This presentation gives the basics of surface plasmon polaritons

1. Theory of Surface plasmon polaritons
Presented by
R.Gandhimathi

2. • Plasmonics deals with optical confinement and guiding of light at the nanoscale metallic structures
• Plasmons are collective electron density oscillations and they are the characteristic of interaction of light and
metallic nanostructures
• The interaction overcomes the diffraction limit for the localization of light into subwavelength dimensions and
enables strong field enhancements
• Light confinement and enhancement of EM field at subwavelengths are utilized in a number of applications
including molecular sensors, photocatalysis, and thin-film solar cells
Overview on Plasmonics
Localized Surface Plasmon Resonance (LSPR)
• Interaction of light with metal nanoparticles produces a collective oscillation
of electrons at a resonant frequency
Surface plasmon polaritons (SPP)
• Surface bound 2D electromagnetic excitations existing at the metal/dielectric
interface
• controls and manipulates the light on the nm scale and exhibit significant
advantages in nanophotonics devices and opens a possibility to perform
ultrasensitive optical measurements

3. Bulk plasmon Surface plasmon polaritons (SPP)
• Longitudinal wave propagates through volume
of the material (𝐸 𝐵𝑢𝑙𝑘 = 𝐸0 𝑒 ሻ𝑖(𝑘 𝑧 𝑥−𝜔𝑡
ሻ
• Longitudinal wave propagates through surface
of the material(𝐸𝑆𝑃𝑃= 𝐸0 𝑒−𝑘|𝑧| 𝑒 ሻ𝑖(𝑘 𝑧 𝑥−𝜔𝑡 )
• Wave propagation above plasma frequency p • Wave propagation below plasma frequency p
• Cannot couple with EM field • Strong coupling with EM field
• Oscillate at p determined by the free
• electron density and effective mass
• Oscillate at SPP determined by the free
• electron density and effective mass
• No confinement • Confined to surfaces that can interact with the
light to produce SPP
Bulk plasmons Vs Surface plasmons
Metal Sphere
Electron cloud
Electro magnetic wave
Localized surface plasmon
Dielectric Medium
Metal
Z
Surface plasmon polaritonsBulk plasmons
Exponential
decay in
dielectric
Exponential
decay in meal
Propagation
direction
▪ EM fields of SPPs are highly confined to the interface
▪ Evanescently decrease in strength away from the interface

4. Methods of excitation of Surface Plasmon Polariton
Criterions to excite SPPs
▪ Refractive index of medium 1
much be larger than medium 2
(n1>n2)
▪ Conservation of energy,
frequency and in plane
momentum
▪ Surface Plasmon Polariton -A hybrid wave or a coupled state between incident photons and collective
electron oscillations. SPP can be excited either by accelerated electrons or light waves
▪ Yet a plane (incident) wave propagating in an uniform medium (refractive index n1) cannot excite the SPP due
to the existing momentum mismatch of SPPs and free-space light
▪ The incident photons in free space has smaller momentum (k0), and this momentum is not enough
to excite SPP
▪ It requires an additional in plane momentum i.e. Conservation of tangential components of the momentum,
wavevector of incident and SPP (kz and kSPP)
0 SPPk k
z SPPk k=
Excitation of SPP by plane wave or light
Mechanism
Coupling of SP wave with the
evanescent wave, which is setup
due to ATR at the base of a
coupling prism

5. With the photons, it cannot be done directly,
but requires a prism, or a grating, or a defect
on the metal surface
Special Techniques used to excite SPP
• Otto configuration
• Kretsch Mann configuration
• Diffraction grating
• Near field of tip
• Notch
• Near field of light source
SPP excitations with accelerated electrons
▪ Technique Used - Electron energy loss spectroscopy (EELS)
▪ EELS-measures the change in kinetic energy of electrons after they
interact with a matter
▪ A Metal foil is irradiated by accelerated electrons and because of
inelastic scattering of electrons, transfer energy as well as momentum
between the incident electrons and the metal film
▪ Energy losses of inelastically scattered electrons gives the
characteristic collective excitations of plasmons
SPP
1 2 SPPE E − =
Energy difference
1, 2,x x SPPP P k− =
In plane momentum
conservation
is given by

6. ▪ When light source falls through a prism on a
metal surface, some lights are reflected on the
metal surface, part of light is absorbed by the
surface electron of that metal piece and resonate
the electron i.e. excitation of SPP
▪ To excite SPP (lower interface), an evanescent
wave is created at the prism/metal interface and
that has to penetrate through the metal layer
Kretschmann configuration (Frustrated Total Internal Reflection)
n1k0sin
Dielectric Evanescent field
of SPP
Prism
1 0 sinzk n k =
z SPPk k=
Phase matching condition
SPR peak
angular
position
▪ In Kretschmann configuration, a thin metal film (few tens
of nanometers) is deposited on top of a prism, no air gap
separation between metal film and prism
▪ A beam of p-polarized light is transmitted through the
prism (permittivity, d > m ) , (angle of incidence (i) >
critical angle (c) at which total internal reflection occurs
▪ At some angle greater than the angle of total internal
reflection (TIR), a sharp minimum is observed in the
reflection coefficient, due to losses in the metal
▪ The excitation of a SPP will show up as a minimum in the
reflected light

7. Reflectance vs angle of incidence for different wavelengths
Merits
▪ Provides the optimal coupling in a broad band frequency range
▪ Easy to control the metal film thickness
▪ It is easier than the air gap control in OTTO configuration
4 ( )
L
Im

 
=
Penetration depth of SPP inside the metal film
Coupling strength ()
▪ Depends the thickness of the metal film used
▪ Directly proportional to the overlapping integral between the field of incident wave and the tail of the SPP
penetrate the metal film and inside the prism
▪ If the metal is too thin, the SPP will be strongly damped because of radiation damping into the prism
▪ If the metal film is too thick, the SPP can no longer be efficiently excited due to absorption in the metal
▪ The optimal coupling could be achieved with the same thickness of metal films for different wavelength of
incident light

8. • In the Otto configuration the tail of an evanescent wave at a glass/air interface is brought into contact with a
metal-air interface that supports SPPs
• It uses a highly refractive prism and SPP is excited optically by the evanescent wave produced at the
prism/dielectric medium
• The excitation is seen as a strong decrease in reflection for the transverse magnetic (TM polarized) light and for
a special angle of incidence
Otto configuration
▪ Metal surface is separated from the prism by air gap
▪ If the in-plane momentum of incident light, equals in plane momentum of SPP,
excitation of surface plasmon polariton will happen. i.e. kz=nk0 sin= kSPP.
▪ The evanescent field created excites surface plasmon at interface dielectric
medium/metal. The air gap between prism and metal determines the coupling
strength among the incident light and SPP
Experimental Set up

9. Angular spectrum of reflectance
• When light with different incident angles is irradiated on the surface of the prism and metal, part of the light,
which matches with the SPR angle SPP is attenuated into the metal to generate SPR and the rest of the light is
reflected
• A photodetector can be placed at the end of the device to capture and monitor the intensity of the reflected
wave
• The reflected light of reduced intensity at the SPR angle would be detected due to the optical absorption by the
metal
Applications of Surface plasmons
▪ Sensors (Ultrasensitive immunoassays)
▪ Fluorescence and SERS Imaging
▪ Catalysis
▪ Light harvesting
▪ Photo thermal therapy
▪ Photo dynamic Therapy
Light source
Metallic film
Prism
Detector
Change in refractive
index of metallic
film with the
attachment of
biomolecules is
detected by the
change in angle of
Surface plasmon
resonance
Bio molecules Mechanism

10. References
1. Daiying Zhang, Liqiu Men and Qiying Chen Microfabrication and Applications of Opto-Microfluidic Sensors, Sensors 2011, 11, 5360-
5382
2. Koichi Okamoto1, Mitsuru Funato, Yoichi Kawakami, and Kaoru Tamada, High-efficiency light emission by means of exciton–surface-
plasmon coupling, S1389-5567(17)30003-5
3. Sujan Kasani, Kathrine Curtin and Nianqiang Wu, A review of 2D and 3D plasmonic nanostructure array patterns: fabrication, light
management and sensing applications, https://doi.org/10.1515/nanoph-2019-0158
4. Daiying Zhang 1, Liqiu Men 2 and Qiying Chen 1,3,*, Microfabrication and Applications of Opto-Microfluidic Sensors, Sensors 2011,
11, 5360-5382;
5. Zhanghua Han and Sergey I Bozhevolnyi, Radiation guiding with surface plasmonpolaritons, Rep. Prog. Phys. 76 (2013) 016402 (37pp)
6. Tahir Iqbal, Propagation length of surface plasmon polaritons excited by a 1D plasmonic grating, Current Applied Physics 15 (2015)
1445e1452
7. Yifen Liu, Jaeyoun Kim, Numerical investigation of finite thickness metal-insulator-metal structure for waveguide-based surface
plasmon resonance biosensing, Sensors and Actuators B 148 (2010) 23–28
8. ANDREAS OTTO, Excitation of Nonradiative Surface Plasma Waves in Silver by the Method of Frustrated Total Reflection, Zeitschrift
ffir Physik 216, 398--410 (1968)
9. Anuj K. Sharma, Rajan Jha, and B. D. Gupta, Fiber-Optic Sensors Based on Surface Plasmon Resonance: A Comprehensive Review
IEEE SENSORS JOURNAL, VOL. 7, NO. 8, AUGUST 2007

11. Thank you