Manufacturing Bits: March 22

Tunable windows; world’s smallest lens; terahertz lens.


Tunable windows
Harvard University has put a new twist on tunable windows. Researchers have devised a new manufacturing technique that can change the opacity of a window. With the flip of a switch, the window can become cloudy, clear or somewhere in the middle.

Tunable windows, which aren’t new, rely on electrochemical reactions. Typically, the glass is coated with materials using vacuum deposition techniques. This technique deposits one molecule at a time, which is an expensive process.

In contrast, Harvard’s tunable window technology makes use of a spraying or peeling process. The window itself consists of glass or plastic. This is sandwiched between transparent and soft elastomers. These are sprayed with a coating of silver nanowires.

Then, a voltage is applied. The nanowires on either side of the glass are energized. The elastomer deforms, thereby causing light to scatter. This, in turn, makes the glass opaque in less than a second.

A new technique can change the opacity of a window (Image: David Clarke/Harvard SEAS)

A new spray-based technique can change the opacity of a window (Image: David Clarke/Harvard SEAS)

“Because this is a physical phenomenon rather than based on a chemical reaction, it is a simpler and potentially cheaper way to achieve commercial tunable windows,” said David Clarke, a professor of materials at Harvard, on the university’s Web site.

World’s smallest lens
The Australian National University (ANU) has developed the world’s thinnest lens–a tiny 6.3nm structure.

Based on a molybdenum disulphide (MoS2) material, the tiny lens could enable a new class of flexible displays and miniature cameras. Researchers claim to have beaten the previous record for a lens, which was a 50nm thick gold nano-bar array.

Tiny lens is imaged on the computer screen (Source: ANU)

Tiny lens is imaged on the computer screen (Source: ANU)

To accomplish its feat, Australian National University used a 2D material called MoS2. MoS2 is a transition-metal dichalcogenide (TMD) material. 2D materials are gaining steam in the R&D labs. The 2D materials include graphene, boron nitride (BN) and the TMDs. Another TMD, molybdenum diselenide (MoSe2), is an attractive material for use in future field-effect transistors (FETs).

As it turns out, MoS2 has good optical properties and a giant optical path length. The material’s refractive index has a value of 5.5. In comparison, diamond has a refractive index of 2.4. Water is 1.3. In optics, the refractive index of a material is a number. This number describes how light propagates through that medium.

MoS2 presents some new possibilities in optics. “The capability of manipulating the flow of light in atomic scale opens an exciting avenue towards unprecedented miniaturization of optical components and the integration of advanced optical functionalities,” said Yuerui Lu from ANU’s Research School of Engineering, on the university’s Web site.

Terahertz lens
Brown University has developed a new type of lens for use in terahertz radiation applications.

Terahertz radiation consists of electromagnetic waves. Operating at frequencies from 0.3 to 3 terahertz, terahertz radiation can pass through clothing, paper, wood, plastic and other materials. All told, the technology is ideal for medical imaging, security, communications and other applications.

The lens from Brown, which performs as well or better than existing terahertz lenses, is made from an array of stacked metal plates based on artificial dielectrics. The lens is made using 32 metal plates. Each plate is 100 microns thick, with a 1mm space between each plate.

In operation, a terahertz beam enters the input side of the device. The beam travels through the spaces between the plates. The concave output side of the device bends the beam to varying degrees.

In the lab, researchers were able to focus a two-centimeter-diameter terahertz beam down to a four-millimeter spot. The radiation transmission was about 80%. In comparison, Teflon-based lenses have similar transmissions, while silicon-based lenses have a transmission loss of about 50%.

Dielectric-based lenses have advantages over Teflon material. With Brown’s lens, the spacing between the plates can be changed. As a result, the device can be calibrated for specific wavelengths. This isn’t possible with existing lenses, according to researchers. “The spirit of this work is to develop a new technology for building terahertz components that might be alternatives to things that exist or that might be new,” said Dan Mittleman, professor of engineering at Brown, on the university’s Web site. “That’s important for the terahertz field because there aren’t a lot of off-the-shelf components yet.”

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