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Laser-induced femtosecond damage thresholds of Au and Ag coated pulse compression gratings were measured using 800 nm laser pulses ranging in duration from 30 to 200 fs. These gratings differ from conventional metal-on-photoresist pulse compression gratings in that the gratings patterns are generated by etching the fused silica substrate directly. After etching, the metal overcoating was optimized based on diffraction efficiency and damage threshold considerations. The experiment on these gratings was performed under vacuum for single-shot damage. Single-shot damage threshold, where there is a 0% probability of damage, was determined to be within a 400–800 mJ/cm2 range. The damage threshold exhibited no clear dependence on pulse width, but showed clear dependence on gold overcoat surface morphology. This was confirmed by electromagnetic field modeling using the finite element method, which showed that non-conformal coating morphology gives rise to significant local field enhancement near groove edges, lowering the diffraction efficiency and increasing Joule heating. Large-scale gratings with conformal coating have been installed successfully in the 500 TW Scarlet laser system.
© 2013 Optical Society of America
1. Introduction
Since chirped-pulse amplification (CPA) lasers [1] became the workhorse of modern high-intensity short-pulse laser experiments, further technological advances on such systems have proceeded rapidly. This progress is culminating in several >10 petawatt-scale laser development projects currently under way, each requiring high efficiency, broadband, damage-resistant pulse compression gratings (PCG) of increasing size [2]. The gratings, particularly the final one in the pulse compressor of these large-scale short-pulse laser systems, are typically the least damage-resistant and most expensive optical elements in the laser chain. As a result, the maximum allowable fluence through the system as a whole is dictated directly by the size and robustness of the PCGs. Therefore, a better understanding of the damage mechanism and a more accurate estimate of the laser damage threshold (LDT) of PCGs are both critically important for the economic feasibility and longevity of PW-class short-pulse laser systems.
It is well known that the damage mechanisms for femtosecond laser interaction differ from those for nanosecond or longer pulses [3–8]. Nanosecond pulses cause damage by material heating resulting in melting or ablation; this involves laser energy being absorbed by electrons which, on this timescale, stay in thermal equilibrium with the surrounding atomic lattice. In contrast, femtosecond pulses couple energy into the material by ionizing atoms (via multi-photon ionization initially, and then by avalanche ionization in the case of dielectrics) and accelerating the freed electrons (second step in dielectric, first step in metals) in the laser field. After gaining energy from the laser, these non-thermal electrons de-excite via e-e collisions to a thermal bath which deposits its energy into the surrounding lattice via electron-phonon collisions. This dissipation of energy occurs over a very short length scale compared to that of longer pulses, and the result is highly-deterministic damage sites with relatively less morphological change to the surrounding area. This behavior has led to intense research in the areas of femtosecond etching and lithography [9, 10]. For reflection gratings, the damage mechanism is further complicated by surface morphology and any layered structure [4].
Following some pioneering work on metal overcoat damage [3] and optimum grating design [4, 5], much of the work on PCGs has focused on multilayer dielectric gratings [11–14], which function via interference from layers of material with alternating high and low indices of refraction. The dielectric material allows for a higher damage threshold than metal overcoated gratings [15], but has been unsuitable for short-pulse lasers due to its narrow high-diffraction-efficiency bandwidth–this number has typically been 40 nm or less [11, 12], although recent results are promising [16]. Nevertheless, these dielectric gratings can be difficult to manufacture on the scale necessary for high-power systems, and may suffer delamination due to internal stress buildup when exposed to vacuum [11], so metal-overcoated PCGs remain the standard for broadband short-pulse (below 100 fs) high-power systems.
Another aspect of short-pulse laser damage involves the nonlinear index of refraction. For air this parameter is small (4×10−23 m2/W), but when coupled with intense, short-pulse light it can cause nonlinear-phase accumulation. This phase leads to breakup of the beam into multiple filaments that can then self-focus. The result is a spatially-varying fluence that may, in localized regions, exceed the expected fluence for a given laser energy and spot size. Compression and the subsequent propagation of short pulses must therefore be performed under vacuum, typically <10−4 Torr. The vacuum system provides an ultra-clean environment that is simultaneously thermally and atmospherically stable. For LDT determination with short pulses, the laser is typically focused to ∼10 microns in diameter on the surface of a test optic. The pulses therefore may accumulate significant non-linear phase, even during propagation through air, which can cause an inaccurate measurement of LDT. For maximum operational relevance and accuracy LDT of critical optical components for PW scale lasers should be determined under vacuum. To the best knowledge of the authors no other published work exists for vacuum LDT determination of PCGs.
The following is a study of the damage threshold of several types of metal-on-dielectric PCGs in vacuum. Au-coated gratings are used almost exclusively on all high-power systems at near-IR wavelengths with pulse widths shorter than 100 fs due to their high reflectivity and longevity when compared to other metallic coatings, and to their broad bandwidth and resilience to vacuum environments when compared to multilayer dielectric gratings (as mentioned previously). Despite their ubiquity, Au-coated gratings do have some distinct disadvantages. Firstly, known short pulse LDT values are around 200 mJ/cm2, whereas multilayer dielectric gratings have a typical LDT between 1–2 J/cm2 [12]. Another main problem is grating blackening under prolonged vacuum operation [17–19]. Vacuum contamination on optical surfaces is also known to lower the damage threshold substantially [20]. For these reasons, short pulse fluence (beam normal) on the PCGs are kept strictly below 100 mJ/cm2. Additionally, expensive and cumbersome RF plasma cleaning systems have to be installed to keep PCGs operational for a prolonged period of time [17].
2. Grating fabrication
There are two main methods for manufacturing reflective diffraction gratings: mechanical ruling and holographic surface relief. Mechanically-ruled gratings have been in use for well over a century, and much progress has been made toward increasing the maximum ruling density and precision as well as improving the uniformity with which grating surfaces can be etched [21]. Although high groove quality is achievable with modern advances, this mechanical etching method can still produce gratings of insufficient quality for some applications [21].
An alternative to mechanically-ruled gratings utilizes the fringe pattern arising from the interference of two monochromatic collimated beams of UV light. Here the substrate (again, often fused silica) is coated with a layer of photoresistive material, which has the property that its chemical bonds are broken upon irradiation with UV light. After illumination, a chemical developer etches away the broken bonds, forming the desired grating structure. A metallic film can then be applied to the photoresist layer. In theory, this method can produce ruling patterns devoid of any groove placement errors, potentially at a faster rate than mechanical ruling [21]. However, it requires precise and homogeneous light exposure. Furthermore, the photoresist layer may modify the damage properties of the grating.
Plymouth Grating Laboratory has developed a serialized grating fabrication technique that performs energetic-sputter coating of metals. This technique utilizes improvements on the Nanoruler metrology system [22], which uses scanning beam interference lithography as described above and is equipped to create grating patterns on optics larger than 300 mm. This precise ruling system, along with their newly-developed energetic-sputtering technique for smoother overcoats, allows for more uniform final grating structures that improve diffraction efficiency, as shown in Fig. 1. This technique was used to create the gratings used in the Scarlet laser system.
Fig. 1 (a) SEM image of the conventional sputter-coated grating exhibiting a nonconformal groove structure that will decrease the diffraction efficiency. (b) SEM image showing the energetic sputter-coated grating with a more uniform groove structure, from Plymouth Grating Laboratory.
The gratings tested here had a groove density of 1480 lines/mm and were all made on a 2 inch diameter fused-silica substrate by Plymouth Grating Laboratory. All gratings were tested with p-polarized light at 46° angle of incidence. A bare (no protective dielectric overcoat) Au-coated fused-silica mirror (which was coated simultaneously along with the gratings) was tested as a control sample.
3. Experimental setup
The experiment was conducted using the front-end of the Scarlet laser. The Femtolasers Femtopower front-end consists of a titanium:sapphire system that utilizes a 9 pass amplifier situated between a glass stretcher and a prism compressor. This generates 800 μJ pulses at a repetition rate of 1 kHz. The beam has a 55 nm bandwidth centered on 800 nm. The distance between the compressor prisms can be manipulated to vary the pulse width from 25 to 200 fs. Single-shot operation was achieved by manipulating the timing of two Pockels cell triggers in the front end amplifier.
The front-end beam was sent to a vacuum chamber as shown in Fig. 2(a), where samples were tested at a pressure of around 0.1 Torr. A f/30 (300 mm) achromatic lens on a translation stage focused the beam onto the sample at an angle of 46°. For the data presented here, the fluence on target was varied by a zero-order half-waveplate and a broadband beamsplitting thin-film polarizer. When a greater range of fluences was desired, the focusing lens could be shifted along the laser axis to vary the fluence while keeping the pulse energy constant (used for LDT testing of samples not discussed here). This required detailed characterization of the beam waist, the data for which is shown in Fig. 2(b), showing near-perfect Gaussian beam characteristics. The target was positioned on a jig consisting of two vacuum picomotor-controlled translation stages, allowing for horizontal and vertical movement under vacuum. The jig maintained the angle of incidence within 0.5° throughout its full range of motion. Damage was observed using an in-situ, 10×, infinity-corrected, long working distance, plan-apochromatic microscope objective (Mitutoyo) with a 3.5 μm depth of focus over a 0.5 mm field of view. The image was relayed onto a 16-bit camera (Basler A641f) outside vacuum using a 200 mm focal length achromatic lens. Damage was defined to be a visible change in the surface morphology or reflectivity visible through this in-situ microscope, an example image from which is shown in Fig. 2(c). Damages were subsequently documented with a separate microscope with up to 400× magnification. The pulse width on target, which was set by changing the separation of the prisms in the front-end compressor, was measured using an autocorrelator within the vacuum chamber. The pulse widths used were 30, 100, and 200 fs. Single-shot pulse energy was measured with a small-percentage pickoff optic sending light to a calibrated photodiode.
Fig. 2 (a) Schematic of vacuum chamber for damage testing. Of note are the autocorrelator to measure pulse width and the 10× microscope objective for in-situ damage verification. (b) Focal spot evolution and respective fit showing M2 < 1.2. (c) Example in-situ damage image in false color.
4. Results
Figure 3 shows the single-shot damage probability with respect to fluence in mJ/cm2 for the Au and Ag conformal gratings and the non-conformal Au grating, as well as a bare-Au control sample. Though samples were tested at 46°, all fluences reported here have been converted to the beam-normal value. LDT is expressed in terms of the peak fluence for a Gaussian beam profile, , where E is the energy in the pulse, and w0 is the Gaussian waist radius [23]. The damage probability in the femtosecond regime is not linear with fluence [24], and so the curve fits here are generated from sigmoid functions constrained to asymptote at 0 and 100 percent damage probabilities. Data was collected by observing damages at specific irradiation fluence on an area of the sample, providing a percentage of shots at a fluence level that resulted in visible damage. Between 10 and 20 shots were sampled for damage probability determination at each fluence. After irradiation the sample was translated by 100 μm to eliminate multi-shot effects [25–27] and to ensure that each exposure was conducted on a debris-free surface. Figure 4 shows the dependence of pulse width on the damage threshold. All curves exhibit a similar shape with limited variations in the damage threshold level. These results obtained with current gratings show a somewhat higher LDT than previous measured values for Au-coated gratings [3]. Neauport et al. [13] tested the LDT for Au-on-photoresist gratings with 1740 lines/mm at 72.5° angle of incidence with 500 fs pulses and reported 0.67 J/cm2, which would correspond to 0.28 J/cm2 after correcting for the incidence angle, lower than the LDT of gratings tested here. For comparison with dielectric and metal dielectric gratings tested by Canova et al. [12], the LDT after correcting for their incidence angle is ∼ 0.67 J/cm2.
Fig. 3 Damage threshold data with curve fits for non-conformal (NC) and conformal (C) samples irradiated with 30 fs pulses. For the Au-coated gratings, the conformal coating has a higher damage threshold than the non-conformal coating. Each data point represents 15 shots with a fluence error of ± 0.02 J/cm2.
Fig. 4 Pulse width dependence of damage threshold by sample. While the conformal (C) coating clearly has a higher damage threshold than the non-conformal (NC) coating, there is no clear pulse width dependence of the damage threshold observed for either grating type. Each data point represents 15 shots with a fluence error of ± 0.02 J/cm2.
The damage threshold data for the samples tested is summarized in Table 1; these numbers represent the fluence level at which 0 percent of shots resulted in damage. Safe operation is possible on the conformal Au grating at a fluence of 200 mJ/cm2, while maintaining a 2× safety factor. To determine the long-term parameters of these gratings, multiple shot (1000 shots per test point) LDT measurements were also performed, which resulted in ∼20 % lower LDT on all gratings tested here. Although published data on multiple fs shot LDT measurements on metals is scarce [28], based on theoretical work by Wang et al [29] and works on dielectrics [30, 31], multiple shot LDT is predicted to be lower than the single-shot LDT due to heat buildup in the metals film from successive shots.
Table 1. Single-shot damage thresholds (0% damage probability) in mJ/cm2 for three gratings tested at multiple pulse widths.
No LDT testing on the PCG samples were performed in air. However, to test how short pulse LDT is affected by vacuum, other PCG samples and bare metal mirrors have been tested previously in air. Compared to the vacuum LDT, atmospheric LDT was always found to be lower, typically by 30–40%. This is attributed to the generally uncontrolled nature of atmospheric experiments on surfaces due to exposure to dust and other airborne particles, humidity, and also non-linear phase accumulation at higher fluences.
A pair of 1480 lines/mm conformal Au gratings of the type studied here have been used to compress full energy (24 J uncompressed, 43 nm bandwidth, 130 mm beam diameter) pulses to 50 fs. The all-aluminum-construction compressor chamber in Scarlet is kept below 2 × 10−7 Torr with oil-free turbomolecular pumps backed by a dry screw pump. The highest beam normal average fluence at short pulse intensity experienced by the smaller grating at the final pass is 130 mJ/cm2. The beam profile incident on these gratings resembles a ”flat-top” profile, and therefore the peak fluence is closer to the average fluence than in the case of a Gaussian beam. The grating sizes are 340 mm (w) × 420 mm (h) and 560 mm (w) × 420 mm (h). Analysis of both gratings at a wavelength of 805 nm and 46° angle of incidence shows an average diffraction efficiency of ∼93%. An autocorrelation trace lineout is shown in Fig. 5. The compressor efficiency is 65% and has remained unchanged over a year. After final optimization the system is anticipated to yield compressed pulses down to 30 fs.
Fig. 5 Autocorrelation trace of 50 fs pulse after compression by conformal Au gratings at the Scarlet laser facility.
5. Discussion
Damage morphology was observed to depend on the surface structure of the tested grating. For the Au gratings, the in-situ camera revealed that damages had a similar morphology and change in reflectivity when compared to those observed on the bare-Au mirror. In all of these cases, the observed reflectivity change was due solely to an ablation of the Au layer and not a result of direct damage to the groove pattern on the substrate below; this can be seen in Fig. 6, which shows a SEM image of a conformal Au damage site from a 950 mJ/cm2 pulse. Therefore, an advantage of this type of grating is the possibility of recoating at a fraction of the cost of replacing the entire optic. Since these measurements, full-size gratings have been fully-stripped of their coatings and then successfully recoated.
Fig. 6 SEM image showing intact fused silica substrate at a damage site on the conformal Au grating, produced from a 950 mJ/cm2 pulse.
Numerical simulations were conducted in order to assess the effects of surface morphology variations of the gold coating on grating diffraction efficiency and laser damage threshold. In particular, careful attention was paid to detect any field enhancement effects caused by nonconformal groove features, which were not present in the near-sinusoidal gratings reliefs discussed by Boyd et al [4]. The simulations were performed using the commercial finite elements method package Comsol Multiphysics 4.3a. A 2D Frequency Domain study in the Radio Frequency module of the software was used. The simulation domain was terminated by periodic boundary conditions, with the period set to P = 675 nm in the direction along the grating surface, and a periodic input/output port in the direction normal to the surface. The same port is used for injecting the incident wave and for collecting the reflected waves. The periodic ports can accommodate an incoming plane wave at any angle of incidence with any preset number of diffractive orders; it does so by being transparent to the radiation along those modes and tracking the power flow among them. The incoming monochromatic plane wave with wavelength 800 nm is incident at an angle θ = 46° with respect to grating normal. Only the zeroth (specular) and the negative first diffractive orders are propagating in reflection for the design wavelength of λ =800nm. These propagating diffractive orders are accounted for by the periodic port boundary condition. All higher diffractive orders are evanescent in the simulation and do not reach the input/output port, which is placed at a distance of Δz = 3λ from the grating.
Gold is modeled using the complex permittivity interpolated from tables generated by Pa-lik [32]. The surface shape of the gold layer is traced out by using second order (quadratic) Bezier curves so that the outline is consistent with SEM images of the groove profile (Fig. 1). Three different curves corresponding to different levels of conformal coating are simulated. Less conformal coatings form a more pronounced gold lump between the grating ridges. Reflectance into the zeroth and negative first diffractive orders is obtained from the scattering parameter output of the code. The results of the simulation are shown in Fig. 7, where three conformal gold coverages are shown in (a), ranging from least conformal (C1) to most conformal (C3). Note that the least conformal grating showed the strongest field enhancement at the gold surface. This translates into the highest Joule heating of the grating. Table 2 details the absorptivity A, grating efficiency R−1 (i.e. scattering into the desirable negative first diffractive order), and parasitic (specular) reflectivity R0. These values were calculated in the simulation and also measured on the actual gratings that corresponded to the simulated curvatures. The inferred absorption from the reflectivity measurements is also shown in the table for comparison, as well as the corresponding measured LDT.
Fig. 7 (a) Shapes of the three different gold surfaces corresponding to differently-conforming Au layers. (b)–(d) Color-coded magnetic field enhancement |H/H0| for the gratings C1–C3, respectively, centered around one diffraction ridge (visible at the bottom of each image). Here H0 is the magnetic field of the p-polarized light wave with λ = 800 nm, incident at an angle θ = 46°. The max value of |H/H0| is indicated above each color bar, and decreases for the more conformal curvatures. Other wavelengths within the experimental range were also simulated, with similar field enhancement results to those shown here.
Table 2. Measurements and simulation calculations of the absorptivity A, negative first diffraction order reflectivity R−1, and specular reflectivity R0 for three different grating profiles (with C3 being the most conformal), as well as measured LDT values for the relevant profiles
In measurement, the highest damage threshold was obtained for the conformal grating, which had a negative first order diffraction efficiency of 92%. The slightly non-conformal grating, which exhibited a significantly lower damage threshold, had 84% efficiency. These gratings had reflection efficiencies of 0.5% and 4%, respectively. These measurements were performed using broadband femtosecond pulses with a spectrum centered on 800 nm, with rms error < 0.5% for all measurements. Any loss due to scattering was not measured, and assumed to be minimal due to the smooth surface of the gold coating. It is apparent that the model somewhat underestimates the diffraction efficiency of the conformal and non-conformal gratings when compared with model gratings C3 and C2, respectively. This is most likely because of two reasons: the uncertainty of curve-fitting of grating surface profiles, and the fact that diffraction efficiency as a function of wavelength is not constant. However, the model does qualitatively capture the physical picture, i.e. that the non-conformal grating with higher absorbance and lower diffraction efficiency undergoes higher Joule heating via energetic electrons trapped by field enhancement.
Additionally, the increased damage threshold of the Ag-coated grating (∼ 600 mJ/cm2) is of note. Originally the experiment was intended to test the effect of conformal Au coatings, but promising initial results prompted a test of the conformal Ag coating as well. Traditionally, Ag coatings are not widely used for large gratings due to a susceptibility to tarnish over time. However, the observed intact substrate upon damage of the conformal coating type (seen in the Ag coating as well as Au) suggests recoating as a cheap alternative to replacement of the optic, potentially making Ag-coated conformal gratings a viable alternative to the current standard Au type. More study is required to decrease the cost of recoating further, and to preserve the reflective qualities of Ag for a longer duration. One possibility is adding a thin dielectric coating on top of the Ag coat. This procedure has increased the damage threshold at longer pulse lengths by a factor of two [33], and on Ag may have the added benefit of preventing tarnish.
6. Conclusion
Damage threshold results for various gratings with metal reflective coatings have been determined. The data shows no clear pulse width dependence on LDT for pulses between 30–200 fs. Simulations show that the decreased diffraction efficiency and damage threshold of the nonconformal grating is due to field enhancement resulting from convex surface morphology of the groove coating valleys, which the conformal coating method eliminates. The conformal Au grating on fused silica has the advantage of possible restoration of its diffraction efficiency fully by stripping the damaged Au layer and recoating, even after the metal coating sustains significant damage. Due to their higher LDT, compressors consisting of conformal-groove gratings should be able to safely operate at fluences of 200 mJ/cm2 (as opposed to the current typical value of 100 mJ/cm2), reducing the beam aperture and all post-compressor optical component sizes, which allows for significant cost savings in design, construction, and maintenance of PW-class laser systems.
Acknowledgment
This work was supported by US Department of Energy grant no. DE-FC52-06NA26262, and by the Air Force Office of Scientific Research grant no. FA9550-12-1-0454. The authors would like to thank Jerry Britten of LLNL and Mike Storm of OSU for valuable comments and discussions.
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