We investigate the effects of the laser repetition rate on the creation of angle-independent colours on bulk silver samples, and characterize the coloured surfaces in terms of the associated morphology and oxidation products produced. The laser used produces pulses 10 ps in duration at λ = 1064 nm, and the repetition rate was varied over the range from 5 to 400 kHz. Decreasing the laser repetition rate creates a colour palette on silver with a significantly wider gamut, including green and cyan colours, which were previously difficult to obtain. Scanning electron microscope analyses of these surfaces show an increase in topographical features (roughening) with decreasing laser repetition rate. Chemical analyses of the coloured areas show that the amounts of oxide and carbonate species formed depend on the laser repetition rate.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The collective oscillation of the electron cloud around a metal ionic core, also known as a surface plasmon, allows for the manipulation of light at the sub-wavelength scale . Plasmon resonances are sensitive and can be tuned by varying characteristics such as particle size, shape and permittivity [1–3]. In the past decade, the nanostructuring of metallic surfaces or the use of sub-wavelength metallic structures have seen growing applications, such as in photo-chemistry [4–6], colouring [7–12], medicine [13,14], sensing [15,16] and anti-counterfeiting [17,18].
The fabrication of nanostructured plasmonic surfaces is usually achieved by lithographic techniques [10,11]. Recently, ultrafast lasers demonstrated the ability of producing nanostructures by direct-laser writing of metal surfaces to create colours [19–23]. These colours are believed to be plasmonic in origin due to the random re-deposition and fusion of metallic nanoparticles onto the metal substrate following surface laser irradiation and ablation . The colours rendered depend strongly on the type of laser used to produce them, and on the laser parameters which are typically determined through trial and error [19–23]. Moreover, compared to colours generally produced by periodic nanostructured surfaces, such as laser-induced periodic surface structures (LIPSS) [19,24], the randomness of the re-deposited metal nanoparticles makes the rendered colours insensitive to viewing angle.
To date, a comprehensive understanding of the effects of laser parameters on the direct-laser colouring of metals is still lacking. In particular, the effects of the laser repetition rate are ill-understood; yet this parameter is of high importance as it controls throughput, yield, energy delivery and the timing of the delivery. It is expected that the colours produced on the surface of metals exhibit a temporal dependence on the deposition of laser energy thereon. The machining of materials using high laser repetition rates is known to increase the bulk and surface temperatures due to heat accumulation [25–27]. The increase in temperature and the response (deformation) of the bulk are determined in part by the laser repetition rate.
Guay et al.  showed through computer simulations that the plasmonic resonances and colours originating from arrays of re-deposited nanoparticles are highly sensitive to embedding depths as small as 0.5 nm. Specifically, the plasmon resonance is sensitive to the angle formed between the surface-bound nanoparticles and the substrate. Thus the presence of metallic nanoparticles of lower melting temperature than that of the bulk [28,29], combined with the sensitivity of the colours to slight sintering variations or changes in the nanoparticle contact angle, indicate that all laser parameters in the laser-colouring process are impactful, including the laser repetition rate. Furthermore, laser colouring of metals is commonly achieved in ambient air. The ionization of material and air during the irradiation/ablation process suggests the creation of exotic chemical species . To date, little chemical analyses have been done on laser-coloured, or more generally, laser-nanostructured surfaces [31,32]. In previous work, colours produced on metals by ultrafast lasers were thought to be solely rendered plasmonically, although similar colour palettes could be produced by controlling the thickness of metal-oxide layers [33–35]. Thus, the presence of chemical species on laser-machined surfaces and their effect on colour or on the behavior of nanostructured surfaces are of strong interest. Moreover, the amount of chemical species formed during the ablation process could produce purely plasmonic , core/shell [36–38] or semiconductor [39–41] (i.e. quantum dot) nanostructures. Each of these different compositions can exhibit a different and unique behavior, but all of them are in principle capable of producing structural colours.
In this paper, we investigate the effects of laser repetition rate on the production of colours on silver (Ag). In parallel, we also study the effect of the laser repetition rate on the morphology and surface chemistry of the coloured silver surfaces. For the same total accumulated fluence  delivered to the surface, the colours, morphology and surface chemistry are observed to change depending on the time between subsequent laser shots. With decreasing laser repetition rate, we observe a widening of the colour gamut to include obtain green and cyan colours (previously difficult to obtain). Scanning electron microscope (SEM) images of the colours produced using the same total accumulated fluence but with different laser repetition rates reveal the creation of significantly different nanostructures. Spectroscopic analyses of the coloured surfaces reveal that an increasing amount of silver oxide and silver carbonate species are formed by increasing the time between laser shots (i.e. lower laser repetition rates). The morphological and chemical analyses of the surface suggests that the origin of the colours to be morphological in nature.
2. Experimental sections
2.1 Laser specifications and fabrication
The machining of the silver samples was carried out by irradiating the surface with 1064 nm light from a Duetto mode-locked laser (Nd:YVO4, Time-Bandwidth Product) and a pulse duration of 10 ps. The repetition rate of the laser is tunable from 50 to 8200 kHz. For the lower repetition rates, a pulse picker option was used to reduce the laser repetition rate by an integer factor N. In our experiments, the repetition rate of the laser was fixed to 400 kHz and the pulse picker was used to control the laser repetition rate at the output. This approach ensured that the laser pulse energy remained the same throughout the experiments. This was verified by reading a factor of 2 drop in the average power (3A-P-QUAD, Ophir) for every factor of 2N of the pulse picker. The operation of the pulse picker was also verified and monitored using an oscilloscope (tds3033b, Tektronix). The silver samples were 38 mm in diameter, 3 mm thick and of 99.99% purity. The flat silver surfaces were produced via a high-tonnage press with a resulting silver surface roughness of 20-100 nm (average). Prior to machining the silver surfaces, the surfaces were cleaned using a multistep cleaning approach: (step 1) isopropanol wash, (step 2) acetone wash, (step 3) de-ionized water rinse, (step 4) second isopropanol wash and (step 5) dried using nitrogen. For transport, the silver samples were placed in nitrogen filled plastic capsules and placed in nitrogen filled bags. Colours machined by the raster-scanning of the silver surfaces were achieved following the same method previously described in ref , where the different line spacing is observed to produce distinct nanostructures that are unique to each colour [22,23]. The laser was fully electronically integrated and physically enclosed by a third party for industrial applications (GPC-PSL, FOBA). To machine the silver surface, the laser light at the output was expanded from a 3 to 12 mm diameter spot using a fixed beam expander (LINOS G038662000, QIOptiq) and directed into the back of the XY galvanometric mirrors (TurboScan 10, Raylase). The light was then focused onto the silver surface using an F-theta lens (f = 254 mm, Rodenstock). The light was displaced over the surface in a top-to-bottom raster-scan pattern at a maximum speed of 3000 mm/s. For accurate determination of the silver surface, the surface of the silver was located using a touch probe system. Throughout the experiments, the polarization of the laser light was kept parallel to the machining direction. For machining, the samples were placed on a 3-axis stage of resolution of 1 µm in the lateral and axial directions. The laser power was computer-controlled via a user interface and calibrated using a power meter. A Gaussian beam radius of ~28 µm was obtained using a semi-logarithmic plot of the square diameter of the modified region, measured using a SEM, as a function of laser pulse energy .
The Lightness (L), Chroma (C) and Hue (H) values of each coloured square were measured using a Konica Monilta CR-241 chroma meter in the CIELCH colour space, 2 observer and illuminant C (North sky daylight). The Hue is representative of colour associated with a 360° polar scale . The LCH values were converted to the XYZ tristimulus colour space using Matlab for the plotting of the Commission Internationale de l’Éclairage (CIE) diagrams. The reflectance measurements of the coloured silver surfaces were carried out using a CARY 7000 UV-Vis-NIR spectrometer (Agilent Technologies) equipped with an integrating sphere detector (Labsphere) to collect both the specular and diffuse reflectance signals from the samples. The reflectance data was corrected against reference samples of silicon and silver. Grazing angle measurements of the coloured surfaces were performed using an FT-IR spectrometer (Nexus 870, Thermo Nicolet) with the incident light source hitting the surface of the sample at an angle of 80°. In this approach (RAIRS - Reflection Absorption Infrared Spectroscopy), the sharp angle is utilized to predominantly generate an electric field that is oriented perpendicular to the probed surface, such that vibrational modes of the chemical species on the surface that have transition dipole moments parallel to the electric field are excited, and featured in the IR spectra . Each spectrum is the average of 256 accumulations recorded with a spectral resolution of 2 cm−1, taken from a coloured square of surface area of 0.5 cm2. A smooth and polished silver sample was used for background correction. High-resolution SEM (Gemini SEM 500 FESEM, Zeiss) images of the coloured surfaces were obtained by using secondary electron imaging mode. To obtain additional information on the nanoparticles deposited within the machined surface, SEM images of the surfaces were taken with a stage tilt of 70°.
3. Results and discussion
3.1 Laser repetition rate dependent colours
Significant progress has been made towards the understanding of the colourimetric response of laser coloured silver and copper surfaces. However, studies focusing on the colouring of such surfaces by ultrafast lasers have overlooked the effects of the laser repetition rate  as the pulse-to-pulse separation time was longer than the thermal expansion time of the metal . Yet, the heating of a material with increasing laser repetition rate is well known [25–27]. In addition, due to the low melting point of nanoparticles with respect to the bulk [46,28,29], the effect of cumulative heating on the metallic nanostructures responsible for the colours should not be neglected. Moreover, the interaction of the laser pulse with the ablation plume and the ejected debris occurring within the same time scale should also be considered . In our previous work , we presented the colouring of silver using a fixed repetition rate in order to leave out any temporal effect on the energy deposition. With this approach, each colour could be linked to a unique total accumulated fluence (TAF) value . The TAF was observed to control the nanoparticle density re-deposited on the surface and from this observation a model based on nanoparticle sizes and distributions was proposed suggesting that the perceived colours are plasmonic in origin . Even though the model simplified the disordered topography of the machined surfaces and neglected the presence of oxides and carbonates, it nonetheless successfully reproduced the salient features observed in measured reflectance spectra . The TAF relation is given by:
The surfaces shown in Fig. 1(a) were irradiated with a laser TAF of 5.16 J/cm2 and a laser pulse energy of E = 15 µJ. The laser pulse energy and TAF were kept constant throughout the experiments using the laser pulse picker to control the repetition rate at the output of the laser. A decrease of half in the average power for each integer of 2 on the pulse picker was confirmed using a power meter, ensuring constant pulse energy at the laser output. To deliver the same TAF to the surface, the laser repetition rate was changed by f' = f/N and the marking speed was modified proportionally as v' = v/N, where N is the pulse picker number. Replacing the modified repetition rate and marking speed into Eq. (1), we obtain the following relation:Fig. 1(a), significant changes in colour can be seen for the same TAF deposited on the surface, but using different laser repetition rates. This suggests a strong temporal dependence on colour formation most likely from cumulative heating of the surface, which could in turn affect the contact angle of the nanoparticles sitting on the substrate.
The colours are observed to evolve in a counter-clockwise manner on a colour Hue polar plot representation (i.e. yellow → purple → blue) with decreasing laser repetition rate, see Fig. 2(a). The rate of change in Hue with decreasing laser repetition rate is observed to increase as the line spacing decreases. In addition, a full Hue rotation can be observed for the line spacing Ls = 5 µm. Figures 1(b)-1(d) show reflectance spectra from surfaces with the line spacings of Fig. 1(b) Ls = 5 µm, Fig. 1(c) Ls = 8 µm and Fig. 1(d) Ls = 13 µm, using different laser repetition rates. Features in the reflectance spectra can be followed as the repetition rate changes, and thus as the time between laser pulses delivered to the surface changes. The features are observed to red-shift with decreasing laser repetition rate. The red-shift in the features are smallest at the highest repetition rates (400 to 200 kHz), but are significantly larger for laser repetition rates lower than 50 kHz.
Radial plots of Hue (θ) as a function of the logarithm of the TAF (r) using different laser repetition rates are shown in Fig. 2(a). The TAF deposited on the surface increases radially outward. The range of colours covered by the highest repetition rate of 400 kHz is observed to be significantly limited and reduced to the yellow Hue region. Alternatively, with decreasing rate much wider colour palettes are obtained. We previously reported the colouring of silver using a fixed repetition rate of 50 kHz, where only a small number of green and cyan colours could be obtained (when the process was modified by changing the angle of polarization with respect to the machining direction or by performing multiple overlapping passes) . In Fig. 2(a), the colour palette (Hue) is extended simply by decreasing the repetition rate of the laser. Furthermore, the Chroma values of the cyan and green colours obtained using lower repetition rates are higher than those obtained with previously reported non-burst colouring techniques . Additionally, the TAF used to produce the full range of colours is significantly reduced. For example, at Ls = 15 µm and f = 50 kHz, a yellow colour is obtained. By decreasing the laser repetition rate and marking speed accordingly (keeping the same TAF), a blue colour is obtained instead, which would have normally required a higher TAF (i.e. smaller line spacing). This feature can be observed by the tightening of the spiral in the radial plots at lower laser repetition rates, see Fig. 2(a).
In addition, with decreasing rate a full 360° rotation in Hue is achieved, see Fig. 2(b). At much lower rates, the gain in the colour range is not as significant, however, control of the slope in the Hue region of 235° to 110° (cyan and green) could be useful since they are the most difficult colours to obtain . By controlling the slope, the sensitivity of the colours to slight variations in the TAF could be decreased in favour of reproducibility while extending the colour range obtainable, see Fig. 2(b). As a result, in an industrial environment where the ambient conditions and vibration may not be well controlled, variations in laser output and its effect on the colour (i.e. ΔE) could be minimised.
Of note is the reduction of the palette with increasing repetition rate. The colour palette is observed to collapse and to produce only yellow colours for rates higher than about 200 kHz, as seen on Fig. 3(b). Figure 3(a) is a CIE xy chromaticity diagram of the colours obtained using different repetition rates. The points are representative of the colours shown in the previous figures. The area covered by the colours produced with the laser repetition rate of 400 and 200 kHz is seen to collapse into a small region of the CIE diagram in the yellow section. An increase in the area covered by the colours is observed by decreasing the laser repetition rate from 200 to 50 kHz. No significant gain in area is observed after decreasing the repetition rate below 50 kHz. Although, lowering the repetition rate below 50 kHz does not expand the covered area, more colours (i.e. markers) are added (Fig. 2), resulting in a complete rotation (ring) in the CIE diagram. However, the increase in the green and cyan regions is not as well captured in the CIE diagram. This is due to the low Chroma values of the green and cyan colours since Chroma values in the CIE diagram always increases outward.
3.2 Surface characterization
High magnification images of the surfaces, as given in Fig. 4, show that the nanostructures produced by the lower laser repetition rates are more disorganized. While round particles are distinctively present for all laser repetition rates, the underlying roughness is significantly different. With increasing laser repetition rate, the roughness is observed to smooth out. In a previous publication, this roughness was modeled as nanoparticles with a statistical distribution of embedding ratios . The effect of embedding was demonstrated in computer simulations to play an important role in determining which colours might be observed. In fact, changes in embedding as small as 0.5 nm were enough to significantly shift the plasmon resonance and consequently the far-field colours. The embedding could suggest that a high enough temperature was produced on the surface having the effect of sintering the nanoparticles whose melting temperature is lower due to their size [28,29].
The direct laser machining to produce coloured surfaces is conducted in ambient air. The ionization of silver atoms and the ejection of charged silver nanoparticles during ablation  favours the creation of Ag-based chemical species . The species can form a broad range of nanoparticles, including purely metallic , core/shell [36–38], semiconductor [39–41] or more chemically-complex structures . Combinations of different types of nanoparticles can also produce colour . Additionally, metals processed using direct-laser machining or electro-deposition also produce colours but via oxide formation [33,34,51].
We performed chemical analyses of our coloured surfaces using grazing angle reflection absorption infra-red spectroscopy (RAIRS). Colours produced using two distinct sets of laser parameters were characterized to determine their surface chemistry. Figure 5(a) is a plot of the RAIRS measurements from samples fabricated with a line spacing Ls = 5 µm and different laser repetition rates. Figure 5(b) is a plot of the RAIRS measurements from a full colour palette obtained on the surface of silver by varying the line spacing, Ls, from 2 to 11 µm, and using a fixed laser repetition rate of 50 kHz. The absorbance results shown in Fig. 5 are plotted over the 500 to 2000 cm−1 spectral range. In both cases, two main absorbance bands centered at 560 cm−1 and 1450 cm−1 appear . The peak at 565 cm−1 corresponds to the longitudinal optical vibration of the Ag-O bond in silver oxide, Ag2O . The broad feature centered at 1450 cm−1 is the result of two distinctive spectral contributions at 1400 cm−1 and 1465 cm−1. The origin of these bands were formally assigned to the splitting of asymmetric stretching of the carbonate ion, C, which would otherwise be centered at 1410 cm−1 . Alternatively, in another study it was shown that the splitting can result from the formation of basic silver carbonate, AgOHAg2CO3, which can readily evolve when silver carbonate, Ag2CO3, is exposed to water vapor . As the ejection of the nanoparticle is conducted in an ambient environment the formation of such chemical species is expected. In the past, the exposure of the coloured silver surface to humid conditions consistently showed a significant increase in carbonate signals that are consistent with the doublet observed in the literature . Therefore, we assign the two spectral contributions at 1400 cm−1 and 1465 cm−1 to basic silver carbonate. The creation of these chemical species during machining is currently the subject of a separate study. No chlorine or sulfur compounds were detected on the silver surfaces prior or following the laser machining using X-ray photoelectron (XPS), IR and Raman spectroscopic techniques.
The RAIRS measurements in Fig. 5 show a significant variation in the presence of chemical species with changing laser repetition rate, but that the same chemical species are relatively insensitive to changes in line spacing. In Fig. 5(a), a concomitant decrease in peaks assigned to silver oxide and basic silver carbonate is observed as the laser repetition rate is increased from 5 to 400 kHz. Surfaces machined at 400 kHz showed only very weak signals from carbonates and the total disappearance of oxides. The results clearly show that longer times between pulses can result in significant increases in the amounts of detected oxides and carbonates. For Ag2O to dissociate into pure Ag a temperature of ~400 °C  is required on the surface. Figure 5(a) suggests that the dissociation temperature is reached when the time between laser pulses is reduced to 10 µs (i.e. 100 kHz), where silver oxide and basic silver carbonate begin to be suppressed. This chemical information and trend would support the heating of the bulk and the smoothing or sintering of the roughness in Fig. 4. The dissociation of the chemical species could be understood by the systematic erasing of the chemicals by the following laser pulses and subsequent laser lines due the heating of the bulk in close proximity of the previously formed chemical compounds.
Referring back to Fig. 1, the different colours generated by the machined surface of silver are produced using a fixed laser repetition rate and by changing only the line spacing. Similarly, the same color palette is created when the line spacing is fixed to Ls = 13 µm and the laser repetition rate is changed (horizontal and vertical white dashed lines, respectively). In contrast, the oxide and carbonate content of the colours obtained by changing the line spacing was not found to vary appreciably compared to those obtained by changing the laser repetition rate. The strong colour response to variations in line spacing, combined with a relatively consistent surface chemistry, suggests that the observed colours are not the result of thin-film interference effects or varying amounts of silver oxide or carbonate. Furthermore, the colours produced using a repetition rate of 5 kHz and 400 kHz are both yellow in Hue, yet they have very different surface chemical composition, which suggests the colours to be morphological in nature. Additionally, SEM images of the coloured surfaces (see Fig. 4 and previous work [22,23]) consistently show the formation of nanoparticles on a roughened surface. The spectroscopic results, however, show that silver oxide and basic silver carbonate are present on these surfaces. The creation of these chemical species likely results from the ionization and ejection of charged particles through ambient air during laser ablation . It is unlikely that all of the silver material ejected from the surface would be oxidized. Instead, a surface composed of regions of silver, as well as silver oxides and silver carbonates are to be expected. We cannot discount the possibility that the ejected material takes the form of core/shell nanoparticles, which is not uncommon to form during laser ablation [54,55]. For silver in particular, even more chemically-complex nanoparticles could be formed . The nanoparticle model developed based on statistical measurements of the coloured surfaces [22,23] should include this new chemical information - a mixture of silver, silver oxide and silver carbonate nanoparticles should be considered. Additionally, the formation of chemical species on the nanostructured surface warrants further research within the context of plasmon-assisted photochemistry.
The colour response, morphology and chemistry of laser-machined bulk silver surfaces were observed to be dependent on the repetition rate of the marking laser. Incremental reduction in the repetition rate was observed to broaden the colour palette on silver covering the evasive and difficult-to-obtain green and cyan colours before completing a full 360° rotation on the colour wheel. For the same total accumulated fluence, analysis of the surface reveals significant differences in the nanostructures produced within the irradiated areas. The nanostructured surfaces are observed to become smoother with increasing laser repetition rate. The reduction of chemical species combined with smoothing of the surface suggests local temperatures in excess of 400 °C, which is enough to sinter the heat sensitive nanoparticles. Additionally, due to the full 360° rotation in colour, the presence of 2 distinct chemical signatures for the same colour supports the morphological nature of the colours.
Canada Research Chair program; Natural Sciences and Engineering Research Council of Canada (NSERC); Canada Foundation for Innovation (CFI); Ontario Ministry of Research, Innovation and Science (MRIS)
We acknowledge the Royal Canadian Mint. We also acknowledge Dr. Antonino Calà Lesina, Dr. Lora Ramunno and Anthony Olivieri, Martin Charron and Graham Killaire from the Centre for Research in Photonics at the University of Ottawa (Canada).
The authors declare no conflicts of interest.
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