Hemispherical total reflectivity of copper, nickel, and tungsten in ablation by nanosecond Nd:YAG laser pulses in air of atmospheric pressure is experimentally studied as a function of laser fluence in the range of 0.1–100 J/cm2. Our experiment shows that at laser fluences below the plasma formation threshold the reflectivity of mechanically polished metals remains virtually equal to the table room-temperature reflectivity values. The hemispherical total reflectivity of the studied metals begins to drop at a laser fluence of the plasma formation threshold. With increasing laser fluence above the plasma formation threshold the reflectivity sharply decreases to a low value and then remains unchanged with further increasing laser fluence. Computation of the surface temperature at the plasma formation threshold fluence reveals that its value is substantially below the melting point that indicates an important role of the surface nanostructural defects in the plasma formation on a real sample due to their enhanced heating caused by both plasmonic absorption and plasmonic nanofocusing.
© 2011 OSA
Nanosecond laser ablation of solids is used in numerous applications such as laser processing of materials , pulsed-laser film deposition [2,3], manufacturing of nanomaterials , surface micro/nanostructuring [4–14], laser-induced breakdown spectroscopy [15,16], fabrication of diamond-like materials [17,18], biomedicine , and others. Although the nanosecond laser ablation was a subject of many studies, the reflection of high-intensity nanosecond laser pulses remains a poorly studied issue. In the past, the reflection of intense laser pulses by metals was first studied by Bonch-Bruevich et al. , where a substantial drop of the total reflectivity during submicrosecond spikes in a millisecond Nd-glass laser pulse was experimentally found. For studying reflection, the authors used an integrating sphere technique that allowed measuring the total reflectivity (both specular and diffuse components). Basov et al.  studied the total reflection of 15-ns Nd-laser pulses from Cu, Sn, and Al in a laser intensity range of 3 × 107 to 3 × 1010 W/cm2 and reported a sharp decrease of the total reflectivity to ~0.1 for ablation in a vacuum. Ready  investigated the time-resolved reflection of metallic surfaces during irradiation by 100-ns-duration pulses from a CO2-TEA laser and showed that the specular reflectivity reduction is mostly due to redistribution into diffuse reflectivity. The behavior of the specular reflection during 60-ns Nd:YAG laser pulses in ablation under vacuum conditions has been reported by Zavecz et al. . However, the interpretation of the experiments on the specular reflection is complicated because surface structures induced during ablation can significantly enhance the diffuse component of the reflected light . Dymshits  studied the reflection of a 30-ns Nd-laser pulse from a thin aluminum film ablated in vacuum. The reflected laser light was collected over a solid angle of about 1 sr. Vorob’ev  carried out a comparative study on the hemispherical total reflection of 45-ns ruby laser pulses in ablation of Cu in air and vacuum. Both time-resolved and time-integrated experiments showed a substantial decrease of the total reflectivity at the plasma formation threshold. At laser fluence above about 15 J/cm2, the time-integrated reflectivity of copper was measured to be about 0.3 and 0.2 in vacuum and air, respectively. At present, nanosecond Nd:YAG laser is the most widely used laser for ablation of materials as compared with other nanosecond lasers. In many applications, nanosecond laser ablation of materials is carried out in air of atmospheric pressure. However, as seen from previous works [20–25], the reflection/absorption of the nanosecond Nd:YAG laser pulses for ablation of the materials in air has not been yet studied despite a crucial role of the reflection/absorption in ablation.
In this work, we investigate the hemispherical total reflection of the nanosecond Nd:YAG laser pulses in ablation of Cu, Ni, and W into air of the atmospheric pressure. The hemispherical total reflection is studied as a function of laser fluence in a range of 0.1–100 J/cm2. In our study, we use the samples with the surfaces that are not ideal and have initial surface defects, impurities, oxides, and adsorbates (as in many practical cases of materials processing). The obtained experimental results show that the reflectivity of metals begins to drop at a laser fluence of the plasma formation threshold. With increasing laser fluence above the plasma formation threshold, the reflectivity of the studied metals drops rapidly to a low value of about 0.10–0.15 that remains unchanged with further increasing laser fluence. The computation of the surface temperature at the plasma formation threshold shows that its value is below the melting point that indicates an important role of the surface imperfections in the plasma formation on the real samples.
The experimental configuration is sketched in Fig. 1 . An Nd:YAG laser that produces 1.3 J in a 50-ns FWHM (Full Width at Half-Maximum intensity) pulse is used for ablation of the samples. The main laser beam is focused onto the sample with a lens. Reflected laser light is collected using an ellipsoidal light reflector technique [25, 26]. The sample is positioned in an internal focal point of the light reflector. To reduce laser light backscattering through the entrance hole in the ellipsoidal reflector, the laser beam incident upon the sample is aligned at 19 degrees from the normal to the sample. Energy of the reflected laser pulse, Erefl, is measured by an energy meter located in the external focal point of the reflector. A cutoff filter is placed in front of this energy meter for blocking the plasma radiation. Energy of the laser pulse incident upon the sample, Einc, is measured using an 8%-beamsplittter and energy meter. Using this method, the total reflectivity, R, (a sum of specular and diffuse components of the reflected light) can be found as R = Erefl/Einc. Laser fluences, F, in the range of 0.1–100 J/cm2 are set by both attenuation filters and varying the distance between the focusing lens and sample. All experiments are carried out in air of the atmospheric pressure. After each laser shot the sample was translated in order to expose an undamaged surface area to the next laser shot. Along with the reflection measurements, a surface damage and plasma formation thresholds are also determined. The surface damage threshold is found as the lowest laser fluence resulting in a surface damage that can be discerned under an optical microscope. The plasma formation threshold is determined by detecting the onset of a bright violet flash from the irradiated spot  using a photomultiplier (PMT) with a filter that blocks wavelengths longer 0.45 μm. The samples were mechanically polished and then cleaned with a tissue wetted with methanol.
3. Results and discussion
The plasma formation thresholds averaged over ten measurements were measured to be 2.05, 0.9, and 0.95 J/cm2 for Cu, Ni, and W, respectively. The values of the damage threshold were found to be only slightly lower than those for the plasma formation threshold. Similar relation between the damage and plasma formation thresholds was previously observed in ablation of Al . The reflectivity as a function of laser fluence in ablation of Cu, Ni, and W in air of the atmospheric pressure is shown in Fig. 2 . It is seen that the reflectivity of the studied metals remains constant at low laser fluences. At these low fluences, the irradiated surface does not undergo any surface damage and the reflectivity values are 0.9, 0.72, and 0.6 for Cu, Ni, and W, respectively. These reflectivity values agree with available table values of the room-temperature reflectivity for mechanically polished surfaces [28,29]. The plots of R(F) in Fig. 2 show that the reflectivity begins to decrease rapidly at a threshold fluence of 2.0, 0.9, and 0.9 for Cu, Ni, and W, respectively. These threshold fluences of a sharp reflectance drop coincide with the measured plasma formation thresholds within the experimental uncertainty. As can be seen in Fig. 2, as the laser fluence increases further, the reflectivity drops (to about 0.19, 0.14, and 0.11 for Cu, Ni, and W, respectively) and then remains unchanged with further increasing fluence.
The above observations indicate the correlation between the reflectivity drop and plasma formation. In general, the reflectivity reduction can be caused by Drude’s temperature dependence of the optical constants and absorption of laser light in a laser-induced plasma. In order to ascertain the role of Drude’s temperature dependence of the optical constants on the reflectivity, we computed the surface temperature, Tsurf, of the samples at the plasma formation threshold fluences using the following formula Fig. 3 , where it is seen that the maximum surface temperature is about 210, 500, and 700 °C for Cu, Ni, and W, respectively. These surface temperature values are significantly smaller than the melting points of studied metals (1083, 1453, and 3410 °C for Cu, Ni, and W, respectively). An experimental study  showsthat the reflectivity of a polished Cu sample smoothly decreases by about 2% in a temperature range of 20–400 °C. Experimental data on the absorptivity of tungsten  show an increase of the absorptivity from 0.38 to 042 in the temperature range between 20 and 2100 °C; and this absorptivity increase is smooth (without any significant change at the temperature of about 700 °C). The reflectivity of liquid nickel is 0.68  that is slightly smaller the table value of the room-temperature reflectivity (0.72) . Hence, the reflectivity drop occurring in our experiment at 210, 500, and 700°C on Cu, Ni, and W surfaces cannot be explained by Drude’s temperature dependence and is indeed caused by the plasma effect. The plasma formation observed in our experiment at low surface temperature indicates that the imperfections on the sample surface play an important role in inducing an optical breakdown. For example, such surface structural defects as nanoscratches, nanoprotrusions, and nanopits commonly present on the mechanically polished surfaces can be locally heated to a high temperature due to plasmonic absorption [33–35] and plasmonic nanofocusing . These “hot nanospots” on cold (on average) surface can be sources of both thermally ionized species and thermionically emitted electrons, which due acceleration through inverse-bremsstrahlung mechanism can trigger an avalanche air optical breakdown. When the plasma forms in front of the irradiated sample, the reflection and absorption of laser light by the sample dramatically changes due to absorption of the laser light in the plasma. For ablation into the background gas, the reflection/absorption of laser energy by the sample is more complicated than in the vacuum due to generation of laser-supported absorption waves (laser-supported combustion wave and laser-supported detonation wave) [37,38]. Under these conditions, the reflection of the laser beam occurs from a sample-plasma system . Assuming negligible laser light scattering from particulates ejected from the sample and negligible reflections at both air/air-plasma and air-plasma/vapor-plasma boundaries, the laser beam reflection will occur as schematically shown in Fig. 4 . Although the plasma reduces the laser energy that arrives at the sample surface, it can contribute to energy deposition into the sample through the transfer of a fraction of its stored thermal energy to the sample [25,27,39].
Previously, a number of theoretical models that include the absorption of laser radiation in the plasma produced by ablation into vacuum and background gas [40–50] have been developed. However, satisfactory understanding of reflection/absorption of the laser energy is still lacking and the value of the laser energy absorbed by the sample actually remains a parameter of intuitive choice. We believe that our experimental data can be useful for further advancing theoretical models of the nanosecond laser ablation.
In this work, the total reflectivity of the mechanically polished Cu, Ni, and W samples in ablation by nanosecond Nd:YAG laser pulses in air of the atmospheric pressure is experimentally studied as a function of laser fluence in the range of 0.1–100 J/cm2. Our experiment shows that at laser fluences below the plasma formation thresholds the reflectivity of the studied metals remains virtually equal to the table room-temperature reflectivity values for mechanically polished surfaces. The total reflectivity of the studied metals begins to drop at the laser fluence of the plasma formation threshold. With increasing laser fluence above the plasma formation threshold, the reflectivity drops sharply to a low value (to about 0.19, 0.14, and 0.11 for Cu, Ni, and W, respectively) and then remains unchanged with further increasing laser fluence. The computation of temperature of the irradiated surface at the plasma formation threshold fluence shows that the surface temperature is substantially below the melting point that indicates an important role of the surface nanostructural defects in the plasma formation on the real sample due to their enhanced heating caused by both plasmonic absorption and plasmonic nanofocusing.
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