Ultra-compact air-mode photonic crystal nanobeam cavity integrated with bandstop filter for refractive index sensing

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 7.2×10 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.

Large-Dynamic-Range Dual-Parameter Sensor Using Broad FSR Multimode Photonic Crystal Nanobeam Cavity

“…2(b), the dielectric mode band edge of the CLC-PCNC shows a distinct structural sensitivity which is conducive to generate broad frequency intervals with small amplitude structure modulations. As comparisons, most reported PCNCs with moderate FSR performance exhibited close-knit band edges (air mode or dielectric mode) in the band diagram [11], [35]- [38].…”

“…Photonic crystal (PC) based optical sensing technologies especially of high integration on-chip sensing [1]- [11] and multifunctional detection [12]- [14] are attracting extensive attentions in these years. While most of the reported works focused on integration of multiple sensor units based on single-mode regime, with which one sensor unit only performs one function on one mode.…”

“…In the multimode regime, multiple resonant modes are simultaneously used for multi-parameter sensing. While one challenge is that for most reported PCNCs based sensors, the integration and detection range are greatly limited by the modest FSR (usually around 5 nm-40 nm) of the resonant modes if not using any additional filters [8]- [11], [27]. As known, additional filters may add additional negative effects to the sensing system, such as inevitably leading to transmission reduction of the sensing signal, bringing additional complexity to the sensing system, increasing the difficulty for practical fabrication, and so on.…”

Applications of Photonic Crystal Nanobeam Cavities for Sensing

“…As for the multiplex RI sensing with the use of multiple parallel nanobeam cavities, it is possible for the sensing signal of each nanobeam cavity to interfere with others due to the existence of multiple resonances of each nanobeam cavity. Therefore, several studies have been carried out on filtering out the resonances of unwanted orders of a single nanobeam cavity without sacrificing the sensing performance [ 91 , 92 , 93 ]. The schematic of a typical approach is shown in Figure 9 b.…”

Deterministic aperiodic photonic crystal nanobeam supporting adjustable multiple mode-matched resonances

“…While the fundamental mode is confined at the center, field distribution of the higher-order mode shifts away, making the overall field overlap between different modes negligible. Another potential alternative is to use both dielectric and air modes in a tapered nanocavity; however, since dielectric and air modes are mainly focused in the dielectric and air regions, respectively, the overlap between the two types of modes is also limited [7,11]. Modes with different orthogonal polarizations can be supported in a single nanocavity, but the different polarizations also limit the field overlap [12].…”