Shifting plasmonics towards the infrared: the quest for alternative materials

One of the key features of plasmonics is the confinement of the optical energy into very small volumes (the so called hot-spots) that can be further engineered by means of nanoantennas. In the small modal volume of the hot... [ view full abstract ]

One of the key features of plasmonics is the confinement of the optical energy into very small volumes (the so called hot-spots) that can be further engineered by means of nanoantennas. In the small modal volume of the hot spots, plasmons can boost energy transfer, trigger chemico-physical process, provide field enhancement for nonlinear and second-order phenomena that are often difficult or even impossible to generate. These features depend on the nanoantenna geometry and on the dielectric function of the metal. Due to the intrinsic optical properties of noble metals these concepts are mostly exploited in the visible/near-infrared range. Therefore, finding valuable materials and device concepts for mid-infrared plasmonics is still an open issue. Here we show that the introduction of unconventional mid-IR materials, such as nanoporous gold and heavily-doped germanium (doping concentration of 10^20 cm^-3), can pave the way to overcome the aforementioned limitations. Indeed the dielectric function can be tuned and optimized for the infrared range, acting on the physical parameters of the material development process. To this aim, we measure the optical response of both materials by means of Fourier Transform Infrared Spectroscopy and retrieve their dielectric function. In the case of nanoporous gold, array of 3D vertical nanoantennas have been fabricated (fig. 1a). The resonance wavelength has been properly tuned in order to further promote the coupling between surface plasmons and 7 nm of SiO2, used as probing layer (fig. 1b). In a standard surface-enhanced infrared absorption experiment, we demonstrate a 3-fold enhancement with respect to bulk gold. In the case of doped germanium, bow-tie antennas are designed, fabricated (fig. 2a), and embedded in a polymer. Nanoantennas were investigated with a near-field mid-IR nanoimaging technique in which the absorbed electromagnetic energy is measured locally with a scanning probe tip coupled to a quantum cascade laser in pulsed operation at Lambda = 5.8 um. We demonstrate the existence of an electromagnetic energy density hotspot in the antenna gap of diameter below 100 nm and confinement volume 105 times smaller than Lambda^3 (fig.2 b and 2c).