Microknot optical resonators and their applications

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PhD "ZOOM" SEMINAR
Wednesday, June 24, 2020 at 14:00
Microknot optical resonators and their applications
Alexandra Blank
PhD of Dr. Yoav Linzon
Resonating microstructures defined locally on optical fiber tapers are an attractive venue for researchers in the recent years. These engineered microstructures find use in various applications, including photonic filters, lasers, optomechanical light-matter interaction devices, and various sensing elements mimicking human smell and taste sensing system (gas and liquids sensors), known as electronic noses and electronic tongues.
The presented work is devoted to experimental and numerical investigations of the behaviour of microknot resonators (MKRs) defined locally on optical fiber tapers as a promising platform for gas and liquid sensing applications. We demonstrate the two-probe localized heating technique for MKR fusing intended for improving its optical performance and mechanical stability in size and circular profile. We show that an above-threefold dynamical range enhancement has been consistently achieved. The fused MKRs are found to maintain phase stability, as opposed to the unfused knots exhibiting a random phase drift. We prove that e-fusing improves the mechanical strength in the MKR, providing it with transferability that is of importance in sensing applications.
We investigate fused MKRs’ characteristics for in-liquid sensing both local, where the analyte is delivered to the packaged device and the remote sensing. We have suggested and demonstrated two deployment schemes, namely, folded configuration intended for remote sensing, and straight configuration on either hydrophilic or hydrophobic substrate intended for packaged integration of photonic sensors. We found that in folded configuration reversibility remained sustainable over approximately 50 cycles of operation, whereas in most cases of the straight configuration on glass substrate transmission drops to the noise level after a few dewetting cycles. We show that a hydrophobic PDMS substrate can be advantageous relating to surface chemistry as compared to a typical hydrophilic glass substrate. For all those configurations we demonstrated their persistent sensing capabilities and defined unique recognition plots in principle components space specific to pure and diluted volatile liquids tested.
We demonstrate a durable and simple humidity sensing approach based on index-sensitive interference spectroscopy of surface stress birefringence and incorporating tapered microfibers on a silicon substrate coated with an active polymer layer. We show theoretically that the transmission spectrum of coated tapers possess interference patterns induced by the stress applied to the taper due to the coating weight load. The coated device demonstrated persistent detection capability in humid environment and a linear response to calibrated analytes. For each volatile organic analyte tested we defined a calibration plot in principal component space.
Microstructured tapers, when untreated, amenable to low performance in terms of resonance depth (dynamical range), Q and mechanical fragility. In this work we performed a novel comprehensive numerical study of the photonic transmission in manually prepared microknot resonators with different contact coupling area geometries and refractive index variations. Using selective modifications of the MKR coupling area and index profile, the transmission characteristics were studied, and a recipe for the experimental realization of high quality-factor resonators is prescribed.
We present results of the numerical study of the photonic transmission in 3D resonating microstructures defined on optical tapered fibers as functions of both radius and doping and deduce their sensitivities to localized refractive index variations in terms of resonance shifts. Based on the analysis of the transmission characteristics of polymer-coated microstructures, we demonstrated a linear response to the refractive index gradient in NIR wavelength range.
We envision that simple and robust devices with improved optical and mechanical characteristics will be harbored in next generation sensors, as well as optomechanical applications incorporating microfiber-based resonating structures.
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