Invisibility carpet cloak can hide objects from visible light

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When an input beam (black arrow) reflects off (a) a bump without a cloak, the bump causes a perturbation. When the beam reflects off (b) a bump covered by a cloak, the cloak masks the bump, and the reflected beam is reconstructed as if the bump did not exist. (c) Light after reflection from a flat mirror, a bump without a cloak, and a cloaked bump, at three different wavelengths. Image credit: Majid Gharghi, et al. 2011 American Chemical Society

(PhysOrg.com) -- Most of the invisibility cloaks that have been demonstrated to date conceal objects at frequencies that are not detectable by the human eye. Designing invisibility cloaks that can conceal objects from visible light has been more challenging due to the strict material requirements. But in a new study, researchers have fabricated a carpet cloak that can make objects undetectable in the full visible spectrum.

The researchers, led by Prof. Xiang Zhang at the University of California, Berkeley, and Lawrence Berkeley National Laboratory, have published their study in a recent issue of Nano Letters.

As the researchers explain, most previous invisibility cloaks have used metallic metamaterials for cloaking at microwave frequencies. But at optical frequencies, the metal absorbs too much light and leads to significant metallic loss, and Berkeley and other groups have had to design dielectric cloaks at infrared frequencies. More recently, researchers at University of Birmingham (UK) have experimented with using uniaxial crystals as the cloak material, which can enable cloaking in visible frequencies, but only for a certain polarization of light.

In the current study, the researchers used a technique called quasi conformal mapping (QCM) to conceal an object with a height of 300 nm and a width of 6 µm underneath a reflective ?carpet cloak.? The carpet itself has the appearance of a smooth optical mirror, so that the object and the bump that the object makes underneath the carpet are undetectable by visible light.

?The carpet cloak means that you conceal the object under a layer, which we call carpet, but you see the carpet like a normal mirror, as if it is flat with no bump caused by putting the object underneath,? Zhang told PhysOrg.com. ?This way, the observer won't recognize something is concealed underneath.?

In order to guide visible light around the concealed object, the researchers had to make light travel at different speeds while approaching the bump. They achieved this by designing the materials to have a variable refractive index, transforming them into metamaterials, since they don?t appear in nature. The researchers placed a silicon nitride waveguide on a transparent nanoporous silicon oxide substrate that they specially developed to have a much lower refractive index than that of the waveguide. Using nanofabrication techniques, the researchers etched tiny holes into the nitride to make a desired pattern, giving the waveguide the cloaking refractive index profile.

?The concept of the carpet cloak was originally suggested so that you can design a certain pattern for a given size of the bump, and hide an object of arbitrary shape under that,? Zhang said. ?If you need to make a bigger size bump to hide a bigger object, a new hole pattern will be required.?

With this refractive index profile, along with the transparency of both the waveguide and the substrate, the cloak could completely conceal an object by producing a light beam profile identical to a beam reflected from a flat carpet with no object underneath.

?This device is among the first cloak devices that operate at visible frequencies; the other very recent visible light cloaks operate based on a principle that relies on a certain polarization of light, whereas the quasi-conformal-based principle does not rely on the polarization,? Zhang said. ?Of course, the waveguide geometry entails different operation for different polarizations, which is extrinsic to the QCM design.?

In addition to cloaking, the new technique provides an important step toward implementing optical transformation structures in the visible range. Using transformation optics (TO), researchers can manipulate light for applications such as powerful microscopes and computers.

?The carpet cloak is an example of a wide family of devices that can be made based on transformation optics,? Zhang said. ?Besides invisibility, all kinds of optical illusion schemes can be made based on the concept, where the observer receives a different impression when looking at an object. The capability to manipulate light propagation can be used in energy devices, optical computing devices, and beyond, wherever it is desired to have full control on the light path; TO lets us redirect light and re-route it.?

More information: Majid Gharghi, et al. ?A Carpet Cloak for Visible Light.? Nano Letters. DOI: 10.1021/nl201189z

Copyright 2011 PhysOrg.com.

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Nano-LEDs emit full visible spectrum of light

 

(Left) A single nanodisk-nanorod LED viewed with a field-emission scanning electron microscope. (Right) Some colors of light emissions from nanodisk-nanorod LEDs - violet, blue, cyan, green, and yellow - viewed with an optical microscope. Image credit: Lu, et al. 2011 American Institute of Physics

Physicists from Taiwan have designed and fabricated nano-sized light-emitting diodes (LEDs) that emit light spanning the entire visible spectrum. Although the tiny full-color LEDs aren't intended for commercial lighting applications, they should be useful in high-resolution microscopy and subwavelength photolithography.

The researchers, Yu-Jung Lu, et al., from National Tsing-Hua University in Hsinchu, Taiwan, have published their study on the nano-LEDs in a recent issue of Applied Physics Letters.

The new nano-LEDs have a unique structure that consists of 40-nm-thick nanodisks sandwiched between two layers of nanorods, resulting in a nanodisk-in-nanorod geometry. The nanodisks are made of indium gallium nitride (InGaN), a semiconducting material that is widely used in LEDs and solar cells, while the nanorods are made of gallium nitride (GaN). However, InGaN LEDs capable of emitting light of the entire visible spectrum have not been achieved until now.

?The InGaN/GaN nanodisk/nanorod structure is similar to a well-known quantum well structure, but in a reduced dimensionality (reduction in lateral sizes),? coauthor Shangjr Gwo, a physics professor at National Tsing-Hua University, told PhysOrg.com. ?The InGaN nanodisks sandwiched between the p- and n-GaN regions act as the full-color visible-light emitters when electrons and holes are injected across the p-n junction at a forward bias voltage. The electroluminescent light comes from the electron-hole recombination within the InGaN nanodisks.?

As the researchers explained, the key to achieving full-color LEDs was overcoming large lattice strains, which degrade long-wavelength emissions. The InGaN/GaN nanorod system resolves this issue due to the strain relaxation in the nanostructured geometry.

The researchers hope that these full-color nano-LEDs can be used in high-resolution imaging techniques that can resolve ultrasmall subwavelength features of objects. To do this, these techniques must overcome the diffraction limit, which is a fundamental limit on imaging resolution caused by the spreading out ? or ?diffraction? ? of waves. Imaging techniques can get around this limit by using evanescent waves, which reveal information on objects? subwavelength features, but also decay exponentially away from the object. Due to the short range of the evanescent waves, imaging techniques that detect them are based on near-field optics.

One of these techniques is scanning near-field optical microscopy (SNOM), which uses a tiny probe to generate and retrieve evanescent waves. One of the biggest challenges in SNOM is getting a light source that is small and versatile enough to work on this probe, and that?s where the new nano-LEDs come in. While previous research has demonstrated the advantages of using nano-LEDs on the probes, this is the first time that a nano-LED with a full-color range has been available.

?For microscopy, we can use the nano-LED as a localized excitation light source at a chosen wavelength to selectively excite specific fluorescent molecules,? Lu said.

In their study, the researchers experimentally demonstrated using the nanodisk-in-nanorod LEDs for subwavelength photolithography, in which light is used to create a pattern on a light-sensitive material. They predict that, by fabricating the nano-LEDs onto the SNOM probe tips, they could achieve better spatial control for future subwavelength photolithography.

?For the applications of photolithography, the freedom of using nano-LEDs at any wavelength broadens the choice of photoresist and allows for the control of their photo-response,? Lu said.