1. Introduction

Femtosecond laser-matter interactions have attracted broad interests in both fundamental [1] and applied [2] research. Despite of extensive studies, however, many fundamental aspects still remain unclear. For example, it is commonly believed that one of the most important advantages of femtosecond laser ablation is that the energy deposited by the ultrashort laser pulses does not have enough time to move into the bulk sample during the pulse. Therefore, all the absorbed pulse energy is used for ablation and the residual thermal energy remained in the bulk sample should be negligible. More recently, however, we found that a significant amount of residual thermal energy is deposited in metal samples following multi-shot femtosecond laser ablation [3]. One contributing factor for the multi-pulse induced enhancement in energy deposition is shown to be the increased absorptance due to surface structural modifications, but we have shown that alone cannot fully account for the observation [3, 4]. In this paper, to eliminate the effects of absorptance enhancement due to surface structural modifications, we perform a systematic study of energy coupling to metals following single-pulse femtosecond laser ablation. Based on the multi-shot study, we would expect that the residual thermal energy coupling should be reduced following single-pulse ablation since the enhanced absorptance does not play a role anymore. Surprisingly, a significantly enhanced residual energy is also observed following single-pulse ablation in various gases. This implies that, in addition to the direct absorption of laser energy, other mechanisms may play roles in the significantly enhanced residual energy coupling found in this current study. Since energy absorption is the first step and the foremost question in laser-matter interactions, it is critically important to further study the physical mechanism of the enhanced thermal energy deposition following femtosecond laser ablation.

2. Experimental setup

Experimentally, we use an amplified Ti:sapphire laser system generating 65-fs pulses with pulse energy up to 1.5 mJ at a 1-kHz repetition rate with the central wavelength at 800 nm. A fast electromechanical shutter is used to select a single pulse. Laser beam is focused onto a mechanically polished bulk sample at normal incidence. The residual thermal energy coupling following femtosecond laser ablation is measured as follows. When a solid sample is ablated with a single femtosecond pulse, a fraction of the absorbed laser energy is carried away by the ablated material while the rest of the absorbed energy is retained in the sample. This retained energy dissipates into the bulk of the sample and remains as residual thermal energy. Additionally, residual energy can also include other contributions, such as re-deposition of ablated material back onto the sample surface and exothermic chemical reactions. To characterize residual thermal energy deposition, we define a residual energy coefficient (REC), K, as K=E_R/E_I, where E_R is the residual energy remaining in the sample and E_I is the incident pulse energy. To measure E_R, we use a calorimetric method that has been described in our previous multi-pulse studies [3,4]. For single-shot experiment, we have improved both the sensitivity and accuracy of our experiment. To improve our calorimeter sensitivity, we use thin foil samples to reduce the heat capacity of the calorimeter. We have also upgraded the sensor pre-amplifier by using ultra-stable resistors with low-temperature resistance coefficient of 2 ppm/°C, and also place our calorimetric head into an isoperibol enclosure to minimize environmental thermal interferences. This gives us a 10% measurement uncertainty for residual energy E_R>0.8 mJ. There could be a greater uncertainty for measuring lower residual energy. However, this is relatively unimportant because the change of residual energy to be discussed later in our paper is much larger than our experimental uncertainty. To measure the laser pulse energy, E_I, incident upon the sample, a fraction of the incident pulse energy is split off by a beamsplitter and diverted to a pyroelectric joulemeter. The pyroelectric joulemeter used (Coherent model J25LP-4A) has a ±2% accuracy down to 1 µJ level. The measurement error of E_I is estimated to be about 5%. In order to optimize the accuracy of our pulse energy measurement, we vary laser fluence by only changing the distance between the focusing lens and the sample rather than reducing the pulse energy. In this study, all the measurements are performed with the sample placing before the focal plane, and a lower fluence is obtained by placing the sample closer to the focusing lens.

3. Results and discussion

We study REC as functions of laser fluence F, pressure P, and different types of ambient gases. Figs. 1 and 2 show the residual energy coefficient versus laser fluence, K(F), at various pressures of ambient air for Zn and Pt, respectively. The ablation and plasma formation thresholds determined in 1-atm air are also indicated in Figs. 1 and 2. The ablation threshold is determined visually as the onset of the surface modification and subsequently verified under an optical microscope. The plasma formation threshold F_pl is determined by the onset of violet radiation from the ablated area [2, 5]. By definition, REC should be equal to the absorptance of a metal when it is irradiated by low-fluence laser light that does not cause any surface modification. We can see from Figs. 1 and 2 that, in the range of low fluence below the ablation threshold, REC is independent of fluence and air pressure and has a value of 0.44 and 0.34 for Zn and Pt, respectively. These values agree well with the table absorptance values of 0.39 and 0.3 for mechanically polished bulk samples of Zn and Pt, respectively [6, 7]. When fluence is above the ablation threshold, REC decreases with F for the two metals both in air and in vacuum since a fraction of the absorbed laser energy is carried away by the ablated material. The behavior of decreasing REC remains similar for both air and vacuum up to a certain fluence value before the two curves start to deviate from each other. In air, REC starts to abruptly increase, while in vacuum it continues to decrease. In Figs. 1 and 2, we mark the threshold value where REC begins to increase in air as F_enh. From Figs. 1 and 2, REC for Zn and Pt reaches a maximum value of about 0.9–0.95 at F=10 J/cm², i.e., about 90–95% of the incident laser energy is retained in the sample as residual thermal energy following femtosecond laser ablation in air. In contrast to the ablation in air, we observe a significant decrease (by a factor of about 1.6 for Pt and 4 for Zn) of residual thermal energy coupling in vacuum. Data in Figs. 1 and 2 indicate that there are two distinct regimes of ablation in a gas medium. In the lower fluence regime where F_abl<F<F_enh, ambient gas does not appear to play an essential role since the dependence of REC on fluence in air is virtually the same as in vacuum. However, in the higher fluence regime where F>F_enh, the ambient gas appears to play an important role since the dependence of REC on fluence in air is very different from that in vacuum.

 

Fig. 1. Residual energy coefficient for zinc as a function of laser fluence in 1-atm air and in vacuum (P=0.01 Torr). The thresholds marked in the plot are determined in 1-atm air.

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Fig. 2. Residual energy coefficient for platinum as a function of laser fluence at various air pressures. The thresholds marked in the plot are determined in 1-atm air.

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Figure 2 also shows the measurement of K(F) for Pt at P=1 Torr. We can see that the enhanced residual thermal energy deposition, although less than that in 1-atm air, occurs also at this reduced air pressure. To determine the effect of different ambient gas on residual thermal energy deposition, we measure K(F) dependence in various inert gases, such as He, Ne, and Ar at atmospheric pressure, and we find that the enhanced thermal coupling occurs in all three gases (see Fig. 3). We also perform similar measurements with other metals, including Ti, Bi, and Sn, and all these metals exhibit a similar general behavior in residual energy coupling as in Zn and Pt. This leads us to conclude that the enhanced residual energy deposition is a general phenomenon for femtosecond laser ablation of metals in a gas medium.

The thermal response of metals following high-intensity laser irradiation has been studied in the past with microsecond [8] and nanosecond [9] laser pulses, and enhanced thermal coupling to metals has been found in those experiments at laser intensities above the plasma formation threshold. To explain those observations, it has been suggested that energy transferring to a metal sample from laser-induced plasmas plays a key role [8]. Although the enhanced energy coupling occurs above the plasma formation threshold in our experiment, the plasma-assisted mechanisms in long-pulse experiments do not necessarily apply to ultrashort femtosecond pulses for the following reasons. First, there is no interaction of femtosecond laser pulse with ejected material because the hydrodynamic expansion of the ablated material occurs long after the termination of the femtosecond pulse. Secondly, most of the common plasma phenomena for longer pulse ablation, such as laser-supported combustion and detonation waves [10], cannot be induced by a femtosecond laser pulse. In general, two types of plasmas can be produced during a femtosecond laser pulse: (1) solid-density plasmas in the surface layer of the sample and (2) ambient gas plasmas. Solid-density plasmas can be induced in both air and vacuum; in our experiment, however, the enhanced thermal coupling occurs only in air and therefore, we believe solid-density plasmas do not play a key role in the enhanced energy coupling and we will subsequently focus our discussion on the effects of ambient air plasmas.

 

Fig. 3. Residual energy coefficient for platinum as a function of laser fluence in He, Ne, and Ar at a pressure of 1.08 atm.

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In our experiment, the enhanced thermal energy coupling in 1-atm air is observed in the intensity range of 10¹³–10¹⁴ W/cm². At these intensities, direct ionization of the ambient gas can occur via multiphoton and tunneling ionization during a femtosecond laser pulse [11, 12], i.e. prior to hydrodynamic expansion of ablated material. Furthermore, a metal surface can facilitate air ionization in the following two ways. First, an abundant amount of electrons at our intensity range can escape the metal surface through multiphoton photoelectron and thermoionic emission [13]. Secondly, if solid-density plasma is induced by a femtosecond laser pulse, this plasma will emit UV radiation. Therefore, besides the direct ionization by strong laser fields, additional air ionization can be produced by both energetic electrons and UV photons produced from the metal surface. We note that the air plasma formation prior to ejection of material from the sample has been observed in the past using picosecond laser pulses [14]. The ionization of the ambient gas during a femtosecond laser pulse can generate a high-pressure plasma layer near the sample surface. The high-pressure of the air plasma will remain acting on the sample surface for a time τ required for the rarefaction wave to travel from the plasma periphery to the center of the irradiated spot. The time τ is given by [10] τ=r/a_p, where r is the radius of the irradiated spot and a_p is the sound speed in the plasma. For our experiment, τ is estimated to be on the order of 10 ns with r=50 µm and a_p=5 km/s [15], and this agrees with the time delay between the femtosecond pulse and the onset of material ejection measured to be 5–50 ns [16]. The generation of the high-pressure air plasma can affect the thermal energy coupling to the sample in the following two ways. First, a certain amount of the laser energy stored in the plasma can be transferred to the metal sample during and after the laser pulse. Secondly, the high-pressure of the ambient gas plasma applied to the sample surface after the pulse can restrain the expansion of the ablated material, and this will enhance the thermal energy coupling to the sample due to an enhanced redeposition of the ablated material compared with the vacuum. To verify this speculation, we take SEM images of the redeposited material on the Pt sample in both air and vacuum. Since the redeposition by a single pulse is very difficult to observe clearly, we use multi-pulse measurements but with a long time delay between successive pulses (~1 s) so that the plasma effects from a previous pulse will fully relax and the redeposition from multi-pulse ablation can be considered as an accumulation of single-pulse ablation. From Fig. 4, we can see that the amount of the material redeposited around the crater in air is much greater than in vacuum. This SEM study leads us to conclude that one of the channels for the enhanced residual thermal coupling in air is the enhanced redeposition of the ablated material back onto the sample surface. However, to account for the large enhancement in REC in air (~95% in Pt), the energy contained in the ablated material should be very large, and this requires an enhanced energy deposition into the irradiated area on the sample. Such an enhancement can be due to both energy transfer from the air plasmas as discussed above and an enhanced light absorption by the sample at the intensity range of 10¹³–10¹⁴ W/cm² resulting from the formation of high-density plasma of the metal sample as reported in Ref. [1]. Thus, we suspect that the enhanced thermal coupling in air is due to both energy transfer from air plasma and enhanced redeposition of ablated particles. Finally, we would like emphasize that our above analyses are consistent with our observation of the existence of the two distinct regimes of ablation in a gas medium as discussed earlier. In the lower fluence regime where F_abl<F<F_enh, ambient gas does not appear to play an essential role since the dependence of REC on fluence in air is virtually the same as in vacuum. We believe that this is because the gas is mostly neutral or only weakly ionized in the lower fluence regime and the absorption of laser light is low. However, in the higher fluence regime where F>F_enh, the ambient gas plays an important role as the dependence of REC on fluence in air is very different from that in vacuum. We believe that this is because the ambient gas becomes highly ionized in the high fluence regime and starts to play a significant role in ablation and thermal energy coupling. However, further extended studies such as dynamics of both air plasma and ablated material plasma, mass and temperature of redeposited material, the effects of dielectric constant change of the sample material on the direct absorption of laser light by the sample, absorption of laser energy in the air plasma-sample system, and the effects of ambient gas plasma on beam propagation are needed to fully understand mechanisms of enhanced thermal coupling following ablation in a gas medium.

 

Fig. 4. SEM images of the redeposited material on Pt samples following multi-pulse ablation at F=1.4 J/cm² in (a) vacuum and (b) air. The amount of material redeposited around the crater in air is greater than in vacuum.

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4. Conclusions

In summary, we perform a systematic study of residual energy coupling to metals following single-pulse femtosecond laser ablation. A significantly enhanced residual energy is observed following single-pulse ablation in various gases but not in a vacuum. Therefore, for ablation in a gas medium, our observation is in direct contrast with the previous belief that residual energy deposition is negligible following femtosecond laser ablation. The ambient gas plasmas produced near the sample surface is believed to play a key role in the enhanced residual energy coupling. Although further studies are necessary along this direction, our study reveals, for the first time, an enhanced energy coupling in high-intensity femtosecond laser-metal interactions and provides new guidelines for a broad range of technological applications, such as designing optimal thermal loading conditions in femtosecond laser micro- and nano-machining.