Integration of Nanoemitters onto Photonic Structures by

Article Cite This: J. Phys. Chem. C 2019, 123, 14669−14676

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Integration of Nanoemitters onto Photonic Structures by Guided Evanescent-Wave Nano-Photopolymerization Giuseppe Emanuele Lio,*,†,‡ Josslyn Beltran Madrigal,† Xiaolun Xu,† Ying Peng,† Stefano Pierini,†,§ Christophe Couteau,† Safi Jradi,† Renaud Bachelot,† Roberto Caputo,‡ and Sylvain Blaize*,† †

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Light, Nanomaterials and Nanotechnologies (ex-LNIO), CNRS, University of Technology of Troyes (UTT), 12 rue Marie Curie, Rosières-près-Troyes 10430, France ‡ CNR-Nanotec and Department of Physics, University of Calabria, Arcavacata di Rende 87036, Cosenza, Italy § Laboratoire Kastler Brossel, Sorbonne University, CNRS, ENS-PSL Research University, Collége de France, Paris 75005, France ABSTRACT: In this article, we demonstrate the feasibility of self-positioning nanoemitters onto optical waveguides by visible-light nanoscale photopolymerization. A light-sensitive material containing nanoemitters is photopolymerized at interfaces by using the evanescent field of the light propagating in photonic structures. By exploiting this method, it is possible to pattern polymeric ridges containing CdSe/ ZnS nanocrystals (NCs) directly on top of optical guiding structures. Photopolymerization experiments have been performed in the case of a single ion-exchange glass waveguide (IEx WG) and of a double waveguide made of the IEx WG with a thin titanium dioxide film fabricated on top of it. Atomic force microscopy (AFM) and spectroscopy analyses highlight the reliability and reproducibility of the fabrication technique. Continuous ridges of controlled thickness have been realized on top of a single waveguide interface with thicknesses as small as 18 nm, thus thick enough to contain only a photoluminescent NC monolayer in the vertical direction. In the presence of the double waveguide (TiO2 layer on top of the IEx WG), AFM measurements reveal that the thickness of the photopolymerized ridge has a sinusoidal modulation. This is due to a light beating phenomenon theoretically predicted in the case of light propagating in coupled waveguides. In our case, the light beating can be efficiently exploited to photopolymerize ridges with modulated thickness. Overall, the flexibility of the reported nanoscale fabrication technique paves the way toward the controlled positioning of single nanoemitters in proximity of nanoantennas or other elaborate plasmonic/photonic structures.

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INTRODUCTION Photonics has proven to be a key enabling technology for many applications. The use of integrated optics is of tremendous importance and has relevant application in many systems.1−3 Nanoemitters and nanosources of light are crucial elements for applications in biology imaging, nanomedicine, quantum technologies, or sensing.4−10 The integration of nanoemitters with guiding structures seems thus the natural step forward for miniaturization. Nevertheless, it is by no means easy to combine these two elements together in a reproducible and controlled way. Most techniques are based on elaborate electron-beam lithography methods for positioning emitters11,12 or by using dispersion-type techniques where nanoemitters are just sprinkled on top of photonics structures.13−16 Nanotips have also been used for nanoemitter positioning but have not made the process easy and reproducible. In our work, we describe an efficient and easy way to position nanosources of light on top of guiding structures such as high-index thin films/nanolayers and glass waveguides. For that, we exploit the evanescent near-fields at the surface of photonic structures that, interacting with a light© 2019 American Chemical Society

sensitive polymer/resin, mold a given structure. Spatially controlled photopolymerization thus represents an interesting tool for the micro- and nanopatterning of polymers17 and hybrid materials including polymers containing quantum dots with high spatial resolution.18,19 The size of the photopolymerized structures could even reach few nanometers in case of plasmon-induced photopolymerization.20−22 In microfabrication by photopolymerization processes, a light beam triggers a polymerization reaction, which results in solidification of the liquid material in the irradiated areas, while nonirradiated areas remain unchanged and can be washed out by suitable organic solvents. When a droplet of resin is exposed to impinging light, physical chemistry mechanisms as cross-link formation are involved.23,24 This is also valid while polymer curing is due to the enhanced electric field produced by nanoparticles, nanorods, or plasmonic sources.25−27 The related technique is indicated as photolithography, and more Received: April 20, 2019 Revised: May 21, 2019 Published: May 22, 2019 14669

DOI: 10.1021/acs.jpcc.9b03716 J. Phys. Chem. C 2019, 123, 14669−14676

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The Journal of Physical Chemistry C

are characterized by a gradient of the refractive index (Δn = 0.7) responsible for trapping the light (Figure 1a). The

or less complex structures can be realized by coupling light with specific wave fronts and vitrifying only the cured polymer. Based on this process, micro-optical elements (microlenses and waveguides) can be also fabricated at the extremity of optical fibers.28,29 Because of the volume of the irradiation beam that is diffraction limited, the size of the polymer features in the farfield configuration is limited to few hundreds of nanometers. The confinement of the polymerized volume can be tuned by introducing some physicochemical effects such as chemical quenching by oxygen30 and the decrease of light penetration with increasing absorptivity of the material in accordance with the Beer−Lambert law.31 Additional decrease in the polymer feature sizes is possible by introducing free radical inhibitor molecules into the formulation. This results in a drastic decrease of the local density of the free radical and thus in more confined polymer structures.32 Two-photon polymerization is an alternative approach to microfabrication that has been introduced 20 years ago.33−35 It allows writing submicrometer polymer structures beyond the diffraction limit in a 3D manner due to the threshold behavior and nonlinear nature of the process. As a result, the technique provides much better structural resolution and quality than the well-known stereolithography method. However, the spatial resolution in the z direction is limited due to the rugby ball-shaped laser beam profile. One of the most promising approaches for the fabrication of ultrathin polymer films is the use of evanescent waves to induce photopolymerization. Indeed, during photopolymerization by evanescent waves (PEW), the energy received by the material exponentially decays with distance from the interface, which results in a strong confinement of the photopolymerized thickness36,37 or a simple way to map surface plasmon polaritons38 due to the very high sensitivity of the electric field perturbation. Coupling interference photolithography with PEW allows thus patterning thin polymer films in a spatially controlled manner (76 nm polymer line width in a grating of 160 nm pitch).39 However, the fabrication of such thin polymeric structures requires the use of prisms with a refractive index (np) higher than that of the photopolymer (nph). PEW takes place at the interface of the prism/photopolymer, and the polymeric thin film is laminated to the prism surface.35 Another possible configuration for the fabrication of polymeric thin films is on a free-standing substrate. It requires the use of different layers with different refractive indices as follows: prism/liquid/substrate/photopolymer with np = nliquid = nsubstrate ≥ nph.36 Unfortunately, the imposed constraints in terms of the refractive index make this configuration not suitable for the integration of polymeric nanostructures on top of optical waveguides and/or photonic structures. In this paper, we propose an original method for achieving spatially controlled nanofabrication on top of optical waveguides. This novel method exploits the evanescent fields associated to the light propagating at the interface between the waveguide and a photopolymerizable material. These fields allow curing a thin layer (few nanometers) of polymer containing quantum dots on the waveguide surface.

Figure 1. Sketches of the samples involved in the PEW experiments showing a (a) laser beam entering a typical IEx WG. Here, the shaded area (from blue to yellow) visible on the sample surface represents the increasing value of the refractive index that passes from 1.5 to 1.57. The dashed arrow represents the propagation direction of the green laser beam responsible of the PEW mechanism; (b) PEW experiment on a single waveguide where a photoresin droplet is cast directly on top of the IEx WG; (c) PEW experiment on a double waveguide where the photoresin droplet is cast on a 85 nm TiO2 thick slab fabricated directly on top of the IEx WG.

refractive index changes its value from 1.5 in the glass bulk to 1.57 in the proximity of the surface, where the area is doped with ions. The two main experiments, reported in the following, depict how the PEW mechanism takes place due to the evanescent field associated with the guided light. In one case, polymerized ridges are directly fabricated on top of a bare IEx WG (single waveguide experiment, Figure 1b); in another case, the cured ridges are obtained on the surface of a double dielectric waveguide, made of a nanolayer of titanium dioxide (TiO2), placed on top of an IEx WG (Figure 1c).

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EXPERIMENTAL PROCEDURE AND RESULTS To implement the novel method, ion-exchange waveguides (IEx WGs)40−42 have been involved. These guides are fabricated by inducing an exchange of ions (Ag ions from molten salts with Na ions contained in the glass substrate) directly on the substrate surface (glass), which results in the realization of low-loss waveguides.43−45 The obtained IEx WGs 14670


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sample for the PEW experiment is prepared by casting a tiny amount of photoresin on the surface of the sample in correspondence of the IEx WG. In order to precisely check the position of the tapered fiber with respect to the IEx WG and the droplet deposition on the sample surface, a chargecoupled device (CCD) camera mounted above the sample equipped with a microscope objective (20× magnification) has been adopted (Figure 2b). The starting point in implementing the novel nanofabrication method is the empirical search of the lowest energy dose corresponding to the minimum achievable polymer ridge thickness. Indeed, in a typical photopolymerization process, light induces the polymerization reaction when the exposure energy dose exceeds a given threshold. Experiment wise, this threshold is related to the exposure time and the incident laser power according to the relation te × P. Operatively, to find this threshold, a reference time te = 60 s has been fixed, and the corresponding minimum power value has been found as the one below which no ridges were observable by atomic force microscopy (AFM) measurements. Results of this approach suggest an empirical threshold P value of 0.56 mW for the resin without NCs and of 0.70 mW in the presence of NCs. The fact that the energy dose threshold is higher in the presence of NCs is due to the fact that these partly absorb the incident light.19 Once the threshold for both resins has been determined, a second experiment was performed to verify the variation of the ridge morphology and size with a 5× te increase. All fabricated ridges have been characterized by AFM in terms of their thickness (t) and width (w). Figure 3a,b shows the AFM results in the case of the first photoresin (without NCs), while Figure 3c,d shows the results obtained using the photoresin that includes NCs. The key parameters of the obtained ridges are resumed in Table 1. From Table 1, it is possible to observe that the ridge width varies from 1.8 to 3.3 μm; these values are comparable with the typical core width of the ion-exchange waveguide (about 2 μm). In terms of ridge thickness, we measured values comprised between 18 and 240 nm. In particular, 18 nm represents a noticeable result as it is just slightly larger than the diameter of the involved NCs (12 nm,46−48). Such a thin polymeric ridge doped with nanoemitters could be fabricated even in proximity of optical nanostructures which could result in breakthrough applications such as control of the directivity of nanoantennas 49 or light emission driven by Mie resonances.50 We envisage the extension of our approach to these cases by combining the present results with the precise positioning of nanoemitters on plasmonic nanostructures achieved in a previous work.21,22 So far, no aging effects have been observed for the realized ridges probably because the photoresin used in this work is not exposed to high light intensity and long exposure time during the curing process,51−53 neither to thermal or mechanical stresses afterward. Photoluminescence Characterization. Once the curing parameters have been fixed providing the minimum thickness and width of the polymer ridge, a photoluminescence experiment has been performed to verify the presence of the NCs in the cured polymer. The experiment has been conducted by means of a home-built confocal microscope equipped with a spectrometer with a Peltier-cooled CCD sensitive camera. A specific area of the sample (whose size depends on the objective magnification) can be excited by blue light (λ = 405 nm). The emission spectrum is then recorded by

During the experiments, two different photopolymers have been utilized. The first one is a mixture of polymer PETA (pentaerythritol triacrylate) + 4% of MDEA (methyl diethanolamine) + 0.5% eosin Y; the second one is PETA + 4% MDEA + 0.5% eosin Y + 1% CdSe/ZnS nanocrystals (NCs); the percentages indicate molar concentrations. Both resins absorb visible light in the range from 450 to 550 nm. The first one shows a peak centered at about 525 nm; the second one, because of the presence of NCs, red-shifts its absorption peak of about 10 nm (Figure 2a).

Figure 2. (a) Micrograph shows the two absorption curves for the photoresin with and without NCs (red dashed and blue solid line, respectively). Both curves present a maximum absorption value at about λ = 530 nm. (b) Sketch of the setup utilized for the PEW experiments consisting of two lasers (λ = 632 nm and λ = 532 nm). The power of both lasers can be controlled by dedicated attenuators, while the exposure time (te) for the photopolymerization process is exactly determined by using an electronic shutter, placed along the green laser path, that remotely allows a sharp turning on and off of the laser beam. An optical fiber coupler combines both beams in a collinear path through an optical fiber put in front of the sample. The black solid arrow represents the propagation direction of the laser beams.

PEW Experiments on Single Waveguide. The experimental setup utilized for implementing the PEW nanofabrication technique comprises two lasers. A transverse magnetic (TM)-polarized green laser (λ = 532 nm) whose light is strongly absorbed by the curable material and hence used to induce the photopolymerization. A second laser (λ = 632 nm), at whose wavelength the curable mixture is not sensitive, that ensures good alignment between the green laser beam and the sample during curing. The collinear alignment between the two lasers is made by means of an optical coupler that combines the beams in the same path through a tapered fiber. The power of both lasers can be controlled by dedicated optics, while the exposure time (te) is exactly determined by using an electronic shutter, put along the green laser path, that remotely allows a sharp turning on and off of the laser beam. The detector placed behind the sample allows measuring the incident power (P) when the sample is not present. Finally, the 14671


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a spectrometer using a high-pass band filter (λ > 500 nm). The spectrum collected from the polymer ridge, directly fabricated on top of the bare IEx WG (Figure 3d), is indicated by the red solid line in Figure 4a. The measured curve is quite broad

Figure 4. (a) Emission spectrum of the photopolymerized ridge on the IEx WGs (red solid line) compared to the emission of the photoresin (blue dots, peak at 550 nm) and of the PETA + NCs (green dots, peak at 590 nm); (b) fluorescence images collected by the confocal microscope in the case of 20× magnification (top) in which the whole ridge is observed and a localized intense spot emitted by a portion of the ridge (bottom) observed with 60× magnification.

because it is the convolution of several contributions. In order to identify each contribution in this curve, the fluorescence emissions from drops of PETA + NCs (green curve) and PETA + 0.5% eosin Y (blue curve) have been separately collected. By superposing the three spectra together (Figure 4), we can distinguish the emission of the NCs at about 590 nm (green curve) as a shoulder of the red curve, while a peak at 550 nm of the red curve partly overlaps the maximum emission of eosin Y (blue curve). Finally, the first part of the spectrum for λ < 520 nm can be attributed to the MDEA emission.54,55 Figure 4b shows the fluorescence images collected by the same confocal microscope at different magnifications. In case of 20× magnification, the emission of the whole cured ridge is observed (Figure 4b, top), while a quite intense emission spot is evident by focusing onto a specific part of the ridge with higher magnification (60×; Figure 4b, bottom). PEW Experiments on Double Waveguide. Once the nanofabrication has been verified directly on top of the IEx WG, the next step was to obtain a thin nanolayer of the polymer containing NCs on top of a double dielectric waveguide made of a TiO2 thin slab and the underlying IEx WG. The adoption of the PEW technique in such a double waveguide system represents a simple, yet efficient, way to potentially access a new scenario of applications in integrated photonics. Indeed, silicon waveguides are typically involved in telecommunications or photonic applications for their low-loss features in the infrared range.56−58 Titanium dioxide offers the possibility to realize low-loss waveguides able to propagate single- or multimode visible light.59−61 In the specific case of the PEW experiment, the presence of TiO2 is particularly convenient because, due to its high refractive index (n ≈ 2.3) with respect to that of the IEx WG, it ensures an optimal

Figure 3. AFM images and related cross-section profiles of the polymeric ridges fabricated by PEW experiments performed on the single waveguide (IEx WG). The (a,b) and (c,d) micrographs, respectively, refer to the ridges obtained by using the photoresin without and with NCs. The (a,c) and (b,d) micrographs are, respectively, related to ridges obtained with an exposure time of te = 300 s and te = 60 s. In the case of the resin without NCs, the incident power has been fixed to a value of P = 0.56 mW, while a slightly higher value, P = 0.56 mW, was chosen in the case of the resin with NCs because these partly absorb the incident light.

Table 1. Key Parameters of the PEW Experiments Performed on the Single Waveguide and Ridge Sizes of Thickness and Width Deduced by the AFM Results for the Analyzed Cases (Polymer without and with NCs)a power (mW) 0.56 0.56 0.70 0.70

exp. time (s)

t (nm)

PETA + MDEA + Eosin Y without NCs 300 260 ± 2 60 24 ± 2 PETA + MDEA + Eosin Y with NCs 300 87 ± 2 60 18 ± 2

w (μm) 1.5 ± 0.02 1.43 ± 0.02 1.62 ± 0.02 1.25 ± 0.02

a

Here, the power represents the input power of the laser measured before the waveguide.

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analysis, performed on a double waveguide with the same features, revealed the phenomenon. The aim of that characterization experiment was to measure the evanescent electric field associated to the light propagating in the TiO2 guide.64 In order to theoretically validate the results of that SNOM analysis, we have also performed a finite element method simulation (COMSOL Multiphysics) of the same propagation experiment. In the first case, we only considered the light propagation in the single IEx WG (Figure 6a). In this case, the

confinement of light and an efficient coupling with the glass waveguide underneath, as previously demonstrated.62−64 For what concerns the fabrication of the double waveguide, this was conducted by means of electron beam lithography and physical vapor deposition. A thickness of 85 nm was chosen for the TiO2 slab because this guarantees an optimum working range for wavelengths from 450 to 800 nm and single-mode propagation.62,63 For the PEW nanofabrication of a polymer ridge on the realized double waveguide, the photoresin doped with NCs has been exclusively utilized. The experiment followed the same steps as in the case of the single waveguide except for the fact that the droplet of resin has been cast on the surface of the TiO2 slab. For what concerns the curing parameters, time te = 300 s and an input power of 0.7 mW were chosen as values already tested by the experiments on the single waveguide when using the photoresin with NCs. The AFM analysis of the realized ridge evidenced a new, peculiar element because of a periodically modulated morphology with a periodicity along the ridge of about 5.6 μm (see Figure 5a). In particular, the

Figure 5. Results of the PEW experiment on the double waveguide. (a) optical micrograph of the surface of the TiO2 slab with the fabricated photopolymerized ridge (10× magnification); (b) AFM measurement of the same photopolymerized ridge on the TiO2 slab. A morphology with modulated thickness is evident, showing a periodicity of d = 5.6 μm along the ridge; (c) cross-section profiles of the fabricated ridge indicating points with maximum and minimum thickness; red and black solid lines, respectively.

Figure 6. Results of the FEM numerical simulations of the singlemode propagation of light (λ = 532 nm) in single and double waveguides performed by using COMSOL Multiphysics. (a) When light propagates only in the single waveguide, the evanescent field associated with it is confined only in the IEx WG; (b) light propagating in the double waveguide (85 nm TiO2 slab fabricated on top of the IEx WG) experiences an optical beating that is due to the presence of two coupled interfaces; (c) in this simulation, the photopolymerization process is considered. Here, a droplet of the photosensible resin is placed on top of the whole system, and the light propagating in the double guide maintains the beating behavior also in the polymeric material.

profile reported in the micrograph of Figure 5c shows points of different thicknesses with maximum (tH = 64 nm ± 2) and minimum (tL = 45 nm ± 2) values. In order to interpret this result, it is worth considering that the photopolymerization is induced by the evanescent field associated to the propagating wave. As such, it is possible to intuitively deduce that the obtained morphology represents the cured replica of a periodical beating of the evanescent field deriving from the optical coupling between the two different guides (TiO2 slab and the IEx glass waveguide underneath) during the light propagation. Indeed, a first observation of this light beating was carried out during past experiments in which a SNOM

beating is not present, and we have a confirmation of the continuous cured morphologies (constant thickness) observed in Figure 3. In the second simulation, where the double waveguide is considered, the presence of the optical beating is quite evident (Figure 6b). This result is in agreement with both the SNOM analysis64,65 and the present PEW experiment on the double waveguide. The simulation also confirms the experimentally observed beating period of about 5.6 μm for a TM polarization direction of the propagating green light (Figure 5b). The fact that the resulting morphology is a cured replica of the sinusoidal behavior of the evanescent field has been verified by 14673


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a third COMSOL simulation that takes into account the presence of the photoresin drop on top of the TiO2 slab during the propagation of light in the double waveguide. In the presence of the resin droplet, light also propagates through the polymer, maintaining in the material the beating behavior of the associated evanescent field and curing the polymer consequently (Figure 6c).

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CONCLUSIONS In conclusion, we demonstrated a novel method of nanofabrication at the interfaces of photonic structures successfully implemented by exploiting the evanescent field associated to the light waves guided by those same photonic structures. In such a way, a suitable photoresin, directly put on top of the waveguide, is cured resulting in polymeric nanoridges as thin as 18 nm. These experiments have been repeated in the presence of a photoresin grafted with light emitting nanosources, that is, semiconductor colloidal NCs made of CdSe/ZnS. By spectroscopy analysis of the realized ridges, we revealed the presence of the NCs within them. The method has also been exploited in the presence of an additional high-index dielectric waveguide (TiO2) realized on top of the IEx WG. This time, the optical microscope and AFM measurements demonstrate that the propagation in both waveguides acquires a different behavior, showing periodical beatings. This is a signature that both waveguides are coupled together and that the evanescent wave allows us to observe such effects. We think that our method can be very powerful for positioning single emitters at a given place, in particular, at the tip of a nanoantenna66,67 for plasmonics and quantum optics applications.68−74

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.E.L.). *E-mail: [email protected] (S.B.). ORCID

Giuseppe Emanuele Lio: 0000-0002-8925-7202 Safi Jradi: 0000-0003-1712-8986 Renaud Bachelot: 0000-0003-1847-5787 Roberto Caputo: 0000-0002-0065-8422 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank A. De Luca for fruitful discussions and support. This project is financially supported by the ANR (French research agency) and the NRF (National Research Foundation, Singapore) through the international “ACTIVENANOPHOT” (alias MUHLYN) funded project (ANR-15CE24-0036-01). We thank the GrandEst regional platform “Nanomat” for nanofabrication and nanocharacterization. The authors also thank the “Area della Ricerca di Roma 2”, Tor Vergata, for the access to the ICT services (ARToV-CNR) for the use of the COMSOL Multiphysics Platform, OriginLab, and Matlab.

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REFERENCES

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