Squeezing Light Into Tiny Channels

Nanofocusing and optical mode properties of the organic hybrid gap plasmon waveguide on the silicon platform used for degenerate four-wave mixing. (Source: Nielsen, IPL)

By forcing light to go through a smaller gap than ever before, researchers have paved the way for computers based on light instead of electronics. Light is desirable for use in computing because it can carry a higher density of infor­mation and is much faster and more efficient than conven­tional electronics. However, light does not easily interact with itself, so while it can be used to move infor­mation quickly, it is not very good at processing infor­mation.

In order to use light for processing on micro­chips, several important obstacles need to be overcome. For example, light can be made to interact using parti­cular materials, but only over relatively long distances. Now, however, a team from Imperial College London has made a signi­ficant step forward by reducing the distance over which light can interact by 10,000 fold. This means that what pre­viously would have taken centi­metres to achieve can now be realised on the micro­metre scale, bringing optical processing into the range of electrical tran­sistors, which currently power personal computers.

Michael Nielsen, from the Depart­ment of Physics at Imperial, said: “This research has ticked one of the boxes needed for optical computing. Because light does not easily interact with itself, infor­mation sent using light must be converted into an elec­tronic signal, and then back into light. Our tech­nology allows processing to be achieved purely with light.” Normally when two light beams cross each other the indi­vidual photons do not interact or alter each other, as two electrons do when they meet. Special non­linear optical materials can make photons interact, but the effect is usually very weak. This means a long span of the material is needed to gradually accu­mulate the effect and make it useful.

However, by squeezing light into a channel only 25 nano­metres wide, the Imperial team increased its intensity. This allowed the photons to interact more strongly over a short distance, changing the property of the light that emerged from the other end of the one-micro­metre-long channel. Control­ling light on such a small scale is an important step is the con­struction of computers that use light instead of elec­tronics. Electronic computing is at the limit of effi­ciency; while it is possible to make a faster elec­tronic processor, the energy cost of moving memory data around the computer any faster is too high.

To make computers more powerful, proces­sors are instead made smaller, so more can fit into the same space, without increasing proces­sing speed. Optical proces­sing can generate little to no heat, meaning using light can make computers much faster and more efficient. The team achieved the effect by using a metal channel to focus the light inside a polymer previously inves­tigated for use in solar panels. Metals are more efficient at focussing light than tradi­tional transparent materials, and are also used to direct elec­trical signals. The new tech­nology is there­fore not only more effi­cient, but can be inte­grated with current elec­tronics.

Rupert Oulton, from the Depart­ment of Physics at Imperial said: “The use of light to transfer infor­mation has gotten closer to our homes. It was first used in trans­atlantic cables, where capa­city was most crucial, but now fibre optic broadband is being installed in more and more streets in the UK. As our hunger for more data increases, optics will need to come into the home, and even­tually inside our computers.” As well as providing an important step towards optical computing, the team’s achieve­ment poten­tially solves a long­standing problem in nonlinear optics. Since inter­acting light beams with different colours pass through a nonlinear optical material at different speeds, they can become ‘out of step’ and the desired effect can be lost. In the new device, because the light travels such a short distance, it does not have time to become out of step. This eli­minates the problem, and allows non­linear optical devices to be more versatile in the type of optical processing that can be achieved. (Source: IPL)

Reference: M. P. Nielsen et al.: Giant nonlinear response at a plasmonic nanofocus drives efficient four-wave mixing, Science 358, 1179 (2017); DOI: 10.1126/science.aao1467

Link: Dept. of Physics, Imperial College London, London, UK

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