WO2016090496A1 - Laser-induced metallic surface colouration processes, metallic nanoscale structures resulting therefrom and metallic products produced thereby - Google Patents

Abstract

Described are various embodiments of laser-induced metallic surface colouration processes, metallic nanoscale structures resulting therefrom and metallic products produced thereby.

Description

LASER-INDUCED METALLIC SURFACE COLOURATION PROCESSES,

METALLIC NANOSCALE STRUCTURES RESULTING THEREFROM AND

METALLIC PRODUCTS PRODUCED THEREBY

FIELD OF THE DISCLOSURE [0001] The present disclosure relates to metal surface colouration, and in particular, to laser-induced metallic surface colouration processes, metallic nanoscale structures resulting therefrom and metallic products produced thereby.

BACKGROUND

[0002] Colours can generally be imparted to a metal surface through the use of organic paints, films, etc. However, such techniques also compromise the purity of the metal, are prone to fading or progressive removal, and can dull or altogether eliminate the general reflective metallic appearance of the surface.

[0003] Holographic effects can also be imparted to metal surfaces by generating nano-scale diffraction gratings into the metal surface, such as by forming metallic nano- scale grooves or ripples in the surface to exhibit colourations whose visual appearance depends on the orientation of these grooves or ripples with respect to viewing angle.

[0004] U.S. Patent No. 8,685,185 to Guo et al. for Femtosecond Laser Pulse Surface Structuring Methods and Materials Resulting Therefrom describes such examples produced by exposing a gold surface to femtosecond laser pulses. Also described is the production of blackened or golden aluminum surfaces that do not change as a function of viewing angle (i.e. fixed colouration).

[0005] Fan et al. further reports the production of fixed colourations on copper by exposing copper surfaces to picosecond laser pulses (J. Appl. Phys. 115, 124302 (2014) and 114, 083518 (2013)), and attributes these colourations to the production of nanoparticle (NP) distributions on the copper surface with densities in the order of 1 ΝΡ/μιη². [0006] This background information is provided to reveal information believed by the applicant to be of possible relevance. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art.

SUMMARY

[0007] The following presents a simplified summary of the general inventive concept(s) described herein to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to restrict key or critical elements of the invention or to delineate the scope of the invention beyond that which is explicitly or implicitly described by the following description and claims.

[0008] A need exists for new laser-induced metallic surface colouration processes, metallic nanoscale structures resulting therefrom and metallic products produced thereby that overcome some of the drawbacks of known techniques, or at least, provide a useful alternative thereto. Some aspects of this disclosure provide examples of such new processes, structures and products.

[0009] In accordance with one aspect, there is provided an object having at least one metallic surface portion, said at least one metallic surface portion having a first coloured region and a second coloured region each of which having distinct colouration; each of said first coloured region and said second coloured region being defined by respective metallic nanostructures generated via a plurality of consecutively spaced-apart laser irradiation lines, wherein a first line spacing for said first region is distinct from a second line spacing for said second region so as to produce said distinct colouration.

[0010] In accordance with another aspect, there is provided an object having a coloured metallic surface region defined by a metallic nanostructure generated via a crossing of consecutively spaced-apart laser irradiation lines. In one such embodiment, the crossing is a first crossing, and wherein the object further comprises a second coloured metallic surface region of distinct colouration defined by a respective metallic nanostructure generated via a second crossing of consecutively spaced-apart laser irradiation lines, wherein a crossing angle of said first crossing is distinct from that of said second crossing resulting in production of said distinct colouration.

[0011] In accordance with another aspect, there is provided an object having a metallic surface of fixed colouration defined by a metallic nanostructure generated via laser irradiation; said metallic nanostructure being at least in part defined by a surface density of metallic nanoparticles (NP) above about 5 Ρ/μπι² for particles having a radius ranging from about 10 to about 200 nm, or above 200 Ρ/μπι² for particles having a radius from about 4 to about 10 nm.

[0012] In one embodiment, the surface density is between about 5 and 200 Ρ/μπι² for particles having a radius ranging from about 10 nm to about 200 nm, or between about 200 and 7000 Ρ/μπι² for particles having a radius from about 4 to about 10 nm.

[0013] In one embodiment, the surface density is between about 10 and 70 Ρ/μπι² for particles having a radius ranging from about 10 to about 200 nm, or between about 1000 and 5500 Ρ/μπι² for particles having a radius from about 4 to about 10 nm.

[0014] In one embodiment, there is provided an object having a metallic surface of fixed colouration defined by a metallic nanostructure generated via laser irradiation; said metallic nanostructure being at least in part defined by a surface density of metallic nanoparticles (NP) of between about 5 ΝΡ/μπι² and about 65 ΝΡ/μπι² for particles having a radius ranging from about lOnm to about 75nm, and a surface density of between about 1000 ΝΡ/μπι² to about 5100 ΝΡ/μπι² ίθΓ particles having a radius below lOnm.

[0015] In one embodiment, a Chroma of said fixed colouration is at least about 2, when measured using CIELCH.

[0016] In one embodiment, a Chroma of said fixed colouration is at least about 10, when measured using CIELCH, for a measured Hue between -90 and 90.

[0017] In accordance with another aspect, there is provided an object having a metallic surface of fixed colouration defined by a metallic nanostructure generated via laser irradiation; said metallic nanostructure being at least in part defined by distributed metallic nanoparticles having a mean wall to wall distance of between about 1 and about 100 nm, about 5 and about 50 nm, or about 10 and about 40nm, for particles having a radius between about 10 to about 200 nm; or a mean wall to wall distance of between about 0.1 and about 10 nm, or about 2 and about 6 nm for particles having a radius between about 4 and about 10 nm.

[0018] In accordance with another aspect, there is provided an object having a metallic surface of fixed colouration defined by a metallic nanostructure generated via laser irradiation; said metallic nanostructure being at least in part defined by distributed metallic nanoparticles having a mean wall to wall distance of between about 10 and 120 nm for particles having a radius between about 10 to about 75 nm; a mean wall to wall distance between about 2 and 8.5 nm for particles having a radius above 75nm; and a mean wall to wall distance of between about 0.1 and about 10 nm for particles having a radius below 10 nm.

[0019] In one embodiment, a Chroma of said fixed colouration is at least about 2, when measured using CIELCH.

[0020] In one embodiment, a Chroma of said fixed colouration is at least about 10, when measured using CIELCH, for a measured Hue between -90 and 90.

[0021] In accordance with another aspect, there is provided an object having a coloured metallic surface region defined by a metallic nanostructure generated via a laser irradiation pattern, wherein said laser irradiation pattern involved laser irradiation in accordance with said pattern using consecutive laser pulse bursts, each of said bursts consisting of two or more consecutive laser pulses.

[0022] In accordance with another aspect, there is provided a method for colouring a metallic surface comprising: selecting from multiple designated laser irradiation patterns a given laser irradiation pattern corresponding to a desired fixed metallic colouration using one or more predefined laser irradiation parameters; irradiating at least a portion of the metallic surface in accordance with said selected irradiation pattern and said preset laser irradiation parameters to form a corresponding metallic nanostructure thereon, wherein said metallic nanostructure so formed affects an absorption spectrum of the portion so as to exhibit said desired fixed metallic colouration.

[0023] In accordance with another aspect, there is provided an object having a coloured metallic surface region defining a relief profile and defined by a metallic surface nanostructure generated via a laser irradiation pattern, wherein the coloured metallic surface region exhibits one or more colour gradients in accordance with said relief profile.

[0024] In accordance with another aspect, there is provided an object having a coloured metallic surface region defining a relief profile and defined by a metallic surface nanostructure generated via a laser irradiation pattern, wherein the coloured metallic surface region exhibits a substantially fixed colouration irrespective of said relief profile.

[0025] In accordance with another aspect, there is provided an object having a gold surface portion, said surface portion having a fixed colouration defined by a metallic nanostructure generated via laser irradiation; said metallic nanostructure being at least in part defined by a surface density of metallic nanoparticles.

[0026] In accordance with one embodiment, the fixed colouration is defined by a measurable Hue in the green or blue region of the visible spectrum.

[0027] In accordance with one embodiment, the laser irradiation comprises burst laser irradiation. [0028] Other aspects, features and/or advantages will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

[0029] Several embodiments of the present disclosure will be provided, by way of examples only, with reference to the appended drawings, wherein: [0030] Figures 1A to 1C are schematic diagrams of different laser irradiation patterns, in accordance with some embodiments, showing an interline spacing (/) and an intraline spacing (d) for non-overlapping, partially overlapping and burst-mode patterns, respectively; [0031] Figure 2 are coloured photographs of various silver surface samples coloured in accordance with one embodiment via picosecond laser irradiation with a constant line spacing of ΙΟμιη, a fluence of 1.12 J/cm², and marking speeds varying from 5mm/s to 32mm/s corresponding to surface area colouration rates ranging from about 20s/mm² to about 3 s/mm²; [0032] Figures 3A and 3B are coloured photographs of various silver surface samples coloured, in accordance with one embodiment, via picosecond laser irradiation for different line spacings ranging from Ι μπι to 16μιη for a laser fluence of 1.67 J/cm² and a marking speed of 50 mm/s in Figure 3 A, and ranging from Ι μπι to 25μιη for a laser fluence of 29.75 J/cm² and a marking speed of 1000 mm/s in Figure 3B, which speeds and spacings effectively correspond to surface area colouration rates ranging from about 20s/mm² to about 1.25s/mm², and from about 1 s/mm² to about 0.04 s/mm², respectively;

[0033] Figure 4A is a data plot of Lightness versus Hue for different marking speeds on silver surface samples, in accordance with one embodiment, wherein increasing Lightness can be observed for increasing marking speeds and wherein different colours (Hues) with a same marking speed and fluence result from changes in line spacing;

[0034] Figure 4B is a data plot showing a general increase in Lightness observed for various silver surface samples with increasing laser fluence, wherein plotted Lightness was calculated by taking the average Lightness over data within specific Hue ranges, such as Yellow: 30-90 Hue, Red: -30-30 Hue, Magenta: 300-360 Hue, and Blue: 240-300 Hue;

[0035] Figure 4C is a data plot of Chroma versus Hue for different marking speeds on silver surface samples, in accordance with one embodiment, wherein Chroma is shown to be less affected by fluence than Lightness and wherein different colours (Hues) with a same marking speed and fluence result from changes in line spacing;

[0036] Figure 5 are SEM images of laser-coloured silver surfaces obtained using a laser fluence of 2.36 J/cm² and a marking speed of 50 mm/s with a line spacing of (a) 1 μπι, (b) 2 μπι and (d) 15 μιη; a significant difference in particle density and the amount of larger nanoparticles is observed; both are reduced with increased line spacing; (c) is a close up of (a) revealing the presence of a wide distribution of NP within the modified region;

[0037] Figure 6 are SEM images of laser-coloured silver surfaces obtained using a laser fluence of 29.75 J/cm² and a marking speed of 1000 mm/s with a line spacing of (a) 1 μπι, (b) 2 μπι and (d) 15 μπι; a significant difference in particle density and the amount of larger nanoparticles is observed. Both are reduced with increased line spacing, (c) is a close up of (a) revealing the presence of a wide distribution of NP within the modified region; [0038] Figure 7 are SEM images of laser-coloured silver surfaces obtained using the following parameters: (a) marking speed of 11 mm/s, fluence of 1.12 J/cm² and line spacing of 15 μπι; (b) marking speed of 50 mm/s, fluence of 2.36 J/cm² and line spacing of 2 μπι; (c) marking speed of 1000 mm/s, fluence of 29.75 J/cm² and line spacing of 15 μπι; (d) is a high magnification of (b); wherein each image clearly shows an accumulation of smaller NPs over other formed larger nano and microstructures, suggesting a deposition or otherwise formation of these smaller NPs during subsequent line irradiations;

[0039] Figure 8 is a photograph of a complex laser colouration design reproducibly achieved on a silver surface by varying one or more irradiation patterns and/or parameters for different colouration regions defined by the design, in accordance with one embodiment;

[0040] Figures 9A to 9C are graphs of typical variations in colouration Lightness, Chroma and Hue achieved as a function of laser pulse density, in this example at a fluence of 1.66 J/cm² and marking speed of 50 mm/s for various silver surface samples, in accordance with one embodiment;

[0041] Figures 10A to IOC are respective photographs of 2 irradiation lines drawn on respective silver surface areas and separated by a distance of (a) 250um, (b) 200um and (c) 150um; the lines were made at a fluence of 16.90 J/cm², frequency of 50 kHz and a marking speed of 100 mm/s;

[0042] Figure 11 is a photograph of laser-coloured silver samples for a series of fixed metallic colourations achieved using a picosecond laser operating at a wavelength of 355nm, as compared to the 1064nm, in accordance with one embodiment; [0043] Figure 12 is a photograph of laser-coloured silver samples in which both a line spacing and the irradiation line direction was changed relative to the fixed polarization direction to produce different colourations, in accordance with one embodiment;

[0044] Figures 13A to 13C are photographs of different colourations achieved on silver surface samples using various 2-pass (13A and 13B) and 3-pass (13C) cross- hatching irradiation patterns with cross-hatching angles ranging from 0 degrees to 90 degrees, in accordance with one embodiment;

[0045] Figure 14 is a graph of spectrophotometer reflection spectra measured for different silver samples coloured via 2-pass cross-hatching irradiation lines having 13μπι spacing for different cross-hatching angles varying from 0 to 90 degrees, in accordance with one embodiment;

[0046] Figures 15A and 15B are graphs of spectrophotometer reflection spectra measured for laser-coloured silver samples using 2 and 3-pass cross-hatching irradiation patterns, in accordance with one embodiment;

[0047] Figures 16A and 16B are photographs of laser-induced colourations achieved on silver using burst-mode irradiation at frequencies of 200kHz and 600kHz respectively, in accordance with one embodiment; [0048] Figure 16C is a photograph of laser-induced colourations achieved on gold using burst-mode irradiation, in accordance with one embodiment;

[0049] Figure 17A is a plot of Chroma values achievable on silver with and without burst mode irradiation across the entire Hue range (-180 to 180) and showing a significant increase in attainable Chroma values using burst-mode irradiation;

[0050] Figure 17B is a plot of Lightness values achievable on silver with and without burst mode irradiation across the entire Hue range (-180 to 180) and showing a broader range of achievable Lightness across the whole spectrum using burst-mode irradiation;

[0051] Figure 18 is a photograph of laser-coloured silver samples irradiated in non- burst mode each with a total fluence of 1.74 J/cm² and an interline spacing of 6 um, and with correspondingly varying frequencies of 200 kHz, 100 kHz, 66.6 kHz, 40 kHz, 28.6 kHz, 10 kHz and 5kHz, and marking speeds of 200 mm/s, 100 mm/s, 66.6 mm/s, 40 mm/s, 28.6 mm/s, 10 mm/s and 5 mm/s, respectively;

[0052] Figures 19A and 19B are SEM photographs taken at different magnifications of a given silver sample, wherein Figure 19A depicts a first NP regime of larger particles, and wherein Figure 19B depicts a second NP regime of smaller particles;

[0053] Figures 20A, B and C are nanoparticle density histograms for different silver sample laser irradiation line spacing' s of 5um (A), lOum (B) and 30um (C), respectively, each for a fluence of 1.12 J/cm² and marking speed of l lmm/s, covering a range of nanoparticle radii across three identified NP size regimes, namely defined by small, medium and larger sized NPs;

[0054] Figures 21A and 21B are photographs of silver coins illustratively coloured using burst-mode irradiation using the cross-hatching method, in accordance with one embodiment;

[0055] Figures 22A and 22B are photographs of high relief silver coins coloured in accordance with one embodiment to produce colour gradients; [0056] Figure 23 is a photograph of a high relief silver coin coloured in accordance with one embodiment to produce fixed colours irrespective of surface relief;

[0057] Figure 24 is a schematic diagrams of a laser system operated in standard (non- burst or Burst 1) and burst (e.g. Burst 2, 3, 4, ...) modes, in accordance with one embodiment;

[0058] Figures 25A and 25B are photographs of single laser pulse irradiation results seen in bright field mode under an optical microscope without colour correction, where Figure 25 A shows the colouration result of 50 pulses at 19.09 J/cm² on silver and Figure 25B shows the colouration result of 100 pulses with a fluence of 19.09 J/cm² on gold;

[0059] Figures 26A to 26C are SEM photographs of different silver samples irradiated in accordance respective 5-fold burst irradiation patterns, namely at a common marking speed of lOOmm/s and fluence of 5.16 J/cm², with line spacing of lum, 3um and 9 um, respectively;

[0060] Figures 27A and 27B are SEM photographs of different silver samples irradiated with and without bursts, respectively, showing what ultimately appears as a "cleaner" surface for burst mode irradiation than without, namely showing how burst mode irradiation results in the manifestation of fewer relatively larger Ps;

[0061] Figures 28A and 28B are SEM photographs of different silver samples irradiated with respective 2-pass cross-hatched patterns having respective line crossing angles of 90 and 15 degrees;

[0062] Figures 29A to 29D are photographs of different silver samples showing how layered colourations can be used to achieve different results, as clearly visible when comparing the samples of Figures 29A and B, as compared to the samples of Figures 29C and 29D in which the second layer appears to completely erase the colour induced by the first layer;

[0063] Figures 30A to 3 OF are SEM photographs of different silver samples irradiated at a fluence of 1.12 J/cm² and a laser marking speed of 11 mm/s, with lines spacings of (A) 5 um, (B) 10 um and (C) 15 um, with their respective higher magnification images shown in D, E and F;

[0064] Figure 31 is a plot of observable Hue as a function of a total accumulated fluence for various silver samples under different irradiation parameters;

[0065] Figures 32A and B are plots of mean particle radius for small and medium sized P regimes, respectively, as a function of irradiation line spacing on silver samples;

[0066] Figures 32C and 32D are plots of mean particle distance for small and medium sized NP regimes, respectively, as a function of irradiation line spacing on silver samples;

[0067] Figures 33A and 33B are plots of particle density variations with total accumulated fluence for small and medium NP sizes, respectively, on silver samples;

[0068] Figures 34A and 34B are simulated results for a silver surface in the absence and presence of smaller NPs, respectively, which are believed to act as plasmonic couplers between medium-sized NPs, in accordance with embodiment;

[0069] Figures 35 A and 35B are simulated results for a silver surface exposed to laser irradiations for line spacings of 3um and 5um, respectively, showing a simulated controllable impact on surface reflectance when combining small NPs with medium NPs;

[0070] Figure 36 is a plot of a variation in a simulated plasmonic peak for a silver surface having a NP distribution similar to those observed for irradiations at a 5um line spacing, fluence of 1.67 J/cm^A2 and marking speed of 50 mm/s, as a function of an embedding distance for small NPs produced on this surface;

[0071] Figure 37 is a plot of various reflectance spectra produced by FDTD simulations reproducing the full NPs network on the surface of silver using the measured values for a fluence of 1.67 J/cm^A2 at a marking speed of 50 mm/s and a line spacing of 5um, showing a significant shift in the plasmonic resonance for embedding distances as small as 0.5nm; [0072] Figure 38 is a plot comparing measured and simulated Hue values for different P particle arrangements, in which a simulated embedding of small Ps on silver by 1.5 - 2.5 nm shows good agreement with experimental results; and

[0073] Figure 39 and 40 are SEM photographs of silver samples exposed to burst- mode laser irradiation and showing underlying structures believed to be associated with the improved Chroma achievable using burst-mode irradiation.

DETAILED DESCRIPTION

[0074] Generally, the following description is directed toward the fixed colouration of metallic surfaces by way of laser irradiation, whereby such irradiation results in the formation of a metallic surface nanostructure that is at least in part responsible for this fixed colouration. By varying certain parameters associated with the laser irradiation, such as an irradiation pattern, speed, fluence and the like, different colourations were achieved due, as is currently understood, to variations in the metallic surface nanostructures so produced. Accordingly, in one embodiment, a deterministic approach can be applied in the selection of colouration parameters such as Hue, Chroma and Lightness, to be reproducibly applied from surface to surface. Observable colouration characteristics such as Hue, Chroma, Lightness, and/or combinations thereof, will be used in some examples to characterize and concretely describe new metallic product colouration classes or regimes as specific embodiments of the invention and falling within the general scope and nature of the present disclosure.

[0075] As will be appreciated by the skilled artisan, colouration parameters such as Chroma, Hue and Lightness can generally be defined and measured for comparative purposes in defining and identifying products and features falling within the general scope and nature of the present disclosure. For example, Chroma is generally accepted to define a quantified value of colour saturation, wherein a higher Chroma value will generally be associated with a better colouration. In the present context, significantly higher Chroma values have been observed than previously expected possible, particularly for certain Hue ranges heretofore difficult if not impossible to achieve via laser-induced colouration. On that note, Hue is generally accepted to define a quantifiable colour scale where each colour is associated to a certain angle for a 360-degree rotation. Lightness on the other hand is a quantified value of colour Lightness with respect to white, wherein the higher the Lightness value, the whiter the colour will be. As will be appreciated by the skilled artisan, other colouration scales and values may be considered to characterize products produced by the embodiments considered herein, and that, without departing from the general scope and nature of the present disclosure.

[0076] For simplicity, the following will make general reference to "fixed colours" or "fixed colouration" which will be understood by the skilled artisan to refer to material colourations that remain consistent no matter the viewing angle, that is, to be distinguished from known holographic metal colouring effects. Accordingly, as used herein the term fixed, and variations thereof, as it relates to colours and colourations shall be taken to mean colours that remain substantially constant and independent of sample viewing orientation.

[0077] Without wishing to be bound by theory, it is believed that the metallic nanoscale structures produced and responsible for the metal surface colouration in these embodiments are the result of metallic nanoparticle re-deposition and/or recrystallization on the metal surface following laser-induced ablation and/or localized melting, which in the examples described below, takes place in air in a non-vacuum environment. Namely, one interpretation is that laser-induced ablation and particle re-deposition of the metal surface occurs using low picosecond laser pulses such that the resultant surface structure appears coloured. An alternative interpretation rather suggests a localized melting and subsequent highly non-equilibrium metal solidification mechanism as responsible for the formation of the observed nanostructures deemed responsible for the metallic surface colouration so provided. Regardless of the underlying mechanism for nanostructure formation, the colouring effects are deemed to be structural rather than chemical. In other words, the various colourations achieved in the below examples are believed to be produced by a laser induced nano-scale texturing of the metal surface which, in the least, alters the absorption spectrum of the metal surface so to produce different reflected colourations. [0078] In this regard, the term "irradiation" will be used genetically herein to refer to the exposure of certain metallic surfaces to laser pulses which are characterized, alone or in combination, as sufficient to generate the desired nanoscale structures therein. In that context, and without forgoing the above, the term "ablation" may be used to describe a result of such irradiation as referring to material alterations generally, rather than to any specific process of material alteration. Specifically, "ablation" is defined as occurring by experimental observation, i.e., by the onset of surface damage or alteration to the material being processed, where the surface damage or alteration is typically observed by eye or by SEM analysis, as illustrated herein. Thus the term "ablation" is generic, and is not used to refer to a specific physical process of material alteration, for example, the specific physical process of vaporization or other form of removal of material from a surface, etc.

[0079] The metallic surface alterations considered herein are generically and interchangeably referred to as nanostructures and/or nanoscale structures, but may also be referenced as nanoscale roughness, nano roughness, nanoscale texture or nanotextures without loss of generality. As will be appreciated by the skilled artisan, the term "nano" is generally used to define structures/features having a dimension less than about 100 nm or 200 nm, for example). Generally, these nanostructures will consist of one or more nanoscale features that may include, but are not limited to, nanoparticles, nanoprotrusions, nanospheres (nano-ovoids), nanopores, nanocavities, etc. While the below description contemplates one theoretical explanation for the production of metallic surface colourations due to the metallic nanostructures generated by laser irradiation, which generally discuss the impact of a distribution of nanoparticles or like nano-scale features produced, deposited or otherwise resulting from distinct laser irradiation patterns and/or parameters, other explanations may also or alternatively prevail as being more accurate or predominant in producing the observed metallic surface colourations. Further, while certain nano-scale features/structures may be predominant in the production of certain fixed colourations, other nano-scale and/or micro-scale features/structures may be concurrently generated to produce desired compounding colouration or finishing effects, such as compounding holographic effects produced by the concurrent formation of surface ripples or grooves, frost-finish effects (e.g. provided by the concurrent production of micro-scale structures such as a distribution of larger light-scattering microparticles), textured colouration gradient effects (e.g. provided by irradiating profiled surfaces under certain illumination parameters), etc.

[0080] Generally speaking, the characterization of described nano- and microstructures are based on scanning electron microscopy (SEM) imaging and analysis, which SEM analysis may also be used to establish qualitative and/or quantitative metrics for characterizing the surfaces, processed and products encompassed with the following description of illustrative embodiments.

[0081] As will be demonstrated below with reference to the following examples, the metal colouration processes discussed herein can more or less predictably produce a selectable fixed metallic surface colouration and/or effect as a function of a designated laser irradiation pattern, such as picosecond pulsed laser irradiation pattern, along with designated laser irradiation parameters conducive to producing such colourations. As will be appreciated by the skilled artisan, such laser irradiation parameters should include those conducive to producing surface nanostructures as those illustrated in the appended Figures. Reasonable ranges for such parameters are nonetheless provided in the below examples to provide guidance in this respect, though it will be expected that some may vary depending on the type of irradiation laser and equipment being used, the materials being irradiated and their respective properties, and various other experimental and/or industrial conditions in which the surfaces are irradiated in accordance with the various processes encompassed by the embodiments discussed to produce metallic surface colourations and commercial products subject thereto, as considered herein.

[0082] While most colouration results presented herein result from laser irradiation, in accordance with the various methods and techniques described herein, of metallic silver surfaces, described results achieved on metallic gold surfaces further depict the applicability of the techniques described herein to gold surfaces, and may also be considered for the colouration of other metals such as copper, titanium, palladium, platinum, ruthenium, rhodium, tantalum, iron, niobium and alloys thereof, for example. For greater clarity, unless otherwise stated, results presented herein were obtained suing silver surface samples. As will be appreciated by those skilled in the art, reference herein to silver and gold surfaces or surface portions should be understood to make reference to silver and gold metal surfaces and not surfaces that are otherwise silver or gold coloured.

[0083] In some embodiments, a metallic surface portion can be irradiated in accordance with a selected irradiation pattern consisting of consecutive picosecond laser irradiation lines defining a succession of adjacent and/or overlapping laser pulse irradiations both along a same irradiation line (i.e. as defined by laser spot size, laser repetition rate and line scanning speed) and between successive irradiation lines (i.e. as defined by laser spot size and line spacing). Figure 1A provides a simplified example of an irradiation pattern 100 produced on a metallic surface portion 102 by the combination of successive parallel irradiation lines 104 having an interline spacing /, each defined by a succession of irradiation spots 106 defined by an inter-spot or intraline spacing d (i.e. where d = marking speed/frequency). Practically, respective irradiation pattern spacings are defined as a distance between adjacent peak irradiations. Accordingly, where an effective spot size of the laser irradiation is greater than the intraline and/or interline spacing, such as depicted in the schematic irradiation pattern 108 of Figure IB, the irradiation pattern will result in partially overlapping irradiations, which, in some examples, appears to enhance some desirable colouration characteristics.

[0084] In some embodiments, a selected irradiation pattern may further or alternatively consist of overlapping sets of picosecond laser irradiation lines, overlapping either by rescanning the same surface portion with two or more sets of consecutive lines in the same orientation, scanning each line two or more times before moving on to the next line, and/or by crossing respective sets of irradiation lines at a predetermined angle in accordance with a cross-hatching irradiation pattern. Such effectively overlapping irradiation patterns have been shown in some examples, as discussed further below, to produce or improve certain colouration parameters.

[0085] In some embodiments, a selected irradiation pattern may further or alternatively consist of burst laser irradiation lines, in which successive laser irradiation pulses, rather than to be produced one-by-one as successive single pulse irradiations, are produced in bursts of two or more consecutive pulses such that a temporal spacing between consecutive pulses within a same burst is considerably shorter than a temporal spacing between successive bursts. In other words, each picosecond pulse is effectively divided in two or more distinguishable sub-pulses to form a multi-pulse burst. For example, in the below examples, each sub-pulse within a given n-pulse burst will be characterized by a total irradiation energy that is a corresponding fraction (//«) of the total irradiation energy that would otherwise have been available in a single pulse burst (n=l). In other words, a 2-pulse burst will include two sub-pulses of half comparative energy. In the below examples, the temporal spacing between each sub-pulse within a given burst is governed by the oscillator frequency of the laser, and in these examples, is approximately 12.8 ns. It will be appreciated that while the current installation produces intra-burst pulses of equal energy (i.e. energy of each intra-burst pulse is equal to the total burst energy divided by the number of intra-burst pulses), other burst energy distribution profiles may also be considered without departing from the general scope and nature of the present disclosure. [0086] Figure 24 schematically illustrates a laser system operated in a standard (e.g. Burst 1) versus burst (e.g. Burst 2, 3, 4, ...) mode. For example, a standard oscillator can generate a train of pulses periodically released by a switch that either periodically releases single pulses (e.g. standard or Burst 1 mode) to a transient amplifier that ultimately produces output pulses at a given repetition rate (e.g. 50 kHz) to be externally manipulated in a desired illumination application, or that periodically releases 2 or more consecutive pulses to form a train of pulse bursts, again defined by a repetition rate dictating an inter-burst period, but also defining an intra-burst period between intra-burst pulses. Generally, each burst will effectively encompass the same total energy as that of a single pulse, by subdivided between the intra-burst pulses of each burst. Accordingly, the operation of the irradiation laser in burst-mode may allow for successive intra-burst pulses to more effectively interact with the melt, ejected particles and/or plasma induced by the previous pulses. For greater clarity in the present context, it is to be understood that a burst mode is one in which the timescale between the intra-burst pulses allows for the interaction with the melt, ejected particles and/or plasma induced by the previous pulses of that same burst. [0087] Using such burst irradiation thus results in the production of an irradiation pattern consisting of successive spatially grouped irradiations corresponding to each successive burst. A schematic representation of an exemplary burst-mode irradiation pattern 110 is shown in Figure 1C, in which successive two-pulse bursts irradiate the surface region of interest, thus defining not only an intraline spacing d and an interline spacing /, but also an inter-burst spacing b, which is considerably shorter than the intraline spacing d. For instance, as will be appreciated by the skilled artisan, while the burst-mode pattern illustrated in Figure 1C shows grossly distinguishable intra-burst pulses, given the 12.8ns separation between intra-burst pulses, an actual spatial separation for a given line speed of approximately 50mm/s is roughly in other order of a nanometer, and thus orders of magnitude shorter than an inter-burst spacing. Regardless, such effectively overlapping irradiation patterns, at times in combination with other pattern characteristics such as cross-hatching, have been shown in some examples to produce or improve certain colouration parameters. For instance, using burst-mode irradiation, such closely spaced intra-burst irradiations may effectively act as a local repetition rate in which a substantially same spot on the sample is irradiated two or more times with correspondingly lower energies, thereby not necessarily affecting an effective irradiation fluence at that spot, but nonetheless providing what may be characterized as a softer irradiation pattern to positive effect, to be discussed below. [0088] As will be discussed in greater detail below, various combinations of the above irradiation pattern options, with option to further adjust irradiation characteristics by adjusting other laser irradiation parameters such as fluence, polarization, frequency, etc., were surprisingly observed to generate myriad colourations and colouration effects heretofore unavailable using traditional methods, not to mention the ability to carefully and reproducibly fine tune such colourations for a given product design or requirement, a relevant feature for commercial and industrial applications. While some initial work, as noted above, reports preliminary results in the fixed picosecond laser colouration of copper surfaces and the fixed femtosecond laser colouration of aluminum, these early reports are mostly inconsistent with the results discussed herein, and produce particularly sparse results on achievable colourations with little to no indication as to an expected reproducibility of these results or an ability to adequately control a prescribed colouration. One observation as to the limited applicability of their results appears to be related to their focus on characterizing select surface nano-texturing and irradiation parameters deemed to be at best secondary in the present work, if not entirely misleading in directing results away from the particularly successful colourations achieved using the processes considered herein, these being characterized by improved Chroma (i.e. depth of colour saturation), reduced and/or controllable Lightness, and/or a greater spread of achievable colouration Hues heretofore never observed using laser colouration techniques, for example.

[0089] For instance, it was unexpectedly observed that certain colouration characteristics could be achieved and/or improved by optionally and selectively: overlapping multiple irradiation lines in a same or distinct (e.g. crossing) line directions; controlling an irradiation line spacing to correspondingly adjust colouration, and that, over a range of line spacings optionally encompassing much closer irradiation lines at times using correspondingly lighter irradiation fluences (e.g. effectively producing partially overlapping irradiation lines); operating the irradiation laser at comparatively lower repetition rates and speeds so as to significantly partially overlap successive irradiations along a same irradiation line; operating the irradiation laser in burst mode so to produce multiple partially overlapping irradiation pulses within each burst; and/or adjusting other irradiation parameters in isolation or in combination to reproducibly create a desired colouration and/or colouration effect.

[0090] Further, as will be described in greater detail below with reference to some examples, some of the techniques described herein may also be applied to achieve colouration on textured, rough or otherwise profiled surfaces, and in some respects, yield particularly interesting results as a function of such surface profiles. For example, application of the methods described herein have been shown to yield desirable colour gradients when applied to a surface having a designed relief, such as shown in Figure 22, for example. Likewise, colourations may be applied to frosted or otherwise textured surfaces to produce desirable effects. Ultimately, the techniques described herein allows for the reliable production of quality colouration results on flat and polished surfaces and rough/textured and unpolished surfaces alike, a characteristic particularly amenable for industrial and commercial applications.

[0091] As will be described in greater detail below, it is believed that by controlling an irradiation pattern and/or parameters as discussed above, a metallic nanostructure produced in/on the irradiated metal surface using such patterns/parameters will also be correspondingly controlled so to effectively and reproducibly control a prescribed laser- induced colouration of the irradiated metal surface. In some embodiments, these metallic nanostmctures are characterized by a distribution of nanoparticles (NP) or like features of varying densities, whereby upon controlling a density of such nanoparticles, a colouration may also be reliably controlled. For instance, desirable colourations were achieved upon producing NP densities above 1000ΝΡ/μιη² and ranging from about 1000 or 2000 to about 4000, 5000 or even 5500 ΝΡ/μιη², and/or with mean inter-particle spacings ranging from about 2 to about 8 nm wall to wall. These and other such NP distribution characterizations will be discussed in greater detail below with reference to the following examples.

[0092] Figure 17 provides a general overview of some of the various colourations achieved using the techniques, features and options described herein. As can be seen from these results, achievable colourations extend across the full spectrum of possible Hue values with a Chroma over 2 achievable across the board. For certain Hue values, such as those ranging between about -90 to 90, Chroma values over 10 where achieved, and over 15 when operating in burst mode.

EXAMPLES

[0093] The following provides various exemplary results achieved using the picosecond laser irradiation techniques and patterns discussed above, alone or in combination, using the following experimental setup.

[0094] In particular, metal samples (such as gold and silver in the below examples) were exposed to laser irradiation at a wavelength of 1064 nm emitted from a Duetto (Nd: YV04, Time Bandwidth Product) Q-switching laser, operating at a repetition rate of 50 kHz and producing 10 ps pulses with a peak power of 15W, that was focused on the silver surface using F-theta lenses (f=163 mm, Rodenstock). The laser was fully electronically integrated and enclosed by a third party for industrial applications (GPC- PSL, FOBA). For accurate focusing, the surface of the samples was determined using a touch probe system. The silver samples (z=8 mm) were of 99.99% purity. Samples were not polished prior to machining to meet requirements of reproducibility in industrial applications.

[0095] For machining, the samples were placed on a 3-axis stage with resolution of Ι μηι in both the lateral and axial directions. The samples were raster scanned using galvanometric XY mirrors (Turboscan 10, Raylase) displacing the beam in a top to bottom fashion with a mechanical shutter blocking the beam between each successive line. As shown in Figure 1A, the irradiation lines 102 were raster scanned from right to left, as discussed above, where d is the spacing between each successive laser shot on a single line and / is the spacing between each successive line. [0096] Laser power was computer controlled via a laser interface and calibrated using a powermeter (3A-P-QUAD, OPHIR). A Gaussian beam radius of approximately 14 μιη was obtained from a semilogarithmic plot of the square diameter of the modified region, measured with a scanning electron microscope (SEM), as a function of energy following known methods. [0097] High-resolution SEM (JSM-7500F FESEM, JEOL) images were obtained using secondary electron imaging (SEI) mode. The colours were quantified using a Konita Minolta CR-241 Chroma meter with the CIELCH colour space, 2 observer and illuminant C (North sky daylight).

[0098] For most results presented, the repetition rate of the laser was set to 50kHz, corresponding to a single shot every 20 μβ. Contrary to previous reported results on copper stating no heat accumulation effect ensued due to the timescale of lattice energy transfer, results on silver discussed herein suggest a much different picture. After changing the repetition of the laser from 50 kHz, to 100 kHz, and 200 kHz, colours were observed to fade and became yellow-brown (e.g. see Figure 18). The experimental conditions were such that the pulse energy and the number of shots were kept constant throughout each laser frequency, an indication that there is indeed a cumulative effect that should be taken into consideration. Laser frequencies of 200 kHz and higher, also failed to produce colours other than the yellow-brown colour, except, as will be discussed in greater detail below, when using burst-mode irradiation (e.g. 2-8 fold bursts). For example, in the photographed samples of Figure 18, the effect of higher frequencies on colour in single pulse (i.e. burst 1) mode can be observed for samples irradiated at a constant fluence of 1.74 J/cm² and an interline spacing of 6 um. For these samples, the frequency and speed were changed simultaneously in order keep the intraline spacing constant, so that only the time between two successive pulses was changed. From left to right, the frequencies are 200 kHz, 100 kHz, 66.6 kHz, 40 kHz, 28.6 kHz, 10 kHz and 5kHz, and the speeds are 200 mm/s, 100 mm/s, 66.6 mm/s, 40 mm/s, 28.6 mm/s, 10 mm/s and 5 mm/s, respectively. For all parameters, a similar trend was observed, where colours made at high frequencies (over 100 kHz) faded and turned yellow. [0099] Figure 2 shows different colourations obtained on silver with a fixed fluence of 1.12 J/cm² and a line spacing of 10 μπι, as a function of irradiation line speeds ranging from 5 mm/s to 32 mm/s (intraline spacing ranging between about 0.1 μπι and 0.64μπι). As the scanning speed of the mirrors increases the colours change from dark gray, through blue, purple, violet, pinks and ultimately yellow. Slower transition from one colour to another can be interpolated with smaller speed jumps. At this low fluence, the colours are accompanied by a metallic finish, which is distinctive from colours obtained at higher fluence.

[00100] To study colours made at higher speeds, which may be preferred for industrial applications, mirror speeds were increased as well as fluence to remain above ablation threshold. Figures 3 A and 3B show colours obtained with a fluence of 1.67 J/cm² and 29.75 J/cm² with marking speeds of 50 mm/s and 1000 mm/s, respectively, both having similar accumulated fluence. The accumulated fluence is calculated using the following relation:

Face N_ef X Fl_aser where N_efr is the number of effective laser shots delivered within the local region and Fiaser is the laser fluence.

[00101] The different colours were obtained by varying the line spacing by intervals of 1 μπι, providing irradiation patterns having an interline spacing ranging from Ι μπι to 16μιη and an intraline spacing of about Ι μπι for results in Figure 3 A, and an interline spacing ranging from Ι μπι to 25 μιη and an intraline spacing of about 20μιη for results in Figure 3B.

[00102] As can be seen in these results, the colours are observed to change starkly with increasing line spacing, demonstrating the colors' sensitivity to such small changes. Considering that both line spacing and speed can be linked to a number of effective shots delivered to the surface, changing them independently has a similar effect on colour transition while the values in Chroma and Lightness may differ greatly. For instance, colourations observed in Figure 3A tend to be much more saturated than those in Figure 3B produced at higher speeds (i.e. higher intraline spacings). Irrespective, providing independent but potentially concurrent control over line spacing and speed can provide for greater control and selection of a desired colouration outcome. Depending on the fine tuning ability of either parameter, one can thus for example coarsely adjust a line spacing to predefined setting and then fine tune the desired results as a function of scan speed, which in some installations, may be more readily fine tunable. [00103] To put these results into context, colouring using a scan speed of 1000 mm/s corresponds to a colouring time of about 20 to 100 seconds to cover an equivalent face surface area of an American quarter (462.24 mm²), an acceptable time for practical industrial applications.

[00104] Colors were also obtained for marking speeds as high as 3000 mm/s at similar effective fluences (results not shown). Lighter colours are obtained for higher speeds and fluences, in addition to the increased spread of colours over larger line spacings. Depending on the intended application, control of Lightness can serve to further manipulate colourations while increases in line spacing can significantly reduce marking time. Interestingly, compared to the lower speeds and fluences, the finished surface for higher fluences is generally matte and does not have a shiny metallic finish suggesting two types of colouration regimes dependent on laser fluence. Furthermore, a significant reduction in Chroma values was observed with increasing fluence for the same marking speed. [00105] From the above, one reasonably anticipates that more colours and a more progressive colour transition can also be obtained using smaller spacings. For observed fluences, an optimal range of laser pulse density was found for colourisation. This range, usually starting near the ablation threshold with a yellow colouration, typically ended with a blue colouration, after which an increase in pulse density only resulted in a decrease in Chroma and ultimately a loss in colour and transition to a grayscale. Nevertheless, for femtosecond lasers plagued with low pulse energies and so left to be used at low repetition rates, achieving high marking speeds while remaining above the ablation threshold is currently not an option.

[00106] Figure 4A presents a fraction of the colours obtained where different colour Lightness and Hues are obtained by changing laser parameters such as line spacing, speed and fluence. The produced colours showed no variability on the quality of the sample's surface. WDS analysis of the different colours showed no difference in oxidation, ruling out oxidation as the mechanism responsible for the colours.

[00107] Figure 4B is another data plot showing a general increase in Lightness with increasing laser fluence, wherein plotted Lightness was calculated by taking the average Lightness over data within specific Hue ranges, such as Yellow: 30-90 Hue, Red: -30-30 Hue, Magenta: 300-360 Hue, and Blue: 240-300 Hue).

[00108] Figure 4C is a data plot of Chroma versus Hue for different marking speeds, in accordance with one embodiment, wherein Chroma is shown to be less affected by fluence than Lightness and wherein different colours (Hues) with a same marking speed and fluence result from changes in line spacing.

[00109] In Figure 31, a plot of Hue vs total accumulated fluence is provided for different laser parameters, again showing the impact of controlling fluence, and in particular a total accumulated fluence, on observable Hues, which demonstrates not only the range of colouration results achievable using this process, but also the controllable nature thereof by carefully adjusting irradiation parameters to predictively adjust a total accumulated fluence on the target surface and, in this example, consequently control a resulting colouration Hue. As will be described in greater detail below, different control parameters may be invoked to control a total accumulated fluence on the irradiated surface, such as line spacing and marking speed, which may impart different colouration results as noted above. For example, and as will be discussed in greater detail below, control of the total accumulated fluence can impact a P density on the irradiated surface, which according to some observations is at least in part responsible for the different colouration results observed herein. In any event, this and other results presented herein promote the implementation of a controllable process for the colouration of a metal surface across a broad range of colours and characteristics.

[00110] It is believed that variations in the nanostructures produced by the various laser irradiation patterns and parameters discussed above are the most likely mechanism for causing the perceived colourations. In particular it is believed that variations in NP size and volume/surface fraction (i.e. density) may be responsible, alone or in combination, for at least some of the colourations observed.

[00111] In support of this observation, Figures 25 A and 25B provide photographs of single laser pulse irradiation results seen in bright field mode under an optical microscope without colour correction, where Figure 25A shows the colouration result of 50 pulses at 19.09 J/cm² on silver and Figure 25B shows the colouration result of 100 pulses with a fluence of 19.09 J/cm² on gold. The colouration gradients shown in these pictures is consistent with those shown in Figure 3 as further evidence of colour variations as a function of particle densities, where lower particle densities are attributed to a plasmon peak absorption in the high energy end of the visible spectrum, displaying a yellow finish, and higher particle densities to an absorption in the low energy end of the visible spectrum, responsible to the blue colour. This further supports that the control of particle densities through variations in irradiation pattern can create a wide range of colours. The change in colour in Figure 25B also demonstrates the ability to colourize other metals such as gold.

[00112] With reference to Figures 5 and 6, SEM analysis of the different line spacings revealed significant changes in area fraction and size distribution of NPs as well as the number of larger NPs covering the surface. For example, the particle density is observed to decrease significantly by changing the line spacing from (a) 1 μπι to (b) 2 μπι for both 50 mm/s and 1000 mm/s, as shown in Figures 5 and 6 respectively. In addition, a significant drop in the quantity of larger NPs can be observed. The difference in area fraction and quantity of larger NPs is less abrupt with increasing line spacing (d). Figures 5c and 6c provide respective close-ups of the Ι μπι line spacing samples showing the presence of a wide distribution of NP sizes aggregated on the surface of the irradiated region, which are believed to be responsible for the lower Chroma values of areas irradiated with a higher laser pulse density. Some laser-induced periodic surface structures (LIPSS) are also observed at the surface for small line spacing, but are seen to disappear as the spacing is increased, transitioning to a perceptively more melted surface. Interestingly, no changes in colour perception as a function of viewing angle were observed even in samples where LIPSS were present, potentially attributable to the large quantity of nanostructures covering the LIPSS, also known as nanoparticle covered LIPSS or NC-LIPSS. [00113] Figure 30 provides further SEM images illustrating how the number of relatively smaller NPs increases with increasing line spacing while the medium and larger NPs tend to decrease with increasing line spacing. The SEM pictures at Figure 30 were taken for line spacings of (A) 5 um, (B) 10 um and (C) 15 um at a fluence of 1.12 J/cm^A2 and a laser marking speed of 11 mm/s, with their higher magnification images shown in Figures 30D, E and F.

[00114] Generally, SEM analysis reveals the presence of uniformly distributed NP on the metal substrate within the irradiated regions. Large arrays of closely spaced nanoparticles are observed to cover the entire metal surface. It is commonly believed that the NPs ejected during the ablation process redeposit themselves on the metal's surface following ablation in air, contrary to PLD in vacuum, due to the atmospheric pressure and responsible for the colouring of metals. Strangely in this case, the close proximity, uniform distribution and absence of staked NPs show more resemblance to films created by sputtering. Nevertheless, clear indication of subsequent ejection of NPs with each successive line can be observed by the accumulation of smaller NPs on top of the large clusters (e.g. see Figure 7). Furthermore, as seen in Figures 10A to IOC, respective photographs of 2 irradiation lines drawn on respective silver surface areas and separated by a distance of (a) 250um, (b) 200um and (c) 150um and made at a constant fluence of 16.90 J/cm^A2, frequency of 50 kHz and a marking speed of 100 mm/s. As can be seen in these photographs, as the distance is decreased between the 2 lines the blue region is extended by approximately 30% (Figure 25C), providing supporting evidence as to the potential impact of an increase in particle accumulation from adjacent lines in shifting resulting colourations.

[00115] The change in particle densities, inter-particle distance and/or particle size with line spacing, as seen in Figures 5, 6, and 30, gives rise to different perceived colours. SEM analysis of the irradiated surfaces showed NP densities for larger particles (e.g. 10 to 200 nm) on the order of 10 to 100 NPs per μπι², and more particularly from 10 to 70 NPs per μπι², and for smaller particles (e.g. 4 to 10 nm) on the order of 200 to 7000 NPs per μπι², and more particularly 1000 to 5500 NPs per μπι², which densities are drastically higher than those previously reported on the colouration of copper and may explain the distinct colouration regime observed in some of the examples provided herein. This may in fact result from the basis that, for large volume/surface fractions, NPs can no longer be treated individually due to near field interactions with surrounding NPs, which generally results in shifted plasmon resonances from that of a lone NP. Corresponding mean wall- to-wall interparticle distances were also observed to range between about 11 and 35 nm for larger NPs, and about 2 and 6 nm for smaller NPs. It should be noted that these results were more or less consistent for prescribed colourations using non-burst, burst and/or cross-hatching irradiation patterns.

[00116] Upon further investigation, three NP size regimes could be defined, a small NP regime again defined for particles around or below 10 nm in radius (e.g. under 10.7 nm in one study); a medium NP regime defined by particles between lOnm and 75nm in radius (e.g. 10.7 to 75 nm in one study); and a large NP regime for particles larger than 75 nm in radius.

[00117] As compared to prior reports on metallic colourations, those achieved in the present context at lower fluences, for example, generally appear to result from smoother and uniform NP distributions. Further, in addition to the smaller NPs, larger NPs were also observed to cover the irradiated surfaces, ranging up to 234nm, 455nm and 1465nm for marking speeds of l lmm/s (1.12 J/cm²), 50 mm/s (2.36 J/cm²) and 1000 mm/s (29.75 J/cm²). The presence of larger NPs at higher fluences appears to correspond with the increase in Lightness observed in Figure 4, and may thus be at least in part responsible for this increased Lightness as larger NPs are more likely to scatter incoming radiation conjointly with the rougher melted surface so produced by higher fluences.

[00118] In traditional pulsed laser deposition (PLD), such larger particles are generally unwanted, however, in the present context, the controlled generation of such larger NPs may in fact be desired in providing yet further control over the resulting colouration of the metal surface of interest, i.e. extending the available colour palette to include selectable colours of higher Lightness.

[00119] Figures 19A and 19B show two different magnifications of a given sample. In Figure 19A, a first regime of formed nanoparticles is shown with particles ranging in size from about lOnm to about lOOnm, whereas in Picture 19B, a second regime of smaller NPs is shown with particles seemingly embedded in the substrate with a mean radius of about 6nm (e.g. ranging from about 4 to about lOnm).

[00120] Figures 20A, B and C show different NP radius histograms for different irradiation line spacings of 5um (A), lOum (B) and 30um (C), respectively, each for a fluence of 1.12 J/cm² and marking speed of l lmm/s. These histograms clearly show the different particle size regimes noted above, i.e. for small, medium and larger sized NPs. These results confirm the general existence of different NP regimes and provides respective illustrative distribution and size ranges therefor. [00121] For example, summed NP densities in the low, medium and larger NP size regimes defined above were tabulated for various samples as ranging from about 1000 to about 5100 NP/um^A2 for smaller NPs, and between about 5 to about 65 NP/um^A2 for medium NPs, depending on colour.

[00122] Further observations were made on the basis of the NP size regimes noted above. For example, as shown in Figures 32A and B, respectively, a mean particle radius for smaller and medium NPs was found to remain more or less constant as a function of line spacing, suggesting that small and medium particle size variations were not predominantly responsible for colour changes. On the other hand, and with reference to Figure 32C and D, respectively, a mean particle distance between particles in the small (3 to 6 nm) and medium (30 to 70nm) size regimes appeared to vary with line spacing suggesting a correlation between particle density and colouration changes.

[00123] As introduced above, to account for these observations and those presented in Figure 31 showing a predictable relationship between colouration Hue and a total accumulated irradiation fluence, Figures 33A and 33B provide variations in small and medium NP densities, respectively, as a function of total accumulated fluence. As expected, NP densities in both regimes appear to vary significantly with total accumulated fluence, which, as noted above, appears to be strongly linked to observed variations in colouration Hue.

[00124] With reference to Figures 9A to 9C, typical Lightness, Chroma and Hue as exhibited by laser-coloured silver surfaces are provided as a function of laser pulse density, in these examples achieved at a constant fluence of 1.67 J/cm² and marking speed of 50 mm/s while changing the line spacing. Similar trends were also observed for different fluences and marking speeds demonstrating more or less an optimal or preferable range of effective laser pulse densities in which colouration Hue, Chroma and/or Lightness can be more readily controlled and adjusted over a wider range of such colouration parameters. In other words, certain ranges in pulse laser densities are shown to be particularly sensitive to variations in producing a correspondingly wide range of colouration parameters within those ranges. [00125] To further investigate the potential impact each particle size regime may have on colouration, particularly as it relates to the small NPs first observed and controllably studied as reported herein, various simulations were conducted. In one interpretation, it was observed that the presence and controlled variation of these small NPs may in fact be predominantly responsible for the range of colour observed, and in particular the range colour parameters so observed. To do so, finite difference time domain (FDTD) simulations were conducted to simulate the response of the observed nanostructure arrays discussed herein to incoming light, in this particular case, for a silver surface. In the simulation results illustrated in Figure 34, the nature of the observed colours is believed to be plasmonic, wherein the medium NPs do not appear to couple (A) in the absence of the smaller NPs (included in B), and thus, cannot produce the full spectrum of colours observed and discussed herein without participation from these smaller NPs. In other words, the smaller NPs appear to act as plasmonic couplers between medium sized NPs.

[00126] Figures 35 A and 35B provided further simulated results for a silver surface exposed to laser irradiations for line spacings of 3um and 5um, respectively, showing a simulated controllable impact on surface reflectance when combining small NPs with medium NPs.

[00127] In order to further investigate the principles associated with colouring due to the formation of the nanostructures observed and discussed herein, further simulations were conducted to consider the potential impact the shape of these small NPs may have on overall colouration results. Generally, the small NPs were observed to manifest themselves as more or less rounded protrusions atop the irradiated surface, which can be approximated as nanospheres in one example that sit atop, but generally somewhere below the surface, i.e. embedded to a certain degree within the surface. While the concept of embedded nanospheres is useful in the qualification of the observed nanostructures for simulation purposes, it will be appreciated by the skilled artisan that the actual shape and disposition of the formed NPs may vary to some extent from this idealized description without departing from the general scope and nature of the present disclosure. For example, while the concept of embedded spheres provides a useful simulation tool, observed NPs were less likely to consist of actual spheres embedded within the surface by the irradiation process, but more likely involve the formation of rounded protrusions whose observable extent may be more or less described using the simulated example described below.

[00128] On that basis, Figure 36 provides FDTD simulation results for a silver surface having a NP distribution similar to those observed for irradiations at a 5um line spacing, fluence of 1.67 J/cm^A2 and marking speed of 50 mm/s, in this case plotting variations in a simulated plasmonic peak as a function of an embedding distance for produced small Ps. As can be seen from these results, the colour (Hue) from the reflectance spectra of the system is observed to vary with embedding distance, the Hue values transitioning from yellow to blue with increasing embedding distance signifying a blue shift in the plasmonic resonance.

[00129] As further illustrated in Figure 37, the reflectance spectra produced by FDTD simulations reproducing the full NPs network on the surface of silver using the measured values for 1.67 J/cm^A2 at 50 mm/s and a line spacing of 5um show a significant shift in the plasmonic resonance for embedding distances as small as 0.5nm. Figure 38 further compares measured and simulated Hue values for different NP particle arrangements, in which a simulated embedding of small NPs by 1.5 - 2.5 nm shows good agreement with experimental results. A small NP embedding factor in this range is also consistent with preliminary focused ion beam (FIB) analyses conducted to quantify this particular factor. This observation again supports the interpretation that the distribution of small NPs is at least in part responsible for the manifestation of the broad colouring ranges observed and discussed herein.

[00130] With reference to Figure 11, a photograph of laser-coloured silver samples is provided showing a series of fixed metallic colourations achieved using a picosecond laser operating at a wavelength of 355nm, as compared to the 1064nm wavelength light used to achieve the results discussed above, wherein different colours were achieved using different line spacings ranging from ΙΟμπι to 34μπι with a constant marking speed of 400mm/s and repetition rate of 50kHz. Corresponding changes in NP volume/surface fractions were also observed consistent with results presented above, with colours exhibiting more of a metallic finish.

Colouration Dependence on Laser Polarization and Beam Shape

[00131] With reference to Figure 12, a photograph of laser-coloured silver samples is provided in which a line spacing and a relative polarization angle and line direction were changed to produce different colourations, wherein different line spacings produced greatest colouration variability, as discussed above, and wherein adjustment of the relative polarization angle and/or line direction produced a finer tuning of resulting colouration, thus providing yet another predictably controllable parameter to adjust colouration results.

[00132] With reference again to the photograph of Figures 25A and 25B, a slight asymmetry can be observed in the laser pulse from the irregular shape of the ring surrounding the ablation spot. From this, one observes that controlling the pulse shape can contribute to controlling a resulting particle distribution, thus allowing for further control of colour.

Cross-Hatching

[00133] As introduced above, selection of an appropriate irradiation pattern consistent with a desired colouration may also include selection of a particular cross-hatching pattern, wherein a first set of irradiation lines is first provided in a first line direction, and wherein a second (or third and beyond) set of irradiation lines is then provided in a second line direction at an angle to the first direction.

[00134] With reference to the photographs of Figures 13A and 13B, different 2-pass cross-hatching angles ranging from 0 degrees to 90 degrees are observed to produce colourations on silver of increasing Lightness for a same line spacing, repetition rate and fluence. Each square was made at a speed of 11 mm/s, fluence of 1.12 J/cm², and repetition frequency of 50kHz. Interline spacing of 13 um and 17 um were used to produce the blue (B) and pink (A) colours respectively. The colour was then further tuned by changing the angle between each pass as noted on top of each square. Similarly, with reference to Figure 13C, different 3 -pass cross-hatching angles ranging from 0 degrees to 90 degrees were also observed to produced variable colourations on silver samples with a marking speed of 11 mm/s, fluence of 1.12 J/cm², a repetition frequency of 50kHz and an interline spacing of 13 um. Accordingly, while a particular colouration may be selected and grossly tuned using line spacing or the like, a fine tuning as to a desired colouration effect or characteristic, such as Lightness, may be further finely adjusted as a function of cross-hatching angle.

[00135] Figure 14 further demonstrates the adjustability of resulting colouration as a function of cross-hatching angle, showing the measured spectrophotometer reflection spectra for different silver samples coloured via 2-pass cross-hatching irradiation lines having 13μπι spacing for different cross-hatching angles varying from 0 to 90 degrees and irradiation patterns as described above with reference to Figures 13 A and 13B.

[00136] With reference to Figures 15A and 15B, further spectrophotometer reflection spectra measured for laser-coloured silver samples using cross-hatching irradiation patterns are provided in which 2 and 3 -pass cross-hatching pattern colouration results are compared for 0 and 90 degree cross-hatching angles (Figure 15 A) and for 0 and 45 degree cross-hatching angles (Figure 15B). In these results, a distinctive blue-shift is observed between 2 and 3 passes, suggesting yet another technique for fine tuning desired colourations and achieving heretofore unavailable colouration results (e.g. see transitions shown in photographs of Figures 13B and 13C).

[00137] In Figures 28A and 28B, SEM photographs are provided for different samples irradiated with respective 2-pass cross-hatched patterns having respective line crossing angles of 90 and 15 degrees, clearly showing manifestation of the respective cross- hatched patterns in the resulting nanoscale structures formed on each sample. Burst-Mode Colouration

[00138] As introduced above, different colouration characteristics may be achieved using burst-mode irradiation as compared to standard-mode irradiation. Using such irradiation patterns can provide certain improvements and/or advantages over conventional methods, not to mention the ability to achieve certain colourations heretofore impossible using other techniques.

[00139] For example, in some embodiments, burst-mode irradiation for metal colouration can be applied over a greater range of laser repetition rates (e.g. burst rates) and consequently use such increased rates to produce a different range of colourisation, while also potentially increasing machining rates and thus reducing production times. For example, in some embodiments, colouration becomes effectively impossible or at least impractical at repetition rates above 200kHz. On the other hands, significant results were observed using burst-mode irradiation for repetition rates up to 800 kHz, a rate inaccessible to standard-mode irradiation methods on silver likely due to cumulative heat effects generally expected at those rates using standard-mode irradiation patterns.

[00140] Furthermore, it was unexpectedly observed that the use of burst-mode irradiation could lead to enhanced Chroma values over a wide range of achievable Hues, presenting an interesting option for not only accessing previously unachievable metal colouration such as red and green, but also deeper and more vibrant colours as compared to other techniques, such as non-burst irradiation controls that were reportedly more conducive to controlling Lightness, as discussed above, which may still be vibrant but lighter in colour. Accordingly, the ability to control both Chroma and Lightness can significantly extend colour palettes obtainable on metals such as silver. One interpretation is that selecting a burst-irradiation pattern allows for greater suppression of larger nanoparticles generally deemed responsible for increased colour Lightness and consequently resulting in darker colours.

[00141] For example, as clearly seen in Figure 17A, which plots measured Chroma values achieved with and without burst mode irradiation across the entire Hue range (- 180 to 180), burst mode irradiation can be observed to significantly increase attainable Chroma values across the range. Furthermore, as illustrated in Figure 17B, a broader range of achievable Lightness across the whole spectrum is observed when irradiating a silver surface using burst irradiation. [00142] With reference to Figures 16A and 16B, photographs are presented of laser- induced colourations achieved using burst-mode irradiation at frequencies of 200kHz and 600kHz respectively, and using various irradiation parameters as discussed herein. These photographs show a wide spread of achievable colourations, and that, even at much higher repetition rates than otherwise observed using non-burst mode irradiation, while providing a similar control over colouration characteristics.

[00143] Figure 16C is a photograph of laser-induced colouration on a gold surface using burst-mode irradiation (4-fold burst at 50kHz) with different colours achieved by changing the line spacing between 100 and 230um. Similar P distributions were observed in these samples as reported above with respect to silver.

[00144] Figures 26A to 26C provide SEM photographs of different silver samples irradiated in accordance respective 5-fold burst irradiation patterns, namely at a common marking speed of lOOmm/s and fluence of 5.16 J/cm², with line spacing of 1, 3 and 9 um, respectively. Accordingly, the laser shot density increases from A to C. Under some conditions, such as the ones depicted in Figure 26A, multiple holes and creases appear, which could be responsible for the absorption of more of the incoming light, resulting in a lower Lightness, which may ultimately be at least partly responsible for the increased range in achievable Lightness using burst mode irradiation.

[00145] Figures 39 and 40 provide further SEM photographs of different silver samples exposed to burst-mode irradiation, showing further periodic structures generally only manifested, at least to this extent, under burst-mode irradiation. For instance, ripples or like structures were observed that were larger, equal and much smaller than the irradiation wavelength. Again, such underlying structures are believed to be at least in part responsible for the increase in Chroma achievable using burst-mode irradiation.

[00146] Figures 27A and 27B provide SEM photographs of different samples irradiated with and without bursts, respectively, showing what ultimately appears as a "cleaner" surface for burst mode irradiation than without, namely showing how burst mode irradiation results in the manifestation of fewer of the relatively larger Ps. [00147] Particular applications of burst-mode colouration are provided in Figures 21 A and 2 IB in which a fixed deep/dark red colouration and a deep/dark purple colouration where applied to the surface of a silver coin, clearly exhibiting the merits of including burst-mode irradiation as an option for generating fixed metallic colourations. In this example, the red colouration was achieved using a 2-pass 90 degree cross-hatching pattern using an 8-fold burst mode, a fluence of 4.16 J/cm², a marking speed of 88 mm/s and a line spacing of 10 um; whereas the blue colouration was achieved using a 2-pass 90 degree cross-hatching pattern using a 5-fold burst mode, a fluence of 3.00 J/cm², a marking speed of 44 mm/s and a line spacing of 10 um. Overlapping Colourations

[00148] In accordance with one embodiment, yet further colourations may be achieved by overlapping irradiation patterns. For example, Figures 29A to 29D shows how layered colourations can be used to achieve different results. In Figures 29A and B, the influence of the first layer on the final colour is clearly visible. In Figures 29C and 29D, however, the second layer appears to completely erase the colour induced by the first layer. The following parameters were used in each case. In Figure 29A, the first layer was achieved using 1 um line spacing, a frequency of 50 kHz, Burst 1, fluence of 1.30 J/cm², and marking speed of 400 mm/s, whereas the second layer was achieved using 1 um line spacing, 50 kHz, Burst 1, fluence of 1.18 J/cm², and a marking speed of 400 mm/s. In Figure 29B, the first layer was achieved using 1 um line spacing, a frequency of 50 kHz, Burst 8, a fluence of 11.39 J/cm², and a marking speed of 3000 mm/s; whereas the second layer was achieved using 1 um line spacing, a frequency of 50 kHz, Burst 1, a fluence of 1.30 J/cm², and a marking speed of 400 mm/s. In Figure 29C, the first layer was achieved using a line spacing of 1 um, a frequency of 50 kHz, Burst 8, a fluence of 11.39 J/cm², and a marking speed of 400 mm/s; whereas the second layer was achieved using a 1 um line spacing, a frequency of 50 kHz, Burst 8, a fluence of 11.39 J/cm², and a marking speed of 3000 mm/s. In Figure 29D, the first layer was achieved using a line spacing of 1 um, a frequency 50 kHz, Burst 8, a fluence of 11.39 J/cm², and a marking speed of 3000 mm/s; whereas the second layer was achieved using a 1 um line spacing, a frequency of 50 kHz, Burst 8, a fluence of 11.39 J/cm², and a marking speed of 400 mm/s. Colouring Applications

[00149] With reference to the photograph of Figure 8, a first example of a complex colouration design is provided, whereby a series of distinct fixed colourations were applied to a flat silver surface. For instance, the laser-coloured silver surface included selection of a number of distinct irradiation patterns and/or parameters designated within a database of such patterns/parameters to correspond with preset colourations selected for this design. By effectively programming the laser-colouration equipment to reproduce the selected irradiation sequences for each coloured region of interest, multiple sets of the same design were made and each were observed to be substantially identical as it relates to perceivable colouration, and that, irrespective of the original metallic surface, thus promoting the proposed process for industrial application.

[00150] In this particular example, the various colourations were achieved as follows: Dark blue - 8-fold burst mode, marking speed of 50 mm/s, fluence of 6.20 J/cm², line spacing of 13 um, frequency of 50 kHz; Light Blue: single pulse mode, marking speed of 1000 mm/s, fluence of 29.75 J/cm², line spacing of 10 um, frequency of 50 kHz; Shiny Blue (top head and back): single pulse mode, marking speed of 11 mm/s, fluence of 1.16 J/cm², line spacing of 10 um, frequency of 50 kHz; White: 8-fold burst mode, line spacing of 1 um, frequency of 50 kHz, fluence of 11.39 J/cm², marking speed of 3000 mm/s; Black: single pulse mode, line spacing of 1 um, frequency of 50 kHz, fluence of 11.39 J/cm², marking speed of 3000 mm/s.

[00151] Generally speaking, the laser colouration equipment, which may include various computer controllable sample displacement mechanisms such as a high precision 3-axis stage to displace the irradiation beam across the various colouration regions of interest in accordance with selected irradiation patterns (e.g. line spacing, cross-hatching, etc.) as well as various controllable means for controlling pulse irradiation parameters such as repetition rate, fluence, burst, etc. can be configured to receive control inputs from a computation device to sequentially produce the various colourations prescribed by the design. For example, a user interface, such as a graphical user interface (GUI) or other interface may be configured to receive as input the intended design outline as well as its various colouration regions so defined, and further provide selectable colouration options, be it in the form of a user-selectable colouration palette and/or other available colouration effects/preferences, to be associated with each such colouration region. Once selected, the various colouration selections, each associated with a respective irradiation pattern and associated parameters, can be assigned to each region, thus directed as a control sequence to the laser colouration equipment to impart the selected colourations. As will be appreciated by the skilled artisan, colouration parameters may be stored in a system-accessible database or listing in association with perceivable colouration indexes, thumbnails, descriptions, characteristics or the like that accessible to the user for selection via the GUI. Upon confirming a selected colouration design, the corresponding parameters may be automatically conveyed to the colouration equipment for execution.

[00152] With reference to the photographs of Figures 22A and 22B, the above- described fixed colouration techniques are applied to a high relief frosted sample, such as a stamped silver coin. It will be understood that the term "relief is intended to refer to the raised or three-dimensional image found on a coin's field while the term "frosted" is intended to refer to areas on a coin surface that have been intentionally roughened to produce a dull appearance, for example. In these examples, the fluence of the laser as applied to the coin surface varies as a function of the relief to generate colour gradients consistent with this relief, effectively automatically applying a fluence-dependent colouration variation to regions of changing relief. Accordingly, the entire coin, or portions thereof, can be scanned using constant irradiation pattern and parameters irrespective of profile to produce an alternative colouration effect.

[00153] With reference to the photograph of Figure 23, the same coin is rather coloured while monitoring the surface's relief and adjusting a focus of the irradiating laser accordingly so to substantially maintain a constant colouration within a given region irrespective of profile variations.

[00154] Accordingly, various colouration techniques and variations can be achieved upon high relief surfaces by again further combining predictable colouration patterns and/or parameters with relief-sensitive characteristics such as fluence. [00155] It will be appreciated that while examples provided herein focused on the colouration of various coins, that the application of the embodiments described herein are not limited as such. For example, metal colourations for different types of objects, which may include coins, medals, tokens as well as other objects having at one metallic surface, can also be considered without departing from the present disclosure.

[00156] Likewise, while silver coins are considered in these examples, results such as those shown in Figures 16C and 25B depict the applicability of the techniques described herein to gold surfaces, and may also be considered for the colouration of other metals such as copper, titanium, palladium, platinum, ruthenium, rhodium, tantalum, iron, niobium and alloys thereof, for example. As will be appreciated by those skilled in the art, reference herein to silver and gold surfaces or surface portions should be understood to make reference to silver and gold metal surfaces and not surfaces that are otherwise silver or gold coloured.

Colouration Ranges and Parameters [00157] For the sake of illustration, below are various colouration results and currently achievable ranges illustrating the strength and breadth of the methods and techniques described herein.

[00158] In Tables 1 and 2, below, different colouration parameters, such as minimum and maximum Lightness (Lmin, Lmax) and maximum Chroma (C) values are provided for different Hue ranges corresponding generally to different colours in the full spectrum of visible colours, as achieved and observed using different irradiation parameters as described above in accordance with different embodiments for single pulse irradiation lines (i.e. Burst 1) and burst-mode irradiation lines (i.e. Bursts 2 to 8), respectively.

Table 1: Burst 1 Mode

Blue 210-270 40.01 59.98 14.42

Magenta 270-330 32.71 52.76 25.66

Table 2: Burst Mode (2 - 8)

[00159] While the present disclosure describes various exemplary embodiments, the disclosure is not so limited. To the contrary, the disclosure is intended to cover various modifications and equivalent arrangements included within the general scope of the present disclosure.

Claims

What is claimed is: 1. An object having at least one metallic surface portion, said at least one metallic surface portion having a first coloured region and a second coloured region each of which having distinct colouration;

each of said first coloured region and said second coloured region being defined by respective metallic nanostructures generated via a plurality of consecutively spaced- apart laser irradiation lines, wherein a first line spacing for said first region is distinct from a second line spacing for said second region so as to produce said distinct colouration.

2. The object as defined in claim 1, wherein each of said first coloured region and said second coloured region exhibit fixed colouration.

3. The object as defined in either one of claim 1 or claim 2, wherein at least one of said first line spacing and said second line spacing is between about 0.1 and about 300um.

4. The object as defined in claim 3, wherein said metallic surface is silver, and wherein each of said first line spacing and said second line spacing is between about lum and about 50um.

5. The object as defined in claim 3, wherein said metallic surface is gold, and wherein at least one of said first line spacing and said second line spacing is between about lOOum and 220um.

6. The object as defined in any one of claims 1 to 5, wherein said metallic nanostructure is at least in part defined by a surface density of distributed metallic nanoparticles between about 5 and about 7000 P/um²

7. The object as defined in any one of claims 1 to 5, wherein said metallic nanostmcture is at least in part defined by a surface density of distributed medium metallic nanoparticles of between about 5 and about 100 P/um², or between about 5 and about 65 NP/um², for particles having a radius from about 10 to about 75nm; and by a surface density of small metallic nanoparticles of between about 200 and about 7000 NP/um², or between about 1000 and 5100 NP/um² for particles having a radius below 10 nm.

8. The object as defined in any one of claims 1 to 7, wherein said metallic nanostmcture is at least in part defined by distributed metallic nanoparticles having a mean wall to wall distance of between about 10 and about 120 nm, for particles having a radius between about 10 to about 200 nm.

9. The object as defined in any one of claims 1 to 8, wherein said metallic nanostmcture is at least in part defined by distributed metallic nanoparticles having a mean wall to wall distance of between about 0.1 and about 10 nm, or about 2 and about 8 nm for particles having a radius below about 10 nm.

10. The object as defined in any one of claims 1 to 7, wherein said metallic nanostmcture is at least in part defined by distributed metallic nanoparticles having a radius below about 75nm.

11. The object as defined in claim 10, wherein said metallic nanostmcture is at least in part defined by distributed metallic nanoparticles having a radius below about lOnm.

12. The object as defined in any one of claims 1 to 11, wherein a Chroma of at least one of said first coloured region and said second coloured region is at least about 2, when measured using CIELCH.

13. The object as defined in any one of claims 1 to 12, wherein a Chroma of at least one of said first coloured region and said second coloured region is at least about 10, when measured using CIELCH, for a measured Hue between -90 and 90.

14. The object as defined in any one of claims 1 to 13, wherein at least some of said consecutively spaced-apart laser irradiation lines comprise burst laser irradiation lines, wherein said burst laser irradiation lines involved laser irradiation along said burst laser irradiation lines using consecutive laser pulse bursts, each of said bursts consisting of two or more consecutive laser pulses.

15. The object as defined in any one of claims 1 to 14, wherein said consecutively spaced-apart laser irradiation lines comprise picosecond pulse laser irradiation lines, for example defined by pulses of pulse duration between about 1 ps and 999 ps, or of between about 1 ps and 100 ps, or of about 10 ps.

16. The object as defined in any one of claims 1 to 15, wherein the object is a coin, medal or token comprising the at least one metallic surface portion.

17. The object as defined in any one of claims 1 to 16, wherein the metallic surface portion is of a metallic surface material selected from the group consisting of gold, silver, copper, titanium, palladium, platinum, ruthenium, rhodium, tantalum, iron, niobium and alloys thereof.

18. The object as defined in any one of claims 1 to 17, wherein the metallic surface portion is a gold surface portion.

19. An object having a coloured metallic surface region defined by a metallic nanostructure generated via a crossing of consecutively spaced-apart laser irradiation lines.

20. The object as defined in claim 19, wherein said crossing is a first crossing, and wherein the object further comprises a second coloured metallic surface region of distinct colouration defined by a respective metallic nanostructure generated via a second crossing of consecutively spaced-apart laser irradiation lines, wherein a crossing angle of said first crossing is distinct from that of said second crossing resulting in production of said distinct colouration.

21. The object as defined in 19 or claim 20, wherein at least some of said consecutively spaced-apart laser irradiation lines comprise burst laser irradiation lines, wherein said burst laser irradiation lines involved laser irradiation along said burst laser irradiation lines using consecutive laser pulse bursts, each of said bursts consisting of two or more consecutive laser pulses.

22. The object as defined in any one of claims 19 to 21, wherein said consecutively spaced-apart laser irradiation lines are defined by a line spacing between 0.1 and 300um.

23. The object as defined in any one of claims 19 to 22, wherein said metallic nanostructure is at least in part defined by a surface density of distributed metallic nanoparticles between about 5 and about 5500 P/um².

24. The object as defined in any one of claims 19 to 22, wherein said metallic nanostructure is at least in part defined by a surface density of distributed medium metallic nanoparticles of between about 5 and about 100 NP/um², or between about 5 and about 65 NP/um², for particles having a radius from about 10 to about 75 nm; and by a surface density of small metallic nanoparticles of between about 200 and about 7000 NP/um², or between about 1000 and 5100 NP/um² for particles having a radius below lOnm.

25. The object as defined in any one of claims 19 to 24, wherein said metallic nanostructure is at least in part defined by distributed metallic nanoparticles having a radius below about 75nm.

26. The object as defined in claim 25, wherein said metallic nanostructure is at least in part defined by distributed metallic nanoparticles having a radius below lOnm.

27. The object of any one of claims 19 to 26, wherein said consecutively spaced-apart laser irradiation lines comprise picosecond laser irradiation lines, for example defined by pulses of pulse duration between about 1 ps and 999 ps, or of between about 1 ps and 100 ps, or of about 10 ps.

28. The object as defined in any one of claims 19 to 27, wherein a Chroma of said coloured region is at least about 2, when measured using CIELCH.

29. The object as defined in any one of claims 19 to 27, wherein said Chroma is at least about 10, when measured using CIELCH, for a measured Hue between -90 and 90.

30. The object as defined in claim 29, wherein the object is a coin, medal or token comprising the coloured metallic surface region.

31. The object as defined in any one of claims 19 to 30, wherein the coloured metallic surface region is of a metallic surface material selected from the group consisting of gold, silver, copper, titanium, palladium, platinum, ruthenium, rhodium, tantalum, iron, niobium and alloys thereof.

32. The object as defined in claim 31, wherein said metallic surface material is gold.

33. An object having a metallic surface of fixed colouration defined by a metallic nanostructure generated via laser irradiation;

said metallic nanostructure being at least in part defined by a surface density of metallic nanoparticles (NP) above about 5 Ρ/μπι² for medium particles having a radius ranging from about 10 to about 75nm, and above 200 Ρ/μπι² for small particles having a radius below about lOnm.

34. The object as defined in claim 33, wherein said surface density is between about 5 and 65 Ρ/μπι² for said medium particles, and between about 200 and 7000 ΝΡ/μπι² for said small particles.

35. The object as defined in claim 34, wherein said surface density is between aboutlOOO and 5100 Ρ/μπι² for said small particles.

36. The object as defined in any one of claims 33 to 35, wherein a Chroma of said fixed colouration is at least about 2, when measured using CIELCH.

37. The object as defined in claim 36, wherein said Chroma is at least about 10, when measured using CIELCH, for a measured Hue between -90 and 90.

38. The object as defined in any one of claims 33 to 37, wherein the metallic surface is one of a gold surface and a silver surface.

39. The object as defined in any one of claims 33 to 38, wherein the object is a coin, medal or token comprising the metallic surface.

40. An object having a coloured metallic surface region defined by a metallic nanostructure generated via a laser irradiation pattern, wherein said laser irradiation pattern involved laser irradiation in accordance with said pattern using consecutive laser pulse bursts, each of said bursts consisting of two or more consecutive laser pulses.

41. The object as defined in claim 40, wherein said laser irradiation pattern consists at least in part of a set of consecutively spaced-apart laser irradiation lines.

42. The object as defined in claim 40, wherein said laser irradiation pattern consists at least in part of a cross hatching of two sets of consecutively spaced-apart laser irradiation lines angled relative to one another.

43. The object as defined in any one of claims 40 to 42, wherein the coloured metallic surface region exhibits a measurable Lightness between about 20 and 100, or between about 24 and 96.

44. The object as defined in any one of claims 40 to 43, wherein the coloured metallic surface region exhibits a measurable Chroma of at least 2.

45. The object as defined in any one of claims 40 to 44, wherein said metallic nanostmcture is at least in part defined by a surface density of distributed metallic nanoparticles between about 5 and 65 Ρ/μπι² for particles having a radius ranging from about 10 to about 75nm, and a surface density of distributed metallic nanoparticles between about 1000 and 5100 Ρ/μπι² for particles having a radius below about lOnm..

46. The object as defined in any one of claims 40 to 45, wherein said metallic nanostmcture is at least in part defined by distributed metallic nanoparticles having a radius below about 75nm.

47. The object as defined in any one of claims 40 to 46, wherein the object is a coin, medal or token.

48. The object as defined in claim 47, wherein the coin is manufactured of a metal selected from gold, silver, copper, titanium, palladium, platinum, ruthenium, rhodium, tantalum, iron, niobium and an alloy thereof.

49. The object as defined in claim 48, wherein said metal is gold.

50. The object as defined in any one of claims 40 to 49, wherein said coloured metallic surface region is further defined by an underlying periodic metallic structure.

51. A method for colouring a metallic surface comprising: selecting from multiple designated laser irradiation patterns a given laser irradiation pattern corresponding to a desired fixed metallic colouration using one or more predefined laser irradiation parameters;

irradiating at least a portion of the metallic surface in accordance with said selected irradiation pattern and said preset laser irradiation parameters to form a corresponding metallic nanostructure thereon, wherein said metallic nanostructure so formed affects an absorption spectrum of the portion so as to exhibit said desired fixed metallic colouration.

52. The method as defined in claim 51, wherein said one or more preset laser irradiation parameters comprise a preset irradiation fluence.

53. The method as defined in claim 51 or claim 52, wherein said one or more preset laser irradiation parameters comprise a preset irradiation line scanning speed.

54. The method as defined in any one of claims 51 to 53, wherein said one or more preset irradiation parameters comprise a preset total accumulated fluence.

55. The method as defined in any one of claims 51 to 54, wherein said irradiating comprises irradiating said portion using consecutive picosecond duration laser pulses.

56. The method as defined in claim 55, wherein said irradiating comprises irradiating the metallic surface using consecutive bursts of said consecutive picosecond duration laser pulses, and wherein each of said bursts consists of two or more said picosecond duration laser pulses.

57. The method as defined in claim 56, wherein said irradiating comprises selectively irradiating the metallic surface using consecutive bursts of one or more of said consecutive picosecond duration laser pulses, wherein said one or more preset laser irradiation parameters comprise a selectable number of said picosecond duration laser pulses within each of said bursts.

58. The method as defined in any one of claims 51 to 57, the method further comprising selecting at least one of said one or more preset laser irradiation parameters as a function of at least one of a desired Lightness and Chroma of said desired fixed metallic colouration.

59. The method as defined in any one of claims 51 to 58, wherein said given irradiation pattern comprises a set of consecutively spaced-apart irradiation lines having a designated line spacing.

60. The method as defined in claim 59, wherein said given irradiation pattern comprises crossing-hatching irradiation lines angled relative to said set.

61. The method as defined in any one of claims 51 to 60, further comprising:

selecting a distinct irradiation pattern corresponding to a distinct fixed metallic colouration; and

irradiating an other portion of the metallic surface in accordance with said distinct irradiation pattern to form a correspondingly distinct nanostructure thereon that affects an absorption spectrum of said other portion so to exhibit said distinct fixed metallic colouration.

62. The method as defined in claim 61, further comprising maintaining said one or more preset laser irradiation parameters upon irradiating said other portion.

63. The method as defined in claim 61, further comprising changing at least one of said one or more preset laser irradiation parameters prior to irradiating said other portion consistent with said distinct fixed metallic colouration.

64. The method as defined in any one of claims 51 to 63, wherein the metallic surface consists of a metallic coin, medal or token surface.

65. The method as defined in any one of claims 51 to 64, wherein the metallic surface is one of a gold surface and a silver surface.

66. The method as defined in claim 51, wherein the metallic surface is at least partially defined by a relief profile, the method further comprising irradiating said metallic surface without adjusting said irradiation parameters across said relief profile so to produce colour gradients in accordance with said relief profile.

67. The method as defined in claim 51, wherein the metallic surface is at least partially defined by a relief profile, the method further comprising adjusting said irradiation parameters consistent with said relief profile so to maintain a substantially constant colouration across said relief profile.

68. An object having a coloured metallic surface region defining a relief profile and defined by a metallic surface nanostructure generated via a laser irradiation pattern, wherein the coloured metallic surface region exhibits one or more colour gradients in accordance with said relief profile.

69. An object having a coloured metallic surface region defining a relief profile and defined by a metallic surface nanostructure generated via a laser irradiation pattern, wherein the coloured metallic surface region exhibits a substantially fixed colouration irrespective of said relief profile.

70. An object having a gold surface portion, said surface portion having a fixed colouration defined by a metallic nanostructure generated via laser irradiation; said metallic nanostructure being at least in part defined by a surface density of metallic nanoparticles.

71. The object as defined in claim 70, wherein said fixed colouration is defined by a measurable Hue in the green or blue region of the visible spectrum.

72. The object as defined in claim 70 or claim 71, wherein said laser irradiation comprises burst laser irradiation.