The rapid and on-going development of micro-miniature optical electronic devices is helping to usher in a new era of photonic computers and light-based memories that promise super-fast processor speeds and ultra-secure communications. However, as these components are shrunk ever further, fundamental limits to their dimensions are dictated by the wavelength of light itself. Now researchers at ETH Zurich claim to have overcome this limitation by creating both the world's smallest optical switch using a single atom, and accompanying circuitry that appears to break the rules by being smaller than the wavelength of the light that passes through it.

The exponential growth of data and its accompanying reception and transmission around the world, has meant that the severe bandwidth limitations of copper-based networks have been largely eschewed in favor of high-capacity optical systems. And, as more and more photonics-based electronic devices and processors come on line, almost all conventional wiring will follow suit and eventually be replaced by optical fibers, waveguides, and other light-carrying conduits. Some future connecting devices, however, will still require the conversion of electrical signals to light, as do current data transmission systems, and it is one of these interim components – the modulator – that researchers at ETH Zurich are seeking to miniaturize.

Modulators are the devices responsible for converting electronic signals into optical ones. They do this by turning a laser on and off at the frequency of an incoming signal, thereby "modulating" the light to create an optical replication of the input transmission. Incorporated in data centers in the tens of thousands across the world, modulators are relatively large as far as electronic devices are concerned, at about three centimeters or so wide. In the numbers in which they are employed, this requires a lot of space to house them and, if they are to be useful in future photonics-based devices, their size needs to be significantly reduced.

Building on previous work carried out by the ETH Zurich research team under the auspices of Professor Jürg Leuthold, the new research took an already micro-miniature version of a modulator that was just 10 micrometers across, and used the lessons learned in constructing this device to further reduce the size of the new modulator by a factor of 1000. That is, the team produced a device that was actually smaller than the 1.55 micrometer wavelength of laser light used in optical data transmissions. At this size, as previously mentioned, fundamental constraints generally preclude the use of any optical device smaller than the wavelength of light that it transmits. But the new device was able to do this, much to the surprise of the researchers themselves.

"Until recently, even I thought it was impossible for us to undercut this limit," said Professor Leuthold.

To achieve this hitherto unlikely breakthrough, senior scientist at ETH Zurich, Alexandros Emboras, reconfigured the construction of the modulator to use two minuscule pads, one composed of silver and the other made of platinum, placed on the top of an optical waveguide constructed of silicon. With the pads arranged mere nanometers apart, the silver pad was constructed with a small protuberance on one side that stretched across the gap to almost touch the platinum pad, thus creating a space just about an atom's thickness wide. If the silver pad now has a voltage applied to it, ideally a single silver atom will be drawn toward the furthermost point of the pad and stay there (in practice, a few atoms may do this, but at this scale the difference between one or two atoms is largely negligible).

The presence of the silver atom so close to the platinum pad effectively creates a circuit between the two pads – a single atom switch – and electrical current is able to flow between them. When the voltage is removed, the silver atom retracts, thus opening the circuit. According to the researchers, this switching function is capable of being performed millions of times per second.

To allow for the transmission of light through the exceptionally narrow channel created by the closely positioned pads (and which is smaller than the wavelength of the light being transmitted), the device relies on the behavior of light at atomic levels when traveling across a metallic surface. When the waveguide directs laser light above the metallic surface of the input channel, the light is converted into a surface plasmon.

In other words, when the incoming laser light strikes the atomic surface layer of the metal, its energy creates an electromagnetic field that essentially gives rise to electrons that oscillate at the frequency of the laser light. As the resulting electron oscillations are much smaller than the wavelength of the light, they can travel through the gap to the other side. When they reach the other side, the electron oscillations are reconverted into optical signals, thereby allowing a circuit smaller than the wavelength of light to effectively pass that light through it.

Applying a voltage to the pads turns the atom-sized switch completely on or completely off, there is no intermediate state, so that this part of the circuit also acts to effectively create digital signals composed of ones or zeros.

"This allows us to create a digital switch, as with a transistor," said Professor Leuthold. "We have been looking for a solution like this for a long time."

Not yet developed enough for production – despite the fact that it operates in the megahertz range and at room temperature (in stark contrast to similar quantum effect devices that require cryogenic cooling to work) – there remains the requirement to both improve the transition speed into the gigahertz to terahertz range for optimum data transmission efficiency, as well as to improve the lithography techniques used to produce the device.

The results of this research were recently published in the journal Nano Letters.

Source: ETH Zurich

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