High-Dielectric 3-D Printable Materials for Laser Accelerators

1. High-Dielectric 3-D Printable Materials for Laser Accelerators Ethan Walker Postdoctoral Research Associate MST-7 8/16/18

2. 3-D Printing Photonic Band Gap Structures For Dielectric Laser Accelerators • Micron - scale features can be printed using Nanoscribe • Previous work has demonstrated ability to print “woodpile” accelerator structure • Printable resins must be susceptible to radical polymerization 2-photon polymerization of monomer resin Simakov, E. I.; Andrews, H. L.; Herman, M. J.; Hubbard, K. M.; Weis, E. AAC Conference Proceedings 2016.

3. Material Requirements for a Functioning Accelerator • High dielectric constant (ε` > 4) / refractive index (n > 2) • Low loss tangent (tan δ< 0.001) / NIR absorbance (k < 0.001) at 2μm (laser frequency) • High breakdown voltage/ laser damage threshold (Fth > 0.19 J/cm2) • Appropriate optical properties • Resistant to laser damage

4. Refractive Index Requirement • n > 2 necessary to confine accelerating field to central channel

5. Refractive Index Requirement • n > 2 necessary to confine accelerating field to central channel

6. Computational Screening for High-Refractive Index Polymers Candidates = Generic polymer backbone with various substituent groups X = Pilania, G.; Weis, E.; Walker, E. M.; Gilbertson, R. D.; Muenchausen, R. E.; Simakov, E. I. Scientific Reports2018, 8, 9258.

7. Computational Screening for High-Refractive Index Polymers Candidates = Generic polymer backbone with various substituent groups X = Pilania, G.; Weis, E.; Walker, E. M.; Gilbertson, R. D.; Muenchausen, R. E.; Simakov, E. I. Scientific Reports2018, 8, 9258.

8. The Challenge of High Refractive Index

9. The Challenge of High Refractive Index

10. The Challenge of High Refractive Index

11. The Challenge of High Refractive Index

12. Why Nanoparticles? • Combines high-n inorganics with printable organics • Nanoparticles necessary to prevent light scattering • Enable uniform distribution within a microstructure • Nanocrystals enable tuning of absorption frequency • The challenge is high loading while avoiding aggregates Particle aggregates > 200nm in size will cause scattering of accelerator beam

13. Two Paths to Nanoparticle Hybrids • Lead-impregnated polymer printed from lead-containing monomer • Nanoparticles formed in-situ with hydrogen sulfide gas • Polymer matrix prevents aggregation • Nanoparticles synthesized and incorporated into monomer resin • Enables a wide range of particles including germanium • Ligand shell needed to prevent aggregation

14. This Sort of Thing Has Been Done Before… In-situ synthesis of PbS particles in Pb - containing urethane yields n > 2 ~4 nm PbS particles formed in a radical - polymerized lead methacrylate (●) Pb Precursor (▲) Converted to PbS Wang, J.-Y.; Chen, W.; Liu, A.-H.; Lu, G.; Zhang, G.; Zhang, J.-H.; Yang, B. Journal of the American Chemical Society2002, 124, 13358. Lü, C.; Guan, C.; Liu, Y.; Cheng, Y.; Yang, B. Chemistry of Materials2005, 17, 2448.

15. Synthesis of Polymer / Nanoparticle Hybrid Step 1: Polymerization of the metal / monomer complex • Initial synthesis / characterization of material as thin film • Metal – crosslinked polymer is insoluble • To obtain a thin film, must be polymerized as thin film

16. Synthesis of Polymer / Nanoparticle Hybrid Step 1: Polymerization of the metal/ monomer complex Free-standing film of Pb polyacrylate Thin film on Silicon

17. Synthesis of Polymer / Nanoparticle Hybrid Step 2: Hydrogen sulfide exposure to form nanoparticles

18. In-Situ Nanoparticle Synthesis Step 2: Hydrogen sulfide exposure to form nanoparticles • Optical absorbance consistent with 3-4 nm PbS particles • Little absorbance out at 2 microns

19. In-Situ Nanoparticle Synthesis Step 2: Hydrogen sulfide exposure to form nanoparticles • Degree of conversion can be inferred from carboxylic acid

20. In-Situ Nanoparticle Synthesis Step 2: Hydrogen sulfide exposure to form nanoparticles • Photopolymerization apparently enables greater conversion

21. What Level of Conversion is Needed? • Refractive index can be predicted based on volume fraction of components • For acrylate-based composite, high level of conversion to nanoparticles required

22. What Level of Conversion is Needed? Some other polymer composites may be more forgiving Poly(vinyl mercaptan) Synthesis of corresponding Lead polymer

23. Nanoparticle Hybrids and the Other Materials Requirements • Transparency: • Requires limited aggregation • Absorbance profile can be tuned through size • Breakdown Voltage: • Laser damage threshold of particles poorly understood • Previous results demonstrated 5% Ge nanoparticle composite above required damage threshold • Printing: • Printing blends requires nanoparticle transparency at much lower wavelength (~700 nm) • In-situ nanoparticle synthesis circumvents this problem

24. Alternate Option: Conjugated Polymers • Sufficient refractive index and transparency expected • Not expected to survive laser • Nonlinear optical properties might also be problematic • Related option: extended aromatic complexes • More robust, but not as high refractive index Zinc phthalocyanine

25. Conclusions: • Printable materials with these requirements is difficult but (probably) not impossible • The most promising route appears to involve polymer-nanoparticle hybrids with PbS or Ge • Two different routes are being explored: solution and in-situ synthesis • Other options may be interesting, but hinge on laser damage threshold

26. Acknowledgements: • Ghanshyam Pilania • Eric Weiss • Christina Hanson • Robert Gilbertson • Ross Muenchausen • EvgenyaSimakov

27. Why Not Focus on a Less Toxic Metal?

28. Prospects for Nanoparticle Hybrids • Literature precedent for high-n hybrids • Expected n calculated for a range of different PbS polymer hybrids