Enhancing extraction of light from metal composite structures for plasmonic emitters using light-coupling effect Nan-Fu Chiu,* Cheng-Du Yang, Yi-Lun Kao, and Kuan-Lin Lu Institute of Electro-Optical Science and Technology, National Taiwan Normal University, Taipei 11677, Taiwan * [email protected]

Abstract: This work demonstrates the efficiency and directionality of a method of extracting light from thin-film emissive devices by near-field evanescent waves in plasmonic emitters used in metal composite grating structures. A near-field evanescent wave can induce a surface plasmon wave on the surface of a metal under resonant conditions. Enhancing the near-field evanescent wave generates strong far-field nonlinear optical effects. This effect is highly efficient in some plasmonic emitter structures. Theoretical and experimental results demonstrate that such a metal composite grating structure exhibits good performance, a high coupling ratio, a small coupling angle, enhanced light extraction and a small FWHM. It also improves luminous efficiency, emitter angle, and directivity. ©2015 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (250.5403) Plasmonics; (350.2770) Gratings; (260.2030) Dispersion; (260.3800) Luminescence.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

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1. Introduction Surface plasmon polaritons (SPPs) [1–4] are collectively moving electrons that propagate as a wave along an interface between metals. The propagation and energy distribution of the waves are controlled by the species and the structure of the metals along whose interface they travel. The unique absorption properties [5] of plasmons are exploited using various light concentration techniques, such as those used in solar cells [6, 7] and photodetectors [8–10]. Plasmon resonance is critical in the extraction, excitation and manipulation of light in optoelectronic components in many optical devices, including laser diodes [11, 12], lightemitting diodes [13–17], and biosensors [18–20], in which light-matter interactions occur. Therefore, in recent years, researchers have begun investigating emission mechanisms in plasmonics. By increasing the observed brightness of thin-film emissive devices in far-field optics, plasmonics provide both directional emission and wavelength dispersal [19, 21–23]. This paper describes the interactions between excitation and extraction, which are nonlinear optical effects in an organic-based plasmonic emitter. This interaction involves the grating coupler-based surface plasmon-coupled emission (SPGCE) [21–29] of light that passes through a metal. In this study, two composite structures with photoemissive organic

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Received 26 Dec 2014; revised 16 Feb 2015; accepted 13 Mar 2015; published 6 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009602 | OPTICS EXPRESS 9603

molecules, tris-(8-hydroxyquinoline) aluminum (Alq3), adjunct to a grating material, gold (Au) and a photo resister (PR), were fabricated, for the possible enhancement of directional angle and light intensity by the surface plasmon resonance of the excitation and extraction modes at the dielectric/metal interface. Further enhancement of directional angle and light intensity arises from the interactions of SPPs given the appropriate effective refractive index and thickness of the layered structure. The SPGCE corresponded to the resonant condition of surface plasmon modes on the Au/dielectric interface. Various gratings are designed because light-emitting organic molecules in a grating coupler result in different far-field and near-field optical characteristics. This study discusses excitation and extraction in a plasmonic structure and luminescence properties, energy conversion, wavelength dispersion, and mechanisms of enhanced light extraction. To elucidate the properties of the magnetic field distributed in metal composite grating structures and the limitations of nonlinear far-field optical effects, the excitation and extraction modes are modeled in computer simulations. Analytical results can help in understanding the emitting particularity and radiative decay rates, and in improving the spatial distribution of emission. The model elucidates the evolution of the dispersion relation and measurements and theoretical fitting and directional emission. Notably, coupler emission phenomena are readily extended to applications in SP-coupled fluorophores with different wavelength emissions at different viewing angles. We believe that the combination of plasmonic emitter components in a single structure provides new opportunities for the application of active electroluminescence in a biosensor in novel devices. 2. Experimental setup and theoretical model A grating coupled SPR (GCSPR) [30, 26] is based on a periodic and symmetrical lamellar reflecting surface. Reflected light comprises an infinite sum of plane waves that propagate at various angles from the z axis of the grating. Equation (1) is the grating reflection law. Figure 1(a) shows the ∆k vectors of the plane waves. In this method, the efficiency and intensity of the first-order term is as high as 95%, and the second-order term is 76% of the incident intensity [31]. GCSPR with strong first-order reflection and one of its applications are discussed. sin θ R( m ) = sin θ i + k / / = k x ± mth

mλ Λ

2π 2π = ε eff k0 sin(θ i ) ± mth Λ Λ

(1) (2)

where θi is the angle of incidence; θ(m)R is the angle of reflection of the mth order and depends on the wavelength, Λ is the pitch of the grating and εeff is the effective permittivity of a metal/dielectric medium grating nanostructure [32]. When a light wave with wavevector (kx) is incident on the surface of the grating, diffraction yields a series of diffracted waves. Equation (1) presents the wavevector of the diffracted light (k//). When Λ is too large, θ(m)R = θi, θ(m)R is imaginary, so k// becomes a surface evanescent wave that propagates along the x axis. A surface plasmon that propagates (ksp) along a grating is affected by the dielectric on the opposite side of the metal surface and satisfies, k sp = k0

ε m ε eff ε m + ε eff

(3)

where εm is the permittivity of the thin metal film, and εeff is the effective permittivity of a metal/dielectric grating nanostructure. For a wavevector matching condition in an in-plane wave vector and a SP wave vector in grating structures, k// must equal ksp. Therefore, the resonance angle (θsp) is given by [27],

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Received 26 Dec 2014; revised 16 Feb 2015; accepted 13 Mar 2015; published 6 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009602 | OPTICS EXPRESS 9604



2

 k sp   λ  cos(ϕ ) ±   −  sin(ϕ )  Λ Λ k   0   

θ sp = arc sin 

λ

2

   

(4)

where φ is the azimuthal angle from 0 to 360. In the GCSPR structure, if φ = 0, then the surface plasmon resonance angle (θsp) has a high coupling ratio and the full width at half maximum (FWHM) of the resonance dip is relatively small. Controlling the azimuthal angle is not only of interest in the context of basic science, but also has many applications. The thin-film emissive devices of a metal composite grating structure has a surface plasmon grating coupled emission (SPGCE) mode. The grating samples herein were prepared using an electron beam lithography system [21]. A fluorophores organic semiconductor material, Alq3, was used to excite SPPs by grating-coupled emission. Such emissions correspond to the resonant condition under the SP modes at the Au/Alq3 interface for the emission of light, as presented in Fig. 1(b). The presence of a surface electric field changes the emission wavelength by coupling both excitation and emission modes, and a specific absorption wavelength can be observed in the grating structure. Figures 1(c) and 1(d) show SEM images of the cross-sections of the Au/Au-grating/Alq3 and Au/PR-grating/Alq3 structures, respectively.

Fig. 1. Simplified GCSPR and SPGCE models. (a) GCSPR mechanisms; the incident wavelength at a specific angle can be coupled to the GCSPR structure. (b) The SPGCE structure is based on Alq3 molecules in the active layer, which provide a non-oriented internal light source that generates SPPs on metal/dielectric interfaces and emits detectable radiation. SEM images of cross-sections of gratings in (c) [Si /Au-film / grating (Au) / Alq3] structure, (d) [Si /Au-film / grating (photo resister, PR) / Alq3] structure. In (d), a 10 nm-thick ultra-thin platinum (Pt) film increases the reflection of secondary electrons, which enhances image contrast.

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Received 26 Dec 2014; revised 16 Feb 2015; accepted 13 Mar 2015; published 6 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009602 | OPTICS EXPRESS 9605

3. Results and discussion 3.1 Controlling resonance angle of GCSPR structures To explain the improved detection SPR resonance angle that is achieved by azimuthal rotation, the angular and coupler yields are measured. The SPR resonance characteristics for various azimuthal angular rotations from 0 to 50 degree, presented in Fig. 2, are analyzed. In Figs. 2(a)-2(d), the measurements of the reflectivity spectra are based on a zero-order reflected configuration for angular interrogation. The analysis yields SP resonant angles (θ) for various azimuth angles (φ) and the p-polarization of light with λ of 650 nm, 700 nm, 580 nm, and 600 nm, respectively.

Fig. 2. Results of analysis of azimuthal angle and SPR characteristics based on reflectivity spectra. 2(a, b) Au/Au-grating structure. 2(c, d) Au/PR-grating structure. Measurements of the reflectivity spectra for angular interrogation showed that the variable azimuth φ for the λ was 650 nm in Fig. 2(a), 700 nm in Fig. 2(b), 580 nm in Fig. 2(c), and 600 nm in Fig. 2(d), ppolarization.

Figures 2(a) and 2(b) plot the reflectivity spectra in angular interrogation for the azimuthal angle ranges from 0 to 50 degrees as a function of the SP resonance angle present a 3-D image of an Au/Au-grating structure sample. The experimental results indicate that a larger azimuthal angle results in a larger SPR resonance angle and a smaller FWHM. Variation of reflectance with change in angle of incidence of light with a wavelength of 650 nm and 700 nm, as displayed by the (red) SPR curves in Figs. 2(a) and (b), respectively. The relevant experimental results in Figs. 2(a) and 2(b) yield SPR angles of 13 and 20 degrees, respectively, at an azimuthal angle of zero. Figures 2(a) and 2(b) plot the reflectivity spectra in angular interrogation for the azimuthal angle ranges from 0 to 50 degrees as a function of the SP resonance angle present a 3-D image of an Au/PR-grating structure sample. The results indicate that a larger azimuthal angle yields a larger FWHM at the SPR angle, as presented in Figs. 2(c) and 2(d). Variation of reflectance with change in angle of incidence of light with a

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Received 26 Dec 2014; revised 16 Feb 2015; accepted 13 Mar 2015; published 6 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009602 | OPTICS EXPRESS 9606

wavelength of 580 nm and 600 nm, as displayed by the (red) SPR curves in Figs. 2(c) and (d), respectively. For an incident azimuthal angle was zero, Fig. 2(c) yields the results obtained with an SPR angle of 22 degree, and Fig. 2(d) yields the results obtained with an SPR angle of 20 degree. The results obtained from the Au/PR-grating structure reveal an angle shift in the opposite direction to that revealed by the results obtained from the Au/Au-grating structure. This experimental result is inferred from the derived Eq. (4). 3.2 Emission of light by SPGCE structures To analyze surface plasmon excitation and elucidate the phenomenon that is responsible for surface plasmon extraction by the grating structure. Photoluminescence (PL) measurements are made based on an analysis of specific resonance spectra and the enhanced extraction of light in the SPGCE structure. The dispersion relation for the scattered light is obtained using the PL method, by which excited SPPs enhance the radiation of light and control the directional emission. Figure 3 presents a 3D dispersion relation and the SPR resonance characteristics of the grating. The excitation of a surface plasmon results in the transfer of absorbed energy.

Fig. 3. The dispersion of 3D SPGCE structures was measured in relation to the absorption and emission characteristics. These figures are explained by the excitation and extraction of Alq3 molecules in the composite structure and by emissions at different resonance angles. Figures 3(a)-(c) Au/Au-grating/Alq3 structures; Figs. 3(b)-(d) Au/PR-grating/Alq3 structures. Figures 3(a) and 3(b) present the wavelength of the incident light from 450 to 700 nm at which resonant spectral characteristics were obtained at incident angles from 10 to 60 degrees. Figures 3(c) and 3(d) present the behaviors of an SPP at a metal/organic interface that is generated by an SP emission mode. The insets in Figs. 3(c) and (d) present 2D dispersion relation images based on PL emission spectra characteristics.

Figures 3(a) and 3(b) plot the 3D resonance angle and absorption characteristics at various incident wavelengths from 450 to 700 nm. Figures 3(a) and 3(b) plot the wavelength of dispersion relative as a function of the experimentally determined angle of SP absorption for #231463 - $15.00 USD (C) 2015 OSA

Received 26 Dec 2014; revised 16 Feb 2015; accepted 13 Mar 2015; published 6 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009602 | OPTICS EXPRESS 9607

Au/Au-grating/Alq3 and Au/PR-grating/Alq3 structures, respectively. Clearly, the experimental results correlate closely with the white line in Fig. 3 (a), which shows two wavevectors of the −1st order for the SPP1(Au/Alq3) mode and the −2nd order for the SPP2(Au/Alq3) mode. Figure 3(b) presents two wavevectors one of the −1st order for the SPP1(Au/Alq3) mode and one of the −1st order for the SPP2(Au/PR) mode. Figures 3(c) and 3(d) present a 3D dispersion relation and PL emission spectra characteristics. In Figs. 3(c) and 3(d), Au/Au-grating/Alq3 and Au/PR-grating/Alq3 structures are shown to increase the external coupling efficiency of SPGCE structures, as determined from an experimentally obtained spectrogram of emission angle vs. SP resonance wavelength. Additionally, surface plasmon extraction can increase luminous efficiency and highly directed radiation [21, 33, 34]. According to Figs. 3(a) and 3(c), the Au/Au-grating/Alq3 structure has two wavevectors for −1st order of SPP1(Au/Alq3) and −2nd order of SPP2(Au/Alq3), closely corresponding to the SPR energy modes. Figure 3(c) clearly shows SPP1(Au/Alq3) coupling by Bragg scattering of -Δk (−2π/Λ). The inset in Fig. 3(c) plots a 2D dispersion relation image obtained from the PL emission spectra at various viewing angles. In Figs. 3(b) and 3(d), although the Au/PR-grating/Alq3 structure exhibits resonant spectral response characteristics, it exhibits two −1st orders of SPR absorption by the SPP1(Au/Alq3) and SPP2(Au/PR) interfaces. Figure 3(d) presents the emission properties of an Au/PR-grating/Alq3 structure with a gap of 30 nm (SPBG-1). The SPBG-1 gap supports Bragg scattering of −1st order, which results in an SPP1(Au/Alq3) interaction between the scattered branches. The -Δk component of the SPP2(Au/PR) branches has a gap of 12 nm (SPBG-2). The inset in Fig. 3(d) plots a 2D dispersion relation image based on PL emission spectra at various viewing angles. 3.3 Fitting of experimental SPGCE curves Figures 4(a) and 4(b) show the SPPs propagating along a grating surface to tunability, in which its wave vector depends on the resonance. Such a scheme allows for a momentummatch between Alq3 emission and SPPs. Consequently, a tunable SPP mediated light emission is achieved. Using an SPGCE structure, the optical parameters that are obtained by fitting the SPR dispersion relation curves in both experiments can be understood as causing an increase in the emission angle. The theoretical analysis, in which curves are fitted to resonance spectra, uses a relationship between the dispersion relation and the emission angle of the SPGCE structure. This study also evaluates a refractive index n(PR) of 1.56 RIU (refractive index unit) for the photo resister (PR); n = 0.355 and k = 2.695 for Au and n = 1.724 and k = 0.0033 for Alq3 at a wavelength of 530 nm. The overall effective refractive indices of the SPP at the Au film interface are given by n + ik (Au/Au-grating/Alq3 structure) = −2.756 + i1.209 and n + ik (Au/PR-grating/Alq3 structure) = 0.08 + i0. The theoretical dispersion relation is estimated by solving Eqs. (2), (3) and (4) to obtain analytical expressions for the plasmon modes in SPGCE structures. Obtained from the SPGCE measurements, the peak emission wavelength at each emission angle is utilized to calculate the dispersion curve of the SPR mode with m = ± 1 and Λ = 500 nm using Eq. (2). Figure 4(a) presents the Au/Au-grating/Alq3 structure dispersion relation determined from measurements and by theoretical fitting. The SP resonance frequency condition is εAu(ω) + εAlq3(ω) = 0, where εAu and εAlq3 are the real parts of the dielectric constants of Au and Alq3, respectively. From the measurements made on the Au/Au-grating/Alq3 structure, the peak emission wavelength at each emission angle can be used to fit the dispersion curve. This result is related to the theoretical dispersion relation of SPGCE at the −1st order of the SPP1(Au/Alq3) interface with m = ± 1 for a wavelength of 530 nm by a matching momentum, ΔKSP(Au/Alq3) of 21.41 μm−1. The results reveal that the fitted mean deviation wave vector ΔK at various wavelengths is 0.32 μm−1. Figure 4(b) plots the dispersion relation of the Au/PRgrating/Alq3 structure that is obtained from the measured and fitted data. The overall reflective index of the structure has three components that correspond to the Au-film, the

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Received 26 Dec 2014; revised 16 Feb 2015; accepted 13 Mar 2015; published 6 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009602 | OPTICS EXPRESS 9608

lamellar grating PR and Alq3. The SP frequency condition is given by εAu(ω) + εPR(ω) + εAlq3(ω) = 0. Based on measurements of the Au/PR-grating/Alq3 structure, the peak emission wavelength at each resonant angle can be used to fit the dispersion relation that is given by Eq. (2). The fitting result is related to the theoretical dispersion relation of SPGCE at the −1st orders of the SPP1(Au/Alq3) and SPP2(Au/PR) interfaces with m = ± 1 and a wavelength of 530 nm by matching momentum, ΔKSP(Au/Alq3) of 19.296 μm−1 and ΔKSP(Au/PR) of 19.688 μm−1, respectively. The results reveal that the fitted mean deviations ΔKSP(Au/Alq3) and ΔKSP(Au/PR) of the wave vector at various wavelengths are 0.37 μm−1 and 0.265 μm−1, respectively. This is owing to that the SP modes on troughs of the grating structure have a band gap, which opens up between these lower and higher energy bands. Correlating our data with this equation reveals that the experimental results are close to the theoretical dispersion relation (KSP) of a SP-coupled emission at Au/Alq3 and Au/PR interfaces. Fitting the experimental SPR curves using the theoretical model that is described above yields the optical parameters that specify the emission. The fitting result can be explained with reference to the theoretical SPGCE model of the dispersion relation, which is plotted in Fig. 1(b).

Fig. 4. Figures give fitting results and theoretical interpretation. It is angular frequency vs. wave vector for the measured data (block square) and fitting data (blue and green circle), the theoretical dispersion relation on interface surface plasmon dispersion relation Au/Alq3 (hollow pink triangle) and Au/PR (pink triangle) of 1 order, Au/Alq3 (hollow orange triangle) and Au/PR (orange triangle) of −1 order, and the light in vacuum (red line). The data are taken from the sample with a 500 nm pitch. The dependence of the fitting results on dispersion relation is shown in Fig. 4(a) and Fig. 4(b) for Au/Au-grating/Alq3 and Au/PR-grating/Alq3 structure. The fitting results closely correspond to the theoretical dispersion relation.

3.4 Simulations of SPGCE coupling at near-field interfaces The thin-film emissive devices of the metal composite grating structure were modeled in the finite difference frequency domain (FDFD). Simulations were performed to calculate

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Received 26 Dec 2014; revised 16 Feb 2015; accepted 13 Mar 2015; published 6 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009602 | OPTICS EXPRESS 9609

electromagnetic field distributions and SP excitation in Alq3 organic covered with laminar diffraction grating and interfaces. The components of the scattered electromagnetic field that contribute to the Poynting vector are calculated using plasmonic and Mie scattering control of far-field interference theory [35, 36]. The SP modes propagated in these simulations were analyzed using Drude model and Bloch boundary conditions, to calculate the SP modes and the excitation and extraction modes. A TM-polarized plane wave source with visible wavelengths (400~700 nm) was launched with normal incidence onto the SPGCE surface from the Au/Alq3 film. The uniformity of the structure and the electromagnetic field in the z direction is assumed and a magnetic field parallel to grating line is considered.

Fig. 5. The FDTD simulation results for electric field intensity in SPR propagating modes are shown in the calculated band diagram. Poynting vector distributions in Figs. 5(a)-5(c) correspond to Au/Au-grating/Alq3 structure and those in Figs. 5(d)-5(f) correspond to Au/PRgrating/Alq3 structure. Poynting vector plots reveal that a fraction of the energy of the evanescent field reaches the metal surface, while the remainder is reflected and propagates along the metal surface in the x-y-z axle.

#231463 - $15.00 USD (C) 2015 OSA

Received 26 Dec 2014; revised 16 Feb 2015; accepted 13 Mar 2015; published 6 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009602 | OPTICS EXPRESS 9610

Accordingly, the considered field components are Ex, Ey and Hz and the distribution of the Poynting vectors |P(x,y,z)| = |E(ω) × H(ω)| describes the excitation and extraction modes, as presented in Fig. 5. The distribution of |Px| reveals that a fraction of the energy of the evanescent field reaches the metal/Alq3 slab, while the rest turn around and energy straddle the slab along the z-axis. Figures 5(a)-5(c) show the Au/Au-grating structure. The SP extraction frequency is tuned to the Alq3 emission, and the light emission intensity can be increased by over an order of magnitude when non-radiative paths are bypassed by the decoupling of energy into SP extraction modes. Figures 5(d)-5(f) show the Au/PR-grating structure. The reflective index has two components that are associated with Au and the PRgrating. The extraction frequency is determined from εAu(ω) + εPR(ω) + εAlq3(ω) = 0. Since the SP modes at the peaks and troughs of the periodic structure have different energies, a band gap exists between these lower and higher energy bands, which are denoted by ω+ and ω-, respectively, as presented in Figs. 3(c) and 3(d). Studying the special characteristics of SP resonance modes of a nanostructure is important. The SP excitation frequency condition is εAu(ω) + εAlq3(ω) = 0, where εAu and εAlq3 are the real parts of the dielectric constants of Au and Alq3, respectively. In the near-field region, the distribution of light intensity and the electric field radial component in the propagation direction is at a fixed location in space. Figure 5 presents simulation results concerning the Poynting vector |P| in the gap between the Au slab and Alq3, and also in the region immediately underneath the slab. Figures 5(a) and 5(d) present the results for the Poynting vector component |Px|; Figs. 5(b) and 5(e) show the same for Poynting vector component |Py|, and Figs. 5(c) and 5(f) show the same for Poynting vector component |Pz|. 4. Conclusions This work demonstrates the diversity and complexity of the excitation and extraction modes of a plasmonic emitter at a resonance wavelength that is close to the wavelength of Alq3 molecules light. The SP polarization with azimuthal rotation was also studied as the dependence of the resonance dip on angles of incidence increased. The results herein also confirm that a plasmon can improve emitter efficiency and tuning directionality in light mechanisms. Directional emission enhances the spectral band-gap response. Experimental results demonstrate that the excitation and extraction of SPPs emit visible light and enhance the light-coupling effect. The near-field evanescent wave generated by the plasmonic emitter clearly had strong far-field nonlinear optical effects. This method is effective in plasmonic emitter applications and it has other applications, including in fluorescence discoloration biosensors and organic light-emitting diodes. Acknowledgments This project is supported in part by the Ministry of Science and Technology of the Republic of China, Taiwan, for financially supporting this research under Contract No. MOST 1032221-E-003 −008, NSC 102-2221-E-003-021, and NSC 99-2218-E-003-002-MY3.

#231463 - $15.00 USD (C) 2015 OSA

Received 26 Dec 2014; revised 16 Feb 2015; accepted 13 Mar 2015; published 6 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.009602 | OPTICS EXPRESS 9611

Enhancing extraction of light from metal composite structures for plasmonic emitters using light-coupling effect.

This work demonstrates the efficiency and directionality of a method of extracting light from thin-film emissive devices by near-field evanescent wave...
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