Scientists have created a first building block for an optical switch that consumes record-low energy – a big step forward in gearing optical circuits towards faster long-distance communication networks.
Optical or ‘photonic’ circuits control the flow of light the way an electrical circuit controls the flow of electricity, only much faster – 10 to 100 times faster. Optical circuits are also more energy-efficient than electrical circuits because they have lower heat loss, better signal-to-noise ratios and are less susceptible to interference.
Photonic technologies are currently used mostly for long-distance telecommunications through optical fibres, according to Vincenzo Savona from École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. “There, an optical switch is needed to encode a digital signal – for example a sequence of 0 and 1 – on a light beam. This is done by switching the light passage on and off,” he explains. The switch – the optical equivalent of an electronic transistor – is effectively a tiny cavity that traps the photons of light within a space of a few nanometres.
Creating a faster and more energy-efficient switch
As recently reported in the journal Applied Physics Letters, Savona and colleagues have created a ‘photonic crystal nanocavity’ (PCN), fabricated from a silicon slab, that requires a record-low amount of energy to operate as a switch.
“[A switch] needs to be fast, but also to consume as low power as possible,” Savona says. “Our system improves on previous ones in these two respects. Hence, our result should be preliminary to the development of better performing optical devices for faster long-distance communications.”
One of the challenges in creating optical circuits is precisely their efficiency in terms of speed and energy consumption. These two features are linked together, as an optical circuit’s total absorbed power depends on the energy required by a single ‘switch’ operation multiplied by the number of operations per second. Consequently, for a ‘nanocavity’ to be used as a switch in an optical circuit, it should be designed for minimal switching energy.
Minimising switching energy with small size and high Q factor
Savona and colleagues’ new PCN requires very low energy for acting as a switch, thanks to its record-small size combined with a very high quality or Q factor. The Q factor is 500,000, which means that a photon will bounce back and forth inside the tiny optical cavity five hundred thousand times before escaping.
A small amount of incoming light will change the wavelength of the trapped light, owing to the optical properties of the cavity material. These properties are referred to as ‘non-linear’, meaning that if a small amount of light can make the optical cavity resonate, a higher light intensity can cause it to switch between two different states – ultimately blocking or allowing the flow of light. And the small size of the PCN produces a higher light intensity for the same energy.
“The non-linearity is proportional to the intensity and the effect is stronger if you allow for longer build-up times,” explains Savona in a press release. “This is because light interacts longer with the material that provides the non-linearity.”
Savona’s colleague Romuald Houdré adds: “In this work we have achieved non-linear effects at a record-low intensity of light. Our structure is also one of the smallest ever designed to show such record non-linear properties, and it may be built using standard nanofabrication technology. This is a very important step along the road to optical circuits, as small size, speed and low power consumption are key requirements for the realisation of an efficient optical switching nano-device.”
Adapted from EPFL Mediacom