1. APPLICATION ORIENTED FUNCTIONAL MATERIALS FOR FEMTOSECOND LASER 3D MICRO- AND NANOFABRICATION
L. Jonušauskas, E. Skliutas, S. Varapnickas, S. Rekštytė and M. Malinauskas
Department of Quantum Electronics, Vilnius University, Saulėtekio Ave. 10 Vilnius LT-10222, Lithuania

2. Introduction
Additive manufacturing: quickly developing and highly promising area of industry;
Ultrafast laser allows to achieve great fabrication precision (up to tens/hundreds of nm) and true 3D structures;
Currently most of the materials used are from lithography or 3D printing and not perfectly suitable for some areas of interest;
Subtractive vs Additive 1 2 3
Goal of this work: show an approach of tailoring the material for application rather than use standard material and try compensate its deficiencies. 4

3. 3D laser lithography Photochemical reactions in the focal volume:
Laser writing:
Multiphoton absorption;
Formation of a radical;
Radical-monomer reaction;
Propagation;
(b) Development
(c) Finished structure

4. Materials used
Hybrid organic-inorganic zirconium containing photopolymer SZ2080. Acquired from IESL-FORTH, Heraklion;
Hydrogel Poly(ethylene glycol) (PEG-DA);
Formlabs Flexible produced and distributed by Formlabs for their 3D printers;
Polydimethylsiloxane (PDMS) in pure form;
UV PDMS from Microresist GmbH.

5. Proposed approaches Removing photoinitiators;
Adding noble metal nanoparticles;
Structuring of elastic materials;

6. Photopolymerization of pure material
Photoinitiator – an important additive in pre-polymers used to make them photosensitive to UV radiation;
Absorption and high chemical activity makes them undesirable in some applications (microoptics, biomedicine, etc.);
Femtosecond laser pulses allow to cross-link materials even without photoinitiators;

7. Fabrication windows SZ2080 + 1 wt% IRG Pure SZ2080
Green – complete structure, yellow – with deficiencies, red – overexposed/heavily damaged, black – no structure;
Intensity (I=Ppulse/Sspot) required to structure pure material is higher;
Fabrication window of non photosensitized SZ2080 is only 12.5% smaller than that of photosensitized counterpart;

8. Microoptical and biomedical structures out of pure SZ2080
Optical applications:
Biomedical structures:

9. Polymer – gold nanoparticle (Au NP) nanocompound
Nanoparticles allow to control light in nano- and macroscale;
Plasmonic resonance results in locally enhanced field;
Possible application in additive manufacturing: wavelength sensitive agent which changes photosensitivity of the material;
Additional functionality of finished structures;

10. Effects of doping SZ2080 with Au NP
Fabricated feature size depends on the concentration of Au NP;
Au NP can replace around 0.1 %wt IRG (515 nm);
Combining photoinitiator and Au NP does not have additional effect of photosensitivity;
SZ Au NP mixture have good long term (up to 10 months) colloidal stability;

11. 3D structures out of Au NP doped SZ2080
Comparison of structures produced out of SZ2080 doped with 3.9 · 10−3 wt% Au NPs ((a) and (d)), SZ2080 with 0.1 wt% IRG ((b), ((e)) and pure SZ2080 ((c), (f)). Structures produced from the material combining polymer and Au NPs show no defects. I = 0.61 TW/cm2 .

12. PEG-DA-700 + Au NP PEG-DA-700 - Hydrogelic material;
Au NP was in water solution;
Observable difference in optical damage threshold;
Poor quality of the finished structures;
Bad long term colloidal stability;

13. Theoretical explanation for polymer - NP interaction
NP generated near field reaches polymerization threshold before the laser pulse.
It initiates local polymerization and free electron generation;
Distance between nanoparticles when concentration is ~10−3 wt% is around hundreds of nm.
Generated free electrons interact with strong laser pulse field and increase the strength of a avalanche ionization;

14. Elastic materials
Most lithographically processable materials are hard in their final form;
Some of these materials can be shaped in structures which are elastic due to its architecture (chainmail);
Many applications could benefit from true mechanically flexible materials;

15. Laser produced structures out of flexible polymers
Formlabs Flexible
Pure PDMS
UV PDMS
Liquid during fabrication – difficult to manufacture;
High shrinkage;
Narrow fabrication window;
Poor adhesion with the substrate;

16. Conclusions and outlook
1) Most of the enhancements presented here are possible due to usage of femtosecond (300 fs) laser;
2) Pure material can be processed applying 3DLL with fabrication window staying relatively large – 12.5% of photosensitized material;
3) Usage of very low concentrations (10−3 wt% ) of Au NP allows to control photosensitivity of the material;
4) Elastic 2.5D and 3D structures can be shaped using tightly focused ultrashort laser pulses;
Further challenges such as in-depth understanding of some material/ultrafast laser pulse interaction or elastic materials practical for laser processing will be the driving force behind further research in this field;

17. Acknowledgements
We acknowledge ECs Seventh Framework Programme Laserlab-Europe IV JRA support BIOAPP (EC-GA ).
We also thank Friedrich Waag (University of Duisburg-Essen) for the TEM-measurements.
L. Jonušauskas, M. Lau, P. Gruber, B. Gökce, S. Barcikowski, M. Malinauskas and A. Ovsianikov, Plasmon assisted 3D microstructuring of gold nanoparticle-doped polymers, Nanotechnology 27(15), (2016).
R. Buividas, S. Rekštytė, M. Malinauskas, and S. Juodkazis, Nano-groove and 3D fabrication by controlled avalanche using femtosecond laser pulses, Opt. Mater. Express 3(10), (2013).
Some structures were fabricated cooperating with high-tech firm “Femtika” and using their “Laser Nanofactory” setup.
Authors would like to thank “Microresist“ GmbH for providing free sample of UV PDMS for laser structuring experiments.

18. Thank You!

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20. Resolution array structures
Green – complete structure, yellow – with deficiencies, red – overexposed/heavily damaged.

21. Nanoparticles and sample characterization
Left: Normalised extinction spectra of Au NP in isopropanol and SZ2080 doped with Au NP Green dashed line - laser wavelength applied for excitation (515 nm). Insets show HR-TEM images of particles used in this work.
Right: resolution measurement using resolution bridges: (a) – principle, (b) – example of SEM micrograph.

22. Experimental setup