Abstract

We propose and investigate an ultra-compact air-mode photonic crystal nanobeam cavity (PCNC) with an ultra-high quality factor-to-mode volume ratio ($Q/V$) by quadratically tapering the lattice space of the rectangular holes from the center to both ends while other parameters remain unchanged. By using the three-dimensional finite-difference time-domain method, an optimized geometry yields a $Q$ of $7.2\times {10}^6$ and a $V\sim 1.095{(\lambda /n_{Si})}^3$ in simulations, resulting in an ultra-high $Q/V$ ratio of about $6.5\times {10}^6{(\lambda /n_{Si})}^{-3}$. When the number of holes on either side is 8, the cavity possesses a high sensitivity of 252 nm/RIU (refractive index unit), a high calculated $Q$-factor of $1.27\times {10}^5$, and an ultra-small effective $V$ of $\sim 0.758{(\lambda /n_{Si})}^3$ at the fundamental resonant wavelength of 1521.74 nm. Particularly, the footprint is only about $8\times 0.7{\mathrm{\mu m}}^2$. However, inevitably our proposed PCNC has several higher-order resonant modes in the transmission spectrum, which makes the PCNC difficult to be used for multiplexed sensing. Thus, a well-designed bandstop filter with weak sidelobes and broad bandwidth based on a photonic crystal nanobeam waveguide is created to connect with the PCNC to filter out the high-order modes. Therefore, the integrated structure presented in this work is promising for building ultra-compact lab-on-chip sensor arrays with high density and parallel-multiplexing capability.

© 2017 Optical Society of America

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