Designing two-dimensional temperature profiles using tunable thermoplasmonics.

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.

Superresolution stimulated Raman scattering microscopy using 2-ENZ nano-composites.

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.

Nonlinear Raman Effects Enhanced by Surface Plasmon Excitation in Planar Refractory Nanoantennas.

We consider a nonlinear mechanism of localized light inelastic scattering within nanopatterned plasmonic and Raman-active titanium nitride (TiN) thin films exposed to continuous-wave (cw) modest-power laser light. Owing to the strong third-order nonlinear interaction between optically excited broadband surface plasmons and localized Stokes and anti-Stokes waves, both stimulated and inverse Raman effects can be observed. We provide experimental evidence for coherent amplification of the localized Raman signals using a planar square-shaped refractory antenna.