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We report the design of one dimensional photonic crystal nanobeam cavities with elliptical holes that is fully encapsulated in the water environment and can confine the light in the low index region. The proposed structure, based on the principle of gentle confinement of the electromagnetic field, is designed by tapering the width of the host photonic crystal waveguide away from the center of the cavity while keeping other parameters constant. With elliptical-hole low-index mode nanobeam cavities and tapered waveguide widths, through three dimensional finite-difference time-domain simulations, large band-gap and high reflectivity are achieved to confine the optical mode. Also, the electric field is confined in the elliptical holes, which helps to enhance the sensitivity. The simulation results demonstrate that we achieve the highest quality factor of 1.35×10 5 when 15 taper segments and 15 additional mirror segments are placed on both sides of the host waveguide. A sensitivity of 390 nm/RIU (refractive index unit) and mode volume of 2.23 (λ res /n si ) 3 are achieved in the water environment, thus showing exceptionally good on-chip sensing properties with respect to high sensitivity, small footprints and masses.

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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 7.2×10 and a V∼1.095(λ/n) in simulations, resulting in an ultra-high Q/V ratio of about 6.5×10(λ/n). 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 1.27×10, and an ultra-small effective V of ∼0.758(λ/n) at the fundamental resonant wavelength of 1521.74 nm. Particularly, the footprint is only about 8×0.7  μm. 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.

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Abstract:We simulated an efficient method for the sensor array of high-sensitivity single-slot photonic crystal nanobeam cavities (PCNCs) on a silicon platform. With the combination of a well-designed photonic crystal waveguide (PhCW) filter and an elaborate single-slot PCNC, a specific high-order resonant mode was filtered for sensing. A 1ˆ3 beam splitter carefully established was implemented to split channels and integrate three sensors to realize microarrays. By applying the three-dimensional finite-difference-time-domain (3D-FDTD) method, the sensitivities calculated were S 1 = 492 nm/RIU, S 2 = 244 nm/RIU, and S 3 = 552 nm/RIU, respectively. To the best of our knowledge, this is the first multiplexing design in which each sensor cite features such a high sensitivity simultaneously.