Upright standing gold monopole nanoantennas are fabricated by irradiation of thin gold films with single pulses of fs-laser radiation. The resulting antennas exhibit extinction resonances in the mid infrared spectral rage for p-polarized light under grazing incidence. Due to the free charge carriers in the surrounding gold film of the antenna, the resonance condition of the thin-wire monopole antenna can be explained by introducing image charges yielding an observable resonance wavelength of four times the antenna length. The antenna length is controlled coarsely by the focusing numerical aperture and fine by the pulse energy of the laser pulse producing the structure. An additional ultrafine tuning of the resonance wavelength with a sub-10 nm resolution is realized by an additional coating process subsequent to the laser structuring.

An aperture-type scanning near-field optical microscope (a-SNOM) is readily used for the optical and optoelectronic characterizations of a wide variety of chemical, biological and optoelectronic samples with sub-wavelength optical resolution. These samples mostly exhibit nanoscale topographic variations, which are related to local material inhomogeneity probed either by an optical contrast or by secondary effects such as photoconductivity or photoluminescence. To date, in the interpretation and evaluation of the measurement results from a-SNOM or derived methods, often only the local material inhomogeneity is taken into account. A possible influence of the optical interaction between the scanning probe and the surface topography is rarely discussed. In this paper, we present experimental and theoretical investigation of the effects of nanoscale topographic features on a-SNOM measurement results. We conduct local photocurrent measurements on a thin-film solar cell with an a-SNOM as the illumination source. A clear correlation between the photocurrent response and local topography is observed in all measurements with a signal contrast of up to ∼30%, although the sample features homogeneous permittivity and electrical properties. With the help of finite-difference time-domain (FDTD) simulations, this correlation is reproduced and local light coupling is identified as the mechanism which determines the local photocurrent response. Our results suggest that a-SNOM-based measurements of any sample with material inhomogeneity will be superimposed by the local light-coupling effect if surface topography variation exists. This effect should always be taken into consideration for an accurate interpretation of the measurement results.

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