Directional Emission of Fluorescent Dye-Doped Dielectric

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Functional Nanostructured Materials (including low-D carbon)

Directional Emission of Fluorescent Dye-Doped Dielectric Nanogratings for Lighting Applications Antonio Ferraro, Dimitrios C Zografopoulos, Marc A. Verschuuren, Dick K.G. de Boer, Frank Kong, H. Paul Urbach, Romeo Beccherelli, and Roberto Caputo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08971 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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ACS Applied Materials & Interfaces

Directional Emission of Fluorescent Dye-Doped Dielectric Nanogratings for Lighting Applications Antonio Ferraro1,2, , Dimitrios C. Zografopoulos2, Marc A. Verschuuren3, Dick K.G. de Boer4, Frank Kong5, H. Paul Urbach5, Romeo Beccherelli2, and Roberto Caputo1,* 1

Department of Physics, University of Calabria, Via Ponte Bucci Cubo 33b, 87036 Rende, Italy.

2

Consiglio Nazionale delle Ricerche, Istituto per la Microelettronica e Microsistemi, Via del Fosso del

Cavaliere 100, 00133 Rome, Italy. 3

Philips Group Innovation – Intellectual Property & Standards, De Lismortel 31 (building 76), 5612 AR

Eindhoven, The Netherlands. 4

Philips Lighting Research, High Tech Campus 7, 5656AE Eindhoven, The Netherlands.

5

Delft University of Technology, Department of Imaging Physics, Lorentzweg 1, 2628CJ Delft,

The Netherlands

KEYWORDS. Nanophotonics, lighting, diffraction grating, fluorescence, photoluminescent materials

ABSTRACT. By structuring a luminescent dielectric interface as a relief diffraction grating with nanoscale features, it is possible to control intensity and direction of emitted light. The composite structure of the grating

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is based on a fluorescent dye (Lumogen F RED 305) dispersed in a polymeric matrix (polymethylmethacrylate, PMMA). Measurements demonstrate a significant enhancement of the emitted light for specific directions and wavelengths when the grating interface is compared to non-structured thin films, made of the same material. In particular, the maximum enhancement of photoluminescence for a given pump wavelength is obtained at an angle of incidence that is close to the Rayleigh anomaly condition for the first-order diffracted waves. In this condition, the maximum extinction of incident light is observed. Upon excitation with coherent and monochromatic sources, photoluminescence plots show that the Rayleigh anomalies confine the angular interval of emitted light. Being the anomalies directly related to the pitch of the diffraction grating, the system can be thus implemented as an optical device whose directional emission can be designed for specific applications. The exploitation of nanoimprinting techniques for the fabrication of the luminescent grating enables production of the device on large areas, paving the way for low-cost lighting and solar applications.

INTRODUCTION. Nowadays, cost-effective and low-power devices able to control the light emission are of paramount importance for display, lighting and solar energy harvesting applications1–3. Much effort and different approaches are focused on improving the properties, in terms of intensity and directionality, of light sources employing luminescent materials. These are, for instance, employed to enhance the sunlight absorption in photovoltaic cells or to control lighting emission in the so-called white light-emitting diodes (LED)4–6. The exploitation of different typologies of plasmonic resonances, arising at the interface between metals and dielectrics, is one of such approaches. This has been widely studied in the context of what is termed as surfaceenhanced fluorescence7–10. The most common type is based on the coupling between metallic nanoantennas and dye luminescent materials, which results in great enhancement of the emitted light intensity11–13. However, the latter is confined in a very narrow spectral band, which can limit its use. Moreover, this approach is time


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consuming and costly since it involves several fabrication steps with nanometric or deep sub-micron resolution. Another commonly used approach relies on the use of guided mode resonances14–16 in conjunction with nanoparticles arrays. In this case, by only varying the thickness of the guided layer, it is possible to tune the emission spectra of the device. A complementary route is based on a suitable choice of molecules embedded in the devices, such as rare earth complex or quantum dots 17–21. In this work, we present a proof-of-principle device able to control the light emission by employing a purely dielectric diffraction grating doped with an organic fluorescent dye. The proposed device, fabricated by means of relatively low-cost nanotechnology fabrication techniques, exploits a light-matter interaction that allows the control of the direction and intensity of the emitted light. Specifically, the first-order Rayleigh anomalies of the diffraction grating confine the emission of light in a limited angular region. This physical behavior is evidenced by angle-resolved measurements and confirmed by numerical studies. The proposed approach, which does not involve any metallic patterns requiring additional fabrication steps, allows for a possible reduction of both -production time and costs.

SAMPLE FABRICATION. The proposed device is schematically illustrated in Figure 1(a,b). As a first step in the fabrication process, poly(methyl methacrylate) (PMMA) Plexiglass 7N22 and Lumogen F RED 305 dye (F305, Figure 1(c))23 (Evonik Röhm GmbH and BASF Co.) were dissolved in toluene and dichloromethane respectively, then mixed together, resulting in a final 9 wt% dye concentration. The dye concentration corresponds to the threshold between maximum dye emissivity and concentration quenching, which leads to a decrease of the intensity of the emitted light. The absorption and emission spectra of the dye are shown in Figure 1(d), indicating that maximum absorption and emission occur at 578 nm and 613 nm, respectively. The PMMA-F305 composite solution was spin-coated on a glass substrate Schott AF4524 having thickness



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ts=0.35 mm and refractive index ns=1.52, resulting in a final thin layer with thickness td=200 nm, as illustrated in Figure 1(b). The specific F305 dye was chosen for its good solubility in PMMA and for the convenient absorption and emission bands. In principle, any fluorescent dye showing good stability in PMMA or other transparent polymers is applicable to the described approach, once a fabrication protocol has been defined.

Figure 1: (a) Schematic 3D drawing of the proposed diffraction grating. The fluorescent dye-doped grating is illuminated by a green light plane wave laser source and directionally emits red light owing to photoluminescence. (b) Cross-section of the grating unit cell and definition of the relative geometrical and material parameters. (c) Molecular structure of the


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fluorescent dye Lumogen F Red 305 (IUPAC name: perylene-1,8,7,12-tetraphenoxy-3,4,9,10-tetracarboxylic acid bis(2',6'-diisopropylanilide)) employed in the study, and (d) its light absorption and emission spectra.

Next, a diffraction grating was fabricated on top of the thin film by using the substrate conformal imprint lithography (SCIL) technique25. According to the process flow indicated in Figure 2, a layer of SiO2-based solgel material with ~100 nm thickness is first deposited by spin-coating on top of the PMMA-F305 thin film.

Figure 2: (a) through (e). Process flow of the Substrate Conformal Imprint Lithography (SCIL) technique, developed by Philips Research, which was employed for the fabrication of the device.



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Within one minute from spin coating, the sol-gel layer is imprinted using a patterned polydimethylsiloxane (PDMS) stamp characterized by 580 nm pitch, 50% duty cycle and a 70 nm peak to valley height. During the imprint time (30 mins) the sol-gel forms a rigid glass-like structure that does not deform during removal of the stamp, nor due to surface tension after the stamp release. The stamp is then detached from the hardened sol-gel layer by gently peeling it off from the side. The residual sol-gel material is removed by means of reactive-ion etching (RIE) by using a mixture of CF4 and N2 in a 1:2 flow ratio, at a pressure of 12 Torr and RF-power of 50 W for 3 mins. Finally, a low-pressure oxygen RIE (70 W, 60 sec) is used to transfer the sol-gel pattern onto the underlying PMMA layer. The resulting grating is schematically illustrated in Figure 1, described by the following set of parameters: p=580 nm, w=290 nm, td=200 nm, ta=70 nm, and ts=0.35 mm that represent its period, stripe width, PMMA+F305 thickness, sol-gel thickness and substrate thickness respectively. The geometrical properties of the sample were confirmed by atomic force microscope (AFM) analysis by means of a Veeco Dimension 3100 scanning probe microscope, using a Si “high aspect ratio” probe in tapping mode on a 5×5 µm2 area. Scanning electron microscope (SEM) characterization of the sample was avoided because the required metallization of the dielectric structure would have irremediably modified its optical features and photoluminescence behavior, hence preventing further experimental investigations. In Figure 3 3D, 2D AFM images show the spatial thickness profile of the investigated sample and the corresponding 1D cross-sections. Total height td+ta=270 nm, pitch p=580 nm and stripe width w=290 nm were measured.


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Figure 3: (a) 3D and (b) 2D AFM images, showing the spatial thickness profile of the investigated sample. From the (c-d) corresponding 1D cross-sections, total height td+ta=270 nm, pitch p=580 nm and stripe width w=290 nm were measured.

OPTICAL CHARACTERIZATION AND DISCUSSION. The diffractive and light emitting properties of the fabricated grating were analyzed by using the setups exemplified in Figure 4. In a first experiment, a broadspectrum probe light generated by a halogen lamp passed through a linear polarizer used to select either s(parallel to the stripes) or p-polarization (perpendicular to the stripes) and then impinged from free space onto the sample from the grating side. Light extinction was measured after the glass substrate, as sketched in Figure 4(a). The sample was mounted on a motorized rotation stage, thus enabling a fine control of the angle of



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incidence θi. Light transmitted directly from the sample, i.e. the zero-order diffraction mode, was then collected by using an UV-VIS fiber coupled spectrometer. The transmittance T(λ) of the fabricated grating was calculated by normalizing the measured intensity to the one transmitted by the bare glass substrate. Finally, the extinction value E(λ) that expresses the reduction in transmitted light intensity due to absorption, reflection or diffraction into higher orders propagating in the grating, was derived by calculating E=1-T(λ).

Figure 4: Schematics of the utilized setup for the (a) extinction and (b) photoluminescence emission measurements of the fabricated photoluminescent grating, where P is a linear polarizer, RS is the rotation stage, θi the angle of incidence, and θs the angle of the measuring spectrometer. In (a) the transmittance of the zero-order diffracted beam was measured in a broad spectrum and for different θi, whereas in (b) the directionality of the emitted light for single-frequency illumination of the sample was investigated in an angle-resolved measurement.

Obtained extinction diagrams, plotted as a function of the angle of incidence in the 500-800 nm spectral range, are reported in Figure 5 for both p- and s-polarized probe light. The experimentally measured values are directly compared with simulations, revealing a satisfactory agreement. Our numerical studies were performed by means of the frequency-domain finite-element method implemented in the commercial software COMSOL Multiphysics. In the simulations, the refractive index for the sol-gel was assumed equal to 1.42, whereas for the real part of the index of the dye-doped PMMA composite, due to the low dye concentration, was assumed equal


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to the refractive index of PMMA (n=1.49). Conversely, the wavelength-dependent imaginary part of the refractive index of the composite was taken as proportional to the absorption spectrum shown in Figure 1(d) and based on the absorption measurements of a reference 200-nm thick film of the PMMA-dye composite. In Figure 5 , a fundamental role is played by the Rayleigh anomalies of the diffraction grating imprinted in the PMMA layer. For a given diffraction order, the anomaly is identified as the boundary between radiative and non-radiative (evanescent) propagation, the latter corresponding to grazing propagation of the considered diffracted wave along the surface of the grating26. The grating equation that expresses the angle of diffraction for the mth order as a function of wavelength λ and angle of incidence θi of the impinging plane wave is given by: ݉ߣ = ‫݌‬ሺ‫ߠ݊݅ݏ‬௠ − ‫ߠ݊݅ݏ‬௜ ሻ,

(1)

where p is the grating pitch. Air is herein assumed as surrounding medium. Rayleigh anomalies occur when sinθm=±1, corresponding wavelengths are calculated as: ௣

ߣோ஺ = ௠ ሺ−‫ߠ݊݅ݏ‬௜ ± 1ሻ,

(2)

The Rayleigh conditions given by Eq. 2 for m=-1,1, and 2 are marked as black lines in the results presented in Figure 5. Experimental measurements demonstrate that the presence of Rayleigh anomalies influences the light absorption mechanism and enhances the extinction of light by the system. The extinction is significantly higher for s-polarization and maximized in the vicinity of the Rayleigh anomalies, with the highest value of 0.7 for normal incidence at 580 nm wavelength. This wavelength coincides with the maximum absorption of the dye material, as shown in Figure 1(d). This result is not accidental. Indeed, if the k-vector of the impinging light fulfills the Rayleigh condition for a given mth diffracted order, most of the energy of that order is forced to propagate along the grating as an evanescent wave and hence localized in proximity of its surface27. By considering Eq. 2, in the approximation of small incident angles and m=±1, it results that λ≈p. Therefore, if the



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value of the grating period falls within the absorption band of the fluorescent material, it is expected that most of the impinging radiation undergoing maximum extinction is trapped by the structure and efficiently exploited for the dye excitation. In other words, it can be argued that in proximity of the Rayleigh anomalies (where light extinction is maximum), the dye excitation has a resonant-type behavior that is proportional to the amount of extinguished light. In order to verify this hypothesis, we performed a series of static fluorescence measurements of the luminescent grating when excited by the CW laser schematically illustrated in the setup of Figure 4(b).

Figure 5: Measured (a,c) and numerically calculated (b,d) power extinction spectra of the grating for both p- and spolarized incident light as a function of wavelength and angle of incidence. The black lines represent the Rayleigh


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anomalies for the m=-1,1, and 2 diffraction orders. The numbers in the parenthesis in (a) denote the propagating grating transmission modes in each region.

As for the excitation wavelength, we preferred not to excite at 578 nm where the extinction is maximum, in order to better discriminate the emitted spectrum from the pump and also because such a wavelength is not easily available. Conversely, green lasers are easily available and their 532 nm wavelength is well outside the emission spectrum of the dye (Figure 1 (d)). In a second experiment, the angle of incidence for s-polarized 532nm light was fixed at certain discrete values by tilting the sample on the rotation stage accordingly. The intensity and directionality of the emitted light in the 550-800 nm spectral window were recorded via angleresolved measurements, by rotating the spectrophotometer in the range (-5; 90)° with 1° steps. The emission angle θs is defined such that θs=0° corresponds to the zero-order transmitted wave, as shown in Figure 4(b). Particularly relevant are the data for the absolute photoluminescence (PL) in case of an incident angle θinc=7°, that is very close to the Rayleigh anomaly for the m=1 order (Figure 5 (c, d)). In Figure 6(a,b), the PL value is plotted in terms of emission angle and wavelength, for the structured (grating) and the uniform thin film (PMMA-F305, td=200 nm) respectively.



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Figure 6: Photoluminescence as a function of wavelength and emission angle at θi= 7˚ incidence s-polarized 532nm light: measured data for (a) patterned grating and (b) uniform film and (c) numerically computed data for patterned grating; (d) measured data for enhancement in the photoluminescence for the patterned grating; the inset in (d) shows a colour photograph of the emitted light pattern from the grating.


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During the experiment, dye molecules absorb light at 532 nm and re-emit at longer wavelengths, with peak emission at 610 nm approximately. Because they are uniformly and randomly distributed in the PMMA film, it is reasonable to assume a Lambertian cosine law for their emission in the x-z plane. As such, in presence of a uniform film, the overall emitted light pattern results Lambertian in emission angle as well (Figure 6(b)). The observed confinement in emitted wavelength is instead due to the emission curve of the dye that is limited to the spectral interval (550; 700) nm with a peak emission amplitude in the interval (570; 630) nm. For what concerns the structured film (Figure 6(a)), most of the emitted light is angularly localized in a triangular region in the x-z plane, i.e. the grating plane of incidence (Figure 1 (a)), delimited by the Rayleigh anomalies. In this case, we can still assume that dye molecules present in the fringes of the grating emit light in a Lambertian distribution. However, due to the presence of the grating, light emitted in all directions is not only transmitted by the film as in the uniform case, but diffracted in a large range of angles (except in the upper region within the Rayleigh lines, λ>580nm, where light is only transmitted). From the comparison of Figure 6(a) and (b), the effect that diffraction produces on the emission pattern is quite impressive. To investigate this optical effect, a calculation of the diffractive properties of the grating by using a rigorous coupled-wave approach was performed. The result of the calculation was utilized to estimate how light is re-distributed by the structured film in angle and wavelength. This was done by qualitatively calculating the grating photoluminescence efficiency by properly weighting the extinction simulations of Figure 5(c) with a Lambertian distribution of the emitted light and by the spectral dependence of the dye emission curve (Figure 1 (d)). The resulting intensity map is reported in Figure 6(c) and shows a noticeable agreement with the experimental measurement of the light emitted by the grating upon laser excitation (Figure 6(a)). In order to calculate the enhancement in photoluminescence between the grating structure and the non-structured thin film, each value of the intensity of the light emitted by the grating, reported in Figure 6(a), was divided by the



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corresponding one from the uniform film in Figure 6(b). We focused on the case of s-polarization, as this resulted in higher extinction values. In order to collect only the emission originating from the surface of the sample, the substrate edges were carefully blackened. Figure 6(d) reports the enhancement in a zoomed area and clearly demonstrates an enhancement factor of approximately 2.5x within the above considered triangular region. It must be stressed that the enhancement results from the grating with duty cycle of 50% (w=p/2), in which the amount of dye molecules is half the one contained in the reference case of the uniform PMMA-F305 film. It follows that the effective enhancement per unit dye volume is twice the values displayed in Figure 6(d), i.e. up to 5x. The fact that the enhancement is angularly localized in the region delimited by the Rayleigh anomalies is particularly interesting as it demonstrates that the device is able to control the directionality of the emitted light. The inset of Figure 6(d) shows a photograph capturing the light emitted by the illuminated sample. As predicted by the experiments, when the system is excited by a green laser, most of this light is absorbed by the luminescent grating and the emitted light is spectrally comprised in a yellow-red interval. To further elucidate the origin of the enhanced photoluminescence, we have calculated the normalized electric field intensity distribution of a thin PMMA film and of the investigated grating made of pure PMMA and solgel material. We have assumed an s-polarized plane wave at 532 nm, with electric field amplitude E0, and impinging at θi=7o. The electric field intensity is proportional to the light intensity absorbed by the dye and therefore provides an estimate of the enhancement factor of the resulting photoluminescence of the excited dye molecules, which can be considered as incoherently radiating dipole sources. Figure 7 presents the spatial profiles of the electric field intensity for both cases. The normalized field intensity in the grating (Figure 7 (b)) locally obtains values nearly up to a factor of 5, compared to the thin film (Figure 7 (a)). By integrating over the PMMA volume the average intensity enhancement is approximately twice as high as in the grating, which can be directly correlated with the experimentally observed enhancement.


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Figure 7: Numerical calculation of the s-polarized electric field intensity enhancement in (a) thin film PMMA 200 nm thick, (b) in a grating with the investigated dimensions composed of PMMA (td=200 nm) and sol-gel (ta=70 nm), both on glass substrate. The angle of incidence is θi=7˚.

As already discussed, the maximum enhancement in emission occurs in proximity of the Rayleigh condition for the incident light where the energy channeled to the dye molecules in the PMMA grating is maximized. This is experimentally verified by repeating the photoluminescence measurements for three more angles of incidence, namely θi=15o, 20o, and 40o, for the same remaining parameters as in Figure 6(d). The results are presented in Figure 8. It is clearly observed that the enhancement is progressively reduced as the angle of incidence increases. This is consistent with the extinction measurements and simulations presented in Figure 5(c,d), where the red dashed line corresponds to the laser wavelength of 532 nm. Interestingly, the stronger emission is still taking place within the Rayleigh delimited region for all three angles, as the grating pitch and related conditions are not modified. Thus, even in the case of non-optimal excitation angle, the emission is still directional due to the presence of a grating structure. As a side-note, the intense region observed in the case of θi=45o corresponds



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to the first-order diffracted wave of the impinging laser field, which saturates the detector. Indeed, for 580 nm pitch and θi=45o, the grating equation (Eq. 1) yields a first order diffraction angle θd=12.13o.

Figure 8: Enhancement in the photoluminescent light intensity for the patterned grating as a function of wavelength and the emission angle, measured at three angles of incidence: (a) θi=15˚, (a) θi=20˚, and (a) θi=45˚.

CONCLUSIONS. The fluorescent emission of a device comprising a nanograting engraved in a dye-doped PMMA thin layer has been studied. Excitation of the structure in proximity of the Rayleigh anomaly condition of the grating leads to enhanced absorption and photoluminescence. Under such conditions, the grating can provide overall enhanced photoluminescence by more than two times, as compared to a thin film of the same thickness with double quantity of fluorescent dye material. By properly adjusting the grating pitch and consequently, its diffraction properties, the emission angle can be selected at a given wavelength. Further experiments are ongoing to define behavior and features of the system when illuminated by non-collimated commercial light sources like LEDs. Fluorescent dye-doped nanogratings provide an efficient light emitting mechanism with tailorable directionality for display technology, lighting applications and smart photovoltaics.

AUTHOR INFORMATION Corresponding Author:


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Dr. Roberto Caputo, email: [email protected]

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

Funding Sources The manuscript has received networking support by the European Cooperation in Science and Technology (COST) within the action: “Integrating Devices and Materials: A challenge for new instrumentation in ICT”, grant number: IC1208

ACKNOWLEDGMENT The author(s) would like to acknowledge networking support by the COST Action IC1208, “Integrating Devices and Materials: A challenge for new instrumentation in ICT”.

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