We experimentally determine the order of multiphoton induced luminescence of aluminum nanoantennas fabricated on a nonconductive substrate using electron-beam lithography to be 2.11 (±0.10). Furthermore, we optically characterize these nanostructures via linear dark-field microscopy and nonlinear multiphoton laser excitation. We hereby observe different spectral response functions that can be seen as a splitting of peak positions when the antenna arm length is increased to Larm > 150 nm which has not yet been reported for aluminum nanostructures.

Resonant optical antennas with nanoscale gaps are of high interest due to their ability to enhance electric fields in localized subdiffraction-limited volumes. They are especially attractive for coupling with quantum emitters. One challenge for applications that exhibit a spectral shift is to fabricate nanoantennas that provide two distinct resonances at the excitation and emission frequency, respectively. We propose a coupled T-shaped nanoantenna structure that provides independently controllable resonances with a common electromagnetic hot spot in the antenna gap. We demonstrate the fabrication of such structures and investigate experimentally and theoretically their spatial, time-integrated spectral- and polarization-dependent electromagnetic field properties. The nanoantennas exhibit two resonances with unique spectral and polarization responses. The resonance wavelengths are independently tailored by varying the geometry of individual T-shaped antennas.

Abstract:As a key feature among metals showing good plasmonic behavior, aluminum extends the spectrum of achievable plasmon resonances of optical antennas into the deep ultraviolet. Due to degradation, a native oxide layer gives rise to a metal-core/oxide-shell nanoparticle and influences the spectral resonance peak position. In this work, we examine the role of the underlying processes by applying numerical nanoantenna models that are experimentally not feasible. Finite-difference time-domain simulations are carried out for a large variety of elongated single-arm and two-arm gap nanoantennas. In a detailed analysis, which takes into account the varying surface-to-volume ratio, we show that the overall spectral shift toward longer wavelengths is mainly driven by the higher index surrounding material rather than by the decrease of the initial aluminum volume. In addition, we demonstrate experimentally that this shifting can be minimized by an all-inert fabrication and subsequent proof-of-concept encapsulation.

We perform two-photon excitation confocal experiments on coupled gold nanoantennas and observe time-integrated luminescence spectra that match plasmonic mode emission in the far-field. We show that the transversal particle plasmon mode can be excited, using excitation light that is cross-polarized with respect to the gold luminescence signal and therefore oriented along the long axis of the dipole gold antenna. We provide evidence for losses in polarization information from the excitation channel to the luminescence response due to the nature of the energy and momentum transfer. Finally, we map out the two-photon induced luminescence intensity profile for a fixed excitation wavelength λ and varying antenna arm length L.

Fabrication of small nanoantennas with high aspect ratios via electron beam lithography is at the current technical limit of nanofabrication and hence significant deviations from the intended shape of small nanobars occur. Via numerical simulations, we investigate the influence of geometrical variations of gap nanoantennas, having dimensions on the order of only a few tens of nanometers. We show that those deviations have a significant influence on the performance of such nanoantennas. In particular, their resonance wavelength as well as the magnitude of absorption and scattering cross section and the electric field distribution in the near field is strongly altered. Our findings are thus of importance for applications based on near field as well as those based on far field interactions with nanoantennas and have to be carefully and individually considered in both cases.