This paper describes a novel chemical etching method to fabricate high quality near-field optical antennas-tapered metallic tips-from gold wire in a reproducible way for optically probing a specimen on the nanoscale. A new type of an electrochemical cell is introduced and different dc and ac etching regimes are studied in detail. The formation and dynamics of a meniscus around a gold wire immersed in an electrolyte when supplying a square wave voltage are considered. We show that in situ etching current kinetics allows one to improve a yield of tips with a well-defined geometry up to 95% by filtering these on the basis of a cutoff current and a power spectrum of etching current fluctuations. As a quantitative measure for estimating the yield we introduce a probability to find tips with curvature radii falling in the range of interest. Testing the tips for a plasmonic effect is implemented with tip-enhanced Raman spectroscopy and sub-wavelength imaging of a thin fullerene film.

Alternative plasmonic materials are gaining more and more interest since they deliver a plethora of advantages in designing of optical metadevices. Among other alternatives, titanium nitride (TiN) has shown an exceptional combination of encouraging properties, such as CMOS- and bio-compatibility, high carrier concentration, tunability and outstanding robustness (high mechanical, chemical and temperature durability). Optical constants of TiN can be tuned at the synthesis stage. This allows for the adjustment of the spectral position of a plasmon resonance within the visible and near-infrared (NIR) range in order to match the desired working wavelength of a particular device. Together, these factors made TiN a popular material of choice in a diversity of recent plasmonic applications. Titanium oxynitride (TiON), which can be produced through the oxidation of TiN, have a great potential to build upon the success of TiN. Recently, it has been demonstrated that TiON thin films can exhibit a negative double-epsilon-near-zero (2ENZ) dielectric function. This unusual behavior of the permittivity opens up novel opportunities for the excitation of the plasmon resonance at several distinct frequencies within the visible and NIR region. Multi-resonant plasmonic components are beneficial for applications, where the enhanced light-matter interaction at multiple frequencies is demanded, such as nonlinear optics, up- and down-conversion, wavelength multiplexing and broadband absorption. This work begins with a brief survey of the recent progress in plasmonics made with TiN-based structures. Then we focus on TiON thin films with the 2ENZ behavior by discussing their potential in plasmonics. The experimental approaches useful for characterization of TiON thin films and the corresponding results are analyzed. These results are valuable for the development of 2ENZ plasmonic materials with large figure-of-merits in a diversity of applications. We believe that 2ENZ media is a powerful concept for multi-resonant plasmonics that will augment the functionalities and extend the operation bandwidth of plasmonic devices.

Heat flow generation and manipulation in nanometer-sized solids using light represents one of the up-and-coming tasks in thermonanophotonics. Enhanced light–matter interaction due to plasmon resonance permits metallic nanostructures to absorb light energy efficiently, and it results in extra optical heating. The net temperature increment of nanostructures is directly dependent on heat exchange with a thermostat. However, to the best of our knowledge, precise tailoring of optical heating at a fixed pump power is still of no practical implementation. In this paper, we focus on the tunable optical heating of a plasmonic nanostructure exposed to moderate light intensity (MW cm⁻²) based on slowing down heat exchange through a 1D waveguide heatsink bridging the nanostructure and the highly thermal conducting thermostat. The rationale for this concept is evidenced through optical heating of a 2D array of stacked titanium nitride (TiN) (plasmonic refractory nanoheater) and height-controlled silicon (Si) (1D waveguide heatsink) cylinders. Depending on the Si pillar height, the temperature rise of a TiN : Si voxel ranges from a few up to thousands of degrees at a fixed pump power. The temperature of the TiN : Si voxel is remotely measured from the Raman shift of the Si pillar. Using ellipsometry, we find a temperature threshold of 400 °C, above which the thin TiN film is chemically degraded due to oxidation. The latter enables fine tailoring of thermal gradients using TiN : Si voxels of equal size but different permittivity. These findings contribute towards the development of tunable thermoplasmonics by demonstrating programmable non-uniform temperature profiles in the steady-state regime under continuous-wave laser illumination for a variety of thermo-optical applications.

Superlensing plays a crucial role in near- and far-field optical imaging with sub-wavelength resolution. One of the ways to expand optical bandwidth is surface plasmon resonances in layered metal-dielectric nanostructures. These resonances are commonly excited at a tunable single frequency. In this study, we propose the concept of a multimode far-field superlens made of a titanium oxynitride (TiON) thin film, that is a disordered metal-dielectric refractory nano-composite. These films exhibit a double epsilon-near-zero (2-ENZ) behavior near the percolation threshold and, therefore, favor super-coupling the incident laser light to surface plasmon resonances, not using such couplers as a prism, a grating, etc. We experimentally observe stimulated Raman gain emission from nano-structured TiON thin films exposed to low-power continuous-wave laser light. It is shown that superresolution of <λ/80 (near-field) and <λ/8 (far-field) is achieved due to both the enhanced third-order optical nonlinearity and the multiplicative nature of four-wave mixing. The multimode tunable far-field superlens will impact emerging diffraction-free far-field optical microscopy, random Raman lasing on meta-atoms and broadband thermophotovoltaics.