The Influence of Nanopore Arrays Disorder On

Nanosphere Photolithography: The Influence of Nanopore Arrays Disorder on Extraordinary Optical Transmission Andrei Ushkov1;2 a, Olivier Dellea3, Isabelle Verrier1, Thomas Kampfe1, Alexey Shcherbakov4 b, Jean-Yves Michalon1 and Yves Jourlin1 c 1Univ. Lyon, UJM-Saint-Etienne, CNRS, Institut d’Optique Graduate School, Laboratoire Hubert Curien UMR 5516, F-42023 Saint-Etienne, France 2Center for Photonics and 2D Materials, Moscow Institute of Physics and Technology, 9 Institutsky Lane, Dolgoprudny 141700, Russia 3Grenoble Alpes Univ., CEA-Liten, 17 rue des Martyrs, 38054 Grenoble, France 4Department of Physics and Engineering, ITMO University, 49 Kronverksky Pr., 197101 St. Petersburg, Russia Keywords: Nanosphere Photolithohraphy, NPL, Extraordinary Optical Transmission, EOT, Disorder, Nanopore Array, Diffraction Graing, Plasmonics, Numerical Simulations. Abstract: We analyze both experimentally and numerically the influence of nanopore arrays disorder on extraordinary optical transmission in samples, fabricated via nanosphere photolithography. Two measures of disorder are considered, the correlations between them are discussed using experimental and numerical data. We propose a theoretical model which takes explicitly the disorder into account, and show how the concurrence between nanopore depth and disorder level defines the quality of EOT excitation. Simulated spectra are in a good agreement with experimental ones. Our results reveal the possibilities of NPL for EOT-based applications and pave the way toward plasmonic devices with a polycrystalline design. 1 INTRODUCTION pos et al., 2008) or Laser Interference Lithography (Ushkov et al., 2020; Cao et al., 2018). Since its discovery in 1998 by Ebbessen (Ebbesen Nowadays the self-assembled colloidal mono- et al., 1998), the Extraordinary Optical Transmission layers attract more and more attention for practi- (EOT) has been studied intensively for numerous ap- cal plasmonic applications. Despite the polycrys- plications in optical filtering (Wen et al., 2019), sens- talline geometry, structures fabricated via colloidal ing (Yeh et al., 2011) and energy transfer (Andrew self-assembly can support resonant optical effects. and Barnes, 2004). Different EOT-compatible sam- Plasmon-mediated resonant transmission was ob- ple designs were considered, for example continuous served, for example, in samples prepared via metal or perforated thin metal films (Ushkov et al., 2019; deposition into the intersticies between colloidal par- Jourlin et al., 2009; Sauvage-Vincent et al., 2013; ticles (Liu et al., 2020a; Jamiolkowski et al., 2019), or Wen et al., 2019; Park et al., 2020; Yue et al., 2014), in continuous metal films covering a self-assembled metallic slit arrays (Deng et al., 2018), deep undu- nanosphere mask (Farcau, 2019; Quint and Pacholski, lations for localized plasmon excitations (Genet and 2014; Zhang et al., 2012) with or without the removal Ebbesen, 2010). In order to fabricate samples and of particles. study the EOT experimentally, various surface nanos- The deposition of close-packed self-assembled tructuration approaches exist: Electron-Beam Lithog- colloidal monolayers on different substrates is per- raphy (Li et al., 2010; Yue et al., 2014), Focused formed by various modifications of the Langmuir- Ion Beam Milling (Ebbesen et al., 1998; Hahn et al., Blodgett technique (Ruan et al., 2007; Dellea´ et al., 2020; Liu et al., 2020b), Soft-Nanoimprinting (Cam- 2014; Lotito and Zambelli, 2016; Vogel et al., 2011). In comparison with conventional periodic a https://orcid.org/0000-0001-8962-1599 structuration methods these approaches possess a b https://orcid.org/0000-0002-9070-5439 high throughput, are well adapted for the curved and c https://orcid.org/0000-0002-7935-2150 non-conventional surfaces (Berthod et al., 2017; Pen- 46 Ushkov, A., Dellea, O., Verrier, I., Kampfe, T., Shcherbakov, A., Michalon, J. and Jourlin, Y. Nanosphere Photolithography: The Influence of Nanopore Arrays Disorder on Extraordinary Optical Transmission. DOI: 10.5220/0010344400460053 In Proceedings of the 9th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2021), pages 46-53 ISBN: 978-989-758-492-3 Copyright c 2021 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved Nanosphere Photolithography: The Influence of Nanopore Arrays Disorder on Extraordinary Optical Transmission dergraph et al., 2013; Bhawalkar et al., 2010) and can process allows the transfer of these polycrystalline be integrated into industrial production lines (Dellea´ monolayers onto the substrate in a continuous man- et al., 2014). ner, which is advantageous for industrial needs (Shav- The disorder should be explicitly taken into ac- dina et al., 2015). The colloidal mask on the resist count for the polycrystalline structures. It was shown surface acts as a microlens array for UV irradiation both numerically and experimentally (Quint and Pa- (see Fig. 1b) and exposes the resist during a cer- cholski, 2014) that the disorder in thin gold film un- tain time texp ∼ 10 s. The exposed sample is then dulations flattens and lowers the optical transmission; cleaned in an ultrasonic bath from the colloidal parti- the randomness in film perforation positions leads to cles and is developed in MF-319 developer at 8 ◦C for a broadband absorption (Fang et al., 2015), polycrys- tdev ∼ 4 s. As a result, nanopore arrays appear in the talline structures were used as light absorbers (Qu and resist, see Fig. 1c. Final fabrication steps (Figs. 1d-e) Kinzel, 2016). are 20 nm-thick aluminum layer deposition via mag- In this work we study the influence of disorder netron sputtering, and the second 600 nm-thick resist on EOT in samples, prepared via an advanced sur- coating to protect the metal and create the symmet- face nanostructuring method called Nanosphere Pho- rical ”Insulator-Metal-Insulator” (IMI) structure with tolithography (NPL) (Zhang et al., 2016). In con- improved EOT behavior. trast to other colloidal-based approaches mentioned above, NPL is much more flexible in the nanotopogra- phy as it employs photonic nanojets for drawing surface motifs. This feature makes it possible to control the surface undulations depth, avoid the photoresist layer perforations and produce nanopores - the geometry more flexible for the tuning of plasmonic re- sponse in comparison with nanoholes (Gartia et al., 2013; Wang et al., 2013). We consider two measures of nanopore arrays disorder, compare them and show how to explicitly take the disorder into account in numerical simulations. Experimental EOT measure- ments are in a good correspondence with calculated spectra. In addition, we propose a phenomenological model, adapted from the 1D case (Nau et al., 2007), for EOT calculations in disordered 2D structures. Our results are promising to reveal the possibilities of NPL for EOT-based applications and pave the way toward plasmonic devices with a polycrystalline design. 2 FABRICATION AND Figure 1: Nanopore array fabrication: a) The glass slide cleaning and a deposition of a photoresist film; b) deposi- CHARACTERIZATION OF tion of colloidal monolayer, UV irradiation; c) the removla SAMPLES of colloidal particles, resist development and formation of nanopores; d) surface metallization; e) second resist film deposition for the symmetrical ”Insulator-Metal-Insulator” In Fig. 1 we present the main technological steps for (IMI) structure. samples fabrication. On a surface of BK7 glass slides 3.7 × 2.5 cm, cleaned in ultrasonic acetone, ethanol An optical microscope and SEM were used to baths and DI-water bath, a 600 nm/250 nm-thick visualize arrays with interpore distances 1:1 µm and Shipley S1805 photoresist layer is spin-coated for 300 nm, respectively, before the final resist coating in 1:1 µm/300 nm microsphere diameter, respectively, Fig. 1e. Four samples S1-S4 were fabricated experi- see Fig. 1a. The resist was soft-baked at 60 ◦C mentally with different interpore distances and disor- for 1 min. The Boostream process, developed in der, see Figs. 2a-d. Nanopore depth was measured us- CEA-Liten (Dellea´ et al., 2014), performs the self- ing AFM, transmission spectra of IMI structures were assembly of silica nano/micro particles with diam- measured using the UV/Vis/NIR spectrophotometer eters of s0 =1:1 µm or 300 nm into close-packed Cary 5000. monolayers on the water surface. The Boostream 47 PHOTOPTICS 2021 - 9th International Conference on Photonics, Optics and Laser Technology walls were slightly slanted. Both methods produced similar spectra, which assures the attained numerical results. The material dispersion is used for simulations, the dispersion of the resist S1805 was measured by ellipsometry. 3.1 Nanopores Per Grain In the context of polycrystalline geometries a natu- ral measure of disorder is an average number N of nanopores per grain, because this value is dimension- less, allows to compare samples with different char- acteristic lengths and is easy to estimate from experimental data. We use the inverse number of nanopores per grain as the first of two parameters of disorder: pd1 ≡ 1=N. The values pd1 for the samples S1-S4, which are specified in Fig. 2f, confirm the visual im- pression that the disorder increases from S1 to S4. In order to clarify the nanopore distribution in high- quality samples, we performed the image-processing- based statistical analysis of large 227 µm×170 µm microscope photographs, nanopore grains with different orientations of hexagonal lattice are depicted by different colors

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