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Optical forces for assembling complex plasmonic nanostructures
- 1. Optical forces for assembling complex plasmonic nanostructures M Sai Nitish Chandra Supervisor : Dr. Shourya Dutta Gupta
- 2. Introduction to plasmonics • Noble metallic nanoparticles (Au, Ag,Pt ) support surface plasmons. • Extremely strong absorbers and scatterers of light. • Nanoparticle optical properties depend on : • Size • Shape • Composition © 2019 nanoComposix, Xingchen Ye et al. Nano Lett., 2013, 13 (2), pp 765–771 Shift towards higher λ with the increase in aspect ratio of Au NRs
- 3. Plasmonic trapping • Trapping nanoparticles using very strong gradient forces that are produced in the near-field of plasmonic nanostructures. b) Focused by a strong lens c) Far-field d) Near-field Raziman TV et al. , Faraday Discuss., 2015, 178, D >> wavelength D << wavelength and strong gradient field a) Homogenous field D < wavelength and limited focused field D > wavelength and focused field • The optical force is directly proportional to the gradient of intensity of light illuminated. F = −𝛻 𝐼
- 4. ? • Strong coupling between the LSPs of the nanostructures. • Hence, a stronger field enhancement. Raziman TV et. al , Faraday Discuss., 2015, 178,
- 5. Shell structure • Initially , a golden shell of inner radius 25nm and outer 35nm was created and x- plane polarized light was illuminated in the z-direction. Top View3D View 25nm 35nm Px ǩ Gold Water
- 6. Resonance in the shell with an opening [1] In work Plane : Cross section of the shell was drawn. [2] It was revolved about z-axis [3] Edges were made smooth by using fillet. Work plane Shell obtained after revolving and applying fillet Shift in resonance Peak after making an opening
- 7. Full structure Fig. (a) Fig. (b) Fig. (c) a) The outermost layer of the structure : PML (Perfectly matched layer) b) A cubical block enclosing the shell and particle for a finer mesh. c) The particle and shell system.
- 8. Geometry of the structure and parameters Top View Parameters Gold The wavelength of light was swept from 500 nm to 800 nm and the • Force • Charge Distribution (Power outflow) • Electric field on the nanoparticle were calculated. Radius of Core 25 nm Radius of Shell 35 nm Initial wavelength 500 nm Final wavelength 800 nm Wavelength step size 10 nm Opening Angle (theta) 45 °
- 9. Shift in resonance peak on trapping A change in the resonance position of the reflectivity spectrum was observed due to introduction of gold nanoparticle. Legend with np without np Field Gradient xy plane yz plane
- 10. Effect of position on force Forces which the nanoparticle experiences when it is placed at different positions in the z- direction:
- 11. Effect of Position on Force Forces which the nanoparticle experiences when it is offset in x-direction: FX FZ Offset x x = 14 nm x = 0 nm 0 4x10-24 0 -10x10-25
- 12. Effect of Position on Force Forces which the nanoparticle experiences when it is offset in y-direction: FY FZ Offset y y = 14 nm y = 0 nm 6x10-25 0 -2.2x10-25 0
- 13. Charge Distribution At λ = 690 nm XY Plane – Top View
- 14. Electric Field The particle is made offset along x-axis by 8nm XY – Plane (Top View) XZ – Plane We can observe that the field enhancement is maximum when it is near the periphery. So, coupling between the nanoparticle and antenna takes place. 10 70
- 15. Electric Field The particle is made offset along y-axis by 8nm XY – Plane (Top View) XZ – Plane (Top View) 50 5
- 16. Applications & scope in future • Optical tweezers • Bio sensors • Nanofactories • Color engineering • Plasmonic solar cells Alexander S. Urban, Nanoscale, 2014, 6, 4458-447
- 17. Conclusion • Shifting of the resonance peak on trapping a NP. • The optical forces were maximum at the resonance. • Magnitude of these forces change when the particle in displaced in x, y, z directions. • The forces push the NP towards periphery (at fixed height).
- 18. References [1] Raziman TV et al. , Faraday Discuss., 2015, 178, 421 [2] Alexander S. Urban, Nanoscale, 2014, 6, 4458-4474 [3] Onofrio M. Maragò1, Nat Nanotechnol. 2013 Nov;8(11):807-19
- 19. Acknowledgement I am very much thankful to Dr. Shourya Dutta Gupta, Pravallika Bandaru and Lavanya Devi for guidance and support during the project.
- 20. Thank You
Editor's Notes
- Noble metal nanoparticles support surface plasmons (oscillations of the conduction electrons at the nanoparticle surface) that result in extraordinary optical properties that are not exhibited by any other class of material. The basis for the effect is the plasmon resonance of the free electrons in the metal nanoparticle, which can be understood by studying the polarizability (the ease with which charges, such as the conduction electrons on the metal nanoparticle surface, undergo charge distribution and form partial dipoles). -- What is a Surface Plasmon Resonance? The remarkable optical properties of plasmonic materials occurs because the conduction electrons on the nanoparticle surface undergo a collective oscillation when excited by light at specific wavelengths (shown below). This oscillation, which is known as a surface plasmon resonance (SPR), results in the unusually strong scattering and absorption of light. When these resonances are excited, absorption and scattering intensities can be up to 40x higher than identically sized particles that are not plasmonic. -- What are surface plasmons: Surface plasmons (SPs) are coherent electron oscillations that exist at the interface between any two materials where the real part of the dielectric function changes sign across the interface . In our case, gold has negative permittivity. And water has positive value.
- The mechanisms of optical trapping can be easily understood – even within the framework of ray optics – if one assumes that each photon forming the optical ray carries some momentum. This is illustrated in Fig. 1, inspired by one of the seminal papers of Ashkin. In a homogeneous field, a particle is just pushed along by the scattering force. (b)In the focus of a strong lens, the gradient forces can laterally trap and catch the particle; these forces can be understood in the framework of ray optics, assuming that momentum is transferred from the incoming ray to the sphere at each reflexion or refraction. © In far-field optics, the focus produced by a lens is limited to about the wavelength l; consequently, a very small particle does not “feel” the gradient forces and is merely pushed along by the scattering force. (d) A plasmonic nanostructure can produce extremely strong field gradients, which are not limited by the illumination wavelength, but simply determined by the geometrical features of the nanostructure. Hence, in that case, it is possible to trap extremely small objects. It is actually a feature of the near-field that it can be confined to dimensions that are determined by the size of the object that produces the near-field, irrespective of the illumination wavelength. This effect is dramatically intensified in plasmonic nanostructures, where the very strong near-field originates from the confinement of polarization charges at the surface of the metal.
- . A localized surface plasmon (LSP) is the result of the confinement of a surface plasmon in a nanoparticle of size comparable to or smaller than the wavelength of light used to excite the plasmon. The LSP has two important effects: electric fields near the particle’s surface are greatly enhanced and the particle’s optical absorption is maximum at the plasmon resonant frequency.
- Biomedical – Plasmonic nanoparticles are photostable and thus can be used as bio-nanoprobes. Plasmonic nanoparticles scatter light vigorously, and hence can be identified easily under dark-field illumination and other sensing techniques. Thus they can be utilized in various in vitro biological applications. Color engineering – The unique optical properties of metal nanoparticles are very useful in color engineering. Here customized nanoparticle formulations are created for the purpose of absorbing and scattering specific wavelengths of light to generate a color. Plasmonic nanoparticles can concurrently absorb and scatter light to offer a bichromic color result. Plasmonic solar cells – Plasmonic nanoparticles possess low absorption property as well as the ability to scatter light back into a photovoltaic structure. Researchers are keen on exploiting these aspects to enhance solar cell efficiency by forcing more light to be absorbed by solar cells