Electro-Optical Nano-Antennas

One of our main interests are electro-optical nano-antennas. These antennas are so small that they are no longer resonant in the MHz or GHz regime, but in the regime above 400 THz, i.e., for visible light. At these frequencies, no semiconductor works anymore and, hence, we had to come up with a new method to electrically drive them. In the following an overview is given how the structures are designed to allow an electrical and optical access as well as the light generation.

The substrates for our nano-antennas are microscope cover slips which are 24 mm in width and height as well as 170 µm in thickness. They are made of transparent BK7 glass such that when mounted on an inverted optical microscope, structures sitting on top of the substrate can still be characterized from below.

A macroscopic view of a samples with three positions (A, B & C).

In order the obtain an electrical connection we prepare the slips with an electrode infrastructure using optical lithography and electron-beam evaporation. The electrodes consist of Chrome as an adhesion layer and a thicker Gold film that is soft for easy contacting and also chemical inert.

Zoom-in to the ‘B’ position which reveals a gold flake lying on the electrode infrastructure. (Note, from here on all pictures are black & white SEM images which were colorized to make them easier to understand.)

Afterwards Gold flakes are chemically grown and suitable ones are selected. They are transferred to the electrode substrate using PMMA and micromanipulators. The flakes are typically around 100 µm in lateral size and 50 nm in thickness, i.e., they have a very high aspect ratio.

A further zoom-in highlights the ground electrode as well as the ends of the six top and six bottom electrodes (o1-o6 & u1-u6).

The samples are then mounted on a conductive holder to structure them with Focused Ion-Beam milling (FIB). This is usually done in several steps starting with coarse isolation of the electrodes down to the fine-cutting of the antenna gap.

A further magnification shows a connected antenna with one connecter going down to the ground stripe and the other up to the ‘o4’ electrode.

After the structuring, the samples are eventually imaged with a Scanning Electron Microscope (SEM) before they are mounted on an inverted optical microscope for the electro-optical characterization.

And finally, we see a close-up of the two antenna arms separated by a 20-nm gap.

Now the ‘o4’ electrode of the ‘B’ position can for example be contacted with a micromanipulator and a voltage be applied. Due to the electrode infrastructure this potential difference directly propagates down to the tiny antenna gap. Hence, when applying 2 V over the 20 nm gap this means for example a field of 100 MV/m. As a reference, spark plugs in car engine are operated up to 35 kV while typically having a gap of around 0.6 mm which equates to ~60 MV/m.

An electrically connected optical antenna featuring a functionalized Gold particle sitting asymmetrically inside the gap. The resulting 1-nm-barrier between the particle and the right antenna arm can be used to generate light.

Even higher fields and smaller distances can be achieved by putting a functionalized particle inside the gap that touches one arm but not the other. When applying a voltage under these conditions, the electrons now not only accumulated at the 1-nm barrier, but they can tunnel through it. While doing so, light is inelastically generated and radiated via the antenna. It can be later collected with the microscope objective and, hence, analyzed.

It is worth to point out that such a barrier is only three Gold atoms wide.

An Yagi-Uda antenna for light consisting of (from left to right) a reflector, a feed element and three directors.

Finally, one can add further functionalities to such a nano light source like a directional emission which is desirable for on-chip data communication. The directionality can be achieved by adding reflectors and directors to the antenna similarly to the Yagi-Uda antennas still presents on many roof tops.