Localized and Propagating Plasmons in Metal Films with Nanoholes

Abstract

The occurrence of plasmon resonances in thin (~20 nm) Al and Au films, perforated with nanoholes, was studied. In both metals, two plasmon resonances were observed: (i) A surface plasmon polariton mode associated with a maximum in extinction and (ii) a localized resonance in the nanohole associated with a minimum in extinction. By varying the diameter of the nanoholes, the scaling of the peak positions of the plasmon resonances was determined as a function of hole diameter. In the large nanohole limit, the plasmon peak positions depend only on the nanohole diameter being independent of the material. On the other hand, for small nanoholes the plasmon peak positions are material and size dependent. In contrast to Al films where the localized plasmons can be excited from the near-IR to the UV, no plasmon resonances were observed for Au at energies above the interband threshold (2.4 eV). The interaction between a distinct interband transition in Al at 1.5 eV and the localized plasmon resonance is considered in detail. We observe for the first time experimentally a noncrossing behavior of the interband transition and the localized plasmon resonance. The energy (size) dependence of surface plasmon peak width, being a measure for the decay/damping of the latter, is very different for the two metals. This can be explained by considering the different decay mechanisms active in the two metals. Apart from these basic plasmonics results, we test the potential of using the shifts of the plasmon resonances in perforated Al films to follow the atmospheric oxidation/corrosion kinetics of Al. The results are quantified by model calculations. The obtained kinetic law for the oxide growth is in good agreement with a previous XPS study on plain Al films. This suggests that the nanohole-induced plasmon resonances can be a sensitive and simple measure for Al corrosion and metal corrosion in general.

Strategies for enhancing the sensitivity of plasmonic nanosensors

“…Additionally, the refractive index sensitivity of gold nanorods is linearly proportional to the aspect ratio, facilitating its tunability [97]. Nanostructures of other shapes, including nanocubes [106][107][108][109], nanoshells [110][111][112], nanodiscs [113][114][115], nanotriangles [116,117], nanostars [118][119][120], nanobipyramids [121,122], nanorice [123,124], nanoholes [125,126], and nanocrescents [56,[127][128][129][130], have also been investigated and expanded to nanocubes and nanocuboids enclosed with concave surfaces (Fig. 1).…”

Robust and Versatile Light Absorption at Near-Infrared Wavelengths by Plasmonic Aluminum Nanorods

“…The resulting repulsive noncrossing energy behavior between the LSPR and the IT has recently been experimentally confirmed in Al films with nanoholes. 23 To identify the contribution of the IT to the total optical absorption, the spectral properties of the nanorods were simultaneously calculated using the dielectric function ε of Al measured by Rakic 36 and the extracted Drude contribution only, ε Drude . To separate the free electron (or Drude) behavior and the interband component, we fit the real and imaginary part of the measured ε with a Drude-Lorentz model, expressed here as a function of the frequency ω: (1) ω p and γ c are the plasma frequency and the collision rate.…”

Plasmon-induced photonic and energy-transfer enhancement of solar water splitting by a hematite nanorod array

“…The long-range ordered gold nanohole array supports both propagating (SPP) and localized (LSPR) plasmon modes, which is the key to overcoming the scattering/absorption trade-off. On a local scale, the nanohole in the metal film leads to a LSPR mode with energy below the band gap of hematite 35,36 , which increases the spectral coverage through PIRET. The long-range order of the nanohole array creates a grating, giving incident light sufficient momentum to excite a SPP mode at the energies above the band edge of hematite.…”