Analysis of ultra-high sensitivity configuration in chip-integrated photonic crystal microcavity bio-sensors

Abstract

We analyze the contributions of quality factor, fill fraction, and group index of chip-integrated resonance microcavity devices, to the detection limit for bulk chemical sensing and the minimum detectable biomolecule concentration in biosensing. We analyze the contributions from analyte absorbance, as well as from temperature and spectral noise. Slow light in two-dimensional photonic crystals provide opportunities for significant reduction of the detection limit below 1 Â 10 À7 RIU (refractive index unit) which can enable highly sensitive sensors in diverse application areas. We demonstrate experimentally detected concentration of 1 fM (67 fg/ml) for the binding between biotin and avidin, the lowest reported till date. V C 2014 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4875903]In recent years, various integrated optical devices have been developed for label-free bio-sensing such as ring resonators, 1 wire waveguides, 2 surface plasmon resonance (SPR), 3 and photonic crystal (PC) microcavities. [4][5][6] The detection principle is based on a change in the refractive index, and hence the transduced signal caused by the specific binding of the biomolecule of interest to its specific conjugate biomolecule receptor bound to the optical device substrate. The device sensitivity is determined by the magnitude of light-matter interaction.For early bio-pathogen detection, a sensor with highest sensitivity is desired. The sensitivity is measured by the magnitude of the resonance wavelength shift for a fixed concentration, as well as the minimum concentration that can be detected. Initial PC designs focused on donor defect modes such as in a L4 microcavity (4 missing holes) 7 or acceptor defect modes 8 as in H1 defect cavities, in a triangular lattice of air holes. Later, designs increased the analyte overlap with cavity modes, also referred as fill fraction, for enhanced sensitivity. 9 Recently, we showed that an increased cavity length results in an increase in the quality factor (Q-factor) of the resonance mode 10-12 that allows smaller changes in concentration to be distinguished. The high Q enhances the interaction time between the optical mode and the analyte while the larger mode volume results in larger fill fraction, both factors resulting in higher sensitivity We demonstrated experimentally in our side-coupled two-dimensional (2D) PC cavity-waveguide architecture that the magnitude of the slow-down factor in the coupling waveguide contributes to enhanced light-matter interaction. 11 Over successive generations, we demonstrated experimentally 50 fM (3.35 pg/ml) sensitivity to the detection of the specific binding of avidin to biotin 12 with a L55 type PC microcavity (55 missing holes).A question still remains about the relative merits of Q-factor, fill fraction, and group index, when considered in conjunction with different sources of noise in measurements, in order to achieve low detection limits in chip-integrated photonic sensors. While an increased modal overlap with the analyte lowers the detection limit...

Advances in optoplasmonic sensors – combining optical nano/microcavities and photonic crystals with plasmonic nanostructures and nanoparticles

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“…(G) Biosensing spectral shifts in L21, L55, L13, and L13 defect holed PhC microcavities. Adapted from [185]. (H) Flexible silicon nanomembrane PhC microcavity (i.e.…”

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