title: Optimization of high-Q coupled nanobeam cavity for label-free sensing.
abstract: We numerically and experimentally investigated the lateral coupling between photonic crystal (PhC) nanobeam (NB) cavities, pursuing high sensitivity and figure of merit (FOM) label-free biosensor. We numerically carried out 3D finite-difference time-domain (3D-FDTD) and the finite element method (FEM) simulations. We showed that when two PhC NB cavities separated by a small gap are evanescently coupled, the variation in the gap width significantly changes the coupling efficiency between the two coupled NB cavities and the resulting resonant frequencies split. Experimentally, we fabricated laterally-coupled PhC NB cavities using (InGaAsP) layer on the InP substrate. For sensing, we showed that the laterally coupled PhC NB cavities sensor exhibits higher sensitivity than the single PhC NB cavity. The higher sensitivity of laterally coupled PhC NB cavities is due to the strong evanescent coupling between nearby PhC NB cavities, which depends on the gap width and it is attributed to the large confinement of the electromagnetic field in the gap (air or liquid). As a result of the lateral coupling, both even (symmetric) and odd (asymmetric) modes exist. We show that even modes are more sensitive than odd modes. In addition, higher-order modes exhibit higher sensitivity. Hence, we characterized and examined the fabricated PhC NB cavity as a label-free biosensor, and it exhibits high figure of merit due to its high Q-factor. This illustrates a potentially useful method for optical sensing at nanoscale.
authors.affiliation: Department of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan. mtyaseen@yahoo.com.|Nano Science and Technology Program, Taiwan International Graduate Program, Academia Sinica, Taipei 115, Taiwan. mtyaseen@yahoo.com.|Research Centre for Applied Sciences, Academia Sinica, Taipei 115, Taiwan. mtyaseen@yahoo.com.
authors.forename: Mohammad Tariq
authors.initials: MT
authors.lastname: Yaseen
authors.affiliation: Research Centre for Applied Sciences, Academia Sinica, Taipei 115, Taiwan. tigpnano2008@gmail.com.
authors.forename: Yi-Chun
authors.initials: YC
authors.lastname: Yang
authors.affiliation: Research Centre for Applied Sciences, Academia Sinica, Taipei 115, Taiwan. mhshih@gate.sinica.edu.tw.|Department of Photonics, National Chiao Tung University, Hsinchu 300, Taiwan. mhshih@gate.sinica.edu.tw.|Department of Photonics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan. mhshih@gate.sinica.edu.tw.
authors.forename: Min-Hsiung
authors.initials: MH
authors.lastname: Shih
authors.affiliation: Research Centre for Applied Sciences, Academia Sinica, Taipei 115, Taiwan. yiachang@gate.sinica.edu.tw.|Department of Physics, National Cheng Kung University, Tainan 701, Taiwan. yiachang@gate.sinica.edu.tw.
authors.forename: Yia-Chung
authors.initials: YC
authors.lastname: Chang
mesh_terms: D015374:Biosensing Techniques / Q000295:instrumentation*; D003198:Computer Simulation; D036103:Nanotechnology / Q000295:instrumentation* / Q000379:methods*; D055095:Optics and Photonics / Q000295:instrumentation*; D012031:Refractometry
keywords: Photonic crystals (PhCs); finite-difference  time-domain (FDTD); label-free sensing; nanobeam (NB) cavity