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.
Highlights
Optical biosensing based on photonic devices such as interferometers [1,2], resonators [3,4], plasmonic structures [5,6], slot waveguides [7,8], and photonic crystals (PhC) [9,10,11,12] has attracted much attention in recent years for lab-on-chip bio applications
The high-Q factors, small mode volumes (of the order of optical wavelength [~(λ/n)x], where λ is the resonant wavelength in vacuum, n is the refractive index of the slab, and the exponent x depends on the design itself.), highly compact and integrative properties of these PhC nanocavities have allowed a wealth of new applications in sensing [13,14,15,16].The sensing concept of these photonic devices is based on the electromagnetic field overlapping with the surrounding medium, which allows label-free detection of the refractive index change
High Q-factor and high figure of merit (FOM) are related and they lead to high-performance label-free optical biosensors
Summary
Optical biosensing based on photonic devices such as interferometers [1,2], resonators [3,4], plasmonic structures [5,6], slot waveguides [7,8], and photonic crystals (PhC) [9,10,11,12] has attracted much attention in recent years for lab-on-chip bio applications. The high-Q factors, small mode volumes (of the order of optical wavelength [~(λ/n)x], where λ is the resonant wavelength in vacuum, n is the refractive index of the slab, and the exponent x depends on the design itself.), highly compact and integrative properties of these PhC nanocavities have allowed a wealth of new applications in sensing [13,14,15,16].The sensing concept of these photonic devices is based on the electromagnetic field overlapping with the surrounding medium, which allows label-free detection of the refractive index change. The overlap of the surrounding medium with the evanescent tail of the propagating optical field in the PhC cavity can lead to a detectable spectral shift in the cavity resonance wavelength when the refractive index of the surrounding changes. The laterally-coupled PhC NB cavity that provides small mode volume [Vmod ~ (λ/n)3], ultra-high Q-factor (>106) and high (Q/Vmod), yields an excellent sensing performance
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