Fabrication of plasmonic structures
Fabrication of plasmonic structures
In this project we are interested in the interaction of metallic nanostructures with light, e.g. with ultrashort laser pulses. These nanostructures can play a crucial role when focusing electromagnetic waved below the diffraction limit of light. As a result nanometer-sized areas can be generated, in which the electric field strength is order of magnitude larger than in the surrounding region. In principal, the electric field strength is inversely proportional to the antenna distance. This field of enormous strength now allows investigating the interaction of light with few or even singling atoms or quantum emitters.
Fabrication process of the antenna structures
These nanostructures are fabricated by milling them from a very thin, few-nanometer thick gold film using ion beam lithography. By using helium ions for the lithography process instead of gallium using the Zeiss Orion Plus, sub-10 nm feature sizes are now producible. In Fig. 1, these new, highly precise plasmonic antennas using Helium-Ion Lithography are compared to standard gallium-ion beam-produced antennas.
By using the newly developed technique of Helium ion beam lithography, plasmonic components can be produced with feature sizes of less than 6 nm. These structures are able to concentrate the electric field within their antenna components extremely efficient.
Nonlinear microscopy with ultrashort laser pulses
We now investigate the interaction by using a spectrally-resolved, nonlinear microscopy (Fig.2). Few-cycle, ultrashort laser pulses from a Ti:Sapphire laser oscillator are focused to their diffraction limit by using an all-reflective Cassegrain objective. The sample is mounted on a piezo stage and can be moved perpendicular to the beam with nm precision. We now detect the third harmonic (TH) of the incident laser light in a spectrometer attached to a liquid-nitrogen cooled CCD camera. The intensity of the TH is herein taken as a measure for the electric field strength within the two components of the nanoantenna.
Our measurement show that for the antennas produced using helium ion lithography the intensity of the TH increased by a factor of three when compared to the antennas produced using standard Ga-FIB (see Fig. 3(a)). This seems to be especially remarkable, since the emission is originating from an even small volume from the gap region..
We simulated the interaction of the light with the nanostructure using a finite element method. These simulations confirm our measurements about the increased confinement of the electric field for the smaller gap structures. In Fig. 3 and, the two-dimensional distribution of the electric field within the nanostructure is shown both for the 20 nm gap antenna produced using Ga-FIB (b) and for the 6 nm antenna produced using helium ions (c). The increase of the field enhancement for the latter structure is clearly visible.
These results show the potential of helium-ion based lithography for the production of plasmonic antennas. We are now able to fabricate plasmonic antennas with an unrevealed spatial precision. Plasmonic structures with tailored optical and electronic properties are now within reach. We therefore hope to push the investigation of light-matter interaction to a new limit.
Our results have now been published in NanoLetters:
H. Kollmann et al., Towards Plasmonics with Nanometer Precision: Nonlinear Optics of Helium-Ion Milled Gold Nanoantennas, Nano Letters 14, 4778-4784 (2014)