3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

CHAPTER 12.1

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities S. Varapnickasa, A. Žukauskasa, E. Brasseletb, S. Juodkazisc,d, Mangirdas Malinauskasa a

Laser Research Center (LRC), Vilnius University, Vilnius, Lithuania University Bordeaux, CNRS, LOMA, Talence, France c Centre for Micro-Photonics, Swinburne University of Technology, Hawthorn, VIC, Australia d Melbourne Centre for Nanofabrication (MCN), Australian National Fabrication Facility (ANFF), Clayton, VIC, Australia b

1 Introduction Trends of evolving technology usually are based on miniaturization and increasing complexity of devices or combining both of them. Three-dimensional (3D) laser structuring of materials employing ultrashort pulses is widely used in photopolymer rapid prototyping [1–5], spanning microoptical elements [6, 7], optical actuators [8–10], microfluidic chips [11, 12], scaffolds for cell growth and tissue engineering [13, 14], templates for plasmonic metamaterials [15, 16], and photonic crystals (PhC) [17, 18]. The technique has originated from nonlinear microscopy providing 3D-confined recording inside a ultraviolet (UV)sensitive polymer [19, 20]. Now it is one of the most precise additive manufacturing technologies ever developed in both scientific and industrial fields [21–25]. Despite already appearing as commercially available setup still some active engineering is being carried out, for instance, implementation of active autofocusing and machine vision for sample detection and positioning [26, 27]. In order to achieve resolution of structuring required for microoptical and PhC structures operational at visible spectral range, the feature dimensions should be controlled with
10 nm spatial precision [28, 29]. Despite recent advances in techniques’ versatility, regarding specifically optical applications, the research inherits some gaps due to an initial pure engineering approach to 3D fabrication. For instance, the materials designed for 3D direct laser writing (DLW) are not fully (or barely at all) characterized [30, 31], and their optical performance and reliability [32, 33] have to be established. Thus, despite the DLW technique being mature and commercially available, some standard knowledge for its application in fabricating microoptical and photonic devices must be considerably improved, namely, the linear and nonlinear properties of the material itself [34], its modulation due to DLW structuring and postprocessing [35], taking into account various side effects and artifacts [36], for

Three-Dimensional Microfabrication Using Two-Photon Polymerization https://doi.org/10.1016/B978-0-12-817827-0.00012-6

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Fig. 12.1.1 Manufacturing steps in 3D DLW lithography. (A) Pinpoint (maskless and selective) exposure by tightly focused femtosecond pulsed beam; (B) removal of unexposed volume by development in organic solvent; (C) revelation of manufactured micro-/nano-object.

example, an optical resistance [37]. These investigation vectors are currently actively performed by many research groups. In Fig. 12.1.1 is demonstrated how optical and thermal resist exposure protocol via tight focusing of high-repetition femtosecond pulses can offer 3D nanofabrication using the popular DLW hybrid organic-inorganic resist SZ2080 [38]. The chosen polymer represents organically modified silicate (ORMOSIL) material class [39]. Optical characterization of the novel material and structures made out of it, specifically for the microoptical and nanophotonic applications, is delivered. Advances in this direction open new prospects for the precise manipulation of light in space, intensity, phase, and spectrum at the microscale [40, 41]. Exact knowledge of real and imaginary parts of refractive index (n + ik) of SZ2080 is the first step in creation of complex and multicomponent optical elements combined with other resists and optical materials.

2 Optical materials The most popular materials in the case of 3D laser photopolymerization are acrylate- and epoxy-based resins and resists developed decades ago before the era of tabletop femtosecond lasers for one-photon stereolithography. The photopolymers are photosensitized for wavelength of excimer laser at 308 nm or i-line of Hg-light source at 360 nm. Photosensitization and initiation of a nonlinear photopolymerization is fundamentally different by its nature from that developed for the one-photon processes in the case of ultrashort laser pulses close to infrared (IR) wavelengths. Excitation of electronic subsystem occurs faster than an uptake of energy into ionic subsystem proceeding via electronion equilibration, recombination, and thermal diffusion in the case of ultrashort sub-1 ps laser pulses. Since thermal processes are very efficient in chemical modifications of materials, by tuning a photoexcitation with a controlled thermal activation the materials’ processing is acquiring new functionalities when femtosecond laser pulses with tens-ofmegahertz repetition rate are used [18, 20, 21, 42–46].

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

Acrylate- or epoxy-based optical glues are attractive due to provision of high optical transparency and straightforward application such as sticking under UV light and unsticking under increased temperature. Next to them are organic-inorganic hybrid materials that are used in optics as both dielectric coatings and optical elements. They are desired in optical/photonic applications due to their material processing and behavior combining the best of both worlds—organic (easy processing) and inorganic (structural rigidity). Furthermore, their properties such as transparency, refractive index, mechanical stiffness, chemical reactivity (inertness)/swelling, temperature resistance, biocompatibility, and many others can be gradually yet strictly controlled by changing the initial ingredient ratios. Lastly, it enables synthesis of optically active (fluorescent) media that can be selectively deposited in a 3D space with
100 nm feature definition and even higher accuracy [47–49]. The combination of two or more hybrid materials with different functionalities in the same system has allowed the preparation of structures with advanced properties and functions. Furthermore, the surface functionalization of the 3D structures opens new avenues for their applications in a variety of nanobiotechnological fields [50]. SZ2080 materials have emerged as the one providing ultralow shrinkage [51] and rigid mechanical properties [52], and are biocompatible in vitro [53] as well as in vivo [54]. Analysis provided here is focused on SZ2080 hybrid photoresist suitable for fabrication of 3D nanostructures/microstructures, especially for optical and photonic applications (Fig. 12.1.2). Simplicity of usage contributes to popularity of this resist. It is, first, spin-coated with a controlled thickness or drop casted on arbitrarily shaped surface on different materials such as glass [55], metal [56], and black Si [57] for mask projection as well as DLW exposure. Resists that have properties comparable with glass (a transparent dielectric) can be used to polymerize complex 3D shapes with high resolution and low surface roughness on mesoscale dimensions ranging from 10 nm to sub-1 mm [58]. Polymerization is usually achieved via photoinitiator additives up to few weight percent to absorb at the wavelength of exposure and to promote polymerization that occurs via chemical bonds opening (formation of radicals) and subsequent crosslinking or to induce crosslinking by highly localized heating. In SU-8, each crosslinking is releasing molecules that opens bonds and promotes polymerization further in a chain reaction fashion. DLW with ultrashort laser pulses at λ1 ¼ 800 and 1030–1060 nm wavelengths were mostly used due to popular and affordable femtosecond laser at that spectral range.

2.1 Transmittance, refractive index, and extinction coefficient of polymers (SZ2080) Stereolithography has started field of 3D printing with single-photon (direct) absorption on the surface of a liquid resin [59]. Recent advances and availability of high-quality microscopes have enabled optics to be used for imaging microscale volumes and it

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Fig. 12.1.2 The transmittance spectrum of the photosensitized SZ2080 and nonphotosensitized SZ2080 thin films (black solid curve shows the transmittance of the UV FS substrate (A)). Refractive index (B) and excitation coefficient (C) of the pure and PI-doped SZ2080 determined from the ellipsometry data. FS, fused silica.

can now perform light energy delivery (absorption) and drive 3D printing on (nano) microscale in a very similar fashion as 3D thermal extrusion produces larger millimeter-scale structures (Fig. 12.1.2) [60]. Understanding of the light-matter interaction under tight focusing, high irradiance/ intensity, radiative and thermal energy transfer in subwavelength volumes, as well as photochemistry will help to create a technology that either defines high fabrication resolution or is optimized for high throughput or can mix both at the required locations on the sample. The high resolution and high throughput have to be managed due to their countercompeting nature. DLW and photocuring of SU-8 without any usual postexposure annealing required for chemical reactions of crosslinking was shown to deliver subwavelength resolution of 3D polymerization [61] due to direct optical action. Currently a significance of thermal, linear, nonlinear, photochemical, and photophysical mechanisms in laser-driven polymerization is strongly debated, but there is no principal disagreement that both direct heating and thermal curing are contributing to the polymerization [62–64] and are compatible with a high-resolution fabrication.

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

2.2 Material resistance under light irradiation An ISO-certified laser-induced damage threshold (LIDT) testing method was applied to characterize photopolymers widely used in 3D laser microlithography/nanolithography. For the first time, commercial as well as custom-made materials, including epoxy-based photoresist (SU-8), hybrid organic-inorganic polymers (OrmoComp and SZ2080), thermopolymer (polydimethylsiloxane (PDMS)), and pure acrylate (poly(methyl methacrylate) (PMMA)), are investigated and directly compared. The presence of photoinitiator molecules within a host matrix clearly indicates the relation between damage threshold and absorption of light. To simulate single-photon and multiphoton absorption processes, optical resistance measurements were carried out at both fundamental (1064 and 1030 nm) and second-harmonic (532 and 515 nm) wavelengths with laser pulse durations in nanosecond (Fig. 12.1.3A–D) and femtosecond (Fig. 12.1.3E–H) regimes. Damage morphology differences from postmortal microscopic analysis were used to discuss the possible breakdown mechanisms. The obtained characteristic values of damage threshold reveal a potential of photopolymers being used in high-power laser applications [37] and are discussed in detail next. The material thickness is determined by the used material’s viscosity and the implemented experimental measurement conditions. The SZ2080 was applied for spin coating as it is used for performing DLW lithographic structuring [21, 38, 65]. In the studied case due to interference effects, the peak intensity of electric field was always somewhere around or coinciding with the surface of the polymer layer. The deviation of film thickness and its possible influence on LIDT value were taken into account and are included within the error bars. In all cases, damages were induced in polymer films and neverdamaged substrates or ionized air. LIDT values of glass substrates or ambient air are

Fig. 12.1.3 DIC (colored triangles) and SEM (gray triangles) images of the laser-induced damage morphology and LIDT values of the pure and photosensitized SZ2080 thin films at 1-on-1 regimein nanosecod (A–D) and femtosecond (E–H) pulse modes at both wavelengths (515 and 1030nm) [34].

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significantly higher than those of the examined polymer films. This was observed during the performed experiments as well as can be found as a well-known fact in literature (see, e.g., Ref. [66]). A nanosecond Q-switched Nd:YAG (SpitLight Hybrid, InnoLas Laser GmbH) and a femtosecond mode-locked Yb:KGW (Pharos, Light Conversion Ltd.) laser system were used for the LIDT measurements according to ISO 21254-2:2011 [67] standard in 1-on-1 and S-on-1 (S ¼ 1000) regimes by using automated test station [68]. In single-shota mode, the nanosecond laser emitted λ ¼ 1064 nm and frequencydoubled λ ¼ 532 nm wavelength pulses with a duration of 11 and 6.2 ns, respectively, at 50 Hz repetition rate. The laser beam was focused down to 250  10 and 133  4 mm2 spots at 1/e2 intensity level in the sample plane, respectively, to the first and second harmonics. In the case of the femtosecond laser system, it provided 343-fs duration pulses at 1030- and 515-nm wavelengths, while repetition rate was set to 50 kHz. The laser beam was focused down to 65  0.2 mm2 beam spot at 1030 nm and 46.5  0.2 mm2 at 515 nm. It featured slightly elliptical Gaussian-like beam profile. However, effective beam diameter was estimated and ellipticity did not influence the results. The measurement procedure was performed in an automated manner by computer-controlled sample positioning, laser fluence attenuation, and damage detection automation. Onlineb damage detection in situ was based on change in optical scattering of irradiated site. Additionally, offlinec damage inspection was performed, registering laser-induced changes with DIC microscope (BX51, Olympus) at 40 magnification. Thin film morphological surface examination was performed with scanning electron microscope (SEM; TM1000, Hitachi). A 20-nm gold film was sputtered on top before the inspection with SEM. The optical resistance (to optical damage) values similar to those in sol-gelsynthesized antireflection (AR) coatings found in literature were observed for SZ2080. Damage threshold of the pure SZ2080 at mostly used testing conditions (1064 nm wavelength, one-on-one regime, nanosecond pulse mode) is equal to
20 J cm2, whereas the LIDT of AR coatings at the same experimental conditions ranges from 10 to 80 J cm2 [69–75]. It is noteworthy that SZ2080 is primarily developed for the DLW 3D lithography applications and it was not intended to be optimized a

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One pulse per spot (one-on-one) measurement regime reveals more fundamental material properties and is linked with the initiation of laser-induced damage. Multishot (1000-on-1) measurement is used to include fatigue effects attributed to defect state generation (bond braking) and possible thermal energy accumulation. For practical applications fatigue effects are important issue; thus, multishot was employed next to single-shot LIDT tests. The online damage detection system is based on optical scattering from irradiated sample surface. A photodiode sensor was employed to track laser-induced surface changes. The damage-detecting optical scattering signal was recorded for every pulse. The offline inspection of irradiated sites is performed after the irradiation exposure under a differential interference contrast (DIC) microscope. Here, criterion of damage is any visible modifications that can be seen with it.

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

for the optical coatings. Thus, evaluation of the SZ2080 and the effect of the photosensitization by means of LIDT reveal high potential of this hybrid inorganic-organic polymer for applications in high-power optical systems. It was demonstrated that existence of the photoinitiator in the host matrix of the hybrid polymer SZ2080 leads to the decreased damage threshold to 10- and 2-folds for the second harmonics at both nanosecond and femtosecond pulse modes [34]. It is known that absorption of the laser beam energy is directly dependent on both laser wavelength and material properties, i.e., an optical band gap Eg. The Eg of SZ2080 + PI is smaller compared with that of the pure SZ2080 [34]. Obviously, higher extinction coefficient results in increase of the absorption, which yields a more efficient laser energy transfer via free electron generation to the lattice that in turn causes optical breakdown and reduction of the damage threshold of the SZ2080 + PI. Hence, photosensitization is beneficial for the efficient photopolymerization (high fabrication throughput and lower employed DLW light intensities); however, it is simultaneously undesirable in terms of applications at green and shorter wavelengths.

3 Microoptical elements and components Early efforts in DLW were focused on PhC applications due to clearly understood potential of localization of focal volume in multiphoton absorption. Since PhC stopgaps occur at wavelength that is approximately twice larger than periodicity of spatial 3D modulation of refractive index, λ ∝ 2a, and volume fraction of the PhC has to be <50%, a high resolution becomes a driving argument in 3D DLW. However, apart from PhC applications, there is much wider demand of larger area/volume fabrication where resolution is not the major factor. Fabrication of optical elements with DLW is downscaled to dimensions from several micrometers to sub-1 mm and can be integrated into complex designs of microoptical and fluidic systems and chips. Unique capabilities of DLW are demonstrated next for specific families of optical elements. SEM images are used to depict their shape, scale, and quality.

3.1 Miniature standard refractive optical elements It is believed that direct laser polymerization will allow fabrication of hybrid microoptics that combines singular optical properties together with functionalities of either a refractive (lenses, prisms, axicons) or a diffractive (gratings) nature. The versatility and applicability of the laser polymerization technique have already been proven by fabricating single optical elements of complex surfaces, including aspherical, Fresnel, and solid immersion lenses. An absolutely unique and silent feature of DLW is the final optical finish of surface that does not require any postprocessing. This makes it possible to make large-curvature optical quality surfaces (Fig. 12.1.4).

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Fig. 12.1.4 Miniature (minimized) conventional optical components. (A) An aspheric lens [76], (B) a prism [28], (C) a grating [48], and (D) conical lens (axicon) [36].

3.2 Singular microoptics Free-form microoptical elements are complicated to produce via standard manufacturing techniques, if indeed it is possible to do so. In contrast, the flexibility and high spatial resolution of DLW lithography perfectly suit such a microfabrication challenge. A demonstrative example is the case of microoptical elements endowed with singular features, which corresponds in practice to abrupt geometrical variations on wavelength scale. Indeed, DLW easily does the job where mechanical tools would give up. This is exemplified in Fig. 12.1.5 where a few realizations are displayed. Fig. 12.1.5A presents a so-called refractive spiral-phase plate viewed from the top. Such an element is characterized by a height that has a linear dependence with respect to the azimuthal angle, which leads to a radial step clearly identified in Fig. 12.1.5A and singular central part where all heights meet. An appropriate choice of the step height allows producing so-called optical vortices from usual Gaussian beams, i.e., light beams carrying a predetermined amount of optical orbital angular momentum (AM) [79]. In practice, the higher is the step height, the larger is the orbital AM imparted to the incident beam, which may eventually be detrimental to the obtention of relatively flat optics. Such a difficulty is circumvented by designing multistep spiral phase plates, as illustrated in Fig. 12.1.5B. By doing so, such singular microoptical elements may find optomechanical applications, for instance, on-demand light-induced rotation of microobjects by using the generated optical vortices [80] or to realize micromechanical rotors to steer fluid at a miniature scale

Fig. 12.1.5 Free-form 3D elements. (A, B) Single-step and multistep spiral-phase plates (SPP) [77]. (C) a freestanding axicon (from top) merge with an SPP (from bottom) [41], and (D) a topological lens (see text for details) [78].

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

[81]. More elaborated singular microoptical elements can also be produced; an example is shown in Fig. 12.1.5C where several functions are combined into a monolithic design, namely, the generation of diffraction-free beams by use of an axicon together with the generation of optical vortices by use of a spiral-phase plate [41]. Another example is illustrated in Fig. 12.1.5D where a lens with multicusp cross-section is shown. Such a topological lens consists of the 3D refractive version of recently introduced closed-path designs endowed with geometrical singularities (e.g., hypocycloids or epicycloids) to shape the topology of light [82, 83]. In view of the growing interest in the topology of light fields, it is very likely that the ability of DLW to fabricate singular microoptical elements with arbitrary complexity will find further applications.

3.3 Polarization microoptical components An important class of free space optical components is polarizing or polarization-sensitive devices. Polarization control has been already demonstrated in both bulk and subwavelength patterned dielectric microstructures. For the bulk microoptical elements, the phase retardation can be introduced by means of total internal reflections inside an optical component. The practical implementation of such component is depicted in Fig. 12.1.6A. The polymer structures consisting of a collimating lens and one (red) or stack of two (green) Fresnel rhombs were integrated directly

Fig. 12.1.6 (A) SEM image of a quarter-waveplate structure (with only one Fresnel Rhomb, left) and of a half-waveplate structure (with two stacked Fresnel Rhombs, right) printed on a glass substrate. (B) 45-Degree-slanted SEM image of a discretized optical spin splitter. (C) Colored SEM image of the polarizing beam splitter on a fiber facet, with the prism highlighted in blue, the lamellar grating in red, and the supporting structure in green. ((A) Reproduced with personal permission from Authors. (B) Reproduced with permission from X. Wang, A.A. Kuchmizhak, E. Brasselet, S. Juodkazis, Dielectric geometric phase optical elements fabricated by femtosecond direct laser writing in photoresists, Appl. Phys. Lett. 110(18) (2017) 181101. ©2017 AIP Publishing. (C) Reproduced with permission from V. Hahn, S. Kalt, G.M. Sridharan, M. Wegener, S. Bhattacharya, Polarizing beam splitter integrated onto an optical fiber facet, Opt. Express 26(25) (2018) 33148–33157. ©2018 Optical Society of America.)

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onto the end facet of a polarization maintaining optical fiber via DLW nanopolymerization technique. The linearly polarized radiation exiting the fiber is collimated by a tailored lens and experiences two total internal reflections inside the Fresnel rhomb at the angles, such that each reflection introduces a π/8 phase shift between s and p polarization components. As a result, a device consisting of one Fresnel rhomb acts as a quarter-wave plate, while two stacked rhombs exhibit a half-wave plate behavior. The phase shift is not based on material birefringence and therefore features a very broad spectral bandwidth. Authors report nearly pure circular polarization observed in a broad spectral bandwidth (>300 nm) [84]. On the other hand, the reported structure is relatively massive, i.e., 0.15 mm in diameter and 0.32 mm in length was required to achieve a quarter waveplate retardance. Microprinting of such volume is time consuming and special dip-in configuration should be applied for DLW process, which is incompatible with some materials and DLW systems. An alternative approach for manufacturing phase optical elements is based on exploitation of the form birefringence, emergent in the structures of two alternating dielectric layers with different refractive indices. In case of dielectric geometric phase optical elements fabricated via DLW of photoresists, structures comprise strands of optically clear polymer (typical n  1.5) separated by submicrometer scale air gaps (n  1). The numerical calculations based on effective-medium-theory (EMT) predicts that induced form birefringence is as large as Δnf  0.1 at a given refractive indices if only a subwavelength operation condition (Λ ’ λ/2) is fulfilled [85]. Thus, the half-wave retardation for visible to near-infrared radiation can be achieved in flat optical components as thin as several micrometers. Moreover, the flexibility of DLW nanolithography enables creation of flat surfaces with in-plane effective optical axis whose orientation angle is spatially modulated. Experimental realization and characterization of diverse geometric phase optical elements fabricated by femtosecond direct laser writing (i.e., optical spin splitters (Fig. 12.1.6B) or spin-to-orbital optical angular momentum converters with different topological charges from 1 to 20 (not shown)) was reported by Wang et al. [86]. On the other hand, the purity parameter of the photopolymerized geometric-phase optical elements of only a few percent was achieved so far. Although such a modest value does not compromise the proof of the principle, further efforts toward efficient and reproducible writing of high aspect ratio (height/period
10) subwavelength gratings are required. Finally, a hybrid optical device—polarizing beam splitter (Fig. 12.1.6C), combined of refractive optical element, i.e., triangular prism and suspended diffractive element, i.e., lamellar grating was manufactured and characterized in a recent work [87]. The design of beam splitter, mounted directly onto the facet of a fiber tip, was chosen so that the beam exiting the total internal reflecting prism would illuminate the grating at the Littrow angle. Authors report polarization purity to be 81% and 82% for transverse magnetic (TM) and transverse electric (TE) polarization, respectively.

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

3.4 Hybrid multifunctional and integrated microoptical components The DLW lithography technique enables fabrication of components having multiple optical functions based on hybrid refractive-diffractive performance. Such designs are promising in microfluidic environments where low refractive index contrasts between liquid and glass substrate are hampering efficient light beam bending that can be solved via diffractive elements incorporated with lenses and prism optical elements. This approach also helps planarization of optics, which is important in fluidic chips. The preparation of the specimen [88] and centering of the microstructure [89] are additional challenges that require special measures. Compact packing of various microlenses is possible with surface filling efficiency reaching 100% without optically nonfunctional flat regions [36, 90]. Self-smoothing phenomenon known as repolymerization [91–93] can induce challenges in clean developing of the exposed specimen, but at the same time it is advantageous for decreasing the surface roughness of the fabricated structures (Fig. 12.1.7A–C). It is noteworthy that microoptical components manufactured via DLW nanolithography could be further improved applying diverse postprocessing procedures. A straightforward method of subsequent sputter coating of 3D-printed microobjects allows to obtain bimaterial (dielectric/metallic) micro-/nano-optical structures [96]. Multilayer dielectric coating of polymer diffractive optical elements by means of physical vapor deposition is also feasible [97]. Another hybrid manufacturing approach combines the additive 3D structuring capability of laser polymerization and the subtractive subwavelength resolution patterning of polymerized structure. As an illustrative example of such hybrid manner of processing, an axicon for the purpose of topological shaping of light with 165 degree full-apex angle after sputter coating with 200-nm gold layer and spiral zone plate (SZP) pattern with vortex order ‘ ¼ 2 milled in the metal coating using focused ion beam (FIB) is depicted in (Fig. 12.1.7D) [98].

Fig. 12.1.7 Various microoptical components combining several functions (A) An array of arbitraryshaped (umbrella-type) microlenses [94], (B) a fraxicon (a Fresnel lens merged with an axicon) [95], (C) a refractive aspherical lens with diffractive grating [94], and (D) an axicon with 165 degree fullapex angle structure after sputter coating with 200 nm of Au and FIB milling of SZP patterns with vortex order ‘ ¼ 2 in the Au layer. (Reproduced with permission from M. Farsari, B. Chichkov, Materials processing: two-photon fabrication, Nat. Photon. 3 (2009) 450–452. © 2016 Optical Society of America.)

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A promising method of polymerized microoptical components postprocessing is calcination of hybrid organic/inorganic polymer via heating in air atmosphere [99]. Resolution of the final 3D structures could be increased up to
40% compared to as-fabricated objects due to the isotropic shrinking. This may help to overcome a limiting factor of the minimal feature size obtainable in DLW fabrication (especially of diffractive optical elements for visible spectral range). The completely different material properties obtained via calcination process are intriguing: heating at temperature near 1000°C results in a structure comprises a glass phase, and for >1200°C a hybrid glass-ceramic phases emerge. This is beneficial for production of high refractive index microoptical components. The aforementioned postfabrication processes are effective toward enhancement of structuring resolution or enrichment microscaled objects with nanoscale features. However, in some cases, the opposite might be desired: to complement a microprecision 3D-printed object with a less precise sub-mm-scaled subsidiary structures in a fast and low-cost manner. By way of illustration, an interesting approach of combining 3D DLW lithography and superfine inkjet printing processes was proposed with the aim to integrate apertures and nontransparent hulls with polymer microoptical components evading any additional alignment procedures. A freestanding monolith optical device, i.e., an asphere with front aperture and opaque hull (Fig. 12.1.8A) demonstrated an improved optical performance compared to those without the ink [100].

Fig. 12.1.8 (A) 3D-printed asphere with front aperture and nontransparent hull. (B) An axicon (from top) merged with an aspherical lens (from bottom) on the tip of a single-mode optical fiber [89]. (C) 3D-printed four-lens system with four different FOVs for foveated imaging directly integrated on CMOS imaging sensor (not shown). The combined footprint is <300 mm  300 mm. ((A) Reproduced with permission from A. Toulouse, S. Thiele, H. Giessen, A.M. Herkommer, Alignmentfree integration of apertures and nontransparent hulls into 3D-printed micro-optics, Opt. Lett. 43 (2018) 5283–5286. © 2018 Optical Society of America. (C) Reproduced with permission from S. Thiele, K. Arzenbacher, T. Gissibl, H. Giessen, A.M. Herkommer, 3D-printed eagle eye: compound microlens system for foveated imaging, Sci. Adv. 3 (2) (2017) e1602655. © 2017 American Association for the Advancement of Science.)

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

Optical fibers serve as standard optical platforms for light energy and information transport and processing. Their widespread usage is underpinned by convenience that the coupled-in light can be delivered to the required destination with negligible losses. At the same time, it is a real challenge to handle strongly diffracting light once it is coupled out from the fiber. The game changes if extra optical components are implemented to control the flow of light guided out of the waveguide (Fig. 12.1.8B). Fiber-tip structures are attractive due to their possible exploitation of end face areas that offer integrated photonic devices that can be applied as concentrators [101], collimators [95], and antireflective coatings [7] as well as sensor devices [102]. No less important is the ability to integrate complete 3D-printed photonic devices with other highly irregular transparent or opaque substructures. Direct printing of multilens microobjectives with different focal lengths (Fig. 12.1.8C) on complementary metal-oxide semiconductor (CMOS) image sensor enables creation of foveated imaging systems [103]. A comprehensive study of hybrid optical assemblies, i.e., free-form lenses, free-form mirrors, and multilens beam expanders integrated to edge-emitting and surface-emitting photonic devices was provided by Dietrich et al. [104]. High coupling efficiencies of up to 88% between edge-emitting lasers and single-mode fibers and a wide choice of microoptical components available to print on demand implies a DLW nanolithography to be a promising technique for automated assembly of photonic multichip systems.

4 Toward GRIN microoptics In the following sections, a refractive index control in volume is demonstrated for complex microoptical elements. This novel approach based on DLW varying the fabrication (exposure) parameters enables creation of free-form elements with the refractive index being selectively tunable in space.

4.1 The need of control over the refractive index Since the advent of the two-photon polymerization (TPP) technique, various research groups targeted applications in the diverse fields and countless examples of the miniaturized copies of the real world at the micrometer scale were demonstrated [4, 21, 105–107]. Even several tabletop 3D DLW systems are already available off-the-shelf at the moment. However, the thoughtful insights in the photophysical and photochemical processes acting during the polymerization are beginning to emerge in recent years. Interest begins to grow up in synthesis of new materials leading to the fabrication throughput and resolution optimization. For instance, Takada et al. have studied the influence of the temperature on the polymerized feature’s (voxel’s) spatial resolution [108], and Malinauskas et al. have investigated the photopolymerization mechanisms at tight focusing [109], while Stocker et al. discovered the dye molecules that can be used as photoinitiators in a

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resolution augmentation through photoinduced deactivation (RAPID) lithography [110]. Photopolymers used for the TPP were characterized for their Young’s modulus by nanoindentation technique [111]. A large contribution to the field of TPP was made by Wegener and coworkers discussing the mechanisms involved during DLW fabrication [112], in situ measurement of temperature [64], polymerization kinetics [113], and stimulated emission depletion (STED)-enhanced lithography [114]. The degree of conversion in crosslinking was investigated using coherent anti-Stokes Raman spectroscopy (CARS) [115], Fourier transform infrared (FTIR) spectroscopy [116], and Raman microspectroscopy [30, 31]. Despite a recent progress, this topic still remains relevant for the further development of TPP technology at both scientific and industrial levels. The determination of interplay between degree of crosslinking and the refractive index is discussed next for the applications in gradient refractive index (GRIN) microoptics. Various methods, some of which were introduced three decades ago, are used for the manufacturing of GRIN elements, e.g., chemical vapor deposition (CVP) [117], ion exchange [118], neutron irradiation [119], and thermal [120], UV [121], and diffusionassisted lithography [122]. However, despite a very recent study [123], all of them lack the possibility to embody arbitrary-shaped components. On the other hand, 3D DLW technique is distinguished for its unique feature to fabricate complex-shaped 3D structures, including free-form microoptical elements. The vast majority of those elements are controlling a light flow via a sophisticated 3D sculptured surface of the polymers with the constant refractive index. Recently, it was demonstrated that a light focusing can be realized with an optically flat element [29]. In this case, the focusing power arises from the phase transformation of the angular light field components. Thus, the propagation of light radiation through the microstructures with the spatially varying refractive index distribution inside material opens new opportunity in design of microoptics.

4.2 μ-Raman measuring methodology Again, the same SZ2080 (IESL-FORTH, Greece) mixed with 2 wt% of the photoinitiator Irgacure 369 (Sigma Aldrich) was used as a model material in the present investigation. The prepolymer is synthesized by mixing methacryloxypropyl trimethoxysilane (MAPTMS), methacrylic acid (MAA), and zirconium n-propoxide (ZPO). In the presence of the light absorption, free radical photopolymerization is initiated by photoinitiator molecule dissociation into two highly reactive radicals via CdC bond breaking. Subsequently, chain propagation is followed by opening C]C double bond and conversion to CdC single bonds in the methacrylic group generating crosslinked network of monomers. The material is prepared by either drop casting or spin coating (30 s at 5000 rpm) it on the soda lime glass substrates followed by prebake at 90°C for 60 min. After the DLW, the samples are kept in the dark for 12 h at ambient conditions as postpolymerization lasts

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

after the exposure. Finally, samples are developed in 4-methyl-2-pentanone for 1 h and dried in critical point dryer (K850, Quorum Technologies). A standard DLW lithography setup (FemtoFAB, Workshop of Photonics Ltd.) equipped with Yb:KGW (Pharos, Light Conversion Ltd.) laser emitting 300-fs laser pulses at a repetition rate of 200 kHz and a second harmonic at 515 nm was used for the fabrication of microstructures for the determination of both degree of conversion and refractive index. The laser beam was tightly focused into the volume of the prepolymer using 63 magnification microscope lens (Plan-Apochromat, Carl Zeiss) with the numerical aperture (NA) of 1.4. Three sets of the identical samples (arrays of the cuboids with 15  15  10 mm3 dimensions) were prepared for the Raman microspectroscopy experiments. Each array was fabricated by varying the average power of the writing laser beam in the range of 20–220 mW (corresponding to 0.28–3.11 TW cm2 intensity at the focus) and the sample translation velocity from 1.5 to 5 mm/s keeping the same lateral dx ¼ 200 nm and axial dz ¼ 250 nm hatching steps. For the refractive index evaluation, an array of the right-angled trapezoidal prisms (a ¼ l ¼ 40 mm, b ¼ 60 mm, and h ¼ 10 mm) was prepared maintaining the same fabrication parameters. Raman spectra of the nonpolymerized prepolymer and polymerized cuboids were measured by a confocal Raman microspectrometer (inVia, Renishaw) in the 830–1920 cm1 frequency range. A continuous-wave (CW) He-Ne laser operating at 632.8-nm wavelength was used for the excitation via 50  microscope lens with the NA of 0.75. A laser beam of 7.5-mW average power (measured before the lens) was focused on the spot of 1.28 mm in diameter that was face-centered in respect of the polymerized cuboid. A Si substrate was used as a standard for the microspectrometer calibration. Before the spectra acquisition, each microstructure was continuously irradiated for 15–30 min in order to reduce the fluorescence background in accordance with the method studied by Palus and Michalska [124]. After the photobleaching, Raman spectra remained stable during acquisition indicating absence of additional polymerization or degradation effects in the sample. The irradiation time for each measurement was set to 10 s and number of accumulations to 10, yielding total accumulation time of 100 s. The fifth-order polynomial function was used for the baseline subtraction. Each peak was fitted using a Lorentzian function, from which the data of the peaks (height, integrated area, full width at half maximum (FWHM), and position) were deduced. Fig. 12.1.9A shows Raman spectrum of the nonpolymerized thin film of the prepolymer SZ2080 in the region of interest (i.e., 1550–1780 cm1 range). Three peaks situated at 1593, 1640, and 1714 cm1 can be clearly resolved. The first one is attributed to the aromatic ring vibrations of the Irgacure 369 photoinitiator, the second to the C]C stretching mode of the crosslinking methacrylate group, and the third to the stretching mode of the carbonyl group C]O. The typical dynamics of the Raman spectra of the cuboids photopolymerized with a different laser power at constant velocity (in this case v ¼ 2.5 mm/s) is shown in Fig. 12.1.9B. Clearly, the decrease of the C]C peak

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Fig. 12.1.9 (A) Raman spectra of the nonpolymerized prepolymer and (B) its dynamics of the cuboids photopolymerized with different laser power at constant velocity (2.5 mm/s). (Inset) Optical microscope image of the fabricated cuboid array.

intensity with the increase of the laser beam power indicating the consumption of the carbon-carbon double bonds during photopolymerization reaction can be observed. Meanwhile, the peak of the carbonyl group remains unaffected (the peak intensity slightly decreases while FWHM increases resulting in the unchanged area of the peak) as it does not participate in the reactions, suggesting it can be used as a reference peak for the calculation of degree of conversion. Besides, the intensity of the peak at 1593 cm1 decreases and it exhibits hyposochromic shift by maximum 4.5 cm1 with increasing writing laser beam power (i.e., exposure dose) indicating the change in its local environment during photopolymerization. Degree of conversion was calculated by comparing the methacrylate C]C stretching mode with a reference band C]O before and after photopolymerization:    SpðC¼CÞ =SpðC¼OÞ η¼ 1  100 (12.1.1) SnðC¼CÞ =SnðC¼OÞ where S is an integrated area under the corresponding peaks (“p” and “n” refer to the polymerized and nonpolymerized samples, respectively).

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

4.3 Spatially selective modulation of refractive index by tuning DLW parameters Influence of the DLW processing parameters on the degree of conversion of fabricated microstructures was investigated. In Fig. 12.1.10, the degree of conversion as a function of the average power/intensity of the writing laser beam and sample translation velocity is plotted. For comparison with recent study performed by Jiang et al. [31], the range of employed velocities by almost two orders of magnitude was increased to expand discussion on photochemical conversion. A rapid increase of the degree of conversion with the increase of laser power can be observed in Fig. 12.1.10 in all cases. Meanwhile, the decrease of the degree of conversion from 68% to 52% with the increase of the writing velocity from 0.05 to 5 mm/s is found as predicted in Ref. [116] and, now, is demonstrated experimentally. Also, the achieved maximum value of degree of conversion is in close agreement with the results published in Refs [30, 116]. It has to be mentioned that experimentally a 100% conversion cannot be reached since the mobility of the polymerizable group molecules is limited by the crosslinked network that becomes more complex with increase of the exposure dose. Thus, the probability of interaction between the free radicals and monomers is significantly reduced. Interestingly, no clear correlation between the minimum degree of conversion with laser power and scanning velocity can be resolved in contrast to the maximum degree of conversion. Also, the trend of P ! max and v ! min indicates the path for the maximum degree of conversion, and thus fabrication of microstructures with the highest robustness according to Ref. [31]. In contrast to report in Ref. [30], no saturation in conversion close to the damage threshold of the prepolymer was observed. Here, the damage threshold is defined as the value above which microexplosions occur during fabrication, but microstructures are still

Fig. 12.1.10 Experimental data of the degree of conversion, η, as a function of the TPP laser power/ intensity, P/I, and linear stage velocity, v.

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recoverable after development. The hatching step in the present study, dx ¼ 200 nm, was different in comparison to that in Ref. [30] at which saturation is mostly pronounced, dx ¼ 100 nm. It is expected that fabrication of the microstructures at constant exposure dose would lead to the similar degree of bond conversion in crosslinking. Accordingly, the exposure dose is proportional to the average laser power and exposure time: D ∝ PNt, where N is the coefficient reflecting a cumulative nonlinearity of the optical absorption and chemical conversion processes, and t ¼ 2 w/v, where 2w ¼ 1.22 l/NA is the diameter of the focused laser beam and v is the sample translation velocity. The plot of the exposure time versus average laser power with the similar degree of conversion values would reveal the scaling mechanism of the process, i.e., the overall nonlinearity N. The obtained results are presented in Fig. 12.1.11. Here, symbols denote the experimental results and the lines represent theoretical curves. Clearly, the linear fits for the five different degree of conversion values are in good agreement with the experimental data, which results in an average value of N ¼ 3.01  0.05 indicating the third-order scaling of the process. The absorption peak of the SZ2080 mixed with the photoinitiator is situated at λ ¼ 390 nm and a two-photon absorption is expected for the fabrication at λ ¼ 515 nm. It should be stressed out that

Fig. 12.1.11 Double logarithmic plot of the constant degree of conversion against average power and exposure time. The solid lines are fits to the experimental data representing the overall nonlinearity N from light absorption to crosslinking (dashed lines corresponding to N ¼ 1 and 2 are shown for direct comparison).

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

evaluation of the degree of conversion counts in the intensity diminution of the C]C bond in the methacrylate group whose absorption is blue-shifted in comparison to the absorption of the photoinitiator. Therefore, the deduced process nonlinearity from the degree of conversion measurements arises from the photodissociation of the C]C bonds in the methacrylate group. Further experiments must be carried out for investigation of scaling laws of exothermic polymerization in different photopolymers. In the following, the optical properties of the photopolymerized SZ2080 based on the Raman microspectroscopy are revealed. The aim is to determine the refractive index of the microstructures corresponding to the broadest interval of the degree of conversion fabricated at constant laser power by alternating the velocity from vmin ¼ 0.05 mm/s to vmax ¼ 2.5 mm/s (Δη  54% at P ¼ 60 mW; see Fig. 12.1.10). It can be noticed that during fabrication the exposed prepolymer tends to occasionally burst into bubbles at the places that are scanned at the vmin and corrugates after the development at the places that were scanned at the vmax. The former is caused due to the exposure dose being close to the damage threshold and the latter is due to shrinkage, which occurs even when sample is developed in a critical point dryer where capillary collapse of the structure by liquid front of a rinse liquid is avoided. Optical properties are evaluated at P ¼ 40 mW corresponding to Δη  45% (indicated by shadowed area in Fig. 12.1.10). The refractive index of the fabricated right-angled trapezoidal prisms was measured with the Michelson interferometer with setup shown in Fig. 12.1.12A.

Fig. 12.1.12 (A) Interferometer setup, (B) 45 tilted scanning electron micrograph (SEM) of the fabricated trapezoidal prism, and (C) interference pattern exhibiting the refractive index change.

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A 633-nm-wavelength He-Ne laser beam with a size of ω0 ≫ b (here b is the longest side of the prism) theretofore spatially filtered and 7.5 expanded is normally incident onto the sample. A light wave passed through the fabricated microstructure (see Fig. 12.1.12B) is magnified with a microscope objective (20, NA ¼ 0.4), split into two waves and reflected backward resulting in a superposition wave whose intensity is subsequently imaged onto a charge-coupled device (CCD) camera with an f ¼ 180 mm lens. Adjusting one of the mirrors (either #1 or #2), an interference pattern with equally spaced straight and parallel lines near the beam axis is obtained (Fig. 12.1.12C). The gradation of the fringes apparent in this picture is the result of interference between the light field propagating through the region around and through the microstructure. Intensity of the light field is related to its phase, φ, and the refractive index can be evaluated as follows: φλ (12.1.2) 2πh where h is the height of the prism and φ ¼ Δ d/d (d is the period of the interference fringes and Δd is the relative deviation of the same line caused by propagation through the prism) [125]. Table 12.1.1 shows the measured refractive index of the five prisms fabricated at different conditions (P ¼ 40 mW with v ¼ 0.05, 0.1, 0.25, 0.5, and 1 mm/s). The surface roughness measured with the optical profilometer (PLm 2300, Sensofar) was taken into account for the estimation of the error bars. Obtained results show that it is possible to achieve refractive index change in the SZ2080 photopolymer up to Δn ¼ n1  n5 ¼ (1.16  0.01)  102 depending on the DLW conditions, i.e., exposure dose. Also, a fairly good agreement between the refractive index measured in this study and from m-line prism coupling experiments (n ¼ 1.504 at λ ¼ 633 nm) [38] was found. A positive correlation with the Pearson coefficient r ¼ 0.94 between the degree of conversion and refractive index exists indicating the increase of n with the growth of η and vice versa. The higher the degree of conversion, the stronger the crosslinked network of monomers is photopolymerized corresponding to increase of refractive index. n¼

Table 12.1.1 Experimentally measured degree of conversion (η) and refractive index (n) values of the trapezoidal prisms photopolymerized at constant power (40 mW) by alternating scanning velocity (v) corresponding to accumulated energy dosage Dacc ¼ Dp  Np, where Dp is the energy dosage per pulse and Np the number of pulses No.

v (mm/s)

Dacc (J/cm2)

η (%)

n

n1 n2 n3 n4 n5

0.05 0.1 0.25 0.5 1

304 152 61 30 15

50.33  1.96 34.78  1.40 23.08  3.60 10.35  2.74 5.49  3.06

1.5112  0.0010 1.5099  0.0010 1.5061  0.0015 1.5046  0.0011 1.4996  0.0012

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

Following the established refractive index data, a GRIN lens with 50  50  5 mm3 dimensions was designed. Fig. 12.1.13A represents the scanning algorithm for DLW. The lens is fabricated in a “zigzag” scanning manner with an increase of exposure dose toward the lens center and decrease next to the opposite edges in a horizontal direction (x-axis), while the exposure dose is constant in a perpendicular direction (y-axis). For the simplicity, to control the exposure dose a varying scan velocity was implemented from v1 ¼ 1.050 mm/s and v2 ¼ 1.042 mm/s to vn ¼ 0.05 mm/s (n ¼ 125) retaining reflection symmetry and hatching steps equal to dx ¼ 200 nm and dz ¼ 250 nm as investigated previously. It is expected that the lens fabricated by the described protocol would inherit

Fig. 12.1.13 (A) Graphical representation of the scanning algorithm for the GRIN lens fabrication (solid arrows show the trajectory of the linear stage motion at corresponding velocities: v1 ¼ 1050 μm/s, …., vn ¼ 50 μm/s, where n ¼ 125), (B) top view SEM image of the polymerized GRIN lens, and (C and D) measured height profile of the lens across x- and y-axes and its deviation from flat surface.

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refractive index modulation only in one direction (in this case, x-axis) similar to a cylindrical lens. In Fig. 12.1.13B, SEM image of the fabricated GRIN lens is shown revealing high quality (no shrinkage and/or microexplosions) and a regular as designed shape. In Fig. 12.1.13C and D, the measured surface profile of the lens along x- and y-axes is shown, respectively. It reveals that the lens is a flat-top element in both directions even though the exposure dose varies continuously in x-axis. This condition ensures that the change in optical properties of the fabricated component is not caused by its surface curvature. Finally, the refractive index of the GRIN lens along x- and y-axes was measured. Fig. 12.1.14A and B shows representative interference patterns obtained independently for both cases by adjusting one of the mirrors in either vertical or horizontal direction. Qualitative analysis of these patterns indicates that fringes in image (A) slightly bend (see curved dashed line) if compared with the reference (solid line). Remembering that n ∝ φ ¼ Δd/d, it gives the direct evidence of the refractive index modulation signatures. Meanwhile, no such behavior between specimen and reference can be observed in the image (B). Therefore, it is possible to conclude that the refractive index is constant across y-axis. Quantitative analysis of the refractive index distribution along both axes is shown

Fig. 12.1.14 Characterization of the GRIN lens across “XZ” (A) and “YZ” (B) axes (image (B) is rotated by 90 degrees). (Left) Interference pattern exhibiting the change of refractive index. (Right) Magnified view of the fringes shown in left panel (solid and dashed lines are guidelines to the eye). (C) Experimental data of the obtained refractive index profile.

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

in Fig. 12.1.14C and confirms the interpretation of the interference pattern images. Obviously, the refractive varies continuously in x direction, but is uniform in y direction as was expected from the previous n measurements in this study. Also, obtained index distribution along x-axis distinguishes for the symmetry within error bars and can be expressed as follows: n(x) ¼ n0 +k1x if x < 0 and n(x) ¼ n0 +k2x if x > 0, where n0 ¼ 1.5159  0.0004, k1 ¼ (6.78  0.37)  104, and k2 ¼ (5.93  0.37)  104. Thus, attained results prove that fabricated microstructure is indeed a GRIN lens. However, peak to valley refractive index value of the GRIN lens Δn ¼ (1.54  0.01)  102 is higher if compared with the previous results Δn ¼ (1.16  0.01) 102. We suppose that the gradient of the DLW exposure dose leads to the diffusion of the molecules toward the lens center during and after fabrication that, in turn, determines the growth of the refractive index. This effect can be accounted for in selection of exposure conditions of the designed GRIN elements. The interplay between the degree of conversion and refractive index of microstructures fabricated at various 3D DLW experimental parameters (the average laser power and sample translation velocity) is revealed. Based on the Raman microspectroscopy results, it was found that degree of conversion is within 6%–70% range and increases together with the applied 3D DLW exposure dose. A refractive index change in the SZ2080 material up to Δn ¼ (1.54  0.01)  102 can be achieved. The linear dependence between the dose and refractive index is a new valuable finding for design of microoptical elements. The 3D DLW technology can be used for the manufacturing of GRIN microoptics in a precisely controlled manner of refractive index due to its point-to-point scanning nature. This could find applications in anamorphic light beam shaping, where spherical aberration-free and plane optical surface components are desirable. The determined scaling of refractive index with exposure dose and crosslinking can be expected from standard scaling of the index with the mass density; however, it is less obvious that a Gaussian intensity profile can define the crosslinking with a good control as shown by the described experiment. Crosslinking of a polymer determines its Young’s modulus and permeability for solvents [126–128], hence opening new capabilities in sensing using fluidic chips and optomechanical platforms [129–131].

5 Conclusions In summary, recent advances of DLW lithography employing ultrafast lasers for microoptics have been demonstrated and discussed from fundamental research and practical applications points of view. The emphasis was given on SZ2080 material optical properties and its possible usage in manufacturing minimized, multifunctional, and integrated optical devices for the spatial and spectral control of light at a microscale. Novel elements such as spiral-phase plates and topological lenses for generation of light beams carrying

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AM are presented. Advanced sample characterization techniques such as μ-Raman spectroscopy methodology were used to reveal bond conversion and crosslinking. It is essential to precisely control the exposure conditions in order to achieve the desired functionalities and, at the same time, it offers vast new possibilities that widen the field of free-form GRIN optics. The effect of photosensitization and exposure conditions on the LIDT of ORMOSIL’s class polymer SZ2080 is discussed. The obtained values of the LIDT are in the range of tens of Joules per square centimeter in the nanosecond pulse mode and one order of magnitude lower for femtosecond pulses. Addition of photoinitiator to polymer was found to decrease the LIDT by 10 and 2 times at the second harmonics, and it has no considerable influence on the fundamental nanosecond and femtosecond pulsed irradiation modes. As one could expect, this corresponds to diminishing of the effective band gap, Eg, of resist due to the photoinitiator-caused absorption. The refractive index n and extinction coefficient k are measured over all optical ranges relevant for fabrication of microoptics. The exact (n, k) values provide practical guidance for fabrication from the pure and photosensitized SZ2080 resists for performance in different environments. Nonphotosensitized materials for the high-power applications can be made since the photosensitization of polymers results in the reduced damage threshold. An incubation model confirmed experimental data that accumulation and fatigue effects are typical for the investigated materials and it is more noticeable for the doped polymer. Results of the damage morphology showed that delamination is responsible for the damage in the nanosecond pulse mode due to excessive heat deposition. Both the LIDT and (n, k) measurements have shown that nonphotosensitized hybrid inorganic-organic polymer SZ2080 is a promising material for both the DLW 3D microstructuring/nanostructuring and optical coating technology. Even though it can be expected that an addition of the photoinitiator as absorber would decrease the LIDT value, this work shows this quantitatively by employing the ISO 21254-2:2011 standardized procedure for common laser pulse durations and wavelengths. These findings have a reference value for the future DLW 3D lithography material development and characterization. Further studies should target LIDT value in volume of resists and its dependence on fabrication parameters. This overview shows current advances in the field of 3D DLW by determination of the degree of conversion in crosslinking and determination of the refractive index and extinction coefficient of the microstructures fabricated by 3D DLW lithography. The relationship between exposure dose and scan velocity is revealed for precise control of the refractive index. The largest change of the index obtained was Δn
Oð102 Þ. Realization of controllable index profile and shape GRIN lens, resulting from a laser-induced local refractive index modification due to controlled monomer crosslinking ratio, is shown. The 3D DLW lithography is a versatile tool for GRIN microoptics.

3D microoptics via ultrafast laser writing: Miniaturization, integration, and multifunctionalities

Acknowledgment This research was funded by a grant “+Tech-” (No. S-MIP-17-99) from the Research Council of Lithuania.

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