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Plasmonic resonances in metallic nanoparticles have been used since antiquity to colour glasses. The use of metal nanostructures for surface colourization has attracted considerable interest following recent developments in plasmonics. However, current top-down colourization methods are not ideally suited to large-scale industrial applications. Here we use a bottom-up approach where picosecond laser pulses can produce a full palette of non-iridescent colours on silver, gold, copper and aluminium. We demonstrate the process on silver coins weighing up to 5 kg and bearing large topographic variations (∼1.5 cm). We find that colours are related to a single parameter, the total accumulated fluence, making the process suitable for high-throughput industrial applications. Statistical image analyses of laser-irradiated surfaces reveal various nanoparticle size distributions. Large-scale finite-difference time-domain computations based on these nanoparticle distributions reproduce trends seen in reflectance measurements, and demonstrate the key role of plasmonic resonances in colour formation.

Picosecond laser pulses have been used as a surface colouring technique for noble metals, where the colours result from plasmonic resonances in the metallic nanoparticles created and redeposited on the surface by ablation and deposition processes. This technology provides two datasets which we use to train artificial neural networks, data from the experiment itself (laser parameters vs. colours) and data from the corresponding numerical simulations (geometric parameters vs. colours). We apply deep learning to predict the colour in both cases. We also propose a method for the solution of the inverse problem – wherein the geometric parameters and the laser parameters are predicted from colour – using an iterative multivariable inverse design method.

NRC publication: Yes

NRC publication: Yes

We report the enhanced photoluminescence of carbon dots formed in situ on silver gratings via plasmonic reduction of a carbonaceous seed layer. This layer transforms into photoluminescent carbon dots in a non‐oxygenated environment, catalyzed by surface plasmon polaritons (SPPs) excited on the grating by a pump laser, as monitored in real‐time via Raman scattering. Pump SPPs subsequently excite the carbon dots, which emit at longer wavelengths into the local electromagnetic environment, which is dominated by SPPs propagating on and diffracting from the grating. We experimentally observe SPP‐enhanced photoluminescence as a significant reduction in the lifetime of the emitters, and as induced coherence in far‐field emission. Purcell factors of ∼70 are deduced from lifetimes of ∼30 ps for carbon dots measured using time‐correlated single‐photon counting. SPPs accelerate radiative decay and produce polarized directional free‐space emission via grating diffraction, conferring characteristics of coherence to a random arrangement of luminescent carbon dots. A method for modeling an ensemble of random, incoherent emitters using finite‐difference time‐domain (FDTD) simulations is proposed, and the results agree with observed far‐field images and measured Purcell factors. Our results provide insight into carbon dot ‐ SPP interactions and have implications for cavity‐enhanced biosensors, quantum emitters, and carbon‐based light sources. Carbon emitters formed in situ on a silver grating via plasmonic catalysis yield enhanced spontaneous emission by preferential radiative decay into surface plasmons. Enhancement manifests as emission exhibiting characteristics of coherence (polarization, directionality), and accelerated lifetimes of ∼30 ps, yielding Purcell factors of ∼70. Findings have implications for cavity‐enhanced biosensors, quantum light sources, and carbon‐based light emitters.

The presence of surface oxides on the formation of laser-induced periodic surface structures (LIPSS) is regularly advocated to favor or even trigger the formation of high-spatial-frequency LIPSS (HSFL) during ultrafast laser-induced nano-structuring. This paper reports the effect of the laser texturing environment on the resulting surface oxides and its consequence for HSFLs formation. Nanoripples are produced on tungsten samples using a Ti:sapphire femtosecond laser under atmospheres with varying oxygen contents. Specifically, ambient, 10 mbar pressure of air, nitrogen and argon, and 10−7 mbar vacuum pressure are used. In addition, removal of any native oxide layer is achieved using plasma sputtering prior to laser irradiation. The resulting HSFLs have a sub-100 nm periodicity and sub 20 nm amplitude. The experiments reveal the negligible role of oxygen during the HSFL formation and clarifies the significant role of ambient pressure in the resulting HSFLs period.

As phase-based sensors, surface plasmon interferometers offer higher sensitivity than resonant or attenuation-based plasmonic sensors. In this paper we realize surface plasmon interferometric sensors based on a multimode nanoslit used as a combiner. The phase difference in the surface plasmon waves, incident on the nanoslit, determines the resonant mode excited therein, and the radiation pattern that emerges therefrom. The device construction integrates on-chip grating couplers, gold sensing and reference surfaces, transparent claddings, sealed microfluidic channels, and a nanoslit in the gold film. The structure can be arrayed with individual microfluidic channels thereby enabling multiplexing. Nanofabrication of the devices using wafer-based processes is discussed in detail. Fabrication involves integration into a full process flow of techniques such as photolithography, electron beam lithography, focused ion beam milling, plasma etching, wafer bonding, and dicing, with several overlay and precision alignment steps. We also describe the design and realization of a test jig useful for mounting a chip under test, providing in-plane sealed microfluidic interfacing to several channels simultaneously, and enabling optical interrogation in the perpendicular direction using microscope objectives. Operation of the devices is demonstrated by refractometric (bulk) sensing experiments. The device concept is of strong interest for multiplexed biosensing applications, and the fabrication flow presented can be scaled to mass-manufacturing.

In this paper we discuss the optical response of laser-written plasmonic colours on silver coated via the atomic layer deposition of alumina. These colours are due to nanoparticles distributed on a flat surface and on a surface with periodic topographical features (i.e. ripples). The colours are observed to shift with increasing alumina film thickness. The colours produced by surfaces with ripples recover their original vibrancy and hue after the deposition of film of thickness ~60 nm, while colours arising from flat surfaces gradually fade and never recover. Analysis of the surfaces identifies periodic topographical features to be responsible for this behaviour. Finite-difference time-domain simulations unravel the role played by the alumina thickness in colour formation and confirm the rotations and recovery of colours for increasing alumina thickness. The coloured surfaces were evaluated for applications in colourimetric and radiometric sensing showing large sensitivities of up to 3.06/nm and 3.19 nm/nm, respectively. The colourimetric and radiometric sensitivities are observed to be colour dependent.

A new process to rapidly obtain high-strength interstitial free (IF) steel was investigated. Thin sheets of IF steel were coated on one or both sides with an amorphous FeC film and subjected to a two-step induction heating cycle (1100 °C followed by an isothermal hold at 780 °C for 2 or 4 min) and a rapid quench in water. Tensile mechanical properties were measured, and a yield stress of 374 MPa and an ultimate tensile strength of 448 MPa were achieved after 2 min of induction heating. After 4 min of induction heating, the yield stress and the ultimate tensile strength drop at 206 and 320 MPa, respectively. During tensile testing, the specimens induction heated for 2 min show Lüdering, which is suppressed when the induction heating is extended to 4 min. Vickers microhardness measurements through thickness confirm that higher mechanical properties are obtained after 2 min of induction heating. Transmission electron microscopy reveals that strengthening results from dislocations, carbon in solid solution, and the precipitation of nanosized TiC particles. A fine microstructure with an average grain size of 15 μm is preserved after the induction heat treatment.

Experiments on the growth and linkage of 10 μm diameter holes laser drilled in high precision patterns into Al-plates were modelled with finite elements. The simulations used geometries identical to those of the experiments and incorporated ductile damage by element removal under the control of a ductile damage indicator based on the micromechanical studies of Rice and Tracey. A regularization of the problem was achieved through an integral-type nonlocal model based on the smoothing of the rate of a damage indicator D over a characteristic length L. The simulation does not predict the experimentally observed damage acceleration either in the case where no damage is included or when only a local damage model is used. However, the full three-dimensional simulations based on the nonlocal damage methodology do predict both the failure path and the failure strain at void linkage for almost all configurations studied. For the cases considered the critical parameter controlling the local deformations at void linkage was found to be the ratio between hole diameter and hole spacing.