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Transcript of Surface Plasmon Spectroscopy Lokanathan Arcot Department of Forest Products Technology School of...
Surface Plasmon Spectroscopy
Lokanathan ArcotDepartment of Forest Products TechnologySchool of Chemical TechnologyAalto University
Course 3130, Dr. Lokanathan Arcot
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Total Internal Reflection
What is Total Internal Reflection ?
Course 3130, Dr. Lokanathan Arcot
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Evanescent Wave
Total Internal Reflection creates an Evanescent Wave
Upon internal reflection the electric and magnetic fields of incident light partially propagate into the upper lower refractive index medium
Lower ’n’(air)
Higher ’n’(glass)
θi > θc
n – Refractive index; θi – Angle of incidence; θc – Angle of reflection
Course 3130, Dr. Lokanathan Arcot
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Evanescent WaveEvanescent means vanishing
Lower ’n’(air)
Higher ’n’(glass)
θi > θc
n – Refractive index; θi – Angle of incidence; θc – Angle of reflection
The intensity of field decays exponentially as a function of distance
Z – distance from surfaceI – intensity of fieldd – arbitrary distance
Dr. Lokanathan Arcot
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What is a Plasmon ?
Electrically neutral volume
Plasma of ElectronsCollective oscillation
Electric field
Dr. Lokanathan Arcot
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What is Surface Plasmon?
Metal
Plasmon-Light coupling /Surface Plasmon Polariton
• MatchingElectron Freq and Light Freq
• Depending on Electronic PropertiesMetal Surface
• Angle of Incidence
Light of appropriate λ excite plasmonsLight Absorption
Dr. Lokanathan Arcot
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Combining Surface Plasmon and Total Internal Reflection
Lower ’n’(air)
Higher ’n’(glass)
θi > θc
Metal
Add a thin film of Metal capable of undergoing plasmonic excitation
Metal film
Dr. Lokanathan Arcot
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Evanescent Wave due to Surface Plasmons Electric-magnetic field due to Plasmonic vibrations propagate into dielectric (Low ’n’ medium)
Z – distance from surface
I – intensity of field
d – arbitrary distance
Absorption of Light of right ’λ’What about maximum absorption of light ?
Course 3130, Dr. Lokanathan Arcot
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Optimal Conditions for Maximum Absorption of Light by Surface Plasmons
Resonance - Condition when maximum transfer of energy between incident
light and surface plasmons occurs- Maximal energy tranfer – maximal absorption of light - SPR is a special case of TIR (what is the difference? )
http://www.rci.rutgers.edu/~longhu/Biacore/pdf_files/SPR_TechNote.pdf
Total Internal Reflection Surface Plasmon Resonance
Course 3130, Dr. Lokanathan Arcot
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Optimal Conditions for Surface Plasmon Resonance
Peak at specific θ due to maximum transfer of energy between incident light and plasmons
Angle of incidence
http://www.rci.rutgers.edu/~longhu/Biacore/pdf_files/SPR_TechNote.pdf
Course 3130, Dr. Lokanathan Arcot
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Optimal Conditions for Surface Plasmon Resonance
Metals should have electronic properties which allow plasmonic excitations based absorption producing sharp peak
Metal film
http://www.sprpages.nl/spr-overview/spr-theory.html
Metals capable of plasmonic excitationsIndium – expensiveSodium – highly reactiveCopper, Aluminum – broad peakSilver – prone to oxidationAu – good choice
Course 3130, Dr. Lokanathan Arcot
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Optimal Conditions for Surface Plasmon Resonance
Thickess of Au film
Sensors 2014, 14(1), 458-467
Reflectance as a function of angle of incidence. Results for three different Au film thickness
Course 3130, Dr. Lokanathan Arcot
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Using SPR for Surface Adsorption StudiesKretschmann configuration
Materials Transactions, Vol. 51, No. 6 (2010) pp. 1150 to 1155
Dielectric(water) ’εd’
(glass Prism)’ np’
(Metal) ’ εm’np – refractive index of Prism (glass)εd – dielectric constant of water or airεm – dielectric constant of metal
Metal is usually 50 nm Au
Course 3130, Dr. Lokanathan Arcot
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Using SPR for Surface Adsorption StudiesKretschmann configuration
Materials Transactions, Vol. 51, No. 6 (2010) pp. 1150 to 1155
Dielectric(water) ’εd’
(glass Prism)’ np’
(Metal) ’ εm’
Electric field parallel to plane of incidence
E
MMagnetic field perpendicular to
plane of incidence
* plane of incidence is plane of this slide
Incident Light is ’p’ polarized or
Transverse Magnetic
The ’p’ polarization ensures maximum
excitation of plasmons whose
evanescent waves propagate into
dielectric medium
Course 3130, Dr. Lokanathan Arcot
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Surface Plasmon ResonanceWavenumber Characteristics
Materials Transactions, Vol. 51, No. 6 (2010) pp. 1150 to 1155
κx – wave vector of light in Prism; κSP – wave vector of surface plasmon; np – refractive index of Prism (glass); nd – refractive index of dielectric; ngold – refractive index of gold; εd – dielectric constant of water or air: εm – dielectric constant of gold;
Dielectric(water) ’εd’
κSP
(glass Prism)
’ κx ’
(Metal) ’ εgold’
θ
κSP
κx
Resonance – magnitude of incident wave vector = magnitude of SP wave vector
κSP = κx Equality – λ, θ, nd
Course 3130, Dr. Lokanathan Arcot
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Using SPR for Surface Adsorption Studies
ResonanceκSP = κx Equality – λ, θ, nd
Basis of SPR sensing mechanism Any deposition of molecules on metal (gold) – dielectric
(water) interface results in change of ’ nd ’ refractive index
How do we maintain the resonance after ’ nd ’ change ?
We can change the θ or λ
Course 3130, Dr. Lokanathan Arcot
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Using SPR for Surface Adsorption Studies
κSP = κx Equality – λ, θ, nd
Quantitative surface adsorption analysis- Refractive index changes linearly with amount of adsorbed molecules- We vary the θ in order to maintain the resonance- How much we need to vary the θ - tells us how much adsorption has occured
Course 3130, Dr. Lokanathan Arcot
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Applications of SPRκSP = κx Equality – λ, θ, nd
Angular Sweep λ constant – study variation
of θ as a function of film propertiesWavelength Sweep
θ constant – study variation of λ as a function of film properties
Course 3130, Dr. Lokanathan Arcot
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Summary of Part I• Total Internal Reflection• Evanescent wave• Plasmon, Surface Plasmon• Evanescent wave of Surface Plasmon• Surface Plasmon Resonance
Optimal angle of incidenceOptimal metal filmOptimal film thickness
• Kretschmann configurationPrism (Au film), dielectric, p- polarized incident light
• Quantitative sensing basisWave vector equality – θ, λ and nd
• Applications – Angular/Wavelength sweep
Course 3130, Dr. Lokanathan Arcot
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A short breakFollowed by
Part IIExamples of SPR studies