Regulating light-matter interaction

  Since our ancestor discovered how to use fire and light in the dark night, human civilizations have been developed with the advances in controlling the light. From the ancient stone lantern which merely lit small areas to LEDs of our times which brighten whole cities, people have gradually conquered the dark night and never ceased from controlling the light. And that was a result of eager for challenges to understand and regulate the interactions between light and matter.

  As an extension of that unrelenting eager, some people have set out a journey of manipulating light-matter interaction which unavailable in naturally occurring or chemically synthesized materials just as the the chemist control electronic, mechanical property of materials by finely utilize the intrinsic properties of materials.

  In the past decades, metamaterials offered an entirely new route to improve the capability of people to manipulate the light-matter interaction at will. Among them, artificial magnetism and negative refractive index hold special interest because they enable various applications in subwavelength resolution imaging[1], electromagnetic cloaking[2], magnetic resonance imaging(MRI)[3].

  To induce negative refraction, various artificial structures have been presented. Among them, split ring resonators (SRRs) are one of the most widely used design in the past decades. However, the reliance on traditional fabrication method intrinsically limits its versatility to Infrared (IR) or lower frequencies and anisotropic structure. The self-assembly of metallic colloids provides a versatile alternative route to the construction of 3D and isotropic negative index metamaterial in optical frequencies [4]. In addition, by dispersing these isotropic clusters, we can produce metafluid – a liquid metamaterials, as easily paintable, spin-coatable, and photopatternable onto a universal substrate, which can provide a practical platform for the versatile translation of metamaterials into various real-world applications.

  To successfully realize negative refraction in liquid phase, one requires a tool which can assemble metallic nanoparticles in exactly desired geometry. In the past few years, due to the lack of this tool, no researches to date have realized negative-index metamaterial in liquid phase. DNA origami provides an ideal platform to realize metafluid by assembling nanoparticles in exact geometry. DNA origami can produce geometrically well defined template and has a ability to assemble target molecule in programmable way. We expect that rationally designed DNA origami allows us to achieve target clusters with high yield and produce strong meta-molecules could be well dispersed within various fluids; thereby we hypothesize our strategies can provide a practical platform for the versatile translation of metamaterials into various real-world applications.

Fig 1. Electric field distribution around gold nanoparticle(AuNP) with various sizes on impinging light(above). The fundamental strategy of our team to realize metafluid: seeding growth method (bottom).

  In our project, our main aim is deterministically arrange gold nanoparticles acting as photonic meta-atom onto DNA origami to realize liquid metamaterial. For this, not only large enough gold nanoparticles(AuNPs)(~60nm which produce strong enough optical response. See above figure 1.) should be used but also the binding yield between DNA functionalized AuNPs and DNA origami structure should be maximized. However, the binding yield between origami structure and relatively large AuNPs (over ~40nm) drop significantly due to steric hindrance and electrostatic repulsion between particles.

  Because formation yield of target cluster exponentially decay as single AuNP binding yield decrease, low binding yield of large AuNPs acts as main obstacle for assembling AuNPs in clusters with high yield. As a brief illustration, if we can locate single AuNP with 70% yield, the final yield of desired geometric clusters(for example, octahedron) drop to 11%. Thus, our team chose seeding growth method which clustering small AuNPs (~18nm) into octahedral with high yield first and further grow it in suspension to larger size(~60nm).

  Our final goal is to realize liquid metamaterial, this is, metafluid, which can pave the way to practical application of metamaterial. To achieve this main goal, we set 6 sub-goals as a guide to our final destination.

• 1. Design DNA origami template for AuNP clusters formation
  
  • Rationally design DNA origami structure is essential to realize our challenge. Following criteria are applied for designing DNA origami structure
    • a) Structural rigidity - DNA origami template should be rigid enough to sustain from external forces applied by growing AuNPs
    • b) Provide versatile functional site to arrange AuNPs into octahedral geometry
    • c) Chemical stability - in AuNP further growth steps, reactant can damage DNA double strand and origami template.


• 2. Seed AuNPs synthesis and DNA functionalization
  • Seed AuNPs should be spherical as possible for homogeneous optical properties
  • DNA functionalization density on seed AuNPs should be maximized while minimizing aggregation between AuNPs in buffer solution


• 3. Simulate the optical properties of designed geometries to verify the feasibility of targeted optical properties from AuNP clusters.


• 4. High-yield fabrication of AuNP clusters in octahderal geometry using DNA origami template


• 5. Futher growth of AuNPs to achieve target size of cluster
  • Even though similar strategy has been briefly presented in previous works using electroless deposition[5][6], this method is highly restricted to deposited structure on substrate or shows relatively low reliability which are not appropriate for our purpose. In addition, normally AuNPs can be grown in salt condition due to its aggregation in buffer or salt containing solution. Thus, the most important criteria for this sub-goal was whether we can grow AuNPs on DNA origami and stabilize them in bulk suspension which contains salt and buffer solution with high sphericity (containing divalent cations and buffer which are destabilize grown AuNPs dramatically)


• 6. Optical properties measurement from single cluster and bulk fluid
  • Main criteria of this goal is whether we can get strong magnetic response from a single cluster and bulk solution which shows direct evidence of metafluid.

• References

[1] Pendry, J. B. (2000). Negative refraction makes a perfect lens. Physical Review Letters, 85(18), 3966–3969.
[2] Schurig, D.; Mock, J.J.; Justice, B. J.; Cummer, S. A.; Pendry, J. B.; Starr, A. F.; Smith, D. R. Science 2006, 314, 977-980.
[3] M. C. K. Wiltshire et al., Science 291, 849 (2001)
[4] V. G. Veselago, Sov. Phys. Usp., 1968, 10, 509-514
[5] Gür, F. N., Schwarz, F. W., Ye, J., Diez, S., & Schmidt, T. L. (2016). Toward Self-Assembled Plasmonic Devices: High-Yield Arrangement of Gold Nanoparticles on DNA Origami Templates. ACS Nano, 6b01537.
[6] Schreiber, R., Santiago, I., Ardavan, A., & Turberfield, A. J. (2016). Ordering Gold Nanoparticles with DNA Origami Nanoflowers, 1–20.