Fabrication and characterization of optical nanofiber interferometer and resonator for the visible range Chengjie Ding, Vivien Loo, Simon Pigeon, Romain Gautier, Maxime Joos, E Wu, Elisabeth Giacobino, Alberto Bramati, Quentin Glorieux To cite this version: Chengjie Ding, Vivien Loo, Simon Pigeon, Romain Gautier, Maxime Joos, et al.. Fabrication and characterization of optical nanofiber interferometer and resonator for the visible range. New Journal of Physics, Institute of Physics: Open Access Journals, 2019, 21 (7), pp.073060. 10.1088/1367- 2630/ab31cc. hal-02276060 HAL Id: hal-02276060 https://hal.sorbonne-universite.fr/hal-02276060 Submitted on 2 Sep 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. PAPER • OPEN ACCESS Fabrication and characterization of optical nanofiber interferometer and resonator for the visible range To cite this article: Chengjie Ding et al 2019 New J. Phys. 21 073060 View the article online for updates and enhancements. This content was downloaded from IP address 134.157.80.196 on 02/09/2019 at 10:38 New J. Phys. 21 (2019) 073060 https://doi.org/10.1088/1367-2630/ab31cc PAPER Fabrication and characterization of optical nanofiber interferometer OPEN ACCESS and resonator for the visible range RECEIVED 26 March 2019 Chengjie Ding1,2, Vivien Loo1,3, Simon Pigeon1, Romain Gautier1, Maxime Joos1,EWu2, REVISED Elisabeth Giacobino1, Alberto Bramati1 and Quentin Glorieux1 7 July 2019 1 Laboratoire Kastler Brossel, Sorbonne Université, CNRS, ENS-PSL Research University, Collège de France, Paris 75005, France ACCEPTED FOR PUBLICATION 2 ʼ 12 July 2019 State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, People s Republic of China 3 Institut Langevin, ESPCI Paris, CNRS, PSL University, Paris 75005, France PUBLISHED 30 July 2019 E-mail: [email protected] Keywords: optical nanofiber, purcell effect, photonic structure, sagnac interferometer Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Abstract Any further distribution of We report the fabrication and characterization of photonic structures using tapered optical this work must maintain fi fi fi attribution to the nano bers. Thanks to the extension of the evanescent electromagnetic eld outside of the nano ber author(s) and the title of two types of devices can be built: a ring interferometer and a knot resonator. We propose a general the work, journal citation and DOI. approach to predict the properties of these structures using the linear coupling theory. In addition, we describe a new source of birefringence due to the ovalization of a nanofiber under strong bending, known in mechanical engineering as the Brazier effect. 1. Introduction Due to their low losses, optical fibers are undoubtedly a medium of choice to transport optical information, making them critical to current telecommunication networks and to the future quantum internet [1]. However collecting a specific state of light, for example a single photon in a fiber with a good coupling efficiency is not an easy task. A typical way to couple light into a fiber is to place the emitter directly at one end of a fiber with or without additional optical elements [2]. An alternative approach recently raised significant interest, by collecting light from the side of a fiber [3, 4]. Indeed, stretching the fiber diameter down to the wavelength scale allows for a coupling between the fiber guided mode and an emitter in its vicinity [5]. In such a nanofiber, the fundamental propagating mode has a significant evanescent component at the glass/air interface, which allows for interacting with emitters on the surface [3, 4, 6–10]. Collection efficiency is limited so far to 22.0%±4.8% for a bare nanofiber [6]. Maximizing this coupling is a challenging task as it requires simultaneously a fine-tuning of the fiber size and the largest possible cross- section for the emitter. To render this ‘injection by the side’ technique more attractive, the collection efficiency has to be increased. One approach to do so is to enhance the effective light–matter interaction. It is commonly done by reducing the mode volume using confined modes of the electromagnetic field rather than propagating modes. It leads, via the Purcell effect, to an increase of the spontaneous emission within the nanofiber confined mode and therefore to an increase of the emitter-fiber coupling [11]. A detailed model predicts more than 90% collection efficiency if one adds an optical cavity of moderate finesse to the nanofiber [11]. Diverse strategies have been investigated to do so. One is to fabricate two mirrors directly in the fiber to add a Fabry–Perot cavity within the nanofiber itself [12]. This strategy requires advanced nanofabrication methods such as femtosecond laser ablation to modify the fiber index. Using a Talbot interferometer, it has been possible to fabricate two fiber Bragg gratings and form an optical cavity with a transmission of 87% for a finesse of 39 [12]. A similar strategy, called nanofiber Bragg cavity, where a focused ion beam mills the nanofiber to create mirrors has shown a Purcell factor and coupling efficiency of 19.1% and 82% respectively [13, 14]. Another solution relies on coupling the nanofiber with a whispering gallery mode resonator with very high quality factor up to 109 [15, 16]. With this strategy, at the difference of previous ones, the cavity is exterior to the nanofiber. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft New J. Phys. 21 (2019) 073060 C Ding et al Figure 1. Optical nanofiber structures. (a) Nanofiber twisted loop: optical microscope image. (b), Sagnac interferometer equivalent optical setup: the light emerging from the port Aout consists of two reflections or two transmissions of the light from incident port Ain through the beamsplitter. (c) Nanofiber knotted loop: scanning electron microscope image. (d) Fabry–Perot ring resonator equivalent optical setup: light coming from Ain that is not directly reflected to Aout by the beam splitter, is trapped in the cavity formed by the beam splitter and the mirrors. In this article, we study an alternative approach, particularly interesting because it does not involve nanofabrication capabilities. The idea is to loop the nanofiber in order to directly create a cavity thanks to the evanescent coupling. Such cavities have been obtained in the telecom range at 1.5 microns using microfiber with a finesse up to 20 [17, 18]. Here we demonstrate the first experimental implementation of this approach in the visible range using a fiber tapered to the nanoscale while maintaining a similar finesse. This is an important step because most of the efficient single-photon emitters work in the visible range. Compared with other nanofiber 3 cavities, our knot structure gives a significant small mode volume(≈50 μm ) and free spectral range ΔυFSR in THz. For example, the nanofiber cavity with two fiber Bragg gratings [19] has a mode volume of about 2.6×104 μm3. The nanofiber cavity with fiber beam splitter gives a big mode volume according to the cavity length of more than two meters [20]. The nanofiber Bragg cavity has the smallest mode volume among the achieved methods for nanofiber based cavity [14], the mode volume is 1 μm3, which is achieved with a fabrication process far more demanding than ours. Moreover, using the linear coupling theory, we present a generalized model for two complementary geometries: twisted and knotted loops, illustrating the crucial role of the topology of the loop formed. While the twisted loop is found to work as a Sagnac interferometer, the nanofiber knot behaves as a Fabry–Perot micro- resonator. In addition, we report here a novel source of birefringence for nanoscale tapered fiber. In such micron-size structures the nanofiber region is put under strong bending constraints and therefore this induces an ovalization of its transverse section, known as the Brazier effect in mechanical engineering [21]. We have estimated that this ovalization leads to a substantial difference in the effective refractive index similar to the the well-known stress induced birefringence [22]. 2. Effective coupling theory approach The manufacturing of nanofibers is a well-controlled process, and it is possible to fabricate fibers with a diameter down to 200 nm [5, 23]. At this size, only the core of the fiber remains, and the surrounding air acts as a cladding. Consequently, there is a strong evanescent field extending around the surface of the nanofiber. The fundamental mode does not correspond anymore to the standard linearly polarized mode LP01. Nevertheless, using Maxwell’s equations the correct propagating mode profile can be precisely characterized [24]. We will consider single mode air-cladding nanofibers only, that is, nanofibers in which the fundamental mode HE11 is the only propagating solution [24]. This is the case if the normalized frequency V with V º-ka n2 1 is lower than the cutoff normalized frequency Vc=2.405, where k is the wavevector, a is the fiber radius, and n is the fiber index. We have bent and twisted manually such nanofibers with great care to realize two miniaturized optical setups: a fiber loop and a fiber knot (figure 1). The common feature of these two structures is that they both present a section where the two parts of the nanofiber touch each other as shown on figure 1.