Silicon nanostructure cloak operating at optical frequencies

Abstract: The ability to render objects invisible using a cloak - not detectable by an external observer - for concealing objects has been a tantalizing goal1-6. Here, we demonstrate a cloak operating in the near infrared at a wavelength of 1550 nm. The cloak conceals a deformation on a flat reflecting surface, under which an object can be hidden. The device has an area of 225 um2 and hides a region of 1.6 um2. It is composed of nanometre 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 paper presents the design and experimental demonstration of a cloak that manipulates light at 1550 nm using transformation optics.
• It employs sub-wavelength silicon posts in an SiO2 medium to create a spatial refractive index variation that effectively conceals deformations.
• Experimental and simulation results validate the cloaking effect, highlighting its scalability and potential for industrial optical applications.

Silicon Nanostructure Cloak Operating at Optical Frequencies

This paper presents the design, fabrication, and experimental demonstration of an optical invisibility cloak employing silicon nanostructures for functionality at optical frequencies. The authors contribute to the burgeoning field of optical cloaking through their innovative application of transformation optics, providing a novel approach to manipulating light paths in the near-infrared spectrum. Specifically, the device operates at a wavelength of 1550 nm and is capable of concealing deformations on reflecting surfaces through precise control of light trajectories.

Key to the functioning of this cloak is its composition from sub-wavelength-scale dielectric structures, specifically silicon posts with a diameter of 50 nm embedded in an SiO2 medium. The cloak covers an area of 225 µm² and is designed to conceal a region measuring 1.6 µm². The density variation of these posts, dictated by transformation optics principles, induces a spatial refractive index variation—a critical mechanism enabling the cloaking effect.

The authors employ a transformation optics framework, applying a coordinate transformation to Maxwell’s equations to achieve an effective index distribution that allows light to traverse the cloak as if it were propagating in a homogenous medium. This optical illusion renders both the cloak and the hidden object imperceptible to external observation. The work builds on previous demonstrations of cloaking devices in the microwave regime, pushing the boundaries into the optical regime, which demands nanometer precision in structuring the cloaking media.

The experimental setup involves a triangular cloaking device placed over a distributed Bragg reflector (DBR) with a deformation. Light is coupled into the system through a tapered silicon waveguide, and the cloak’s effectiveness is evaluated using both simulation and experimental validation with an infrared camera. Simulation results demonstrate that the cloak successfully eliminates power gaps caused by the deformed DBR when the light’s trajectory is modified by the cloak. Consistent with predictions, the experimental data corroborate this effect, showing that the reflective image resembles that of a plane mirror without the presence of any deformations.

A significant strength of this study is the demonstration of an effective anisotropy factor of 1.02 across the cloak, achieved by minimizing the Modified-Liao functional during index distribution calculations. The reported index values range from 1.45 to 2.42, fitting comfortably within the refractive indices of the constituent materials, crystalline silicon, and SiO2. These parameters enable the use of conventional silicon processing techniques, enhancing the practical feasibility of fabricating such devices.

In terms of practical implications, the authors highlight the potential scalability of this cloaking device via cost-effective techniques such as nanoimprinting, expanding its utility across various industrial applications. Furthermore, the concept of using transformation optics to not only conceal but also concentrate light ushers in possibilities for optimally capturing solar energy, revealing broader implications for energy-efficient technologies.

This paper lays a promising groundwork for further exploration in optical metamaterials and cloaking applications. The successful demonstration of an optical frequency cloak opens avenues for advancements in stealth technology, secure communications, and advanced optical devices. Future research can explore enhanced scalability, bandwidth optimization, and multi-frequency cloaking capabilities to further refine and extend the applicability of these findings.