The "light-well": a tuneable nanoscale free-electron light source on-a-chip

Adamo, G., MacDonald, K.F., Zheludev, N.I., Fu, Y.H., Wang, C.M., Tsai, D.P. and García de Abajo, F.J. (2009) The "light-well": a tuneable nanoscale free-electron light source on-a-chip. MRS Materials Research Society Spring Meeting, , San Francisco, CA. 13 - 17 Apr 2009.

Abstract

An experimental demonstration of a new type of electromagnetic radiation source driven by electron beam, a 'lightwell', which can be used as a nanoscale tuneable emitter of surface plasmons and optical radiation, is here reported. His small lateral size, just few hundred of nanometres, and the compatibility of his structure with silicon-based technology, make this device suitable for employment in nanophotonic circuits as chip-scale free-electron light sources, or in densely packed ensembles for optical memory.

The light-well idea can be seen as a nanoscale analogue of a free-electron laser (FEL). In the FEL a beam of electrons is launched through an accelerator towards a magnetic undulator, or "wiggler", which induces a modulation on the electrons along their path by mean of periodic transverse acceleration, thus resulting in emission of photons. In the FEL the wavelength of the emitted radiation can be tuned by adjusting either the energy of the electrons or the parameters of the wiggler. In the 'light-well', similarly, a beam of free-electrons is launched within a tunnel milled in a periodically layered metal-dielectric nanostructure which forces a modulation on the electron beam by mean of changes in the dielectric constant of the surrounding medium and, as a result, optical photons are emitted at a wavelength dependent on the energy of the incident electrons.

Light-wells have been drawn in a stack of 200 nm thick alternating gold and silica layers sputtered onto a silicon substrate. Nano wells were milled through the various layers using a focused ion beam. Measurements aiming at recording emission of optical radiation were performed in hyperspectral cathodoluminescent imaging mode using a scanning electron microscope equipped with a spectrometer and nitrogen-cooled CCD array for detection and analysis of emitted light. When the electron beam is injected into the hole, the spectrum of the optical radiation shows two peaks whose positions depend on the acceleration voltage applied on the electrons. The total emission intensity increases with beam current and with the injection point approaching the wall of the nano-hole: an optical radiation power of approximately 0.1 nW has been reported.

Theoretical descriptions and full numerical calculations, using the Boundary Element Method, BEM, were carried out and are presented along with the experimental data. The BEM solves Maxwell equations in frequency space expressing scattered electromagnetic fields in terms of boundary charges and currents discretized at representative points. The simulations show a reasonable agreement with the experimental results. They also suggest that the 'light-well' can work as a source of photons as well as surface plasmons.

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Contributors

Author: K.F. MacDonald

Author: N.I. Zheludev

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