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. 2017 Jan 2;10(1):12.
doi: 10.3390/ma10010012.

Optically Clear and Resilient Free-Form µ-Optics 3D-Printed via Ultrafast Laser Lithography

Linas Jonušauskas  1 , Darius Gailevičius  1 , Lina Mikoliūnaitė  2 , Danas Sakalauskas  3 , Simas Šakirzanovas  2 , Saulius Juodkazis  4   5 , Mangirdas Malinauskas  6
Affiliations

Optically Clear and Resilient Free-Form µ-Optics 3D-Printed via Ultrafast Laser Lithography

Linas Jonušauskas et al. Materials (Basel). .

Abstract

We introduce optically clear and resilient free-form micro-optical components of pure (non-photosensitized) organic-inorganic SZ2080 material made by femtosecond 3D laser lithography (3DLL). This is advantageous for rapid printing of 3D micro-/nano-optics, including their integration directly onto optical fibers. A systematic study of the fabrication peculiarities and quality of resultant structures is performed. Comparison of microlens resiliency to continuous wave (CW) and femtosecond pulsed exposure is determined. Experimental results prove that pure SZ2080 is ∼20 fold more resistant to high irradiance as compared with standard lithographic material (SU8) and can sustain up to 1.91 GW/cm² intensity. 3DLL is a promising manufacturing approach for high-intensity micro-optics for emerging fields in astro-photonics and atto-second pulse generation. Additionally, pyrolysis is employed to homogeneously shrink structures up to 40% by removing organic SZ2080 constituents. This opens a promising route towards downscaling photonic lattices and the creation of mechanically robust glass-ceramic microstructures.

Keywords: 3D laser lithography; 3D printing; ceramic 3D structures; direct laser writing; hybrid polymer; integrated micro-optics; optical damage; photonics; pyrolysis; ultrafast laser.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
(a) SEM images of resolution arrays of photosensitized (left) and non-photosensitized (right) SZ2080. Structures with severe structural damage (red), with poor (yellow) and good (green) quality are outlined. The structure is considered good if internal single lines are observable and the shape of the cube is as designed. Average laser powers of the bottom and the top of the ΔI are recalculated to the peak intensity Ip (shown at the top); (b) one of the good quality structures in the array is shown in a greater detail; l and d marks the longitudinal and transverse sizes of the lines; and (c) an example of a poor quality structure and (d) the failed one.
Figure 2
Figure 2
(a) reduction of structural quality in the case of photosensitized and pure SZ2080. Photoinitiator Irgacure 369 (IRG) containing cubes degrade slower and in a more progressive fashion. Conversely, the structures out of non-photosensitized material completely break up as soon as the fabrication parameters are not in the ΔI. All scales are 5 μm; and (b) monolithic micro-optical element on the tip of an optical fiber fabricated out of non-photosensitized SZ2080.
Figure 3
Figure 3
(a) feature width d and height l measured in the resolution array at writing speed of 250 μm/s; and (b) aspect ratio of lines produced for cases of 250 μm/s and 500 μm/s speeds.
Figure 4
Figure 4
(a) SEM micrograph of the measured square polymerized structures; (b) RMS calculated for surfaces fabricated with different dx for SZ2080 containing PI and without it; and (c) AFM images of surfaces of pure and photosensitized polymer obtained with highest voxel overlap (dx = 50 nm) as well as one which was produced via homogeneous UV exposure.
Figure 5
Figure 5
Images of the focal plane of a microlens before and after 30 h of exposure to 405 nm CW laser radiation. No significant change in the image at the focal point can be discerned.
Figure 6
Figure 6
SEM images of microlenses before and after 20 h exposition to a loose focusing of 515 nm 300 fs laser radiation and an image of an LED made by the lens. Degradation of a lateral light distribution in the focal plane can be seen both in the structural quality of the lenses and the degraded projected image. The PI containing microlenses were more degraded.
Figure 7
Figure 7
(a) real-time monitoring of a lateral intensity distribution of LED through microlenses produced using photosensitized and pure SZ2080 during irradiation with 515 nm 300 fs pulses at 200 kHz with Ip = 1.91 GW/cm2 (spot radius of 100 μm). Faster deterioration of SZ2080 containing 1 wt % IRG as compared with pure SZ2080 is evident, as the image at the focus starts to degrade after 60 and 20 s (marked by red dashed squares), respectively; (b,c) SEM micrographs of the tested lenses before and after exposure. The photosensitized element is entirely destroyed, while the one produced out of pure SZ2080 exhibits relatively low damage; and (d) start of the microlens degradation for different concentrations of PI in SZ2080, as well as time needed to damage SU8. All scale bars are 10 μm.
Figure 8
Figure 8
Focusing performance of microlenses after exposure to Ip = 1.27 GW/cm2 515 nm 300 fs radiation in continuous (a) and multi-burst mode: 10 s exposure followed by a 10 s pause (b); (c) lateral distribution at the focus of microlenses exposed to 5 min of continuous radiation at Ip = 1.91 GW/cm2 (515 nm 300 fs) achieved with repetition rates of 10, 100 and 200 kHz. The 10 kHz case also serves as a before image, as there were no changes in focusing properties after this experiment.
Figure 9
Figure 9
SEM images of the structure consisting of supporting walls and free hanging ring before (a) and after (b) pyrolysis. Magnification is the same. Fabricated objects appear brighter after pyrolysis, which indicates a change in the electrical conductivity of the material.
Figure 10
Figure 10
(a) SEM micrograph of a photonic crystal prior to (a) and after (b) pyrolysis; (c) the period and line width initially were 500 nm and 295 nm, respectively, which were shrunk to 300 nm and 174 nm after pyrolysis (d); a change of ∼40%. The period/width ratio stayed ∼1.7, indicating a homogeneous reduction in size.
Figure 11
Figure 11
TGA data showing weight loss vs. temperature. The weight loss was 28%; observed shrinkage was ∼40%. The organic component was decomposed and removed by heating.
Figure 12
Figure 12
(a) polymerization reactions initiated by nonlinear absorption of PI molecules and subsequent chemical pathways resulting in a cross linked SZ2080; (b) SZ2080 cross linking without PI. hf is the photon energy.
Figure 13
Figure 13
Schematics of setup used for fabrication and microlens degradation experiments: PS—power control stage, PP—phase plate, G—glass plate, M—mirror, RM—removable mirror, SHC—second harmonic crystal, T—telescope, GS—galvanoscanner, RPM—removable power meter, L—lens, 4F—lens system in 4-F configuration, DM—dichroic mirror, CMOS—CMOS camera used to monitor fabrication process, Obj—objective lens, LED—LED used for sample illumination.
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