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Sensors on the Frontiers of Physics

Photonics Spectra
Sep 2009
New devices manipulate light in unusual ways that can enable ultrasensitive detection, leading toapplications such as quantum communication and quantum computing.

David L. Shenkenberg, Features Editor,

Computer chips based on light as opposed to electricity have only been theorized, but now researchers are actually developing computer chips that will use light for computer functions.

At Yale University in New Haven, Conn., members of Hong Tang’s lab are already taking development of these photonic computer chips a step further. They are using light to power nanomachines built from computer chips, and these nanomachines could have numerous applications, including sensing molecules and even smaller particles.

Existing computer chips such as these run on electricity, but the next generation will run on photons.

The photonic nanomachines are similar to micro- and nanomachines that are powered by electricity, which are formally called micro- and nanoelectromechanical systems, or MEMS and NEMS, respectively. Far from a lab curiosity, MEMS devices have been deployed in automobile air-bag sensors, ink-jet printers and even the motion sensors in Nintendo Wii controllers.

The researchers in the Tang lab have found an optical force that behaves like the Casimir force found in tiny electrical machines. This force is usually weak but becomes significant in micro- and nanodevices. The photonic chip will enable them to study this weak force, as well as to develop applications.

This force is different from the radiation pressure that is used by optical tweezers to move particles. “The new force that we have investigated actually kicks to the side of that light flow,” Tang said.

Enter the matrix

The researchers have used the optical force to move 10 tiny cantilevers on a CMOS chip. Tang said, “The significance of our CMOS platform is that our device is fully compatible with many other devices. You can cascade them, put them in parallel, multiplex, scale up the production.”

As described in the April 26, 2009, issue of Nature Nanotechnology, the light goes through the hollow bore of each nanocantilever and is collected on-chip. The nanocantilevers are of different lengths and therefore resonate at different frequencies, like keys on a xylophone. The system can detect particles 1⁄10,000 the size of an atom, or 0.0001 angstroms. The detection mechanism is based on the deflection of the cantilevers.

The system can operate with inexpensive LEDs as opposed to more expensive laser systems, and at room temperature as opposed to extreme cold – major advantages over detectors with comparable sensitivity, according to the researchers.

In the July 13, 2009, issue of Nature Photonics, they reported that the optical force can be repulsive as well as attractive, a feature that could be used as a routing mechanism for communication between devices that contain computer chips.

In particular, this device possibly could be used as a router for quantum communication, which promises faster and more efficient communication between devices that contain computer chips as well as extremely strong encryption of computer data, which is called quantum cryptography.

Quantum communication, cryptography and computing all are based on the concept that quantum particles such as photons can be in more than one physical state at the same time. Electricity, by contrast, is either on or off – one or the other, but not both. By existing in more than one state at the same time, photons can hasten computer-to-computer communication and make smarter computers that can come up with complex encryption that foreign spies and wanton hackers cannot break.

Holding an ion

Another device that can be used both for sensing and for quantum communication was developed by researchers at the National Institute of Standards and Technology (NIST) in Bolder, Colo., and their colleagues at the University of Erlangen-Nuremberg in Germany. It can trap and hold individual ions above three cylindrical steel electrodes with hollow bores protruding from the device. They call it a “stylus trap” because the steel cylinders trap the ion, and each cylinder reminds the researchers of a stylus.

Laser light and cold temperatures were used to trap the ions using techniques that have been demonstrated previously. The fact that the ions are held above the electrodes is unique. This architecture allows for greater access to the ions.

Using the ion as a probe for electro-magnetic fields, the device can be used to measure forces, especially those oscillating between approximately 100 kHz and 10 MHz. It is about a million times more sensitive than the mechanical sensitivity of a cantilever of an atomic force microscope. Individual photons theoretically could be transferred to the trapped ions with 95 percent efficiency for quantum cryptography, and fluorescent light emitted by the ions could be used in quantum computing. This device is detailed in the June 28, 2009, issue of Nature Physics by senior author David Wineland of NIST and his colleagues.

Ripping a nanozipper

An even more exotic-looking force-sensing device was created by the Oskar Painter group at Caltech. The device consists of two nanoscale strands of silicon with periodic oval holes down the length of the strands. Connected side by side, the strands reminded the researchers of a zipper found on a piece of clothing. The zipper strands even opened up like a zipper when the scientists focused a laser beam down the center of the strands.

However, the researchers said that the zipper does not open as a result of the beam’s path straight down the center. The mechanism is more exotic than that. Some of the photons enter and circulate in the periodic oval holes, and this circulation is what ultimately opens the zipper cavity.

The zipper strands could be used in force sensing; for example, two molecules could be attached to opposite strands, and the force required to open the strands and thereby pull the molecules apart could be calculated from there. The strands also could be used for photonic communication and photonic circuits, as well as for studying fundamental forces.

Applications aside, the cavity is also a marvel of physics. The force of a single photon traveling straight through the cavity is comparable to a force 10 times that of gravity.

A projecting beam or other structure supported only at one end.
1. A localized fracture at the end of a cleaved optical fiber or on a glass surface. 2. An integrated circuit.
radiation pressure
The force exerted on a surface by radiation.
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