Plasmonic nanostar-dimers, decoupled from the substrate, have been fabricated by combining electron-beam lithography and reactive-ion etching techniques. The 3D architecture, the sharp tips of the nanostars and the sub-10 nm gap size promote the formation of giant electric-field in highly localized hot-spots. The single/few molecule detection capability of the 3D nanostar-dimers has been demonstrated by Surface-Enhanced Raman Scattering.

The coupling of an infrared plasmon-polariton excitation of a metal nanoparticle to a vibrational excitation of a similar energy enables strong vibrational signal enhancement. However, the commonly used planar substrates substantially weaken plasmonic resonances because of their polarizability, and, furthermore, a great part of the enhanced near-field is inside the substrate and thus not available for an analyte. In this contribution we report on a way to reduce these undesirable influences of the substrate by fabricating gold nanowires on high pedestals and thus in reduced contact with the substrate. The influence of the height of the pedestal is an important parameter for the plasmonic near-field enhancement, as we show with finitedifference time-domain simulations. Comparing the plasmonic response and the SEIRA activity of the rods prepared by standard electron beam lithography and the rods additionally treated with reactive ion etching to remove the silicon substrate at the hot-spots of the rods reveals not only the change of the plasmonic-resonance spectrum but also interesting differences of the enhanced phonon-polariton signal from the silicon dioxide layer on the silicon substrate and the about 1 order of magnitude stronger vibrational signal enhancement for an adsorbate monolayer.

Top-down fabrication of electron-beam lithography (EBL)-defined metallic nanostructures is a successful route to obtain extremely high electromagnetic field enhancement via plasmonic effects in well-defined regions. To this aim, various geometries have been introduced such as disks, triangles, dimers, rings, self-similar lenses, and more. In particular, metallic dimers are highly efficient for surface-enhanced Raman spectroscopy (SERS), and their decoupling from the substrate in a three-dimensional design has proven to further improve their performance. However, the large fabrication time and cost has hindered EBL-defined structures from playing a role in practical applications. Here we present three-dimensional nanostar dimer devices that can be recycled via maskless metal etching and deposition processes, due to conservation of the nanostructure pattern in the 3D geometry of the underlying Si substrate. Furthermore, our 3D-nanostar-dimer-in-ring structures (3D-NSDiRs) incorporate several advantageous aspects for SERS by enhancing the performance of plasmonic dimers via an external ring cavity, by efficient decoupling from the substrate through an elevated 3D design, and by bimetallic AuAg layers that exploit the increased performance of Ag while maintaining the biocompatibility of Au. We demonstrate SERS detection on rhodamine and adenine at extremely low density up to the limit of few molecules and analyze the field enhancement of the 3D-NSDiRs with respect to the exciting wavelength and metal composition.

Refractorymetal-based broadband absorber/narrowband emitters is a flourishing field in energy harvesting where the physical and chemical stability of the metals at high temperatures provide efficient absorption/emission of solar/ heat energy. [1][2][3] Advancements in solar/ thermophotovoltaics (S/TPV) must be accompanied by thermally stable devices in order to withstand extreme operating conditions. The fundamental limiting factor [Shockley Queisser (SQ) efficiency limit] in traditional single-junction solar cells is its inability in converting the broad solar spectrum into a narrow range of wavelengths defined by the PV cells. [4] Solar photons with energies below the bandgap of the PV cell are not converted, and the photons with energies higher than the bandgap lose their additional energy through a process known as thermalization. In solar thermophotovoltaics (STPV), an intermediate system composed of absorber and emitter is used to overcome the SQ limit by harvesting the solar energy followed by the emission of narrowband radiation. [5] The absorber should provide unitary absorption in the entire solar spectral range over a range of incidence angles with polarization insensitive nature, and minimum thermal reradiation to avoid near-infrared heat radiation at elevated temperatures. Through thermal conduction, the absorbed heat energy is transferred to the emitter, which is tailored to emit a narrowband radiation defined by the bandgap energy of a PV cell. Thus, the absorber needs to withstand high temperatures to transfer a large amount of heat energy to the emitter. The low-loss noble metals are successfully used in unitary absorbers so far, particularly Au and Ag. [6][7][8] However, noble metals are not compatible with hightemperature photovoltaic applications and standard silicon manufacturing processes (complementary metal oxide semiconductor, CMOS, technology), owing to low melting point and diffusion of noble metals into silicon. Annealing the substrates at high temperatures induce oxidation, surface diffusion, corrosion, cracking, and delamination of thin films from the substrate. [7] The situation is even worse in the case of the Broadband absorbers, with the simultaneous advantages of thermal stability, insensitivity to light polarization and angle, robustness against harsh environmental conditions, and large area fabrication by scalable methods, are essential elements in (solar) thermophotovoltaics. Compared to the noble metal and multilayered broadband absorbers, high-temperature refractory metal-based nanostructures with low-Q resonators are reported less. In this work, 3D titanium nitride (TiN) nanopillars are investigated for ultrabroadband absorption in the visible and near-infrared spectral regions with average absorptivities of 0.94, over a wide range of oblique angles between 0° and 75°. The effect of geometrical parameters of the TiN nanopillars on broadband absorption is investigated. By combining the flexibility of nanopillar design and lossy TiN films, ultrabroadband absorption in the vi...

Efficient broadband absorption of visible and near-infrared light by low quality-factor metal-insulator-metal (MIM) resonators using refractory materials is reported. Omnidirectional absorption of incident light for broad angles of incidence and polarization insensitivity are observed for the fabricated MIM resonator. Excellent thermal stability of the absorber is demonstrated at high operating temperatures (800 °C). The experimental broadband absorption spectra show good agreement with simulations. The resonator with 12 nm top tungsten and 100 nm alumina spacer film shows absorbance above 95% in the range of 650 to 1750 nm. The absorption window is tunable in terms of the center wavelength, bandwidth, and the value of maximum absorbance (~98%) by simple variation of appropriate layer thicknesses. Owing to their flexibility, ease of fabrication and low cost, the presented absorbers have the potential for a wide range of applications, including the use in commonly used infrared bands or absorbers for (solar) thermo-photovoltaic energy conversion, where high absorbance and simultaneously low (thermal) re-radiation is of paramount importance.