Jean-Michel Guay

Laser-induced plasmonic colours on metals

The use of metal nanostructures for colourization has attracted a great deal of interest with the recent developments in plasmonics. However, the current top-down colourization methods based on plasmonic concepts are tedious and time consuming, and thus unviable for large-scale industrial applications. Here we show a bottom-up approach where, upon picosecond laser exposure, a full colour palette independent of viewing angle can be created on noble metals. We show that colours are related to a single laser processing parameter, the total accumulated fluence, which makes this process suitable for high throughput industrial applications. Statistical image analyses of the laser irradiated surfaces reveal various distributions of nanoparticle sizes which control colour. Quantitative comparisons between experiments and large-scale finite-difference time-domain computations, demonstrate that colours are produced by selective absorption phenomena in heterogeneous nanoclusters. Plasmonic cluster resonances are thus found to play the key role in colour formation.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.

Femtosecond laser induced surface swelling in poly-methyl methacrylate

We show that surface swelling is the first step in the interaction of a single femtosecond laser pulse with PMMA. This is followed by perforation of the swollen structure and material ejection. The size of the swelling and the perforated hole increases with pulse energy. After certain energy the swelling disappears and the interaction is dominated by the ablated hole. This behaviour is independent of laser polarization. The threshold energy at which the hole size coincides with size of swelling is 1.5 times that of the threshold for surface swelling. 2D molecular dynamics simulations show surface swelling at low pulse energies along with void formation below the surface within the interaction region. Simulations show that at higher energies, the voids coalesce and grow, and the interaction is dominated by material ejection.

Passivation of Plasmonic Colors on Bulk Silver by Atomic Layer Deposition of Aluminum Oxide

We report the passivation of angle-independent plasmonic colors on bulk silver by atomic layer deposition (ALD) of thin films of aluminum oxide. The colors are rendered by silver nanoparticles produced by laser ablation and redeposition on silver. We then apply a two-step approach to aluminum oxide conformal film formation via ALD. In the first step, a low-density film is deposited at low temperature to preserve and pin the silver nanoparticles. In the second step, a second denser film is deposited at a higher temperature to provide tarnish protection. This approach successfully protects the silver and plasmonic colors against tarnishing, humidity, and temperature, as demonstrated by aggressive exposure trials. The processing time associated with deposition of the conformal passivation layers meets industry requirements, and the approach is compatible with mass manufacturing.

Plasmonic colours on bulk metals: laser coloring of large areas exhibiting high topography

The use of metal nanostructures to produce colour has recently attracted a great deal of interest. This interest is motivated by colours that can last a long time and that can be rendered down to the diffraction limit, and by processes that avoid the use of inks, paints or pigments for environmental, health or other reasons. The central idea consists of forming metal nanostructures which exhibit plasmon resonances in the visible such that the spectrum of reflected light renders a desired colour. We describe a single-step laser-writing process that produces a full palette of colours on bulk metal objects. The colours are rendered through spectral subtraction of incident white light. Surface plasmons on networks of metal nanoparticles created by laser ablation play a central role in the colour rendition. The plasmonic nature of the colours are studied via large-scale finite-difference time-domain simulations based on the statistical analysis of the nanoparticle distribution. The process is demonstrated on Ag, Au, Cu and Al surfaces, and on minted Ag coins targeting the collectibles market. We also discuss the use of these coloured surfaces in plasmonic assisted photochemistry and their passivation for day-to-day use. Reactions on silver that are normally driven by UV light exposure are demonstrated to occur in the visible spectrum.

Topography Tuning for Plasmonic Color Enhancement via Picosecond Laser Bursts

We report on the creation of angle-independent colors on silver using closely time-spaced laser bursts. The use of burst mode, compared to traditional non-burst is shown to increase the Chroma (color saturation) by ∼50% and to broaden the lightness range by up to ∼60%. Scanning electron microscope analysis of the surfaces created using burst mode, reveal the creation of 3 distinct sets of laser induced periodic surface structures (LIPSS): low spatial frequency LIPSS (LSFL), high spatial frequency LIPSS (HSFL) and large laser-induced periodic surface structures (LLIPSS) that are 10 times the laser wavelength and parallel to the laser polarization. Nanoparticles are responsible for each plasmonic color and their distributions are observed to be similar for both burst and non-burst modes, indicating that the underlying structures (i.e. LIPSSs) are responsible for the increased Chroma and Lightness. Two-temperature model simulations of silver irradiated by laser bursts show significant increase in the electron-phonon coupling coefficient which is crucial for the creation of well-defined ripple structures. Finite difference time domain (FDTD) simulations of the colored surfaces show the increase in Chroma to be attributable to the HSFL arising in burst mode.We report on the creation of angle-independent colors on silver using closely time-spaced laser bursts. The use of burst mode, compared to traditional non-burst is shown to increase the Chroma (color saturation) by ~50% and to broaden the lightness range by up to ~60%. Scanning electron microscope analysis of the surfaces created using burst mode, reveal the creation of 3 distinct sets of laser induced periodic surface structures (LIPSS): low spatial frequency LIPSS (LSFL), high spatial frequency LIPSS (HSFL) and large laser-induced periodic surface structures (LLIPSS) that are 10 times the laser wavelength and parallel to the laser polarization. Nanoparticles are responsible for each plasmonic color and their distributions are observed to be similar for both burst and non-burst modes, indicating that the underlying structures (i.e. LIPSSs) are responsible for the increased Chroma and Lightness. Two-temperature model simulations of silver irradiated by laser bursts show significant increase in the electron-phonon coupling coefficient which is crucial for the creation of well-defined ripple structures. Finite difference time domain (FDTD) simulations of the colored surfaces show the increase in Chroma to be attributable to the HSFL arising in burst mode.

Plasmonic coloring of noble metals rendered by picosecond laser exposure

We show the angle-independent coloring of metals in air arising from nanoparticle distributions on metal surfaces created via picosecond laser processing. Each of the colors is linked to a unique total accumulated fluence, rendering the process compatible with industry. We report the coating of the colored metal surfaces using atomic layer deposition which is shown to preserve colors and provide mechanical and chemical protection Laser bursts are composed of closely time-spaced pulses separated by 12.8 ns. The coloring of silver using burst versus non-burst is shown to increase the Chroma, or color saturation, by 50% and broaden the color Lightness range by up to 60%. The increase in Chroma and Lightness are accompanied by the creation of 3 kinds of different laser-induced periodic surface structures (LIPSS). One of these structures is measured to be 10 times the wavelength of light and are not yet explained by conventional theories. Two temperature model simulations of laser bursts interacting with the metal surface show a significant increase in the electron-phonon coupling responsible for the well-defined LIPSS observed on the surface of silver. Finite-difference time-domain simulations of nanoparticles distributed on the high-spatial frequency LIPSS (HSFL) explain the increase in color saturation (i.e. Chroma of the colors) by the enhanced absorption and enriched plasmon resonances.

Modelling of Coloured Metal Surfaces by Plasmonics Nanoparticles

Metallic nanoparticles (NPs) dispersed in glass have been used since Romans times to color glasses [1]. With the recent development of metasurfaces, metallic and dielectric nanostructures have been proposed to color surfaces [2]. The excitation of resonant modes in the nanostructures is responsible for selective absorption of the incident light, thus causing the color creation. Top-down techniques based on lithography allow achieving highly saturated colors with a high resolution due to the deterministic patterning, but they are not suited for large-scale applications, such as the coloring of large surfaces. Furthermore, lithographic techniques are not suited to create metallic nanostructures on a substrate of the same metal. Metal nanostructures on metal can generate colors if their shape can support a localized surface plasmon resonance (LSPR). For example, this is valid for NPs of spherical shape slightly embedded on the substrate. In fact, when the embedding increases the resonance condition vanishes (an embedding increase corresponds to a transition from a spherical shape to a hemispherical shape) and the color disappears. This configuration has never been investigated theoretically. We recently proposed a bottom-up laser technique in the picosecond regime to create NPs on the metallic surface (laser-induced nanostructures) through a process of ablation and re-deposition, which is suited for mass production [3]. By tuning the laser properties, it is possible to control the NPs such that their size and density fall in the range of dimensions supporting LSPR, thus producing colors. This is shown in the palette realized on silver at the Royal Canadian Mint in Fig. 1a [3]. SEM images of these colored metallic surfaces reveal the presence of NPs of two sizes, i.e., medium and small, with radii Rm and Rs, respectively. Based on this information, we simulated the optical response of silver NPs distributed on a flat silver surface by using an in-house parallel 3D-FDTD code running on IBM BlueGene/Q (SOSCIP). Medium NPs were embedded by 30% of their radius, while small NPs were embedded by 0.5–3.5 nm. The simulation domain was discretized with a space-step from 0.125 to 0.5 nm to achieve convergent results, and we arranged the NPs following a hexagonal lattice in the xz-plane with a center-to-center inter-distance of Dm and Ds for medium and small NPs, respectively. In order to apply periodic boundary conditions, we needed Dm∕Ds to be an integer number. We modeled silver using the Drude+2CP model [4]. In Fig. 1b we show the field distribution at 390 nm for an xz-plane cut through the center of the small NPs. By averaging the reflectance over the embedding level of the small NPs, we obtained the average reflectance curves and the associated colors shown in Fig. 1c. The averaging washes out the effect of the small NPs, thus highlighting the major role in the color formation played by the medium NPs. The simulated palette in Fig. 1c reveals the same qualitative transition from blue to yellow observed in the experimental palette in Fig. 1a, thus identifying plasmonic resonances in arrangements of NPs as responsible for color creation.