Nicolas Stenger

Three-Dimensional Invisibility Cloak at Optical Wavelengths

Hidden Under the Carpet Transformation optics, combined with the ability to fabricate structures with complex refractive index profiles, allow materials to be formed with fascinating optical properties, such as cloaks where both the object and the cloak concealing the object are rendered invisible. To date, the cloaks have been restricted to two dimensions, which limits realistic applications. Based on a photonic crystal structure with a polymer filling the empty space, Ergin et al. (p. 337, published online 18 March) have designed, fabricated, and realized a three-dimensional cloak operating at optical wavelengths. A structured photonic crystal can be used to cloak an object at optical wavelengths and over a wide viewing angle. We have designed and realized a three-dimensional invisibility-cloaking structure operating at optical wavelengths based on transformation optics. Our blueprint uses a woodpile photonic crystal with a tailored polymer filling fraction to hide a bump in a gold reflector. We fabricated structures and controls by direct laser writing and characterized them by simultaneous high–numerical-aperture, far-field optical microscopy and spectroscopy. A cloaking operation with a large bandwidth of unpolarized light from 1.4 to 2.7 micrometers in wavelength is demonstrated for viewing angles up to 60°.

Tailored 3D Mechanical Metamaterials Made by Dip‐in Direct‐Laser‐Writing Optical Lithography

Dip-in direct-laser-writing (DLW) optical lithography allows fabricating complex three-dimensional microstructures without the height restrictions of regular DLW. Bow-tie elements assembled into mechanical metamaterials with positive/zero/negative Poissons ratio and with sufficient overall size for direct mechanical characterization aim at demonstrating the new possibilities with respect to rationally designed effective materials.

On the practicability of pentamode mechanical metamaterials

Following the theoretical suggestion by Milton and Cherkaev in 1995, we fabricate pentamode metamaterials by dip-in direct-laser-writing optical lithography. Using finite element calculations and geometrical parameters corresponding to our fabricated three-dimensional microstructures, we find that the figure of merit, i.e., the ratio of bulk modulus to shear modulus, can realistically be made as large as about 1,000. This result opens new horizons for transformation acoustics.

Photorealistic images of carpet cloaks

Using home-built dedicated ray-tracing software, we simulate photorealistic images of sceneries in three dimensions including dielectric carpet cloaks--i.e., continuously varying refractive-index distributions that allow for invisibility cloaking of a bump in a metallic carpet. Results for the ideal and for a simplified cloak are shown. The presented material gives a visual and intuitive impression of the performance of different arrangements and might be ideally suited for communicating the concepts of transformation optics to the general public.

Optical microscopy of 3D carpet cloaks:ray-tracing calculations

In a recent publication (T. Ergin et al., Science 328, 337 (2010)), three-dimensional broadband dielectric carpet cloaks have been fabricated and experimentally characterized by optical bright-field and dark-field microscopy using unpolarized light from an incandescent lamp. A direct comparison with theory has not been provided so far. In the present work, we treat the carpet cloak as well as the entire optical microscope within the ray-optics approximation and the cloak within the effective-medium approximation. We find good qualitative agreement between experimental results and our calculations.

From Curved Space to Optical Cloaking

Transformation optics is a powerful approach to manipulate the propagation of electromagnetic waves [1]. Here, the curvature of space is mimicked by an anisotropic metamaterial, which is described by effective medium theory [2, 3]. An interesting application of such metamaterials is optical cloaking. The metamaterial will prevent that light interact with the cloacked object and in the same time will leave the electromagnetic wave unperturbed. We present the basics of transformation optics and two examples of cloaking devices. This talk will be illustrated with full-wave finite element simulations of feasible cloak designs in homogeneous medium approximation as well as in full geometry.