Gold nanoantenna interferes with itself

Although their properties have been studied in many experiments, the precise radiation mechanism of plasmonic nanorods and ridges has remained unknown. AMOLF researchers now have solved this problem, by using an in-home developed experimental technique that allows measuring the antenna response with deep subwavelength spatial resolution.

By exciting gold antennas with a highly energetic nanometer-sized electron beam and subsequent detection of the radiation, they were able to measure which colors the antennas emit and in which direction this emission goes. From these results they were able to deduce how radiation emission exactly occurs in these structures. This finding is potentially important for many applications including sensing and optical computer chips. The research appears today in the journal ACS Nano.

Ridge antennas
The researchers fabricated gold ridge antennas by very precisely removing material around the antenna with a Ga⁺ ion beam. In such a ridge, light can couple to a surface plasmon polariton (SPP) which is a charge wave on the metal surface that travels along the length of the ridge. The end facets of the ridge act as reflecting mirrors for this wave giving rise to standing wave resonances in the antenna, just like sound waves can create standing waves in organ pipes but then 1.000.000 times smaller.

Cathodoluminescence spectroscopy
A novel technique developed at AMOLF, named angle-resolved cathodoluminescence spectroscopy (ARCIS) was used to study the optical properties of these antennas. In this technique the SPPs are generated by a 30 keV electron beam in a scanning electron microscope (SEM). The light emission is collected by a paraboloid mirror and then imaged by a CCD camera. This technique combines deep-subwavelength spatial resolution with the ability to see in which direction light is emitted, making it an ideal technique for studying nanoantennas.

Radiation mechanism
In the article the researchers show that the SPP ‘sticks’ to the waveguide during propagation and can only escape at the end facets, where the SPP is transformed into free space light again. These end facets act as point sources of light waves which interfere with each other, giving rise to intense light emission in some directions and none in others. This effect is very similar to dropping two marbles in water on different positions inducing two circular water waves that interfere constructively in some points and destructively in others. In the future, nanoantennas may be used to the increase the efficiency of LEDs, solar cells and biological sensors.

Reference
T. Coenen, E.J.R,. Vesseur, and A. Polman
Deep-subwavelength spatial characterization of angular emission from single-crystal Au plasmonic ridge nanoantennas
ACS Nano DOI: 10.1021/nn204750d (2012)

Figure 1: (a) Electron microscopy image of a 700 nm long ridge antenna. The insets show a close-up of the structure and a schematic representation of the experiment. The scale bars are 500 nm. (b) The cathodoluminescence spectroscopy technique is able to image the antenna (from the top) with 10 nm spatial resolution. In the image the bright yellow areas correspond to antinodes in the standing wave pattern whereas the red areas correspond to nodes in the pattern. This particular image corresponds to light with a wavelength of 650 nm (red light). (c) We were also able to measure in which direction the light was emitted for different positions on antenna. Yellow corresponds to directions for which the emission intensity is high whereas the black regions represent direction for which the emission intensity is low.