Snowflake-shaped electrodes boost graphene's photoabsorption

January 20, 2017 // By Julien Happich
Thanks to a novel gold fractal metasurface design, an international team of scientists from Purdue University and the Technical University of Denmark have managed to tune graphene's natural wideband optical absorption by over an order of magnitude.

Published in Nano Letters, their article "Enhanced Graphene Photodetector with Fractal Metasurface" describes how the naturally poor photoabsorption of graphene (merely 2.3% of incident light) can be largely enhanced thanks to the metasurface's role in facilitating the generation of electron−hole pairs.

The new approach beats other plasmonic nanostructure-based solutions which tend to be narrowband and polarization dependent, write the researchers, noting that their proposed gold fractal metasurface design has a relatively flat optical absorption in the visible part of the spectrum and is polarization insensitive.

The metasurface consists of 40nm thick and wide gold lines patterned into a densified fractal snowflake geometry through electron beam lithography, out of a Ti (3nm)/Au (40nm) metallization layer.

Conceptual illustration of the graphene-based fractal photodetector. Source Purdue University.

About 10μm in diameter, the tree-like structure incorporated six new branches spreading out from each new root points to four levels, and an additional three-level fractal structure densifying the original fractal concentrically. In their experiment, the fractal metasurface was connected to the drain of a graphene field effect transistor (with a graphene layer grown on a highly p-doped silicon substrate), circled out by a circular gold pattern extending out from the source.

For reference, the researchers also fabricated a tip-and-ring structure (without fractal metasurface) in parallel to the fractal metasurface-enhanced FET.

Scanning electron micrograph (SEM) of the graphene photodetector with the fractal metasurface and tip-and-ring structure (the white scale bar is 10μm); inset image shows the zoomed-in view of gold fractal metasurface (the white scale bar is 1 μm). Source Purdue University.

In agreement with their numerical simulations, they observed that when visible light hits the fractal metasurface, it excites plasmon oscillation in the gold fractal structure, which in turn confines and enhances the electric field of the incident electromagnetic wave within nanometers of the structure. This, they write, contributes to an extensive electron−hole pair generation and elevates the electron temperature through electron−electron interactions in graphene.

The generated carriers are then spatially separated/driven via the built-in electric field (PV) and thermoelectric power differential (PTE) at gold−graphene interface, giving rise to a detectable photovoltage.