1. Introduction

Over the last two decades, laser surface texturing has been increasingly investigated to enable the tailoring of surface functionality of a variety of materials, including tribological properties [1], wettability [2], biological properties [3], and optical properties [4], etc. Among that, Vorobyev et al. conducted pioneering work on turning highly reflective metals to highly absorptive ones by the formation of surface micro/nanostructures under direct femtosecond laser irradiation, creating the so-called “black metals”, e.g., black gold, platinum, titanium, tungsten, and aluminum [5–8]. In this method, the broadband absorption, typically around 85%~95% over the wavelength range from ultraviolet (UV) to near infrared (NIR), i.e., 250~2500 nm, was found to result from the periodic groove structures covered with finer sub-structures at micro- and nanoscales. These sub-structures were spontaneously induced during laser ablation with femtosecond pulses.

However, manufacturing of this kind of black metals by simply focusing femtosecond laser to the sample surface is rather time-consuming, given that the processing speed of a femtosecond laser is usually in the magnitude of micrometers per second. Paivasaari et al. proposed a four-beam interferometric femtosecond laser ablation method in order to increase the processing speed, and formed hole-array structures on stainless steel and copper surfaces [9,10]. Although almost total absorption was obtained for stainless steel in a spectrum band of 200~2300 nm, a gradual increase of reflectance to about 50% at the wavelength of 800 nm was observed for the copper samples. More recently, blackening of copper with a nanosecond (12 ns) Nd:YVO₄ laser was also reported [11]. Constant absorption of over 97% in the spectral range from 250 nm to 750 nm was realized by introducing highly organized periodic microstructure arrays on copper substrates. Nevertheless, a nearly linear increase of reflectance up to 30% was observed within the spectrum of 750~2500 nm.

Due to their unique thermal and electrical conductivity, copper and its alloys are important materials for many applications including solar energy absorbers, thermal radiation sources, and radiative heat transfer devices, etc. Absorption enhancement of copper surfaces plays a key role in increasing the efficiency of these applications. However, high rate (>90%) broadband absorption of copper surfaces in the wavelength range of 250~2500 nm, where most of solar radiation energy is distributed at the sea level [12], has not been reported by either femtosecond or nanosecond laser blackening techniques.

It is known that high throughput can be obtained in nanosecond laser processing at the expense of structure integrity induced by melting, especially causing the nano structuring ability to be deficient, whereas a femtosecond laser is normally good at inducing nano- structures when ablating microstructures with typically very low efficiency. Compared to those, picosecond (ps) laser has the ability to produce complex micro/nano structures comparable to a femtosecond laser. Furthermore, with the development of high power and high repetition rate ps lasers, higher ablation rates can be achieved while keeping good structuring quality. In recent years, increasing research has been focused on ps laser ablation of various materials [13–15]. However, the study on ps laser blackening of copper surfaces has not been reported.

In this paper, we present the interaction of a high power (with an average power up to 100 W) ps laser with Cu in structuring unique surface micro/nano structures for broadband light absorption. Porous coral-like micro structures covered by nanoscale aggregates and corrugations were produced. The work demonstrates that over 97% absorptivity in the visible spectral region and over 90% absorptivity in average in the UV, visible, and NIR regions (i.e., 250~2500nm) can be achieved by this porous coral-like surface structure. These values match the previously reported results on nanosecond laser blackened copper [11], as well as other high reflective metals blackened by femtosecond laser [5–8,16,17]. Additionally, the current picosecond laser processing method produces significantly different surface micro/nano structures from those used to achieve high broadband light absorption with femtosecond lasers or nanosecond lasers, while enabling a processing speed up to tens of millimeters per second, much higher than those reported with femtosecond or nanosecond lasers^{11, 16, 17}. This novelty makes picosecond laser a scientific interesting tool for blackening metal surfaces and also an efficient and affordable method for practical industrial applications.

2. Experimental

The experiments were conducted using an Edgewave picosecond laser with an amplified Nd: YVO₄ laser system which generates 10 ps laser pulses at a maximum repetition rate of 2 MHz at a central wavelength of 1064 nm. Before laser processing, the Cu samples with a dimension of 25 × 25 × 3 mm³ were polished and cleaned ultrasonically with ethanol to remove the oxide and grease on their surfaces. An x-y galvo and an f-θ lens were used to scan and focus the laser beam onto the copper surfaces in a pattern of cross lines in atmospheric environment. The diameter of the focused spot defined by an intensity drop to 1/e² of the maximum value was approximately 30 μm. After laser processing, the sample surfaces were ultrasonically cleaned with ethanol again. The X-ray diffraction (XRD) patterns of the specimens were recorded by a Rigaku SmartLab X-ray diffractometer. The detected diffraction angle (2θ) was scanned from 10° to 90° with a speed of 4°/min for a quantitative measurement. The surface micro/nano structures of the treated samples were studied with a LEO-1530 scanning electron microscope (SEM). A Keyence VHF-500F digital microscope was utilized to image and measure the 3D microstructure and topography of the sample surfaces. In measuring Ra (arithmetical mean deviation) and Rz (profile irregularity), a Talysurf 5P-120 surface topography profilometer was employed. The wavelength dependence of the total reflectance in the UV, visible, and NIR regions was characterized with a spectrophotometer incorporated with an integrating sphere of 60 mm in diameter. After optimizing the ps laser fluence, the repetition rate and the scanning speed to be 25 J/cm², 200 kHz, and 50 mm/s, respectively, this research was focused on varying the scanning interval (I) between two adjacent lines to study the influence of ps laser processing procedure on the reflectance of copper surfaces.

3. Results and discussion

As seen in Fig. 1, ps laser processing with optimized parameters significantly decreases the reflectance of the Cu surfaces in a very broadband spectrum from 250 nm to 2500 nm. Strong light absorption was achieved on initially high reflective polished Cu surfaces. The scanning interval (I) between two adjacent lines has obvious influence on the reflectance. The lowest reflectance was obtained with a scanning interval of 5~10 μm, with over 97% absorptivity in the UV and visible spectral region and over 90% absorptivity in average in the UV, visible, and NIR regions (250~2500 nm) being measured. Due to the decreased reflectance in the visible region, i.e., below 3%, the initial shining surface of the polished Cu sample turns to be pitch black, as shown in the inset of Fig. 1. The reflectance behavior of ps laser processed surfaces with different laser scanning intervals can be separated into three groups: Group 1 with laser beam scanning intervals of 5 μm and 10 μm (less than the laser beam diameter), Group 2 with scanning intervals of 20 μm and 30 μm (equivalent to the laser beam diameter) and Group 3 with scanning intervals of 40 μm and 50 μm (larger than the laser beam diameter). Although the samples of all these three groups possess almost the same low value of reflectance in the UV and visible spectrum regions, obvious differences in the reflectance behavior can be observed in the NIR region. The surface reflectance in Group 1 keeps at a constant low value throughout the NIR spectrum. Similarly, only slight growth can be observed for the reflectance curves in Group 2 in a broad spectrum band except the growth from below 20% to about 30% between 2000 nm and 2500 nm. However, for the surfaces in Group 3, an approximately linear increase appears for the reflectance curves from 750 nm to 2000 nm, showing even a faster increase from about 30% to more than 50% in reflectance in the wavelength region of 2000~2500 nm.

 

Fig. 1 Reflectance as a function of wavelength for samples with different laser scanning intervals. For comparison, the reflectance of polished Cu surface is also plotted. The dashed line is the calculated reflectance curve for I = 40 μm without plateau areas. The inset shows the optical photograph of polished Cu (left) and blackened Cu (right, I = 5 μm).

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To identify the mechanism that causes the above described optical response of ps laser processed copper surfaces with different beam scanning intervals, a detailed study on surface composition and structure was performed. Considering that an oxide layer may form on the surface of copper during and after laser processing [see Fig. 2(a)], quantitative X-ray diffraction (XRD) analyses were performed. Figure 2(b) presents the contrast of surface absorptance as well as the content of oxide (including copper oxide and cuprous oxide) on sample surfaces with different scanning intervals. The absorptance was derived by subtracting the average value of reflectance in the wavelength range of 250~2500 nm from 100%. It is indicated that the evolution trend of the absorptance with scanning interval is not consistent with that of the oxide content. First, as the scanning interval decreases from 50 μm to 20 μm, the content of oxide on the sample surface increases, due to a longer time for oxidation. However, the absorptance does not show the same characteristic. The absorptance of samples with intervals of 50 μm and 30 μm is almost equal to those with intervals of 40 μm and 20 μm, respectively. Besides, more oxide on the copper surface does not yield a higher absorptance, as shown in the comparison of samples with intervals of 10 μm and 20 μm, respectively. More importantly, the limited content of oxide existing on the treated sample surface cannot explain the abrupt increase of absorptance value up to nearly 100%. Thus, it is speculated that, the composition change of the sample surface was not the main reason for the dramatically enhanced absorption. In fact, it has been shown in the process of nanosecond laser blackening of copper that the optical contribution of the oxide layer is on average approximately 3% in the visible range and increases to 10% on average throughout the infrared spectral range [11].The rest of the increased absorption is attributed to the produced surface micro- and nano- structures through laser irradiation.

 

Fig. 2 (a) X-ray diffraction patterns of sample surfaces with different laser scanning intervals. (b) Contrast of absorptance as well as the content of oxide on sample surfaces with different scanning intervals.

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Figures 3 and 4 show the SEM and 3D morphology images of surface structures created with different scanning intervals. With the scanning interval increasing from smaller to greater than of the focus diameter, three kinds of micro structures can be introduced onto the copper surfaces. As in Group 1, with the scanning interval being much smaller than the focus diameter, the entire initial surface was processed and modified by ps laser more than once, some coral-like surface structures composed of micro-cavities with random orientations were produced, as shown in Fig. 3(a) and Fig. 4(a). These micro-cavities consist of small holes with dimensions of 1~30 μm embedding in large hollows with dimensions of 30~100 μm, with micro protrusions and particles surrounding them, forming a kind of porous structure. When the scanning interval was equivalent to the focus diameter as in Group 2, regularly distributed surface structures begin to show up. Highly uniform hole-array structures with dimension of about 20 μm were fabricated, as shown in Fig. 3(b) and Fig. 4(b). When the scanning interval was larger than the focus diameter as in Group 3, only a certain proportion of the original surface can be irradiated directly by the laser focus spot, and the bell mouth-like structures coupled with plateau-like areas were formed, as displayed in Fig. 3(c) and Fig. 4(c).

 

Fig. 3 SEM images of surface structures. Here, the scanning interval for sample (a), (b) and (c) is 5 μm, 30 μm, and 50 μm, respectively.

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Fig. 4 (a), (b), (c) 3D OM morphology images of surface structures formed on copper with different laser scanning intervals. (d) Comparison of the corresponding Ra and Rz together with absorptance.

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Here, the comparison of reflectance obtained when I is smaller than 30 μm and the whole surface is illuminated by the structuring laser beam to that obtained when I is larger than 30 μm and not the complete copper surface is directly structured, may be controversial. This concern can be considered and clarified through the following brief conjecture.

It can be simply assumed that the total reflectance of a surface was the sum of the reflectance of different structure areas weighted by their area proportion. The reflectance of the plateau areas can be represented by the reflectance of the original flat surface, and their proportion on the whole surface can be estimated roughly by the difference between the scanning interval and the focus diameter. Thus, the reflectance of a surface without plateau areas can be estimated, as addressed by the dashed line in Fig. 1. It is not surprising that the calculated curve dropped into negative values in the UV and visible regions considering that the conjecture above apparently overestimated the influence of plateau areas. Indeed, these areas were not as flat and unprocessed as the polished surface and their area proportion was actually much smaller than estimated. For one hand, the focus diameter of 30 μm is not a fixed border of laser beam. There is also laser energy and thus laser influence on the surface at greater diameters due to the intrinsic feature of Gaussian beam. For another hand, the surface area that can be modified by laser was usually more or less larger than the focus diameter due to heat transfer within the treated material. Thus, once the scanning interval was still comparable to (i.e., not much larger than) the focus diameter, the whole sample surface can get modified by laser, as for the conditions with I = 40-50 μm in our experiments. It can be evidenced from both the SEM and morphology images that there also exist plenty of microstructures on the plateau areas and the bell mouth-like structures are packed to each other closely enough. However, even with this clearly overestimated calculation, higher reflectance than the conditions with smaller intervals was still obtained. It can be seen that the calculated curve is just slightly lower than the measured curve for I = 40 μm and still above the curves with I = 5-20 μm in the NIR region. So the influence of the unprocessed area on reflectance can be neglected. Therefore, the reflectance difference between different surfaces can be mainly attributed to the construction form of the structures produced on them.

It is known that when the structure size is much larger than the characteristic wavelength, the ray tracing approach in the geometric optics limit can be used for the prediction of surface reflection, which attributes the suppression of reflection to the multiply reflection effect of the surface structures [18,19]. And the reflection decreases when the surface structures are higher (or deeper). For the porous coral-like structure, the random distributed large hollow and small hole embedding architecture can effectively collect the incident light in a broad spectrum and from all directions. Subsequently, the light incident into the cavities is multiply reflected with enhanced absorption. And the surrounding protrusions and particles can block the collected light from escaping the cavities. Moreover, in Fig. 4(d) it can be seen that the Ra and Rz values of the porous structures are larger than those of the other two structures, meaning that more and deeper trapping cavities are present on the copper surface which further eliminate the reflection. It has been reported that this porous coral-like structure can act as black holes or light traps to seize the external irradiations effectively, resulting in a remarkable decrease in the integrated reflectance over a broadband wavelength range [16].

Compared to the large hollow and small hole embedding structures described above, the light trapping effect of the highly uniform hole arrays is less efficient, resulting in the increase of reflectance from curves of group 1 to group 2 in Fig. 1. And for the open bell mouth structure, the open mouth provides a convenient way for light to escape, and the plateau can directly reflect back the incident light to the outer medium [10]. Thus a relatively high reflectance is performed, especially in the NIR region.

Another consideration to be noted is that, the best results were obtained when the smallest interval was adopted in this research, i.e., I = 5 μm or 10 μm, meaning there was a strong spatial overlapping of two consecutive laser pulses. It seems like that the important point for enhanced absorptance is the use of a uniform exposure of the structuring surface to ps laser pulses. Consequently, it may be anticipated that when I is further reduced (I<5 μm), higher absorptance may be achieved. A little attention needs to be paid to the process of ps laser treatment before illustration of this issue.

For one aspect, under ps laser irradiation with I = 5 μm and 10 μm, the evolution of surface structure is more like a self-assembled process. The pattern of the structure was spontaneously formed rather than determined directly by laser scanning. Namely, sufficient exposure of a surface to ps laser pulses was only one of the possible driving forces for this self-assembled process but not the direct determining factor for the construction form of the structures created. For another aspect, during pulse laser treatment, two factors will actually influence the produced results, namely the pulse intensity and the pulse number per spot (N_pps). The change of scanning speed, scanning interval as well as the pulse repetition rate can all be integrated to the change of N_pps. With speed and repetition rate fixed, a smaller interval results in more pulses and accordingly more laser energy irradiated at one spot. In fact, it was observed that some coarse blocks were formed when I = 5 μm (N_pps≈1440) and 10 μm (N_pps≈720) which were harmful for light absorption by reflecting light directly back to the outer space. These coarse blocks were more developed and pronounced for the condition of I = 5 μm, which is believed to account for the limited improvement of absorptance from the condition of I = 10 μm to that of I = 5 μm as shown in Fig. 1. So it was anticipated that N_pps≈72-1440 was already a saturation value for the enhancement of aborptance under the pulse intensity utilized in the present research. Actually, it has been tried to realize larger N_pps during our experiment which was not reported here for briefness. As a result, either a surface with higher reflectance was formed due to the formation of much larger blocks, or the sample surface was seriously damaged and a big pit was produced. Thus, it will not be beneficial to just reduce the interval further or increase the N_pps, and other modification on the surface structures is needed for higher absorptance, e.g., to eliminate or break the coarse blocks on the surface.

In Table 1, the construction feature and the average reflectance (in the total spectral range 250-2500 nm, as well as its values in specific spectrum regions) of different surface structures as well as the processing efficiency to create them by ps laser are summarized. For comparison, the corresponding performances of ns and fs laser sources were also listed. It can be found that within all the spectra studied in this research, the average reflectance of the porous coral-like structure was comparable to that of the fs laser produced periodic groove structure on Al. Meanwhile the average reflectance of the hole array structure was matchable to that of the ns laser produced cone array structure also on Cu. However, when a view is projected to the processing efficiency, it is inspiring to see that processing efficiency of ps laser is much higher than the other two kinds of laser sources. Specifically speaking, the fs laser processing can turn a shining metal surface with an area of 360 mm² to black within an hour, while the same data for ps laser is 900 mm². And for ns laser, a copper surface of 1260 mm² can be rendered with a broadband absorbance of over 80% between 750 nm and 2500 nm within an hour, while ps laser can make a copper surface of 2700 mm² to reach even a better effect within the same time. In conclusion, to achieve equivalent reflectance, the processing efficiency of ps laser is higher than twice of that of both fs and ns lasers, making the ps laser structuring method more favorable for practical application. For the interferometric fs laser ablation method, there is a possibility that higher processing efficiency can be achieved, however, the reflectance obtained is far from satisfying.

Table 1. Summarizing of obtained surface structures and reflectance with different laser sources.

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Additionally, plentiful nanoscale features, e.g., nanoparticle aggregates and nano-corrugations, covering the surfaces of the micro structures have also been found through more detailed SEM examination as illustrated in Fig. 5. Moreover, for different forms of microstructures, the feature of nanostructures also varies. For the conditions of I = 5-10 μm, nanoparticles with diameter ranging from tens to hundreds of nanometers are the typical characterization, as shown in Figs. 5(a) and 5(b). The formation of nanoparticles under ultrashort laser pulse treatment is a universal phenomenon. It has been reported that nanoparticles with typical radius ranging from few to tens of nanometers can be produced through ablation in vacuum with laser pulses in the 0.1-1 ps temporal range and laser fluence of the order of 0.1-1 J/cm² on both silicon and metal targets [20,21]. Besides, the radius of the produced nanoparticles will increase with the laser fluence. As in the conditions described in this paper, the laser structuring of the copper surface took place in atmosphere. Thus the Cu nanoparticles produced by the ps laser pulses are first ejected with the ablation plume, and then fall back onto the sample surface because of the very high pressure of the atmospheric ambient in which they are produced. The larger size of these nanoparticles compared to those produced in vacuum is ascribed to the higher laser influence used in our experiments (25 J/cm²) as well as the condensation and building up effect during the falling back procedure.

 

Fig. 5 Magnified SEM images of sample surface showing the nanoscale features covering the surface of micro protrusions (a, c, e) and micro cavities (b, d, f). (a) and (b), (c) and (d), (e) and (f) are for surfaces with I = 5 μm, 30 μm, and 50 μm, respectively.

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As I increases (I = 20-50 μm), the feature of nanoparticle aggregates is gradually weakened, with coarsening of particles surrounding the surface of microstructures being observed [Figs. 5(c) and 5(e)]. Instead, the feature of nano-corrugations on the surface of micro protrusions and cavities becomes increasing evident [Figs. 5(d) and 5(f)]. However, those nano-corrugations are a little coarse, compared to the nanoparticles in Fig. 5(b).

It is well known that the nanoscale features can enhance the optical absorption of metal surfaces through the excitation of surface plasmons and the surface plasmon resonance (SPR) mechanism [22–24]. Specifically for copper, with nanoparticles having average sizes (radius) ranging from 1.7 to 6 nm, absorption peaks varying between 593 and 607 nm due to SPR can be obtained, with the wavelength position of the SPR peak shifting to greater wavelength when the particle size increases [22]. The aggregation of these nanostructures can lead to a broadening effect of the resonance bands, in contrast to the isolated nanoparticles with defined and sharp SPR frequencies. Thus broadband enhanced absorption can be induced, especially in the short wavelength spectra. This explains why for all those three kinds of structures above presented comparable low reflectance in the short wavelength regions. The slight difference between them can be explained by the different extent to which the nanostructures developed to. It is apparent that the nanoscale features in Figs. 5(a) and 5(b) are more developed than those in Figs. 5(c)-5(f). This is considered to be another contribution, next to the construction form of porous coral-like structures, to the highest absorptance of conditions with I = 5-10 μm.

4. Conclusions

In summary, we have demonstrated a surface micro/nano structuring technique by utilizing a picosecond laser beam to modify the optical property of copper surfaces. Three kinds of surface microstructures with different construction forms and decorated by plentiful nanoscale features were produced, with their influence on surface reflectance being characterized. It was demonstrated through detailed experiments and discussion that the form and dimensions (especially the depth) of microstructures, as well as the extent to which the nanostructures developed to, have tremendous impact on the absorption property of the copper surface. Specifically by the porous coral-like structure covered with nanoparticle aggregates, over 97% absorptivity in the visible spectral region and over 90% absorptivity in average in the UV, visible, and NIR regions (250~2500nm) can be achieved. Furthermore, with a scanning speed up to tens of millimeters per second utilized, the processing efficiency of this ps laser blackening technique to reach an equivalent absorptivity is higher than twice of that of both fs and ns laser sources. Potential applications of this technique may include solar energy absorbers, thermal radiation sources, and radiative heat transfer devices.