FULL PAPER Photonic Molecules
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Strong Coupling in a Photonic Molecule Formed by Trapping a Microsphere in a Microtube Cavity Jiawei Wang, Yin Yin,* Qi Hao, Yang Zhang, Libo Ma,* and Oliver G. Schmidt role for the efficient intercavity coupling. However, the evanescent field is relatively weak in the reported photonic mole cules because most of the optical field is strongly confined within the coupled cavities (e.g., microdisks,[4,8,11,18] micro toroids,[19] microspheres,[3] microrods,[5,20] and microfibers[7]). As such, one needs a deliberate control on both the cavity geometries and the intercavity coupling gap to ensure a good spectral match and efficient evanescent coupling between the coupled cavities.[18] Moreover, dynamic tuning of the intercavity coupling strength has been investigated in recent years, which were carried out by advanced and sophisticated techniques such as strain tuning,[21,22] acousto-optic control,[23] and precise micromanipulation techniques.[3] To extend and promote the research in the field of photonic molecules, it is of high interest to design novel photonic molecules extending from adjacent solid microcavities to thin-walled hollow cavities which possess intense evanescent field facilitating intercavity coupling and provide novel strategy for tuning of the coupling strength. Microtube cavities, which are formed by self-rolling of pre strained nanomembranes, feature unique properties such as hollow-core structures and ultrathin cavity walls (≈100–300 nm) for the study of light-matter interactions and the integration of “lab-in-a-tube” systems.[24–27] These key merits enable extensive applications ranging from optofluidic sensing,[28–31] single cell analysis,[32] dynamic molecular process detection,[33] photonplasmon coupling,[34] to optical spin–orbit coupling.[35] To combine with other media/objects, luminescent quantum dots,[36,37] quantum wells,[38] and organic molecules[39] have been enwrapped into the microtube wall by the rolling up process, which couple photoluminescence (PL) light to the microtube cavities to support whispering-gallery mode (WGM) resonances.[37,40] In this context, a design and demonstration of photonic molecule based on microtube cavity is of fundamental interest for the study of strong optical coupling and the promo tion of its potential applications. Herein, we report a novel design of photonic molecule by trapping a microsphere cavity into the hollow core of a rolledup microtube cavity. We focus on studying the WGM coupling between the trapped microsphere and microtube cavity with a significant difference of cavity sizes, which is in contrast to pre vious reports where the two externally adjacent microcavities possess highly similar sizes[11,12,41] or slightly mismatched sizes (less than two times).[4–7,18,42–44] Periodic modulations on
A photonic molecule formed by trapping a microsphere cavity into a hollow microtube cavity is demonstrated, which provides a novel design over conventional photonic molecules comprised of solid-core whispering gallery mode microcavities with externally tangent configuration. Periodic spectral modulations of mode intensity, resonant mode shift, and quality factor are observed owing to the largely mismatched cavity sizes. The intercavity coupling strength can be tuned by shifting the excitation position off the tangent point of the microsphere-tube system along the tube axis, rather than the conventional strategy of changing the spacing between coupled cavities. In particular, anticrossing feature of coupled modes is revealed to verify the existence of optical strong coupling in the microsphere-tube system. Numerical simulation results show an excellent agreement with the experimental observations. The present work provides a flexible strategy for designing photonic molecules and tuning the coupling behavior of resonant modes, which is of high interest for both fundamental and applied studies.
1. Introduction Over the last decade, building a system of closely neighbored optical microcavities has been proved to be an effective way to manipulate and regulate resonant light,[1–3] which has indi cated great potentials for various applications such as single mode lasing,[4–7] optical flip-flops,[8,9] self-referenced sensing,[10] coherently transfer excitation of separated quantum emit ters,[11,12] and tunable frequency-comb generation.[13] Optical mode interaction in coupled cavities is an analog to the electron states in a chemical molecule.[14] Thus this kind of system is usually called as “photonic molecule.” The coherent coupling of optical resonant modes and the resultant hybridization effect have been widely investigated both theoretically and experimen tally in photonic molecules consisting of two coupled optical microcavities with identical or closely matched sizes,[15–17] where the evanescent field at the cavity surface plays a key
Dr. J. Wang, Y. Yin, Dr. Q. Hao, Dr. Y. Zhang, Dr. L. Ma, Prof. O. G. Schmidt Institute for Integrative Nanosciences IFW Dresden Helmholtzstr. 20, 01069 Dresden, Germany E-mail:
[email protected];
[email protected] Prof. O. G. Schmidt Material Systems for Nanoelectronics Technische Universität Chemnitz Reichenhainer Str. 70, 09107 Chemnitz, Germany
DOI: 10.1002/adom.201700842
Adv. Optical Mater. 2017, 1700842
1700842 (1 of 8)
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mode intensity, resonant mode shift, quality (Q) factor, and mode splitting were demonstrated owing to the hybridized resonances in the couple cavities which appear periodically depending on the mismatched cavity-sizes. The intercavity coupling strength can be tuned by moving the excitation posi tion off the tangent point of the microsphere-tube system along the tube axis, which equivalently changes the coupling gap spacing and enables the transition between strong and weak coupling regimes. The experimental results show an excellent agreement with the numerical simulation results. Here strong coupling evidenced by spectral anticrossing characteristics was observed only upon specified resonant modes due to selective coupling in such a photonic molecule with largely mismatched cavity-sizes. Our work offers a new design of photonic mole cules and a convenient strategy for fine regulations of mode hybridization by tuning the intercavity coupling.
2. Results and Discussions 2.1. Working Principle Figure 1a schematically shows a photonic molecule formed by trapping a dielectric microsphere into a rolled-up microtube cavity. A laser beam is focused by an objective lens at the tube which works as the optical pump source for exciting the WGM resonances. Due to the difference of cavity sizes, distinct free spectral ranges (FSR), i.e., FSRS and FSRT, can be observed in the resonant spectra which are supported by the micro sphere and microtube cavities, respectively[45] (see Figure 1b). When some discrete resonant modes in each resonant spec trum overlap (or close to each other), optical coherent coupling occurs between the two cavities, which results in delocalized eigenmodes (also termed as “supermodes”).[12,18] As such, in the microsphere-tube system the resonant modes of microtube are
coupled to microsphere with a periodicity of N = [FSRS/FSRT] (“[]” stands for the nearest integer), i.e., the resonant modes are selectively coupled with an interval of N modes in the resonant spectrum. For instance, in the optical coupling at mode M, the optical field can be efficiently coupled into the microsphere, which can be evidenced by the variation of mode intensity and the occurrence of mode splitting. In contrast, for other neigh boring resonant modes (e.g., M + 1, M + 2), the intercavity cou pling is strongly suppressed due to the spectral mismatch of resonant modes supported by the microsphere and microtube cavities. For mode M + N, an efficient coupling occurs again owing to the periodic spectral match in the microsphere-tube system. Intercavity coupling strength is sensitive to the spatial overlap of evanescent fields between the microsphere and microtube cavities, which in turn plays a key role in generating the supermodes in photonic molecules. WGMs in the micro tube are formed by self-interferences of the PL light along a ring trajectory at the excitation position. In our microspheretube system, the resonant trajectory in microtube cavity can be easily tuned by moving the laser excitation point along the tube axis, while the microsphere is fixed inside of the tube. Thus it is natural to utilize the axial dimension of the microtube to tune the coupling strength by changing the axial distance between the laser excitation point and the trapped microsphere. As illus trated in Figure 1c, an excited resonant trajectory coinciding with the tangent point of the microsphere-tube system leads to the maximum spatial overlap where optical interaction occurs in a strong coupling regime. When the excitation position is shifted away from the tangent point, optical coherent coup ling occurs at the closest gap between the microsphere and the resonant orbit in microtube. Once the light is coupled into the microsphere, it resonates along a great circle tilted away from that being excited at position-A, while keeping the con stant optical round-trip length due to the rotational symmetry
Figure 1. a) Schematic of a photonic molecule consisting of a microsphere trapped in a rolled-up microtube cavity. The solid arrow indicates the rolling direction of microtube and the dashed arrows depict a microsphere being trapped into the microtube. b) Schematic showing the resonant modes of the microtube and microsphere with labeled azimuthal mode orders. Inset shows a cross-sectional view of the photonic molecule. c) Schematic showing the varied laser excitation positions and the corresponding coupling regimes. The dashed lines denote the WGM orbit in the microtube and the dotted lines denote the WGM orbit in the microsphere. OB: objective lens.
Adv. Optical Mater. 2017, 1700842
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in sphere (see Figure 1c). A larger coupling gap leads to a decreased mode spatial overlap and therefore a reduced energy transferring, manifesting the interaction in a weak coupling regime. To clarify the transition between the two regimes, it is crucial to characterize the coupling-induced mode splitting and the associated anticrossing feature.
2.2. Fabrication and Characterizations Tubular microcavities were prepared by rolling-up prestrained nanomembranes (see the Experimental Section).[32] Figure 2a shows an optical image of a rolled-up microtube. The outer diameter of the microtube is ≈10.8 µm and the wall thickness is ≈140 nm. The U-shape pattern allows the middle part of the tube to be free-standing after rolling-up, which suppresses the substrate leakage loss. Controlled by a syringe pump (neMESYS 290N), a tapered glass capillary was used to pump colloidal solutions containing polystyrene (PS) microspheres (≈3 µm in diameter, Duke Scientific, standard deviation of size distribu tion
