one order of magnitude larger than the PC-based waveguide. Therefore, the design of an efficient (low loss) spot size converter is a crucial goal in the field of PC; its solution will introduce the PCs devices in the market place. In this regard, several groups 2-7 have tackled this problem by using different approaches. Most of them proposed tapered waveguide structures 2-5 , or by using reflective structures to focus the light into the waveguide.6 A different approach consists of using the anisotropy of the PC's equifrequency surfaces. 7This letter introduces a method that is useful in determining the optimized configuration of a 2D-PC structure capable of performing a requested optical task with high efficiency . The method is illustrated by finding a spot size converter (lens) that has a conversion ratio 11:1. In addition, the designed PC structure that involves a spot-size converter in connection with a PC-based waveguide it is presented. The insertion loss predicted for this new structure is about 13%, which is of the lowest reported by numerical simulations based on different coupling mechanisms. 2-6Our method is based on a binary-coded genetic algorithm (GA), an optimization strategy inspired by Darwinian evolution 8 . This method has been applied to solve a wide variety of problems in different fields like, for example, molecular geometry optimization 9 , material design 10 , and artificial intelligence 11 . In the field of optics, the GA has been employed in the synthesis of Bragg gratings that conform to a particular spectrum, 12 , phase recovering from a fringe pattern 13 , and in designing irregular lateral tapering. 2Although our proposal is general and applicable to any dimensionality, here we analyze 2D-PCs f...

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Design of an acoustic metamaterial lens using genetic algorithms

A survey of acoustic devices for focusing airborne sound is presented. We introduce a new approach to design high quality acoustic lenses based on arrays of cylindrical rigid scatterers in air. A population based stochastic search algorithm is used in conjunction with the multiple scattering theory to optimize a cluster of cylinders that focuses the sound in a prefixed focal point.Various lenses of different sized clusters, for different frequencies and with different focal lengths are presented. In general three focusing phenomena are remarked, focusing due to refraction, diffraction and focusing due to multiple scattering. The dependency on the frequency of the incident sound and the focal distance is analyzed indicating that higher frequencies and smaller focal distances favour larger amplifications in thin lenses based on multiple scattering. Furthermore, the robustness of a designed acoustic lens is studied, examining the focusing effect against errors in the cylinders' positions and their radius. PACS numbers: 43.20.+g,43.90.+v,45.10.Db

An analysis of the reflectance of sonic band-gap crystals consisting of square arrays of rigid cylinders in air is presented. The standing wave formed in front of the structures is studied both experimentally and theoretically. Experiments have been performed with a mobile robotized microphone that obtains pressure maps on the plane perpendicular to the axes of the cylinders. Enhancements of the standing wave ratio (SWR) are observed in frequency regions where attenuation bands appear in zero-order transmission experiments. Also, the SWR presents oscillations that can be related to the finite dimension of the structure (Fabry-Perot effect). Both features are well described by calculations based on a double-scattering approach.

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