Plasmonic nanoantennas are versatile tools for coherently controlling and directing light on the nanoscale. For these antennas, current fabrication techniques such as electron beam lithography (EBL) or focused ion beam (FIB) milling with Ga(+)-ions routinely achieve feature sizes in the 10 nm range. However, they suffer increasingly from inherent limitations when a precision of single nanometers down to atomic length scales is required, where exciting quantum mechanical effects are expected to affect the nanoantenna optics. Here, we demonstrate that a combined approach of Ga(+)-FIB and milling-based He(+)-ion lithography (HIL) for the fabrication of nanoantennas offers to readily overcome some of these limitations. Gold bowtie antennas with 6 nm gap size were fabricated with single-nanometer accuracy and high reproducibility. Using third harmonic (TH) spectroscopy, we find a substantial enhancement of the nonlinear emission intensity of single HIL-antennas compared to those produced by state-of-the-art gallium-based milling. Moreover, HIL-antennas show a vastly improved polarization contrast. This superior nonlinear performance of HIL-derived plasmonic structures is an excellent testimonial to the application of He(+)-ion beam milling for ultrahigh precision nanofabrication, which in turn can be viewed as a stepping stone to mastering quantum optical investigations in the near-field.

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Cd diffusion into GaAs single crystals has been investigated in the temperature range 756–1201 °C. Penetration profiles measured by secondary ion mass spectroscopy and spreading-resistance profiling agree with each other and are numerically reproduced on the basis of the kick-out diffusion mechanism. A major involvement of vacancies via the dissociative mechanism can be excluded. This enables us to deduce from the Cd profiles effective Cd diffusivities (Dseff) as well as Ga diffusivities (DISD) mediated by doubly positively charged Ga self-interstitials (IGa2+). The Dseff values obtained from As-rich and As-poor ambients are mutually consistent. The same holds true for the DISD data. Reduced to 1 atm of As vapor pressure and electrically intrinsic conditions the Cd diffusion coefficient results as 1.76×104 exp(−4.80 eV/kB T) cm2 s−1 and the IGa2+-related self-diffusion coefficient as 3.5×104 exp(−5.74 eV/kB T) cm2 s−1. The present data provide strong evidence that in heavily p-doped GaAs atomic transport related to the Ga sublattice is governed by self-interstitials rather than vacancies.