We report on the near-field coupling of individual gold nanoantennas arranged in tip-to-tip dimer configuration, leading to strong electromagnetic field enhancements in the infrared, which is of great interest for sensing applications such as surface-enhanced infrared spectroscopy. We quantitatively evaluated the enhancement of vibrational excitations of a 5 nm thick test layer of 4,4'-bis(N-carbazolyl)-1,1'-biphenyl as a function of different gap sizes. The dimers with the smallest gaps under investigation (∼3 nm) lead to more than 1 order of magnitude higher signal enhancement with respect to gaps of 50 nm width. The comparison of experimental data and finite-difference time-domain simulations reveals a nonperfect filling of the gaps with sizes below 10 nm, which means that morphological information on the nanoscale is obtained additionally to chemical information.

Two-photon direct laser writing (DLW) lithography is limited in the achievable structure size as well as in structure resolution. Adding stimulated emission depletion (STED) to DLW allowed overcoming both restrictions. We now push both to new limits. Using visible light for two-photon DLW (780 nm) and STED (532 nm), we obtain lateral structure sizes of 55 nm, a Sparrow limit of around 100 nm and we present two clearly separated lines spaced only 120 nm apart. The photo-resist used in these experiments is a mixture of tri- and tetra-acrylates and 7-Diethylamino-3-thenoylcoumarin as a photo-starter which can be readily quenched via STED.

In this paper, we report on the manipulation of the near-field coupling in individual gold nanoantenna dimers resonant in the infrared (IR) spectral range. Photochemical metal deposition onto lithographically fabricated nanoantennas is used to decrease the gap between the antenna arms down to below 4 nm, as confirmed by finite-difference time-domain simulations. The increased plasmonic coupling in the gap region leads to a shift of the surface plasmon resonances to lower energies as well as to the appearance of hybridized plasmonic modes. All of the occurring electron oscillation modes can be explained by the plasmon hybridization model. Besides the bonding combination of the fundamental resonances of individual antennas, also the antibonding combination is observed in the IR transmittance at normal incidence. Its appearance is due to both structural defects and the small gaps between the antennas. The detailed analysis of individual IR antennas presented here allows a profound understanding of the spectral features occurring during the photochemical manipulation process and therefore paves the way to a full optical process monitoring of sub-nanometer scale gaps, which may serve as model systems for experimental studies of quantum mechanical effects in plasmonics.

The trend of mimicking the real biological world has created an intensive search for methods that are able to engineer 3D-structured biological environments using nano- and micro-system technologies. Recently published methods show the design of 3D structures by multi-photon induced polymerization of artificial polymers, such as chemically modified natural polymers. However, limitations of this approach are the long processing time and the fact that no native polymers have been used up to date. In this communication, a case study of multi-photon structuring of unmodified, native proteins (e.g. collagen and fibrinogen) and liquids, such as natural human blood, or cell culture medium supplements, such as fetal calf serum, is presented. Based on a computer-assisted process, the structures are polymerized precisely. Even adhesion and gluing of cells with this technique are possible. These encouraging results open new avenues for further inquiry, which are discussed in the paper

We report on tuning the plasmonic properties of gold nanoantenna arrays resonant in the infrared (IR) spectral region. In particular, we achieve a manipulation of the antenna resonance by decreasing the antenna separation distance via photochemical metal deposition. Narrowing the antenna gaps is monitored using scanning electron microscopy, while increased plasmonic coupling and an associated red-shift of the plasmon resonance is observed by microscopic IR spectroscopy. Since smaller gap sizes lead to enhanced electric fields between the antenna arms, we propose photochemical metal deposition as a fabrication step for surface-enhanced IR spectroscopy (SEIRS) substrates