The enhanced optical absorptance in metals was recently demonstrated using femtosecond laser-induced surface structuring. This structuring was obtained by simply focusing the light to the sample surface. Here we demonstrate more efficient absorptance enhancement using interferometric ablation. This interferometric ablation technique produces deeper surface structures and, consequently, higher absorption than structures obtained by just focusing the light to the surface. We also show the measured reflectance spectra over visible region for unaltered and structured stainless steel and copper samples.
© 2007 Optical Society of America
Recently, it was found that the femtosecond laser generated surface structures enhance the optical absorptance of various metals [1, 2, 3]. Vorobyev et.al. discovered the enhanced energy coupling in femtosecond laser-metal interactions [4, 5, 6]. This enhancement is due to spontaneous generation of nano- and micro-structures and laser-induced periodic surface structures (LIPSS) when surface is repeatedly illuminated with femtosecond laser using sufficient fluence [7, 8, 9]. This phenomenon of enhanced absorptance in metals, sometimes referred as ’black metal’, is believed to have applications in many fields such as sensor technology. However, manufacturing of this black metal by simply focusing the femtosecond light to the sample surface is rather time-consuming.
In this paper we propose more controlled and faster method to alternate the absorptance of metals. Instead of waiting the structures to appear spontaneously under the femtosecond illumination, the surface can be ablated using interference pattern in order to generate the desired structures. We will demonstrate increase in absorption of the metal that is ablated using four beam interference pattern creating hole-array structure to the surface. The absorptance changes using such interference pattern for illumination are greater than ones obtained by just focusing the light onto surface using the same pulse numbers and average fluences. Furthermore, this is the first demonstration of the grating structure in metals acting as a total absorber over the whole visible region of the spectrum. Previously total absorption was obtained using metal gratings due to the surface plasmon generation [10, 11, 12]. Total absorption related to the surface plasmons have a resonant nature and can be obtained only using sub-wavelength grating periods and only for specific incident angle and polarization state of the light. Here absorption is independent of the incident angle and polarization state of the light. Total absorption is obtained for steel sample using grating period of 2 μm. Measured reflectance spectra for structured stainless steel and copper samples over 200-800 nm range are presented.
Laser used in the experiments was CPA Tissa 50 Ti:sapphire oscillator and amplifier system providing 1 mJ pulse energy with repetition rate of 50 Hz at 800 nm central wavelength.
The interferometric ablation set-up is shown in Fig. 1. The femtosecond beam is divided to four beams using a diffractive optical element (DOE) and imaged to the sample using confocal system consisting of two lenses. The period of the interference pattern resulting from such set-up can be calculated by formula d = MΛ/2, where M = f 2/f 1 is magnification of the lens system and Λ is the grating period of the DOE. Period of Λ = 15 μm was used here. Lenses used here were double-convex lenses with focal lengths f 1 =100 - 150 mm and achromatic lens with f 2 = 30 mm. The period of the interference pattern resulting from such set-up was from 1.5 to 2.25 μm. The combination of DOE and confocal imaging system ensures that the femtosecond pulses overlap over their entire cross section on the sample . This makes it possible to ablate large areas while using rather large intersection angles between the beams on the sample. This interferometric ablation setup is also described in details in Refs. [14, 15]. For comparison to the structures produced using interferometric setup, we also generated absorption enhancing surface modifications using traditional method [2, 5]. For this the femtosecond laser light is focused at normal incident to the surface so that the ablated spot size is equal to the one used in the intererometric set-up.
The mechanically polished steel and copper surfaces were used as a sample materials. A 4 × 4 mm2 area of structured surface was manufactured by moving beam with diameter of 350 μm over the sample surface using stop-motion technique so that the ablated spots slightly overlap on the edges. In results presented here, the number of pulses on one spot was 200. The average fluence was 0.2 J/cm2. Here the average fluence is defined as used energy divided by ablated surface area. Note, that although the average energy per unit area is same for both techniques, the peak fluence is four times higher using interferometric ablation. This enables formation of deeper surface structures and consequently higher absorptance. Figure 2 shows the nanostructures and LIPSS generated on the stainless steel sample using focused light and above mentioned fluence and pulse number. In Fig. 3 are shown corresponding structures obtained using interferometric setup. The hole-array structures are clearly deeper than nanostructures obtained using direct focusing due to higher peak fluence when using interferometric ablation. The depth of the holes appears to be essential for our sample to act like an absorber. Using fewer pulses and consequently shallower holes, the structures act as a normal diffraction grating, but when holes get deeper the surface stops diffracting and starts absorbing. The holes shown in the Fig. 3 are approx. 2.5 μm deep. The changing of the grating period from 1.5 to 2.2 μm does not bring any radical changes to the absorptance enhancement.
The reflectance spectrum of the modified sample surface was obtained using a Perkin-Elmer Lambda 18 spectrophotometer together with Labsphere RSA-PE-18 integrating sphere. The use of the integrating sphere allowed us to compare the reflectance spectra of the polished surface with specular and structured surface with diffuse reflection. In Fig. 4 are shown the measured reflectance spectra for polished stainless steel sample and surfaces shown in the Figs. 2 and 3. From the figure one can see that by using the interferometric ablation the steel can be rendered virtually black having only 5-8 % reflectance over the whole visible spectrum of light. The reflectance of the steel sample drops to 10-30 % using surface structures obtained by direct focusing of the femtosecond light. Note, that the reflectance is saturated to this value and it cannot be diminished by applying more pulses using above mentioned fluence. In case of the interferometric ablation using higher fluence values can reduce the reflectance only by few percents. However, with higher fluence this saturation level can be reached using smaller number of pulses. When using direct focusing the reflectance can be reduced near to the saturation level obtained using interferometric ablation, but value of several times higher average fluence must be applied compared to the interferometric ablation.
For copper the changes in absorptance using above mentioned fluence and pulse number are also clear, but less dramatic. The holes are shallower than for steel sample. This is probably due to the almost negligible initial absorption of 800 nm for copper. However, we see no technical constraint for rendering copper black by using higher number of pulses. The reflectance spectrum of the modified sample surface for copper is shown in the Fig. 5.
The reason for this enhancement in absorptance due to the hole-structure is yet unclear. The diffraction theory for ideal hole structure does not support such enhancement of absorptance. Theoretically this kind of metal grating structures can be handled using Fourier modal method, also known as the rigorous coupled-wave approach [16, 17, 18]. Using fourier modal method we have calculated the absorption for such metal grating having cylindrical holes. Calculated results are shown in the Fig. 6 for different hole depths. Parameters used in the calculation are grating period of 2.1 μm and hole diameter of 1.9 μm. Although the grating starts to absorb when holes gets deeper, total absorption is not achieved like in measurements. We believe that the observed total absorption is due to the combined effects of multiple reflections in cylindrical cavities and nanoscale structuring acting as an effective refractive index medium. However, incorporation of such nanostructures to the theoretical model is not straightforward and cannot be presented here.
We have demonstrated the new efficient technique to enhance the absorptance of metals. Total absorption over whole visible region of the spectrum was obtained for stainless steel sample using only modest values of pulse number and fluence. This technique is based on interferometric ablation for structuring hole-array to the sample surface and is more efficient and controlled than just focusing the light to the surface, that have been previously used for absorptance enhancement [2, 3]. The technique may be further improved by optimizing the size of the holes.
This work was supported by TEKES, Finnish Funding Agency for Technology and Innovation. The author acknowledge the Network of Excellence on Micro-Optics (NEMO). http//www.micro.optics.org.
References and links
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