Mid-IR Plasmonic Antennas Fabricated by FIB and EBL: Does an Ultrathin Silicon Oxide Layer Matter?

Lukáš Břínek Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic CEITEC BUT, Brno University of Technology, Brno, Czech Republic Tomáš Šamořil Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic CEITEC BUT, Brno University of Technology, Brno, Czech Republic Ondřej Tomanec Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic CEITEC BUT, Brno University of Technology, Brno, Czech Republic Michal Kvapil Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic CEITEC BUT, Brno University of Technology, Brno, Czech Republic Radek Kalousek Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic CEITEC BUT, Brno University of Technology, Brno, Czech Republic Martin Hrtoň Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic CEITEC BUT, Brno University of Technology, Brno, Czech Republic Jan Čechal Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic CEITEC BUT, Brno University of Technology, Brno, Czech Republic Petr Dub Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic CEITEC BUT, Brno University of Technology, Brno, Czech Republic Jiří Spousta Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic CEITEC BUT, Brno University of Technology, Brno, Czech Republic Peter Varga Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic CEITEC BUT, Brno University of Technology, Brno, Czech Republic Tomáš Šikola Institute of Physical Engineering, Brno University of Technology, Brno, Czech Republic CEITEC BUT, Brno University of Technology, Brno, Czech Republic

One of the most straightforward applications of FIB technology is fabrication of plasmonic nanoantennas by milling their shapes out of metallic thin films. However, we have found that the Mid-IR Au rectangular antennas show different behaviour according to their fabrication origin. These antennas prepared by EBL on transparent samples (e.g. Si) possessed linear scaling of resonance frequency with their arm length. Contrary to that, those prepared by Ga (Xe) FIB milling of the Au layer (60 nm) revealed more complex behaviour. In principle, the resonant peak of relative reflectance of antennas (measured by FTIR) was split into two sub-peaks separated by a dip at λ ≈ 8.2 μm. The short-wavelength sub-peak was moving with the antenna arm length towards higher wavelengths until its motion almost stopped at higher arm lengths. However, the long-wavelength sub-peak almost did not move with the antenna arm length for shorter arm lengths and then started to move towards higher wavelengths.

In the presentation such an unexpected effect will be explained and the proofs for it given. Our previous work [1] indicated that this complex behaviour can be related to a strong coupling of antenna plasmons to the absorbing substrate modified by ions. X-ray Photoelectron Spectroscopy revealed the presence of a silicon-oxide layer having an enhanced thickness (6 nm) with respect to the native oxide (≈ 2 nm) at the silicon surface in the vicinity of antennas fabricated by Ga or Xe ions. FDTD simulations (Lumerical) proved that such an ultrathin oxide layer absorbing in Mid-IR was able to cause the splitting of the resonant peak. Finally, removing the oxide layer by HF etching the dip in the MID-IR resonant peaks disappeared and the antennas started permanently to show up the same spectra like in case of the EBL antennas. The mechanism of the formation of such an oxide layer will be explained as well.

[1] T. Šikola, R. D. Kekatpure, E. S. Barnard, J. S. White, P. Van Dorpe , L. Břínek, O. Tomanec, J. Zlámal, D. Lei, Y. Sonnefraud, S. A. Maier, J. Humlíček, M. L. Brongersma: Appl. Phys. Lett. 95 (2009), 253109.

tomas.samoril@seznam.cz