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The angular dependence of surface-enhanced Brillouin scattering from silver in double resonance configuration
~Solid State Communications, Vol. 70, No. 3. pp. 237-239~ 1989. Printed in Great Britain.
0038-1098/8953.00+.00
Pergamon Press plc
THE ANGULAR DEPENDENCE OF SURFACE-ENHANCED BRILLOUIN SCATTERING FROM SILVER IN DOUBLE RESONANCE CONFIGURATION D.G. Glee&, B. Hillebrands"~ , S. Lee, G.I. Stegeman and J.R. Sambles* Optical Sciences Center, University of Arizona, Tucson, AZ 85721 U.S.A. (Received 20 October 1988 by G. Gfintherodt) We present measurements of the angular dependence of surface-enhanced Brillouin scattering from silver for the configuration in which the fight coupled into and out of the silver film is both resonantly enhanced by the interaction with the surfaceplasmon polariton (SPP) mode. The data is in good agreement with theory and gives an enhancement factor in excess of 350.
provide tile necessary component of monlentum parallel to the surface# The light scattered off the air interface of the fihn was collected at 90 ° to the incident light and frequency analysed using a 5-pass Fabry-Perot interferometer. These results gave an enhanceinent factor of 25 (i25%). A second experiment by the same workers e used SPPs to enhance both the incident and the scattered electric fields, greatly increasing the Brillouin signals as expected. This method was similar to that initially proposed ~ and the enhancement factor observed was 750 (4-25%).
1. Introduction Brillouin scattering has been used for many years to measure the hypersound velocity in transparent materials by frequency analysing the light scattered from them. Initially such measurements were made on transparent materials, but with the advent of high contrast spectrometers, opaque n~aterials could be studied. With the introduction of the tandem Fabry-Perot interferometer the ultra-high contrast enabled measurements on materials which had hitherto not been open to examination, in particular those materials giving very weak Brillouin signals compared to the elastically scattered light. Furthermore, materials having many phonon inodes whos e spectrum previously had complications due to overlapping orders are also available for study because of the large free spectral range of the tandem iuterferometer. 1
Since a single cavity Fabry-Perot interferometer was used, the resulting spectrum was complicated by signals from several orders of the interferometer being present. In the first experiment this was only a minor problem because the only Brillouin signals observed were the Rayleigh mode and the first Sezawa mode from the silver film. However, for the second experiment the BriUouin signal due to bulk scattering in the glass complicated the resulting spectrum. Also a study of the angular dependence of the backscattered light was not possible.
For metals the scattering process is generally the surface ripple mechanism. However, in less absorbing materials there is also a contribution from the elastooptic effect 2 but in general the Brillouin spectra obtained in metals are weak. In order to increase the scattering efficiency it. has been suggested that coupling the incident and scattered light fields to surfaceplasmon polaritons (SPPs) can be used to enhance the signals.3 This method uses the electric field enhancelnenl at t h e metal/air interface, when the light is resonantly coupled into the surface-plasmon mode, to increase the scattered light intensity. The first report of surface-enhanced Brillouin scattering (SEBS) used SPPs to increase the incident electric field at the surface of a silver fihn.4 The fight was coupled into the SPP mode via a henlisphere using the KretschmannRaether attenuated total reflection (ATR) method to
Using a tandem Fabry-Perot interferometer we have been able to measure the angular dependence of the backscattered light in this double SPP resonance configuration and find that the results are in good agreement with theory. 2. Experimental The silver fihn was evaporated at a rate of 6nm/sec onto a 450,450,90 ° crown prism (n=1.52) in a vacuum of 10-4Pa, the film thickness being monitored by a quartz crystal oscillator. A standard reflectivity against incident angle measurelnent was perfor,ned in order to verify that this prism/silver film combination had a strong SPP resonance, z Fitting the reflectivity data, obtained at A = 5145A, to theory gave the thickness of the silver together with the dielectric constant (d = 522.~, e = c, + iei = -9.38 + i0.41). P-polarised light (A = 5145~) from a frequency stabilised single mode argon-ion laser was focused onto the metal fihn through the prism. The backscattered light was collected and frequency analysed using a 3+3
* present address: Thin Film and Interface Group, Department of Physics, University of Exeter, Stocker Road, Exeter, United Kingdom EX4 4QL ** present address: 2. Physikalisches Institut, RWTH Aachen, Templergraben 55, 5100 Aachen, West Germany
237
238
SURFACE-ENHANCED
in
Using a cavity spacing of 3ram for the first FabryPerot enabled the Brillouin signals from both the silver and the glass to be observed• Fig. 1 shows the spectrum obtained when the incident light is resonantly coupled into the S P P mode. The incident laser power was 15roW and the data accumulation time was 1.6 hours. An acousto-optic modulator was used to limit the amount of elastically scattered light incident on the photomultiplier tube, for clarity the effect of this on the central peak has been renmved. The Rayleigh mode and the first. Sezawa mode for the silver fihn are clearly seen at 7.4 and 12.0 GHz, these have velocities of 1810 ± 20ms - I and 2910 ± 30ms -~ respectively. The
I
I
I
Vol.
70, No.
For the measurements of the angular dependence of the intensity of the Brillouin signal from tile silver a cavity spacing of 10ram was used. This enabled the Rayleigh mode and first Sezawa mode to be measured nmre accurately. Measurements were takell at, the same angle at the beginning and end of a set of accumulations to determine whether the alignment could be assumed to be constant. It was found that the relative intensities agreed to within 5 to 10%. Fig. 2 shows the intensity of the Brillouin signal for the Rayleigh mode as a function of the angle of incidence of the light, in tile prism. Tile solid line is a theoretical plot similar to that used ill the second SEBS publication, s It uses the theory of Marvin et al ~ for the interaction of SPPs with SAWs. The fornmla used (equation 4a of Marvin et al) calculates the backscattered light as a function of tile incident and scattered photon monaentum parallel to the fihn, the parameters of the prism/fihn combination (n,d and e) and/3 the ratio of the surface ripples at the glass and air boundaries of tile silver film. A Gaussian angular spread function (half width 0.35 °) has also been included in the theory because the light was focused onto the silver fihn. This has the effect. of broadening the calculated cross-section. The minin m m in the intensity on either side of the resonance are not present in the cross-section unlike the theory curve presented in the earlier SEBS work. s It seems that this arises when only the negative part of the expression for the cross-section is used, a - in equation 4a of Marvin et al. The cross-section used in the calculation of the theory line ill Fig. 2 i s given by a = a + + a - . The values for n,d and ~ were those obtained from a reflectivity against angle fit. The magnitude of the surface ripples at both silver interfaces were taken to be equal, i.e. f~ was set to 1, which is discussed in the earlier work. 4 However in order to specify the angular
When the light was resonantly coupled into tim ~SPP mode the reflected beam had a dark line through its centre indicating the minimunl reflectivity condition. As the prism was rotated away from resonance this line moved towards the extremes of the b e a m profile. Brillouin spectra were collected over a range of angles which corresponded to this line scanning from one side of the reflected beam to the other. This ensured that. sufficient information was obtained about the SPP resonance. The laser power had to be kept. verylow ( ~ 15-mW) When the light was incident on the metal film at angles close to the SPP coupling angle order t.o avoid damaging the silver fihn. Away from resonance, higher powers could be used to reduce the data accunmlation time. 3. Results and Discussion
[
SCATTERING
Brillouin signal due to bulk scattering in the prism is seen at 34.4 GHz which corresponds to a velocity of 5820 ± 60ms -a. These values agree with those previously obtained, s
pass tandem Fabry-Perot and a cooled F W l 3 0 photonmltiplier. Whereas the previous work analysed the ring of re-radiated light using a slit, we used all of the light collected by the first lens in the optical analysis system. Tiffs method contributed to the ease of reproducing the alignment each time the prism was rotated.
IOO0
BRILLOUIN
I
post-
C
800i
-4
-
d3
600-
r-
~oo c
.•_•2500
R
R
I
l
I
I
I
I
2000 t,..
200 -
1
. ~ 1500
40
w-
C
C 0 i__
-40
-30
-20
-I0
I0
20
30
Frequency Shift (GHz)
1ooo
.E 500 0
Figure 1
Brillouin Spectrum for backscattered light from silver when the light is resonantly coupled into and out of the fihn using the Kretschmann-Raether ATR method. R:Rayleigh mode, S:Sezawa mode, L:Longitudinal bulk mode of the substrate (glass prism).
":" on
0
,
4!
~ll
~3
Internal
Figure 2
i ~., ~5 angle
~7
-r I I 49
(degrees)
The angular dependence of the intensity of the Brillouin signals for the backscattered light from the silver film.
Vol.
70~ No. 3
SURFACELENHANCED BRILLOUIN SCATTERING
tion correctly the value of e~ had to be changed from -9.4 to -9.6. This fact that different film parameters are required in order to obtain a good fit to two different types of resonance conditions has been noted before. 4 The reflectivity against angle fit assumes planar interfaces and so the discrepancy could well enter there. Taking the peak value and the far-off resonance value for the Brillouin intensity gives an enhancement factor of 350 (:i:10%). This value represents a lower limit to the enhancenaent factor since the enhancelnent would be far greater if all of the light was coupled into and out of the SPP lnode, i.e. if there was no angular spread in the incident bealn and more importantly if the acceptance angle for the scattered light was significantly smaller. Initial exl)eriments, in the single resonance configuration, showed that the intensity of the first Sezawa mode as a function of angle behaved in a sinfilar way to that of the Rayleigh mode. This indicates that for both modes lhe ripple scattering mechanism is dominant.
239
4. Conclusion We have presented measurements of the angular dependence of surface-enhanced Brillouin scattering from silver in which the light is resonantly coupled into and out of the fihn using the Kretschlnann-Raether ATR method. The data is in good agreement with the theory of Marvin el al s and gives 350 (+10%) as the lower limit to the enhancement factor. Acknowledgement- we wish to thank D.Jarvis for sampie preparation, S.Cowen and S.Elston for film characterisation using SPPs, and Professor F. Nizzoh for a critical reading of the manuscript. One of us (DGG) is supported by a SERC award and received assistance from SERC and the Physics Department at Exeter University for travel to the Optical Sciences Center, Tucson. The work at the Optical Sciences Center has been supported by the Air Force Office of Scientific Research / University hfitiative Program under contract F 49620-86-C-0123.
References [1] J.R. Sandercock, in Light Scattering in Solids 3 edited by M. Cardona and G. Giintherodt (Springer-Verlag Berlin 1982) 173 R. Mock, B. Hillebrands and J.R. Sandercock, J. Phys E 20 (1987) 656 [2] R. Loudon and J.R. Sandercock, J. Plays C 13 (1980) 2609 A.M. Marvin, V. Bortolani, F.Nizzoli, J. Plays C 13 (1980) 299 [3] M. Fukui, O. Toda, V.C.Y. So and (;A. Stegelnan, J. Plays C 14 (1981) 5591
[4] A.L. Moretti, W.M. Robertson, B. Fisher and R. Bray, Plays Rev B 31 (1985) 3361 [5] E. Kretschlnann and H. Raether, Z. Natur. 23A (1968) 2135 [61 W.M. Rol)ertson, A.L. Moretti and R. Bray, Phys Rev B 35 (1987) 8919 [7] W.L. Barnes and J.R. Sambles, Solid State Conmaunications 55 (1985) 921 [8] A.L. Moretti, PhD Thesis, Purdue l!niversity (1985) [9] A. Marvin, V. B0rtolani, F. Nizzofi, G. Santoro and V. Celli, J. Phys C 15 (1982) 3273
0038-1098/8953.00+.00
Pergamon Press plc
THE ANGULAR DEPENDENCE OF SURFACE-ENHANCED BRILLOUIN SCATTERING FROM SILVER IN DOUBLE RESONANCE CONFIGURATION D.G. Glee&, B. Hillebrands"~ , S. Lee, G.I. Stegeman and J.R. Sambles* Optical Sciences Center, University of Arizona, Tucson, AZ 85721 U.S.A. (Received 20 October 1988 by G. Gfintherodt) We present measurements of the angular dependence of surface-enhanced Brillouin scattering from silver for the configuration in which the fight coupled into and out of the silver film is both resonantly enhanced by the interaction with the surfaceplasmon polariton (SPP) mode. The data is in good agreement with theory and gives an enhancement factor in excess of 350.
provide tile necessary component of monlentum parallel to the surface# The light scattered off the air interface of the fihn was collected at 90 ° to the incident light and frequency analysed using a 5-pass Fabry-Perot interferometer. These results gave an enhanceinent factor of 25 (i25%). A second experiment by the same workers e used SPPs to enhance both the incident and the scattered electric fields, greatly increasing the Brillouin signals as expected. This method was similar to that initially proposed ~ and the enhancement factor observed was 750 (4-25%).
1. Introduction Brillouin scattering has been used for many years to measure the hypersound velocity in transparent materials by frequency analysing the light scattered from them. Initially such measurements were made on transparent materials, but with the advent of high contrast spectrometers, opaque n~aterials could be studied. With the introduction of the tandem Fabry-Perot interferometer the ultra-high contrast enabled measurements on materials which had hitherto not been open to examination, in particular those materials giving very weak Brillouin signals compared to the elastically scattered light. Furthermore, materials having many phonon inodes whos e spectrum previously had complications due to overlapping orders are also available for study because of the large free spectral range of the tandem iuterferometer. 1
Since a single cavity Fabry-Perot interferometer was used, the resulting spectrum was complicated by signals from several orders of the interferometer being present. In the first experiment this was only a minor problem because the only Brillouin signals observed were the Rayleigh mode and the first Sezawa mode from the silver film. However, for the second experiment the BriUouin signal due to bulk scattering in the glass complicated the resulting spectrum. Also a study of the angular dependence of the backscattered light was not possible.
For metals the scattering process is generally the surface ripple mechanism. However, in less absorbing materials there is also a contribution from the elastooptic effect 2 but in general the Brillouin spectra obtained in metals are weak. In order to increase the scattering efficiency it. has been suggested that coupling the incident and scattered light fields to surfaceplasmon polaritons (SPPs) can be used to enhance the signals.3 This method uses the electric field enhancelnenl at t h e metal/air interface, when the light is resonantly coupled into the surface-plasmon mode, to increase the scattered light intensity. The first report of surface-enhanced Brillouin scattering (SEBS) used SPPs to increase the incident electric field at the surface of a silver fihn.4 The fight was coupled into the SPP mode via a henlisphere using the KretschmannRaether attenuated total reflection (ATR) method to
Using a tandem Fabry-Perot interferometer we have been able to measure the angular dependence of the backscattered light in this double SPP resonance configuration and find that the results are in good agreement with theory. 2. Experimental The silver fihn was evaporated at a rate of 6nm/sec onto a 450,450,90 ° crown prism (n=1.52) in a vacuum of 10-4Pa, the film thickness being monitored by a quartz crystal oscillator. A standard reflectivity against incident angle measurelnent was perfor,ned in order to verify that this prism/silver film combination had a strong SPP resonance, z Fitting the reflectivity data, obtained at A = 5145A, to theory gave the thickness of the silver together with the dielectric constant (d = 522.~, e = c, + iei = -9.38 + i0.41). P-polarised light (A = 5145~) from a frequency stabilised single mode argon-ion laser was focused onto the metal fihn through the prism. The backscattered light was collected and frequency analysed using a 3+3
* present address: Thin Film and Interface Group, Department of Physics, University of Exeter, Stocker Road, Exeter, United Kingdom EX4 4QL ** present address: 2. Physikalisches Institut, RWTH Aachen, Templergraben 55, 5100 Aachen, West Germany
237
238
SURFACE-ENHANCED
in
Using a cavity spacing of 3ram for the first FabryPerot enabled the Brillouin signals from both the silver and the glass to be observed• Fig. 1 shows the spectrum obtained when the incident light is resonantly coupled into the S P P mode. The incident laser power was 15roW and the data accumulation time was 1.6 hours. An acousto-optic modulator was used to limit the amount of elastically scattered light incident on the photomultiplier tube, for clarity the effect of this on the central peak has been renmved. The Rayleigh mode and the first. Sezawa mode for the silver fihn are clearly seen at 7.4 and 12.0 GHz, these have velocities of 1810 ± 20ms - I and 2910 ± 30ms -~ respectively. The
I
I
I
Vol.
70, No.
For the measurements of the angular dependence of the intensity of the Brillouin signal from tile silver a cavity spacing of 10ram was used. This enabled the Rayleigh mode and first Sezawa mode to be measured nmre accurately. Measurements were takell at, the same angle at the beginning and end of a set of accumulations to determine whether the alignment could be assumed to be constant. It was found that the relative intensities agreed to within 5 to 10%. Fig. 2 shows the intensity of the Brillouin signal for the Rayleigh mode as a function of the angle of incidence of the light, in tile prism. Tile solid line is a theoretical plot similar to that used ill the second SEBS publication, s It uses the theory of Marvin et al ~ for the interaction of SPPs with SAWs. The fornmla used (equation 4a of Marvin et al) calculates the backscattered light as a function of tile incident and scattered photon monaentum parallel to the fihn, the parameters of the prism/fihn combination (n,d and e) and/3 the ratio of the surface ripples at the glass and air boundaries of tile silver film. A Gaussian angular spread function (half width 0.35 °) has also been included in the theory because the light was focused onto the silver fihn. This has the effect. of broadening the calculated cross-section. The minin m m in the intensity on either side of the resonance are not present in the cross-section unlike the theory curve presented in the earlier SEBS work. s It seems that this arises when only the negative part of the expression for the cross-section is used, a - in equation 4a of Marvin et al. The cross-section used in the calculation of the theory line ill Fig. 2 i s given by a = a + + a - . The values for n,d and ~ were those obtained from a reflectivity against angle fit. The magnitude of the surface ripples at both silver interfaces were taken to be equal, i.e. f~ was set to 1, which is discussed in the earlier work. 4 However in order to specify the angular
When the light was resonantly coupled into tim ~SPP mode the reflected beam had a dark line through its centre indicating the minimunl reflectivity condition. As the prism was rotated away from resonance this line moved towards the extremes of the b e a m profile. Brillouin spectra were collected over a range of angles which corresponded to this line scanning from one side of the reflected beam to the other. This ensured that. sufficient information was obtained about the SPP resonance. The laser power had to be kept. verylow ( ~ 15-mW) When the light was incident on the metal film at angles close to the SPP coupling angle order t.o avoid damaging the silver fihn. Away from resonance, higher powers could be used to reduce the data accunmlation time. 3. Results and Discussion
[
SCATTERING
Brillouin signal due to bulk scattering in the prism is seen at 34.4 GHz which corresponds to a velocity of 5820 ± 60ms -a. These values agree with those previously obtained, s
pass tandem Fabry-Perot and a cooled F W l 3 0 photonmltiplier. Whereas the previous work analysed the ring of re-radiated light using a slit, we used all of the light collected by the first lens in the optical analysis system. Tiffs method contributed to the ease of reproducing the alignment each time the prism was rotated.
IOO0
BRILLOUIN
I
post-
C
800i
-4
-
d3
600-
r-
~oo c
.•_•2500
R
R
I
l
I
I
I
I
2000 t,..
200 -
1
. ~ 1500
40
w-
C
C 0 i__
-40
-30
-20
-I0
I0
20
30
Frequency Shift (GHz)
1ooo
.E 500 0
Figure 1
Brillouin Spectrum for backscattered light from silver when the light is resonantly coupled into and out of the fihn using the Kretschmann-Raether ATR method. R:Rayleigh mode, S:Sezawa mode, L:Longitudinal bulk mode of the substrate (glass prism).
":" on
0
,
4!
~ll
~3
Internal
Figure 2
i ~., ~5 angle
~7
-r I I 49
(degrees)
The angular dependence of the intensity of the Brillouin signals for the backscattered light from the silver film.
Vol.
70~ No. 3
SURFACELENHANCED BRILLOUIN SCATTERING
tion correctly the value of e~ had to be changed from -9.4 to -9.6. This fact that different film parameters are required in order to obtain a good fit to two different types of resonance conditions has been noted before. 4 The reflectivity against angle fit assumes planar interfaces and so the discrepancy could well enter there. Taking the peak value and the far-off resonance value for the Brillouin intensity gives an enhancement factor of 350 (:i:10%). This value represents a lower limit to the enhancenaent factor since the enhancelnent would be far greater if all of the light was coupled into and out of the SPP lnode, i.e. if there was no angular spread in the incident bealn and more importantly if the acceptance angle for the scattered light was significantly smaller. Initial exl)eriments, in the single resonance configuration, showed that the intensity of the first Sezawa mode as a function of angle behaved in a sinfilar way to that of the Rayleigh mode. This indicates that for both modes lhe ripple scattering mechanism is dominant.
239
4. Conclusion We have presented measurements of the angular dependence of surface-enhanced Brillouin scattering from silver in which the light is resonantly coupled into and out of the fihn using the Kretschlnann-Raether ATR method. The data is in good agreement with the theory of Marvin el al s and gives 350 (+10%) as the lower limit to the enhancement factor. Acknowledgement- we wish to thank D.Jarvis for sampie preparation, S.Cowen and S.Elston for film characterisation using SPPs, and Professor F. Nizzoh for a critical reading of the manuscript. One of us (DGG) is supported by a SERC award and received assistance from SERC and the Physics Department at Exeter University for travel to the Optical Sciences Center, Tucson. The work at the Optical Sciences Center has been supported by the Air Force Office of Scientific Research / University hfitiative Program under contract F 49620-86-C-0123.
References [1] J.R. Sandercock, in Light Scattering in Solids 3 edited by M. Cardona and G. Giintherodt (Springer-Verlag Berlin 1982) 173 R. Mock, B. Hillebrands and J.R. Sandercock, J. Phys E 20 (1987) 656 [2] R. Loudon and J.R. Sandercock, J. Plays C 13 (1980) 2609 A.M. Marvin, V. Bortolani, F.Nizzoli, J. Plays C 13 (1980) 299 [3] M. Fukui, O. Toda, V.C.Y. So and (;A. Stegelnan, J. Plays C 14 (1981) 5591
[4] A.L. Moretti, W.M. Robertson, B. Fisher and R. Bray, Plays Rev B 31 (1985) 3361 [5] E. Kretschlnann and H. Raether, Z. Natur. 23A (1968) 2135 [61 W.M. Rol)ertson, A.L. Moretti and R. Bray, Phys Rev B 35 (1987) 8919 [7] W.L. Barnes and J.R. Sambles, Solid State Conmaunications 55 (1985) 921 [8] A.L. Moretti, PhD Thesis, Purdue l!niversity (1985) [9] A. Marvin, V. B0rtolani, F. Nizzofi, G. Santoro and V. Celli, J. Phys C 15 (1982) 3273