A plasmonic array, consisting of metallic nanocylinders periodically arranged with a pitch comparable to the optical wavelength, is a system in which both the localized surface plasmon polaritons (SPPs) and diffraction in the plane of the array are simultaneously excitable. When combined with a phosphor film, the array acts as a photoluminescence (PL) director and enhancer. Since the array can modify both excitation and emission processes, the overall modification mechanism is generally complex and difficult to understand. Here, we examined the mechanism by simplifying the discussion using an emitter with a high quantum yield, large Stokes shift, and long PL lifetime. Directional PL enhancement as large as five-fold occurred, which is mainly caused by outcoupling, i.e., the PL trapped in the emitter film by total internal reflection is extracted into free space through the SPPs and diffraction. The present scheme is robust and applicable to arbitrary emitters, and it is useful for designing compact and efficient directional illumination devices.

Metallic nanoparticles possess a very large polarizability, which causes strong light–matter interactions. This is due to surface plasmon polaritons (SPPs), i.e., coherent electronic oscillations in metallic nanoparticles driven by an external electromagnetic field.1 If an optical emitter is located in the proximity of a metallic nanoparticle, the radiative and nonradiative decay rates of the emitter are influenced by the presence of the nanoparticle.2–6 Thus, metallic nanoparticles supporting plasmonic resonance can modify the properties of emitters. Spectral shaping and intensity enhancement have been reported for several types of nanostructures, e.g., single nanoparticles,3,4 dimer and bowtie nanoparticles,5–8 one- and two-dimensional gratings,9–12 and periodic arrays of nanoparticles.13–19 

We focus on periodic arrays of metallic nanoparticles with a pitch comparable to the wavelength of light. In these arrays, strong diffraction in the plane of the array, i.e., the Rayleigh anomaly, mediates the radiative coupling between neighboring SPPs to induce collective plasmonic modes.20–24 Collective plasmonic modes can couple to the emission to enhance its intensity and control its directionality. In sharp contrast to most SPP-coupled emission enhancements, which are effective only for poor emitters with low quantum yield (QY) << 1, collective plasmonic modes enhance the emission intensity of good (QY ∼ 1) emitters, such as Ce3+-doped yttrium aluminum garnet15,16 and luminous dyes,14 and are thus applicable to practical illumination sources.

SPP-coupled photoluminescence (PL) enhancement involves three components, i.e., a pump, QY, and outcoupling enhancements, as described by the following relation:13,17

II0=|E(λex)|2|E0(λex)|2η(λem)η0(λem)Cext(λem)Cext0(λem),
(1)

where E (E0) is the local (incident) electric field at the excitation wavelength (λex). η(η0) is the QY for the emission wavelength (λem) with (without) the plasmonic nanostructure. Cext (Cext0) is the outcoupling constant, i.e., the ratio of PL light extracted from the structure to the total emitted light in the system with (without) the plasmonic nanostructure. Outcoupling is important for systems with flat surfaces, in which a fraction of light is internally reflected at the surface and trapped inside the system. The term |E(λex)|2/|E0(λex)|2 represents the pump enhancement or the increase in PL intensity due to the local enhancement of the field at λex. The term η/η0 corresponds to the increase in the local density of optical states (LDOS). Fermi’s golden rule states that the radiative decay rate increases with increasing LDOS to increase QY, which is the Purcell effect. The term Cext/Cext0 represents the increase in light extraction owing to the presence of nanostructures. There is also a negative aspect associated with metallic nanostructures, i.e., energy dissipation to the metal. This additional nonradiative decay channel severely degrades the QY of otherwise good emitters that have no nonradiative decay paths. To elucidate the effect of each term in Eq. (1), the energy difference between λex and λem of the emitter, i.e., the Stokes shift, should be large such that the terms at λem (η/η0 and Cext/Cext0) are examined while avoiding the effect from |E(λex)|2/|E0(λex)|2. Furthermore, the radiative decay rate should have a different time scale to the energy transfer to metal in order to distinguish between the Purcell effect and energy transfer to metal. Preferably, the emitter should also be highly luminous for easy detection.

Tris(hexafluoroacetylacetonato) europium bis(triphenylphosphine oxide), (Eu(hfa)3(TPPO)2), is an emitter that satisfies these requirements; it has a long PL lifetime (τobs=0.93 ms), a large Stokes shift, and is highly emissive (QY = 0.90).25–30 The PL decays slowly because of the forbidden nature of the 4f−4f transition of Eu3+. The distinct feature of this molecule is its very large absorption coefficient. The organic ligands that coordinate to Eu3+ are designed in such a way that they absorb ultraviolet light and transfer the energy to Eu3+. The fluorine in the methyl group reduces the phonon energy to suppress nonradiative multiphonon decay, and asymmetric coordination around Eu3+ facilitates the radiative decay rates of electric-dipole transitions.

In this study, we adopted Eu(hfa)3(TPPO)2 as an emitter to verify each factor contributing to PL enhancement of coupling with collective plasmonic modes. We selected Al as a plasmonic metal. Al, a low-cost material that is abundant on earth, is plasmonic up to the ultraviolet region and shows reasonably good plasmonic properties in the visible range.31 In addition, Al is processable with selective ion etching, which allows highly accurate fabrication of nanostructured arrays. We designed the Al nanocylinder array to sustain the collective modes at the wavelengths of the Eu3+ emission lines. In this manner, we intentionally exclude the effect of pump enhancement (|E(λex)|2/|E0(λex)|2) and focus on the separation of the two remaining factors at λem, i.e., η/η0 and Cext/Cext0. The thin film of Eu(hfa)3(TPPO)2 was prepared by vacuum evaporation onto the Al nanocylinder array. The sample exhibited directional PL enhancement of as large as five fold under the condition that the collective mode was excited. Measurements of PL decay at λem and optical transmission at λex were conducted to examine the contributions of the QY and pump enhancement terms, respectively. The results suggest that outcoupling (Cext/Cext0) is the main factor of the enhancement in the present system.

To fabricate the Al nanocylinder arrays, we used an Al thin film on a silica glass substrate prepared by a sputtering method. Al nanocylinders arranged in a triangle array with pitches a = 350 and 400 nm were fabricated using nanoimprint lithography, and those with a = 480 nm were prepared using surface conformal imprint lithography32 in combination with reactive ion etching (RIE). The procedure is as follows. First, a resist was deposited on the Al thin film. Then, the surface of the resist was nanostructured by nanoimprint techniques, replicating the surface morphology of a Si mold. The sample was then structured by RIE.

The Eu(III)-complex films on the substrate were prepared by vacuum evaporation of Eu(hfa)3(TPPO)2. A powder of Eu(hfa)3(TPPO)2 was set in a holder within a vacuum chamber (Sanyu), and the chamber was evacuated to 4 × 10−4 Pa. The powder was heated to 338 °C to be sublimated and deposited on the Al nanocylinder array. The dielectric function of the thin films was examined by spectroscopic ellipsometry (FE-5000, Otsuka Electronics Co.) over a wavelength range of 300–800 nm.

The zeroth-order optical transmission was measured as a function of the angle of incidence (θin). For the measurement, we used the 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 as a function of wavelength and θin, T(λ,θin), was obtained by normalizing the transmission of incident light through the sample to that through the glass substrate. The transmission in the ultraviolet region was separately measured by the spectrophotometer (V570, Jasco).

PL spectra were measured as a function of the angle normal to the surface (θem). The sample was illuminated from the substrate side with a He-Cd laser (λex=325 nm, s-polarized) at θin=34°. The PL spectra as a function of θem, I(λ,θem), were collected from the opposite side by a fiber-coupled spectrometer mounted on a computer-controlled rotation arm, which could be rotated about the excitation spot. A film of Eu(hfa)3(TPPO)2 on a flat silica glass substrate without the Al nanocylinder array was used as a reference to evaluate the PL enhancement, I/I0. PL decay at λ=617 nm was measured by using a time-correlated single-photon counting module (Quantaurus-Tau, Hamamatsu Photonics) equipped with a pulsed flash lamp (temporal resolution of 0.5μs) with a band pass filter (center wavelength of 320 nm and full width at half maximum of 40 nm). For the calculation of QY, emission spectra were measured by a spectrofluorometer (FluoroLog-3, Horiba) equipped with a photomultiplier tube (R928, Hamamatsu), and used after correcting for the wavelength-dependence of the detection sensitivity.

Figure 1 shows the excitation and PL spectra of the reference Eu(hfa)3(TPPO)2 thin film deposited on the flat silica glass substrate. The excitation spectrum monitored at λ=613 nm exhibits a broad band centered at λ=320 nm, which corresponds to the absorption by the hfa groups coordinating with Eu3+.33 The peaks at λ=268 and 274 nm are the 7F05K6 and 7F05K5 transitions of Eu3+, respectively.34 The excitation band edge is at λ ∼ 350 nm, and no fluorescence is induced by irradiation with visible light. The PL spectra show narrow peaks due to the 4f–4f transitions of Eu3+. Peaks at 590, 617, and 655 nm are assigned to the 5D07F1, –7F2, and –7F3 transitions, respectively, and are further split by the Stark effect. The Stokes shift is as large as 1.6 eV.

FIG. 1.

Excitation (left plot) and PL (right plot) spectrum for the Eu(hfa)3(TPPO)2 thin film. Excitation (PL) was measured by detection at 613 nm (exciting at 330 nm).

FIG. 1.

Excitation (left plot) and PL (right plot) spectrum for the Eu(hfa)3(TPPO)2 thin film. Excitation (PL) was measured by detection at 613 nm (exciting at 330 nm).

Close modal

Figure 2(a) shows T(λ,θin) of the Eu(hfa)3(TPPO)2 thin film on the Al nanocylinder array plotted in a color scale. The inset shows an SEM image of the array, illustrating Al nanocylinders arranged in a triangular lattice with pitch a = 400 nm. The diameter and the height are 130 and 140 nm, respectively. The x-, y-, and z-axes used in this study are also defined. During the measurement of T(λ,θin), θin was varied in the zx plane to vary the wavevector component of the incident light along the x-axis, and the azimuth angle was fixed (see the inset of Fig. 2(a)). At θin=0°, dips in T appear at λ=580 and 560 nm, corresponding to the SPPs in the Al nanocylinders, which are modulated by the diffraction. The collective plasmonic modes are mediated by diffraction in the plane of the array, and therefore, their dispersion follows that of the Rayleigh anomaly. From the conservation of the parallel component of the wavevector at the surface of the array, the Rayleigh anomaly satisfies the relation kout=kin±G, where kout(=2πn/λ) and kin(=2π/λ×sinθin) are the parallel components of the diffracted and incident wavevectors, respectively, and n is the refractive index surrounding the array. We shall refer to the magnitude of k as k. G = (m1b1, m2b2) is a reciprocal lattice vector where

b1=(2π/a)(x^+y^/3),b2=(2π/a)(x^y^/3),
(2)

and m1 and m2 are a pair of integers defining a diffracted order (see the inset in Fig. 2(b)).35 When kin does not have a component in the y direction, kout is expressed as

kout2=kin2+2(2π/a)(m1+m2)kin+(2π/a)2(m1+m2)2+(2π/a)2(m1+m2)2/3.
(3)

In order to understand the dispersion of the modes, we plotted in Fig. 2(b) the extinction (defined as 1 − T) as a function of the photon energy and kin. The bright color in the plot indicates large extinction. The lines in the figure correspond to the (m1, m2) = (1, 0), (−1, 0), and (1, −1) diffraction orders. In this case, m1 and m2 are equivalent and interchangeable in Eq. (3). The solid (dashed) lines are calculated using n = 1.52 (1.46), which is the refractive index of the thin film (silica glass substrate). Two extinction bands following the dispersion of the Rayleigh anomalies appear in Fig. 2(b), one along (−1, 0), and the other along (1, −1). These bands are due to the collective plasmonic modes. It is noted that because of the interference of two modes, i.e., the SPPs and the diffracted light, the collective plasmonic modes do not exactly coincide with the Rayleigh anomalies, but they red-shift slightly.16 The interference of two modes overlapping both spectrally and spatially causes interesting optical phenomena, such as Fano resonance and electromagnetically induced transparency (EIT).15 

FIG. 2.

(a) Zeroth-order transmission T(λ, θin) of the thin film of Eu(hfa)3(TPPO)2 on an Al nanocylinder array (pitch = 400 nm). The left inset is an SEM image of the array (pitch = 400 nm) with the coordination axes used in this study (scale bar = 500 nm). The right inset is the experimental configuration: The incident light is polarized along the y direction, and the incident angle (θin) was varied to give momentum in the x direction. (b) The extinction, defined as 1 − T, as a function of the photon energy and the in-plane wavevector of the incident light, kin∥. The solid and dashed lines indicate the Rayleigh anomalies with n = 1.52 and 1.46, respectively. The inset illustrates a reciprocal lattice with reciprocal unit vectors. (c) Simulated T(λ,θin) using a 3D finite-element method. The inset shows a sketch of the model for simulation. (d) T(λ, 0°) at shorter wavelengths. Solid and dotted lines denote the Eu(hfa)3(TPPO)2 thin film on the array and on the flat silica glass substrate, respectively. (e) Simulated T, reflectance, and absorption as a function of θin for λ=325 nm. (f) The distribution of light energy under the illumination of a plane wave with λ=325 nm at θin=34.0°. We simulated the square magnitude of the electric field normalized to the incident field, |E|2/|E0|2, in the zy plane, at x intersecting the middle of the nanocylinder.

FIG. 2.

(a) Zeroth-order transmission T(λ, θin) of the thin film of Eu(hfa)3(TPPO)2 on an Al nanocylinder array (pitch = 400 nm). The left inset is an SEM image of the array (pitch = 400 nm) with the coordination axes used in this study (scale bar = 500 nm). The right inset is the experimental configuration: The incident light is polarized along the y direction, and the incident angle (θin) was varied to give momentum in the x direction. (b) The extinction, defined as 1 − T, as a function of the photon energy and the in-plane wavevector of the incident light, kin∥. The solid and dashed lines indicate the Rayleigh anomalies with n = 1.52 and 1.46, respectively. The inset illustrates a reciprocal lattice with reciprocal unit vectors. (c) Simulated T(λ,θin) using a 3D finite-element method. The inset shows a sketch of the model for simulation. (d) T(λ, 0°) at shorter wavelengths. Solid and dotted lines denote the Eu(hfa)3(TPPO)2 thin film on the array and on the flat silica glass substrate, respectively. (e) Simulated T, reflectance, and absorption as a function of θin for λ=325 nm. (f) The distribution of light energy under the illumination of a plane wave with λ=325 nm at θin=34.0°. We simulated the square magnitude of the electric field normalized to the incident field, |E|2/|E0|2, in the zy plane, at x intersecting the middle of the nanocylinder.

Close modal

Figure 2(c) shows T(λ,θin) simulated using a finite-element method (COMSOL Multiphysics). The model structure, from bottom to top, comprises a silica glass substrate/an Al nanocylinder/a thin film of Eu(hfa)3(TPPO)2/air (see the sketch in the inset). Periodic boundary conditions were taken into account in the lateral directions to simulate the array, i.e., a pitch of a = 400 nm in the triangle lattice. The dimensions of the Al nanocylinder are 130 nm × 140 nm (diameter × height), which were deduced from the SEM observation. We simulated the transmission by illumining the plane wave from the air side and monitoring the reflected and transmitted powers. Transmission was calculated with varied film thickness around 400 nm, which was a measured value using a surface profiler. The best agreement with the experiment was obtained when the thickness was 500 nm, as clearly depicted in Fig. 2(c).

Figure 2(d) shows T(λ,0°) measured at the shorter wavelengths around the excitation band of Eu(hfa)3(TPPO)2. The measurements were conducted using another setup because the light source with a variable angle measurement setup did not cover the ultraviolet region. The thin film shows absorption bands at λ ∼ 320 nm and λ<240 nm, corresponding to the electronic transitions of the hfa and TPPO groups attached to Eu3+, respectively.36 Sharp dips in T due to the 4f–4f transition of Eu3+ are also found at λ=268 and 274 nm. For the thin film with (without) the array, T(λ=325 nm, 0°) (=λex in the PL measurement), is 0.0542 (0.0968); therefore, the extinction (1 − T), the energy removed from the incident light, is 0.9458 (0.9032). Given the simulation result that the absorption is largest at θin=0° (Fig. 2(e)), the extinction increases by <4.7% in the excitation condition where λex=325 nm and θin=34°. The very small pump enhancement is visualized by the simulated light energy distribution (Fig. 2(f)). When the light is illuminated under the condition λex=325 nm and θin=34°, the intensity of light in the Eu(hfa)3(TPPO)2 thin film is similar to that of the reference without the array, and no noticeable enhancement is observed.

The PL enhancement as a function of λ and θem is shown in Fig. 3(a). The PL enhancement is defined as the PL intensity normalized to that of the reference Eu(hfa)3(TPPO)2 thin film without the array, I/I0. The black solid and dashed lines are the (−1, 0) Rayleigh anomalies with n = 1.52 and 1.46, respectively. A typical PL spectrum of an Eu(hfa)3(TPPO)2 thin film (the same as that shown in Fig. 1) is shown as a white dotted line. The enhancement follows the Rayleigh anomaly, i.e., large enhancement appears at the angles where the Rayleigh anomaly occurs in the spectrum range of the Eu3+ emission. The maximum enhancement is achieved at θem=21°, where the main emission line at λ ∼ 617 nm is enhanced by as much as five fold. The directionality of the PL is better visualized by the polar plot in Fig. 3(b), representing I(617 nm, θem). An isotropic and Lambertian PL profile is observed for the reference film. In contrast, the PL from the thin film on the array deviates from the Lambertian profile and shows sharp peaks. It is noted that a fraction of PL is trapped in the Eu(hfa)3(TPPO)2 thin film, which works as a slab waveguide because of its higher refractive index (1.52) compared to that of the substrate (1.46). The array extracts the trapped PL in a direction defined by the diffraction. Integration of the intensity over θem between 0° and 90° gives an overall PL enhancement of 1.72 fold in the specific radial direction in the measurement.

FIG. 3.

(a) PL enhancement (I/I0) defined as the PL intensity of the sample normalized to that of the reference Eu(hfa)3(TPPO)2 thin film without the Al nanocylinder array, plotted as a function of wavelength and angle of emission with respect to the angle normal to the surface, θem. The inset is the experimental configuration: We collected the PL polarized along the y direction. The solid and dashed lines indicate the Rayleigh anomalies with n = 1.52 and 1.46, respectively. White dotted line shows a typical PL spectrum of Eu(hfa)3(TPPO)2. (b) Polar plot of I(617 nm, θem). The red and black lines correspond to the PL from the sample and that from the reference, respectively. (c) T(617 nm, θin) (right axis) and I(617 nm, θem)/I0(617 nm, θem) (left axis). (d) Simulated T(617 nm, θin) (right axis) and the square magnitude of the electric field normalized to that of the reference, |Efilm|2/|Ereffilm|2 (left axis), in the Eu(hfa)3(TPPO)2 thin film at λ=617 nm. The arrows indicate local maxima in |Efilm|2/|Ereffilm|2.

FIG. 3.

(a) PL enhancement (I/I0) defined as the PL intensity of the sample normalized to that of the reference Eu(hfa)3(TPPO)2 thin film without the Al nanocylinder array, plotted as a function of wavelength and angle of emission with respect to the angle normal to the surface, θem. The inset is the experimental configuration: We collected the PL polarized along the y direction. The solid and dashed lines indicate the Rayleigh anomalies with n = 1.52 and 1.46, respectively. White dotted line shows a typical PL spectrum of Eu(hfa)3(TPPO)2. (b) Polar plot of I(617 nm, θem). The red and black lines correspond to the PL from the sample and that from the reference, respectively. (c) T(617 nm, θin) (right axis) and I(617 nm, θem)/I0(617 nm, θem) (left axis). (d) Simulated T(617 nm, θin) (right axis) and the square magnitude of the electric field normalized to that of the reference, |Efilm|2/|Ereffilm|2 (left axis), in the Eu(hfa)3(TPPO)2 thin film at λ=617 nm. The arrows indicate local maxima in |Efilm|2/|Ereffilm|2.

Close modal

In order to compare the directionality of PL and the condition of the diffraction, we plot in Fig. 3(c)I/I0(617 nm, θem) and T(617 nm, θin). Three peaks appear in I/I0 at θem=15°, 21°, and 31°, and three dips appear in T around θin=12°, 21°, and 30°. The PL enhancement is apparently accompanied by the dip in T. Figure 3(d) shows a cut in simulated transmission in Fig. 2(c) at λ=617 nm, i.e., T(617 nm, θin), where the three experimentally observed dips are reproduced. The light energy distributed inside the thin film normalized to that of the reference, |Efilm|2/|Ereffilm|2, calculated at λ=617 nm as a function of θin, is also plotted. This value corresponds to the enhancement of light energy inside the thin film by the array. Three local maxima appear, as indicated by arrows, and they are accompanied by dips in T.

To identify the origin of the modes that accompany the dips in T, in Fig. 4, we illustrate the light energy distribution calculated at three local maxima, indicated by arrows in Fig. 3(d). At θin=14.3° and 28.5° (Figs. 4(a) and 4(c)), the light energy is accumulated near the nanocylinder, suggesting a localized nature of the mode. In contrast, at θin=22.0° (Fig. 4(b)) where the enhancement is maximal, the field is not only accumulated on the nanocylinder but also extended into the neighboring nanocylinders, demonstrating radiative coupling between the SPPs. The field distribution clearly shows the excitation of collective plasmonic modes for the condition in which the PL enhancement is a maximum. When the excited Eu emits light with λ=617 nm in the film, a fraction of light excites this collective mode because of the large spatial distribution of light energy in the film. Then the light is preferentially coupled out at θem=22.0°. The outcoupling at θem=14.3° and 28.5° is less because of the smaller light distribution.

FIG. 4.

Distribution of light energy in the zy plane at x intersecting the middle of the nanocylinder. We simulated |E|2/|E0|2 using a plane wave with λ=617 nm at θin=(a) 14.3, (b) 22.0, and (c) 28.5°. The interfaces between the different materials are highlighted by white dotted lines.

FIG. 4.

Distribution of light energy in the zy plane at x intersecting the middle of the nanocylinder. We simulated |E|2/|E0|2 using a plane wave with λ=617 nm at θin=(a) 14.3, (b) 22.0, and (c) 28.5°. The interfaces between the different materials are highlighted by white dotted lines.

Close modal

The PL directionality can be tuned by tuning the dispersion of collective plasmonic modes via the pitch of the array. The SEM image in Fig. 5(a) shows the sample with a = 480 nm. The inset is the fast Fourier transfer image of the SEM image, showing hexagonal spots that indicate a high accuracy of fabrication. Figures 5(b) and 5(c) show T(λ,θin) and I(λ,θem) for the sample with a = 480 nm, respectively. The solid and dotted lines indicate the Rayleigh anomaly with n = 1.52 and 1.46, respectively. T manifests a dip at 617 nm at normal incidence, which is modulated by the Rayleigh anomalies with increasing θin (Fig. 5(b)). Figure 5(c) clarifies that PL enhancement occurs at θem ∼ 2° and λ ∼ 617 nm. The polar plot in Fig. 5(d) demonstrates that the PL direction can be tuned by the pitch. Here, the PL enhancement of the Eu3+ main emission peak corresponding to the 5D07F2 transition (λem between 607 and 625 nm) is plotted for the samples with different pitches, i.e., a = 350, 400, and 480 nm. A maximum appears in I/I0 at θem=42°, 21°, and 2° for a = 350, 400, and 480 nm, respectively.

FIG. 5.

(a) The SEM image of the Al nanocylinder array with pitch a = 480 nm. (b) T(λ,θem) of the thin film of Eu(hfa)3(TPPO)2 on the array (pitch = 480 nm). The solid and dashed lines indicate the Rayleigh anomalies with n = 1.52 and 1.46, respectively. (c) I/I0 as a function of λ and θem. (d) Polar plot of I/I0 integrated at λ between 607 and 625 nm as a function of θem. The pitch of the sample is 480 (blue), 400 (gray), and 350 (red) nm.

FIG. 5.

(a) The SEM image of the Al nanocylinder array with pitch a = 480 nm. (b) T(λ,θem) of the thin film of Eu(hfa)3(TPPO)2 on the array (pitch = 480 nm). The solid and dashed lines indicate the Rayleigh anomalies with n = 1.52 and 1.46, respectively. (c) I/I0 as a function of λ and θem. (d) Polar plot of I/I0 integrated at λ between 607 and 625 nm as a function of θem. The pitch of the sample is 480 (blue), 400 (gray), and 350 (red) nm.

Close modal

Figure 6 shows the PL decay rate for the Eu(hfa)3(TPPO)2 thin film on the array (a = 400 nm) and that on the flat silica glass substrate (reference). The decay curves overlap each other, and can be fitted by the same exponential, yielding a lifetime of τobs=0.71 ms. This result indicates that the array does not affect the emission process. We estimated the QY of the thin film following the scheme by Werts et al., which relates the radiative lifetime τr of the Eu3+ species, the integration of the total Eu3+ emission spectrum (Itot), and that of the magnetic dipole transition, i.e., 5D07F1, (IMD) as follows:37 

1τr=A0MDn3(ItotIMD),
(4)

where AMD0 (=14.65 s−1) is the spontaneous emission probability for the 5D07F1 transition in vacuum. Equation (4) has been verified experimentally and used for the estimation of τr of Eu3+ species, especially Eu3+ complexes containing organic ligands.38 The calculated τr is then related to τobs and the nonradiative lifetime τnr as follows:

1τobs=1τr+1τnr.
(5)

Finally, QY is calculated as the ratio of radiative decay rate kr (=1/τr) to the sum of radiative kr and nonradiative knr (=1/τnr) decay rates, kr/(kr + knr), yielding QY = 0.73 for both the samples. As the Eu3+ complexes are packed densely in the thin film, the obtained value is reasonably high in comparison to that for the diluted solution (QY = 0.90).28 

FIG. 6.

PL decay curves for the Eu(hfa)3(TPPO)2 thin film on the Al nanocylinder array with a = 400 nm (red circles) and on a flat silica glass substrate (black circles).

FIG. 6.

PL decay curves for the Eu(hfa)3(TPPO)2 thin film on the Al nanocylinder array with a = 400 nm (red circles) and on a flat silica glass substrate (black circles).

Close modal

The contribution of the factors in Eq. (1) to the directional PL enhancement in the present system will be discussed in this section. First, the pump enhancement at λex=325 nm can be estimated from Figs. 2(d)–2(f). The Al nanocylinder array increases the extinction by < 4.7% at θin=34°, meaning that the pump enhancement is less than 4.7% in the present experiment. The PL decay rate in Fig. 6 suggests that QY is unchanged by the presence of the array. This suggests no energy dissipation occurs from the excited Eu(hfa)3(TPPO)2 to the Al nanocylinder. This leads to the conclusion that the outcoupling is dominant in the present system; at θem=21° and λem=617 nm, where I/I0 is 5.222, |E|2/|E0|2 ≤ 1.047, and η/η0=1, giving rise to Cext/Cext04.988. This outcoupling comes from the spatial overlap of the collective plasmonic mode over the Eu(hfa)3(TPPO)2 thin film, as seen in Fig. 4. A large fraction of the excited Eu emits light into the collective plasmonic mode, and this mode is preferentially coupled out at θem that corresponds to θin to excite the same mode.

The importance of SPPs in the directional PL enhancement is examined by using a simulation. In Fig. 7(a), we calculated |Efilm|2/|Ereffilm|2 at λ=617 nm for the models with dielectric cylinders of the same geometry and refractive index ncyl. The local maxima appear for all the calculations with varied ncyl, which corresponds to light diffraction in the plane of the array. The maximum |Efilm|2/|Ereffilm|2 increases with increasing ncyl, indicating enhanced diffraction with the increase in the refractive index contrast. The peak is integrated over θin, and plotted as a function of ncyl in Fig. 7(b). The value exceeds that of the Al cylinder only when ncyl>2.2. The superiority of Al as a scatterer becomes clear when the refractive index of the substrate increases. We also calculated |Efilm|2/|Ereffilm|2 for the same geometry with the sapphire substrate (n = 1.8). Al shows the largest |Efilm|2/|Ereffilm|2 in the range of calculation (see the right axis of Fig. 7(b)). Al shows a large scattering ability irrespective of the substrate, as a result of SPPs.

FIG. 7.

(a) Simulated |Efilm|2/|Ereffilm|2 in the Eu(hfa)3(TPPO)2 thin film at λ=617 nm for nanocylinder array with varied refractive index (ncyl). The model geometry is the same as used in Fig. 3(d). (b) Integrated |Efilm|2/|Ereffilm|2 over θin around the peak as a function of ncyl. The refractive indices of the substrate are 1.46 (left axis) and 1.8 (right axis). The integrated area was calculated by a Lorentzian fit with a baseline of |Efilm|2/|Ereffilm|2=1. The dotted line indicates the values for Al (10.7 for SiO2 substrate and 3.4 for sapphire substrate).

FIG. 7.

(a) Simulated |Efilm|2/|Ereffilm|2 in the Eu(hfa)3(TPPO)2 thin film at λ=617 nm for nanocylinder array with varied refractive index (ncyl). The model geometry is the same as used in Fig. 3(d). (b) Integrated |Efilm|2/|Ereffilm|2 over θin around the peak as a function of ncyl. The refractive indices of the substrate are 1.46 (left axis) and 1.8 (right axis). The integrated area was calculated by a Lorentzian fit with a baseline of |Efilm|2/|Ereffilm|2=1. The dotted line indicates the values for Al (10.7 for SiO2 substrate and 3.4 for sapphire substrate).

Close modal

In summary, by coupling Eu(hfa)3(TPPO)2, with its large Stokes shift, high QY, and long PL lifetime, to an Al nanocylinder array, we elucidated the factor that contributed most significantly to the directional PL enhancement. The PL decay and T at λex=325 nm indicated negligible contributions of QY and pump enhancement, respectively. Taking these into account, the directional PL enhancement as large as five fold observed in the present system was determined to be purely due to enhanced outcoupling. Even with good emitters of high QY, the PL intensity is notably improved by using a plasmonic array, which enhances the outcoupling while avoiding unwanted energy dissipation to the metals. Greater enhancement is possible by designing an array with collective modes at both the pump and emission wavelengths. This result is useful for designing a directional, compact, and efficient light source without the use of directors such as mirrors and reflectors.

A part of this work was supported by Kyoto University Nano Technology Hub and NIMS Nanofabrication Platform in the “Nanotechnology Platform Project” sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Financial support from Grant-in-Aid for Scientific Research (B, Grant No. 16H04217) by MEXT, and the Asahi Glass Foundation are acknowledged. SM is grateful for the support from the construction project for the consortium of the fostering of science and technology personnel, “Nanotech Career-up Alliance (Nanotech CUPAL).”

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