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Abstract
Laser-induced damage to the final reflective and diffractive optics limits the total output energy of petawatt laser systems with pulse durations ranging from a few hundred femtoseconds (fs) up to a few tens of picoseconds (ps). In this study, the laser damage to HfO2/SiO2 and Ta2O5/SiO2 multilayer dielectric high-reflectivity (HR) coatings induced by a 1053 nm laser with a pulse width of 8.6 ps was studied to investigate the nano-absorbing precursors in ps regimes. The HfO2/SiO2 HR coating exhibited stronger laser resistance than the Ta2O5/SiO2 HR coating. Flat-bottom pits, pinpoints, and funnel pits were the three typical damage morphologies for the experimental HR coatings. The damage to the HfO2/SiO2 HR coating was primarily dominated by flat-bottom pits, whereas dense pinpoints were the most significant damage for the Ta2O5/SiO2 HR coating. The nano-absorbing precursors introduced by the ion-assisted deposition process were proved to be the damage precursors that trigger pinpoints under a strong electric field intensity (EFI). The nano-absorbing precursors located in the second EFI peak of the SiO2 top layer induced the funnel pits. The funnel pits were expected to be the previous stage of the flat-bottom pits. After they grew along the upward-sloping crack and separated from the interface, the flat-bottom pits were formed. In addition, poor-binding interfaces promoted the formation of flat-bottom pits.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Petawatt lasers with pulse durations that typically range from a few hundred femtoseconds (fs) to a few tens of picoseconds (ps) have important applications in fast ignition, radiation therapy, intense secondary source generation, and particle acceleration [1–3]. These petawatt lasers primarily use chirped-pulse amplification to produce high-energy pulses with short pulse durations [4–6]. Therefore, the limiting factor governing the output power is typically related to the final optics, which includes multilayer dielectric high-reflectivity (HR) coatings [7].
Multilayer dielectric HR coatings operating at a wavelength of approximately 1 µm in petawatt lasers always consist of HfO2 and SiO2 materials [8,9]. To predict the damage performance of meter-scale optics, a raster scan procedure was introduced to evaluate the damage density of small-aperture HfO2/SiO2 HR coatings in hundreds of fs up to tens of ps [8–11]. Damage events originating from low-density nodular defects are believed to occur at fluences significantly lower than the intrinsic laser-induced damage thresholds (LIDTs) at 700 fs for P polarization [9]. In addition to possible nodular ejections, plasma scalds and pinpoints have been observed in the raster scan procedure for HfO2/SiO2 HR coatings at 30 ps for P-polarization [11]. More damage studies on HfO2/SiO2 HR coatings, as well as single layers of HfO2 or SiO2, have been reported for 1-on-1 or N-on-1 test procedures [12–18] . The LIDT of HfO2/SiO2 HR coatings showed an approximate square-root pulse-length scaling in the range of 0.6–100 ps for P-polarization [12]. For pulses shorter than approximately 2.3 ps, the damage exhibited as a well-defined circular profile initiated by electric-field-induced volume breakdown. Between 2.3 ps and 10 ps, venting holes from nano-absorbing precursors near the strong electric field interface between the first HfO2 layer and the second silica layer occurred for S or P-polarized pulses, while shallow quasi-conical craters confined to the SiO2 top layer appeared only for P-polarized pulses [13]. The laser damage growth behaviors of HfO2/SiO2 HR coatings have been investigated based on Gaussian and squared top-hat beam irradiation in the sub-picosecond regime [19–22]. The damage growth threshold independence of the incident beam profile is smaller than the LIDT of the HfO2/SiO2 HR coatings [19,21,22]. This growth behavior is believed to be intrinsic and related to the evolution of the electric field distribution in the sub-ps regime [20].
There are outstanding problems in laser-matter interactions in the regime of a few hundred fs to a few tens of ps. The nano-absorbing precursors with higher density rather than the low-density nodular defects play the dominated role in the ps laser damage. However, current studies on HR coatings primarily focus on the rough transition behavior between intrinsic and defect-dominated laser-induced damage, as well as damage growth in the sub-ps regime, and lack comprehensive and detailed damage characterization related with these invisible nano-absorbing precursors. This results in an unclear understanding of these invisible nano-absorbing precursors, which complicates optimization of the fabrication processes. Moreover, a general design principle for petawatt lasers is to avoid P-polarization reflections, whereas most previous damage studies on HR coatings have focused on P-polarization. In this study, the damage characteristics of HfO2/SiO2 and Ta2O5/SiO2 HR coatings induced by 1053 nm 8.6 ps laser pulses for S-polarization were closely examined to gain insight into the characteristics of these nano-absorbing precursors. The formation reasons for these damage sites were further discussed based on the component analysis and the fundamental damage mechanisms in the picosecond pulse regime. The reduced damage performance of electron-beam (e-beam) ion-assisted deposition (IAD) coating was reported in nanosecond regime [23,24], however the damage morphological differences induced by the IAD process as well the underlying reasons for the reduced LIDTs have not been clarified. Our results proved that the negative influence of the IAD process on the near-infrared 8.6 ps damage resistance originated from the La and B elements introduced by the IAD process.
2. Experiment
2.1 Sample preparation
In this study, HfO2/SiO2 and Ta2O5/SiO2 HR coatings were prepared using e-beam evaporative deposition. The IAD process was accompanied during the deposition of the Ta2O5/SiO2 coatings because the Ta2O5 material is not easily oxidated just using e-beam evaporative deposition [25,26]. The coating stacks of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings were Glass|4 L(HL)^n H4L|Air, where H and L represent high- and low-refractive-index materials, respectively. Both were designed to operate at an angle of incidence of 45 ° and are highly reflective at 1053 nm. The measured spectra of the two HR coatings at an angle of incidence of 45° with S polarization are shown in Fig. 1.
2.2 Experimental setup and method
Laser damage experiments were performed using a 1053 nm laser with a pulse width of 8.6 ps and repetition rate of 1 Hz. The output laser first passes through a variable attenuator and is then focused on the target plane, as shown in Fig. 2. The attenuator comprised a half-wave plate and polarizer. The energy and spatial distributions of each pulse were monitored using two split-off beams reflected by a splitter wedge. The occurrence of damage was determined by an online charge-coupled device (CCD) and was subsequently confirmed by an offline BX53M Olympus optical microscope (OM). The incident laser irradiated the sample with S-polarization at an angle of incidence of 45 °and exhibited a nearly Gaussian distribution at the sample surface. The spot diameter was measured by the method of laser irradiation of a metal target [27] as well as the beam profiler placed at the equivalent optical plane to the location of the sample surface. The beam waist diameter was approximately 117.8 µm at normal incidence. Samples were tested by 1-on-1 procedure. Ten points were irradiated at each energy fluence. The LIDT was obtained by linearly extrapolating the damage probability data to zero damage probability. The minimum fluence that induced 100% damage was referred to as the 100% damage probability threshold. The fluence in our study was given on the coating surface.
A scanning electron microscope (SEM, Zeiss Auriga) combined with a focused ion beam (FIB) was used to characterize the damage topography. The chemical compositions of the coatings were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Fisher). All XPS data were calibrated to the C1s peak at 284.8 eV. The instrument used Ar+ sputter ion beam for depth profiling at a sputtering rate of approximately 0.3 nm/s. The elemental depth profile of the coating was further investigated using time-of-flight secondary ion mass spectrometry (TOF-SIMS, ULVAC-PHI). The instrument had two ion guns (dual-beam depth profiling). We used Bi3+ primary ions as the analysis beam, and an Ar+ sputter ion beam for depth profiling. The sputter rate was approximately 0.23 nm/s, which was slightly different for each coating material. Transmission electron microscopy (TEM, Thermo Fisher) was used for the high-resolution characterization of the internal interface morphology between the layers.
3. Results
3.1 LIDT results
The LIDT results for the experimental samples are shown in Fig. 3. The damage to the HR coatings induced by the 1053 nm 8.6 ps laser pulses exhibited an obvious probability, as shown in Fig. 3(a), demonstrating the dominant role of the defect-driven damage process. The HfO2/SiO2 HR coating exhibited higher damage resistance than the Ta2O5/SiO2 HR coating. The relevant data are summarized in Fig. 3(b). The LIDTs of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings were 4.49 J/cm2 and 3.21 J/cm2, respectively. The 100% damage probability thresholds, which represented the minimum fluence required to induce the intrinsic damage, were 6.56 J/cm2 and 4.26 J/cm2 for HfO2/SiO2 and Ta2O5/SiO2 HR coatings, respectively. The slope of the linear extrapolated damage probability curve was found to be related to the defect density. The slope of the Ta2O5/SiO2 HR coating damage-probability curve was approximately twice that of the HfO2/SiO2 HR coating, indicating that the density of nano-absorbing precursors causing damage was much greater for the Ta2O5/SiO2 HR coating than that for the HfO2/SiO2 HR coating [28,29].
Fig. 3. (a) Damage probabilities of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings. (b) LIDTs, 100% damage probability thresholds and slopes of the linear extrapolated damage-probability curves for the HfO2/SiO2 and Ta2O5/SiO2 HR coatings.
3.2 Damage morphologies
The damage morphologies of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings were characterized, as shown in Fig. 4. In the case of the HfO2/SiO2 HR coating, the typical damage was in the form of flat-bottom pits. The initial damage appears as an isolated flat-bottom pit, occasionally accompanied by a funnel pit, as shown in Fig. 4(a). As the laser fluence increases, more flat-bottom pits occurred in a shot as shown in Fig. 4(b), and finally join together. This results in a large area peeling off the entire surface layer in the irradiated zone, as illustrated in Fig. 4(c). These flat-bottom pits appear similar in size, with the diameters of approximately 5 µm at the top of the pits. However, for the Ta2O5/SiO2 HR coating, the typical damage morphologies are the ubiquitous pinpoints. The initial damage appears as pinpoints, with an occasional funnel pit, as shown in Fig. 4(d). The flat-bottom pits appear subsequently as the fluence increases further, as shown in Fig. 4(e) for the Ta2O5/SiO2 HR coating. The number of pinpoints increases with the irradiation fluence. Similar to the flat-bottom pits, the sizes of these pinpoints seemed to be independent of the irradiation fluence and were approximately 200 nm in diameter. The funnel pits, which were the second damage presented in either the HfO2/SiO2 and Ta2O5/SiO2 HR coatings especially for the lower fluence irradiation, possessed a maximum size of approximately 2 µm at the top of the pit and seemed to originate from a deeper position compared with the pinpoints, as shown in Fig. 4(b) and (e).
Fig. 4. (a-c) SEM images of the damage morphologies for the HfO2/SiO2 HR coating. (d-f) SEM images of the damage morphologies for the Ta2O5/SiO2 HR coating.
The damage to the HfO2/SiO2 HR coating primarily comprised flat-bottom pits. For the Ta2O5/SiO2 HR coating, pinpoints appeared to be more common. The funnel pits were another form of damage, which occurred for both the HfO2/SiO2 sand Ta2O5/SiO2 HR coatings at the irradiated fluence near the LIDTs. Their cross sections were investigated to further understand the formation of these types of damages.
The cross section of the pinpoint for the Ta2O5/SiO2 HR coating presented in Fig. 5(a) is shown in Fig. 5(b). The melting and flow of the material can be observed inside the pinpoint. Statistical cross sections proved that these pinpoints were limited to the first SiO2 layer and did not penetrate to the first interface between the Ta2O5 and SiO2 layers or the first Ta2O5 layer.
Fig. 5. (a) SEM image of a pinpoint for the Ta2O5/SiO2 HR coating. (b) Cross section along the white dashed line in (a).
The cross section of the slightly damaged flat-bottom pit for the HfO2/SiO2 HR coating presented in Fig. 6(a) is shown in Fig. 6(b). The bottom was a circular plate located at the interface between the SiO2 top layer and first HfO2 layer, and there was a small loss of HfO2 materials at the center of the flat-bottom pits. The sidewalls were sloping and smooth and with some splashes of the adherent molten materials. The cross section of the flat-bottom pit for the Ta2O5/SiO2 HR coating in Fig. 6(a) is shown in Fig. 6(b). The bottom layer displayed significant signs of melting and often extended into the first Ta2O5 layer.
Fig. 6. (a) SEM image of a flat-bottom pit for the HfO2/SiO2 HR coating. (b) Cross section along the white dashed line in (a). (c) SEM image of a flat-bottom pit for the Ta2O5/SiO2 HR coating. (d) Cross section along the white dashed line in (c).
The cross section of a typical funnel pit for the HfO2/SiO2 HR coating in Fig. 7(a) is shown in Fig. 7(b). It manifests as a classical funnel shape with a large upper opening and a small lower opening. The bottom of a typical funnel pit in the HfO2/SiO2 HR coating is located at the interface of the SiO2 top layer and the first HfO2 layer, and the first HfO2 layer remains intact, similar to the bottom feature of the flat-bottom pit, as shown in Fig. 6(b). Notably, the separation between two layers at this interface (marked by the solid arrow) and the obvious cracks at the lower part of the SiO2 top layer (marked by the dash arrow) can be observed, as illustrated in Fig. 7(b). The cross section of a typical funnel pit for the Ta2O5/SiO2 HR coating in Fig. 7(c) is shown in Fig. 7(d). The bottom of the typical funnel pit in the Ta2O5/SiO2 HR coating is also located near the interface between the SiO2 top layer and the first Ta2O5 layer, as shown in Fig. 7(d).
Fig. 7. (a) SEM image of a funnel pit for the HfO2/SiO2 HR coating. (b) Cross section along the white dashed line in (a). (c) SEM image of a funnel pit for the Ta2O5/SiO2 HR coating. (d) Cross section along the white dashed line in (c).
4. Discussion
The HfO2/SiO2 HR coating exhibited a stronger damage resistance than the Ta2O5/SiO2 HR coating by the 1053 nm 8.6 ps laser pulses. The pinpoints, flat-bottom pits, and funnel pits were the three typical damage morphologies of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings. The damage to the HfO2/SiO2 HR coating was mainly dominated by flat-bottom pits, whereas dense pinpoints were the most significant damage for the Ta2O5/SiO2 HR coating. Funnel pits were another type of damage found in both the HfO2/SiO2 HR and Ta2O5/SiO2 HR coatings at the irradiated fluence near the LIDTs. The pinpoints were approximately 200 nm in diameter. The diameters at the top of the flat-bottom pits, which were much larger than the pinpoints, were approximately 5 µm. The top diameters of the funnel pits were between those of the pinpoints and flat-bottom pits, namely, approximately 2 µm. The pinpoints were shallow and confined to the SiO2 top layer. The flat-bottom pits and funnel pits damage involved the first interface between the SiO2 top layer and the first high-index material layers. More detailed studies are required to provide insight into these confusing damage initiations.
4.1 Analysis of the formation of the pinpoints
Fig. 8 shows the electric field intensity (EFI) distributions of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings, as well as the cross sections of the pinpoints. The EFI distributions of the coatings shown in Fig. 8(a) and (b) are nearly the same, and the EFI peaks in the SiO2 top layer were 1.25 in Fig. 8(a) and 1.27 in Fig. 8(b). The same two EFI peaks in the SiO2 top layer are located 186 nm and 604 nm from the coating surface, as shown in Fig. 8(c). The statistical results prove that the depth of the pinpoints in the Ta2O5/SiO2 HR coating is close, and around 200 nm as shown in Fig. 8(d). This implies that these pinpoints are related to nano-absorbing precursors in the SiO2 top layer. Combining Figs. 8(c) and 8(d), it is reasonable that these pinpoints are initially located at the first EFI peak. Strong EFI causes rapid ionization of the coating material at the peak position. In addition, nano-absorbing precursors typically have a lower bandgap than the coating materials. Therefore, these nano-absorbing precursor sites exhibit peak ionization rates. A large number of free electrons are first excited at these defect locations through field ionization and avalanche ionization, and rapidly absorb laser energy [30–33]. The energy absorbed by free electrons is transferred to the lattice through electron-phonon scattering, resulting in the deposition of energy at these sites and finally induces a local thermal explosion. Thus, nano-absorbing precursors located at the first EFI peak are damage precursors and trigger the formation of pinpoints.
Fig. 8. (a) and (b): Internal EFI distributions of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings, respectively. (c) EFI distributions of the SiO2 top layer of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings. (d) Cross sections of the pinpoints for the Ta2O5/SiO2 HR coating.
The chemical composition of the SiO2 top layer was analyzed using XPS, which allows the measurement of the chemical shift of the electron binding energy in the inner shell. Fig. 9(a) and (b) show the full spectra of the compositions of the HfO2/SiO2 and Ta2O5/ SiO2 HR coatings at a sputtering time of approximately 600 s, which approximately corresponds to the location of the first EFI peak in the SiO2 layer. No signals from other elements were detected, except for Si and O, as shown in Fig. 9(a). Nano-absorbing precursors are often analyzed based on the nonstoichiometric ratio of the coatings. In general, the nonstoichiometric ratio of SiO2 coatings originates from the presence of incomplete oxides (SiOx). SiOx usually exists in lower valence states such as Si3+ or Si2+, and are characterized by binding energies lower than that of Si4+ [34]. Fig. 9(c) and (d) show the results of the peak fitting of Si2p for the HfO2/SiO2 and Ta2O5/SiO2 HR coatings, respectively, at a sputtering time of approximately 600 s. The spectral feature of Si2p comprises two peaks centered at 102.8 eV and 103.6 eV (Si4+). The Si4+ peak is assigned to SiO2, while the peak at a binding energy of 102.8 eV is assigned to SiOx [35]. The results show that the SiO2 and SiOx contents of the SiO2 top layer of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings are essentially the same, as shown in Fig. 9(c) and (d). Therefore, a slight stoichiometric imbalance exists in the top layers of both coatings. However, the pinpoints are observed only for the Ta2O5/SiO2 HR coating; therefore, the non-stoichiometric ratio in the coatings was not responsible for the occurrence of pinpoints.
Fig. 9. (a) and (b): Full XPS spectrum of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings, respectively at a sputtering time of approximately 600 s. (c) and (d): Si2p spectrum distribution in the SiO2 top layer for the HfO2/SiO2 and Ta2O5/SiO2 HR coatings, respectively, at a sputtering time of approximately 600 s.
Considering the imperfect sensitivity of XPS, we further characterized the elemental composition of the coatings using TOF-SIMS. The measurement sensitivity of XPS is 0.1%, which can reach the ppm level for TOF-SIMS, despite its inability to measure the relative content of elements [36,37]. The elemental depth profiles obtained using TOF-SIMS are shown in Fig. 10. The sputtering depth for the total sputtering time is indicated by the red solid arrow in Fig. 10(a). Fig. 10(b) and (c) show the elements as a function of the sputtering time for the HfO2/SiO2 and Ta2O5/SiO2 HR coatings, respectively. La and B were found to be present only in the Ta2O5/SiO2 HR coating. This was attributed to the LaB6 ion source in the IAD process for Ta2O5/SiO2 HR coating. The corresponding La and B elements identified by TOF-SIMS implied that the LaB6 ion source left La and B impurities in the Ta2O5/SiO2 HR coating. These impurities initiated the formation of pinpoints. Therefore, the present ion source may be unsuitable for preparing the coatings used with picosecond lasers. K and Na elements were also found in the interfaces or the coating layers. However, the variation trends of the signal intensities for K and Na via depth were similar both for the HfO2/SiO2 and Ta2O5/SiO2 HR coatings. They are not responsible for the damage difference between these two HR coatings.
Fig. 10. (a) Schematic of the TOF-SIMS detection of HfO2/SiO2 and Ta2O5/SiO2 HR coatings. (b) TOF-SIMS depth profile of Si, Hf, O, La, and B elements in the HfO2/SiO2 HR coating. (c) TOF-SIMS depth profile of Si, Ta, O, La, and B elements in the Ta2O5/SiO2 HR coating.
Although La and B elements were found to be present in all layers of the Ta2O5/SiO2 HR coating, the initial locations of these pinpoints were in the SiO2 top layer. Therefore, it is believed the influence of IAD process on the picosecond laser damage can be verified by the SiO2 monolayer coatings with and without IAD process deposition. Fig. 11(a)-(c) show the typical damage morphologies of the SiO2 monolayer coating without the IAD process, which are also dominated by morphologies similar to the flat-bottom pits and funnel pits. Some debris re-depositions occur in the damaged region, and no morphologies similar to the pinpoints are observed. The damage morphologies of the SiO2 monolayer coating with IAD process are shown in Fig. 11(d)-(f). In addition to the similar flat bottom pits and funnel pits, high-density pinpoints are observed. Fig. 11(g)–(i) show magnified views of these dense pinpoints. The cross sections of the pinpoints are shown in Fig. 11(j-l). The depths of these pinpoints are shallow and their sizes are on the same scale as those of the Ta2O5/SiO2 HR coating. Therefore, the pinpoints were confirmed to originate from the IAD process. In addition, they first appeared near the LIDT for the Ta2O5/SiO2 HR coating, as shown in Fig. 4, which implied that the LIDT of the Ta2O5/SiO2 HR coating was limited by La and B impurities. This is the primary reason why the LIDT of the Ta2O5/SiO2 HR coating was lower than that of the HfO2/SiO2 HR coating because pinpoints rarely appeared in the HfO2/SiO2 HR coating.
Fig. 11. (a)-(c) Damage morphologies of the SiO2 monolayer coating without IAD process. (d)-(f) Damage morphologies of the SiO2 monolayer coating with the IAD process. (g)–(f) Specific morphologies of the pinpoints. (j)-(l) Cross sections of the pinpoints.
4.2 Formation of the flat-bottom pits and funnel pits
Flat-bottom pits were the most typical damage to the HfO2/SiO2 HR coating. Since the depths of the flat-bottom pits reached the first interface and sometimes slightly affected the surface of the first HfO2 layer, it is insufficient to infer the damage process of flat-bottom pits from the damage morphologies and cross sections. However, similar features were observed in the funnel pits and the flat-bottom pits. Fig. 12(a) and (c) show the cross sections of the funnel pits for the HfO2/SiO2 HR coating. Sloping cracks extending to the coating surface, indicated by the solid green arrows, occur in the lower part of the first SiO2 layer. In addition, the maximum diameter extending to the coating surface along the above the upward-sloping crack is also approximately 5 µm, which is consistent with the top diameter of the flat-bottom pits. Fig. 12(b) and (d) show local images of the bottoms of the funnel pits marked by the red rectangles in Fig. 12(a) and (c), respectively. Significant separation, marked by the green dotted arrows in Fig. 12(b) and (d), is observed at the bottom interface of the funnel pits. This implies interface separation during damage formation. Therefore, after the funnel pits grew further along the above upward-sloping crack and from the interface separation, the final morphology was in accordance with that of the flat-bottom pits. These features indicate that the funnel pits should have been the previous stage of the flat-bottom pits.
Fig. 12. (a) and (c): Overall shapes of the funnel pits. (b) and (d): Locally magnified images of the bottoms of the funnel pits marked by the red rectangles in (a) and (c), respectively.
The bottom of the funnel pits for the HfO2/SiO2 HR coating was located at the interface between the SiO2 top layer and the first HfO2 layer. In addition, the first HfO2 layer remained almost intact. A tiny cavity beneath the funnel pits, which is marked by green circles in Figs. 12(b) and (d), appears to indicate melting of the nano-absorbing center. The locations marked by the red dots in Figs. 12(b) and (d) correspond to the positions where the largest diameter of the tiny cavity is expected to suffer the most severe damage, as well as the initial location of the funnel pits. Figs. 12 (a) and (c) clearly show that the worst damage to the funnel pits for the HfO2/SiO2 HR coating was approximately 610 nm away from the coating surface, near the second EFI peak.
The formation processes of the funnel pits and flat-bottom pits are illustrated in Fig. 13. First, the nano-absorbing centers near the second EFI peak absorb the laser energy and concentrates heat, which allows the melting of a tiny cavity around the nano-absorbing center, as shown in Fig. 13(a). This tiny cavity further expanded to the first interface because of the proximity between the location of the second EFI peak and first interface. The superheated materials in the tiny cavity rapidly increased the internal pressure and finally broke through the surrounding coating confinement, forming funnel pits, as shown in Fig. 13(b). Because the binding force of the interface is not as strong as that of the coating interior, it is around the SiO2 top layer that starts to separate from the interface induced by this internal pressure, followed by a crack extending to the surface around the funnel pits. When the internal pressure was sufficiently large, the funnel pits grew further along the above upward-sloping crack and from the interface separation, forming the flat-bottom pits shown in Fig. 13(c). In addition, the layer at the bottom of the flat-bottom pits suffered minor damage owing to pressure, resulting in a small loss of HfO2 material at the center of the flat-bottom pit, as shown in Fig. 13(c). Fig. 13(d)-(f) visually describes the formation process of the funnel pits and flat-bottom pits in Fig. 13(a)-(c).
Fig. 13. (a)-(c) SEM cross section morphologies of the flat-bottom pits and the funnel pits. (d)-(f) Schematic of the formation of the funnel pits and the flat-bottom pits.
Compared to the HfO2/SiO2 HR coating, the bottom damage of the flat-bottom pits for the Ta2O5/SiO2 HR coating involved the first Ta2O5 layer. This may be because the melting point of Ta2O5 is lower than that of HfO2 and the Ta2O5 layer is more easily melted and destroyed.
The TEM characterization results of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings cross sections were used to analyze the interfaces. As shown in Fig. 14, the interfaces of the HfO2/SiO2 and Ta2O5/SiO2 HR coatings are different. In Fig. 14(a), the interface between the SiO2 top layer and first HfO2 layer is rougher than that between the SiO2 top layer and first Ta2O5 layer, as shown in Fig. 14(b). This phenomenon has also been observed in previous studies. The evaporated SiO2 and Ta2O5 coatings are amorphous, whereas the HfO2 coatings exhibit polycrystalline microstructures. The amorphous films usually show better roughness than the polycrystalline films [38,39]. Therefore, the SiO2-on-Ta2O5 interface is generally smoother than the SiO2-on-HfO2 interface. Comparing the damage and interface characteristics of the Ta2O5/SiO2 HR coating, it appears that the rough interface may represent poorer bonding, resulting in more flat-bottom pits in the HfO2/SiO2 HR coating. Many efforts need to be done on quantitative approximation of the binding strength at these interfaces in our future work.
Fig. 14. (a) TEM cross-sectional morphology of the HfO2/SiO2 HR coating. (b) TEM cross-sectional morphology of the Ta2O5 /SiO2 HR coating.
5. Conclusion
The laser damage characteristics of the HfO2/SiO2 and Ta2O5/SiO2 multilayer dielectric HR coatings induced by S-polarized 1053 nm 8.6 ps laser pulses were investigated. The origins and damage characteristics of the nano-absorbing precursors were closely examined in terms of the damage morphology, elemental composition, and EFI distribution. The conclusions of this study are summarized as follows:
- a) The HfO2/SiO2 HR coating exhibited a stronger laser resistance than the Ta2O5/SiO2 HR coating.
- b) Three typical damage morphologies were observed for the experimental HR coatings: pinpoints, flat-bottom pits, and funnel pits.
- c) The damage to the HfO2/SiO2 HR coating was mainly dominated by flat-bottom pits, whereas dense pinpoints were the most significant damage to the Ta2O5/SiO2 HR coating. Funnel pits were another type of damage found in both the HfO2/SiO2 and Ta2O5/SiO2 HR coatings at the irradiated fluence near the LIDTs.
- d) The nano-absorbing precursors induced by the IAD process were found to be the damage precursors and triggered the formation of pinpoints with the help of the first EFI peak.
- e) The funnel pits were induced by the nano-absorbing precursors located in the second EFI peak in the SiO2 top layer.
- f) The funnel pits were expected to be the previous stage of the flat-bottom pits. They grew further along the upward-sloping crack and from the interface separation, yielding flat-bottom pits.
- g) The HfO2/SiO2 HR coating exhibited more flat-bottom pits, which means that the interface bonding interface force of the HfO2/SiO2 HR coating seemed to be poorer than that of the Ta2O5/SiO2 HR coating in our experiment.
Funding
National Natural Science Foundation of China (52002271); China Postdoctoral Science Foundation (2021M703326); Key foreign cooperation projects of Bureau of International Cooperation of Chinese Academy of Sciences (Grant No. 181231KYSB20210001); Cooperation project with Hong Kong, Macao and Taiwan supported by Science and Technology Commission of Shanghai Municipality (Grant No. 22220760300); CAS Special Research Assistant Project.
Acknowledgments
This research was supported by Nation Natural Science Foundation of China (Grant No. 52002271), CAS Special Research Assistant Project, China Postdoctoral Science Foundation (Grant No.2021M703326), Key foreign cooperation projects of Bureau of International Cooperation of Chinese Academy of Sciences (Grant No. 181231KYSB20210001) and Cooperation project with Hong Kong, Macao and Taiwan supported by Science and Technology Commission of Shanghai Municipality (Grant No. 22220760300).
Disclosures
The authors declare that there are no conflicts of interest related to this study.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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