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Simple thermal detection of surface plasmon-polaritons
Solid State Cor~nunications, Vol.56,No.6, P r i n t e d in Great Britain.
pp.493-496,
]985.
0038-1098/85 $3.00 + .00 P e r g a m o n Press Ltd.
SIMPLE THERMAL DETECTION OF SURFACE PLASMON-POLARITONS R A Innes and J R Sambles Department of Physics, University of Exeter, Exeter, Devon Accepted for publication by C.W. McCo~bie dated 14th July 1985 (Received 12 July 1985 by C. W. McCombie)
Surface plasmon-polaritons (SPPs) excited by electromagnetic radiation ere generally observed (in the ATR configuration) by monitoring the reflected intensity as a function of incident angle or frequency or by monitoring the re-radiated light (due to surface roughness) in a similar manner. In this letter we describe the first thermal detection of SPPs using a thin-film thermocouple of indium tin oxide and the metal (silver or gold) on which the SPP is propagating.
Introduction
Experimental
A surface plasmon-polaziton is a coupled surface charge d e n s i t y - electro~agnetlc field oscillation at the interface between a metal and a dlelectrlc. The dispersion curve ( 1 ) o f these surface waves is such that they cannot be directly excited by electromagnetic radiation since conservation of energy and momentum cannot slmultaneously b e satisfied. Rk:~mver, using the method o f attenuated total reflection (ATR), in w h i c h the p h a s e velocity of the incident radiation may be reduced the required amount, i t i s Possible t o excite these modes. The two experimental arrangements for doing this are those due to Otto (2) and Kretechmann and Raether (3) (Figure is). Both methods make use of the evanescent wave present at total internal reflection to couple into the surface mode. In the former the evanescent wave couples across a thin dielectric gap to the dlelectrlc-metal interface ( thus allowing b u l k metal samples to be studied ) while in the K r e t scha~nn configuration the evanescent wave couples across a thin metal film to the metal-<~lelectrlc interface. This arrangement Is experimentally easier to set up although it i s limited to the study o f thin samples. Using either configuration it is Possible to absorb >99% of the I>-polarised (E-field in the plane of incidence) Incldent radiation into the SPP ~KX~.
The prlsm/film arrangement was met up as shown schematically in Figure (]_b). Hence, it possible t o rotate the crown glass prism, varying e, under constant illumination from a 10 m W He/Ne laser. In order to measure the temperature rise o f the thin gold or silver film a thin-film thermocouple was evaporated o n t o a glass elide w h i c h was then o p t i c a l l y metched to the hypotensuse face o f the prism. This thin-film thermocouple was cxmkoosed o f indium tin oxide (ITO) and the m e t a l o f interest as shown in Figure (2). T o produce the ITO film a 50 nm film o f InSn in the correct proportions was first evaporated onto the glauss elide through an appropriate copper m a s k . This was t h e n oxidised in air overnight at a tel~perature o f 300oC. The appropriate thickness o f metal was then evaporated through a second mask. Both the gold and silver films examined were produced b y fast evaporation (6 nm s -1 ) i n a v a c u u m o f -10 -3 Pa. The sample was then placed in the &TR configuration so that the laser b e a m struck one o f the two mete/ 'pads' adjacent t o the metal/ITO Junctions and the thermal voltage produced as e was v a r i e d was Ronitore~ by an a p p r o p r i a t e l y s e n s i t i v e voltmeter (~V) - I~O, being a selloonductor, gives a large thermal voltage when coupled with a metal.
For the Krete~ geometry Fresnel ' s equations (4) for a layered structure are readily used to c a l c u l a t e t h e reflection coefficient as a function of angle o f incidence e. F r o m such a calculation we find that for metals such as gold o r silver, in the visible region of the electromagnetlc spectrum, a film thickness o f -4Ohm will yield a strong reflectance dip reprementing &lJaoet 100% absorption o f power. This absorption o f energy causes significant heating o f the thin m e t a l film since the surface mode loses most o f its energy via Joule heating. The extent of this Joule heating is governed b y the imaginary part o f the refraction index o f the metal, the thickness o f the film and o f course the incident power. This letter presents direct thermoelectric detection o f such heat ing.
B y p l a c i n g t h e h o t and c o l d 3 u n c ' t i o r m as c l o s e together as possible any changes in ambient te~oerature were readily eliminated and p l a c i n g a glue p l a t e on t h e b a c k o f t h e g l u e slide -IOFm from the surface, stxhilised the envlronment around the thermo~ouple. As in any experiment involving measurements o f such small v o l t a g e signals all wires were electrically screened. It was t h o u g h t that in order to obtain the o ~ l m u m t h e r m a l v o l t a g e t h e beam s h o u l d be i n c i d e n t as c l o s e as ] p o s s i b l e to one j u n c % i o n of the thermocouple. However, it vas found that the metal/ITO ]unction produced a large voltage i n d e p e n d e n t o f e due t o c r e a t i o n o f c h a r g e c a r r i e r
493
SIMPLE T H E R M A L D E T E C T I O N
494
OF SURFACE
V01.
PLASMON-POLARITONS
56, No.
6
Results and Conclusions Typical d a t a for both gold and silver a r e shown in Figure (3) - the solid lines are discussed below. The p e a k corresponds to a thermal voltage of the order of I O ~ V (-0.05 K) The samples were also examined using the conventional reflection measurements (reflectivity, R, versus O) which allow accurate determunation of the dlelectric constant e (= er + iei ) and thickness d of the metal sample. The results o f
/
/
MetaL film
I
SPP on this interface
o) P
(a)
(b) Fig. (a )
(b)
1.
The Kretechmann-Raether arrangement tot the coupling of radiation to the SPP at the metal-air interface. Schematic arrangement of system.
i
44
42
8 b)
i
MetaL
i
i
41
i
46
0 Fig.
2.
N e t a l / s e m : L c o n d u c ~ o r t r a c k a r r a n g e m e n t on t h e hypoteneuse face of the prism.
pairs at the junction so that care had to be taken to ensure that radiation was incident only on the metal pad and not on the junction.
Fig.
3.
Graphs of thermal vo:ttage P (arbitrary units ) (with constant backg}~und level removed ) against e (angle of .incidence inside the prism) for typical samples of a. gold and b. silver. The full llne8 are appropriately ecaleu plots of (1 - R) against e, where R is the experimentally measure4 reflec%ivlty.
Vol.
56, No.
6
SIMPLE T H E R M A L
DETECTION
OF SURFACE
such a n experiment are shown in Figure (4) together with a theoretical curve showing the expected agreement b e t w e e n theory and experiment.
If all the absorbed power goes into Joule h e a t i n g the film then the thermal data should be identical to a (l-R) versus ~ plot, provided that the thermal voltage is linearly proportional to the power absorbed.
This linearity was ascertained b y setting e to the pl~n angle and monitoring the thermal voltage signal as the incident power was varied using crossed polarisers. The solid lines in Figure (3) are plots of (l-R) where R is the re-scaled experimentally measured reflectance. Agreement b e t w e e n the two sets of d a t a is good and well w i t h i n the errors inherent in the experiment, the Most significant of which is the drift of the microvoltmeter. Since the range of angle observation was only a few degrees no Corrections were made for the changes in reflectivity at the p r i s m faces. Note, ~ignlficant
there was, in the original voltage away from the
data, a surface
PLASMON-POLARITONS
495
plauBmon-polazlton resonance condition this o c c u r r e d f o r b o t h p and s p o l a z i s a t l o n (E f i e l d p e r p e n d i c u l a z t o the plane o f i n c i d e n c e ) b u t o f course the s - p o l a z i s a t i o n showed no resonance behaviour. The o b s e r v a t i o n o f much t h e same e f f e c t i n s and p p o l a r i s a t i o n i n d i c a t e d t h a t t h i s b a c k ground v o l t a g e was independent o f the resonance c o n d i t i o n and was t h o u g h t t o be due t o l i g h t b e i n g scattered onto the metaLl/IVO junction thus producing a voltage signal from p a i r creation, as mentioned before. In order to provide a good comparison of theory with data it is essential that this extra contribution to the apparent thermal voltage be taken into account. This was done by simply removing a constant background level. With this level removed from the data, excellent coa~paxison between the thermal data and the reflectivity data was aChieved. These results, like the recent 19aotcacoustic results of Rothenhausler et al (5) illustrate clearly how m a t c h i n g to the surface plasmon-polaritons may be used to generate other than radiative signals from a p u r e l y radiative input, with very high coupling efficiencies - in this c~se greater than 90% of the incident radiation power is converted into heat.
The authors acknowledge the h e l p of Mr D Jarvis in sa~Dle fabrication and useful discussions with Mr W Barnes ar~ Mr K Welford.
1.2
Medium
;
Relot.ive permittivity Thickness ReoL I Imoginory
°
3
i.ooo0o 0.o0000 0.09000o~
.-,z_
0.8 to 0.6
0.4
0.2
Theoretical 1
41.8
42.22
T
4264
T
T
43.06
43.48
AngLe (degrees) Pig.
4.
Reflectivity, R, v e r s u s e for a silver sample. The contlnuous curve is a theoretical fit, the data having been corrected for reflectlons at the alr/prism interfaces.
43.9
496
SIMPLE THERMAL DETECTION OF SURFACE PLASMON-POLARITONS
Vol. 56, No. 6
References
i.
See eg Reether H.
Phys. Thin FIIIB 9 (1977),
4.
Kovacs G. Ch. 4, "Electromagnet~c Surface Modes", F~. ~ A (1982) Wiley.
5.
P~thenhausler B., Rabe J., K o r p 1 ~ P. and Knoll w. Surf. Scl. 137 (1984) 373.
145.
2.
Otto A.
Z. Physlk 316 (1968) 3 9 8 .
3.
Kretgchmann E. and Raether H.
23.__~a(1968) 2135.
Z. Naturforsch
pp.493-496,
]985.
0038-1098/85 $3.00 + .00 P e r g a m o n Press Ltd.
SIMPLE THERMAL DETECTION OF SURFACE PLASMON-POLARITONS R A Innes and J R Sambles Department of Physics, University of Exeter, Exeter, Devon Accepted for publication by C.W. McCo~bie dated 14th July 1985 (Received 12 July 1985 by C. W. McCombie)
Surface plasmon-polaritons (SPPs) excited by electromagnetic radiation ere generally observed (in the ATR configuration) by monitoring the reflected intensity as a function of incident angle or frequency or by monitoring the re-radiated light (due to surface roughness) in a similar manner. In this letter we describe the first thermal detection of SPPs using a thin-film thermocouple of indium tin oxide and the metal (silver or gold) on which the SPP is propagating.
Introduction
Experimental
A surface plasmon-polaziton is a coupled surface charge d e n s i t y - electro~agnetlc field oscillation at the interface between a metal and a dlelectrlc. The dispersion curve ( 1 ) o f these surface waves is such that they cannot be directly excited by electromagnetic radiation since conservation of energy and momentum cannot slmultaneously b e satisfied. Rk:~mver, using the method o f attenuated total reflection (ATR), in w h i c h the p h a s e velocity of the incident radiation may be reduced the required amount, i t i s Possible t o excite these modes. The two experimental arrangements for doing this are those due to Otto (2) and Kretechmann and Raether (3) (Figure is). Both methods make use of the evanescent wave present at total internal reflection to couple into the surface mode. In the former the evanescent wave couples across a thin dielectric gap to the dlelectrlc-metal interface ( thus allowing b u l k metal samples to be studied ) while in the K r e t scha~nn configuration the evanescent wave couples across a thin metal film to the metal-<~lelectrlc interface. This arrangement Is experimentally easier to set up although it i s limited to the study o f thin samples. Using either configuration it is Possible to absorb >99% of the I>-polarised (E-field in the plane of incidence) Incldent radiation into the SPP ~KX~.
The prlsm/film arrangement was met up as shown schematically in Figure (]_b). Hence, it possible t o rotate the crown glass prism, varying e, under constant illumination from a 10 m W He/Ne laser. In order to measure the temperature rise o f the thin gold or silver film a thin-film thermocouple was evaporated o n t o a glass elide w h i c h was then o p t i c a l l y metched to the hypotensuse face o f the prism. This thin-film thermocouple was cxmkoosed o f indium tin oxide (ITO) and the m e t a l o f interest as shown in Figure (2). T o produce the ITO film a 50 nm film o f InSn in the correct proportions was first evaporated onto the glauss elide through an appropriate copper m a s k . This was t h e n oxidised in air overnight at a tel~perature o f 300oC. The appropriate thickness o f metal was then evaporated through a second mask. Both the gold and silver films examined were produced b y fast evaporation (6 nm s -1 ) i n a v a c u u m o f -10 -3 Pa. The sample was then placed in the &TR configuration so that the laser b e a m struck one o f the two mete/ 'pads' adjacent t o the metal/ITO Junctions and the thermal voltage produced as e was v a r i e d was Ronitore~ by an a p p r o p r i a t e l y s e n s i t i v e voltmeter (~V) - I~O, being a selloonductor, gives a large thermal voltage when coupled with a metal.
For the Krete~ geometry Fresnel ' s equations (4) for a layered structure are readily used to c a l c u l a t e t h e reflection coefficient as a function of angle o f incidence e. F r o m such a calculation we find that for metals such as gold o r silver, in the visible region of the electromagnetlc spectrum, a film thickness o f -4Ohm will yield a strong reflectance dip reprementing &lJaoet 100% absorption o f power. This absorption o f energy causes significant heating o f the thin m e t a l film since the surface mode loses most o f its energy via Joule heating. The extent of this Joule heating is governed b y the imaginary part o f the refraction index o f the metal, the thickness o f the film and o f course the incident power. This letter presents direct thermoelectric detection o f such heat ing.
B y p l a c i n g t h e h o t and c o l d 3 u n c ' t i o r m as c l o s e together as possible any changes in ambient te~oerature were readily eliminated and p l a c i n g a glue p l a t e on t h e b a c k o f t h e g l u e slide -IOFm from the surface, stxhilised the envlronment around the thermo~ouple. As in any experiment involving measurements o f such small v o l t a g e signals all wires were electrically screened. It was t h o u g h t that in order to obtain the o ~ l m u m t h e r m a l v o l t a g e t h e beam s h o u l d be i n c i d e n t as c l o s e as ] p o s s i b l e to one j u n c % i o n of the thermocouple. However, it vas found that the metal/ITO ]unction produced a large voltage i n d e p e n d e n t o f e due t o c r e a t i o n o f c h a r g e c a r r i e r
493
SIMPLE T H E R M A L D E T E C T I O N
494
OF SURFACE
V01.
PLASMON-POLARITONS
56, No.
6
Results and Conclusions Typical d a t a for both gold and silver a r e shown in Figure (3) - the solid lines are discussed below. The p e a k corresponds to a thermal voltage of the order of I O ~ V (-0.05 K) The samples were also examined using the conventional reflection measurements (reflectivity, R, versus O) which allow accurate determunation of the dlelectric constant e (= er + iei ) and thickness d of the metal sample. The results o f
/
/
MetaL film
I
SPP on this interface
o) P
(a)
(b) Fig. (a )
(b)
1.
The Kretechmann-Raether arrangement tot the coupling of radiation to the SPP at the metal-air interface. Schematic arrangement of system.
i
44
42
8 b)
i
MetaL
i
i
41
i
46
0 Fig.
2.
N e t a l / s e m : L c o n d u c ~ o r t r a c k a r r a n g e m e n t on t h e hypoteneuse face of the prism.
pairs at the junction so that care had to be taken to ensure that radiation was incident only on the metal pad and not on the junction.
Fig.
3.
Graphs of thermal vo:ttage P (arbitrary units ) (with constant backg}~und level removed ) against e (angle of .incidence inside the prism) for typical samples of a. gold and b. silver. The full llne8 are appropriately ecaleu plots of (1 - R) against e, where R is the experimentally measure4 reflec%ivlty.
Vol.
56, No.
6
SIMPLE T H E R M A L
DETECTION
OF SURFACE
such a n experiment are shown in Figure (4) together with a theoretical curve showing the expected agreement b e t w e e n theory and experiment.
If all the absorbed power goes into Joule h e a t i n g the film then the thermal data should be identical to a (l-R) versus ~ plot, provided that the thermal voltage is linearly proportional to the power absorbed.
This linearity was ascertained b y setting e to the pl~n angle and monitoring the thermal voltage signal as the incident power was varied using crossed polarisers. The solid lines in Figure (3) are plots of (l-R) where R is the re-scaled experimentally measured reflectance. Agreement b e t w e e n the two sets of d a t a is good and well w i t h i n the errors inherent in the experiment, the Most significant of which is the drift of the microvoltmeter. Since the range of angle observation was only a few degrees no Corrections were made for the changes in reflectivity at the p r i s m faces. Note, ~ignlficant
there was, in the original voltage away from the
data, a surface
PLASMON-POLARITONS
495
plauBmon-polazlton resonance condition this o c c u r r e d f o r b o t h p and s p o l a z i s a t l o n (E f i e l d p e r p e n d i c u l a z t o the plane o f i n c i d e n c e ) b u t o f course the s - p o l a z i s a t i o n showed no resonance behaviour. The o b s e r v a t i o n o f much t h e same e f f e c t i n s and p p o l a r i s a t i o n i n d i c a t e d t h a t t h i s b a c k ground v o l t a g e was independent o f the resonance c o n d i t i o n and was t h o u g h t t o be due t o l i g h t b e i n g scattered onto the metaLl/IVO junction thus producing a voltage signal from p a i r creation, as mentioned before. In order to provide a good comparison of theory with data it is essential that this extra contribution to the apparent thermal voltage be taken into account. This was done by simply removing a constant background level. With this level removed from the data, excellent coa~paxison between the thermal data and the reflectivity data was aChieved. These results, like the recent 19aotcacoustic results of Rothenhausler et al (5) illustrate clearly how m a t c h i n g to the surface plasmon-polaritons may be used to generate other than radiative signals from a p u r e l y radiative input, with very high coupling efficiencies - in this c~se greater than 90% of the incident radiation power is converted into heat.
The authors acknowledge the h e l p of Mr D Jarvis in sa~Dle fabrication and useful discussions with Mr W Barnes ar~ Mr K Welford.
1.2
Medium
;
Relot.ive permittivity Thickness ReoL I Imoginory
°
3
i.ooo0o 0.o0000 0.09000o~
.-,z_
0.8 to 0.6
0.4
0.2
Theoretical 1
41.8
42.22
T
4264
T
T
43.06
43.48
AngLe (degrees) Pig.
4.
Reflectivity, R, v e r s u s e for a silver sample. The contlnuous curve is a theoretical fit, the data having been corrected for reflectlons at the alr/prism interfaces.
43.9
496
SIMPLE THERMAL DETECTION OF SURFACE PLASMON-POLARITONS
Vol. 56, No. 6
References
i.
See eg Reether H.
Phys. Thin FIIIB 9 (1977),
4.
Kovacs G. Ch. 4, "Electromagnet~c Surface Modes", F~. ~ A (1982) Wiley.
5.
P~thenhausler B., Rabe J., K o r p 1 ~ P. and Knoll w. Surf. Scl. 137 (1984) 373.
145.
2.
Otto A.
Z. Physlk 316 (1968) 3 9 8 .
3.
Kretgchmann E. and Raether H.
23.__~a(1968) 2135.
Z. Naturforsch