Power/Performance Bits: Aug. 27

Möbius strip ties liquid crystal in knots
By tying knots in liquid crystals using a miniature Möbius strip made from silica particles, University of Warwick researchers hope to understand how their intricate configurations and unique properties can be harnessed in the next generation of advanced materials and photonic devices.

Given that liquid crystal is an essential material in modern life – the flat panel displays on our computers, TVs and smartphones all make use of its light-modulating properties – by controlling the alignment of these molecules, scientists can literally tie them in a knot. To do this, they simulated adding a micron sized silica particle – or colloid – to the liquid crystal, which disrupts the orientation of the liquid crystal molecules.

Using a theoretical model, the University of Warwick scientists have taken this principle and extended it to colloids which have a knotted shape in the form a Möbius strip. A Möbius strip with one twist does not form a knot, however with three, four and five twists it becomes a trefoil knot (like an overhand knot with the ends joined together), a Solomon’s knot or a cinquefoil knot respectively.

Liquid crystal knots created around miniature Mobius strip particles (simulation). Different knots are produced by strips with different numbers of twists. The central part of the knot is shown in red around the strip in blue. Examples are shown for (A) two (Hopf link), (B) three (trefoil knot), (C) four (Solomon knot). (Source: University of Warwick)

By adding these specially designed knotted particles they force the liquid crystal to take on the same structure, creating a knot in the liquid crystal.

‘Groovy’ hologram
In a discovery that could be important for applications such as high-resolution lithography, applied physicists at the Harvard School of Engineering and Applied Sciences have demonstrated that they can change the intensity, phase, and polarization of light rays using a hologram-like design decorated with nanoscale structures.

As a proof of principle, the researchers have used it to create an unusual state of light called a radially polarized beam, which—because it can be focused very tightly—is important for applications like high-resolution lithography and for trapping and manipulating tiny particles like viruses.

They believe this is the first time a single, simple device has been designed to control these three major properties of light at once. Phase describes how two waves interfere to either strengthen or cancel each other, depending on how their crests and troughs overlap; polarization describes the direction of light vibrations; and the intensity is the brightness.

Left: holographic component fabricated by ion milling with a focused ion beam a 150-nanometer-thick gold film deposited on a glass substrate. A laser beam is partially transformed into a radially polarized beam as it traverses the device. The wide grooves create the donut-shaped intensity profile, known as a vortex, while the sub-wavelength nanometer grooves in the inset determine locally the radial polarization, which is perpendicular to the grooves. Right: The computed characteristic beam cross-section; the blue arrows indicate the radial polarization. (Source: Harvard)

Using these novel nanostructured holograms, the Harvard researchers have converted conventional, circularly polarized laser light into radially polarized beams at wavelengths spanning the technologically important visible and near-infrared light spectrum. When light is radially polarized, its electromagnetic vibrations oscillate inward and outward from the center of the beam like the spokes of a wheel, and this unusual beam manifests itself as a very intense ring of light with a dark spot in the center.

The researchers also said it is noteworthy that the same nanostructured holographic plate can be used to create radially polarized light at so many different wavelengths. Radially polarized light can be focused much more tightly than conventionally polarized light, thus enabling many potential applications in microscopy and nanoparticle manipulation.

~Ann Steffora Mutschler