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We experimentally investigate surface plasmon coupled emission (SPCE) in the Kretschmann geometry using thin conjugated polymer films. The broad spectral emission from the polymer films usefully demonstrates how the spectral behaviour of SPCE follows clearly from the underlying dispersion of surface plasmon polaritons supported by the configuration. We pay particular attention to how the spectral bandwidth of the underlying emissive layer impacts on the overall performance. Overall the work highlights the many factors that may be considered when designing optimum optoelectronic devices based on metal-organic multilayers.
©2011 Optical Society of America
1. Introduction
Energy transfer through dipole excitation is a basic mechanism to excite light-guided modes in photonic structures, including conventional waveguides as well as plasmonic waveguides. Surface plasmon polaritons (SPPs), which are resonant electron gas oscillations coupled to electromagnetic fields, are a particularly intriguing energy transfer channel [1–3]. SPP modes, as well as electron-hole excitations, can be created via an oscillating dipole near a metal surface. More generally, energy transfer channels can be controlled by designing the geometry of the guiding structures and, additionally, energy transfer into a particular energy channel can be promoted [4–6]. This approach has been well utilized in many kinds of optoelectronic device, e.g., light-emitting diodes [7,8] and photovoltaics [9]. Understanding the role of the energy transfer in active metallic structures is of great importance not only for optimizing conventional devices, but also for the development of new nanophotonic applications such as superlenses [10,11] and nanoplasmonic lasers [12].
Surface plasmon mediated emission through a thin metal film has been well known as a unique plasmonic energy transfer phenomenon [13–16]. This effect originates from strong near-field interactions between the oscillating dipole and the plasmonic media. Surface plasmon coupled emission (SPCE) has been studied using the prism-based Kretschmann configuration. The SPP fields excited by the near-field interaction are decoupled to the photon continuum via a prism. SPCE has been demonstrated using various metals including aluminum, gold, silver, and platinum [17–23], in combination with several active materials, for instance, dye molecules [21–23], inorganic quantum dots [24], chemiluminescent molecules [25], biomaterials [26], and photo-switching materials [27]. Also the effect of the specific multilayer geometry has been found to be crucial in determining the efficiency of SPCE [20–22].
There has been a growing interest in the use of organic materials as active media in the context of organic photonics, due to advantages of cost effectiveness, and excellent optical properties [28–30]. By adopting a concept of combining the organic polymer and nanoplasmonic systems, the possibility of a new optical plasmonic source has been suggested [31]. Here we report a study of surface plasmon coupled emission using optically excited fluorescent organic polymer films by means of de-coupling via a prism. The polymer used is the Red F conjugated copolymer, showing a broadband red emission that serves as an effective local probe to usefully examine the SPCE processes. A number of experimental and modeling techniques are brought together in the present study to clearly illustrate some of the general aspects of the SPCE process and, along with the previous studies, the intention is to provide some further considerations that need to be considered in order to fully optimize of the use of the SPCE process. The results highlight some of the many opportunities that exist when employing hybrid metal/polymer arrangements especially as active materials in plasmonic nanodevices.
2. Plasmonic coupling via dipole excitation
Bound electron–hole pairs (excitons) in an active medium are generated by optical or electrical pumping and their energy can be transferred to collective electron oscillations at the metal/dielectric interface through resonant coupled transitions. In the first instance, we present a description of SPCE in terms of power dissipation from an electric dipole using Ford and Weber’s model [1]. Figure 1 shows the calculated dissipated power density spectrum of a dipole parallel and perpendicular to a Lumogen Red (also known as Red F [30,32–34]) polymer film / Ag / BK7 prism multilayer system, depicted in the inset, for a free-space emission wavelength of 675 nm, which resides within the Lumogen Red polymer film emission bandwidth. In this particular configuration, the thickness of the polymer film is 38 nm, and of the Ag film 50 nm. Such a power dissipation spectrum, plotted against the normalized in-plane wavevector with respect to in vacuo, k0, allows identification of the various energy transfer channels of the systems, including SPPs [2]
Fig. 1 Normalized dissipated power density spectrum of a point dipole located in the middle of a Lumogen Red (38 nm) film atop a silver (50 nm) coated BK7 prism (c.f. schematic on right). The dissipated power is plotted as a function of the normalized in-plane momentum at 675 nm (c.f. Fig. 2 for Lumogen Red emission spectrum). The black curve and red curve correspond to perpendicular and parallel dipole orientation (c.f. schematic), respectively. The peak in the grey colored region in momentum space corresponds to surface plasmon polaritons excited at the silver/polymer interface, which can be coupled to the prism. The higher-momentum peak corresponds to SPPs at the silver/prism interface, which are dissipated as heat. k0 is light momentum in vacuum. The dissipated power density spectrum was obtained using a silver electric permittivity εAg = -20.5 + 0.8i and a Lumogen Red polymer refractive index of 1.86.
Two distinct peaks in the region where the normalized in-plane momentum exceeds 1, correspond to SPP modes. Of the two SPP modes, only the SPP mode in the grey window, which is a leaky mode localised at the silver/polymer interface, can be coupled through the thin metal film to a propagating field. This is in essence the process that has been termed SPCE [13–16], and results from a competition between the SPP field generation and the coupling process, with a strong dependence on geometrical parameters, such as metal film thickness [20].
3. Measurement configuration
The experimental set-up used to measure the angle-dependent emission properties from a prism-based structure is shown in Fig. 2(a) . The layer structure was fabricated directly onto a BK7 prism. Each sample comprised a thin silver layer that was thermally evaporated directly onto the BK7 prism. The light-emitting polymer material was then spin-coated onto the silver layer to a thickness of nominally 38 nm for all structures studied. All fabrication stages were carried out inside a glove-box under a nitrogen atmosphere (actively purged of oxygen and water) with additional structures, on flat BK7 substrates, fabricated at the same time to confirm thicknesses. To record the angular radiation profiles from the structures, a rotating detection unit was used to collect emission from both the prism side (0°~90°) and the air-side (90°~180°); the prism being fixed at the axis of rotation. At the heart of the detection unit lay either a silicon photodiode or a fibre-coupled spectrometer, selected according to the nature of the measurement. A set of additional elements were also positioned in the rotation unit and comprised a polarizer and a long-pass filter to block any direct radiation from the pump beam and a 10 nm band-pass filter centered around the peak of the polymer emission. The polymer film was excited, at normal incidence from the air-side, by a solid-state laser diode operating at 532 nm. Standard lock-in techniques were used to improve the signal to noise ratio in the measurement, with the signals driving the (modulated) pump laser and those from the corresponding modulated fluorescence both fed into a lock-in amplifier.
Fig. 2 (a) Schematic of the prism structure and configuration for obtaining the angular distribution of the fluorescence from the polymer (Lumogen Red) layer. (b) Normalised fluorescence spectrum of the Lumogen Red polymer (red line). The spectral width of the bandpass filter is marked by the blue hatching and labeled BPF. The chemical structure of the thiophene-benzothiadiazole-thiophene (TBT) red-emitting chromophore unit of Lumogen Red - present at a 5% fraction in the polymer - is also shown above the spectral data.
The fluorescence spectrum from the light-emitting polymer (Lumogen Red) is shown in Fig. 2(b). As is typical of many conjugated polymers the fluorescence has a wide spectral bandwidth that in this instance is around 100 nm full width at half maximum (FWHM), and acts as a useful local source with which to probe the SPP dispersion. To examine the effect of the spectral bandwidth on the angular distribution of SPCE, a bandpass filter (FWHM = 10 nm) was also used in front of the photodiode (c.f. the blue region labeled BPF in Fig. 2(b)).
4. Modeling of angular emission distribution
Numerous approaches for describing the angular distribution of emission in multi-layer structures have been proposed and employed over the years, see e.g [35–37]. We implemented a technique, successfully used for previous SPCE investigations by Luan et al [21], based on the Lorentz reciprocity theory [35]. The basic approach assumes that the normalized angular emission distribution, from a dipole within a multi-layer, can be calculated from the incoming fields, propagating from infinity and experiencing interference within the multiple layers. For each polarization, the resultant field at the dipole position,, may be found and a normalized emitted power distribution may be obtained as follows,
The dipole orientation, perpendicular and parallel to the interface, is denoted by the subscripts. Since a realistic emitter has a finite emission bandwidth the effects of spectral bandwidth on the overall emission distribution need to be considered. In view of this we describe an overall power distribution as comprising all spectral components within a specified bandwidth, i.e.where is a calculated angular emission distribution at a specific λ, is a weighting factor reflecting a normalized fluorescence spectrum of a molecule, and is the responsivity of the detector that may also be wavelength dependent. Radiation patterns of a dipole are analyzed for transverse magnetic (TM) and transverse electric (TE) polarization, where TM polarized light is defined by its electric field being parallel to the propagation plane (x-z plane as depicted in Fig. 2(a)). For the case of a dipole oriented parallel to the interface, both TM and TE polarized radiation can be excited.In Fig. 3 we show the calculated angular radiation distributions for both TE- and TM-polarizations along with the calculated angular reflectivity curve for a structure comprising a 40 nm silver film and 38 nm layer of polymer material; the calculations shown here are only for a single wavelength (675 nm). These spectra exemplify the emission properties we have access to in the later experimental studies and highlight the rather unique characteristics the SPCE process yields; Strongly polarized and directed emission is clearly seen for observation from the prism side, matched in angle to the characteristic reflectivity response associated with excitation of a SPP.
Fig. 3 (a) Calculated TM-polarized angular reflectivity curve and the TM-polarised angular radiation patterns for a wavelength of 675 nm. The structure modeled comprises a 40 nm thick silver film in the geometry depicted in Fig. 2. (b) The radiation profiles for both TE- and TM-polarized emission from the structure, displayed in a polar diagram. Both the reflectivity curve and radiation curves were calculated using a silver electric permittivity εAg = −20.5 + 0.8i and a Lumogen Red polymer index of 1.86.
5. Surface plasmon coupled emission
5.1 Directional emission
The expected angular radiation profile, shown in Fig. 3, clearly illustrates the unique angular emission properties characteristic of SPCE phenomena, particularly the directed TM-polarised emission exiting the prism side of the structure. Our experimental results, collecting the radiation over the full spectral range of Lumogen Red emission with a Si-photodiode, are shown in Fig. 4 and found to be entirely consistent with the expectations of SPCE. Specifically both Figs. 4(a) and 4(b) show virtually complete suppression of any TE-polarised emission from the prism side, in contrast to the TM-polarised emission that clearly shows the increased out-coupling due to SPPs supported within each structure. From the air side (i.e. 90° to 180°) both TE- and TM-components are present and broadly follow a Lambertian (i.e. radially uniform) distribution. In Fig. 4(d) the data for the 53 nm Ag film is displayed in a polar diagram and perhaps more visually confirms the unique angular emission profile arising from SPCE. As with earlier studies [13,17–23], the results present a clear example of SPP-mediated energy-transfer from an active source (polymer film) and effectively through a metal film to an overlying higher index medium. One can speculate, as others have commented [20–22], that such directed and polarized emission may find use in several applications, not least as a basic out-coupling arrangement. In view of this it is useful to question how efficient the out-coupling process is, and how it depends on (say) the metal thickness? To address this, one option is to simply find the ratio of emission component in the prism side compared to the total emission into both the prism and air (rprism); this quantity has been recorded for a series of structures where the silver film thickness has been varied from 30 - 75 nm and the results are shown in Fig. 4(c). We find for silver film thicknesses up to 45 nm, at least half the emitted radiation is coupled into the prism side, this fraction declines somewhat as the silver thickness increases above 45 nm. These observations are well described by the method outlined in the previous section and represented here by the solid curve in Fig. 4(c). Indeed the trend of rprism with silver film thickness closely follows the shape of the integrated power into the prism whereas the total power emitted to the air side remains largely constant as the silver thickness increases.
Fig. 4 Measured angular emission profiles for both TE- and TM-polarized emission for (a) 30 nm and (b) 53 nm thick silver films; the out-coupling angle range of 30° to 90° and 90° to 165° corresponds to prism and air-side, respectively. (c) The fraction of integrated intensity emitted to the prism side rprism as a function of silver thickness. Filled squares are experimental data and line is calculated. (d) The experimental angular spectra data from (b) plotted as a polar diagram.
The reduced SPCE with silver thickness may also be understood from the observations of coupling to SPPs using conventional prism-coupling techniques (see e.g [38].) where coupling to the SPP from the prism side reduces with increasing metal thickness from the optimum thickness; in the case of SPCE we are operating in reverse i.e. coupling to the prism from the SPP. To further emphasize the link we note that for a silver thickness of 45 nm, the minima in the reflectivity spectra (similar to that shown in Fig. 3(a)), approaches zero indicating optimum coupling to the SPP [38]. This same silver thickness is very similar to the value found when rprism falls below 0.5 in Fig. 4(c). The connection with the more familiar prism-coupling view appears to be quite useful when considering the effect of losses on the out-coupling SPCE performance. For example if we reduce the imaginary part of the silver permittivity by (say) one-half, the required thickness of the (modified) silver film to achieve critical-coupling now increases to around 60 nm. Using the same modified permittivity and repeating the dipole emission calculations, we find a similar curve to that shown in Fig. 4(c) although now shifted to the right to indicate rprism falling below 0.5 for a thickness of around 60 nm. Other SPCE properties are also modified by the (somewhat artificially) reduced loss, e.g. the FWHM of the angular radiation profile reduces by around 10% for all thicknesses examined (c.f. Fig. 7(b)).
5.2 Spectral properties: SPP Dispersion
Our structures in Fig. 2(a) are based around planar layers and as such support a continuous band of SPP modes that spans the bandwidth of the polymer emission. The results presented in Fig. 4, carried out using a Si-photodiode, thus record an integrated SPCE process that comprises a series of different SPP contributions spectrally weighted by the underlying intensity of the polymer emission. Replacing the Si-photodiode with a fibre-fed spectrograph and charge coupled device (CCD) spectrometer, enables us to record spectrally resolved angular radiation and thereby reveal the underlying individual spectral contributions (Fig. 5 ). In essence, using the broad emission from the polymer as a local source we may probe the SPP dispersion that is chiefly responsible for the SPCE (Fig. 6 ). Data were recorded for a number of structures with different silver thicknesses. A typical set of spectra, for dAg = 63 nm, are displayed in Fig. 5. The peak response observed in each spectrum indicates the optimum coupling wavelength (momentum) for SPCE at each selected collection angle. Superimposing the spectra taken for the different observation angles, the envelope of the peak responses is seen to largely follow the underlying intensity distribution from the polymer emission (cf. Fig. 2(b)).
Fig. 5 Measured emission spectra at different out-coupling angles in the range 48.3° (black solid line) to 62.7° (blue solid line) for dAg = 63 nm. The spectra are shown for 62.7°, 61.2°, 59.6°, 58°, 56.4°, 54.8°, 53.2°, 51.6°, 49.9° and 48.3° (from left to right). The 56.4° spectrum is highlighted in red.
Fig. 6 Wave vector (real part) dispersions for leaky (red line) and bound (blue) modes for dAg = 63 nm. Transfer matrix calculations (solid lines) are plotted together with experimental data (symbols). The light lines in vacuo (labeled c, black line) and in the prism (labeled c/np, grey line) are also shown. The schematics in the lower half of the figure show the transverse field patterns for the leaky and bound modes (the arrows indicate transverse energy flow).
The connection between the spectrally resolved profiles shown in Fig. 5 and the underlying SPP modal structure may be further appreciated through mapping to the SPP dispersion diagram for the structure. As anticipated in the discussions around Fig. 1, while two SPPs are supported and able to couple with the emitting dipoles in the polymer layer, only one responsible for the strongly polarized and directional emission is associated with the SPCE process, i.e. the leaky-mode appearing in the shaded region in Fig. 1. Computing the dispersion diagram for both modes, i.e. the leaky- and bound-modes [39,40], and transferring the wavelength at which the peak response occurs for each angular spectrum to this diagram, using the corresponding wave vector, where εp is the prism material (BK7 glass) permittivity, it becomes immediately clear that the leaky-mode is responsible for SPCE (Fig. 6). Overlaying the results on the dispersion curves also clearly shows why the peak of the spectral response occurs at shorter wavelengths for larger observation angles (c.f. Fig. 5). A larger wavevector component (larger angle) is required to excite the higher energy (shorter wavelength) SPPs.
5.3 Sensitivity to the emission spectrum
We now examine in greater detail some of the properties of SPCE that may be useful for application. Particular attention is given to the spectral bandwidth of the local source and how this may influence the search for optimum performance using SPCE extraction. In Fig. 7(a) we show the measured SPCE angular radiation profiles along with the reflectivity spectra recorded at 675 nm, for a structure containing a 37 nm thick silver film; the two radiation profiles shown are recorded in the absence (red trace) and presence (blue trace) of a 10 nm bandpass filter in front of the detector. As expected from the earlier results in Figs. 5 and 6, there is a broader angular profile when the emission is unfiltered. This arises because more leaky-mode SPPs contribute to the SPCE signal if a larger range of wavelengths (wave vectors) is collected. In this respect filtering the output using a bandpass filter also mimics the performance that would be expected from an emissive layer with a narrower spectral content. The directional quality of the SPCE emission, i.e. the FWHM in degrees, is therefore firmly linked to the spectral content of the active layer. Another contributing factor, albeit to a lesser extent, is the thickness of the silver film. Figure 7(b) tracks the FWHM (°) of the angular emission profile as a function of the silver film thickness. Regardless of the spectral content, structures with thinner silver films tend to exhibit broader angular emission profiles. This observation appears consistent with the trend found in conventional SPP prism-coupling studies where the resonance width of reflectivity spectra appears broader for thinner films, behaviour often rationalised in terms of a greater radiation loss, see e.g [41].
Fig. 7 (a) Experimental angular reflectivity (black dashed line, 675 nm light) and prism side radiation profiles (red and blue lines) for a 37 nm thick silver film. The red and blue lines are angular radiation distributions for the unfiltered Lumogen Red fluorescence spectrum and for the 10 nm band-pass filtered emission as shown in Fig. 2(b). (b) Angular full-width half-maximum values as a function of silver film thickness. The red square and blue circle data points are experimental data in the absence and presence, respectively, of the band-pass filter. The black and grey solid lines are corresponding results from modeling.
As a potentially useful out-coupling element it is instructive to examine the dependence of the SPCE peak emission intensity on silver film thickness, again particularly from the point of view of the spectral content of the polymer emission. Figure 8 shows this dependence, normalized to the maximum measured intensity. With the 10 nm bandpass filter in place, the SPCE has maximum intensity at around 50 nm silver thickness, a value that is close to that required to maximally couple incident radiation to SPPs in conventional prism-coupling experiments [38]. When, however, the full (unfiltered) polymer spectral bandwidth is considered, the maximum peak intensity now appears for a slightly thinner silver thickness (~40 nm). In both cases, away from the optimized thickness, intensity drops, reflecting less efficient SPP coupling. This again mirrors the situation for the reflectivity minimum observed in SPP prism coupling. Modeling (solid lines in Fig. 8) faithfully reproduces the experimental observations.
Fig. 8 Silver film thickness dependence of the prism side emission peak intensity. The two cases shown are without (upper plot, red square data) and with (lower plot, blue circle data) 10 nmband-pass filtering. The solid lines are the calculated dependences.
6. Conclusion
In conclusion, we have experimentally investigated the SPCE process using conjugated polymer films as emission layers deposited atop silver coated glass prisms. Our clear demonstration of SPCE for these layers allowed us to explore the sensitivity of the SPCE process to the parameters of the structure (silver film thickness) and to the bandwidth of the emission layer (10 nm bandpass filtering versus full polymer bandwidth). Overall SPCE provides a good platform to better understand SPP-mediated energy-transfer mechanisms and thereby can help to identify optimal geometries for a range of out-coupling and fluorescence sensing applications.
Acknowledgments
This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC) through grants EP/C539508, EP/C539494 and EP/H000917. The authors would also like to thank the Sumitomo Chemical Company, Ltd for providing the Lumogen Red polymer used in this work. DDCB is Lee-Lucas Professor of Experimental Physics.
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