Surface Plasmon Resonance. Magneto-optical enhancement and

Surface Surface PlasmonPlasmon Resonance. Resonance. MagnetoMagneto--optical optical

enhancement and other enhancement and other possibilitiespossibilities

Applied Science Department

The College of William and Mary

May 2008 - Surface-Plasmon circuitry

T. W. Ebbesen, C. Genet, S. I. Bozhevolnyi

PlasmonicsRecently surface plasmons have attracted significant attention for a variety of exciting applications (e.g. metamaterials, “cloaking”, etc.)

Outline

• Introduction: Surface Plasmon resonance based sensors • Magneto-plasmonic sensors• Au-Co-Au Trilayers• Gratings• Au-Co nanocomposites• What is next?• Conclusions

1. Surface 1. Surface plasmonplasmonresonance for resonance for

biosensingbiosensing

Surface Surface PlasmonPlasmon ResonanceResonanceWhen light strikes a conducting thin film it is possible to excite a surface plasmon polaritoni.e. charge oscillations in the metal that lead to evanescent surface electromagnetic waves propagating along a metal/dielectric interface.

For the surface plasmon resonance to be excited, the incident light wave vector must match the surface plasmon resonance momentum. This is possible when:

The surface plasmon resonance is highly confined at the interface, and therefore is very sensitive to the dielectric optical properties.

ħkSP> ħk0

How can SPR be excited?

Application: How is SPR used in Application: How is SPR used in biobio--sensing?sensing?

A glass slide with a thin gold coating is chemically modified to be able to bind to specific bio-agents. The slide is mounted onto a prism.

Light passes through the prism and slide, reflects off the gold and passes back through the prism to a detector

Changes in reflectivity versus angle or wavelength give a signal that is proportional to the volume of bio-agent bound near the Au surface.

FundamentalsFundamentalsWhen surface plasmon resonance is excited, it radiates light backwards.

The electromagnetic field is highly enhanced at the metal/ dielectric surface interface

Au filmprism0

200400

600800

0.0

0.5

1.0

60

70

80

90

Ref

lect

ivity

Incidence angle (d

eg)

Au thickness (A)

The Au films thickness can be optimized to achieve full extinction in the reflected beam-> this is the optimum excitation condition for surface plasmon resonance

2. Magneto2. Magneto--optical optical effects and surface effects and surface plasmonplasmon resonanceresonance

Fundamentals Fundamentals

Since the electromagnetic field is strongly enhanced inside the Au film when the surface plasmonresonance is excited, the introduction of a magnetic film can cause strong enhancement of its magneto-optical activity.

C. Hermann, PRB 63, 235422 (2001)

Au Au

Co

MagnetoMagneto--optical Kerr effectoptical Kerr effectThe light that is reflected from a magnetized surface The light that is reflected from a magnetized surface can change in both polarization and reflectivity.can change in both polarization and reflectivity.This results from the offThis results from the off--diagonal components of the diagonal components of the dielectric tensor dielectric tensor εε..MOKE can be further categorized by the direction of MOKE can be further categorized by the direction of the magnetization vector with respect to the the magnetization vector with respect to the reflecting surface and the plane of incidence.reflecting surface and the plane of incidence.

Transverse MOKETransverse MOKEWhen the magnetization is perpendicular to the plane When the magnetization is perpendicular to the plane of incidence and parallel to the surface it is said to be of incidence and parallel to the surface it is said to be in the in the transversetransverse configuration.configuration.In this geometry, the MOKE effect results in a In this geometry, the MOKE effect results in a change in reflectivity that is proportional to the change in reflectivity that is proportional to the component of magnetization that is perpendicular to component of magnetization that is perpendicular to the plane of incidence and parallel to the surface.the plane of incidence and parallel to the surface.Further, the surface Further, the surface plasmonplasmon is also affected:is also affected:

J. B. González-Díaz et al., PRB 76, 153402 (2007).

System studiedSystem studied

Au (20 nm)

Au (3 nm)Co (2.5 - 6 nm)

Au-Co-Au tri-layer samples were grown on glass with DC sputtering. Accurate control of the growth rate allowed precise control of the layers thickness. Au and Co thickness were designed to achieve:

•Optimum excitation of the surface plasmon resonance

•Maximum enhancement of the MO activity.

PreparationPreparationPreparation: sputtering deposition

Sputtering System

Base pressure 10-9 Torr(UHV).

Rheed, Quadrupole in-situ.Substrate temperature:RT-

700ºC.6 magnetron sputtering gunsGas: Ar, etc...

Deposition rates (PAr=5.10-3

Torr)

Au→0.32 Å/sCo→0.066 Å/sCr→0.13 Å/sNi→0.12 Å/sAg→1.03 Å/s

High purity and thickness control!

Characterization

H

prism

solenoid

H

detector

Hpp

pp

RR

∆

HeNe laser

Custom SPR station

Custom SPR station

Au-Co-Au

glass

Au (20 nm)Co (2-10 nm)Au (3 nm)

30 35 40 45 50 55 6044

45

46

Co thickness (A)

Ang

le (d

eg)

-2.350

-2.039

-1.728

-1.417

-1.106

-0.7950

-0.4840

-0.1730

0.1380

0.4490

0.7600

30 35 40 45 5055

60

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

44

45

46

Angle

(deg

)

Co thickness (A)

pp

pp

RR

∆ pp

pp

RR

∆

2.1 2.8

1.6

2.4

3.2

2.0 2.5 3.0

3

4

5

n 2.8 nm n 3.5 nm n 5nm n 6 nm n Bulk

n

Energy (eV)

k

Deviation of the optical constants from bulk values

The thickness of all the metallic layers was designed to achieve full extinction of the reflected intensity.

42 44 46

0.0

0.2

0.4

R (0

)

incidence angle (deg)

6 nm Co 5 nm Co 4 nm Co 2.8 nm Co 2. 5 nm Co

44.1 44.4 44.7-0.01

0.00

0.01

0.02

R (0

)


Results (I)Results (I)

40 45 50-2

-1

0

1

2

3

4

R (H

)-R

(0) (

‰ )


6 nm Co 5 nm Co 4 nm Co 3 nm Co 2.5 nm Co

40 45 50

0.2

0.4

0.6

R (H

)-R

(0) (

‰ )

Incidence angle (deg)


Transverse Kerr magneto-optical signalWith no prism (i.e. the Au surface plasmon is not excited)

With prism (plasmon excited)

The measured signals are normalized with respect to the incident excitation. When the surface plasmon is excited we observe ~ one order magnitude enhancement in the transverse magneto-optical Kerr signal.

Results (II)Results (II)

Combining the enhancement of the MO effect and the extinction ofthe reflected beam, a remarkable enhancement of the relative field-dependent variation of the reflectivity is obtained

( ) (0)(0)

pp pp s pp

pp pp

R R M RR R

∆ −=

42 44 46 48-100

-50

0

50

100

150

R (H

)-R

(0)/

R (0

) (%

)


6 nm Co 5 nm Co 4 nm Co 2.8 nm Co 2.5 nm Co

42 44 46

0

5

10

R (H

)-R

(0)/

R (0

) (%

)


6 nm Co 5 nm Co 4 nm Co 2.8 nm Co 2.5 nm Co

Results (III)Results (III)

Reflectivity (R)Reflectivity (R) FieldField--dependent dependent ∆∆R/RR/R

40 45 50 55 60 65 70 75

Rpp


air water n=1.33 water+glycerine n=1.3375water+glycerine n=1.3475

40 45 50 55 60 65 70 75-0.5

0.0

0.5

1.0

1.5 air water n=1.33 water+glycerine n=1.3375water+glycerine n=1.3475

∆R

pp/R

pp


Measurements in water solutions

Angle shiftAngle shift

1%

pp

pp

d

RR

Sensitivity RIUn

−

∆∂

= = ⋅∂

MO-SPR (Cr-Co-Cr-Au) →19,100% RIU-1

B. Sepúlveda et al. Opt. Letters 31, 1085 (2006).

SPR →3,900 % RIU-1

J. Homola et al. Sens. and Act. 54, 3 (1999).

280,000 % RIU-1 in air

170,800 % RIU-1 in water

air

Sensitivity: Typical metric to compare sensors

020406080100

1

10

100

1000

10000

airwaterSen

sitiv

ity (%

RIU

-1)

Co Thickness (Angs.)

Grown 3 samples with the Co film placed in three different positions

Co (2.8 nm)-Au (23 nm)Au(11.5 nm)-Co (2.8 nm)-Au (11.5 nm)Au(20 nm)-Co (2.8 nm)-Au (3 nm)

The Physics: The Physics: PlasmonPlasmon Excitation, Electric Field Excitation, Electric Field depth dependence and Magnetodepth dependence and Magneto--optical enhancementoptical enhancement

40 45 50

0

6

12

Rpp

101

308

Co

2.8n

m-A

u23n

m

theta

Rpp 101308 Co 2.8nm-Au23nm Rpp 101508 Au 11.5nm-Co 2.8nm-Au 11.5nm c Rpp 101408 Au 20nm-Co 2.8nm-Au 3nm

Reflectivity. We note that the position of the minimum changesbecause the conditions to excite the plasmon have changed.

40 45 50-3

-2

-1

0

1

2

DR

pp/R

pp 1

0130

8 C

o 2.

8nm

-Au2

3nm

theta

DRpp/Rpp 101308 Co 2.8nm-Au23nm DRpp/Rpp 101508 Au 11.5nm-Co 2.8nm-Au 11.5nm DRpp/Rpp 101408 Au 20nm-Co 2.8nm-Au 3nm

Experimental magneto-optical data

∆Rpp ∆Rpp/Rpp

40 45 50-10000

0

10000

20000

DR

pp 1

0130

8 C

o 2.

8nm

-Au2

3nm

theta

DRpp 101308 Co 2.8nm-Au23nm DRpp 101508 Au 11.5nm-Co 2.8nm-Au 11.5nm DRpp 101408 Au 20nm-Co 2.8nm-Au 3nm

40 45 50-4

-2

0

2

4

6

Der

ivat

ive

Rpp

Co2

.8-A

u23

theta

Derivative Rpp Co2.8-Au23 Derivative Rpp Au 11.5-Co2.8-Au11.5 Derivate Au20-Co2.8-Au3

The derivative of Rpp: does not evolve in the same manner asthe experimental ∆Rpp -> the changes observed in ∆Rpp, i.e. the magneto-optical response, are not related to modification of the plasmonexcitation in the samples.

40 45 50-4

-2

0

2

4

6

DR

ppR

pp C

o2.8

-Au2

3A

DRppRpp Co2.8-Au23 DRppRpp Au11.5-Co2.8-Au11.5 DRppRpp Au20-Co2.8-Au3

40 45 50

-0.0005

0.0000

0.0005

0.0010

0.0015

0.0020

DR

pp C

o2.8

-Au2

3

A

DRpp Co2.8-Au23 DRpp Au11.5-Co2.8-Au11.5 DRpp Au20-Co2.8-Au3

Simulations (transfer matrix formalism)

40 42 44 46 48 500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Air i

nter

face

Au2

0-C

o2.8

-Au3

angle (deg)

Air interface Au20-Co2.8-Au3 Co (34 A from air) Au20-Co2.8-Au3 Glas interface Cortes Au20-Co2.8-Au3

40 42 44 46 48 500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

angle (deg)

Air i

nter

face

Cor

tes

Au11

.5C

o2.8

Au11

.5

angle (deg)

Air interface Cortes Au11.5Co2.8Au11.5 Co (244 A from air) Cortes Au11.5Co2.8Au11.5 Glas interface Cortes Au11.5Co2.8Au11.5

40 42 44 46 48 500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Air

inte

rface

Cor

tes

Co2

.8A

u23

angle (deg)

Air interface Cortes Co2.8Au23 Co (244 A from air) Cortes Co2.8Au23 Glas interface Cortes Co2.8Au23

Electric Field

40 42 44 46 48 500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6E

angle (deg)

Co (244 A from air) Cortes Co2.8Au23 Co (244 A from air) Cortes Au11.5Co2.8Au11.5 Co (34 A from air) Au20-Co2.8-Au3

Electromagnetic field in the middle of the Co film in the 3 samples

Thus, the field enhancement due to SPP excitation enhances also the MOKE

40 45 50-10

0

10

20

30

40

50

60

70

Rot

atio

n

angle (deg)

rotacion Au20Co2.8Au3 rotacion Au115-Co28-Au115 rotacion Co2.8Au23

40 45 50-28-21-14-707

142128354249

ellip

ticity

angle (deg)

elipticidad Au20Co2.8Au3 elipticidad Au115-Co28-Au115 elipticidad Co2.8Au23

Simulations polar

Our simulations also indicate dramatic enhancement of the polar Kerr rotation andellipticity. We will investigate this experimentally.

Au (9.5-10.5 nm)

Au (3 nm)Co (2.8 nm)

•Adhesion of Au on glass is poor. Tri-layers are degraded when exposed to a water flux.

•Cr has been extensively used to improve the adhesion of Au on glass, but it is a highly absorptive metal and therefore it broadens the surface plasmon resonance peak.

•At present time the common belief has been that the introduction of Cr layers decreases the sensitivity of these kind of sensors.

•We have demonstrated that this is not true.

Adhesion issues: Cr-Au-Co-Au

Cr (3 nm)

In order to explore the material for a possible bio-sensing application there are additional concerns.

45 48

0.0

0.2

R(0

)


Rpp Cr 3 nm-Au 10.5 nm-Co 2.8 nm-Au 3 nm Rpp Cr 3 nm-Au 9.5 nm-Co 2.8 nm-Au 3 nm Rpp Cr 3 nm-Au 9 nm-Co 2.8 nm-Au 3 nm

39 42 45 48 51 54 57 60 63

0.0

0.2

0.4

0.6

0.8

1.0

R(0

)


Rpp Cr 3 nm-Au 10.5 nm-Co 2.8 nm-Au 3 nm normalized Rpp Cr 3 nm-Au 9.5 nm-Co 2.8 nm-Au 3 nm normalized Rpp Cr 3 nm-Au 9 nm-Co 2.8 nm-Au 3 nm normalized

The thickness of the layers was once more designed to achieve the full extinction of the reflected intensity

Results: Cr-Au-Co-Au

Transverse Kerr magneto-optical signal

•A small decrease in the normalized signal is observed due to increased absorption in the Cr buffer layer


42 45 48

0

2

R (H

)-R

(0) (

‰)


∆Rpp Cr 3 nm-Au 10.5 nm-Co 2.8 nm-Au 3 nm∆Rpp Cr 3 nm-Au 9.5 nm-Co 2.8 nm-Au 3 nm∆Rpp Cr 3 nm-Au 9 nm-Co 2.8 nm-Au 3 nm

40 45 50-2

-1

0

1

2

3

4

R (H

)-R

(0) (

‰ )



Recall transverse Kerr effect Without Cr buffer layer

Yet, combining the enhancement of the MO effect and the extinction of the reflected beam, again a remarkable enhancement of the relative variation of the reflectivity is obtained.

40 45 50 55 60

0

50

100

150

200

250

DRpp/Rpp Cr 3 nm-Au 10.5 nm-Co 2.8 nm-Au 3 nm DRpp/Rpp Cr 3 nm-Au 9.5 nm-Co 2.8 nm-Au 3 nm DRpp/Rpp Cr 3 nm-Au 9 nm-Co 2.8 nm-Au 3 nm

R (H

)-R

(0)/

R (0

) (%

)


40 45 50 55-2

-1

0

1

2

3

4

5

6

DRpp/Rpp Cr 3 nm-Au 10.5 nm-Co 2.8 nm-Au 3 nm DRpp/Rpp Cr 3 nm-Au 9.5 nm-Co 2.8 nm-Au 3 nm DRpp/Rpp Cr 3 nm-Au 9 nm-Co 2.8 nm-Au 3 nm

R (H

)-R

(0)/

R (0

) (%

)incidence angle (deg)


Sensitivity

1%

pp

pp

d

RR

Sensitivity RIUn

−

∆∂

= = ⋅∂

020406080100120

1

10

100

1000

10000

Cr-Au-Co-AuAu-Co-Au

Sen

sitiv

ity (%

RIU

-1)

Co Thickness (Angs.)

280,000 % RIU-1 in air

703,000 % RIU-1 in air

SPR →3,900 % RIU-1


Detection limitDetection limit

MO-SPR (Cr-Co-Cr-Au) →19,100 % RIU-1

B. Sepúlveda et al. Opt. Letters 31, 1085 (2006).

SPR →3,900 % RIU-1


280,000 % RIU-1 in air

170,800 % RIU-1 in water

703,000 % RIU-1 in air ∆nmin=1.42 x10-7 RIU

∆nmin=3.57 x 10-7 RIU

∆nmin=5.85 x 10-7 RIU

∆nmin= 5 x 10-6 RIU

∆nmin= 5 x 10-5 RIU

•A large enhancement of the magneto-optical response of Au-Co-Au trilayers with and without Cr buffer layer was obtained when the surface plasmon resonance was excited.

•Layer thickness was designed to achieve maximum extinction of the reflected beam.

•Combining both effects, a remarkable enhancement of the relative change in reflectivity (∆Rpp/Rpp) was obtained.

•This feature can significantly improve the detection limit in sensors based on surface plasmon resonance.

ConclusionsConclusions

WORK IN PROGRESSWORK IN PROGRESS

We have achieved field modulated enhanced SPR in We have achieved field modulated enhanced SPR in trilayeredtrilayered AuAu--CoCo--Au samples and also with Au samples and also with trilayerstrilayersgrown on a Cr buffer layer. We are now testing these grown on a Cr buffer layer. We are now testing these sensors in liquids.sensors in liquids.

We are also investigating the use of diffraction We are also investigating the use of diffraction gratings gratings nanonano--patterned on the sensor surface to patterned on the sensor surface to couple the light to the surface couple the light to the surface plasmonsplasmons. This . This approach can eliminate constrains on the thickness of approach can eliminate constrains on the thickness of the films deposited and the kind of substrate used. the films deposited and the kind of substrate used.

Diffraction gratings and Diffraction gratings and plasmonsplasmons

Nano-patterning

We have explored e-beam lithography to nanopattern magneto-plasmonicmaterials with two goals in mind:

Use diffraction gratings for photons-plasmonscouplingExplore localized enhancement of the electromagnetic field to further enhance the magneto-optical activity

0 9 18 27 36 45 54 63 72 81

4.8

5.2

5.6

6.0

6.4

6.8

7.2

7.6

inside outside

Rpp

theta

Au film with e-beam patterned grating

800 nm

Au-Co-Au trilayer and Nano-patterned grating on top

0 10 20 30 40 50 60 70 80

grating continuous film

Ref

lect

ivity

(arb

. uni

ts)

θ incidence angle (deg.)

0 20 40 60 800.0

0.1

0.2

0.3

0.4

0.5

0.6

grating continuous film

∆R

/R R

elat

ive

chan

ge in

re

flect

ivity

θ incidence angle (deg.)

Deposition temperature RT, 300 C, 600 C

•Alternative solution for the adherence issue

•Decrease Co Absorption

•Easier to prepare

Au 95% - Co 5 % → Au 40% - Co 60 %

Au-Co nanocompositesSputtering codeposition of Au and Co

50 nm thick films

0 500 1000

0

50

100

600 C

300 C

Hei

gth

(nm

)

Profile (nm)

RT

RTRT

300 C300 C

600 C600 C

Good adhesion to glass.

Au-Co nanocomposites: morphology

-5 0 5

-500

0

500

-5 0 5 -5 0 5

Ms

(em

u/cc

)

50% Co-50% Au30% Co-70% Au

H in plane H perpendicular

H (kOe)

10% Co-90% Au

300 C samples

0 10 20 30 40 500

200

400

600

Ms

(em

u/cc

)

Co C oncentration (% )

•In plane magnetic anisotropy

•Magnetic moment scales with Co concentration

Au-Co nanocomposites: MO-SPR

Au 20%- Co 80 % grown @ 300 C

30 40 50 60

0.00

0.02

0.04

0.06

R(H

)-R

(0) /

R(0

)

incidence angle (deg)30 40 50 60

R(0

)


Measurements in air

The futureInvestigate the metal-insulator transition in VO2 films grown on glass.The films will be excited with IR laser radiation following Cavalleri’s work (PRL, 2001)We will then investigate the effect of this MI transition on plasmonicstructures deposited/patterned on VO2 films.

Documents

Surface Plasmon Resonance. Magneto-optical enhancement and