1. Nano-Plasmonics Jinesh. K.B.
Indian Institute of Space-Science and Technology (IIST)
Thiruvananthapuram.

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Nano-plasmonic Devices

3. Lycurgus cup, an ancient (4th century Roman) glass cup with outside and inside illumination
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The reason for this extraordinary optical phenomenon was gold and silver nanoparticles in the glass!
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In 1857, Faraday observed a color change in colloidal solutions with metallic particles of different sizes.
In 1908, Gustav Mie came up with a satisfactory explanation based on the electromagnetic theory for this phenomenon.

4. Light scattering by metallic nanoparticles
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5. What are plasmons? Waves of the free electron sea of a metal
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Plasma frequency
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Due to the boundary conditions, the electrons form a standing wave pattern on the metallic surfaces.
These waves can be described by quantum mechanics, as particles with certain momentum ħk, and thus the name “plasmons”.
Plasmons are quantized density waves of free electrons in metals.
Plasmons at the metal surface is called Surface plasmons.

6. Will light directly couple with plasmons?ħk
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The condition for coupling the light with the plasmon:
Propagation constant of the plasmon = wave vector of incident light
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When the incident light couples with the surface plasmons, it creates a plasmon resonance, called surface plasmon resonance (SPR).

7. Frequency-dependent dielectric functions of metals
The dielectric function of a material is
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8. Plasmon is the natural oscillation wave of the free electron gas in a metal.
Free electron density of Gold = 5.9x1028 m-3. p = 1.37 x Rad/s
(2.2 x 1015 Hz)
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For comparison, a green light ( = 500 nm),  =3.77x1015 Rad/s or
 = 6 x 1014 Hz
p of gold corresponds to 137 nm, which lies in the UV region.
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Normal light incident on a planar film does not excite surface plasmons (because the propagation vector of incident light k sin is always lower than the propagation constant  of the plasmons).
Hence, special dielectric-matching techniques are used to excite plasmon resonances on thin film surfaces.

9. Exciting SPR: Kretschmann configuration
Propagation constants matching by
 = (k sin) ().
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Generation of evanescent waves
50 nm gold
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10. SNOM Photon tunnelling
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A He-Ne laser beam incident on an uncoated prism at an angle greater than the critical angle.
Photon tunnelling
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11. A He-Ne laser beam incident on a prism-face where 53 nm silver is coated. The tailing (exponential decay) in the right image is due to the SPP generation.
A He-Ne laser beam incident on an uncoated prism at an angle greater than the critical angle.
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The photon incident at critical angle simply tunnel through the metallic layer and excites the surface plasmons.

13. Scanning Near-field Optical Microscopy
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The optical probe is brought near to the evanescent field of the SPP’s (within a distance z, and scanned over the surface. For gold and silver this distance is <100 nm.
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14. SNOM machine from WITech
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AC-mode SNOM image of
polystyrene spheres.
SNOM machine from WITech

15. Exciting SPR: Grating configuration
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The condition for phase matching is =k sin, which can be achieved by patterning the metal surface.
The metal surface is grooved with a structures of periodicity a
When a light of wave vector k falls on the grating, phase-matching takes place whenever the following condition is satisfied
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16. Surface plasmon propagation in Nanoparticle arrays
A laser beam (polarized in x-direction, 815 nm) is focused on the right array; SPP transmitting to the right array . A SNOM image.
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Arrays of holes (diameter is 250 nm) at a pitch of 760 nm. Two arrays are separated by 30 um.

17. Localized surface plasmons in Nanoparticles
The electron cloud of an array of metallic nanoparticles respond to the EM waves
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Plasmon propagation constant in a nanoparticle matches with incident wave vector without additional matching configurations, due to their geometrical features (size, shape etc.)
Due to the resonance of polarization, the scattering theory predicts a field enhancement at small (point-like) particles.
The nanoparticle acts like an electric dipole, resonantly absorbing and scattering electromagnetic fields.

18. Scattering of light on point particles and spheres
This quasi-static approach for point-like particles is not valid for larger particles, when size comparable to the wavelength of light and a rigorous electromagnetic approach is needed to solve it  Mie’s solutions
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Why milk looks white?
Why honey appears dark?
Why clouds are sometimes white and sometimes dark?
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Rayleigh scattering from gas molecules
Mie scattering from water droplets (white or grey color of the clouds)

19. Nanoparticle Plasmon Resonance
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Scattering depends on the shape and size of nanoparticles (example on top: silver nanoparticles) AIP 2002
Scattering depends on the shape and size of nanoparticles (example on top: silver nanoparticles) AIP 2002

20. Plasmonic Bragg Reflector: nanoparticle plasmonic arrays
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Parallel lines of particles (diameter 140 nm), line-spacing 350 nm constitute a “lattice”.
Plasmon falling on this “lattice” undergo reflection. Here the plasmon falls on the “lattice” at an angle of 60o .
The reflection coefficient was 90% in the geometry.
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This showed that SPP can be directed like normal light, but on surfaces, using simple structural elements.

21. Focusing and directing SPP
Hole-array
(hole dia. – 200nm) on 50 nm silver film
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250 nm wide stripe waveguide
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Yin et al (2005), ACS
The hole array acts as an array of point sources for the plasmons

22. Photonic band-gap
A structure (1D, 2D or 3D) that offers a bandgap to the electromagnetic waves is called a photonic bandgap crystal or simply a photonic crystal.
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The surface of a grating is an example of a 1D or 2D photonic crystal. g
Consider the SPP travelling r to the grating lines
The wave vector associated with the grating is
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Relative phases of the modes
Amplitude of the SPP is
Amplitude of harmonic components of the corrugation

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Reflected SPP
Propagating SPP + Reflecting SPP  Standing wave by constructive interference, represented by
and
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One of these standing waves coincides with the peaks of the grating, the other coincides with the troughs.

24. This difference of energy creates an optical “bandgap”
The field distribution and hence, the energies of these two waves are different (compare with the wavefunction of an electron in a crystal lattice).
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This difference of energy creates an optical “bandgap”
The grating perturbs the fields associated with the surface charges ; due to the relative distortion is field distributions, they are associated with different energies.
Optical bandgap
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A. P. Hibbins, PhD Thesis, University of Exeter

25. “atom” size: Ø 378 nm, 100 nm height Made on 40 nm gold film.
Bozhevolnyi and co-workers showed (2001) Photonic bandgap in a triangular array of gold nanoparticles
[Marquart et al., 2005]
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Lattice spacing: 900 nm
“atom” size: Ø 378 nm, 100 nm height
Made on 40 nm gold film.
SPP generated via prism coupling; wavelengths used are 1550 nm and 1600 nm.
1550 nm
1600 nm
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Significant attenuation of the plasmons generated by incident light of 1600 nm explicitly shows the onset of the photonic bandgap in this structure.

26. Plasmon propagation through channels
If a micro-channel is constructed in a lattice of nanostructures by removing a few of them, plasmonic waves can be confined inside.
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The bandgap for different directions in the Brillouin zones are different. So, at the bending, some of the SPP is lost.
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A channel is made in an array of gold dots separated by 950 nm and a light of 1500 nm is coupled through Kretschmann configuration.
An important analogy between optical band gap and electronic bandgap:

27. Naturally occurring Photonic crystals:
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Yizhou Li et al. Phys. Rev. E (2005)
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28. Surface Enhanced Raman Scattering (SERS)
When a molecule is places on a rough surface, the local plasmons enhance the scattering cross section, enabling higher Raman intensity. The enhancement factor is proportional to the fourth power of the electric field
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29. 2. Gas sensing using plasmonics
The metal-dielectric interface is sensitive to the plasmon generation. Gas molecules absorbed on the dielectric alter the dielectric permittivity of the insulator and reflectivity changes.
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Bingham et al., JACS 2010

30. Plasmonics for biological applications: enhanced fluorescence
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Khlebstov et al, Journal of Quantitative Spectroscopy & Radiative Transfer (2010)

31. Cancer cell detection using enhanced fluorescence
Comparison of scattering between gold and ploystyrene particles of the same size.
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NPs are conjugated to anti-epidermal growth factor receptor (anti-EGFR) antibodies via nonspecific adsorption to recognize the EGFR proteins on the cervical carcinoma cells and tissues.
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32. Our work: Towards graphene nanoplasmonics
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To study the plasmon generation in graphene and graphene decorated with nanoparticles
Plasmons induced on graphene by electron bombardment from a scanning tunnelling microscope
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Plasmon generation and propagation will shed light on the possibility of bandgap engineering in graphene, for the applications in fast switching transistors and sensors.

33. Optical nano-imaging of gate-tunable graphene plasmons
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IR beam (λ0 = 9.7 μm)
Near-field image
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Calculated optical density of states
J. Chen et al, Nature 2012

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Thank you
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