A comparative study on reflection of nanosecond Nd-YAG laser pulses in ablation of aluminum in air and in vacuum under the same other experimental conditions is performed. We find that, hemispherical total reflectivity of aluminum undergoes a sharp drop at the plasma formation threshold both in the air and in vacuum. The initial large value (0.8) of aluminum reflectivity decreases to a level of about 0.14 and 0.24 for ablation in the air and in vacuum, respectively. These decreased reflectivity values remain virtually unchanged with further increasing laser fluence. The reflectivity drop in the air is observed to be sharper than in vacuum. Our study indicates that the reflectivity drop is predominantly caused by absorption of the laser light in plasma. Nano/micro-structural defects present on practical sample surfaces play the important role in the plasma formation, especially for the ablation in the air, where the plasma formation threshold is found to be by a factor of 3 smaller than in vacuum.
©2013 Optical Society of America
Nanosecond laser ablation  is widely used in many applications such as film deposition [2,3], manufacturing of nanomaterials [4,5], surface micro/nanopatterning [4,6–11], biomedicine , diamond-like materials , laser marking , microfluidics , and others. A very recent advancement is nanosecond laser blackening of metals  that has been developed following femtosecond laser blackening of metals [17–19]. Although the nanosecond laser ablation has been extensively studied for many years, a number of its physical processes are not yet well understood. This motivates further research efforts [20,21]. Among the nanosecond lasers, the Nd:YAG lasers are the most widely used tools in processing of materials. For further advancement in processing of materials with the nanosecond Nd:YAG lasers, a more comprehensive knowledge on ablation is needed. Absorption/reflection of laser light from the irradiated sample is a key physical process in ablation. However, this process is not yet well studied [22–24]. The reflection of high-intensity nanosecond Nd:YAG laser pulses by metals has been first studied in a work by Basov et al. , where a significant decrease of the total hemispherical reflectivity to a level of about 0.1 was observed for ablation in a vacuum at a laser intensity of 3 × 1010 W/cm2. In a study , time-resolved specular reflection of 60-ns Nd:YAG laser pulses from copper and tantalum has been investigated for ablation in vacuum at laser intensities in the range of 106 –109 W/cm2. A significant decrease of the specular reflection was reported. The reflection of 30-ns Nd-laser pulses in ablation of a thin aluminum film in a vacuum at laser fluence in the range of 0.3 – 400 J/cm2 has been studied in , where the reflected laser light was collected over a solid angle of about 1 sr. A significant drop of the reflection observed in this experiment was explained by the formation of a plasma core near the sample surface. Previous studies performed on the reflection of high-intensity nanosecond laser pulses show an important role of the laser-induced plasmas. It is known that plasma phenomena for ablation in vacuum and in air (or other ambient gas) significantly differ. In ablation under vacuum conditions, the plasma is composed of only ionized species of the ablated material. While for ablation in the air, the plasma consists of ionized species of both the ablated material and air. Furthermore, plasma dynamics is also significantly differs due to laser-supported absorption waves generated in the air [25,26]. These distinctions in the plasma properties may cause distinctions in the reflection behaviors for ablation in vacuum and air. In this work, we perform a comparative study on the reflection of the nanosecond Nd-YAG laser pulses in ablation of aluminum in air and in vacuum under the same other experimental conditions. Our study shows essential differences in the reflection of the laser light for ablation in the air and in vacuum.
The schematic of the experimental setup used in this study is shown in Fig. 1. A Q-switched Nd:YAG laser that generates 50-ns FWHM (Full Width at Half-Maximum intensity) pulses at a wavelength of 1064 nm is used for ablation of a sample. The laser beam is focused onto the sample with a lens. To collect the reflected laser light we use an ellipsoidal light reflector [27,28]. The sample is positioned in an internal focal point of the ellipsoidal reflector and tilted relative to the laser beam axis for reducing laser light backscattering through the entrance hole in the reflector. Both the reflector and sample are placed in a vacuum chamber pumped to a pressure of 3.6 × 10−3 Torr or filled with air at atmospheric pressure. Energy of the reflected laser pulse, Erefl, is measured using an energy meter located in the external focal point of the ellipsoidal reflector. A cutoff filter placed in front of the energy meter blocks the plasma radiation. Since the ellipsoidal reflector absorbs a small amount of the laser light reflected from the sample and there are also losses due the chamber rear window and plasma radiation cutoff filter, we calibrated our measuring setup at low laser fluence using mechanically polished metal samples, the reflectivity of which was measured using a Perkin-Elmer Lambda 900 spectrophotometer with an integrating sphere. Laser pulse energy incident upon the sample, Einc, is measured using an 8%-beamsplittter and energy meter. The hemispherical total reflectivity, R, (a sum of specular and diffuse components of the reflected light) is found as R = Erefl/Einc. The incident laser fluence, F, is determined by dividing the incident laser pulse energy by the laser spot area. The laser fluence incident upon the sample is varied by inserting calibrated attenuation filters and changing the distance between the focusing lens and sample. The total reflectivity is studied in a laser fluence range between 0.2 and 200 J/cm2. After each laser shot, the studied sample is translated to a fresh spot on the sample surface. The studied metal is mechanically polished bulk aluminum. In this study, we also determine both surface damage and plasma formation thresholds. We find the surface damage threshold as the lowest laser fluence that causes a surface damage discerned under an optical microscope. The plasma formation threshold is determined similar to a work  by detecting the onset of a bright violet flash from the irradiated spot using a photomultiplier with a filter that blocks the wavelengths longer 0.45 μm. Time-integrated photography is used to take images of laser-induced plasma plumes . The room-temperature total reflectivity of a mechanically polished aluminum sample at the laser wavelength of 1064 nm is measured to be 0.81 using the Perkin-Elmer Lambda 900 spectrophotometer with an integrating sphere.
3. Results and discussion
Figure 2 presents the plots of the total reflectivity of aluminum as a function of laser fluence for ablation in air and in vacuum. At low laser fluences, the values of the reflectivity are the same for the air and vacuum and equal to 0.8 in an agreement with the room-temperature reflectivity measured with the Perkin-Elmer Lambda 900 spectrophotometer. This reflectivity value does not change with increasing laser fluence from 0.2 J/cm2 to Frefdrop = 1.1 J/cm2 and 3.1 J/cm2 in air and in vacuum, respectively. Above these threshold fluences, the reflectivityundergoes a sharp drop to a level of about 0.14 and 0.24 in the air and in vacuum, respectively. These low values of the reflectivity remain virtually unchanged with further increasing laser fluence up to the highest studied laser fluence value of about 200 J/cm2. The plasma formation thresholds were found to be Fpl = 1.1 J/cm2 and 3.1 J/cm2 in the air and vacuum, respectively. Similar values of Frefdrop, and Fpl indicate that the reflectivity drop is associated with the plasma onset for ablation both in the air and in vacuum. The damage thresholds were determined to be Fdam = 1.1 J/cm2 and 2.1 J/cm2 in the air and in vacuum, respectively. Therefore, our experiments reveal the following relations, Fdam ≈Fpl ≈Frefdrop for air and Fdam < Fpl ≈Frefdrop for vacuum.
To explain these observations, we compute the surface temperature at the damage thresholds Fdam = 1.1 J/cm2 and 2.1 J/cm2 for the ablation in the air and in vacuum, respectively, using the following formula 32]. Furthermore, nanostructural defects typically present on the real surfaces can be also heated to a high temperature due to plasmonic absorption and nanoheating . These “hot micro/nanospots” on the cold (on average) surface can thermionically emit priming electrons for triggering an avalanche air optical breakdown through their acceleration via inverse-bremsstrahlung mechanism. The other factors, which facilitate the optical breakdown of the air in front of a metal surface, are the constructive interference of the incident and reflected laser beams  and high-field emission of priming electrons from nano- and micro-protrusions. The study  shows that the optical breakdown of the air near the metal surface results in a significant thermal energy transfer from the air plasma to the metal surface. On the contrary, there is no noticeable thermal energy transfer to the sample from the plasma formed in vacuum. Therefore, our observation that the damage threshold in air (Fdam = 1.1 J/cm2) is significantly lower than that in vacuum (Fdam = 2.1 J/cm2) can be explained by “plasma-assisted etching” of the sample in air due to enhanced thermal coupling [29,34].
Our observation that Fpl ≈Frefdrop for ablation both in the air and in vacuum indicates that the reflectivity decrease is associated with the plasma formation in front of the irradiated sample. Figure 2 shows that the reflectivity drop is more abrupt in the air than in vacuum. Furthermore, the reflectivity decreases to a significantly smaller value in the air than in vacuum. These observations are explained by an essential distinction of the plasma plumes generated in the air and in vacuum. This distinction is clearly seen in Fig. 3 that shows time-integrated photographs of luminous plasma plumes produced in the ablation of aluminum in the air and vacuum under the same other experimental conditions. It is seen that the size of plasma in air is significantly larger than that in vacuum despite the fact that the ablated material has no resistance to expand into vacuum. The larger size of the plasma plume in the air is explained by the generation of laser-supported absorption waves (laser-supported combustion wave and laser-supported detonation wave) propagating in the air [25,26]. Webelieve that both more abrupt and more significant drop of the reflectivity in the air is caused by these laser-supported absorption waves that increase the plasma optical thickness. Under conditions of the plasma presence in front of the sample and low laser light scattering from the ablated material, the reflection of the laser beam occurs from a sample-plasma system as schematically shown in Fig. 4 for ablation both in the air and in vacuum. Therefore, the time-integrated reflectivity of sample-plasma system measured in our experiment is given byEq. (2) shows that R depends on both the total optical thickness of the plasma θ and the surface reflectivity Rs. In the absence of plasma, the Eq. (3) reduces to R = Rs. In general, the surface reflectivity Rs depends on both the surface temperature Tsurf and the change of the surface morphology during the laser pulse. The interband absorption of aluminum occurs at the wavelength of 800 nm. Therefore, at the Nd:YAG laser wavelength (1064 nm), the absorption/reflection of aluminum is described by Drude’s free-electron model. In the near infrared, the Drude temperature-dependent reflectivity of an ideally smooth and clean metal surface is given by 28], the surface irradiated with nanosecond laser pulses undergoes surface relief changes during the laser pulse. In other words, the reflection of the nanosecond laser light occurs from a non-stationary plasma-sample interface that makes the study of Rs a very complicated task. To our knowledge, an only attempt to gain insight into the behavior of Rs has been previously reported in , where the value of Rs above the plasma formation threshold was estimated to be about the same as the value of Rs below the plasma formation threshold in ablation of copper with nanosecond ruby laser. If this behavior of Rs holds for Nd-YAG laser pulses as well, then the drop of R in our study is predominantly caused by absorption of the laser light in plasma.
In this study, we perform a comparative study on the reflection of nanosecond Nd-YAG laser pulses in the ablation of aluminum in the air and in vacuum under the same other experimental conditions. Our study shows that the reflectivity of mechanically polished aluminum remains virtually equal to the table room-temperature value with increasing laser fluence up to the plasma formation threshold. The values of the plasma formation threshold were found to be significantly different for the ablation in air (Fpl = 1.1 J/cm2) and in vacuum (Fpl = 3.1 J/cm2). At the plasma formation threshold, the reflectivity begins to drop both in the air and in vacuum. With increasing laser fluence above the plasma formation threshold, the reflectivity undergoes a sharp drop to a level of about 0.14 and 0.24 in the air and in vacuum, respectively. These reflectivity values remain virtually unchanged with further increasing laser fluence up to the highest studied fluence value of about 200 J/cm2. Furthermore, the reflectivity drop in the air is sharper than in vacuum. Our study indicates that the surface nano/micro-structural surface defects play the important role in the plasma formation, especially for the ablation in the air, where the plasma formation threshold is found to be by a factor of about 3 smaller than in vacuum.
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