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
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.
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).
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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.
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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.
.
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