May 19, 2011

Three Dimensional Invisibility in the Visible Spectrum

(a) Colored oblique-view electron micrograph of the polymer reference (top) and carpet cloak (bottom) structures (fabricated on glass substrate and coated with 100 nm gold). The scale bar corresponds to 10 µm. (b) Corresponding focused-ion- beam cuts of nominally identical structures. The scale bar corresponds to 2 µm. (c) and (d) are true-color optical micrographs of the structures in (a) taken with an optical microscope under circularly polarized illumination at 700-nm wavelength. Note the identical distortions due to the bump in both structures in (c) when inspected from the air side (serving as a control experiment). When inspected from the glass-substrate side in (d), the reference structure (top) still shows pronounced dark stripes. In sharp contrast, the stripes essentially disappear for the cloaking structure (bottom). (e) and (f) are ray-tracing calculations corresponding to (c) and (d), respectively.

Optics Letters - Three-dimensional polarization-independent visible-frequency carpet invisibility cloak (5 pages)

We miniaturize all features in a previously introduced polarization-independent three-dimensional carpet invisibility cloak by more than a factor of two. This leads to operation wavelengths in the visible. The structures are characterized by electron and optical microscopy. In contrast to our previous work at infrared wavelengths, we can directly measure two-dimensional images at visible frequencies, perform control experiments from the backside, and we can compare the images with theory. We find excellent agreement. Furthermore, we study the wavelength dependence in the range from 900 nm down to 500 nm. Cloaking action deteriorates as the woodpile stop band at around 575 nm is approached.

The minute invisibility cloak produced by Fischer and Ergin is smaller than the diameter of a human hair. It makes the curvature of a metal mirror appear flat, as a result of which an object hidden underneath becomes invisible. The metamaterial placed on top of this curvature looks like a stack of wood, but consists of plastic and air. These “logs” have precisely defined thicknesses in the range of 100 nm. Light waves that are normally deflected by the curvature are influenced and guided by these logs such that the reflected light corresponds to that of a flat mirror.

“If we would succeed again in halving the log distance of the invisibility cloak, we would obtain cloaking for the complete visible light spectrum,” says Fischer.

Transformation optics can be viewed as a powerful tool for designing optical systems. The strength of this tool is emphasized by invisibility cloaking which was long believed to be impossible. In particular, the carpet cloak concept has inspired experiments by several groups. In brief, by tailoring a refractive-index profile, the carpet cloak makes a bump in a metal mirror (the carpet) appear flat. This allows for hiding objects underneath this carpet. However, the "holy grail" of a macroscopic three- dimensional (3D) cloak for unpolarized visible light has not been accomplished so far. Our 2010 work has demonstrated a microscopic 3D carpet cloak for unpolarized light down to wavelengths of about 1.5 µm. More recent work by other groups has demonstrated macroscopic carpet cloaks at visible frequencies, however, only for one linear polarization of light (due to the birefringent calcite used) and in an effectively two-dimensional configuration.

In this Letter, we miniaturize our 2010 structure by more than a factor of two, leading to the first 3D carpet cloak for unpolarized visible light. This also allows for refined characterization via optical microscopy. Comparison with ray-tracing calculations shows that the cloaking performance is close to the theoretical expectation.

The polymer structures in this work are fabricated by stimulated-emission-depletion (STED) inspired direct laser writing (DLW) closely following along the lines of our previous work with two modifications. First, we employ a different photoinitiator in a similar monomer, i.e., 0.25% wt 7-diethylamino-3-thenoylcoumarin in pentaerythritol tetraacrylate. This ketocoumarin offers a better ratio between depletion efficiency and undesired absorption of the 532-nm continuous-wave depletion laser (which limits resolution). Second, we not only improve the lateral but also the axial resolution by using a suitable phase mask known from STED microscopy . After exposure, the photoresist structures are developed in 2-propanol and dried super-critically with CO2

We have fabricated and characterized the first three-dimensional polarization-independent carpet cloak for visible light. The concepts of transformation optics combined with the fabrication technology used here might also allow for realizing other optical devices such as, e.g., unusual flat lenses

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