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 ?


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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


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Evanescent WaveEvanescent means vanishing

Lower ’n’(air)


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


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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


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Combining Surface Plasmon and Total Internal Reflection

Lower ’n’(air)


θi > θc

Metal

Add a thin film of Metal capable of undergoing plasmonic excitation

Metal film


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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 ?


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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


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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


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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


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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


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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


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Using SPR for Surface Adsorption StudiesKretschmann configuration



(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


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Surface Plasmon ResonanceWavenumber Characteristics


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;


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


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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


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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


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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


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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


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A short breakFollowed by

Part IIExamples of SPR studies

surface plasmon spectroscopy lokanathan arcot department of forest products technology school of...

Documents

surface plasmon resonance

maximum absorption of

light freq

light of appropriate

function of angle

film of metal capable

optimal conditions

oxidation au good choice