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Bristol team looks to Chalcogenides for optical networks

Chalcogenide phase change materials could allow low cost dynamic optical switching

Above: Sketch of a tunable Ge2Sb2Tereflectarray. Each subarray consist of two dielectric loaded antennas and four patch antennas

Research by Maciej Klemm and Martin Cryan from the University of Bristol has shown that chalcogenide phase change materials such as Ge2Sb2Te5 could potentially be used to transform high-speed optical neworks.

Chalcogenide compounds are used in rewritable optical disks and will be used in next generation phase change memory with high speed electronic switching. By applying heat, these semiconductors can be switched between an amorphous (glassy) and a crystalline state, changing their optical and electrical properties and allowing the storage of information.

The Bristol researchers have adapted these approaches to develop a low cost, high-speed optical switching technology based on dynamic optical beam shaping and steering using PCM-based optical antennas and antenna arrays. 

Optical antennas and antenna arrays offer many possibilities in manipulating light at sub-wavelength scale. Similar to their microwave counterparts, optical antenna arrays can shape and steer beams by controlling the phase of each element. In order to demonstrate this, the researchers modelled and simulated a phase-change material-based tunable reflectarray (an antenna that uses array of elementary antennas as reflecting surfaces) configured to form a dynamic optical link between a source and two receivers.

Ge2Sb2Te5 was chosen due to its commercial availability, fast tuning time between two states, large number of switching cycles, and stability. The reflectarray was  based on a structure where 60nm Ge2Sb2Te5 is sandwiched between two 30nm silver layers. The Ge2Sb2Te5 layer provides a tunable dielectric environment for the reflectarray. 

The major challenge in the design was the high dielectric constant of the amorphous Ge2Sb2Te5 layer, which prevents a full 2Ï€ phase control when using only single type of antenna patch antenna. Therefore, a second type of antenna, a dielectric loaded antenna, had to be used to provide the missing phase shift angles. 

The researchers found the proposed reflectarray was able to switch the optical link between multiple optical receivers simply by changing the refractive index of the material.

Maciej Klemm, senior research fellow in the Department of Electrical and Electronic Engineering, said: "Phase change materials together with nanoantennas have the potential to provide fast, dynamic optical switching with very low power consumption levels. Once switched, PCMs require no power to maintain their switched state. This could be a breakthrough technology for applications such as data centres where power consumption is of critical importance."

'Phase change material based tunable reflectarray for free-space optical inter/intra chip interconnects' by Longfang Zou, Martin Cryan, and Maciej Klemm appeared in Optics Express.

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