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Abstract
We report the influence of polarization on the damage mechanism of oxide thin films submitted to multiple pulses in the sub-picosecond regime. We have exposed single layer coatings of oxide materials and multilayer stacks (mirrors) to multiple laser pulses at 1030nm, 500fs, and the events on the tested sample sites were recorded in situ with high resolution microscopy. For multiple shots while keeping the fluence below the single shot threshold, damage on the film begins to form and for some of the samples the damage growth follows polarization dependent patterns. This damage growth was investigated and our results match with the assumption that the existence of nano-defects contributes to the early stage of the formation of damage, in which the energy absorption in a defect site causes local nanoablation at a laser fluence under the intrinsic ablation threshold and nanovoid formation. Based on the simulation of the interference of the scattered wave by the nanovoid with the incident wave, we obtain good correlation between simulated and observed damage growth behavior. This process leads to the formation of specific damage morphology that is strongly dependent on the polarization of the incident wave.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The concept of chirped pulse amplification (CPA) technique [1] paved the way of constructing terawatt or petawatt laser system having pulse duration in ultra-short regime [2]. One of the main bottlenecks towards achieving high power laser output is laser-induced damage (LID) incurred in the optical components while interacting with the high intensity laser beam. The corresponding minimum level of radiation for which some physical alteration takes place on the optics is called laser-induced damage threshold (LIDT). LID sites scatter light and absorb energy thereby reducing output beam power and can initiate further damage resulting in the ultimate failure of the optics [3–7]. So for practical reasons, it is crucial to estimate the damage thresholds and understanding the damage processes for materials which are extensively used e.g. in laser optics. This in turn can help the manufacturer for constructing optics with very high threshold that are suitable for high power laser applications [8]. Understanding the underlying physics of laser-induced damage mechanism for optical components is an active research area since the advent of powerful laser systems [9–14]. The damage processes for transparent dielectric materials at ultrafast timescale are mainly driven by nonlinear photoionization [15–17] and electron impact avalanche ionization [18, 19]. Photoionization refers to the process where electrons are directly excited from the valence to the conduction band by the laser field [20,21]. Because a single photon of visible light does not have enough energy to excite an electron in a transparent material, multiple photons are required. Avalanche ionization involves free-carrier absorption followed by impact ionization [22]. An electron already in the conduction band of the material linearly absorbs several laser photons and consequently move to higher energy states in the conduction band. This process requires seed electrons. These initial electrons are provided either by thermally excited carriers, carriers resulting from photoexcitation by multiphoton or tunneling ionization [11, 23] or from pre-existing defects [24]. As a consequence the damage threshold is the function of both the material characteristics and the parameters associated with the laser beam [11,15,25–29].
Likewise the dependencies of LIDT and damage mechanism on the materials and beam characteristics, the damage morphology also has dependencies on these parameters and it gives insights about the initiation of the damage mechanism [30–33]. For some materials, special kind of damage morphologies that exhibit regular pattern [34] are observed on the surface at laser fluence under the threshold value for single pulses. This pattern is also called laser induced periodic surface structure (LIPSS). Fluence, pulse number and polarization of laser beam have very important role in the formation of such nanostructures on the surface [35]. The physics behind the formation of LIPSS is complex [36] and has been a subject of investigation during few last decades. Their origin can be related to near-field local enhancement in the vicinity of inhomogeneous scattering centers [37]. H. Shimizu et al. discussed the role of defects to the formation of LIPPS [38]. Due to localized defects, the damage threshold value can be reduced at the defect site, causing nanoablation and formation of nanovoids. These nanovoids, acting as scatterers, initiate the formation of nanostructure damage patterns with growth direction that depends on the orientation of electric field of the incident radiation. Contribution of nanodefects to LIPPS formation through local electric field enhancement is also discussed in the work of Kafka et al. [39] on laser damage of optical coatings in ultrashort regime. In our investigations on the laser-induced damage of optical coatings in the sub-picosecond regime, we found similar evidences of nanoablation on defect sites followed by polarization dependent damage growth (example in Fig. 1) on dimensions much smaller than the effective size of the beam spot. Relatively similar damage morphologies are reported in the work by Durak et al. [40]. They appear to be the first step of damage growth before catastrophic damage of the optical component and it is of important interest to understand their initiation and evolution under laser irradiation. Even if no periodic structures are observed, the process has similarities to polarization dependent LIPSS formation, mainly an electromagnetic origin [41,42] and the objective of this paper is to explore the physical origin of this polarization dependent damage growth. For that purpose in the first part of this paper, we report on our experimental procedures to observe polarization dependent laser induced damage when the sample is irradiated by sub-picoseconds pulses. Then a model based on near-field numerical simulations is presented to explain the damage behavior that have been investigated during the experiments. We discuss on the comparison of experiments and simulations conducted on Hafnia samples and provide explanations on the origin of the observed structures.
Fig. 1 An example of a polarization dependent damage pattern, observed on a HfO2 coating (thickness 131nm, deposited by Ion Beam Sputtering) submitted to 100 pulses at 1030nm, 500fs, 45 deg of incidence, 2.5J/cm2. The polarization direction is perpendicular to the observed structures.
2. Experiments and methods
2.1. Samples
In these experiments, we have used single layer samples of different oxide materials which have been grown by Ion Beam Sputtering or Electron Beam Deposition on fused silica substrates. Refractive index and thickness of the samples are known, however we do not have details on the deposition parameters. The main idea in this investigation was to see if the behavior of polarization dependent damage could be observed on several materials or the same material deposited in different conditions. These different high index/high bandgap materials are of particular interest for the production of laser coatings used in high power applications. The relevant parameters of the samples are presented in Table 1. Although we have tested all of the samples, extensive experiments and calculations were carried out only for HfO2 sample referred as “IBS-1” in the Table. Discussion on the observed behavior on other samples is conducted in section 3.
Table 1. Parameters of the sample
2.2. Experimental setup
The measurement of damage growth dynamics were carried out with the schematic setup as shown in Fig. 2.
Fig. 2 Basic schematic of the experimental setup: λ/2, half waveplates; LP, linear polarizer; BS, beam splitter; M1,2,3,4,5, steering mirrors; PC1,2, pyroelectric cells; L, focus lens; Obj., microscope objective; T.L., tube lens; F1,2,3, filters; BD, beam dump. Inset diagram describes the sample area with respect to the waist position of laser beam.
The experiment consists of ytterbium doped diode pumped chirped pulse amplification laser source (s-Pulse HP, Amplitude Systèmes) operating at 1030nm. It is able to deliver pulses with maximum energy of 1mJ. Large part of the beam energy is directed to the sample and focused using a 150mm focus lens to perform laser damage experiments. A small portion is directed to a reference pyroelectric cell (PE9, Ophir) (PC1) for measuring fluence in real time. Shot to shot energy was recorded with a removable pyroelectric cell (PC2) placed on the sample path right after the focusing lens. The pulse energy was controlled with a half-wave plate and polarizer system which is placed right after the laser source. Another half-wave plate placed before the focus lens is used for selecting appropriate orientation of linearly polarized beam. The sample was placed on motorized stage with surface normal making an angle 45° with the beam path. With this setup it is possible to record and observe simultaneously in situ the evolution of damage sites with an optical microscope (BX51, Olympus) equipped with a 20 x objective (Nikon FLN Plan Epi Infinity Corrected Obj.) and a CCD Camera (GS-U3-41C6C-C, FLIR). Filters were inserted between the objective and the tube lens to block the pump beam scattering (KG3, Schott) and minimize the contribution from the broadband thermal radiation from the plasma in case such event occurs (FL514.5-10, Thorlabs). The video camera is synchronized with the electronic shutter used for triggering laser pulse. When the pulse interacts with the sample and modifies the sample surface, the image of the interaction event is captured by the camera and recorded after each laser shot.
2.3. Beam characterization
The effective area is the main quantity that characterizes the spatial fluence profile of a pulsed laser beam in a specific transverse plane (in a laser spot) for LIDT experiments, as specified in the LIDT ISO 21254 standards [43]. The effective area is directly related to the peak fluence and the pulse energy of a laser spot and the procedure for calculating the effective beam area can be found for instance in [44,45].
From effective area, the effective beam diameter can be estimated. The diameter can be found in [45] and is given by
and the beam area at 1/e is given byThe CCD image of the beam spot of our laser system is shown in Fig. 3(a). The beam was Gaussian-shaped. By using the pixel size of the CCD sensor and the method presented in [45], the effective diameter of the beam in the focal plane was 65μm ± 2μm measured at 1/e at normal incidence. The duration of laser pulse during the experiments was measured by a single shot autocorrelator (ASF-50, AVESTA). From the autocorrelation trace, Fig. 3(b), pulse duration at Full width at half maximum (FWHM) was of 570fs ± 20fs, estimated from hyperbolic sech2 fit.
Fig. 3 Spot size and pulse duration: (a) CCD image of beam intensity distribution at the sample location (b) Autocorrelation trace.
2.4. LIDT measurements
There are several damage test procedures that are usually used for laser damage tests: 1on1, Son1 and Raster Scan [43]. The 1on1 procedure consists of irradiating a great number of sites (at least ten) on the sample with single shots. By counting the number of damaged regions at each fluence F, a damage probability curve P(F) is then obtained. Though this procedure helps to obtain damage threshold with high precision in sub-picosecond regime, it does not correspond to practical applications where the components are exposed to a large number of laser shots. In this case, the Son1 test procedure (where a burst of pulses is used instead of one shot) is adapted, but the test conditions (particularly the repetition rate) must be close to operational conditions due to incubation effects caused by multiple shots [46]. The experimental setup as shown in Fig. 2 has been designed to operate in the 1on1 and the Son1 mode, the raster scan procedure being more adapted to the case where extrinsic defects are involved [47]. In this study, the test procedure Son1 is used to investigate the damage morphology and damage growth with respect to the polarization state of the laser. Son1 measurements were done with low frequencies (10Hz) for practical reasons of recording the complete video sequences of laser shots to observe damage occurrence and monitor damage growth.
By knowing both the pulse energy and the spatial profile of the beam, it is possible to calculate the fluence in the focal plane in terms of normal beam fluence, i.e. measured in a plane that is normal to the beam axis.
Fluence on the sample surface is found by
θAOI, the angle of incidence. The measurement of Fsurface was carried out for both S and P polarized beam. To ensure S-polarized beam the position of the waveplate (near focus lens, L in Fig. 2) was kept at 0°. And for P-polarized light the waveplate was rotated by an angle of 45°.To calculate the intrinsic LIDT of the sample, it is necessary to know the electric field inside the film to take into account the real fluence in the material. This was estimated by transfer matrix method (TMM) using Matlab. The distribution of Electric Field inside the film for the case of both s and p polarized beam is shown in Fig. 4 for one of the samples.
Fig. 4 Electric field inside the film (HfO2 thin layer with layer thickness of 131nm and refractive index of 1.94). (a) When the incident beam is s-polarized (b) For p-polarized beam.
The intrinsic LIDT (LIDTint) tells us about the material irrespective to the polarization of beam, the thickness of the coating and the angle of incidence.
The EFI (Effective Field Intensity) is expressed by the following definition: EFImax is the maximum value of normalized electric field intensity inside the film found from numerical simulations.3. Experimental results
LIDT experiments have been conducted with S and P polarizations. Different threshold values have been measured for the different polarizations but after taking into account the value of normalized electric field, the intrinsic damage threshold, as defined previously was found to be the same in case of S and P polarization for all the tested samples. An example of comparison of measured and intrinsic LIDT can be found in Table 2.
Table 2. Comparison of single shot LIDT in S and P polarization
When the samples were submitted to multiple pulse irradiations, a decrease of the laser-induced damage threshold (LIDT) is observed with the applied number of pulses (example in Fig. 5), as it has been reported in many studies (Ref. [8,40,46] as some examples). We have not found significant difference in LIDTint between both polarizations states, however we have found significant differences in the damage growth.
For p-polarization of incident laser beam, the typical damage pattern on a Hafnia sample is shown in Fig. 6. In Fig. 6(a), a black spot which is essentially a damage site (in the figure, inside the red circle) formed on the sample surface, close to the center of the beam, is observed after 190 pulses in the particular reported case. Then this damage spot begins to grow vertically as indicated in Figs. 6(b)–6(f) and the rate of damage growth is identical in both ±y direction.
Fig. 6 Evolution of damage for p-polarized laser beam with fluence 2.84J/cm2: (a) Formation of nanovoid after 190 pulses arrived on the sample surface and in (b–d) subsequent growth of the size of the damage spot. (Optical microscopy)
For s-polarized laser beam on the same sample, damage spot appears after 80 pulses interact with the sample as shown in Fig. 7(a). The damage spot elongates horizontally with the successive incoming pulses as depicted in Figs. 7(b)–7(f). However in this case it is interesting to note that damage grows asymmetrically in ±x direction: damage is not growing at the same rate in the 2 directions (Visualization 1).
Fig. 7 Evolution of damage for s-polarized laser beam with fluence 3.3J/cm2: see Visualization 1 (a) Formation of damage spot after 80 pulses and (b–f) subsequent growth of the damage spot. (Optical microscopy)
These two previous examples are typical of our observations on different samples. These structures appear at the laser damage threshold (as reported for instance on Fig. 5) and are unstable: they grow with the number of applied pulses. It should be noticed that at some point the damage grows catastrophically and without high resolution microscopy the occurrence of the linear structures in the first stages of the damage growth could go unnoticed which could explain it was not reported previously (except one image in Ref. [40], as discussed in the introduction).
We have studied the growth rate at the laser damage threshold: it can be estimated from the relationship between feature length and the number of laser shots. The damage length is found from the SEM (TM-1000, Hitachi) micro-graph as shown in Fig. 9. Using this length as a reference, the recorded video was analyzed and the evolution of damage length was estimated. A plot as shown in Fig. 8 was then obtained for the case of a Hafnia sample. The relationship between the damage growth and the No. of laser shot is nearly linear. The slope of the linear fit essentially indicates the growth rate. For s-polarized beam, the average damage growth rate is 40nm/shot. More precisely according to the axis convention, along −x it is 30nm/shot and along +x it is 10nm/shot. For p-polarization the rate is 28nm/shot (14nm/shot on both +y and −y direction).
Fig. 8 Average damage growth distance with respect to No. of laser shot: (a) When the incident beam is s-polarized (b) For p-polarized beam.
Fig. 9 SEM micro-graph: (a) Damage for p-polarization, fluence 2.84J/cm2 and No. Pulse=1000 (b) Damage for s-polarization, fluence 3.3J/cm2 and No. Pulse=1000.
4. Numerical model
Our base assumption is that there are defects present on the thin film and nanovoids are formed with the reduced threshold due to the defect site as discussed in sec.1. The initial diameter of the nanovoid is in the sub 100nm range, which is estimated from the AFM measurement(Dimension Edge, Brucker) of the damage feature as shown in Fig. 10. Ablation of material at this scale can be due to electric near field enhancement by nanoparticles [48].
Fig. 10 Characterization of the damage by Atomic Force Microscopy (Dimension Edge, Bruker). Left figure: surface topography; Right: Depth profile. It must be noticed that the result is a convolution of the real profile with the AFM tip profile, thereby the width should be less than 150nm in this case. (fluence 2.84J/cm2 and No. Pulse=1000).
The nanovoid or hole so produced now acts as the basis for the damage growth. Scattering takes place due to the interaction of the nanovoid and the incident radiation. The successive laser pulses interact with this hole thereby producing specific distribution of electric field depending on the polarization of laser beam. Where the intensity is higher that the intrinsic LIDT threshold, material ablation takes place. This makes the hole larger and the process continues with the incident of next pulses.
First, we will discuss about the geometry of the model and then the physics associated with this model. The model geometry is shown in Fig. 11.
Fig. 11 Model geometry for the simulation (a) Whole computational domain (b) Different layers (c) Thin film with nanovoid.
The physical domain is surrounded by perfectly matched layer (PML). The topmost layer in the physical domain is air from which laser source is launched. We use index value, na = 1 for air and nb = 1.45 for the dielectric substrate of fused silica. The index for HfO2 thin film is nfilm = 1.94. Here the scatterer is a void region which is assumed to be air having index identical to air. We have assumed a cylindrical symmetry based on the assumption that the void is a result of an explosive process. A conical crater would have been a better choice but the cylinder was of interest for computational considerations (meshing of the structure).
We use COMSOL multiphysics for our numerical simulation [49]. This model computes the total field in two steps. In the first step, it solves electromagnetic waves in frequency domain which produces a background field assuming a plane wave passing through a damage free thin film, incident on the substrate, and secondly with considering this background field, the total field is calculated when the defect is present in the thin film. The hole in the thin film essentially functions as scatterer on the substrate. If no scatterer is present, then background field is simply the incidence plane wave. With the scatterer present on a substrate, the analytical expression for the background field becomes more complicated. It needs to be the correct superposition of an incident and a reflected wave in the free space domain (Air domain), and a transmitted wave in the substrate. One easy way to avoid calculating the analytical background field is to use a full field solution of the problem without the scatterer. To achieve this full field solution, the simulation is set up with two Port conditions. One defines the incident plane wave and allows for specular reflection. The other absorbs the transmitted plane wave. Perfectly Matched Layers ensure perfect absorption of the energy on the boundaries.
The propagation vector k⃗ and the polarization of incident field are the input parameters for the input/output ports. According to the convention of co-ordinate system used in Fig. 11(c), the propagation vector k⃗ of the incident field is
where, k0 is the wave number in vacuum, ϕ and θ are azimuthal and polar angle of incidence respectively. For S-polarization the electric field vector at the plane of incidence is given by and for P-polarization the expression of electric field vector at the plane of incidence is given by We can also manually set the port power. Where, I0 is the intensity of the incident field and A is the area of the boundary where the port is set. From the port power the amplitude E0 of the incident field is derived. Then another port which is placed at the substrate for absorbing the field. For defining the parameters of this port it is again necessary to find the direction of propagation vector in substrate.In the substrate, propagation vector k⃗b is given by
The x and y component of k⃗ are the same for both air and substrate. The only change is for the z component. Which is given by With kb = k0nb and θb = sin−1(nasinθ). θb represents the polar angle of incidence in the substrate.5. Simulation results
With all the assumptions and parameters as discussed in section 4, we run the simulation for different cases for each polarization (p-type and s-type). The axis convention as shown in Fig. 12(a) and Fig. 14(a) are identical to that of our laboratory axis. So the numerical outcomes are directly comparable with the experimental results. It should be noted that in this approach, a homogeneous intensity is used, as opposed to a Gaussian distribution as in experiments. The beam diameter being 65μm at 1/e we can estimate that for damage patterns such as those shown on Fig. 9 the fluence variations are less than 10% between the center and the tips.
Fig. 12 Intensity distribution for p-polarization (a) Axis convention (b) with diameter = 150nm hole size (c) with hole size (dx = 150nm, dy = 178nm) (d) with hole size (dx = 150nm, dy = 206nm). Simulated plane is at z = −50nm.
Considering p-polarization, simulation results are shown in Fig. 12. In Fig. 12(b), the field distribution exhibits high values in the y direction, which suggests that the material ablation might take place along this direction. A 1D line plot is also shown on Fig. 13. It is a bit difficult to assign with precision the extend of the ablation zone, but the E-field values are 20 to 30% higher in the extend of few ten of nanometers, which correlates to the growth rates of 28nm/shot (14nm in each direction). It appears that the nature of damage growth is essentially compatible with the observation. Now in the next step at Fig. 12(c) we increase the size of the nanovoid along ±y direction (along high intensity direction) by an amount compatible to the damage growth rate as found in section 3. In Fig. 12(d), similar procedure is followed as of Fig. 12(c). In both cases the intensity distribution remains the same, meaning the damage is also along same direction. An interesting point to notice is the reduction of local intensity in the horizontal directions, preventing any damage growth in the direction. For s-polarization the field distribution is shown in Fig. 14. A 1D line plot is also shown on Fig. Fig. 15. Here one noticeable thing in Fig. 14(b), that the intensity distribution along −x direction is different from +x direction. Now we increase the size of the defect along the high intensity direction in Figs. 14(c) and 14(d), the pattern again remains the same and damage structure runs along horizontal direction only (x direction). The symmetric field distribution of our simulation result for p-polarization and asymmetric field as found for s-polarized light, strongly corroborate the experimental damage observation: same growth rate in ±y directions for p-polarization and different damage growth rate in ±x directions for s-polarization.
Fig. 14 Intensity distribution for s-polarization (a) Axis convention (b) with diameter = 150nm hole size (c) with hole size (dx = 190nm, dy = 150nm) (d) with hole size (dx = 230nm, dy = 150nm).
We have also performed the simulation for the case of an extended damage, typical of our observation, as shown in Fig. 16. This confirms that the field is relatively high at tip whatever the size of the line, suggesting that material ablation might continue to grow along this direction. Experimentally it is however observed that damage growth stops at some point, because of the limited size of the beam.
6. Discussion
From these simulations and experimental observation we can then have a better understanding of the polarization dependent damage patterns that were observed on the samples, which can be summarized as follow:
- Under multiple shots one or several localized damage appear, referred as nanovoid, on some defect or inhomogeneities in the film. The damage size can be less that 100nm.
- The nanovoid acts as a scatterer that modifies the E-field distribution in its vicinity. At some locations that depend on the polarization and angle of incidence, the local intensity can exceed the intrinsic LIDT threshold which induce local nanoablation and extension of the nanovoid.
- The process repeat itself at each new pulse and the nanovoid extends along the direction of intensity enhancement by the structure.
- At some point the damage can grow catastrophically because of initiation of other nanovoids and merging of multiple structures.
The experiments were however conducted on a limited set of samples and we have investigated the extension of these results to other samples. We summarize in Table 3 our observations on different single layer samples. The polarization dependent damage pattern (damage starting from a nanovoid and growing as vertical or horizontal lines) has been observed on many samples from different materials and deposited in various conditions. In some cases however it was not observed. We can speculate that if defects act as a starting point for the damage process, different samples may have different defects favorable or not to this damage process.
Table 3. Report on the observation of polarization dependent patterns
More complicated structures that single ’lines’ can be observed, as shown for instance on Fig. 1 where some of the damage ’lines’ exhibit a fine substructure with multiple parallel lines. This could be related to irregular shape of the initiating scatterer or multiple scatterers but these are just speculations at this point.
In actual laser systems, multi-layer dielectric coatings are used and it is of interest for applications to study the possibility to observe similar growth phenomena on such systems, even if as discussed in the introduction such structure is reported in Ref. [40] for the case of a Metal Mulitdielectric mirror. For that purpose we have tested 6 different High Reflecting coatings, one of the main component used in laser systems. The samples have different designs but are all made of HfO2/SiO2, the main combination for high power applications [50]. From our analysis the polarization-dependent damage growth is observed, but only on 2 of the samples out of 6 (see example Figure 17). These 2 samples have both a HfO2 film as the last layer (interface between air and coating) whereas the 4 others have a SiO2 layer as the last layer. These results suggest that such damage growth can be observed on mirrors for high power lasers operating in the short pulse regime since optimized designs do not include a SiO2 cap layer [51].
Fig. 17 Observation of the polarization dependent laser damage patterns on a HfO2/SiO2 mirror, with HfO2 as the last layer.
7. Conclusion
In conclusion, we have reported on a nanostructured damage pattern which grows along a polarization dependent direction. This specific pattern was observed on single layers of different dielectric materials, as well as on multilayer stacks. In order to undertsand the orgin of this laser damage behavior, numerical simulations were conducted under the assumption that the first step of damage is a nanovoid in the film. The numerical simulations correlate with the experiments and confirm that this type of damage on the surface of the sample is related to near field ablation based on the electric field distribution around the damage site. It is highly probable that this phenomena and the observed structures have a general type, but might go unnoticed in many experiments because they appear at the onset of catastrophic damage.
Acknowledgments
We are grateful to the following laboratories and companies for providing some of the samples: Laboratoire Matériaux Avancés (LMA), Laser Zentrum Hannover (LZH), CILAS, REOSC.
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