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. 2019 Feb 14;6(3):634–641. doi: 10.1021/acsphotonics.8b01407

Tamm Plasmons Directionally Enhance Rare-Earth Nanophosphor Emission

Dongling Geng 1, Elena Cabello-Olmo 1, Gabriel Lozano 1,*, Hernán Míguez 1,*
PMCID: PMC6488127  PMID: 31049366

Abstract

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Rare-earth-based phosphors are the materials on which current solid-state lighting technology is built. However, their large crystal size impedes the tuning, optimization, or manipulation of emitted light that can be achieved by their integration in nanophotonic architectures. Herein we demonstrate a hybrid plasmonic–photonic architecture capable of both channeling in a specific direction and enhancing by eight times the emission radiated by a macroscopically wide layer of nanophosphors. In order to do so, a slab of rare-earth-based nanocrystals is inserted between a dielectric multilayer and a metal film, following a rational design that optimizes the coupling of nanophosphor emission to collective modes sustained by the metal–dielectric system. Our approach is advantageous for the optimization of solid-state lighting systems.

Keywords: optical Tamm plasmons, rare-earth nanophosphors, nanophotonics, photoluminescence enhancement, color converters, lighting

The rapid progress of solid-state lighting technology and full color displays is pushing the development of novel bright luminescent materials with superior efficiency and higher color purity.1−5 Rare-earth (RE) luminophores, able to emit light in the visible spectral range under the excitation of ultraviolet (UV) or blue light, are widely used as down-shifters for lamps and displays for their exceptional optical, mechanical, and chemical properties.6−8 Compared with standard RE powders, phosphor films based on RE nanoparticles (so-called nanophosphors) feature many advantages including higher degree of uniformity and lower loss diffusion.9−11 However, although RE nanosources are interesting for the preparation of versatile transparent coatings with advanced properties, low brightness and limited efficiency associated with their small particle size are hindering the integration of such nanophosphor films in actual devices.

Nanophotonics represents an alternative path to develop light sources of improved performance without changing the chemical composition or degrading the efficiency of the emitters.12,13 In fact, dielectric multilayers have proven useful recently to modify the chromaticity and the directionality of embedded nanophosphors.14−16 Also, metallic surfaces and nanostructures that sustain surface plasmon polaritons (SPs) are known to be beneficial to enhance light–matter interaction, offering a high degree of control over light emission.17 Indeed, plasmonic resonances provide a resonant near-field coupling between electromagnetic radiation and localized surface charges in the metal–dielectric interface that result in a modification of the emission rate and directionality of nearby-located emitters.18−25 Nevertheless, collective resonances entail large field enhancements that extend away from the metal surface, influencing the emission of collections of emitters distributed over large areas, which is of interest for most applications.26−29 In this context, optical Tamm plasmons (TPs) are electromagnetic modes confined at the interface between a one-dimensional photonic crystal (1DPC) and a metal layer.30−32 Their hybrid plasmonic–photonic character bestows reduced dissipation and efficient coupling of localized energy to free-space radiation on TPs compared to standard SPs, as it has been already proven for single-photon extraction, adaptable fluorescent coatings, microcavity lasers, and thermal emitters.33−37

Herein we demonstrate that the efficient coupling between TPs and nanophosphors leads to an enhanced directional emission of RE emitters. To illustrate the effect, we integrate a transparent phosphor film based on GdVO4:Eu3+ nanoparticles into a hybrid plasmonic–photonic architecture comprising a 1DPC that combines ZrO2 and SiO2 layers fabricated by wet deposition techniques and an evaporated noble metal layer, i.e., Ag or Au. In order to maximize the luminous power output of the nanophosphor films, we carefully design the architecture layout using an in silico approach. An in-depth analysis of both spectroscopic and photophysical properties of the fabricated structures reveals that the nanophosphor emission is strongly modified by TPs. Excited RE cations distributed over the emitting layer deposited between the 1DPC and the metal film can couple to TPs, which radiate into free space efficiently. Coherent scattering from the 1DPC molds the directionality of this emission into particular directions, resulting in a maximum value of 8-fold emission enhancement in a direction close to the normal when light emitted by the nanophosphors couples to a silver-based TP.

Nanophosphors employed in our investigation were synthesized following a method described elsewhere using a solvothermal route and poly(acrylic acid) as the functionalization agent, which not only has an important effect on the morphological characteristics of the final nanocrystals but also increases the stability of the suspension.38 The photoluminescence excitation and emission spectra of as-prepared GdVO4:Eu3+ phosphors are shown in Figure 1a. The excitation spectrum was recorded by monitoring the characteristic emission wavelength (λ) at 615 nm ascribed to 5D0 → 7F2 transition of Eu3+ cations. The charge transfer of O2––V5+ inside a [VO4]3– group is responsible for the broad band observed in the excitation spectrum.39 The emission spectrum was measured upon 276 nm excitation of Eu3+ cations through energy transfer from the GdVO4 host.40 It is well known that the 5D0 → 7F2 transition is hypersensitive to the local chemical bonding environment of the Eu3+ ion. As a result, the most intense emission line peaks at λ = 615 nm due to the absence of an inversion symmetry in the europium crystal site (D2d symmetry).41Figure 1b displays the transmission electron microscopy image of the GdVO4:Eu3+ nanocrystals dispersed in methanol. Synthesized nanophosphors feature an almost equiaxial shape with a mean size of 35 nm. Notice that small particle size and high colloidal stability are key ingredients to prepare emitting coatings of optical quality employing these nanoparticles.

Figure 1.

Figure 1

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(a) Excitation (black line) and photoluminescence spectra (gray line) of GdVO4:Eu3+ nanophosphors in methanol. (b) Transmission electron micrograph of such nanophosphors.

In order to fabricate a multilayer structure that sustains TPs, a 1DPC was first built by the alternate deposition of SiO2 and ZrO2 layers, as shown in Figure 2a–c. The choice of SiO2 and ZrO2 is based on the refractive index contrast and high transparency in the UV range that these dielectric materials feature. Before evaporating the noble metal layer, we deposited a thin film of GdVO4:Eu3+ nanophosphors over the photonic multilayer. Notice that dielectric layers are fabricated using a facile wet deposition route that allows the preparation of uniform layers of high optical quality with a film thickness that can be accurately controlled, which contrasts with other metal–dielectric systems whose fabrication is typically more involved. In order to improve the efficiency of the emitters, the nanophosphor film was annealed at 400 °C. It is important to mention that these nanophosphor layers are both bright and efficient. Indeed, high brightness is attained despite the low thickness of the emitting film (∼100 nm), being the origin 2-fold. On the one hand, GdVO4 absorbs very effectively UV light due to an efficient excitation of the activators (Eu3+) through transfer from the host (GdVO4). On the other hand, Eu3+ cations are highly efficient when they are introduced as dopants of GdVO4. However, the aforementioned heating process brings along an increase in the nanoparticle size that leads to a surface roughness enlargement. As a result, the estimated root-mean-square roughness (Rrms) is 26 nm. In order to reduce the surface roughness of the emitting film, we deposited a dense ZrO2 layer, as depicted in Figure 2d. As a result, the average thickness of the film increases up to ∼170 nm, while the Rrms reduces to 4 nm. Surface analysis of the nanophosphor films before and after the deposition of such leveling layer is provided in the Supporting Information. It should be remarked that this smoothing step is crucial to eventually obtain a structure capable of sustaining TPs. The thickness of each layer in the stack can be precisely tuned through the deposition conditions, which allows an accurate control over the optical response of the system. Finally, a 150 nm thick layer of Ag or Au was evaporated to coat the multilayer (Figure 2e). Figure 2f depicts a sketch of the final hybrid Ag–dielectric system. Notice that the metal layer is evaporated with a mask, which will allow direct comparison with the emission of the same nanophosphor layer from an uncoated region, i.e., without any interaction with the metal. Figure 2g and h display scanning electron microscope images of a cross section of the fabricated multilayer. Smooth and continuous interfaces between different types of layers constituting the system reveal the structural quality of the sample. The thickness of each film in the stack is designed using a computer code based on a genetic algorithm that seeks to maximize the spectral overlap of the optical resonances of the ensemble with the most intense emission band of Eu3+ ions at the spatial region where such cations are distributed. Although according to the design the unit cell of the 1DPC is composed of 91 nm thick SiO2 (n = 1.50 at λ = 615 nm) and 84 nm thick ZrO2 (n = 1.89 at λ = 615 nm) layers, the resulting fabricated multilayer is not fully periodic but slightly aperiodic instead. A ZrO2-coated nanophosphor film deposited atop such a multilayer presents a thickness of 167 nm and a refractive index of n = 1.69 at λ = 615 nm. Full details of the optimization steps can be found in the Methods section.

Figure 2.

Figure 2

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(a–f) Fabrication scheme of the hybrid metal–dielectric architecture. (g, h) Scanning electron micrographs of a cross section of the complete structure (g) and a closer look at the Ag–nanophosphor–1DPC interfaces (h).

First, we performed a spectroscopic analysis of light reflected by the photonic multilayer (see Figure 3a) fabricated following the guidance of the calculations. The red line in Figure 3b shows the experimental reflectance spectrum of the 1DPC with the thin nanophosphor film on top. Interference between beams reflected and transmitted at each interface of the stack inhibits the propagation of a certain wavelength range in the direction along which the refractive index varies, which results in the broad and almost flat band of high reflectance observed in Figure 3b. Measurements agree well with the simulated reflectance spectrum (gray line in Figure 3b). Figure 3c shows the reflectance spectrum of the hybrid system (blue line). It can be observed that the metal–dielectric stack features a narrow reflectance dip at λ = 615 nm inside the Bragg band, which is the fingerprint of the optical TP. TPs are formed at the interface between the multilayer and the metal. They can be excited from free space and are sustained for both polarizations, in contrast to SPs. Although the main spectral features experimentally observed are fairly reproduced by the calculations (gray dashed line in Figure 3c), the comparison reveals that the theoretical Tamm mode is narrower and deeper, which we attribute to the fact that the fabricated metal–dielectric interface is not perfectly flat. Indeed, when the roughness of the ZrO2-coated nanophosphor film is considered in the model by averaging over 167 ± 7 nm, the calculated reflectance dip slightly broadens and significantly decreases (black dashed line in Figure 3c), as in the experiment. However, the agreement between measurements and calculations is still far from ideal due to the simplistic nature of our approach. In order to shed more light on the physical origin of this mode, we calculated the spatial and spectral distribution of the normalized electric field intensity enhancement (|E|2/|E0|2) along a cross section of the metal–dielectric structure, as displayed in Figure 3d. We considered a plane wave incident to the normal to the stack. Although the field profile extends over a distance that is much larger than the spatial region where the emitters sit, our design entails that the maximum of the field intensity is attained where the nanophosphors are distributed at the main emission line of the Eu3+ (λ = 615 nm). In contrast to the field intensity profile associated with a regular plasmon resonance, the hybrid character of TPs entails a field enhancement that decays over a longer distance from the metal–dielectric interface, as shown in Figure 3d. Similar results are obtained if the multilayer is coated with gold (see the Supporting Information for further details).

Figure 3.

Figure 3

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(a) Sketch of a Ag–dielectric structure and its reference composed of 10.5 periods of a 1DPC along with a thin layer of nanophosphors. (b) Experimental (red line) and simulated (gray dashed line) reflectance spectra of the fully dielectric system at normal incidence. Thickness values of each layer in the stack are provided in the Methods section. (c) Experimental (blue line) and simulated reflectance spectra of the hybrid Ag–dielectric system when the thickness of the nanophosphor layer was considered to be 167 nm (gray dashed line) and 167 ± 7 nm (black dashed line). (d) Calculated spatial and spectral distribution of the normalized electric field intensity across the section of the Ag–dielectric structure, assuming a plane wave propagating along the direction perpendicular to the multilayer surface.

In what follows we evaluate the photophysical properties of the fabricated metal–dielectric systems and quantify the improvement to the light emission associated with the TPs. Figure 4a shows the photoemission spectra of the nanophosphor film embedded in the metal–dielectric stack measured in the normal direction (blue line) from the glass side. The comparison with the reference sample (red line in Figure 4a) measured from the air side, i.e., same emitting layer over the 1DPC in a fully dielectric system, shows a many-fold enhancement of the photoluminescence (PL) in a narrow spectral region close to 615 nm, with the rest of the emission spectrum remaining mostly unaffected. Notice that if the reference was measured from the glass side, the emission would be blocked by the photonic multilayer, attaining an artificially larger emission enhancement. Such emission enhancement originates from the efficient coupling of TPs to free-space radiation because of their hybrid plasmonic–photonic character. RE cations can decay into such modes, which radiate, enabling a precise spontaneous emission control, as our results demonstrate. We also characterized the variable-angle PL enhancement (PLE), which is defined for each angle of emission as the PL of the emission layer in the metal–dielectric system normalized by the PL of the nanophosphors devoid of the metal layer. PLE is thus a relevant quantity since it accounts for the improvement associated with the presence of the metal film and, hence, of the TP. It accounts for phenomena taking place both at the excitation and emission frequencies. However, the fraction of the excitation light absorbed by the nanophosphor layer in or devoid of the presence of the metal film is essentially the same, which allows us to rule out the effect of pump enhancement in PLE measurements. Figure 4b displays the angular dependence of the PLE from 0 deg to 40 deg. The maximum 8-fold emission enhancement attained at λ = 615 nm at 0 deg shifts to shorter wavelength with the angle following the dispersion of the TP. Results are consistent with the calculations of the spectral dependence of the spatially integrated electric field intensity enhancement (EFIE) as a function of the angle as shown in Figure 4c. This comparison is supported by the Lorentz reciprocity principle,42 which is commonly employed to evaluate the performance of an emitter placed in a certain optical environment.43,44 A certain deviation is observed between the maximum PLE (∼8) measured and the EFIE calculated (∼30), which we attribute mainly to the uneven nature of the metal–nanophosphor interface, since the efficient excitation of TPs strongly depends on the flatness of the surface between the dielectric multilayer and the metal film. Au–dielectric systems exhibit similar behavior (see the Supporting Information), although maximum emission enhancement values attained are lower due to the fact that gold features larger ohmic losses than silver in this spectral range.

Figure 4.

Figure 4

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(a) Photoluminescence (PL) spectra of the Ag–dielectric (blue) and the reference system (red) measured along the normal direction. We also show a sketch of the experimental configuration employed to excite and collect the emission from both reference and hybrid systems. (b) Unpolarized photoluminescence enhancement as a function of the angle for the Ag–dielectric structure. The panel on the right shows the PL enhancement spectrum at 0 deg. (c) Calculated angular-dependent unpolarized electric field intensity integrated into the spatial region of the Ag–dielectric structure where the emitters sit normalized to the same quantity in the fully dielectric structure. The panel on the right displays the electric field intensity enhancement spectrum calculated at 0 deg.

Finally, we have measured the PL dynamics of the samples. Figure 5a shows the time-dependent PL of the nanophosphor film in the fully dielectric reference system (red symbols) and when the metal layer has been deposited onto the nanophosphor layer coated Bragg mirror (blue symbols). Fittings, shown as dashed lines, were performed assuming a sum of two log-normal distributions of decay rates (see the Supporting Information) since, typically, two different environments can be assumed for RE cations doping a crystalline matrix.14 Our results indicate that, as shown in Figure 5b, (i) a narrow distribution of rates can be associated with the transition of cations that sit in the bulk of the crystal structure and (ii) a broad distribution of higher rates associated with transitions of cations located close to grain boundaries. We observe that the average lifetime associated of the transition of Eu3+ cations in the reference system is 0.51 ms, whereas this value reduces to 0.38 ms when the same cations are integrated in the metal–dielectric system. Similar results are attained for the Au–dielectric stack as discussed in the Supporting Information. This result, concomitant with the reported increase of nanophosphor emission intensity in metal–dielectric structures, could be attributed to the higher local photon density of states that arise as a result of the presence of Tamm plasmons at the interface between the metal and the multilayer. However, the effect of additional nonradiative decay pathways related to the presence of the metal cannot be discarded. Furthermore, the interplay between these effects and those caused by the different response of the cations as a function of their intragrain spatial distribution complicates even more the analysis of the origin of the observed changes in the PL decay dynamics. The only clear, conclusive, observation is that those cations located in the bulk of the crystallites, hence much less sensitive to modifications of the chemical environment, show a narrower rate distribution as well as an increase of its most frequent value, which rises from 1.7 ms–1 to 2.1 ms–1 in the presence of the metal. The increase of this radiative rate can then be soundly ascribed to the coupling of the emission to the TP.

Figure 5.

Figure 5

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(a) Time-dependent photoluminescence of nanophosphors integrated in a Ag–dielectric (blue) and a fully dielectric system (red) along with their fits to a sum of two log-normal distributions of decay rates. (b) Log-normal distributions of the decay rates corresponding to the fits shown in (a) using the same color code.

Our results demonstrate that nanophosphor films can be integrated in the vicinity of the interface of a periodic multilayer and a metal film, which sustain optical Tamm plasmons. A careful design and fabrication of metal–dielectric structures allows tuning the spectral position of plasmonic–photonic modes to match the main emission line of Eu3+-based nanophosphors. As a result, the photoluminescence of the nanophosphors is significantly enhanced, up to 8-fold. This behavior originates from the interaction of the emission of the nanoemitters with collective resonances of the metal–dielectric system. Field profiles associated with optical Tamm plasmons extend away from the metal surface, yielding an efficient coupling of localized energy into well-defined directions of free space. Finally, the analysis of the emission dynamics indicates that the average lifetime decreases in the presence of the metal, which we attribute in part to an enhanced radiative decay rate mediated by the Tamm plasmon. Our findings bring to light that hybrid metal–dielectric systems sustaining plasmonic–photonic modes provide new opportunities for an accurate control of the photoemission of rare-earth nanosources distributed over large areas, which could open new ways of designing novel solid-state lighting devices.

Methods

Materials:

Gadolinium(III) nitrate hexahydrate (Gd(NO3)3·6H2O, Aldrich, 99.9%), europium(III) nitrate pentahydrate (Eu(NO3)3·5H2O, Aldrich, 99.9%), sodium orthovanadate (Na3VO4, Aldrich, 99.9%), poly(acrylic acid) (PAA, Aldrich, average Mw ≈ 1800), ethylene glycol (EG), zirconium(IV) n-propoxide 70% in 1-propanol (Aldrich), tetraethyl orthosilicate (TEOS, Aldrich), triblock copolymers Pluronic F127 (Mw ≈ 12 600), 2,4-pentanedione (acetylacetonate, acac, AlfaAesar), SiO2 (LUDOX TMA, Aldrich), absolute ethanol, methanol, HCl (3.571 mol/L, Panreac), and Milli-Q water.

Nanophosphor Synthesis

A solvothermal method was used to synthesize the GdVO4:Eu3+ nanophosphors in our present work. Suitable amounts of RE nitrates were dissolved in EG (2.5 mL). The doping concentration of Eu3+ in the GdVO4 host was fixed at 10% in molar ratio. To facilitate the dissolution of reagents in EG, the solutions were mildly heated (∼80 °C) under magnetic stirring. In a separate vial, a weighted amount of Na3VO4 and 2 mg·mL–1 of PAA were dissolved in an EG–H2O mixture (1 mL EG + 1.5 mL H2O). After cooling to room temperature, both solutions were then admixed with magnetic stirring. In the final solutions, the total RE and the Na3VO4 concentrations were kept constant at 0.02 and 0.1 mol·L–1, respectively, whereas the final EG–H2O volumetric ratio was 3.5:1.5. The as-prepared solutions were then aged for 3 h in tightly closed test tubes using an oven preheated at 120 °C. Then, the resulting dispersions were cooled to room temperature, centrifuged to remove the supernatants, and washed twice with ethanol and once with distilled water. Finally, the precipitates were redispersed in methanol with a concentration of 1% wt.

ZrO2 Precursor Sol Synthesis

Briefly, a mixture of ethanol, acac, zirconium n-propoxide, HCl, water, and F127 in a molar ratio of 40:1:1:1:20:0.005 was prepared. Zirconium propoxide, acac, and F127 were dissolved in 80% of the total ethanol, and the mixture was stirred for 1 min. HCl and water dissolved in the remaining ethanol were added dropwise while stirring to the first solution. The final sol was then stirred for 1 h.

SiO2 Precursor Sol Synthesis

For the synthesis of the SiO2 sol, we used 9 mL of tetraethyl orthosilicate and 68 mL of absolute ethanol. After some minutes under vigorous stirring, 3.44 mL of H2O and 0.16 mL of HCl (0.05 mol·L–1) were added. The resultant solution was stirred 1 day before use.

Deposition of Optical Multilayer

Optical multilayers were fabricated by alternately depositing the ZrO2 and SiO2 layers using a dip-coater with the corresponding precursors. The SiO2 dense layer was employed to prevent the infiltration of the ZrO2 precursor sol into SiO2 nanoparticle layers, which are inherently porous. The withdrawal speeds of the substrate in ZrO2 sol, SiO2 nanoparticle suspension (1 wt % in methanol), SiO2 sol, and GdVO4: Eu3+ nanophosphor suspensions were respectively 120, 120, 50, and 120 mm·min–1. The multilayer starts with the deposition of the ZrO2 sol on top of a piece of clean quartz substrate. This first layer was treated at 500 °C for 30 min. Next, a layer of SiO2 nanoparticles was deposited on top of the ZrO2 layer in several steps of the same suspension to increase its thickness with a waiting time of 60 s between dips to ensure solvent evaporation. After that, a dense SiO2 layer was coated with a soaking time of 30 s in the corresponding precursor sol; then a 10 min heat treatment at 500 °C followed to stabilize the layer. This process was repeated until 10 unit cells were deposited and another ZrO2 layer was coated to complete the structure. Then, a nanophosphor film was deposited in several steps with 5 min of intermediate heat treatment at 400 °C between dips, and a 30 min heat treatment at 500 °C was performed to improve its structural stability. To planarize the surface of a nanophosphor layer, a thin layer of dense ZrO2 was coated in three steps with a withdrawal speed of 20 mm·min–1, and a 10 min heat treatment at 400 °C after each dip was employed to increase the stability. Finally, the Ag or Au layer with a thickness of 150 nm was evaporated to finish the sample. The sample without coating the metal layer was chosen as the reference.

Structural Characterization

The shape of the nanoparticles was examined by transmission electron microscopy (TEM, Philips 200CM). FESEM images of the multilayer films deposited onto glass were taken by using a Hitachi S4800 microscope.

Surface Characterization

The surface of the nanophosphor films was characterized by AFM (Park Systems XE-100) working at tapping mode.

Optical Characterization

The excitation and emission spectra as well as the lifetime of these samples were measured with a Horiba JobinYvon spectrofluorometer (Fluorolog FL3-11). Reflectance spectra were measured using a Cary 7000 series UV–vis–NIR spectrophotometer.

Calculations

Spectral dependence of the spatial distribution of the electric field intensity along with the reflectance was calculated using an in-house code based on the transfer matrix approach. A genetic algorithm was employed to find the structural parameters that yield a targeted optical response. The refractive indexes for the SiO2 and ZrO2 employed were estimated from experimental data using a Cauchy formula. The nanophosphor layer index was also estimated from experimental data using the Forouhi–Blommer dispersion formula. Metal indexes were taken from the literature.45,46

Our design method is based on a continuous feedback between calculations and measurements. As an initial step, we design the hybrid system based on a fully periodic multilayer. In particular, the lattice parameter of the photonic crystal is designed such that the forbidden band overlaps with the spectral range where the main emission peaks of Eu3+ cations are, while the higher diffraction orders of the structure do not block their photoexcitation. As a result, we obtain layer thickness values of ZrO2 (84 nm) and SiO2 layers (91 nm). Then, we fabricate a photonic multilayer based on this design, fit its experimental reflectance using a genetic algorithm, and obtain the multilayer given by quartz substrate/ZrO2 (81 nm)/SiO2 (83 nm)/ZrO2 (79 nm)/SiO2 (102 nm)/ZrO2 (84 nm)/SiO2 (88 nm)/ZrO2 (89 nm)/SiO2 (108 nm)/ZrO2 (76 nm)/SiO2 (96 nm)/ZrO2 (93 nm)/SiO2 (91 nm)/ZrO2 (81 nm)/SiO2 (68 nm)/ZrO2 (69 nm)/SiO2 (77 nm)/ZrO2 (80 nm)/SiO2 (99 nm)/ZrO2 (87 nm)/SiO2 (99 nm)/ZrO2 (101 nm). From this aperiodic layout, we optimize the thickness of the nanophosphor layer in order to maximize the electric field intensity enhancement in the layer at the wavelength where the nanophosphor emission peaks and find an optimal value of ∼170 nm (see Supporting Information).

Acknowledgments

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (NANOPHOM, grant agreement no. 715832) and the Spanish Ministry of Economy and Competitiveness under grant MAT2017-88584-R.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b01407.

  • Additional figures (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Author Contributions

† D. Geng and E. Cabello-Olmo contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

ph8b01407_si_001.pdf (637.3KB, pdf)

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