Lorenzo Sanchis

Acoustic cloak for airborne sound by inverse design

This Letter presents practical realization of a two-dimensional low loss acoustic cloak for airborne sound obtained by inverse design. The cloak consists of 120 aluminum cylinders of 15 mm diameter surrounding the cloaked object—a cylinder of diameter 22.5 cm. The position of each cylinder in the cloak is optimized using the data from two different techniques: genetic algorithm and simulated annealing. The operation frequency of this cloak is 3061 Hz with the bandwidth of about 100 Hz. Being a multi-step approach to the desired cloaking, the inverse design is also valid, in principle, for non-symmetric cylinders and even for three-dimensional objects.

Inverse design of photonic crystal devices

This paper deals with the inverse design in the field of photonic crystal (PC)-based devices. Here, an inverse method containing a fast and accurate simulation method integrated with a competent optimization method is presented. Two designs yielded from this conjunction of multiple scattering theory with a genetic algorithm are analyzed. The potential of this approach is illustrated by designing a lens that has a very low F-number (F=0.47) and a conversion ratio of 11:1. We have also designed a coupler device that introduces the light from an optical fiber into a PC-based waveguide with a predicted coupling efficiency that exceeds 87%.

Three-dimensional acoustic lenses with axial symmetry

In this paper a technique to design three dimensional (3D) devices to focus acoustic waves composed of scattering elements is proposed. The devices are designed and optimized in two dimensions (2D) with the help of a genetic algorithm and the 2D multiple scattering formalism. The transition from 2D to 3D is made by applying a rotation operation to the optimized design, thus passing from a set of 2D circular scatters to their equivalent 3D concentric rings of circular section and finite dimensions, considerably improving its performance. The method has been applied to the design and theoretical characterization of a single-focus acoustic lens and a tunable lens capable of changing the focal length with frequency. A prototype lens was fabricated using aluminum rings clamped to a rigid frame, obtaining a good agreement between theory and experiment.

Genetic algorithm designed silicon integrated photonic lens operating at 1550 nm

We experimentally demonstrate a photonic integrated lens made of holes in a silicon slab operating at λ0=1550 nm. The lens has been designed using a genetic algorithm in conjunction with the two-dimensional multiple scattering theory and fabricated using silicon-on-insulator technology. scanning near field optical microscopy measurements have been performed in order to measure the light intensity distribution on the device surface. The obtained full width at half maximum of the focus is 0.23 λ0, which is in good agreement with three-dimensional finite-difference time-domain simulations, and overcomes the diffraction limit in air, where the measurements are made.

Properties of silicon integrated photonic lenses: bandwidth, chromatic aberration, and polarization dependence

Abstract. We analyze the properties of silicon integrated photonic lenses based on scattering optical elements. The devices have been inverse-designed by combining genetic algorithms and the multiple scattering theory. These lenses are able to focus an infrared plane wave front on a position freely determined during the design stage. The nanofabricated silicon integrated lenses have proved effective over a large range of wavelengths, measured to be of the order of 100 nm. The lenses show chromatic aberration, with a displacement of the position of the focus measured to be higher than 1.5 μm when the wavelength varies from 1500 to 1600 nm. Moreover, we analyze the polarization of the focused beam thanks to a polarization-sensitive scanning near-field optical microscope. The measurements show that the lenses focus on a definite point only for the design’s polarization. The properties of these lenses enable them to assume the function of a nanofocusing device in silicon-on-insulator integrated optics.

Comment on : Theory of tailoring sonic devices : Diffraction dominates over refraction

Recently, Garcia et al. [Phys. Rev. E 67, 046606 (2003)] studied theoretically several acoustic devices with dimensions on the order of several wavelengths. Those authors also discussed experimental results previously reported by several of us [Phys. Rev. Lett. 88, 023902 (2002)]] and concluded that it is diffraction rather than refraction that is the dominating mechanism explaining the focusing effects observed in those experiments. In this Comment we reexamined their calculations and discussed why some of their interpretations of our results are misleading.

Genetic algorithms applied to the design of 3D photonic crystals

We aim at determining the optimal configuration of photonic crystal structures capable of carrying out a certain optical task. An exhaustive search would require a high computational cost, in this work we show how genetic algorithms can be applied to reliably find an optimal topology of threedimensional photonic crystals. The fitness, representing the performance of each potential configuration, is calculated by means of finite element analysis. Different experiments are presented in order to illustrate the potential of this 3D design approach.

Refractive acoustic devices for airborne sound

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.

Experimental demonstration of a three-dimensional acoustic cloak based on a cancelation effect

A three-dimensional acoustic cloak has been designed, fabricated, and experimentally characterized. The cloak is made of 60 concentric tori acoustically rigid that surround a sphere of radius 4 cm, which is considered as the cloaked object. The major radii and positions of the tori along the symmetry axis are determined by a cancelation condition; i.e., the scattering cross section by the sphere and the tori must be zero. An optimization algorithm that combines a genetic algorithm and simulated annealing is employed to satisfy such a condition. The operational frequency of the one-directional cloak is 8.67 kHz with a bandwidth of about 120 Hz.

Inverse design of photonic devices by using a genetic algorithm

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 photonic-crystal-based wave guide with a coupling efficiency over 87% for a wavelength that can be tuned to 1.5 λ.