We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nanoporous silicon oxide substrate with a very low refractive index (n less than 1.25). The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies.
(a) Cross sectional schematic of the cloak device implemented in a silicon nitride waveguide on a low index nanoporous silicon oxide substrate; the nitride layer and the nanoporous oxide layer are 300 nm and 5 to 10 μm thick, respectively. The hole pattern allows for index modulation by varying the solid filling fraction. The holes vary in size from65 to 20 nm. The inset shows an SEM image of the low index nanoporous silicon oxide substrate. (b) AFM image of the hole pattern as transferred to the electron beam resist after development; the smallest holes appear shallow due to incomplete penetration of the AFM tip, but based on the uniform pattern transfer we conclude that they are completely through the resist. (c) SEM image of the device, consisting of roughly 3000 holes; the optical transformation device is surrounded by a triangular background index region not pictured.
(a) Index variation for the optical transformation of a “bump” to a flat plane, normalized to index; the effective index of the cloak is found by multiplying the index shown by the desired background index; the inset schematically shows the transformation performed. (b) Requirements on the index variation and substrate index for implementing the cloaking transformation in the infrared (silicon waveguide on silicon oxide substrate) and the visible (silicon nitride waveguide on low index substrate). Solid line indicates the index of the unmodified material, while the shaded region below the solid line shows the range of indexΔn that must be available. At visible frequencies, a new substrate material is needed as the index variation required for a silicon nitride waveguide goes below the index of the conventional silicon oxide substrate. The inset shows the feature size requirement for scattering free propagation of light in the metamaterial as a function of wavelength, which is used as a design criteria for the cloak.
We have demonstrated a full visible spectrum transformation optical device that is capable of cloaking any object underneath a reflective carpet layer. In contrast to the previous demonstrations that were limited to infrared light, this work makes actual invisibility for the light seen by the human eye possible. In addition to successful demonstration of cloaking, the fabrication scheme employed here is a significant step toward general implementation of optical transformation structures in the visible range. The considerable index contrast between the waveguide and the substrate enables realization of large index variations in the metamaterial device. In addition, the nitride waveguide on the low index substrate provides a platform for on-chip photonic devices in the visible range, which have been traditionally achievable in silicon waveguide on oxide for the infrared.