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
)
incidence angle (deg)
Results (I)Results (I)
40 45 50-2
-1
0
1
2
3
4
R (H
)-R
(0) (
‰ )
incidence angle (deg)
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)
6 nm Co 5 nm Co 4 nm Co 3 nm Co 2.5 nm Co
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
) (%
)
incidence angle (deg)
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
) (%
)
incidence angle (deg)
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
incidence angle (deg)
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
incidence angle (deg)
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
)
incidence angle (deg)
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
)
incidence angle (deg)
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
Results: Cr-Au-Co-Au
42 45 48
0
2
R (H
)-R
(0) (
‰)
incidence angle (deg)
∆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) (
‰ )
incidence angle (deg)
6 nm Co 5 nm Co 4 nm Co 3 nm Co 2.5 nm Co
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
) (%
)
incidence angle (deg)
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)
Results: Cr-Au-Co-Au
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
J. Homola et al. Sens. and Act. 54, 3 (1999).
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
J. Homola et al. Sens. and Act. 54, 3 (1999).
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
)
incidence angle (deg)
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.