Integration of Nanoemitters onto Photonic Structures by Guided

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

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 J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 26, 2019

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The Journal of Physical Chemistry

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∗,† †Light, Nanomaterials and Nanotechnologies (ex-LNIO), CNRS, University of Technology of Troyes (UTT), 12 rue Marie Curie Troyes, France. ‡CNR-Nanotec and Department of Physics, University of Calabria, 87036 Arcavacata di Rende (CS), Italy ¶Laboratoire Kastler Brossel, Sorbonne University, CNRS, ENS-PSL Research University, Collége de France,Paris 75005, France. E-mail: [email protected]; [email protected]

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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 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. AFM and spectroscopic 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 nanocrystal monolayer in the vertical direction. In presence of the double waveguide (T iO2 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 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 towards the controlled positioning of single nanoemitters in proximity of nanoantennas or other elaborate plasmonic/photonic structures.

Introduction Photonics has proven to be a key enabling technology for many applications. The use of integrated optics is of tremendous importance and 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 miniaturisation. Neverthe2


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less, 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 emitters 11,12 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 again not making 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 lightsensitive polymer/resin, mold a given structure. Spatially controlled photopolymerization thus represents an interesting tool for the micro- and nano-patterning of polymers 17 and hybrid materials including polymer containing QDs with high spatial resolution. 18,19 The size of the photopolymerized structures could even reach few nanometers in case of plasmoninduced photopolymerization. 20–22 In micro-fabrication by photopolymerization processes, a light beam triggers a polymerization reaction, which results in a solidification of the liquid material in the irradiated areas while non-irradiated areas remain unchanged and can be washed out by suitable organic solvents. When a droplet of resin is exposed to an 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 or less complex structures can be realized by coupling light with specific wavefronts and vitrifying only the cured polymer. Based on this process, micro-optical elements (micro-lenses and waveguides) can be also fabricated at the extremity of optical fibers. 28,29 Due to the volume of the irradiation beam that is diffraction limited, the size of the polymer features in the far field configuration is limited to few hundreds of nanometers. The confinement of the polymerized volume can be tuned by introducing some physico-chemical effects such as the chemical quenching by oxygen 30 and the decrease of light penetration with increasing absorptivity of the material in accordance with the Beer-Lambert law. 31 Ad-

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ditional 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 free radical and thus in a more confined polymer structures. 32 Two photon polymerization is an alternative approach to micro-fabrication that has been introduced 20 years ago. 33–35 It allows writing sub-micrometer 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 thickness 36,37 or a simple way to map surface plasmon polaritons, 38 due to the very high sensitivity of the electric filed perturbation. Coupling interference photolithography with PEW allows thus patterning thin polymer films in a spatially controlled manner (76 nm polymer linewidth in a grating of 160 nm pitch). 39 However, the fabrication of such thin polymeric structures requires the use of prisms with refractive index (np ) higher than that of the photopolymer (nph ). The PEW takes place at the interface 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 follow: prism/liquid/substrate/photopolymer with np = nliquid = nsubstrate ≥ nph . 36 Unfortunately, the imposed constraints in term of 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

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and a photopolymerizable material. These fields allow curing a thin layer (few nanometers) of polymer containing quantum dots on the waveguide surface.

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 are characterized by a gradient of refractive index (∆n=0.7) responsible for trapping the light (Fig. 1a). The 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 photopolymerization by evanescent waves (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, Fig. 1b); in another case, the cured ridges are obtained on the surface of a double dielectric waveguide, made of a nanolayer of titanium dioxide (T iO2 ), placed on top of an IEx WG (Fig. 1c).

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Figure 1: Sketches of the samples involved in the PEW experiments showing: (a) a laser beam entering a typical ion-exchange waveguide (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) the PEW experiment on a single waveguide where a photo-resin droplet is cast directly on top of the IEx WG; (c) the PEW experiment on a double waveguide where the photoresin droplet is cast on a 85nm T iO2 thick slab fabricated directly on top of the IEx WG.

During the experiments, two different photopolymers have been utilized. The first one

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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 nm to 550 nm. The first one shows a peak centered at about 525 nm; the second one, due to the presence of NCs, redshifts its absorption peak of about 10 nm (Fig. 2a).

PEW experiments on single waveguide The experimental setup utilized for implementing the PEW nanofabrication technique comprises two lasers. A 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 the 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 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 CCD camera mounted above the sample equipped with a microscope objective (20x magnification) has been adopted (Fig. 2b).

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(a) Absorption (a. u.)

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Max. Absorption l=530

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Without NCs With NCs

0.6 0.4 0.2

0 350 400 450 500 550 600 650 700 750 800

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Figure 2: a)The micrograph shows the two absorption curves for the photo-resin 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.

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 8


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corresponding minimum power value has been found as the one below which no ridges were observable by 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 presence of NCs. The fact that the energy dose threshold is higher in 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 5x te increase. All fabricated ridges have been characterized by AFM in terms of their thickness (t) and width (w). Figs. 3a,3b show the AFM results in case of the first photo-resin (without NCs), while Figs. 3c,3d show instead the results obtained using the photo-resin that includes NCs. The key parameters of the obtained ridges are resumed in Table 1.

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Figure 3: AFM images and related cross-section profiles of the polymeric ridges fabricated by PEW experiments performed on single waveguide (IEx WG). The (a b) and (c d) micrographs respectively refer to the ridges obtained by using the photo-resin without and with NCs. The (a c) and (b d) micrographs are respectively related to ridges obtained with an exposure time te =300 s and te =60 s. In case of the resin without NCs, the incident power has been fixed to a value P =0.56 mW while it was chosen a slightly higher value, P =0.56 mW, in case of the resin with NCs because these partly absorb the incident light. 10 ACS Paragon Plus Environment

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Table 1: Key parameters of the PEW experiments performed on single waveguide and ridges sizes of thickness and width deduced by the AFM results for the analyzed cases (polymer without and with NCs). Here, the power represents the input power of the laser measured before the waveguide. Power (mW) Exp. time(s)

t (nm)

w (um)

PETA+MDEA+Eosin Y without nanocrystals 0.56

300

260±2

1.5±0.02

0.56

60

24±2

1.43±0.02

PETA+MDEA+Eosin Y with nanocrystals 0.70

300

87±2

1.62±0.02

0.70

60

18±2

1.25±0.02

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 2µm). In terms of ridge thickness, we measured values comprised between 18 nm 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 like 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 photo-resin 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 afterwards.

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 nanocrystals in the cured polymer. The experiment has been conducted by 11



means of a home-built confocal microscope equipped with a spectrometer with a Peltiercooled 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 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 (Fig 3d), is indicated by the red solid line in Fig. 4a. The measured curve is quite broad 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 (Fig. 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 the 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 a 20x magnification, the emission of the whole cured ridge is observed (Fig. 4b, top), while a quite intense emission spot is evident by focusing onto a specific part of the ridge with higher magnification (60x; Fig. 4b, bottom).

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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 confocal microscope in case of a 20x magnification (top) in which the whole ridge is observed and a localized intense spot emitted by a portion of the ridge (bottom) observed with a 60x magnification.

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 polymer containing NCs on top of a double dielectric waveguide made of a T i02 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 13



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 instead the possibility to realize low-loss waveguides able to propagate single or multi-mode visible light. 59–61 In the specific case of the PEW experiment, the presence of a T iO2 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 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 T iO2 slab because this guarantees an optimum working range for wavelengths from 450 nm to 800 nm and single mode propagation. 62,63

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0

Lowest point Highest point

tH tL

x (mm) Figure 5: Results of the PEW experiment on double waveguide. (a) optical micrograph of the surface of the TiO2 slab with the fabricated photopolymerized ridge (10x magnification); (b) AFM measurement of the same photopolymerized ridge on T iO2 slab. A morphology with modulated thickness is evident showing a periodicity 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.

For the PEW nanofabrication of a polymer ridge on the realized double waveguide, it has been exclusively utilized the photo-resin doped with NCs. The experiment followed the same steps as in case of the single waveguide except for the fact that the droplet of resin has been cast on the surface of the T iO2 slab. For what concerns the curing parameters, a time te =300 s was chosen and an input power of 0.7 mW, as values already tested by the experiments on single waveguide when using the photo-resin 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 Fig. 5a). In particular, the profile reported in the micrograph of Fig. 5c shows points of different thickness with 15



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 (T iO2 slab and the IEx glass waveguide underneath) during the light propagation. Indeed, a first observation of this light beating was done during past experiments in which a SNOM 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 T iO2 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 a first case, we only considered the light propagation in the single IEx WG (Fig. 6a). In this case, the beating is not present and we have a confirmation of the continuous cured morphologies (constant thickness) observed in Fig. 3. In a second simulation, where the double waveguide is considered, the presence of the optical beating is quite evident (Fig. 6b).

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Figure 6: Results of the FEM numerical simulations of the single mode 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 T iO2 slab fabricated on top of the IEx WG) experiences a 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 photo-sensible resin is placed on top of the whole system and the light propagating in the double guide maintains the beating behaviour also in the polymeric material.

This result is in agreement with both the SNOM analysis 64,65 and the present PEW experiment on 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

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light (Fig. 5b). The fact that the resulting morphology is a cured replica of the sinusoidal behaviour of the evanescent field has been instead verified by a third COMSOL simulation that takes into account the presence of the photo-resin drop on top of the T iO2 slab during the propagation of light in the double waveguide. In presence of the resin droplet, light also propagates through the polymer maintaining in the material the beating behaviour of the associated evanescent field and curing the polymer consequently (Fig. 6c).

Conclusion 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 photo-resin, directly put on top of the waveguide, is cured resulting in polymeric nano-ridges as thin as 18 nm. These experiments have been repeated in presence of a photo-resin grafted with light emitting nanosources, i.e. semiconductor colloidal nanocrystals made of CdSe/ZnS. By spectroscopic analysis of the realized ridges, we revealed the presence of the NCs within them. The method has also been exploited in presence of an additional high-index dielectric waveguide (T iO2 ) realized on top of the IEx WG. This time, 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 effect. We think that our method can be very powerful for positioning single emitters at a given place, in particular at the tip of a nano-antenna 66,67 for plasmonics and quantum optics applications. 68–74

Acknowledgement 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 18


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Foundation, Singapore) through the international “ACTIVE-NANOPHOT” (alias MUHLYN) funded project (ANR-15-CE24-0036-01). We thank the GrandEst regional platform “Nanomat” for nanofabrication and nanocharacterisation. 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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Integration of Nanoemitters onto Photonic Structures by Guided

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