[Paper Review] Cloaking at Optical Frequencies

Lucas H. Gabrielli, Jaime Cárdenas+2 more|arXiv (Cornell University)|Apr 23, 2009

Metamaterials and Metasurfaces ApplicationsMaterials Science29 references12 citations

TL;DR

This paper demonstrates a broadband optical cloak made of nanoscale silicon structures with spatially varying densities, engineered via transformation optics to manipulate light trajectories. The cloak successfully conceals surface deformations on a reflective plane, rendering the hidden object undetectable under optical illumination.

ABSTRACT

1 The ability of rendering objects invisible using a cloak – not detectable by an external observer – for concealing objects has been a tantalizing goal 1-6. Here, we demonstrate a cloak operating in the optical regime. The cloak operates at a wide bandwidth and conceals a deformation on a flat reflecting surface, under which an object can be hidden. The device is composed of nanometer size silicon structures with spatially varying densities across the cloak. The density variation is defined using transformation optics to define the effective index distribution of the cloak. The prospect of optical cloaking has recently become a topic of considerable interest. Through the use of transformation optics 7-11, in which a coordinate transformation is applied to Maxwell’s equations, several designs for such a device were created 12-23. These designs are based on the idea of manipulating the structure of the cloaking medium so that the trajectory of light after interacting with the cloak is the same as that in an empty medium, without the cloak nor the object underneath. The external observer is therefore unaware of the presence of the cloak and the object. Such cloaks were recently experimentally demonstrated in the microwave regime using metamaterial structures with feature sizes in the millimeter to centimeter scale 24-25. Pushing this

Motivation & Objective

• To achieve optical cloaking in the visible spectrum using nanofabricated materials.
• To overcome the limitations of previous microwave-frequency cloaks by scaling down to optical wavelengths.
• To design a cloak that operates over a wide bandwidth and conceals surface irregularities.
• To implement transformation optics principles in a practical, nanoscale metamaterial structure.

Proposed method

• Employing transformation optics to derive the required effective index distribution for guiding light around a hidden object.
• Designing a cloak with spatially varying silicon nanostructures to emulate the desired effective index profile.
• Using nanofabrication techniques to realize subwavelength silicon structures with controlled density gradients.
• Validating the cloak's performance through electromagnetic simulations of light scattering and propagation.
• Ensuring the cloak preserves the phase and direction of incident light, mimicking free space propagation.
• Focusing on broadband operation by optimizing the geometry and material properties of the nanostructures.

Experimental results

Research questions

• RQ1 Can transformation optics principles be practically implemented at optical frequencies using nanoscale materials?
• RQ2 Is it possible to create a broadband optical cloak that conceals surface deformations on a reflective plane?
• RQ3 How can nanoscale silicon structures be engineered to produce the required effective index distribution for cloaking?
• RQ4 To what extent does the cloak maintain the original wavefront of incident light, ensuring invisibility?
• RQ5 Can such a cloak function without significant scattering or detectable distortions under optical illumination?

Key findings

• The cloak successfully conceals a deformation on a flat reflecting surface under optical illumination, rendering it undetectable to an external observer.
• The device operates over a wide bandwidth, indicating robust performance across multiple optical frequencies.
• The spatially varying density of silicon nanostructures effectively emulates the required effective index distribution for cloaking.
• Light incident on the cloak follows trajectories identical to those in free space, preserving phase and direction.
• The design demonstrates the feasibility of optical cloaking using nanoscale silicon structures, advancing practical realization beyond microwave regimes.
• The results validate the application of transformation optics in the optical domain using engineered metamaterials.