Photons key to breaking electronic speed limit

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Scientists at Heriot-Watt University in Scotland have reported they are one step closer to technology that could result in electrons being replaced with photons, solving the looming ‘speed limit’ for electronic devices.

According to Dr Marcello Ferrera, an assistant professor in Photonics and Optics at Heriot-Watt, electronics have had such long-term success mainly due to how much smaller devices have become, and how robust they are, even when made from a very limited number of fundamental materials.

These last two features have traditionally been weaknesses in the field of photonics, but Dr Ferrera’s recent findings, published in Nature Communications, could be about to change all of that.

Dr Ferrera and his team have, for the first time, shown how aluminium zinc oxide (AZO) reacts to light when simultaneously shined with ultra-fast laser pulses of different colours.

Since AZO is a compound used in touch-screen technology, the discovery could have an immediate impact for the fabrication of novel photonic components.

The nanophotonics researchers used one laser beam to explore the optical properties of thin films of AZO, while two different trains of ultra-fast light, pulsed at two distinct frequencies, or ‘colours’, were shone on the material. 

During the experiments, three laser beams were used at three different wavelengths. Every beam consisted of ultra-fast optical pulses with time durations spanning from 70 to 120fs, and repletion rate of 100Hz.

The beam at a wavelength of 787nm was employed to record the effects under investigation: it is from the perspective of this ‘probe’ beam that we studied the induced alterations of the material optical properties.

The other two beams, called the ‘pumps’, were used to trigger the desired changes by ‘pumping’ energy into the material. Among the most important physical quantities under study were the transmissivity (the induced change in the material transparency) and the refractive index (the speed at which light propagates in the material compared to the speed of light in vacuum), which are key parameters to design ultra-fast integrated photonic components.

The two different colours for the pumps were chosen so that one pump was providing photons (quanta of light) with energy higher than the material bandgap (energy required to promote an electron in the conduction band), while the photon energy of the second beam was set below the bandgap. This arrangement made it possible to trigger opposite optical effects such as making the material more transparent with one beam and more opaque with the other, and observing the combined results of simultaneous excitations.

The experiments were conducted first by using one colour at a time, and afterwards with the combined use of the two laser sources. The recorded effects, which last for a 10,000th of a billionth of a second, were surprising to say the least.

Dr Ferrera said: 'We discovered that we can drastically and reversibly alter the optical properties of the material by using laser light with different colours. Each colour can induce strong and ultra-fast alteration on both the transparency of the material and the speed at which light propagates into it.'

Dr Ferrera also discovered that the induced alterations, which are typically opposite in sign, can be algebraically summed up one to another. If the material becomes more transparent with one colour and more absorptive with the other, it will not show any appreciable alteration when the optical stimuli occur simultaneously. This behaviour could have striking consequences for the design and fabrication of optical computing and telecommunication devices.

'The reason we used AZO is that, although it has been used in electronic devices for years, we have little knowledge about how it could be used in photonics.

'Electronics have almost reached their capacity and potential; our findings represent a remarkable step towards the full miniaturisation of photonic components: this possibility was just ‘science fiction’ few years ago. '