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Nanoscale light source can change its colour

30 Jul 2009

Tiny 'free electron laser' could find applications in displays, medical diagnostics.

An international collaboration claims to have made the first tuneable nanoscale light source that is driven by free electrons. Light is created by directing a beam of electrons through a tiny aperture that has been drilled into a stack of alternating gold and silicon-dioxide layers. Interaction between the electron beam and the alternating layers generates visible and infrared light emission.

The device resembles a free-electron laser in which a beam of electrons passes through an alternating magnetic field – causing the electrons to "wiggle" and emit light.

The invention could lead to an on-chip light source for nanophotonic circuits, according to the partnership, which involves researchers at the University of Southampton, UK, National Taiwan University and theorists at CSIC in Madrid, Spain.

"Nanoscale devices require nanoscale light sources, and tunability would provide a range of opportunities, such as spectroscopic lab-on-a-chip devices for medical diagnostics," said Southampton's Kevin MacDonald.

Next-generation displays could also benefit from a tunable light source, according to MacDonald. Switching to this type of device could eliminate the need for separate pixels that deliver different colours of light, such as red, green and blue.

The team fabricated their "lightwells" by depositing alternating 200 nm thick gold and silicon-dioxide layers onto a silicon substrate, before a focused ion beam milled a 700 nm diameter hole in the metal-dielectric stack.

Researchers at Southampton then used an electron microscope to fire a beam into the device. In principle, the beam could instead be supplied by an integrated free-electron emitter – a technology that has already been developed for microelectronic and flat-panel display applications.

As electrons pass through the aperture they create a dipole, owing to the presence of "image charges" in the gold layers. This dipole oscillates, producing light emission thanks to the alternating dielectric environment encountered by the electron as it passes through the well.

By adjusting the energy of the beam from 20–40 keV, the emission is tuned from the red to the near infrared. "However, with adjustments in structural periodicity we anticipate that lightwells could operate anywhere from the ultraviolet range to the terahertz domain," MacDonald told our sister website physicsworld.com.

Twin peaks

Two broad emission peaks were produced by the structure: one that shifted from 830–750 nm and the other 910–800 nm as the electron energy increased. The number of emission peaks depends on the physical dimensions of the device.

The emission lines are broader than 150 nm, which may be too wide for some applications. However, it should be possible to produce narrower emission lines by simply extending the length of the well.

The efficiency of the light generation process is very low, with just 2–4 photons produced for every 100,000 electrons injected. Substantial improvements are possible, however, by optimizing the lightwell geometry, material composition and pumping regime.

A fuller understanding of the emission process requires the inclusion of the more complicated interaction between the electron and the dielectric silicon-dioxide layers. Relativistic corrections also need to be included in the calculations, along with the light-guiding properties of the silicon-dioxide layers, and the interaction of metal-dielectric interfaces with surface plasmons – which are collective oscillations of electrons.

Nikolay Ledentsov, chief executive of the laser manufacturer VI Systems, thinks that the development of the lightwell is an interesting piece of fundamental research. However, he says that because this emitter is a plasmonic structure, it will be hampered by losses that could prevent deployment in commercial applications. Realizing single-wavelength emission is another obstacle, alongside higher efficiencies for every part of the system.

The work can be accessed on the arXiv server. It is currently under review for journal publication.

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