Researchers of China and the US state that using silicon nanopillars infrared light can be bent by 90° as it travels through the material without being reflected at all. These half-micron-wide pillars are made from silicon, rather than the metal components typically found in metamaterials, because absorption losses are lower. This discovery can lead to a novel approach to optical beam steering, which usually requires specially crafted "metamaterials". The line of pillars could be used to bend beams of light in photonic circuits, possibly helping to steer light inside the components used in optical networks.

Working

Each pillar can resonate when excited by infrared light by setting up a standing wave inside it depending on the wavelength of the incident light in silicon, which is about one-third of its value in air. Each pillar essentially behaves like an antenna, absorbing light and re-emitting it in a form possessing the symmetry of the standing wave inside the cylindrical nanostructures. The light from the pillars interfere constructively or destructively, depending on the direction of the incident light. The pillars resonate like an electric dipole and when the pillars arranged in a line get excited by infrared light at 1550 nm. Emissions from each dipole interfere constructively when light of this wavelength is used and a single beam of light emerges from the same side of the normal as the incoming beam which is known as negative transmission. The outgoing beam makes a sharp turn, lies on the same side of the normal as the incident beam and the conventional reflected and transmitted beams disappear. When 2362 nm light hits the line of silicon nanopillars, an isotropic standing wave is formed and radiation exits in four orthogonal directions.

Applications

Production of nanopillar array is relatively easy, because each pillar has the same radius. Light beams can be steered through photonic circuits and used in photonic-crystal structures, which guide radiation using an array of small holes with diameters similar to the wavelength of light in free space. Further light guiding is possible with arrays that contain fewer elements.

Gold nanopillars with reverse optical behaviour

Scientists in the UK and Russia have fabricated a material with a negative permeability at visible wavelengths. Gold pillars The material consists of an array of about 1 million pairs of tiny gold pillars of about 100 nm height and width on a glass substrate occupying a total area of about 1mm2.

Working

When the pillars are illuminated with green light (TM polarization) an “antisymmetric plasmon resonance” is excited between each pillar pair inducing a high frequency electric current. The magnetic moment associated with this current gives rise to negative permeability having behaviour, such as optical impedance matching - a cloaking effect where reflection from the sample is totally suppressed.Applications This discovery could be a milestone for optics and could help realise the visible left-handed materials exhibiting a negative refractive index which promises the perfect lens and provides the possibility for making new optical devices such as spacers and nanolasers.

Cadmium sulfide nanopillars for solar panels

Researchers of University of California have made small 'nanopillars' placed on aluminum foil with a new method that could lead to about 300% more efficient solar panels than previous methods that used similar nanostructures. The nanopillars are made of cheaper, lower-quality materials than those used in conventional silicon and thin-film technologies. Nanopillar semiconductor arrays should be more efficient in absorbing light and have a greatly enhanced ability to separate electrons from holes and collect them as electrical current.

Working

The solar cells are made of uniform 500-nanometer-high pillars of cadmium sulfide embedded in a thin film of cadmium telluride. Both materials are semiconductors used in thin-film solar cells. A new, controlled process has been developed to make large-scale modules of dense, highly ordered arrays of single-crystal nanopillars. Electron rich cadmium sulfide pillars were grown on a template of anodized aluminum with geometrically ordered pores. A thin layer of hole-rich cadmium telluride was then deposited to cover the nanopillars. Electron-hole pairs are generated in the CdTe and the electrons flow down through the nanopillars to the aluminum contact below, while the holes are conducted to thin copper-gold electrodes placed on the surface of the CdTe layer. So far the nanopillar solar cells have been shown to have an efficiency of about 6%, but the researchers say that further improvements are possible. They are aiming for a doubling of the efficiency by using indium oxide, a transparent material, for the top surface, letting more light through.

Quartz nanopillars for precise imaging

Observing individual molecules in a complex environment by fluorescence microscopy is becoming increasingly important in biological and medical research, for which critical reduction of observation volume is required. In this context, rresearchers of Stanford University have used nanopillars that glow very precisely in such a way as to allow biologists, neurologists and other researchers a deeper, more precise look into living cells. The nanopillar structures offer many developments promising for the study of human cells.

Working

The Stanford team has successfully employed quartz nanopillars that glow just enough to provide light to see by, but weak enough to punch below the 400-nanometer barrier. The field of light surrounding the glowing nanopillars – known as the "evanescence wave" – dies out within about 150 nanometers of the pillar. Voilà – a light source smaller than the wavelength of light. The Stanford researchers estimate that they have shrunk the observation volume to one-tenth the size of previous methods. To create their nanopillars, a sheet of quartz is spray with fine dots of gold in a scattershot pattern – Jackson Pollock-style, then the quartz is etched using a corrosive gas when the gold dots shield the quartz directly below from the etching process, leaving behind tall, thin pillars of quartz. The height of the nanopillars can be controlled by adjusting the amount of time the etching gas is in contact with the quartz and the diameter of the nanopillars by varying the size of the gold dots. Once the etching process is complete and the pillars are created, a layer of platinum is added to the flat expanse of quartz at the base of the pillars.

Applications

The nanopillar imaging technique is promising in non-invasive cellular studies as a living neuron can be cultured on the platform and observed over long periods of time. The nanopillars essentially pin the cells in place to help in the study of neurons in particular, which tend to move over time due to the repeated firing and relaxation necessary for study. By modifying the chemistry on the surface of the nanopillars they could attract specific molecules to study even within the crowded and complex environment of a human cell. The specific chemical modification of the nanopillar surface makes it possible to locally recruit proteins of interest and simultaneously observe their behavior within the complex, crowded environment of the cell.

Nanopillars grown on polymer films

Scientists at the California Institute of Technology have uncovered the physical mechanism by which arrays of nanoscale pillars can be grown on polymer films with very high precision, in potentially limitless patterns. This nanofluidic process developed could some day replace conventional lithographic patterning techniques now used to build three-dimensional nano- and micro scale structures for use in optical, photonic, and biofluidic devices.

Fabrication

The fabrication of high-resolution, large-area nanoarrays relies heavily on conventional photolithographic patterning techniques, which involve treatments using ultraviolet light and harsh chemicals that alternately dissolve and etch silicon wafers and other materials. Photolithography is used to fabricate integrated circuits and micro electromechanical devices, for example. However, the repeated cycles of dissolution and etching cause a significant amount of surface roughness in the nanostructures, ultimately limiting their performance. This process is also inherently two-dimensional, and thus three-dimensional structures must be patterned layer by layer. In an effort to reduce cost, processing time, and roughness, researchers have developed molten films can be patterned and solidified in situ, and in a single step.