1. Surface Plasmon Spectroscopy Lokanathan Arcot Department of Forest Products Technology School of Chemical Technology Aalto University

2. Course 3130, Dr. Lokanathan Arcot 2 Total Internal Reflection What is Total Internal Reflection ?

3. Course 3130, Dr. Lokanathan Arcot 3 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) θ i > θ c n – Refractive index; θ i – Angle of incidence; θ c – Angle of reflection

4. Course 3130, Dr. Lokanathan Arcot 4 Evanescent Wave Evanescent means vanishing Lower ’n’ (air) Higher ’n’ (glass) θ i > θ c n – Refractive index; θ i – Angle of incidence; θ c – Angle of reflection The intensity of field decays exponentially as a function of distance Z – distance from surface I – intensity of field d – arbitrary distance

5. What is a Plasmon ? Electrically neutral volume Plasma of Electrons Collective oscillation Electric field Dr. Lokanathan Arcot 5

6. What is Surface Plasmon? Metal Plasmon-Light coupling /Surface Plasmon Polariton Matching Electron Freq and Light Freq Depending on Electronic Properties Metal Surface Angle of Incidence Dr. Lokanathan Arcot 6 Light of appropriate λ excite plasmons Light Absorption

7. Combining Surface Plasmon and Total Internal Reflection Dr. Lokanathan Arcot 7 Lower ’n’ (air) Higher ’n’ (glass) θ i > θ c Metal Add a thin film of Metal capable of undergoing plasmonic excitation Metal film

8. Evanescent Wave due to Surface Plasmons Dr. Lokanathan Arcot 8 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 ?

9. Course 3130, Dr. Lokanathan Arcot 9 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_fi les/SPR_TechNote.pdf Total Internal Reflection Surface Plasmon Resonance

10. Course 3130, Dr. Lokanathan Arcot 10 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_fi les/SPR_TechNote.pdf

11. Course 3130, Dr. Lokanathan Arcot 11 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 excitations Indium – expensive Sodium – highly reactive Copper, Aluminum – broad peak Silver – prone to oxidation Au – good choice

12. Course 3130, Dr. Lokanathan Arcot 12 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

13. Course 3130, Dr. Lokanathan Arcot 13 Using SPR for Surface Adsorption Studies Kretschmann configuration Materials Transactions, Vol. 51, No. 6 (2010) pp. 1150 to 1155 Dielectric (water) ’ε d ’ (glass Prism) ’ n p ’ (Metal) ’ ε m ’ n p – refractive index of Prism (glass) ε d – dielectric constant of water or air ε m – dielectric constant of metal Metal is usually 50 nm Au

14. Course 3130, Dr. Lokanathan Arcot 14 Using SPR for Surface Adsorption Studies Kretschmann configuration Materials Transactions, Vol. 51, No. 6 (2010) pp. 1150 to 1155 Dielectric (water) ’ε d ’ (glass Prism) ’ n p ’ (Metal) ’ ε m ’ Electric field parallel to plane of incidence E M Magnetic 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

15. Course 3130, Dr. Lokanathan Arcot 15 Surface Plasmon Resonance Wavenumber Characteristics Materials Transactions, Vol. 51, No. 6 (2010) pp. 1150 to 1155 κ x – wave vector of light in Prism; κ SP – wave vector of surface plasmon; n p – refractive index of Prism (glass); n d – refractive index of dielectric; n gold – refractive index of gold; ε d – dielectric constant of water or air: ε m – dielectric constant of gold; Dielectric(water) ’ε d ’ κ SP (glass Prism) ’ κ x ’ (Metal) ’ ε gold ’ θ Resonance – magnitude of incident wave vector = magnitude of SP wave vector κ SP = κ x Equality – λ, θ, n d

16. Course 3130, Dr. Lokanathan Arcot 16 Using SPR for Surface Adsorption Studies Resonance κ SP = κ x Equality – λ, θ, n d Basis of SPR sensing mechanism Any deposition of molecules on metal (gold) – dielectric (water) interface results in change of ’ n d ’ refractive index How do we maintain the resonance after ’ n d ’ change ? We can change the θ or λ

17. Course 3130, Dr. Lokanathan Arcot 17 Using SPR for Surface Adsorption Studies κ SP = κ x Equality – λ, θ, n d 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

18. Course 3130, Dr. Lokanathan Arcot 18 Applications of SPR κ SP = κ x Equality – λ, θ, n d Angular Sweep λ constant – study variation of θ as a function of film properties Wavelength Sweep θ constant – study variation of λ as a function of film properties

19. Course 3130, Dr. Lokanathan Arcot 19 Summary of Part I Total Internal Reflection Evanescent wave Plasmon, Surface Plasmon Evanescent wave of Surface Plasmon Surface Plasmon Resonance Optimal angle of incidence Optimal metal film Optimal film thickness Kretschmann configuration Prism (Au film), dielectric, p- polarized incident light Quantitative sensing basis Wave vector equality – θ, λ and n d Applications – Angular/Wavelength sweep

20. Course 3130, Dr. Lokanathan Arcot 20 A short break Followed by Part II Examples of SPR studies