As shown in previous reports,4–7
the occurrence and angular distribution of SPCE can be predicted from the reflectivity curves of the thin silver and gold films. Knowing of the widespread use of UV fluorophores, we considered the use of different films of different metals. Aluminum was an obvious choice given the high reflectivity of aluminum mirrors at short wavelengths. An additional advantage of aluminum is that on exposure to air it forms a thin oxide film with a uniform self-limiting thickness. In our case, the aluminum film was coated with SiO2
. The use of aluminum with fluorescence was not obvious because of earlier reports of strong quenching on aluminum surfaces.15
However, several reports described the transmission of ultraviolet light by thin aluminum films as seen by excitation of fluorescence with total internal reflection,16
attenuated total internal reflection,17
or light scattering.18
Additionally, aluminum has been suggested for use in SPR,19
and calculations of the spatial distribution have been reported for fluorophores near an aluminum surface.20
However, we are not aware of any experimental reports of SPR or SPCE using aluminum films.
Based on the absence of UV absorption by aluminum, we decided to calculate the reflectivity curves for thin aluminum films on quartz. These calculations were performed using commercial software (TFCalc) or our own equivalent programs. Reflectivity curves are shown in for various thicknesses of aluminum. These curves were calculated using the dielectric constant for aluminum at 370 nm. These results show that the deepest reflectivity minimum is for an aluminum thickness of 20 nm, which is the thickness we chose for our coated slides.
Calculated reflectivity at 370 nm for a three-phase system of quartz–aluminum–air, for different thicknesses of aluminum. For 370 nm, we used εm = −17.02 + i2.95.
We were aware of aluminum oxidation, which can change an effective thickness of the metal layer. In the vapor deposition procedure, the slides were protected with a 100-Å SiO2
layer, twice as thick as we used previously for silver mirrors.5–7
The second reason for using thicker protection layer is to avoid the well-known quenching effect of aluminum.15,20,21
Because of earlier reports of quenching on aluminum surfaces, we first examined the free-space emission of spin-coated samples on quartz and an aluminum-coated quartz (). Emission spectra were recorded for illumination and observation from the same side of the sample. Hence, plasmons were not created by the incident light and the emission of 2-AP was the free-space (FC) emission mostly uninfluenced by the metal. The emission spectrum of the spin-coated sample on quartz or aluminum was essentially the same as the spectrum observed for 2-AP in water in a square cuvette. This result is not in conflict with the reports of quenching on aluminum because our sample was distal from the metal surface by 100 Å and SPCE is thought to occur for distances up to 200 nm. Quenching probably occurs at shorter distances. The background emission from a reference slide without 2-AP (, dotted line) is significantly lower than the observed 2-AP emission.
Figure 2 Front face emission spectra of 2-AP in water (top) and spin-coated from 1% PVA on a 20-nm aluminum film (bottom). The spectrum of 2-AP in water (top) is identical to the spectrum measured in square geometry using a 1-cm square cuvette. The spectrum of (more ...)
The presence of a sharp reflectivity minimum for 20-nm aluminum () suggested the presence of SPCE at the plasmon angle for the emission wavelength. The angle-dependent intensities are shown in . The emission was found to be sharply distributed around ±59° from the normal axis. We were unable to measure the intensities for all angles on the sample side of the metal, but it appears that roughly 30% of the total emission from 2-AP appears as SPCE. This percentage does not represent the maximum coupling efficiency but is the result found with our particular thicknesses of aluminum, SiO2, and PVA.
Angle-dependent emission intensity of 2-AP at 370 nm with reverse Kretschmann excitation.
Next, we stored the samples in a dry environment at room temperature and measured SPCE once again next day. The measurement was highly reproducible in both angular distribution and intensity. Later, we washed out the PVA from the slide and spin-coated once again with 1% PVA solution doped with 2-AP as described in Materials and Methods. We repeated the SPCE measurements, and we were pleasantly surprised that the aluminum-coated slide with SiO2 protection can be reused. We were not able to reuse the silvered slides with 50-Å SiO2 protection. We concluded that there is no significant damage to the aluminum layer when protected with 100-Å SiO2 coating.
Emission spectra of the SPCE are shown in . Emission was only observed when the emission polarizer was in the p-orientation, meaning parallel to the plane of observation. The signal was over 20-fold less for an s-oriented polarizer. The high p-polarization of the signal proves its origin with surface plasmons and not free-space emission of 2-AP.
Emission spectra of the SPCE from 2-AP with reverse Kretschmann excitation. The polarizer (pol.) orientation refers to the emission polarizer.
Examination of shows that there is symmetry about the axis normal to the metal surface. There is no reason for the SPCE to occur in a plane, but rather should appear as a cone with equal intensities at all azimuthal angles around the normal axis. Because of the large angle θF it is difficult to capture the emission from the entire cone. We were able to capture and image the SPCE using a parabolic reflector (). The cone of emission collected using the parabolic reflectance is seen in the lower panel. The angular distribution appears to be narrow, but this is in part due to the reflector, which is focusing this distribution. These images show that a relatively simple optical configurations can be used to capture a large fraction of the total emission.
Parabolic reflector used to capture SPCE (top) and the cone of emission for 2-AP observed using the reflector (bottom).
While we observed directional emission from 2-AP on aluminum, the 59° maximum angle was ~16° larger than that calculated for 20-nm aluminum on glass, which was ~43° (). This difference is a result of the presence of two dielectric layers, a protective 10 nm of SiO2 and the PVA doped with 2-AP. Hence, we repeated the reflectivity calculations using the five-phase system shown in , with an assumed PVA thickness of ds = 26.3 nm. In this case, the reflectivity minimum for p-polarized incident light was at 59° (), in precise agreement with the observed value. An s-polarized incident light did not show a calculated reflectivity minimum.
Calculated reflectivity at 370 nm for the five-phase system shown in .
It is known that the angle of minimum reflectivity of silver and gold films depends on the incident wavelength.22
As a result, different wavelengths radiate at slightly different angles in the prism.5
shows the free-space emission spectra of 2-AP (top) and the emission spectra for different observation angles in the prism (bottom). The spectra are blue shifted at larger angles (62°) and red shifted at smaller angles (56°) compared to the central angle of 59°. This shift is not due to different local environments of 2-AP but rather to the intrinsic wavelength dispersion of SPCE.
Surface plasmon-coupled emission spectra of 2-AP recorded at different observation angles θF. The angles are defined on the figures.
In earlier studies of fluorophores near metallic particles,23,24
we found decreases in lifetimes as compared to fluorophores distant from the particles. Hence, we examined the lifetime for 2-AP (). The amplitude-weighted lifetime for the free-space emission was found to be 0.53 ns (top). The amplitude-weighted lifetime for SPCE was found to be 0.43 ns (bottom). In both cases, the lifetimes were multiexponential (). Compared to the dramatic reduction in lifetime near metallic particles,23,24
the effect of the aluminum film is minimal. At this time we do not fully understand how the surface plasmons and fluorophores interact to determine the SPCE intensity decay.
Frequency-domain intensity decays of 2-AP with RK excitation. Top, FS. Bottom, SPCE.
Multiexponential Intensity Decay Parameters for 2-AP in PVA on the Aluminum Film
It is of interest to consider the range of distances over which fluorophores couple with surface plasmons. This distance is likely to be related to the depth of the surface plasmon evanescent field in the sample. Hence, we calculated the penetration depths for gold, silver, and aluminum for several wavelengths (). These calculations show similar penetration depth in water for all three metals. The penetration depths of the evanescent waves are similar in PVA. Additionally, the optical properties of aluminum suggest that SPR and SPCE can occur down to 280 nm. This result suggests that SPR and SPCE will be possible using the intrinsic tryptophan fluorescence from proteins.
Penetration Depth of the Surface Plasmon Evanescent Field into the Sample