- Email: [email protected]
Photoluminescence from dye molecules on silver gratings
a __
I January
l!iB d
1996
-
OPTICS COMMUNICATIONS
__
ELYEVIER
Optics Communications
132
( 1996) l47- I54
Photoluminescence from dye molecules on silver gratings S.C. Kitson, W.L. Barnes, J.R. Sambles Depor~nenf I$Ply.Gt:s. hiwr.sity of’E.reter. Stocker Rwd. Exeter, EX4 4QL.
UK
Received 5 June 1995
Abstract Excited dye molecules on metallic gratings can relax by generating surface plasmon polaritons modes can scatter from the grating presents an experimental emission is dominated the azimuthal momentum
emitting
light in well-defined
study of the emission by light re-radiated
properties
direction
conditions
by SPPs. The direction
that govern the coupling
polarisation.
of a thin layer of laser dye on a silver grating. and the polarisation
This paper
It is found that the
of the emission are found to depend on
angle between the emitted light and the grating grooves. The experimental matching
( SPPs). These non-radiative
and with a characteristic
results are explained
in terms of the
between photons and SPPs.
1. Introduction In this paper we examine how proximity to a silver grating modilies the photoluminescence from a thin layer of laser dye. Proximity to a metal surface significantly quenches the emission of light from a dye layer; the energy is nonradiatively absorbed by the metal [ I]. There arc several processes that contribute to this absorption [ 2 1. If the molecule is very close to the metal (less than 10 nm) then the dominant process may be direct excitation of electron-hole pairs [ 31, at slightly larger separations Joule heating of the metal dominates. Of particular interest for this paper, however, is the possibility of energy being adsorbed via the excitation of surface plasmon polaritons (SPPs) If the surface is planar then these SPPs cannot reradiate. They have too much momentum to couple to bulk radiation, so the energy is ultimately dissipated as heat within the metal. However, if the surface is corrugated then the SPPs may scatter from the grating resulting in emission in well-defined directions and with a characteristic polarisation. The grating matches the momentum of the SPP to the component of the 0030.4018/96/$11.00 Q 1996 Elsevier Science B.V. All rights reserved .wI/o030-40 I X( 9s )00490-4
photon momentum in the plane of the grating, yielding the coupling equation [ 41: /&sin 0= 3-k,,, t_ nG ,
(1)
where k,, is the wavevector of the incident photon, 0 is the emission angle, kspp is the SPP wavevector, tr is an integer and G is the grating vector (equal to 2rr/grating pitch). We, therefore, expect strong emission at the angles 6,given by this equation. The sample geometry that we have examined experimentally is illustrated in Fig. I, namely a thin layer of laser dye DCM deposited onto the surface of a silver grating. In this case direct luminescence from the dye is strongly inhibited due to nonradiative transfer of energy to the metal. The radiation from the sample is, therefore, dominated by the re-radiation from SPPs. First we examine the emission at zero azimuthal angle, that is in the plane perpendicular to the grating grooves. This situation has, to some extent, been studied by other workers [5-S]. We extend this work by examining how the radiation pattern depends on the azimuthal angle of the emission. We find that both the peak emission angle and the polarisation of the emis-
S.C. Kitson et d. / 0ptic.s Cmtnunicufions
148
( Fig. I. The
sample geometry.
1-
Slllca
The polar emission angle is 0, the
azimuthal angle is I$.
sion vary with the azimuthal angle. The results can be understood by considering the boundary conditions that govern the coupling between photons and SPPs.
122 (19961 147-154
tomultiplieron the exit slits. A Glan-Thomson polariser placed in front of the spectrometer selected either the p- or the s-polarised component of the emission. A filter on the entrance splits prevented argon ion emission reaching the detector. The sensitivity of the detection system depended on the polarisation of the collected light. To measure this, 664 nm light from a diode laser was diffused to randomise the polarisation and then made incident onto the collection lens. The signal was recorded with the polariser set to pass first p- and then s-polarised light. The results showed that the system was a factor of 3.6( f 0.2) times more sensitive to p-polarised than spolarised light of this wavelength. This factor was used to correct the relative intensities of the emission for the two polarisations.
3. Sample preparation 2. Experimental The dye layer was excited optically using an argon ion laser. The experimental measurements consisted of recording the luminescence intensity as a function of the emission angle for different wavelengths and polarisations. In doing this, it was important that the angle of incidence and polarisation of the incident beam were kept fixed during data acquisition [ 9, IO]. The experimental arrangement used is illustrated in Fig. 2. The sample was mounted on an X-Z stage in a holder that allowed the azimuthal angle of the sample to be set to a precision of k 0.5”. The dye layer was excited by 488 nm emission from a 1 W argon ion laser made incident on the sample via a multimode optical fibre. The fibre was attached to the sample holder via a mount which included a graded index rod lens to focus the light onto the sample. In this way the argon ion beam moved with the sample as it was rotated during data acquisition. The angle of incidence and polarisation of the beam were thus held fixed. The X-Z stage was fixed to a computer controlled rotating table to allow the angle of emission to be scanned. A lens collected the emission and focused it onto the slits of the spectrometer, the collection angle being limited to 1.5” by an aperture. The argon ion laser beam used to excite the dye layer was mechanically chopped at about 900 Hz so that phase sensitive detectors could he used to record the signal from the pho-
A grating profile was holographically produced in a photoresist layer deposited on a silica substrate. The modulated surface profile was then transferred to the silica surface by etching with a beam of argon atoms in a high vacuum chamber. An optically thick silver film was subsequently evaporated onto the surface of the grating. This was then coated with a thin layer of the laser dye DCM by spin deposition from a solution of the dye in methanol. A solution concentration of 0.4 mgmll’ was used with the substrate spun at 3000 rpm. To characterise the sample, angle dependent rellectivity data were recorded for 488 nm and 664 nm light. A p-polarised beam incident in the plane + = 0” and at the appropriate angle 13(Eq. ( I ) ) can resonantly excite SPPs. This absorbs energy from the beam producing a dip in the reflectivity. Fitting multi-layer grating theory [ 11,121 to the data allows the determination of the optical constants. Data was recorded both before and after coating the silver grating with DCM. Fitting theory to the data for the hare silver gave the grating protile and the optical constants of the silver. These values were then held fixed whilst the thickness and the dielectric permittivity of the dye were adjusted to obtain a good fit to the data for the coated grating. A starting point for the DCM parameters was obtained by using the critical edge technique [ 13,141 to characterise a dye layer deposited onto a silica prism. Table 1 summarises the fitted parameters for the sample. The thick-
S. C. Kitsorr e! (11./ Opiita
C~)tntnlmicrrrions
I22 (1996) lJ7-151
Fibrr adaptor
149
Chopped 488nm light
Sample holder \
Snectrometer
w Rotating table
Fig. 2. The experimental arrangement T&k
I
The fitted optical constants of the sample at the two wavelengths of interest. 488 nm and 664 nm
silver DCM silver DCM
Wavelength (nm)
E,
6,
488 488
-8.3 +0.1 l.S9~00.03 ~ 19.0 f0.1 2.22 * 0.03
0.38 1.32 1.74 0.002
664 664
1600
-
,ooo
(arbitrary
so0
units)
;
600 -
)/ ,’
I (
II-
p-polarised
) ) :
Jotx0
--.
)/ ;\
Emission lzDo _ Intensity
s-polar&d
:;
14ilo-
kO.05 +0.03 +o.os i_ 0.0005
_ ~_-_ ’ Ill
’ IS
1 21)
~_
_,_ ’ 25
^..
‘\
_.--
’ 30
1 35
“’ 40
Emission angle (degrees) Fig. 3. The radiation
pattern for 664 nm emission and an azimuthal angle of zero. Both p- and s-polarised emission are shown.
ness of the dye layer was found to be 1.55 & 0.5 nm. The grating profile is best described as a sum of harmonic components [ 151:
y(x)
=d,
sin G.r+d,
sin(2G.r+&)
f...
(2)
The extraharmonic components arise from the nonlinear response of the photoresist during exposure in the interferometer; in general only the first 2 or 3 components are significant. Fitting theory to the data for the grating used in this study gave: pitch = 820.0 + 0.2 nm, d,= 14.2-tO.5 nm, d,= 1.7kO.2 nm, d,= 0.3 f 0. I nm, c#+= - 90” and & = 0”.
4. Radiation pattern for zero azimuthal
angle
The angle dependent radiation pattern was first recorded for light emitted in the plane normal to the grating grooves ( ~$=0”). Fig. 3 shows the data recorded for an emission wavelength of 664 nm. Data were recorded for both the p- and s-polarised components of the emission, the relative intensities have been corrected for the polarisation sensitivity of the detection system. The p-polarised emission is dominated by two strong peaks, at 13.1” and 35.2”. The stronger peak corresponds to first order coupling between the SPP and photons; the SPPs scatter from the G component of the grating. The smaller peak is a result of SPPs scattering from the 2G component of the surface and is consc-
I so
SC. Kitson et (11./Optics Co,nmrmic,utions 122 (1996) 147-154
quently much weaker. It should be noted that the width of the emission peaks is dominated by the 1.5” collection angle. The s-polarised emission shows no resonant features; for zero azimuthal angle SPPs can only couple to p-polarised light, a point that we will return to later. For this sample the dye layer is in contact with the metal film; direct radiation is therefore quenched at the expense of non-radiative energy transfer to the metal. The radiation pattern is consequently dominated by SPP re-radiation; the background direct radiation intensity is an order of magnitude smaller than the light reradiated via the first order SPP mode. DCM emits over a wide range of wavelengths, a radiation pattern similar to Fig. 3 can be recorded for each. A problem encountered in taking such data is the bleaching of the dye layer. At the laser intensities used in these experiments the dye bleaches irreversibly on a time scale of tens of minutes. In order to correct for the effect of bleaching on the emission intensity, the measurement for a particular wavelength was repeated after every few scans. Plotting the area under the peaks for these control scans as a function of run number yields a curve showing the extent of the bleaching. This plot can then be used to correct the intensities of the other data sets. Data was recorded for emission wavelengths ranging from 640 nm to 684 nm. The position of each emission peak is related to the wavevectorof the SPP from which it originates. Eq. ( I ) The peak positions can, therefore, be used to plot the dispersion curve of the mode. The area under each curve is a measure of the total intensity emitted at that wavelength; plotting the area as a function of the wavelength gives an effective emission spectrum for the sample, Fig. 4. So far we have only considered light emitted in the plane perpendicular to the grating grooves. The SPP m-radiation is purely p-polarised in this case, the spolarised emission shows no such resonant features. We now go on to discuss the more general case of light emitted at non-zero azimuthal angles.
5, Radiationpattern
for non-zero azimuthal angle
5.1. The effect on the emission angle The coupling between SPPs and radiation depends on the azimuthal angle 4 of the emission. For 4 = 0”
Fig. 4. The effective emission spectrum for the sample; the area under each radiation curve is plotted as a function of the emission wavelength.
all the diffracted beams lie in the measurement plane and SPPs, which are TM in nature, couple only to ppolarised light. When (b f 0”, the diffracted beams no longer lie in the plane of measurement, and a vector form of the momentum matching condition (Eq. ( I ) ) is required: k,, sin t)= +k,,,
& nG .
(3)
Fig. 5a shows a k-space representation of the grating surface. The solid circle has a radius k, and represents the maximum momentum component in the plane of the grating available to a photon; this corresponds to a photon grazing along the surface. The dotted circles .. have radii of ksrr which is larger than k,, so that the SPP cannot directly couple to even a grazing photon. The grating allows the momentum of the SPP to be enhanced or reduced by integer multiples of G; to represent this the kspp circles are displaced by these multiples along the direction of the grating vector. As shown, the SPP may now couple to a photon with momentum component k,, sin 0 in the plane of the grating and propagating at an azimuthal angle 4. When 4 # O”, we have, therefore, by simple trigonometry, ki sin20= Lspp - n’G’ k 2nGk,, sin 13cos 4 .
(4)
It should also be noted from Fig. 5a that the SPP does not propagate in the direction defined by the azimuthal angle (b. The angle I) between the SPP wavevector and the grating vector is given by cos rcr= G+&sin
0 cos 4 k SW
(51
XC. Kitson et al. /Optics Cotnrnrrnications 122 (1996) 147-154
Fig. 5.
(a) A k-space representation
of the grating, for G > ksr,,, The solid circle has radius k. and represents the maximum momentum component
in the plane available from a photon propagating above the grating. The dotted circles have radius ks,,, and represent the momentum of the SPP mode. An SPP propagating at an angle J, relative to the grating vector may couple, via the grating, to a photon emitted at an angle 8 in a plane defined by the azimuthal angle I$. (b) As (a) but for G < k,,,,,. The SPP may now couple to photons by scattering by either + G or * 2G.
We can consider two distinct cases, that is G > ksrr and G < ks,,,,. If G > ksrr (Fig. 5a) then the SPP can only couple to photons by scattering by )G. As the azimuthal angle is increased the resonance moves to higher angle 8 and there is a maximum value of 4 beyond which SPPs cannot re-radiate. If G
positions of the SPP re-radiation peaks will be given by Eq. (3). This was studied by recording the radiation pattern for 664 nm emission for different azimuthal angles ranging from 0” to 90”. The radiation pattern for each azimuthal angle was dominated by at least one peak corresponding to the re-radiation of SPPs. The angular positions of these emission peaks correspond to the SPP resonance angle 0 in Eq. (3). Fig. 6 shows the measured $-dependence of the peak emission angles; sin 0 is plotted versus 4 on polar axes. Data were only recorded for one quadrant of this graph, from 0” to 90”, but to allow easy comparison with Fig. 5 it has been translated into the other three quadrants. The similarity between the experimental results and Fig. 5b is striking. In this paper we will examine in detail the first order resonance. From Eq. (4) a plot of sin’bversus sin 0~0s 4 should be linear with a gradient of - 2G/ko and an intercept
0.6
Fig. 6. The measured dependence of the peak emission angle 0 on the azimuthal angle 6 of the grating for 644 nm emission.
given by ( I /G) (k&G’). Fig. 7 shows the data for the first order coupled branch from Fig. 6 replotted in this way. The solid line is a theoretical curve plotted according to Eq. (41, the grating pitch and the value of ks,,,, being taken from the fit to the reflectivity data. In order to get good agreement between the data and theory, systematic errors in the measurements of the angles had IO be assumed; the azimuthal angles had to be corrected by 1.0” and the emission angles by OS”, both within the accuracy of the measurements. A sample error bar on one of the data points shows the effect of the angular resolution of the system on the precision of the results. The agreement between the data and the theory is good. although there is a systematic curve to the data. The origin of this is unclear; measurements with better angle resolution may help clarify the issue. In this section WC have seen that the peak emission angle 0 depends strongly on the azimuthal angle 4 of the emission. This dependence can be understood from considerations of the momentum matching condition. Twisting the grating not only affects the emission angle; hreaking the symmetry also affects the polarisation of the emission. 5.2. The effect 011the polnrisation
of the light that couples resonantly to the surface mode varies with azimuthal angle. For an azimuthal angle of 0” the SPP couples to p-polarised light; for $= 90”, however, the mode couples to s-polarised light. At a general value of C$ it is only the component of the photon E-field that is in the plane defined by the substrate normal and the grating vector that may couple resonantly to SPPs. At certain points along the grating surface such an E-field has a component normal to the local surface allowing it to couple to the surface charge oscillation that constitutes an SPP. An E-field parallel to the grooves, however, is always tangential to the local surface and so cannot couple to an SPP. Following this argument, one expects that the polarisation of the light radiated by SPPs will depend on the azimuthal angle of the emission. The emitted light will be polarised such that the E-field is in the planedefined by the substrate normal and the grating vector. Therefore, the polarisation will vary from p-polarisation for light emitted at d, = 0” to s-polarisation for += 90”. The $-dependence of the two polarisation components can be calculated by resolving the E-field into components parallel and perpendicular to the emission plane, the square of these components then gives the intensities of the p-polarised and s-polarised components respectively. Accordingly, the p-polarised component should vary as co?+ and the s-polarised component as sin’+. It should be stressed that the above argument is far from rigorous. A theory is needed that predicts the emission from this system. Modelling the surface charges and the associated E-field may then give an insight into the mechanism that determines the polarisation.
of emission .
Twisting the grating by an azimuthal angle (b#O’ breaks the symmetry of the system; the grating vector G, the SPP wavevector kg,,, and the wavevector of the coupled photon k,, no longer lie in the same plane. Using reflectivity experiments, Bryan-Brown et al. [ 161 showed that, as a consequence, the polarisation
t II0
I
A, IF
I
I, IN,
I
0 (15
I
.
0 IO
9
,
I
I, 15
O.?l)
0
GnOcosg Fig. 7. The emission data from Figure 6 replotted to allow comparison with theory.
Light emitted via SPPs is always polarised such that the E-field of the light is in the plane defined by the grating vector and the substrate normal. Rotating the grating azimuthally, therefore, changes the polarisation of the emitted light from p-polarised at (b=O” to spolarised at &=90”.
5.3. Effect on the area under the emission curves
Fig. 8. (a) Theintensityofp-polxisedemissionas
functionofcos’&
The line is a linear fit to the data points. (b) The intensity of cpolarised emission as function of sin’& The line is a linear fit to the data points.
To measure the polarisation of the emission as a function of the azimuthal angle, the polariser in front of the spectrometer was used to select either the p- or the s-polarised component of the emission. The radiation pattern were recorded for each component for different azimuthal angles ranging from 0” to 90”, again the data were recorded for 664 nm emission. The total intensity emitted in each polarisation component for a given azimuthal angle was obtained by integrating the area under each radiation curve. Plotting this area as a function of the azimuthal angle shows how the intensity of the two polarisation components depend on the azimuthal angle, Fig. 8. The data for ppolarised emission is plotted against COS’~, the s-polarised data is plotted against sin24. Both sets of data fit well to straight lines confirming the expressions derived above. The fact that both of the fitted lines have non-zero intercepts is probably due to a background level on the emission data due to noise or stray light. In this section we have seen that the polarisation of the emission depends on the orientation of the sample.
Another parameter of interest is the area under the emission curve, this gives information on the total intensity emitted at a given azimuthal angle. Fig. 9 shows the total area under the emission curves as a function of the azimuthal angle. The total area was obtained by summing the areas under the curves for both p-polarised emission and s-polarised emission, appropriately corrected for the polarisation sensitivity of the detection system. Emission at a particular azimuthal angle 4 corresponds to SPPs propagating at an angle 4 relative to the grating vector, given by Eq. (5). The intensity emitted at a particular azimuthal angle 4 will, therefore, be proportional to the number density of SPPs propagating at the corresponding angle ti, and to the probability than an SPP will scatter from the grating to result in emission. Consequently, for this sample both these quantities seem to be independent of the direction ot propagation. Of particular interest is the fact that the excited dye molecules appear to couple equally strongly to SPPs propagating in all directions on the grating surface. In this respect, the surface is behaving as if it were uncorrugated. Results for other samples
Fig. 9. Total area under the emission curves as a function of the azimuthal angle.
IS4
SC. Kitson et al. /Optics Communications
[ 91 indicate that this is not the case if the pitch of the grating is close to the emission wavelength. For a pitch of 634 nm and a wavelength of 632.8 nm, the emission intensity increases with azimuthal angle, reaching a maximum at (b = 90”. Further work is needed to study this effect.
122 (1996) 147-154
Acknowledgements The authors are grateful to the EPSRC and the DRA (Malvern) for supporting this research including the provision of a CASE award. References I I) K.H. Drexhage, Progress in Optics 12. ed., E. Wolf (NorthHolland, Amsterdam)
p. 165.
[ 21 1.Pockrand, A. Brillante and D. Mobius, Chem. Phys. Lett. 69 ( 1980) 499.
6. Summary
Locating a layer of excited dye molecules near a metallic surface introduces additional decay routes, principally decay via SPPs. Corrugating the surface allows these SPPs to couple to radiative modes. If the molecule is very close to the metal this re-radiation from SPPs can dominate emission from the sample. The result is a modified radiation pattern. The emission is concentrated in well defined directions corresponding to the resonant coupling between the SPPs and photons. We have studied how this radiation pattern depends on the azimuthal angle of the emission and find that the polarisation and the direction of the emission both vary with 4. The nature of this &dependence can be readily understood by considering the momentum matching conditions that govern the coupling between photons and SPPs. Further work is needed, however, to clarify the mechanism that determines the polarisation of the emission.
[ 31B.N.J. Persson, J. Phys. C I 1 ( 1978) 4251. [41 H. Raether. Surface Plasmons (Springer, Berlin, I988 ). [ 5 1W. Knoll, M.R. Philpott and J.D. Swalen, J. Chem. Phys. 75 (1981) 4795. j6j R.W. GruhlkeandD.G.Hall.Appl. Phys.Lett.53 ( 1988) 1041. 17 1R.W. Gruhlke and D.G. Hall, Phys. Rev. B 40 ( 1989) 5367. ] 8 ] A. Adams, J. Moreland and P.K. Hansma, Phys. Rev. B 25 (1982) 3457. [ 9 ] S.C. Kitson, Molecular Fluorescence Near Metallic Gratings. Ph.D. Thesis ( 1995). University of Exeter. UK. ] IO] SC. Kitson. W.L. Barnes, J.R. Sambles and N.P.K. Cotter. Excitation of molecular fluorescence via surface plasmon polaritons, J. Chem. Phys. ( 1995). submitted. 1I1 I J. Chandezon, M.T. Dupuis, G. Comet and D. Maystre. J. Opt. Sot. Am. 72 ( 1982) 839. [ 12 1N.P.K. Cotter, T. W. Priest and J.R. Sambles, J. Opt. Sot. Am. A 12 (1995) 1097. [ 131F. Yang, J.R. Sambles and G.W. Bradberry, J. Mod. Optics 38 (1991) 1441. [ 141 S.C. Kitson and J.R. Sambles. Thin Solid Films 229 ( 1993) 128. 1151 E.L. Wood, J.R. Sambles, N.P.K. Cotter and S.C. Kitson. Diffraction grating characterisation using multiple wavelength excitation of surface plasmon polaritons, J. Mod. Optics ( 1995). to be published. 1161 G.P. Bryan-Brown, J.R. Sambles and M.C. Hutley. J. Mod. Optics 37 ( 1990) 1227.
I January
l!iB d
1996
-
OPTICS COMMUNICATIONS
__
ELYEVIER
Optics Communications
132
( 1996) l47- I54
Photoluminescence from dye molecules on silver gratings S.C. Kitson, W.L. Barnes, J.R. Sambles Depor~nenf I$Ply.Gt:s. hiwr.sity of’E.reter. Stocker Rwd. Exeter, EX4 4QL.
UK
Received 5 June 1995
Abstract Excited dye molecules on metallic gratings can relax by generating surface plasmon polaritons modes can scatter from the grating presents an experimental emission is dominated the azimuthal momentum
emitting
light in well-defined
study of the emission by light re-radiated
properties
direction
conditions
by SPPs. The direction
that govern the coupling
polarisation.
of a thin layer of laser dye on a silver grating. and the polarisation
This paper
It is found that the
of the emission are found to depend on
angle between the emitted light and the grating grooves. The experimental matching
( SPPs). These non-radiative
and with a characteristic
results are explained
in terms of the
between photons and SPPs.
1. Introduction In this paper we examine how proximity to a silver grating modilies the photoluminescence from a thin layer of laser dye. Proximity to a metal surface significantly quenches the emission of light from a dye layer; the energy is nonradiatively absorbed by the metal [ I]. There arc several processes that contribute to this absorption [ 2 1. If the molecule is very close to the metal (less than 10 nm) then the dominant process may be direct excitation of electron-hole pairs [ 31, at slightly larger separations Joule heating of the metal dominates. Of particular interest for this paper, however, is the possibility of energy being adsorbed via the excitation of surface plasmon polaritons (SPPs) If the surface is planar then these SPPs cannot reradiate. They have too much momentum to couple to bulk radiation, so the energy is ultimately dissipated as heat within the metal. However, if the surface is corrugated then the SPPs may scatter from the grating resulting in emission in well-defined directions and with a characteristic polarisation. The grating matches the momentum of the SPP to the component of the 0030.4018/96/$11.00 Q 1996 Elsevier Science B.V. All rights reserved .wI/o030-40 I X( 9s )00490-4
photon momentum in the plane of the grating, yielding the coupling equation [ 41: /&sin 0= 3-k,,, t_ nG ,
(1)
where k,, is the wavevector of the incident photon, 0 is the emission angle, kspp is the SPP wavevector, tr is an integer and G is the grating vector (equal to 2rr/grating pitch). We, therefore, expect strong emission at the angles 6,given by this equation. The sample geometry that we have examined experimentally is illustrated in Fig. I, namely a thin layer of laser dye DCM deposited onto the surface of a silver grating. In this case direct luminescence from the dye is strongly inhibited due to nonradiative transfer of energy to the metal. The radiation from the sample is, therefore, dominated by the re-radiation from SPPs. First we examine the emission at zero azimuthal angle, that is in the plane perpendicular to the grating grooves. This situation has, to some extent, been studied by other workers [5-S]. We extend this work by examining how the radiation pattern depends on the azimuthal angle of the emission. We find that both the peak emission angle and the polarisation of the emis-
S.C. Kitson et d. / 0ptic.s Cmtnunicufions
148
( Fig. I. The
sample geometry.
1-
Slllca
The polar emission angle is 0, the
azimuthal angle is I$.
sion vary with the azimuthal angle. The results can be understood by considering the boundary conditions that govern the coupling between photons and SPPs.
122 (19961 147-154
tomultiplieron the exit slits. A Glan-Thomson polariser placed in front of the spectrometer selected either the p- or the s-polarised component of the emission. A filter on the entrance splits prevented argon ion emission reaching the detector. The sensitivity of the detection system depended on the polarisation of the collected light. To measure this, 664 nm light from a diode laser was diffused to randomise the polarisation and then made incident onto the collection lens. The signal was recorded with the polariser set to pass first p- and then s-polarised light. The results showed that the system was a factor of 3.6( f 0.2) times more sensitive to p-polarised than spolarised light of this wavelength. This factor was used to correct the relative intensities of the emission for the two polarisations.
3. Sample preparation 2. Experimental The dye layer was excited optically using an argon ion laser. The experimental measurements consisted of recording the luminescence intensity as a function of the emission angle for different wavelengths and polarisations. In doing this, it was important that the angle of incidence and polarisation of the incident beam were kept fixed during data acquisition [ 9, IO]. The experimental arrangement used is illustrated in Fig. 2. The sample was mounted on an X-Z stage in a holder that allowed the azimuthal angle of the sample to be set to a precision of k 0.5”. The dye layer was excited by 488 nm emission from a 1 W argon ion laser made incident on the sample via a multimode optical fibre. The fibre was attached to the sample holder via a mount which included a graded index rod lens to focus the light onto the sample. In this way the argon ion beam moved with the sample as it was rotated during data acquisition. The angle of incidence and polarisation of the beam were thus held fixed. The X-Z stage was fixed to a computer controlled rotating table to allow the angle of emission to be scanned. A lens collected the emission and focused it onto the slits of the spectrometer, the collection angle being limited to 1.5” by an aperture. The argon ion laser beam used to excite the dye layer was mechanically chopped at about 900 Hz so that phase sensitive detectors could he used to record the signal from the pho-
A grating profile was holographically produced in a photoresist layer deposited on a silica substrate. The modulated surface profile was then transferred to the silica surface by etching with a beam of argon atoms in a high vacuum chamber. An optically thick silver film was subsequently evaporated onto the surface of the grating. This was then coated with a thin layer of the laser dye DCM by spin deposition from a solution of the dye in methanol. A solution concentration of 0.4 mgmll’ was used with the substrate spun at 3000 rpm. To characterise the sample, angle dependent rellectivity data were recorded for 488 nm and 664 nm light. A p-polarised beam incident in the plane + = 0” and at the appropriate angle 13(Eq. ( I ) ) can resonantly excite SPPs. This absorbs energy from the beam producing a dip in the reflectivity. Fitting multi-layer grating theory [ 11,121 to the data allows the determination of the optical constants. Data was recorded both before and after coating the silver grating with DCM. Fitting theory to the data for the hare silver gave the grating protile and the optical constants of the silver. These values were then held fixed whilst the thickness and the dielectric permittivity of the dye were adjusted to obtain a good fit to the data for the coated grating. A starting point for the DCM parameters was obtained by using the critical edge technique [ 13,141 to characterise a dye layer deposited onto a silica prism. Table 1 summarises the fitted parameters for the sample. The thick-
S. C. Kitsorr e! (11./ Opiita
C~)tntnlmicrrrions
I22 (1996) lJ7-151
Fibrr adaptor
149
Chopped 488nm light
Sample holder \
Snectrometer
w Rotating table
Fig. 2. The experimental arrangement T&k
I
The fitted optical constants of the sample at the two wavelengths of interest. 488 nm and 664 nm
silver DCM silver DCM
Wavelength (nm)
E,
6,
488 488
-8.3 +0.1 l.S9~00.03 ~ 19.0 f0.1 2.22 * 0.03
0.38 1.32 1.74 0.002
664 664
1600
-
,ooo
(arbitrary
so0
units)
;
600 -
)/ ,’
I (
II-
p-polarised
) ) :
Jotx0
--.
)/ ;\
Emission lzDo _ Intensity
s-polar&d
:;
14ilo-
kO.05 +0.03 +o.os i_ 0.0005
_ ~_-_ ’ Ill
’ IS
1 21)
~_
_,_ ’ 25
^..
‘\
_.--
’ 30
1 35
“’ 40
Emission angle (degrees) Fig. 3. The radiation
pattern for 664 nm emission and an azimuthal angle of zero. Both p- and s-polarised emission are shown.
ness of the dye layer was found to be 1.55 & 0.5 nm. The grating profile is best described as a sum of harmonic components [ 151:
y(x)
=d,
sin G.r+d,
sin(2G.r+&)
f...
(2)
The extraharmonic components arise from the nonlinear response of the photoresist during exposure in the interferometer; in general only the first 2 or 3 components are significant. Fitting theory to the data for the grating used in this study gave: pitch = 820.0 + 0.2 nm, d,= 14.2-tO.5 nm, d,= 1.7kO.2 nm, d,= 0.3 f 0. I nm, c#+= - 90” and & = 0”.
4. Radiation pattern for zero azimuthal
angle
The angle dependent radiation pattern was first recorded for light emitted in the plane normal to the grating grooves ( ~$=0”). Fig. 3 shows the data recorded for an emission wavelength of 664 nm. Data were recorded for both the p- and s-polarised components of the emission, the relative intensities have been corrected for the polarisation sensitivity of the detection system. The p-polarised emission is dominated by two strong peaks, at 13.1” and 35.2”. The stronger peak corresponds to first order coupling between the SPP and photons; the SPPs scatter from the G component of the grating. The smaller peak is a result of SPPs scattering from the 2G component of the surface and is consc-
I so
SC. Kitson et (11./Optics Co,nmrmic,utions 122 (1996) 147-154
quently much weaker. It should be noted that the width of the emission peaks is dominated by the 1.5” collection angle. The s-polarised emission shows no resonant features; for zero azimuthal angle SPPs can only couple to p-polarised light, a point that we will return to later. For this sample the dye layer is in contact with the metal film; direct radiation is therefore quenched at the expense of non-radiative energy transfer to the metal. The radiation pattern is consequently dominated by SPP re-radiation; the background direct radiation intensity is an order of magnitude smaller than the light reradiated via the first order SPP mode. DCM emits over a wide range of wavelengths, a radiation pattern similar to Fig. 3 can be recorded for each. A problem encountered in taking such data is the bleaching of the dye layer. At the laser intensities used in these experiments the dye bleaches irreversibly on a time scale of tens of minutes. In order to correct for the effect of bleaching on the emission intensity, the measurement for a particular wavelength was repeated after every few scans. Plotting the area under the peaks for these control scans as a function of run number yields a curve showing the extent of the bleaching. This plot can then be used to correct the intensities of the other data sets. Data was recorded for emission wavelengths ranging from 640 nm to 684 nm. The position of each emission peak is related to the wavevectorof the SPP from which it originates. Eq. ( I ) The peak positions can, therefore, be used to plot the dispersion curve of the mode. The area under each curve is a measure of the total intensity emitted at that wavelength; plotting the area as a function of the wavelength gives an effective emission spectrum for the sample, Fig. 4. So far we have only considered light emitted in the plane perpendicular to the grating grooves. The SPP m-radiation is purely p-polarised in this case, the spolarised emission shows no such resonant features. We now go on to discuss the more general case of light emitted at non-zero azimuthal angles.
5, Radiationpattern
for non-zero azimuthal angle
5.1. The effect on the emission angle The coupling between SPPs and radiation depends on the azimuthal angle 4 of the emission. For 4 = 0”
Fig. 4. The effective emission spectrum for the sample; the area under each radiation curve is plotted as a function of the emission wavelength.
all the diffracted beams lie in the measurement plane and SPPs, which are TM in nature, couple only to ppolarised light. When (b f 0”, the diffracted beams no longer lie in the plane of measurement, and a vector form of the momentum matching condition (Eq. ( I ) ) is required: k,, sin t)= +k,,,
& nG .
(3)
Fig. 5a shows a k-space representation of the grating surface. The solid circle has a radius k, and represents the maximum momentum component in the plane of the grating available to a photon; this corresponds to a photon grazing along the surface. The dotted circles .. have radii of ksrr which is larger than k,, so that the SPP cannot directly couple to even a grazing photon. The grating allows the momentum of the SPP to be enhanced or reduced by integer multiples of G; to represent this the kspp circles are displaced by these multiples along the direction of the grating vector. As shown, the SPP may now couple to a photon with momentum component k,, sin 0 in the plane of the grating and propagating at an azimuthal angle 4. When 4 # O”, we have, therefore, by simple trigonometry, ki sin20= Lspp - n’G’ k 2nGk,, sin 13cos 4 .
(4)
It should also be noted from Fig. 5a that the SPP does not propagate in the direction defined by the azimuthal angle (b. The angle I) between the SPP wavevector and the grating vector is given by cos rcr= G+&sin
0 cos 4 k SW
(51
XC. Kitson et al. /Optics Cotnrnrrnications 122 (1996) 147-154
Fig. 5.
(a) A k-space representation
of the grating, for G > ksr,,, The solid circle has radius k. and represents the maximum momentum component
in the plane available from a photon propagating above the grating. The dotted circles have radius ks,,, and represent the momentum of the SPP mode. An SPP propagating at an angle J, relative to the grating vector may couple, via the grating, to a photon emitted at an angle 8 in a plane defined by the azimuthal angle I$. (b) As (a) but for G < k,,,,,. The SPP may now couple to photons by scattering by either + G or * 2G.
We can consider two distinct cases, that is G > ksrr and G < ks,,,,. If G > ksrr (Fig. 5a) then the SPP can only couple to photons by scattering by )G. As the azimuthal angle is increased the resonance moves to higher angle 8 and there is a maximum value of 4 beyond which SPPs cannot re-radiate. If G
positions of the SPP re-radiation peaks will be given by Eq. (3). This was studied by recording the radiation pattern for 664 nm emission for different azimuthal angles ranging from 0” to 90”. The radiation pattern for each azimuthal angle was dominated by at least one peak corresponding to the re-radiation of SPPs. The angular positions of these emission peaks correspond to the SPP resonance angle 0 in Eq. (3). Fig. 6 shows the measured $-dependence of the peak emission angles; sin 0 is plotted versus 4 on polar axes. Data were only recorded for one quadrant of this graph, from 0” to 90”, but to allow easy comparison with Fig. 5 it has been translated into the other three quadrants. The similarity between the experimental results and Fig. 5b is striking. In this paper we will examine in detail the first order resonance. From Eq. (4) a plot of sin’bversus sin 0~0s 4 should be linear with a gradient of - 2G/ko and an intercept
0.6
Fig. 6. The measured dependence of the peak emission angle 0 on the azimuthal angle 6 of the grating for 644 nm emission.
given by ( I /G) (k&G’). Fig. 7 shows the data for the first order coupled branch from Fig. 6 replotted in this way. The solid line is a theoretical curve plotted according to Eq. (41, the grating pitch and the value of ks,,,, being taken from the fit to the reflectivity data. In order to get good agreement between the data and theory, systematic errors in the measurements of the angles had IO be assumed; the azimuthal angles had to be corrected by 1.0” and the emission angles by OS”, both within the accuracy of the measurements. A sample error bar on one of the data points shows the effect of the angular resolution of the system on the precision of the results. The agreement between the data and the theory is good. although there is a systematic curve to the data. The origin of this is unclear; measurements with better angle resolution may help clarify the issue. In this section WC have seen that the peak emission angle 0 depends strongly on the azimuthal angle 4 of the emission. This dependence can be understood from considerations of the momentum matching condition. Twisting the grating not only affects the emission angle; hreaking the symmetry also affects the polarisation of the emission. 5.2. The effect 011the polnrisation
of the light that couples resonantly to the surface mode varies with azimuthal angle. For an azimuthal angle of 0” the SPP couples to p-polarised light; for $= 90”, however, the mode couples to s-polarised light. At a general value of C$ it is only the component of the photon E-field that is in the plane defined by the substrate normal and the grating vector that may couple resonantly to SPPs. At certain points along the grating surface such an E-field has a component normal to the local surface allowing it to couple to the surface charge oscillation that constitutes an SPP. An E-field parallel to the grooves, however, is always tangential to the local surface and so cannot couple to an SPP. Following this argument, one expects that the polarisation of the light radiated by SPPs will depend on the azimuthal angle of the emission. The emitted light will be polarised such that the E-field is in the planedefined by the substrate normal and the grating vector. Therefore, the polarisation will vary from p-polarisation for light emitted at d, = 0” to s-polarisation for += 90”. The $-dependence of the two polarisation components can be calculated by resolving the E-field into components parallel and perpendicular to the emission plane, the square of these components then gives the intensities of the p-polarised and s-polarised components respectively. Accordingly, the p-polarised component should vary as co?+ and the s-polarised component as sin’+. It should be stressed that the above argument is far from rigorous. A theory is needed that predicts the emission from this system. Modelling the surface charges and the associated E-field may then give an insight into the mechanism that determines the polarisation.
of emission .
Twisting the grating by an azimuthal angle (b#O’ breaks the symmetry of the system; the grating vector G, the SPP wavevector kg,,, and the wavevector of the coupled photon k,, no longer lie in the same plane. Using reflectivity experiments, Bryan-Brown et al. [ 161 showed that, as a consequence, the polarisation
t II0
I
A, IF
I
I, IN,
I
0 (15
I
.
0 IO
9
,
I
I, 15
O.?l)
0
GnOcosg Fig. 7. The emission data from Figure 6 replotted to allow comparison with theory.
Light emitted via SPPs is always polarised such that the E-field of the light is in the plane defined by the grating vector and the substrate normal. Rotating the grating azimuthally, therefore, changes the polarisation of the emitted light from p-polarised at (b=O” to spolarised at &=90”.
5.3. Effect on the area under the emission curves
Fig. 8. (a) Theintensityofp-polxisedemissionas
functionofcos’&
The line is a linear fit to the data points. (b) The intensity of cpolarised emission as function of sin’& The line is a linear fit to the data points.
To measure the polarisation of the emission as a function of the azimuthal angle, the polariser in front of the spectrometer was used to select either the p- or the s-polarised component of the emission. The radiation pattern were recorded for each component for different azimuthal angles ranging from 0” to 90”, again the data were recorded for 664 nm emission. The total intensity emitted in each polarisation component for a given azimuthal angle was obtained by integrating the area under each radiation curve. Plotting this area as a function of the azimuthal angle shows how the intensity of the two polarisation components depend on the azimuthal angle, Fig. 8. The data for ppolarised emission is plotted against COS’~, the s-polarised data is plotted against sin24. Both sets of data fit well to straight lines confirming the expressions derived above. The fact that both of the fitted lines have non-zero intercepts is probably due to a background level on the emission data due to noise or stray light. In this section we have seen that the polarisation of the emission depends on the orientation of the sample.
Another parameter of interest is the area under the emission curve, this gives information on the total intensity emitted at a given azimuthal angle. Fig. 9 shows the total area under the emission curves as a function of the azimuthal angle. The total area was obtained by summing the areas under the curves for both p-polarised emission and s-polarised emission, appropriately corrected for the polarisation sensitivity of the detection system. Emission at a particular azimuthal angle 4 corresponds to SPPs propagating at an angle 4 relative to the grating vector, given by Eq. (5). The intensity emitted at a particular azimuthal angle 4 will, therefore, be proportional to the number density of SPPs propagating at the corresponding angle ti, and to the probability than an SPP will scatter from the grating to result in emission. Consequently, for this sample both these quantities seem to be independent of the direction ot propagation. Of particular interest is the fact that the excited dye molecules appear to couple equally strongly to SPPs propagating in all directions on the grating surface. In this respect, the surface is behaving as if it were uncorrugated. Results for other samples
Fig. 9. Total area under the emission curves as a function of the azimuthal angle.
IS4
SC. Kitson et al. /Optics Communications
[ 91 indicate that this is not the case if the pitch of the grating is close to the emission wavelength. For a pitch of 634 nm and a wavelength of 632.8 nm, the emission intensity increases with azimuthal angle, reaching a maximum at (b = 90”. Further work is needed to study this effect.
122 (1996) 147-154
Acknowledgements The authors are grateful to the EPSRC and the DRA (Malvern) for supporting this research including the provision of a CASE award. References I I) K.H. Drexhage, Progress in Optics 12. ed., E. Wolf (NorthHolland, Amsterdam)
p. 165.
[ 21 1.Pockrand, A. Brillante and D. Mobius, Chem. Phys. Lett. 69 ( 1980) 499.
6. Summary
Locating a layer of excited dye molecules near a metallic surface introduces additional decay routes, principally decay via SPPs. Corrugating the surface allows these SPPs to couple to radiative modes. If the molecule is very close to the metal this re-radiation from SPPs can dominate emission from the sample. The result is a modified radiation pattern. The emission is concentrated in well defined directions corresponding to the resonant coupling between the SPPs and photons. We have studied how this radiation pattern depends on the azimuthal angle of the emission and find that the polarisation and the direction of the emission both vary with 4. The nature of this &dependence can be readily understood by considering the momentum matching conditions that govern the coupling between photons and SPPs. Further work is needed, however, to clarify the mechanism that determines the polarisation of the emission.
[ 31B.N.J. Persson, J. Phys. C I 1 ( 1978) 4251. [41 H. Raether. Surface Plasmons (Springer, Berlin, I988 ). [ 5 1W. Knoll, M.R. Philpott and J.D. Swalen, J. Chem. Phys. 75 (1981) 4795. j6j R.W. GruhlkeandD.G.Hall.Appl. Phys.Lett.53 ( 1988) 1041. 17 1R.W. Gruhlke and D.G. Hall, Phys. Rev. B 40 ( 1989) 5367. ] 8 ] A. Adams, J. Moreland and P.K. Hansma, Phys. Rev. B 25 (1982) 3457. [ 9 ] S.C. Kitson, Molecular Fluorescence Near Metallic Gratings. Ph.D. Thesis ( 1995). University of Exeter. UK. ] IO] SC. Kitson. W.L. Barnes, J.R. Sambles and N.P.K. Cotter. Excitation of molecular fluorescence via surface plasmon polaritons, J. Chem. Phys. ( 1995). submitted. 1I1 I J. Chandezon, M.T. Dupuis, G. Comet and D. Maystre. J. Opt. Sot. Am. 72 ( 1982) 839. [ 12 1N.P.K. Cotter, T. W. Priest and J.R. Sambles, J. Opt. Sot. Am. A 12 (1995) 1097. [ 131F. Yang, J.R. Sambles and G.W. Bradberry, J. Mod. Optics 38 (1991) 1441. [ 141 S.C. Kitson and J.R. Sambles. Thin Solid Films 229 ( 1993) 128. 1151 E.L. Wood, J.R. Sambles, N.P.K. Cotter and S.C. Kitson. Diffraction grating characterisation using multiple wavelength excitation of surface plasmon polaritons, J. Mod. Optics ( 1995). to be published. 1161 G.P. Bryan-Brown, J.R. Sambles and M.C. Hutley. J. Mod. Optics 37 ( 1990) 1227.