Suitable structures have proved difficult to make using conventional lift-off techniques, such as lithography. In such cases, the aspect ratio of the structure is limited to a fraction of the mask thickness. "Typically, the final height of the structure should be a third of the polymer resist layer," Romain Quidant of Institut de Ciències Fotoniques told "You can increase the polymer thickness, but this strongly reduces the resolution of the writing."

The Barcelona-based team found success using electron-beam induced deposition, a direct writing method that not only allows higher aspect ratios, as demonstrated by the creation of gold structures with an aspect ratio of 5:1, but also suits non-flat surfaces.

Computer control of the apparatus' focused electron beam makes it possible to grow dots at specific locations and with a certain size and aspect ratio. Here, the group injected an organometallic precursor containing gold atoms into the vacuum chamber of a scanning electron microscope. By optimizing conditions such as pressure, electron-beam current and the surface material, molecules were made to decompose into gold clusters scattered within an organic matrix.

The group obtained its best results by depositing structures on glass coated with a 4 nm layer of titanium. A post-annealing process was found to substantially increase the gold purity of the nanodots to a level where the team was able to detect a plasmonic response from its structures.

"The purification can be optimized in a way that allows our nanodots to behave in a similar manner to pure gold structures; here I am talking about plasmon resonances – oscillations of the free charges of the metal, which lead to a dramatic concentration of the surrounding optical field," said Quidant.

The researchers presented their work in Applied Physics Letters.