When light couples to electrons on a surface, their concerted motion can travel as a wave guided by the surface geometry itself. Known as ‘surface plasmons’, these waves could impact the development of telecommunications and computing as in the future data will likely be processed using light instead of electricity. Not only is the use of light more energy-efficient than electricity, it also allows developers to reduce the processors’ size to the nanoscale – a necessary step in the quest to build high-resolution sensors and nano-sized signal processing systems.
The challenge, however, is that to build these nano-sized processors we must first be able to stack different layers of advanced materials and track the guided light as it travels across the layers. Unfortunately, scientists have not been able to accomplish this – until now.
According to a recent study published by the journal ‘Nature Communications,’ researchers have reached a breakthrough for future optical-electronic hybrid computers. Scientists from the Ecole Polytechnique Fédérale de Lausanne (EPFL), working via the EU-funded TRUEVIEW and USED projects, developed an ultrafast technique capable of tracking light and electrons as they travel through a stacked, nanostructured surface.
The USED project focused on the understanding and control of material properties at the atomic level. The project was the first to successfully implement an ultrafast Transmission Electron Microscope (TEM) based on a new design that enables an unprecedented time resolution and sensitivity to magnetic contacts. A TEM is an advanced telescope that allows the user to take femtosecond snapshots of materials with the atomic resolution guaranteed by high-energy electrons. By confining an electromagnetic field on the surface of one single nanowire and imaging its properties in space and energy, the USED-designed TEM takes a snapshot of light itself, simultaneously revealing its quantum and classical nature.
TRUEVIEW, on the other hand, successfully unravelled the working principles of nanoscale-confined optical waves and the manipulation of light in optoelectronic nanostructures. By implementing innovative electron imaging techniques to directly visualise and characterise photonic and plasmonic nanostructures in both space and time with nanometer and femtosecond resolution, the project successfully established the field of ultrafast electron microscopy within the European research community.
Lights, camera, nano-action
Combined with the USED designed TEM, the two projects laid the groundwork for an array of optoelectronic applications, including the ultrafast technique for tracking light and electrons across stacked nanostructured surfaces. The process includes a tiny antenna array consisting of an extremely thin membrane of silicon nitride, which is then covered with an even thinner film of silver. The array’s surface is full of nano-holes, which serve as antennas and allow plasmons to travel across its interface.
These antennas are then lit up by firing ultrafast laser pulses onto the array, followed by ultrashort electron pulses fired across the multilayer stack. This process allows scientists to map the plasmons radiated by the antennas at the interface between the silver film and the silicon nitride membrane. By using the ultrafast PINEM technique, scientists are actually able to film the propagation of the guided light and read its spatial profile across the film. In a sense, the USED and TRUEVIEW breakthrough gives scientists the ability to see through walls – and from here they can design the confined plasmonic fields in multi-layered structures that are needed for the development of optoelectronic devices.