Abstract
We propose a new type of refractive index sensing based on the transverse magnetic (TM) modes in the photonic crystal (PhC) nanobeam (NB) cavity with a horizontal air gap. The electric field of the resonant TM mode is strongly confined within the horizontal air gap present at the PhC NB cavity. In order to increase the quality (Q) factor and the sensitivity (S) of the refractive index change in the air simultaneously, the cavity structure is fully optimized. Because of the trade-off between the Q-factor and S of the TM mode in the PhC NB cavity with an air gap, there is an optimal thickness of the air gap in the dielectric slot. From the numerical simulation results, S can exceed 1000 nm/RIU with Q > 40,000. When the dielectric slot becomes thin, S could be higher than 1200 nm/RIU. For practical applications, we suggest an Si-based PhC NB cavity with a horizontal SiO2 slot structure which can also provide high S with a high Q-factor after a very fine selective wet etching process. This new type of TM resonant mode in the PhC NB cavity can be an ideal platform for compact sensors in photonic integrated circuits for TM waveguide systems.
Highlights
Photonic crystals (PhCs) have been widely studied for their ability to prohibit light propagation due to their photonic bandgap (PBG) [1,2]
This new type of transverse magnetic (TM) resonant mode in the PhC NB cavity can be an ideal platform for compact sensors in photonic integrated circuits for TM waveguide systems
We numerically investigated a new type of ultrahigh sensitive optical sensor based on the TM
Summary
Photonic crystals (PhCs) have been widely studied for their ability to prohibit light propagation due to their photonic bandgap (PBG) [1,2]. As the electric field could be tightly concentrated in the air void by large discontinuity at the high-index-contrast interfaces, the Q/V ratio and optical sensitivity of the resonant mode in PhC cavity structures with an air gap could increase dramatically [15,16]. By introducing the air gap geometry in the PhC cavity, the Q-factor decreased due to a high scattering loss. The deterministic PhC NB cavity with an air gap was proposed to achieve a high coupling efficiency to the waveguide with a high Q factor [17,18]. The Q-factor of the resonant cavity mode in PhC NB structures could be increased by adding more air holes outside the cavity region to reduce the loss along the waveguide
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