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Coloration of a copper surface by nanostructuring with femtosecond laser pulses
Optics and Laser Technology 119 (2019) 105574
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Coloration of a copper surface by nanostructuring with femtosecond laser pulses
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Serguei P. Murzina,b, , Gerhard Liedlb, Robert Pospichalb a b
Samara National Research University, Moskovskoe Shosse 34, Samara 443086, Russia TU Wien, Institute of Production Engineering and Photonic Technologies, Getreidemarkt 9, Vienna 1060, Austria
H I GH L IG H T S
of a copper by nanostructuring with femtosecond laser pulses was studied. • Coloration structures with low spatial frequency were formed. • Surface topology on observed areas of different colours were investigated. • Surface were taken in secondary electron and backscattered electron mode. • SEM-images • Reasons for the colour difference are the surface topology and the oxidation degree.
A R T I C LE I N FO
A B S T R A C T
Keywords: Coloration Femtosecond laser pulse Surface structure Oxidation
The coloration of a copper surface after treatment in air with femtosecond laser pulses at changing only the scanning direction has been studied. Surface structures with low spatial frequency, which were formed by a relative movement of the pulsed laser beam across the sample surface with an energy density below the ablation threshold, changed the sample colour. Four areas have been identified showing different colours such as blueturquoise, orange, grey-green and red-violet. The possible influence of the surface topology on observed areas were investigated by atomic force microscopy. It was found that in a blue-turquoise area of the observed colour perception occurs the maximum average height of the relief and it is 1.7 times higher than in the red-violet area, while gray-green and orange areas have an average height between these values and are much closer to the values of the red-violet area. Scanning electron microscope images of the sample surface were taken in secondary and backscattered electron modes. It was revealed that the sample shows a combined microrelief consisting of low-spatial-frequency laser-induced periodic surface structures with an additional nanoroughness. It was determined that reasons for the colour difference of images are both the surface topology, as well as the degree of oxidation.
1. Introduction Ultrafast lasers have opened up new possibilities in ultra-high-precision micro- and nano-processing [1–3], whereby the pulse width of these lasers can range from several ten femtoseconds to ten picoseconds. Lasers in the longer pulse width range are used for commercial and industrial applications mainly, whereas lasers with pulse widths shorter than picoseconds are mostly used for fundamental research [4,5], respectively. In the case of exposure to a substance of ultrashort pulses, the characteristic time of the transition of laser energy into heat is longer than the pulse duration. Thus, thermal processes can occur after the passage of a laser pulse. In this case, the mechanism for
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processing materials is different than in the case of, for example, nanosecond pulses [6]. Micro/nanostructures on metals and surfaces with unusual optical properties or wettability [7–10] can be obtained by using ultrafast pulse laser sources. The coloration of a metal surface by means of ultrashort laser pulses was first achieved and described in [11]. Such colour marking of surfaces of almost any solid composition is feasible in the formation case of surface-periodic structures. This is particularly important for materials that are weakly oxidized or have an opaque oxide (e.g. copper). A possibility to obtain specific colour patterns using ultra short pulses during the process of material modification by polarization-dependent structure generation is shown in [12]. The orientation of these nanostructures created on a metal surface, the
Corresponding author. E-mail address: [email protected] (S.P. Murzin).
https://doi.org/10.1016/j.optlastec.2019.105574 Received 25 January 2019; Received in revised form 7 May 2019; Accepted 15 May 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 119 (2019) 105574
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for sample processing and the laser beam was focused on the surface of pure copper plates by a metal mirror. The spot size was adjusted to obtain an energy density of 1.05 J/cm2. The dimensions of the spot were 0.26 mm height and 0.27 mm width, with a total spot area of approx. 0.07 mm2. Samples have been generated by scanning with the focused laser beam over a surface of a copper plate. The size of the processed area was 20 × 20 mm2. During laser exposure the plate has been moved 20 mm in the positive x-direction. At the end of the line, the laser was turned off and the plate was displaced by 0.28 mm in the positive y-direction. Following that, the laser was switched on again and the sample was moved in the negative x-direction. The whole processing cycle was repeated until an area of 20 × 20 mm2 had been processed by ultrafast laser pulses. The copper surface was therefore treated with femtosecond laser pulses over an area of 400 mm2. First, samples have been moved during processing in x-direction, and then, after a small displacement in ydirection the direction of movement was reversed. Thus a line-by-line processing, first in +x-direction, followed by processing in −x-direction could be achieved. Speed of motion was 180 mm/min, and the distance between lines was 0.28 mm, with an overlap ∼0%. The experiments have revealed that visible colours depend only on the direction of movement. Fig. 1 shows an image of the treatment zone obtained using a metallographic optical microscope. Within the image, the direction of movement of the sample during laser processing is indicated by an arrow. Fig. 2 shows the zone of a single pulse action on the sample surface. It is known that during normal incidence of femtosecond, linearly polarized laser pulses, low-spatial-frequency laser-induced periodic surface structures (LSFL) are formed on the copper surface, which demonstrate the effect of diffraction staining [22,26]. That is, the appearance of a surface relief is observed, which demonstrate properties of a diffraction grating in the visible range. However, generated at normal incidence, LSFL do not allow obtaining bright colours in the
so-called ripples, depends on the polarization of the laser beam and they show a periodicity that is typically smaller than the laser wavelength and in the range of the visible spectrum. The colour palette that such surfaces may demonstrate, is caused by a grating diffraction effect of the visible light at the laser-induced periodic surface ripples, which has been confirmed in Ref. [13]. This can be used to an adaptive management of the coloration effect during laser processing of pure copper. The realizable colour laser marking technique allows to obtain a high-resolution colour image by a contactless effect on the material. The applied process does not significantly change the physical-chemical properties of the material surface compared to a coloration process by oxidation of the metal surface (temper colour). This phenomenon of surface structuring can be observed in all metals depending on the absorption coefficient at the laser wavelength as well as the ablation atmosphere – vacuum or in presence of a defined gas. Generally, in air nearly all metals with multielectron valence react with oxygen of air or water vapour [14–16]. This topic has been quite extensively covered in the last decade. Studies of the properties of the formed surface as a function of the pulse duration were performed. For instance, it is known that functional copper surfaces combined with brighter structural colours and superhydrophobicity were fabricated not only using femtosecond, but also picosecond laser pulses [17]. It is known that the scan velocity is an important parameter. Ref. [18] shows that during femtosecond laser treatment with scanning, the ripple has a clear morphology in the slower scan velocity regime. Conversely, the ripple has an ambiguous morphology when the scan velocity is high enough. It is known that the incident angle is also an important parameter. Ref. [19] shows a theoretical model of femtosecond laser induced ripple formation on copper for varying incident angle, which is consistent with the results of experimental studies. By changing the angle of incidence of ultrashort pulse beam on the sample, brighter colours can be achieved, especially in the long-wave region of the visible spectrum. Ref. [20] shows a typical optical microscope image of the sample surface representing the parallel micro-trenches inscribed on the target surface by the bidirectional scanning of the laser beam. However, about a colour dependence on the scanning direction, it is not reported. The method of scanning a femtosecond laser beam with an asymmetrical spatial fluence distribution over the sample surface, resulting in a change of the nanoscale surface topology as well as a manipulation of the chemical composition of a titanium surface is presented in Ref. [21]. It can be assumed that observed phenomena can be found not only in titanium, but with other materials too and that individual features will probably also be present [22–24]. Specific features of colorizing copper surfaces and changes in the nanostructure by scanning with a femtosecond laser beam with a nearly Gaussian energy density distribution across the sample surface is studied in Ref [25]. It was revealed that by forward and reverse scanning of a copper surface by an ultrashort pulse beam it was possible to create a combined microrelief consisting of low-spatial-frequency laser-induced periodic surface structures (LSFL) and an additional nanoroughness. It was shown that the observed colour change of the processed surfaces depended on relatively small variations of the nanostructure. The objective of the research presented here is to investigate more deeply the controlled coloration of copper by diffraction effects through nanostructures generated by femtosecond laser pulses below the material ablation threshold, changing only the scanning direction. 2. Results of experimental studies A femtosecond laser was used to perform the experimental studies. The system consists of a Ti: Sapphire oscillator and a multi-pass amplifier with max. 0.8 mJ pulse energy, 1 kHz pulse frequency and a pulse duration < 30 fs. The laser generates pulses in the TEM00-mode, and a pulse contrast > 109: 1. Central emission wavelength was 800 nm, bandwidth approx. 100 nm. A 2D positioning system was used
Fig. 1. Image of the treatment zone obtained using a metallographic optical microscope. Visible colours depend on the direction of movement: 1 – redviolet; 2 – blue-turquoise; 3 – orange; 4 – gray-green. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 2
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Fig. 3. AFM topology images of four sample areas in different colours: blueturquoise (a), orange (b), grey-green (c) and red-violet (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Fig. 2. Zone of a single pulse action on the sample surface.
The action of a femtosecond laser in a multipulse mode leads to the appearance of tracks on the material, which were studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM). In order to investigate the possible influence of the surface topology on the observable colour appearance, an AFM was used to study areas of different colours. Figs. 3 and 4 show AFM topology images and 3D-images of four sample areas in different colours. Fig. 5 shows height profiles of four areas of the sample showing different colours like blue-turquoise, orange, grey-green and red-violet. At first glance, there aren’t significant differences between these height profiles. Therefore, for each graph, the height of the individual peaks was measured as from the height of the local maximum substacting the height of the next local minimum, and the mean value and the corresponding standard deviation for each of the four areas were calculated. The measurement results are presented in Table 1. It was shown that even relatively small changes in the nanostructure can lead to a significant colour change of the surface. The average period appears to be the same for all areas of the sample surface and is 600–640 nm. The maximum average height of the relief occurs in a blue-turquoise area of the observed colour perception and is 1.7 times higher than in the redviolet area. Gray-green and orange areas have an average height
long-wavelength part of the visible electromagnetic radiation [27–29]. By using a femtosecond laser with pulse duration of 30 fs the formation of near-wavelength periodic surface structures makes it possible to increase the brightness of colorized copper surfaces. Accordingly to the many times experimentally confirmed interference model [23,24]; the process of the formation of periodic structures can be schematically represented as follows: the process begins with the appearance of a periodically modulated interference light field in a space near the surface. The reason for its appearance is the interference of an incident light wave with a wave scattered by a certain surface roughness. The interference of an incident wave with resonant components of the diffracted field is most effective. A spatially inhomogeneous heating of the surface occurs in a periodically modulated intensity field. In this case, the temperature distribution along the surface obviously correlates with the distribution of the intensity of the interference light field. If the intensity of the laser radiation is sufficiently large, inhomogeneous heating of the surface can cause inhomogeneous melting, and then evaporation and removal of matter: the interference relief is “remembered”. The above considerations can only be considered in general. For a more rigorous description it is necessary to consider the problem of an inhomogeneous imbedding of the electromagnetic field energy into the irradiated rough surface. The total electromagnetic field on the surface has the character of a periodic structure only if the scattered wave has a different tangential component of the wave vector than the incident wave. This is the situation when light is reflected from an even slightly rough surface: in the reflected light field there are not only mirrored components of the reflected wave, but also components that have experienced diffraction on various Fourier components of the roughness spectrum. Any real rough surface can be represented as a set of sinusoidal gratings with random orientations of the strokes, random periods and relief amplitudes. Then the scattering of the incident light wave on the surface roughness can be regarded as diffraction on various Fourier components of the roughness spectrum. Due to fields addition of incident and surface electromagnetic waves in the skin layer, the formation of interference maxima occurs, and consequently an inhomogeneous or periodic heating of the surface. The results of these experiments show that forward and reverse scanning of a copper surface by a beam of ultrashort pulses with a weakly asymmetric spatial energy distribution in a multipulse mode and an energy density below the material ablation threshold can lead to significant colour differences between different scanning directions.
Fig. 4. 3D-images of four sample areas in different colours: blue-turquoise (a), orange (b), grey-green (c) and red-violet (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 3
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Fig. 5. Height profiles of four areas of the sample showing different colours: blue-turquoise (a), orange (b), grey-green (c) and red-violet (d). Table 1 The average height and average period of the microrelief in areas of different colours. Colour
Average height [nm]
Standard deviation [nm]
Average period [nm]
Blue-turquoise Orange Grey-green Red-violet
260 175 177 151
28 54 45 28
620 640 600 610
between these values and are much closer to the values of the red-violet area. For the examination of laser processed tracks, an analytical FEI Quanta 200 SEM was used. Fig. 6 shows an image of tracks on a sample surface formed as a result of scanning the surface with multipulses from a femtosecond laser. Fig. 6a shows images taken with an ETD – Everhart-Thornley Detector, which is a secondary and backscattered electron detector used in SEMs. Additionally, Fig. 6b shows an image taken with CBS - Circular Backscatter Detector, which is a multi-segment solid state high-efficiency backscattered electron (BSE) detector which is composed of multiple rings which can form images simultaneously. Images reveal that the sample shows a combined microrelief consisting of LSFL with an additional nanoroughness. There are areas with a large number of adhered particles and areas with a smaller number of adhered particles. It appears that the visible red-violet areas have fewer and smaller quantities of adhering particles than the blue-turquoise areas. Performed by energy dispersive X-ray spectroscopy technique, an elemental chemical analysis of the surface has shown that the oxygen content in the blue-turquoise areas (up to 12% by weight) is significantly higher than in the red-violet areas (up to 2% by weight). The adherent particles with a shape close to the spherical were identified as copper oxide. Since the intensity of the backscattered electrons depends mainly on the atomic number of the material, one can distinguish between elements in BSE images. Heavy elements lead to strong backscattering, and that result in brighter images, while light elements cause weak backscattering and, therefore, the image becomes dark. Copper atom is much heavier than oxygen atom, and therefore the backscattered electron image of non-oxidized regions should be much brighter. Figs. 7, 8 show images resulting from secondary (left) and backscattered (right) electrons of four areas. Of course, on the basis of these images it is quite difficult to make a quantitative description, but from a purely qualitative point of view it can be seen that the image of the sample area of red-violet colour is much brighter compared to other areas, whereas the blue-turquoise sample area appears much darker than other areas. Thus, it is shown that the reason for the difference in the colour of the images is also the degree of their oxidation.
Fig. 6. SEM images of nanostructures recorded on the copper surface corresponding to red- violet (up) and blue-turquoise areas (under) taken with secondary (a) and backscattered electrons (b).
3. Conclusions The coloration of a copper surface by nanostructuring with femtosecond laser pulses at changing only the scanning direction in air has been studied. Experimental studies were performed with a femtosecond laser with a pulse duration < 30 fs. Surface structures with a low spatial frequency (LSFL) have been formed by a relative movement of the pulsed laser beam across the sample surface with an energy density below the ablation threshold. The colour appearance and the brightness of the copper surface were changed by almost wavelength-periodic surface structures. Four areas have been identified showing different colours such as blue-turquoise, orange, grey-green and red-violet. Possible influences of the surface topology on observed colours at different areas were investigated by using AFM. It was shown that a significant colour change of the surface can be caused by even relatively small changes in the nanostructure. The average period of the nanostructures appears to be the same for all areas of the sample surface and is in the range between 600 and 640 nm. In a blue-turquoise area of the observed colour perception occurs the maximum average height of the relief and is 1.7 times higher than in the red-violet area, while graygreen and orange areas have an average height between these values and are much closer to the values of the red-violet area. SEM images of the sample surface were taken in secondary and backscattered electron modes. It was revealed that the sample shows a combined microrelief consisting of LSFL with an additional nanoroughness, and the red-violet areas have fewer and smaller quantities of adhering particles than the blue-turquoise areas. Backscattered electrons images show on brighter background dark areas with clearly defined borders. Results indicate that the sample material shows a 4
Optics and Laser Technology 119 (2019) 105574
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Fig. 7. SEM images taken from secondary (left) and backscattered (right) electrons of four areas of sample showing different colours: blue-turquoise (a), orange (b), grey-green (c) and red-violet (d); the brighter areas correspond to lower oxygen content and greater content of copper. SEM magnification: 10,000×.
colour difference of images.
certain heterogeneity in its composition, since areas with a light element – that is oxygen in the common copper matrix – can be detected. An elemental chemical analysis of the surface has shown that the oxygen content in the red-violet areas is significantly lower than in the blue-turquoise areas. It was found that not only the periodicity of the nanostructures but the degree of oxidation is a reason for the observed
Acknowledgements Scanning electron microscopy and atomic force microscopy of Cu samples were performed using facilities at the University Service Centre 5
Optics and Laser Technology 119 (2019) 105574
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Fig. 8. SEM images taken from secondary (left) and backscattered (right) electrons of four areas of sample showing different colours: orange (a), grey-green (b) and red-violet (c); the brighter areas correspond to lower oxygen content and greater content of copper. SEM magnification: 50,000×.
for Transmission Electron Microscopy, TU Wien, Austria.
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Full length article
Coloration of a copper surface by nanostructuring with femtosecond laser pulses
T
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Serguei P. Murzina,b, , Gerhard Liedlb, Robert Pospichalb a b
Samara National Research University, Moskovskoe Shosse 34, Samara 443086, Russia TU Wien, Institute of Production Engineering and Photonic Technologies, Getreidemarkt 9, Vienna 1060, Austria
H I GH L IG H T S
of a copper by nanostructuring with femtosecond laser pulses was studied. • Coloration structures with low spatial frequency were formed. • Surface topology on observed areas of different colours were investigated. • Surface were taken in secondary electron and backscattered electron mode. • SEM-images • Reasons for the colour difference are the surface topology and the oxidation degree.
A R T I C LE I N FO
A B S T R A C T
Keywords: Coloration Femtosecond laser pulse Surface structure Oxidation
The coloration of a copper surface after treatment in air with femtosecond laser pulses at changing only the scanning direction has been studied. Surface structures with low spatial frequency, which were formed by a relative movement of the pulsed laser beam across the sample surface with an energy density below the ablation threshold, changed the sample colour. Four areas have been identified showing different colours such as blueturquoise, orange, grey-green and red-violet. The possible influence of the surface topology on observed areas were investigated by atomic force microscopy. It was found that in a blue-turquoise area of the observed colour perception occurs the maximum average height of the relief and it is 1.7 times higher than in the red-violet area, while gray-green and orange areas have an average height between these values and are much closer to the values of the red-violet area. Scanning electron microscope images of the sample surface were taken in secondary and backscattered electron modes. It was revealed that the sample shows a combined microrelief consisting of low-spatial-frequency laser-induced periodic surface structures with an additional nanoroughness. It was determined that reasons for the colour difference of images are both the surface topology, as well as the degree of oxidation.
1. Introduction Ultrafast lasers have opened up new possibilities in ultra-high-precision micro- and nano-processing [1–3], whereby the pulse width of these lasers can range from several ten femtoseconds to ten picoseconds. Lasers in the longer pulse width range are used for commercial and industrial applications mainly, whereas lasers with pulse widths shorter than picoseconds are mostly used for fundamental research [4,5], respectively. In the case of exposure to a substance of ultrashort pulses, the characteristic time of the transition of laser energy into heat is longer than the pulse duration. Thus, thermal processes can occur after the passage of a laser pulse. In this case, the mechanism for
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processing materials is different than in the case of, for example, nanosecond pulses [6]. Micro/nanostructures on metals and surfaces with unusual optical properties or wettability [7–10] can be obtained by using ultrafast pulse laser sources. The coloration of a metal surface by means of ultrashort laser pulses was first achieved and described in [11]. Such colour marking of surfaces of almost any solid composition is feasible in the formation case of surface-periodic structures. This is particularly important for materials that are weakly oxidized or have an opaque oxide (e.g. copper). A possibility to obtain specific colour patterns using ultra short pulses during the process of material modification by polarization-dependent structure generation is shown in [12]. The orientation of these nanostructures created on a metal surface, the
Corresponding author. E-mail address: [email protected] (S.P. Murzin).
https://doi.org/10.1016/j.optlastec.2019.105574 Received 25 January 2019; Received in revised form 7 May 2019; Accepted 15 May 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
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for sample processing and the laser beam was focused on the surface of pure copper plates by a metal mirror. The spot size was adjusted to obtain an energy density of 1.05 J/cm2. The dimensions of the spot were 0.26 mm height and 0.27 mm width, with a total spot area of approx. 0.07 mm2. Samples have been generated by scanning with the focused laser beam over a surface of a copper plate. The size of the processed area was 20 × 20 mm2. During laser exposure the plate has been moved 20 mm in the positive x-direction. At the end of the line, the laser was turned off and the plate was displaced by 0.28 mm in the positive y-direction. Following that, the laser was switched on again and the sample was moved in the negative x-direction. The whole processing cycle was repeated until an area of 20 × 20 mm2 had been processed by ultrafast laser pulses. The copper surface was therefore treated with femtosecond laser pulses over an area of 400 mm2. First, samples have been moved during processing in x-direction, and then, after a small displacement in ydirection the direction of movement was reversed. Thus a line-by-line processing, first in +x-direction, followed by processing in −x-direction could be achieved. Speed of motion was 180 mm/min, and the distance between lines was 0.28 mm, with an overlap ∼0%. The experiments have revealed that visible colours depend only on the direction of movement. Fig. 1 shows an image of the treatment zone obtained using a metallographic optical microscope. Within the image, the direction of movement of the sample during laser processing is indicated by an arrow. Fig. 2 shows the zone of a single pulse action on the sample surface. It is known that during normal incidence of femtosecond, linearly polarized laser pulses, low-spatial-frequency laser-induced periodic surface structures (LSFL) are formed on the copper surface, which demonstrate the effect of diffraction staining [22,26]. That is, the appearance of a surface relief is observed, which demonstrate properties of a diffraction grating in the visible range. However, generated at normal incidence, LSFL do not allow obtaining bright colours in the
so-called ripples, depends on the polarization of the laser beam and they show a periodicity that is typically smaller than the laser wavelength and in the range of the visible spectrum. The colour palette that such surfaces may demonstrate, is caused by a grating diffraction effect of the visible light at the laser-induced periodic surface ripples, which has been confirmed in Ref. [13]. This can be used to an adaptive management of the coloration effect during laser processing of pure copper. The realizable colour laser marking technique allows to obtain a high-resolution colour image by a contactless effect on the material. The applied process does not significantly change the physical-chemical properties of the material surface compared to a coloration process by oxidation of the metal surface (temper colour). This phenomenon of surface structuring can be observed in all metals depending on the absorption coefficient at the laser wavelength as well as the ablation atmosphere – vacuum or in presence of a defined gas. Generally, in air nearly all metals with multielectron valence react with oxygen of air or water vapour [14–16]. This topic has been quite extensively covered in the last decade. Studies of the properties of the formed surface as a function of the pulse duration were performed. For instance, it is known that functional copper surfaces combined with brighter structural colours and superhydrophobicity were fabricated not only using femtosecond, but also picosecond laser pulses [17]. It is known that the scan velocity is an important parameter. Ref. [18] shows that during femtosecond laser treatment with scanning, the ripple has a clear morphology in the slower scan velocity regime. Conversely, the ripple has an ambiguous morphology when the scan velocity is high enough. It is known that the incident angle is also an important parameter. Ref. [19] shows a theoretical model of femtosecond laser induced ripple formation on copper for varying incident angle, which is consistent with the results of experimental studies. By changing the angle of incidence of ultrashort pulse beam on the sample, brighter colours can be achieved, especially in the long-wave region of the visible spectrum. Ref. [20] shows a typical optical microscope image of the sample surface representing the parallel micro-trenches inscribed on the target surface by the bidirectional scanning of the laser beam. However, about a colour dependence on the scanning direction, it is not reported. The method of scanning a femtosecond laser beam with an asymmetrical spatial fluence distribution over the sample surface, resulting in a change of the nanoscale surface topology as well as a manipulation of the chemical composition of a titanium surface is presented in Ref. [21]. It can be assumed that observed phenomena can be found not only in titanium, but with other materials too and that individual features will probably also be present [22–24]. Specific features of colorizing copper surfaces and changes in the nanostructure by scanning with a femtosecond laser beam with a nearly Gaussian energy density distribution across the sample surface is studied in Ref [25]. It was revealed that by forward and reverse scanning of a copper surface by an ultrashort pulse beam it was possible to create a combined microrelief consisting of low-spatial-frequency laser-induced periodic surface structures (LSFL) and an additional nanoroughness. It was shown that the observed colour change of the processed surfaces depended on relatively small variations of the nanostructure. The objective of the research presented here is to investigate more deeply the controlled coloration of copper by diffraction effects through nanostructures generated by femtosecond laser pulses below the material ablation threshold, changing only the scanning direction. 2. Results of experimental studies A femtosecond laser was used to perform the experimental studies. The system consists of a Ti: Sapphire oscillator and a multi-pass amplifier with max. 0.8 mJ pulse energy, 1 kHz pulse frequency and a pulse duration < 30 fs. The laser generates pulses in the TEM00-mode, and a pulse contrast > 109: 1. Central emission wavelength was 800 nm, bandwidth approx. 100 nm. A 2D positioning system was used
Fig. 1. Image of the treatment zone obtained using a metallographic optical microscope. Visible colours depend on the direction of movement: 1 – redviolet; 2 – blue-turquoise; 3 – orange; 4 – gray-green. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 2
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Fig. 3. AFM topology images of four sample areas in different colours: blueturquoise (a), orange (b), grey-green (c) and red-violet (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Fig. 2. Zone of a single pulse action on the sample surface.
The action of a femtosecond laser in a multipulse mode leads to the appearance of tracks on the material, which were studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM). In order to investigate the possible influence of the surface topology on the observable colour appearance, an AFM was used to study areas of different colours. Figs. 3 and 4 show AFM topology images and 3D-images of four sample areas in different colours. Fig. 5 shows height profiles of four areas of the sample showing different colours like blue-turquoise, orange, grey-green and red-violet. At first glance, there aren’t significant differences between these height profiles. Therefore, for each graph, the height of the individual peaks was measured as from the height of the local maximum substacting the height of the next local minimum, and the mean value and the corresponding standard deviation for each of the four areas were calculated. The measurement results are presented in Table 1. It was shown that even relatively small changes in the nanostructure can lead to a significant colour change of the surface. The average period appears to be the same for all areas of the sample surface and is 600–640 nm. The maximum average height of the relief occurs in a blue-turquoise area of the observed colour perception and is 1.7 times higher than in the redviolet area. Gray-green and orange areas have an average height
long-wavelength part of the visible electromagnetic radiation [27–29]. By using a femtosecond laser with pulse duration of 30 fs the formation of near-wavelength periodic surface structures makes it possible to increase the brightness of colorized copper surfaces. Accordingly to the many times experimentally confirmed interference model [23,24]; the process of the formation of periodic structures can be schematically represented as follows: the process begins with the appearance of a periodically modulated interference light field in a space near the surface. The reason for its appearance is the interference of an incident light wave with a wave scattered by a certain surface roughness. The interference of an incident wave with resonant components of the diffracted field is most effective. A spatially inhomogeneous heating of the surface occurs in a periodically modulated intensity field. In this case, the temperature distribution along the surface obviously correlates with the distribution of the intensity of the interference light field. If the intensity of the laser radiation is sufficiently large, inhomogeneous heating of the surface can cause inhomogeneous melting, and then evaporation and removal of matter: the interference relief is “remembered”. The above considerations can only be considered in general. For a more rigorous description it is necessary to consider the problem of an inhomogeneous imbedding of the electromagnetic field energy into the irradiated rough surface. The total electromagnetic field on the surface has the character of a periodic structure only if the scattered wave has a different tangential component of the wave vector than the incident wave. This is the situation when light is reflected from an even slightly rough surface: in the reflected light field there are not only mirrored components of the reflected wave, but also components that have experienced diffraction on various Fourier components of the roughness spectrum. Any real rough surface can be represented as a set of sinusoidal gratings with random orientations of the strokes, random periods and relief amplitudes. Then the scattering of the incident light wave on the surface roughness can be regarded as diffraction on various Fourier components of the roughness spectrum. Due to fields addition of incident and surface electromagnetic waves in the skin layer, the formation of interference maxima occurs, and consequently an inhomogeneous or periodic heating of the surface. The results of these experiments show that forward and reverse scanning of a copper surface by a beam of ultrashort pulses with a weakly asymmetric spatial energy distribution in a multipulse mode and an energy density below the material ablation threshold can lead to significant colour differences between different scanning directions.
Fig. 4. 3D-images of four sample areas in different colours: blue-turquoise (a), orange (b), grey-green (c) and red-violet (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 3
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Fig. 5. Height profiles of four areas of the sample showing different colours: blue-turquoise (a), orange (b), grey-green (c) and red-violet (d). Table 1 The average height and average period of the microrelief in areas of different colours. Colour
Average height [nm]
Standard deviation [nm]
Average period [nm]
Blue-turquoise Orange Grey-green Red-violet
260 175 177 151
28 54 45 28
620 640 600 610
between these values and are much closer to the values of the red-violet area. For the examination of laser processed tracks, an analytical FEI Quanta 200 SEM was used. Fig. 6 shows an image of tracks on a sample surface formed as a result of scanning the surface with multipulses from a femtosecond laser. Fig. 6a shows images taken with an ETD – Everhart-Thornley Detector, which is a secondary and backscattered electron detector used in SEMs. Additionally, Fig. 6b shows an image taken with CBS - Circular Backscatter Detector, which is a multi-segment solid state high-efficiency backscattered electron (BSE) detector which is composed of multiple rings which can form images simultaneously. Images reveal that the sample shows a combined microrelief consisting of LSFL with an additional nanoroughness. There are areas with a large number of adhered particles and areas with a smaller number of adhered particles. It appears that the visible red-violet areas have fewer and smaller quantities of adhering particles than the blue-turquoise areas. Performed by energy dispersive X-ray spectroscopy technique, an elemental chemical analysis of the surface has shown that the oxygen content in the blue-turquoise areas (up to 12% by weight) is significantly higher than in the red-violet areas (up to 2% by weight). The adherent particles with a shape close to the spherical were identified as copper oxide. Since the intensity of the backscattered electrons depends mainly on the atomic number of the material, one can distinguish between elements in BSE images. Heavy elements lead to strong backscattering, and that result in brighter images, while light elements cause weak backscattering and, therefore, the image becomes dark. Copper atom is much heavier than oxygen atom, and therefore the backscattered electron image of non-oxidized regions should be much brighter. Figs. 7, 8 show images resulting from secondary (left) and backscattered (right) electrons of four areas. Of course, on the basis of these images it is quite difficult to make a quantitative description, but from a purely qualitative point of view it can be seen that the image of the sample area of red-violet colour is much brighter compared to other areas, whereas the blue-turquoise sample area appears much darker than other areas. Thus, it is shown that the reason for the difference in the colour of the images is also the degree of their oxidation.
Fig. 6. SEM images of nanostructures recorded on the copper surface corresponding to red- violet (up) and blue-turquoise areas (under) taken with secondary (a) and backscattered electrons (b).
3. Conclusions The coloration of a copper surface by nanostructuring with femtosecond laser pulses at changing only the scanning direction in air has been studied. Experimental studies were performed with a femtosecond laser with a pulse duration < 30 fs. Surface structures with a low spatial frequency (LSFL) have been formed by a relative movement of the pulsed laser beam across the sample surface with an energy density below the ablation threshold. The colour appearance and the brightness of the copper surface were changed by almost wavelength-periodic surface structures. Four areas have been identified showing different colours such as blue-turquoise, orange, grey-green and red-violet. Possible influences of the surface topology on observed colours at different areas were investigated by using AFM. It was shown that a significant colour change of the surface can be caused by even relatively small changes in the nanostructure. The average period of the nanostructures appears to be the same for all areas of the sample surface and is in the range between 600 and 640 nm. In a blue-turquoise area of the observed colour perception occurs the maximum average height of the relief and is 1.7 times higher than in the red-violet area, while graygreen and orange areas have an average height between these values and are much closer to the values of the red-violet area. SEM images of the sample surface were taken in secondary and backscattered electron modes. It was revealed that the sample shows a combined microrelief consisting of LSFL with an additional nanoroughness, and the red-violet areas have fewer and smaller quantities of adhering particles than the blue-turquoise areas. Backscattered electrons images show on brighter background dark areas with clearly defined borders. Results indicate that the sample material shows a 4
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Fig. 7. SEM images taken from secondary (left) and backscattered (right) electrons of four areas of sample showing different colours: blue-turquoise (a), orange (b), grey-green (c) and red-violet (d); the brighter areas correspond to lower oxygen content and greater content of copper. SEM magnification: 10,000×.
colour difference of images.
certain heterogeneity in its composition, since areas with a light element – that is oxygen in the common copper matrix – can be detected. An elemental chemical analysis of the surface has shown that the oxygen content in the red-violet areas is significantly lower than in the blue-turquoise areas. It was found that not only the periodicity of the nanostructures but the degree of oxidation is a reason for the observed
Acknowledgements Scanning electron microscopy and atomic force microscopy of Cu samples were performed using facilities at the University Service Centre 5
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Fig. 8. SEM images taken from secondary (left) and backscattered (right) electrons of four areas of sample showing different colours: orange (a), grey-green (b) and red-violet (c); the brighter areas correspond to lower oxygen content and greater content of copper. SEM magnification: 50,000×.
for Transmission Electron Microscopy, TU Wien, Austria.
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