Actively steering 'virtual particles' key to advances in optoelectronics

The steady improvement in speed and power of modern electronics may soon hit the brakes unless new ways are found to pack more structures into microscopic spaces. Unfortunately, engineers are already approaching the limit of what light – the choice tool for 'tweezing' tiny features – can achieve. But there may be a way of reaching beyond this diffraction limit by precisely steering, in real time, a curve-shaped beam of virtual particles known as surface plasmons.

This technique, described in the Optical Society's (OSA) journal Optics Letters, opens the possibility of even smaller, faster communications systems and optoelectronic devices. Examples of optoelectronic devices used today include photodiodes such as solar cells, integrated optical circuits used in communications, and charged-coupled imaging devices at the heart of cell phone cameras and receivers on the world’s most advanced telescopes. This method also may yield new, important tools for research in chemistry, biology, and medicine.

The key to this innovation is the ability to manipulate a blended stream of light and plasma actively, known as a plasmonic Airy beam. The beam, owing to the laws of electromagnetism, travels, not in a straight line like a beam of light, but rather in an arc.

As the beam first strikes a metal surface (typically at an irregular feature called a grating structure), it stirs up small waves of electrons at the metal-insulator interface. These waves, which can be thought of as 'virtual particles' known as surface plasmon polaritons (SPPs), then follow the curved trajectory of the Airy beams. And, just as ocean waves move objects on the surface of the water, the SPPs can be directed to manipulate ultrafine-scale features on the surface of a metal.

SPPs are already essential elements in the design and manufacture of optoelectronic devices. The reason they're so critical is that they can affect extremely small-scale objects, smaller than the diffraction limit, or half of the wavelength of light used to create SPPs.

The current systems, however, have a significant drawback: they required fixed, permanent nanostructures to direct the SPPs. This lack of flexibility severely limits their uses in nano-system design and manufacture. But by being able to manipulate the Airy beam, and therefore the SPPs, in real time, the new design gives scientists on-the-fly control.

'We have demonstrated a new way of routing the flow of surface plasmons without any guiding structures,' said Xiang Zhang, who led this research and is the director of the NSF Nanoscale Science and Engineering Center at Berkeley and a faculty scientist with the Materials Sciences Division of the Lawrence Berkeley National Laboratory.

Currently, permanent guiding structures, such as waveguides, lenses, beam splitters, and reflectors are used, which cannot be reconfigured; once the structure is fabricated it cannot be changed in real time.

By using computer-controlled optics, however, the research team has developed a way to steer and manipulate the beams, precisely directing their trajectories to specific spots on an optical surface and adjusting them as needed. Due to their unique arc-shaped paths, the beams have the added ability to bypass surface roughness and defects, or even vault over obstacles.

'These on-the-fly adjustments are extremely desirable,' commented Zhigang Chen, a principal investigator with the Department of Physics and Astronomy at San Francisco State University (SFSU). 'They enable reconfigurable optical interconnections in ultra-compact integrated photonic circuits, which are at the core of many high-speed computing technologies. They also would enable on-chip nanoparticle manipulations for chemical, medical, or biological research purposes.'

The Airy beams used to direct the flow of plasmons also remain coherent, not fanning out or distorting as they travel along their curved trajectories, much in the same way that laser light remains coherent even after travelling great distances.

Further examples of potential applications of the research include designing practical reconfigurable plasmonic devices for ultra-compact integrated photonic circuits; in biology and chemistry, researchers may establish new tools for dynamically manipulating nanoparticles or molecules, and improving the performance of sensors; and the ultrafine wavelength nature of surface plasmons makes them a promising tool for future nanolithography or nanoimaging applications.

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