# Analysis of laser-induced damage threshold of circular grating waveguide structures exposed to sub-picosecond laser radiation centered at a wavelength of 1030 nm

Denys Didychenko, Fangfang Li, Anton Savchenko, Janis Zideluns, Marek Stehlik, Laurent Gallais, Julien Lumeau, Fabien Lemarchand, Petri Karvinen, Markku Kuittinen, et al.

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Denys Didychenko, Fangfang Li, Anton Savchenko, Janis Zideluns, Marek Stehlik, et al. Analysis of laser-induced damage threshold of circular grating waveguide structures exposed to sub-picosecond laser radiation centered at a wavelength of 1030 nm. Applied Physics B - Laser and Optics, 2025, 131 (3), pp.39. 10.1007/s00340-025-08414-1. hal-05213440

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# Analysis of laser-induced damage threshold of circular grating waveguide structures exposed to sub-picosecond laser radiation centered at a wavelength of 1030 nm

Denys Didychenko<fnref n="1" /> · Fangfang Li<fnref n="2" /> · Anton Savchenko<fnref n="3" /> · Janis Zideluns<fnref n="4" /> · Marek Stehlik<fnref n="4" /> · Laurent Gallais<fnref n="4" /> · Julien Lumeau<fnref n="4" /> · Fabien Lemarchand<fnref n="4" /> · Petri Karvinen<fnref n="2" /> · Markku Kuittinen<fnref n="2" /> · Christof Pruss<fnref n="3" /> · Damien Jamon<fnref n="5" /> · Thomas Graf<fnref n="1" /> · Marwan Abdou Ahmed<fnref n="1" />

Received: 18 December 2024 / Accepted: 6 February 2025 / Published online: 12 February 2025
© The Author(s) 2025

## Abstract

The laser-induced damage threshold of a grating waveguide output coupler (GWOC) exposed to laser radiation at a wavelength of 1030 nm and with a pulse duration of 500 fs was investigated. The GWOC is a combination of a sub-wavelength circular grating and a partial reflector based on a Nb₂O₅ and SiO₂ multilayer sequence. It was designed to be used as an output coupler of a thin-disk laser cavity for the generation of beams with radial polarization. The results revealed a laser-induced damage threshold (LIDT) fluence of 0.36 J/cm<fnref n="2" /> for single-pulse tests and 0.26 J/cm<fnref n="2" /> for multiple-pulse conditions with up to 1000 shots. These threshold values are comparable to those of an unstructured output coupler with Nb₂O₅ and SiO₂ coating layers, highlighting the minor influence of the grating on the LIDT.

## 1 Introduction

Radially and azimuthally polarized laser beams were proven to have specific benefits for the performance of material processing [1–4]. Since the intracavity power of correspondingly polarized lasers may reach high peak levels, especially in pulsed operation, it is important to use optical components with a high laser-induced damage threshold (LIDT). Grating waveguide output couplers (GWOC) are special elements that can be used as an output coupler of

the laser cavity [5]. It consists of a sub-wavelength grating combined with a fully dielectric multi-layer partial reflector. The benefit of integrating the polarizing function into the output coupler is to allow the implementation of the semiconductor saturable absorber mirror (SESAM) as an end-cavity mirror to generate high-power radially polarized ultrashort pulses [6]. A high-power, radially polarized continuous-wave Yb:YAG thin-disk laser using such a grating waveguide mirror has already been demonstrated, achieving up to 1 kW of average output power [7]. Additionally, using other crystals as gain media, such as Yb:Lu₂O₃ ceramics, the generation of beams with radial polarization with up to 175 W of average output power was reported in [8].

The combination of materials used for GWOCs is typically based on Ta₂O₅/SiO₂ or Nb₂O₅/SiO₂ multilayer stacks [9, 10]. The LIDT of Nb₂O₅-based resonant reflection grating used as an angular filtering mirror [11] was reported in nanosecond pulse operation to be 3.1 J/cm<fnref n="2" /> for P- (TM-) polarization and 0.7 J/cm<fnref n="2" /> for S- (TE-) polarization [12]. Furthermore, the LIDT of thin-film single layer Nb₂O₅ has also been determined in sub-picosecond pulse operation to be 0.82 J/cm<fnref n="2" /> [13]. However, to the best of our knowledge, the LIDT of circular GWOCs based on a Nb₂O₅ and SiO₂ multilayer, especially in view of sub-picosecond pulsed laser operation, has not yet been studied in detail.

<footnote>Marwan Abdou Ahmed
marwan.abdou-ahmed@ifsw.uni-stuttgart.de</footnote>

<footnote n="1">¹ Institut für Strahlwerkzeuge, University of Stuttgart,
Pfaffenwaldring 43, 70569 Stuttgart, Germany</footnote>

<footnote n="2">² Center for Photonics Sciences, University of Eastern Finland,
P.O. Box 111, Joensuu FI-80101, Finland</footnote>

<footnote n="3">³ Institut für Technische Optik, University of Stuttgart,
Pfaffenwaldring 9, 70569 Stuttgart, Germany</footnote>

<footnote n="4">⁴ Aix Marseille Univ, CNRS, Centrale Med, Institut Fresnel,
Marseille, France</footnote>

<footnote n="5">⁵ Université Jean Monnet Saint Etienne, CNRS, Institut d'optique Graduate School, Laboratoire Hubert Curien UMR 5516, Saint-Etienne F-42023, France</footnote>
<figure><img src="image_2.png" /><figcaption>Fig. 1 Design (a) and photo (b) of the GWOC structure</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2 Calculated and measured reflectivities of the GWOC for TE and TM polarized radiation together with the calculated and measured reflectivities of the OC for TM polarized radiation</figcaption></figure>

In the following, we therefore report on the LIDT and investigations on the origin of the laser-induced damage of a GWOC based on a Nb₂O₅ and SiO₂ multilayer. This GWOC is meant to be used as an intracavity output coupler for the generation of radially polarized ultrashort pulses in Yb-doped (Yb:YAG and Yb:Lu₂O₃) thin-disk laser oscillators emitting at a wavelength of 1030 nm. Achievement of stable high-power laser operation requires an understanding of the LIDT to evaluate the energy density limits (LIDT) of the device before its implementation in the laser system. The LIDT of the fabricated structure was investigated using single and multiple-shot configurations. The damage morphology was analyzed using optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results are compared to those obtained for the same coating sequence but without a grating.

## 2 Design and testing procedure

### 2.1 GWOC design and manufacturing

The structure of the GWOC under test and a photo of the fabricated sample are shown in Fig. 1. The GWOC was designed using simulation codes based on rigorous coupled-wave analysis (RCWA) [14]. A fused silica substrate (refractive index, *n*=1.45) with a diameter of 1.5 inches was first structured with a grating having a groove depth of 35 nm and a period of 840 nm. Subsequently, 12 alternating quarter-wave layers of niobium pentoxide (Nb₂O₅, *n*=2.27) and silicon dioxide (SiO₂, *n*=1.48) were deposited on top of the grating by means of the magnetron sputtering technique [15].

The GWOC was designed to achieve a reflectivity of 95% for the radially (TM) polarized radiation and <25% for the azimuthally polarized radiation, both at a wavelength of 1030 nm. The low reflectivity for the azimuthally (TE) polarized radiation, which is required to obtain a radially polarized output, is achieved by coupling the incident beam to a leaky mode of the GWOC [5, 16]. Figure 2 shows that the reflectivity calculated for the radially (TM) polarized radiation agrees well with the measurements. The measured reflectivity for the azimuthally (TE) polarized radiation at the wavelength of 1030 nm is higher than expected. This difference is mainly attributed to the fabrication tolerances of the GWOC, particularly the etching depth and the sidewall angles of the grating. Nevertheless, the measured reflectivity of 44% is still sufficiently low to ensure that only radially polarized modes can oscillate inside the laser cavity. Furthermore, the reflectivity for TM polarized radiation was measured outside the grating area – unstructured areas acting like a normal output coupler (OC). As can be seen in Fig. 2, no difference is observed when compared to the measured values on the structured area.

The sample used for our test was fabricated using nano-imprint lithography [17]. The process involves producing the imprint master through electron-beam lithography on a silicon wafer. The stamp was then created on cyclo olefin
polymer sheets through hot embossing. The latter was then
used to transfer the structures onto the fused silica sub-
strate via the nanoimprint technique, followed by reactive
ion etching to precisely define the grating structures inside
the substrate. Finally, the structured fused silica was coated
with the Nb<sub>2</sub>O<sub>5</sub> and SiO<sub>2</sub> multilayers using plasma-assisted
reactive magnetron sputtering [9, 18, 19].

## 2.2 LIDT experimental setup

Figure 3 illustrates the arrangement used to determine the LIDT. The laser pulses were provided by a diode-pumped Yb:KYW laser (Amplitude, S-Pulse HP) operating at a wavelength of 1030 nm with a pulse duration of 500 fs. A comprehensive description of the setup and procedures can be found in [20].

In our experiments, the linearly polarized laser beam had
an effective beam diameter (d<sub>eff</sub>) of 78 µm on the sample
surface, determined according to the ISO standards 21254
[21] (which differs from the Gaussian beam diameter speci-
fied in ISO 11146 [22] by a factor of √2). The angle of
incidence (AOI) was adjusted to be less than 0.5° to be as
close as possible to the operational conditions of the grating,
which is commonly used at normal incidence while avoid-
ing back reflection of the probe beam into the laser system.
The sample was mounted on a motorized XY-stage, allow-
ing for precise and smooth tuning of the sample's position
in horizontal or vertical directions while maintaining the
beam path unchanged. To monitor the induced damages
in situ, a microscope with a 20x magnification was used to
observe the sample surface. Additionally, the sample was
also checked with an ex situ high-resolution microscope
after the experiments.

## 2.3 Experiment procedure and LIDT analysis

The methodology to determine the LIDT of optical com-
ponents involves a comprehensive approach designed to
evaluate their resilience to laser exposure under various con-
ditions. Our experiment used several configurations, includ-
ing single and multiple shots (10, 100, 1000). To ensure the
statistical reliability of the results, the effect of each applied
level of the pulse fluence was assessed across at least ten
separate positions on the sample. This approach provides
a broad dataset for analyzing the damage probability and
accounts for potential variability in the sample's fabrication
or material properties. The investigation was focused on the
TM (radial) polarization. Since the GWOC has circular grat-
ing lines, the linearly polarized probe beam was positioned
at the edges of the grating area, where the grating lines have
the largest radii of curvatures (RoC) and can be considered
to be nearly straight within the probed spot. Tests were also
performed outside the grating area (OC) to facilitate the
analysis and interpretation of the experimental results.

## 3 Experimental results and discussions

Figure 4 summarizes the LIDT fluences as a function of the number of laser pulses for both the GWOC and the OC areas of the sample. This comparison allows to assess the influence of the presence of the grating on the LIDT. In Fig. 4(a), the data correspond to fluences for which delamination of the top layer was observed. Figure 4(b) shows the LIDT fluences corresponding to a discoloration of the sample.

For single-shot exposures, the fluence leading to delami-
nation and discoloration of the top layer was found to be
0.36 J/cm² for both the GWOC and the OC. Under 1000-
on-1 testing conditions, this value decreased by 28% to

<figure><img src="image_4.png" /><figcaption>Fig. 3 Schematic configuration (not to scale) used for the determination of the LIDT</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 4 LIDT fluences as a function of the number of pulses for which top layer delamination (a) and structure discoloration (b) are observed</figcaption></figure>

0.26 J/cm² for the GWOC. In contrast, the decrease for the OC was only 11%, resulting in a fluence of 0.32 J/cm². The fluences leading only to irreversible discolorations of the sample were measured to be 0.28 J/cm² for the GWOC and above 0.32 J/cm² for the OC, independent of the number of shots. The accuracy of the LIDT measurements was estimated to be 9% according to [20]. These results indicate a minor difference in the LIDT fluence between the GWOC and the OC. This slight variation is attributed to the fact that the incident radiation does not couple into the multilayer waveguide when no grating is present.

Figure 5 shows a detailed examination of the damage morphology on both the GWOC and the OC. Fig. 5(a) and (e) display high-resolution optical microscope images, and Fig. 5(b) shows an SEM image of the GWOC. The shown area was exposed to single shots at different levels of pulse fluence. Optical microscope analysis reveals structural modifications for all fluences above 0.28 J/cm². The SEM analysis was performed using the Phenom XL G2 system and was operated in a low vacuum condition to minimize the charging effect. By this, no metallization was applied to the sample. The results of the SEM analysis indicate that the delamination of the top layer occurred at fluences of 0.38 J/cm² and above. The morphology of the damage was either bulging or complete delamination of the top layers. No surface damage was detected for fluences lower than or equal to 0.36 J/cm².

Figure 5(c) and (f) show the high-resolution optical microscope images, while Fig. 5 (d) displays an SEM image of the OC. Similar to the GWOC, top layer delamination, primarily in the form of bulging, was observed at the LIDT fluence of 0.37 J/cm² and above. For fluences above 0.32 J/cm², discoloration was visible in the optical microscope

images. No structural modifications were observed for fluences below 0.32 J/cm².

Figure 6(a) shows a magnified SEM image of the damage obtained at a pulse fluence of 0.4 J/cm². This area was further analyzed with an Atomic Force Microscope (AFM, Dimension 3100d Veeco) at three different planes (labeled 1, 2 and 3 on Fig. 6(a)). Figure 6(b) shows the obtained scan results confirming an average depth of approx. 230 nm. Considering the nominal thicknesses of SiO₂ and Nb₂O₅ layers to be 174 nm and 114 nm, respectively, the AFM profilometric analysis together with the SEM analysis confirmed that the damage led to the full delamination of the SiO₂ layer and partial damage of the Nb₂O₅ layer. These observations indicate that damage likely initiates in the Nb₂O₅ layer, appearing in the form of the optical structure discoloration, and progresses to delamination of the overlying SiO₂ layer once a certain threshold is reached.

As shown in Fig. 7, a distinct damage morphology was observed in experiments involving multiple laser shots. For the 10-on-1 tests conducted near the damage threshold, delamination of the top layer was observed. In this test series, the damage typically has a circular shape, with an average diameter of approximately $φ_x=φ_y=8$ µm, as determined by high-resolution optical microscopy. However, for the 100-on-1 tests, the damage profile evolves into an elliptical shape, with $φ_x=8$ µm and $φ_y=11$ µm. The pattern becomes even more pronounced during the 1000-on-1 tests, where the extent of the damage further increased to about $φ_x * φ_y=9 * 22$ µm.

These observations indicate that the dimension of the damage increases in y-direction depending on the number of shots, indicating a directional dependence of the damage which is determined by the applied laser polarization. In fact, the elongation of the damage occurs in a direction
<figure><img src="image_6.png" /><figcaption>**Fig. 5** High-resolution optical microscope (a, c, e and f) and SEM (b and d) images of the laser exposed areas on the GWOC (a, b and e) and OC (c, d and f) at different fluences</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 6 SEM image of the damaged area at the fluence of 0.4 J/cm² (a) and its profilometric analysis for three different planes (b)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 7 Optical microscope images of the damages taken after: (a) 10 (b) 100 and (c) 1000 shots</figcaption></figure>

which is perpendicular to the polarization orientation of the incident laser beam as reported based on experimental and theoretical investigations in [23].

Our experimental results confirm the simulation analysis shown in Fig. 8. The simulations show the calculated electric field distribution inside the GWOC and the OC, revealing a strong field amplitude in the high-index material as well as at the SiO₂/Nb₂O₅ interface. These simulation results suggest a field enhancement that led to the morphological changes observed with the SEM and AFM analysis reported in Figs. 5 and 6, respectively. The characteristic bulging and the measured depth of the damage, corresponding to the SiO₂ and Nb₂O₅ layers, corroborate the simulations. The fluence levels resulting in structural discoloration reveal the Nb₂O₅ layer as a critical factor in the damage mechanism, probably due to its lower LIDT fluence compared to the low-index SiO₂ material [15].

To further compare the experimental results to simulations, we used the empirical equation

$$LIDT = 0.63 (\pm 0.031 Jcm^{-2}eV) Eg - 1.61 (\pm 0.14Jcm^{-2})$$

given in [24] to estimate the theoretical LIDT for single-pulse irradiation with a pulse duration of 500 fs at a wavelength of 1030 nm, where Eg is the bandgap of the material. For our multilayer stack, consisting of SiO₂ and Nb₂O₅, niobium pentoxide has a lower bandgap (Eg=3.46 eV) compared to silicon dioxide (Eg=7.75 eV) [25, 26] making it sensitive to laser-induced damage. Considering the maximum electric field intensity in the Nb₂O₅ layer, the calculated damage threshold was found to be 0.26 J/cm², which is consistent with the measured result corresponding to the discoloration of the Nb₂O₅.

These observations suggest that the enhanced electric field intensity at material interfaces plays an important role in damage initiation. Combined with the lower laser damage threshold of Nb₂O₅, this factor is expected to significantly contribute to the observed delamination mechanism. The comparison between the theoretical data and the observations from SEM and AFM provides a good insight into the damage processes affecting grating waveguide structures (GWOC in the present case) and highlights the importance of material composition for their durability and functional threshold in laser applications. It was verified that the
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 8** Calculated electric field amplitudes inside the GWOC (a) with a magnified view of the top layers (b), and inside the OC (c) with a magnified view of the top layers (d)</figcaption></figure>

presence of the grating structure does not significantly affect the LIDT fluence when compared to the OC. These findings open up further possibilities to explore alternative materials for GWOC fabrication that will enhance its damage threshold. This can be achieved by selecting designs that minimize electric field enhancement in the grating region. Regarding high-index coating materials, Ta₂O₅ and HfO₂ offer promising options, due to their larger bandgaps [26] compared to the Nb₂O₅. This material combination is expected to increase the laser damage threshold of the GWOCs by a factor of at least four, approaching values higher than 1 J/cm². This will be the subject of our future investigations for the next generations of grating waveguide structures.

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

## 4 Conclusion

In conclusion, the LIDT and damage mechanisms of a circular GWOC were investigated and compared to those of the corresponding OC without grating. Obtained results reveal only a minor influence of the grating on the LIDT. Under single-shot exposure, delamination occurred at a fluence of 0.36 J/cm² for both the GWOC and the OC. For 1000-on-1 conditions, the LIDT fluence decreased by 28% for the GWOC (0.26 J/cm²), and by 11% for the OC (0.32 J/cm²). The fluence required for the irreversible discoloration was found to be 0.28 for the GWOC and 0.32 J/cm² for the OC. These findings define the energy density limits that can be applied to the device in the laser system.

Damage analysis performed using optical microscopy and SEM confirmed these results. AFM of the delaminated areas revealed a damage depth of approximately 230 nm, indicating full delamination of SiO₂ and partial damage of the Nb₂O₅ layer. This highlights the increased susceptibility
of Nb₂O₅ to laser damage due to its lower bandgap. Simulations of electric field distribution supported these observations, showing field enhancement at material interfaces. Future research will focus on enhancing the LIDT of grating waveguide structures by optimizing their design and exploring alternative materials.

**Author contributions** DD wrote the manuscript and conducted the laser experiment, analysis and interpretation of the data. MAA provided the overall design of the grating waveguide structure, including the calculated reflectances and the Electrical field distribution inside the structure. JZ, FabL provided the coating of the substrates. DJ conducted the precise ellipsometry measurement of the coated materials. FL fabricated the grating waveguide structure. MS assisted in the preparation of the experimental setup. LG provided the experimental setup and assisted in the interpretation of laser damage results. AS assisted in analysis (SEM, AFM) of the laser damages. LG, MAA, TG assisted in the preparation of the manuscript. All authors reviewed the final manuscript.

**Funding** Open Access funding enabled and organized by Projekt DEAL. This work was supported by funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 813159. MK also acknowledges the financial support of the Academy of Finland through the flagship on "Photonics research and innovation" (PREIN, 346518).

**Data availability** No datasets were generated or analysed during the current study.

**Declarations**

**Competing interests** The authors declare no competing interests.

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