Calcination-Enhanced Laser-Induced Damage Threshold of 3D Micro-Optics Made with Laser Multi-Photon Lithography

: Laser Direct Writing (LDW), also known as 3D multi-photon laser lithography of resins, is a promising technique for fabricating complex free-form elements, including micro-optical functional components. Regular organic or hybrid (organic–inorganic) resins are often used, with the latter exhibiting better optical characteristics, as well as having the option to be heat-treated into inorganic glass-like structures particularly useful for resilient micro-optics. This work is a continuation of our SZ2080™ calcination development of micro-optics, speciﬁcally studying the Laser-Induced Damage Threshold (LIDT). Such sol–gel-derived glass 3D micro-structures, particularly those that undergo heat treatment, have not been well-characterized in this respect. In this pilot study, we investigated the LIDT using the Series-on-One (S-on-1) protocol of functional micro-lenses produced via LDW and subsequently calcinated. Our results demonstrate that the LIDT can be signiﬁcantly increased, even multiple times, by this approach, thus enhancing the resilience and usefulness of these free-form micro-optics. This work represents the ﬁrst investigation in terms of LIDT into the impact of calcination on LDW-produced, sol–gel-derived glass micro-structures and provides important insights for the development of robust micro-optical devices.


Introduction
The field of laser multi-photon lithography, also known as Laser Direct Writing (LDW), is witnessing rapid progress [1], with an increasing number of micro-optical devices [2] being produced at the micro-scale. These devices include conventional [3] and Fresnel micro-lenses [4], holographic elements [5], polarization-controlled lens arrays [6], metaoptics [7], and multi-component systems [8], most notably, micro-objectives [9], non-linear excitation imaging systems [10], as well as anti-reflective-coated complex systems [11]. Furthermore, grayscale lithography has been utilized to create step-free micro-lens systems either by using increased resolution [12] or variable laser fluence [13]. The LDW method may also be used as an alternative for "classic" lithography, especially flexible maskless diffractive imaging systems [14]. Furthermore, precise systems such as microfluidic devices with complex geometries and high aspect ratios [15], as well as actively tunable photonic crystals with sub-micron features [16], also benefit from LDW.
Distinct benefits of the LDW method are its <200 nm resolution [17]; the ease of manufacturing compared to traditional methods in the micro-and nano-scale; the vast availability of organic and hybrid resins; and various post-processing steps, such as optical coating deposition [11] and calcination into inorganic glass [18][19][20]. However, one problem that has been often overlooked with micro-optics produced by LDW is their laser-induced damage threshold (LIDT) behavior. This can pose a significant limitation for applications that involve modern high-intensity pico-and femtosecond pulses since high LIDT is not always guaranteed.
Classical measurement of LIDT for coating (thin film) and bulk objects is standardized [21] and industrially beneficial; however, it is rarely utilized for LDW-fabricated elements. Several attempts have been made to measure such elemental LIDT, and the results have shown variations based on various factors, such as the resin type used (organic or hybrid) [22][23][24], the presence of a photo-initiator (PI) [25], and the type of structure produced, whether it is a thin-film [24] or bulk object [26] or a free-form device [27,28]. While some methods can increase LIDT, such as using resins without PI [29] or a less organic composition [30], we propose an alternative approach. We aim to create purely inorganic structures using a heat-based post-processing method (calcination) while retaining the benefits of the LDW method [31].
For hybrid prepolymers such as SZ2080™ [32], calcination above 1000°C results in an inorganic composite glass or glass-ceramic phase while retaining the printed geometry with homogeneous and repeatable shrinking [33]. It is often assumed that transparent glassy structures should feature higher LIDT values and, therefore, must be more resilient to high-intensity radiation, but this idea has not been tested before [27]. However, highly mechanically resilient silicon oxycarbide structures have already been fabricated by utilizing calcination [34].
In this paper, we aim to fabricate suspended and functional structures-namely, microlenses-heat-treat them, and confirm the useful increase in LIDT. Figure 1 illustrates the process and the resulting structures.

Fabrication
To produce the lenses, we used the low-shrinkage organic-inorganic prepolymer SZ2080™ [35]. The preparation and exposure conditions ( Figure 1a) were selected following the methodology reported in [35], using a 63x 1.4 NA Plan-Apochromat immersion oil objective (Zeiss), 517 ± 10 nm wavelength, and 144 fs pulse duration with a repetition rate of 76 MHz. Scanning speed was set to 2000 µm/s utilizing Aerotech IFOV technology; beam intensity was −0.4 TW/cm 2 . Development was carried out in Methyl isobutyl ketone for 15 min. The lenses were printed on a quartz substrate, with the final baseline geometry being a plano-convex, 50 µm diameter, 300 µm focal length lens with a thickness of approximately 2 µm. To support the lenses above the quartz substrate, pillars were printed with an inclination angle of 35° [33] and a total height close to 30 µm, giving us pristine non-calcinated (NCA) lenses. Heat treatment was performed at 1100 • C with a rise time of 12 h and held for 3 h, resulting in calcinated (CA) lenses. Transitional reactions with the ambient atmosphere and carbon-induced darkening are expected at around 500-600 • C. The final transparent phase, characterized in this work, is generated at the highest treatment temperature. (Figure 1b overviews the calcination transition).

LIDT Metrology
Non-calcinated (NCA) and calcinated (CA) samples were qualitatively examined and exposed to the probe beam in damage tests in an array form (Figure 1c). Qualitative characterization of their ability to form an image of an object placed under microscope illumination was performed in a bright-field microscope to confirm their imaging function before and after LIDT measurements. See Figure 2a for the illustrated concept. The imaging function was used to confirm the occurrence of significant and catastrophic damage events (Figure 2b-e). After, they were characterized using a Scanning Electron Microscope (Model Hitachi TM1000 SEM, Figure 3). Tests were performed on arrays of micro-lenses. Arrays, mainly composed of 16 lenses, were divided in half to account for damage experiments for NCA (reference) and CA (test) micro-optics. Tests were performed in the following sequence: an array of lenses was printed; half of the lenses were exposed before calcination; then, the array was calcinated as described previously; and finally, the second half of the lenses was exposed. The laser system parameters used for all experiments were as follows: wavelength λ 1 = 1030 nm and λ 2 = 515 nm, repetition rate f = 200 kHz, pulse duration τ = 300 fs, Plan-Apochromat Zeiss 20x objective (0.8 NA). S-on-1 damage tests [21] were performed with both wavelengths, exposing the lenses for 50 ms and 5 s, corresponding to 10,000-on-1 and 1,000,000-on-1 pulses.
S-on-1 testing was applied in two regimes (as illustrated in Figure 3f) by varying the applied beam diameter. The first regime is referred to as a local-damage protocol, where the beam diameter is around 4 µm (1/e 2 intensity level) on the sample. The second regime is referred to as delocalized, where the probe beam diameter is 20 µm, attained by shifting the (relative to the lens) focus position of the laser beam. The delocalized regime was used to demonstrate the expected behavior where the full aperture of the lens is used, such as in high-intensity focusing, while the local regime shows the behavior of a highly focused laser beam on the optics themselves.

Morphology
The morphology of the observed damage is depicted in Figure 3. It shows an example of a pristine lens (Figure 3a) with a diameter of 50 µm that shrinks to 30 µm (40% shrinkage, expected and matching [33]) after calcination. The localized damage regime results in small (<1 µm) damage sites, while the delocalized-damage regime produces large (approx. 10 µm) damage sites for NCA lenses (Figure 3b). The latter results in catastrophic damage, as most of the aperture becomes distorted and the lenses lose their imaging function. However, CA lenses, as shown in Figure 3d,e, retain their imaging function and exhibit small-diameter ablation sites similar to those produced by fs-laser surface ablation [36]. The morphology does not vary significantly depending on the wavelength used. The only observed difference for NCA lenses is the prominent brown discoloration, particularly for λ = 1030 nm. Previous research by Jonusauskas et al. [37] had concluded that the damage mode of non-calcinated SZ2080™ structures in the femtosecond regime is volume heat accumulation. By its nature, SZ2080™ should have a similar heat conductance to other hybrid polymers such as PDMS ( [38], 0.2 W/m*K) or Ormocomp (0.4 W/m*K estimated from [39]). It was suggested that better heat load mitigation might increase LIDT. Butkute et al. [27] explained the phenomena as Coulomb explosion-driven dielectric breakdown. In both cases, calcinated Si-Zr glass micro-structures excel in LIDT by potentially having better heat transfer characteristics (SiO 2 -ZrO 2 composites [40] 1 to 3 W/m*K). In addition, the minimum bond energy for Si-O (89 kcal/mol) bond in glass is higher than for C-C (80 kcal/mol) or C-O (79 kcal/mol) bonds that constitute the weaker link [41]. Figure 4 presents results of LIDT measurements, which include a combination of localized and non-localized damage regimes for NCA and CA lenses, as well as exposure to 10 4 -on-1 and 10 6 -on-1 pulses at 515 and 1030 nm wavelengths. The LIDT of NCA lenses at the localized regime was measured to be F = 0.12-0.17 J/cm 2 and F = 0.6-0.8 J/cm 2 at 515 and 1030 nm, respectively. The delocalized regime resulted in LIDT values of F = 0.07-0.08 J/cm 2 and F = 0.28-0.31 J/cm 2 . The results show that local regime LIDT values in calcinated micro-optics exhibit the highest increase. Using a 300 fs laser pulse duration at 1030 nm, a 3-fold increase in the laser-induced damage threshold was observed, from F = 0.6-0.8 J/cm 2 to F = 2.3-2.7 J/cm 2 . This increase was the highest among all measured values and was consistent across the entire exposure duration range from 50 ms to 5 s. The use of a second harmonic (515 nm) resulted in the highest percentile increase in measured LIDT, with a 6-fold increase in LIDT observed across all exposure durations, from F = 0.12-0.17 J/cm 2 to F = 0.8-0.9 J/cm 2 .

LIDT Values
However, when exposed in the delocalized regime, with an increased exposure area on the CA lens compared to NCA counterparts, a decrease in measured damage thresholds is observed. For the 1030 nm wavelength, the effect of calcination was less pronounced, with only a 1.5 times increase in LIDT observed from F = 0.28-0.31 J/cm 2 for NCA to F = 0.38-0.47 J/cm 2 for calcinated lenses, while a 515 nm wavelength exhibited a 2.7 times increase in LIDT-from F = 0.07-0.08 J/cm 2 for NCA to F = 0.19-0.21 J/cm 2 for CA lenses.

Discussion
The results provided above are interesting in multiple ways. To begin with, the LIDT values for NCA localized damage regimes were consistent with previously reported scientific data, with F 1030 = 0.57 J/cm 2 and F 515 = 0.13 J/cm 2 [24,25,30]. Secondly, the reported CA values exceed all known previous observations for NCA SZ2080™, consistently and regardless of irradiation area or testing process. Our findings demonstrate a significant increase in damage threshold, reaching a 3-6 times increase as a conservative estimate, which supersedes all measured values before. The maximum measured LIDT value at λ = 1030 nm is F = 2.74 J/cm 2 , which is relatively large and approaches the level of fused silica [26] F = 3.11 J/cm 2 .
In contrast, the LIDT measured using the non-localized regime is lower compared to known reported values [24,25,30]. The previous experiments that employed S-on-1 damage testing protocols only exposed the samples to up to 1000 pulses. The current tests featured at least by an order larger pulse amounts (10 4 -on-1 and 10 6 -on-1 protocols). Therefore, current measurements of the degradation effects in time offer a novel insight into calcinated hybrid polymer fatigue behavior.

Conclusions
In this study, we have reported laser-induced damage threshold measurements of SZ2080™ material after calcination for the first time. Our findings demonstrate a significant increase in damage threshold, reaching 3-6 times as a conservative estimate, which supersedes all measured values before. These results are promising, as they suggest that the LDW method combined with heat treatment can offer a technologically viable pathway to producing optical-grade glass-level performance in micro-optical elements for use in harsh environments in visible and IR wavelengths, as previously postulated.
As this pilot study covers only femtosecond LIDTs, further research is needed for other significant regimes, such as the nanosecond and continuous-wave damage tests. In summary, our findings contribute to the field by providing new insights into the performance of SZ2080™ material after calcination and offer exciting possibilities for the development of high-performance micro-optical elements. Furthermore, increased LIDT usually results in an increase in the optics' useful lifetime, particularly useful in femtosecond applications [42]. Finally, increased fluence pump sources may be utilized in Glass-on-Glass [43] LDW fabricated micro-lasers, consisting of high-resilience polymers [44].

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Abbreviations
The following abbreviations are used in this manuscript:

LIDT
Laser-Induced Damage Threshold LDW Laser Direct Writing S-on-1 Series-on-One PI Photo-Initiator CA Calcinated NCA Non-Calcinated SEM Scanning Electron Microscope