Thin films containing light emitters act as light-to-light converters that absorb the incident light and emit luminescence. This well-known phenomenon is photoluminescence (PL). When a photoluminescent film is notably thinner than the absorption length of emitters, it exhibits weak absorption of incident light. The absorption can be increased by depositing the thin film on a plasmonic array of metallic nanocylinders arranged with a specific periodicity. The array couples the incident light into the thin film, facilitating the plasmon-enhanced absorption by the emitters in the film. In this study, we demonstrate both experimentally and numerically the plasmon-enhanced absorption of a rhodamine 6G-containing film that is thinner than its absorption length using a periodic array of Al nanocylinders. The experimental results demonstrate that the spectrally integrated PL intensity is increased up to 3.78 times. In addition to enhanced absorption, the array is also found to diffract the PL into a direction determined by the periodicity, thereby facilitating the multiplied enhancement of PL. The combination of the two factors yields a PL intensity enhanced up to 10 times at a specific angle and wavelength. Numerical simulations combining the carrier kinetics with full-wave electromagnetics in the time-domain support the experimental observations.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Interaction of quantum emitters with metallic nanostructures has been a key topic in plasmonics [1–7]. The effect of a metallic nanostructure on photoluminescence (PL) is best illustrated by the three optical parameters: absorption, quantum yield (ratio between the radiative and non-radiative decay rates), and outcoupling [6,8–11]. All the preceding works follow this scheme, although it is not often described explicitly. The metallic nanostructures reported in the previous works include optical bow-ties [12–14], Yagi-Uda antennas [15,16], plasmonic metasurfaces [17–20], and plasmonic arrays of nanoholes [21–26] /nanocylinders [27–30]. Among them, plasmonic nanocylinder arrays refer to the metallic nanocylinders arranged periodically with lattice constants on the wavelength scale. In such arrays, the plasmonic-photonic hybrid modes, i.e., lattice plasmons, represented by the coexistence of both localized surface plasmon polariton (LSPP) and diffraction modes [31–35] facilitate coherent and strong interaction between the incident light and the array. The array was used in conjunction with an emitter film for enhanced PL intensity . The main contribution of the plasmonic array to PL enhancement originates from outcoupling whereby a fraction of PL that would otherwise be confined within the emitter film in the absence of the array is scattered out of the film into a specific direction predefined by the periodicity. As the array can enhance the outcoupling of the generated PL towards free space (light extraction), plasmonic arrays are especially useful to enhance the PL of high quantum yields emitters [36–39].
In this study, we examine the absorption enhancement contributed by the plasmonic array. The effect of absorption enhancement is especially critical for the films thinner than the absorption length (i.e., the distance over which light propagates before its intensity becomes 1/e of its original value). In such thinner films, the plasmonic array can act as an absorption enhancer: It diffracts the excitation light into the plane of the array thereby enlarging the effective thickness of the film and increasing the absorption. The absorption enhancement is very useful for efficient light-to-light conversion in solar luminescent converters [40,41] and light-to-carrier conversion in solar cells [42–44]. In our experiments, we deposited a poly(methyl methacrylate) (PMMA) film containing rhodamine 6G (R6G) dye, referred to as PMMA + R6G film, which was thinner than its absorption length on a plasmonic array. R6G was particularly chosen because of its high quantum yield and broad excitation band in the visible range that allowed us to examine the effect of absorption enhancement on PL using a range of excitation wavelength. Furthermore, we validated the experimental observations with multiphysics numerical simulations coupling the carrier kinetics of quantum emitters with full-wave electromagnetics in the time-domain. Specifically, the rate equations of carrier kinetics (derived from a density matrix formalism) are combined with a classical Maxwell solver in a joint finite-difference time-domain (FDTD) multiphysics framework using the auxiliary differential equations (ADE) approach [45–47]. The correlation between absorption enhancement and PL intensity enhancement was clarified both by the experiment and the simulation.
2.1 Sample preparation
We prepared a square array of Al nanocylinders with a period of a = 400 nm that sustained hybrid modes in the visible range of wavelength. This array was designed to show hybrid modes at the spectral ranges of both excitation and emission of R6G, as will be experimentally shown in section 4.1. Al thin films were prepared on SiO2 glass substrates using a sputtering method. Al nanocylinders were fabricated using nanoimprint lithography in conjunction with reactive ion etching (RIE). The procedure is described in detail as follows. First, a resist was deposited on the Al thin film. Then, the surface of the resist was nanostructured using nanoimprint techniques (EntreTM3, Obducat) replicating the surface morphology of an Si mold. The Al thin film with a patterned resist on the top was then structured by RIE (RIE-101iPH, Samco). The array structures were examined using a scanning electron microscopy (SU8000, Hitachi).
A PMMA + R6G film containing 0.8 wt% R6G was spin-coated on the array. The thickness of the film was estimated with a surface stylus profiler (ASIQ KLA Tencor). The dielectric function of the thin film was examined using spectroscopic ellipsometry (FE-5000, Otsuka Electronics Co.) over a wavelength range of 300~800 nm (see Fig. 1).
2.2 Optical characterization
Zeroth-order optical transmission spectra of the samples were measured as functions of the angle of incidence θin. For the sake of measurement, we used an s-polarized collimated beam from a halogen lamp with a beam diameter of ~0.5 mm. The sample was mounted on a computer-controlled rotation stage. The absolute zeroth-order transmission (T) was obtained by normalizing the transmission of incident light through the sample to that through a glass substrate. The specular reflection (R) was measured at θin = 5°, and the absorptance (A) at θin = 5° was deduced from the relation: A = 1 – T – R.
The PL spectra were measured by illuminating the sample with a laser diode (LD). Three different LDs with λex = 445, 473, and 532 nm were used. The LD output (s-polarized) was incident from the substrate side at θin = 5° unless otherwise noted, and the PL spectra were collected from the opposite side by a fiber-coupled spectrometer at an angle θem from normal to the sample surface. A film of PMMA + R6G prepared on a flat SiO2 glass substrate without the Al nanocylinder array was used as a reference to evaluate the PL enhancement. The PL decay at λ = 570 nm was measured using a time-correlated single-photon counting module (Quantaurus-Tau, Hamamatsu Photonics) equipped with a pulsed light-emitting diode (LED) (temporal resolution of < 1 ns) with λex = 470 nm.
3. Numerical simulations
We used a semi-classical model in which the dye molecules were treated quantum-mechanically, and the electromagnetic wave was treated classically in the time domain [42–44]. The unit cell of the model is shown in Fig. 2(a). The top panel shows the top view of the structure. The nanocylinder has a diameter of 150 nm and a height of 150 nm. The array has a 400 nm period in both x and y directions. Periodic boundary conditions are applied in the lateral directions to simulate the periodicity of the array. The bottom panel shows the cross-section of the structure consisting of an SiO2 glass substrate and an Al nanocylinder covered with a 700-nm-thick PMMA + R6G film. The thickness obtained from the surface stylus profiler measurement varied from place to place on the film, and we used a representative value of 700 nm. We used the value of dispersive permittivity of Al taken from ellipsometry measurements. A constant refractive index of 1.45 was used for SiO2 glass and 1.00 for air. The incident light was linearly-polarized with the electric field parallel to the y-direction, while θin was defined in the z-x plane. The absorptance A was simulated by using the calculated values of the transmittance T and reflectance R in the relation A = 1– T– R. For the calculation of A, the PMMA + R6G film was treated as an effective medium with a dispersive permittivity whose value was taken from the fit to the ellipsometry measurement data (see Fig. 1). We simulated the absorption spectra at θin = 5° for the PMMA + R6G films on the Al nanocylinder array and that on the flat glass substrate.
Since spontaneous emission is a stochastic process, a direct implementation of it by a time-domain modeling approach is associated with a significant computational load. To simplify the complexity of the problem and to overcome the computational cost, we phenomenologically incorporated the spontaneous emission into our FDTD solvers by introducing randomly-distributed and randomly-oriented classical quantum emitters (electric dipoles) with a Lorentzian dispersion centered at an emission frequency .
In the model, shown in Fig. 2(b), we assumed a four-level scheme of R6G, with the pumping transition terminating at a quasi-stable short-lived state above the metastable state . The rate equations are as follows:
Positions of dipoles, orientation, as well as their polarization were randomly chosen during simulation. In the excitation process, the active medium was pumped from the substrate side with an angle of 5° in the z-x plane, and the emission was collected from the other side of the sample. Three pump wavelengths were used for the excitation source: λex = 445, 473, and 532 nm. The random dipoles were used to stimulate the radiation emission with a wavelength centered at 589 nm. Numerous simulations were carried out each time with different dipole properties, and all the results were averaged. That randomization and averaging were used to eliminate the coherence in the numerical model of spontaneous emission. The main parameters of numerical simulations are outlined in Table 1.
4. Results and discussion
4.1 Optical absorption
Figure 3(a) shows the SEM image of the array used. The Al nanocylinders (150 nm in diameter) are arranged in a square lattice with a period of a = 400 nm. The SiO2 glass substrate is flat, and the horizontal bright and dark lines are due to the charge build-up of the insulating glass substrate.
Figures 3(b) and 3(c) show the optical transmittance (T) as a function of θin for the array before and after the deposition of PMMA + R6G film. Also shown are the conditions for the in-plane diffraction. The in-plane diffraction condition is referred to as Rayleigh anomaly, which satisfies the following relation for a square lattice when the light is incident parallel to the x-direction ,Fig. 3(c)) because of the mitigation of refractive index mismatch between the up- and substrates of the array by the film deposition. Figure 3(c) also shows several features that are unassignable to the in-plane diffractions. They are most probably due to excitation of quasi-waveguide modes: the PMMA + R6G thin film acts as a slab resonator to trap the incident light. Figure 3(d) compares T at θin = 0°. For the array before the film deposition (top panel), asymmetric shape of the dip with a steep edge at λ = 580 nm clearly shows that the LSPP of Al nanocylinders is modulated by the in-plane diffraction. After the deposition (bottom), the main dip redshifts to λ = 630 nm because of the increase in the refractive index of the medium around Al nanocylinders. At shorter wavelengths, several features newly appear. Absorption of R6G should appear at around λ = 532 nm, but it is weaker compared to the other resonances associated with the array (see Fig. 4 for detail) and is indistinguishable to the other resonances.
Figure 4(a) shows absorptance A at the angle of incidence θin = 5°, obtained from the relation A = 1 – T – R, where T and R represent transmittance and reflectance, respectively. The spectrum for the PMMA + R6G film on a flat glass (AR6G, represented by a grey area) shows the absorption peak at λ = 530 nm, which is typical of R6G molecules. Using the AR6G and the obtained film thickness (t = 700 nm), the absorption length (ℓa) is calculated using the relation, 1 – A = exp(–t/ℓa). The values of ℓa are larger than the film thickness at the three excitation wavelengths used in this study: As listed in Table 2, we obtained ℓa = 2690, 2690, and 2090 nm for λ = 445, 473, and 532 nm, respectively. Thus, the absorption is found to be moderate indicating the scope for further enhancement. The absorption of the film deposited on the array of Al nanocylinders (Aarray + R6G) is obviously larger than AR6G. The spectral shape appears irregular owing to the excitations of LSPP and light diffraction of different orders in the array. Here it is noted that the array sample shows light scattering and A = 1 – T – R does not hold in a rigorous sense for the specular components measured in this experiment. Here we used the term A for the sake of simplicity.
Figure 4(b) shows the enhancement of absorptance by the array, defined as Aarray + R6G/AR6G. Aarray + R6G/AR6G is found to be 2.32, 1.81, and 1.04 for λ = 445, 473, and 532 nm, respectively. It is noted that at θin = 5° and λ = 532 nm, there is nearly no change in absorption as inferred from the value of Aarray + R6G/AR6G ~1. The increase in Aarray + R6G compared to AR6G suggests that a part of the incident light, which would have otherwise transmitted through the film, is scattered by the array within the PMMA + R6G film and eventually absorbed by the R6G dye molecules and/or the Al nanocylinders.
4.2 PL enhancement
Figure 5 summarizes the PL properties of the PMMA + R6G film on the array under the excitation with three LDs of different wavelengths: λex = 445, 473, and 532 nm. Figure 5(a) shows the PL enhancement, defined as the PL from the PMMA + R6G film on the array (Iarray + R6G) normalized to that from the reference film on a flat substrate (IR6G), i.e., Iarray + R6G/IR6G, as a function of emission angle, θem (see the inset for the definition). Also shown are the in-plane diffraction conditions as in Figs. 3(b) and 3(c). At θem = 0°, notable enhancement is observed, which blueshifts with the increase in θem following the (0, ± 1) diffraction order. Another line of enhancement along the (−1,0) order is also notable. This similarity in angular profiles between the PL enhancement and the diffraction indicates the outcoupling effect.
Figure 5(b) compares the PL spectra at θem = 0°. The dotted lines and the grey area represent Iarray + R6G and IR6G, respectively. The reference shows a broad PL spectrum centered at λ = 563 nm, which is typical of R6G. In addition to the peak of this broad spectra, there are extra PL peaks observed at approximately λ = 576 and 585 nm for Iarray + R6G. It is noted that these peaks are associated to diffraction; the (−1,0), (0, ± 1), and (1,0) diffraction orders are degenerate at λ = 580 nm and θem = 0°.
In order to confirm the outcoupling effect, we plot Iarray + R6G/IR6G at θem = 0° and T at θin = 0° (Fig. 5(c)). The spectral positions of the dips in T (at λ = 572 and 585 nm) correspond to the extra peaks in Iarray + R6G/IR6G thereby clearly indicating the outcoupling effect: The dips in T mean that the light is coupled inside the thin film for the specific condition of incidence. Conversely, when the PL is generated inside the film, a fraction of the PL that possesses the corresponding λ is coupled outside the film at the corresponding direction, i.e., the array acts as a spatial and spectral filter for PL.
Although the spectral shapes are similar, the degree of enhancement depends notably on λex. While the largest value of Iarray + R6G/IR6G, as large as 10 times at λ = 585 nm (see Fig. 5(c)), is found for λex = 445 nm, the smallest value is found for λex = 532 nm. This variation comes from the absorption enhancement at each λex. Figure 5(d) compares the Iarray + R6G/IR6G averaged over λ = 545 to 650 nm and the Aarray + R6G/AR6G at the three λex. It is found that Iarray + R6G/IR6G remarkably scales with Aarray + R6G/AR6G. As it is also inferred from Fig. 4(b) that Aarray + R6G/AR6G = 1.04 (close to unity) for λex = 532 nm, it is evident that the PL enhancement observed for this wavelength is not owing to the absorption enhancement but owing to the outcoupling aspect. For λex = 445 and 473 nm, the integrated values of Iarray + R6G/IR6G are 3.78 and 3.20 times, respectively, implying that the enhancement originated from both outcoupling and absorption. As the outcoupling depends only on the PL process and does not depend on the excitation conditions, the difference in the values of Iarray + R6G/IR6G between the three conditions is attributed to absorption enhancement. Compared to λex = 532 nm where Iarray + R6G/IR6G = 1.39 and absorption enhancement is negligibly small as explained above, the Iarray + R6G/IR6G for λex = 445 nm (473 nm) is 2.72 (2.30) times larger while Aarray + R6G/AR6G being larger by 2.23 (1.74) times. The slight differences between Iarray + R6G/IR6G and Aarray + R6G/AR6G indicate that the fraction of Aarray + R6G absorbed by R6G dyes varies with λex; not only the R6G dye molecules, but the metallic nanocylinders can also absorb.
We investigated the other enhancement factor, quantum yield, using PL decay measurement. Figure 6 shows the PL decay curves excited with a pulsed LED with λex = 470 nm for the PMMA + R6G films deposited on the Al nanocylinder array (red line), and on the glass substrate (black line) as a reference. The PL lifetimes (τ) for the film on the array and reference are τarray + R6G = 2.15 ns and τR6G = 2.34 ns, respectively. We speculated the decrease in τarray + R6G equally occurs for all the three excitation wavelengths used, because the PL lifetime only depends on the density of optical states in the initial and final states of the transition and is not dependent on the excitation wavelength. The decrease in τ is only 8%, which is small with respect to the spectrally-integrated PL intensity enhancement up to 3.78 times. We speculated that the quantum yield modification plays a minor contribution to the PL enhancement in the present system.
4.3 Comparison of experimental results with numerical simulations
Figure 7 compares the simulated absorption spectra for the PMMA + R6G films on the Al array (Aarray + R6G) and on the flat substrate (AR6G), where the dispersive permittivity of PMMA + R6G film was taken from measurement data of spectroscopic ellipsometry (see Fig. 1). Absorption of the PMMA + R6G film on the flat substrate, AR6G, shows an absorption band centered at λ = 530 nm, which is typical for the R6G dye. Unlike the experimental spectrum (Fig. 4), the simulated absorption spectrum for the case of the flat substrate is found to exhibit a fringe-like pattern indicating the effect of Fresnel reflectance at the surfaces. This fringe is not observed in the experiment owing to the surface corrugation. In contrast, Aarray + R6G exhibits a considerable increase in the magnitude and also the additional peaks. This simulation qualitatively reproduces the experimental absorption spectra in Fig. 4(a), including the spectral positions of the in-plane diffractions; the discrepancy may be attributable to the film surface morphology. It is also noted that the simulation contains both specular and diffuse components, while the experiment contains specular component only. This difference also causes the discrepancy. A comparison between the curves of Aarray + R6G and AR6G clearly manifests the increased absorption by the R6G molecules attributable to the functioning of the PMMA + R6G film as a slab waveguide that receives the incident light scattered by the Al nanocylinder array. At each excitation wavelength, we calculated the simulated absorption enhancement Aarray + R6G/AR6G that was found to be 8.1, 18.7, and 2.6 at λ = 445, 473, and 532 nm, respectively.
Figure 8 shows the simulated PL spectra excited at the three different wavelengths. The solid line and the grey area represent the PL from the PMMA + R6G film on the array (Iarray + R6G) and the reference film on the flat substrate (IR6G), respectively. The reference shows a broad PL spectrum centered at λ = 583 ~596 nm, which is typical of R6G. In addition to this broad peak, extra PL peaks at approximately λ = 600 ~612 nm are observed for the spectra of PMMA + R6G film on the arrays. The PL spectra reproduce the experimental results qualitatively although the simulated PL shows a single additional peak at λ = 600 ~612 nm and the experimental PL shows a main peak at λ = 585 nm with a subpeak at λ = 576 nm. The discrepancy in spectral positions, as well as the number of the peaks, may be attributable to the fitting parameters adopted, including λ21, the number density of the dipole and the geometrical parameters (dimensions) of the Al nanocylinders. A better fit could be achieved by finely tuning these parameters. We were not keen on further optimization because the simulated results already reproduced the optical phenomenon in line with the theoretical expectations. We evaluated the PL intensity enhancement Iarray + R6G/IR6G averaged over λ = 545 to 650 nm from the spectra in Fig. 8, and it was found to be 2.7, 2.8, and 2.2 times for λex = 445, 473 and 532 nm, respectively.
Figure 9 compares the absorption enhancement and the PL enhancement. Except for the difference in the order of the simulated and the experimental results (see Fig. 5(d)) of PL enhancement which is attributed to the difference in the surface flatness between the sample and the model as explained above, the correlation between the absorption and the PL enhancements is found to be qualitatively reproduced. The discrepancy in the magnitude of Aarray + R6G/AR6G between the experiment and the simulation also comes from the surface corrugation. The flat surface of the simulated structure results in the Fresnel-like spectral fringe for the reference film that underestimate AR6G, which results in the larger Aarray + R6G/AR6G for the simulation. The simulation confirms that the variation of PL enhancement with the excitation wavelength originates from the dependency of the absorption enhancement at the wavelength.
In summary, we evaluated the absorption enhancement of the light emitter films thinner than the absorption length deposited on a plasmonic array of Al nanocylinders, both experimentally and numerically. For this purpose, a PMMA + R6G film was deposited on the square array of Al nanocylinders with a 400 nm periodicity. The array was designed to support hybrid modes in a spectral range that covered the excitation and emission wavelength of R6G, thus; it can influence both processes via absorption enhancement and outcoupling, respectively. The PL enhancement at the three excitation wavelengths (λex = 445, 473, and 532 nm) was found to scale with the absorption enhancement. The results helped to establish that the array scattered the excitation light that was eventually reabsorbed by the R6G molecules in the film, thereby enhancing the PL intensity. The comparison to other materials, including plasmonic Ag and nonplasmonic TiO2 and Si, would give further insight into the effect of LSPPs on the absorption enhancement. Our study also brings out the importance of quantum emitters with a small Stokes shift, such as the R6G dye used in this work, as a key utility in controlling both the plasmon-enhanced absorption and PL outcoupling by a single array.
Grant-in-Aid for Scientific Research (B, 16H04217 and Exploratory, 17K19176) by MEXT; Precursory Research for Embryonic Science and Technology (PRESTO, JPMJPR131B) from JST; the Asahi Glass Foundation; Nippon Sheet Glass Foundation for Materials Science and Engineering; Nanotech CUPAL (SM and SI); Defense Advanced Research Projects Agency (DARPA/DSO) Extreme Optics and Imaging (EXTREME) Program, (HR00111720032, A.V.K and S.I.A).
A part of this work was supported by Kyoto University Nano Technology Hub and NIMS Nanofabrication Platform in the “Nanotechnology Platform Project”, Japan.
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