Tunable light source reaches nanoscale milestone

Marie Freebody, mariefreebody@physics.org

In the race to develop ever-smaller and -faster optical devices, researchers have built the first tunable nanoscale light source driven by free electrons. Dubbed the “light-well” by its creators, the novel emitter could one day be used as an on-chip light source for nanophotonic circuits or in large assemblies for optical memory or display applications.

Dr. Kevin MacDonald, senior research fellow at the University of Southampton and co-inventor of the light-well, said that tunable chip-scale light sources could open up a range of possibilities in nanophotonics, including spectroscopic lab-on-a-chip devices for medical diagnostics. In other sectors, such devices could, for example, eliminate the need for individually colored pixels in next-generation displays. Instead, a single light-well at each pixel could emit the full range of colors required.

The light-well, seen here in an artist’s impression, is a tunable chip-scale light source that could open up a range of possibilities in nanophotonics. Images courtesy of the Optoelectronics Research Centre, University of Southampton.

The most significant aspect of the light-well is that it combines nanoscale size with tunability. Many of today’s electron-beam-driven radiation sources, such as synchrotrons and free-electron lasers, are inherently tunable but are generally macroscopic – often facility-scale systems. On the other hand, many of the nanoscale light and surface plasmon sources that have been proposed in recent years are fixed-wavelength.

The Southampton group, with partners in Taiwan and Spain, describes its work in a paper on the arXiv server; the paper is currently under review for journal publication. Although the study is at the proof-of-principle stage, the team sees a path toward chip-scale integration.

Anatomy of a light-well

“In our experiments, we used a scanning electron microscope to drive light-well emission,” MacDonald said. “However, chip-scale electron sources are already well developed for vacuum-microelectronic and flat panel display applications. The task of integrating such sources with light-well structures is not trivial but should certainly be possible.”

The internal periodic structure is visible in this electron microscope image of an experimental light-well.

The light-well comprises a 700-nm hole drilled through a stack of alternating layers of gold and silica, each with a thickness of 200 nm. Although structurally simple, the light-well’s emission is not yet fully understood and remains a key challenge for the team.

In a broad approximation, light is generated by a beam of electrons directed through the tiny aperture in the metal-dielectric stack via the formation of dipoles between electrons and their “mirror images” in the wall of the well. As the electrons travel down the well, the alternating dielectric environment causes the dipole to oscillate and emit light. In the experimentally demonstrated structure, adjusting the energy of the electron beam from 20 to 40 keV enables the team to tune the wavelength of the emitted light from red to the near-infrared. Varying structural periodicities will provide access to different spectral ranges.

The light-well provides an output intensity of 250 W/cm² at the peak emission wavelengths, which corresponds to an efficiency of around 3 × 10^–5 photons per electron. MacDonald admits that there is considerable work to be done before commercialization can be considered. The light-well structure must be optimized to maximize output intensity and achieve narrower emission linewidths, and it must be engineered for integration with chip-scale electron sources.

“We are currently working on numerical and analytical studies in preparation for the fabrication of new sample structures and further experimental investigations,” MacDonald concluded. “For example, we expect to achieve narrower linewidths in longer wells.”