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1. Fixed wavelength interferometer sensors for low-cost chem-bio sensing applications

   https://doi.org/10.1117/12.3003036  

   Shen, Jianhao; Gauna, Juan C.; Rai, Arushi; McClimans, Liam; Lawandi, Roseanna; Perera, Asela; Kango-Singh, Madhuri; Chakravarty, Swapnajit (March 2024, Proceedings of the SPIE)

   Miller, Benjamin L.; Weiss, Sharon M.; Danielli, Amos (Ed.)

   We experimentally demonstrated slow wave enhanced phase and spectral sensitivity in asymmetric Michelson interferometer sensors with a phase sensitivity 277,750 rad/RIU-cm and theoretical phase sensitivity as high as 461,810 rad/RIU-cm. In the context of low-cost chip integrated photonic packaged sensors, in this paper we will experimentally demonstrate a method for active tuning of interferometer fringes using phase change materials that will potentially overcome fabrication induced variation of interference fringe wavelengths, thus allowing sensor chip packaging with a fixed wavelength laser and available integrated photodetectors. 

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2. Sub-wavelength waveguide Michelson interferometer sensors

   https://doi.org/10.1117/12.3003051  

   Perera, Asela; Shen, Jianhao; Chakravarty, Swapnajit (March 2024, Proceedings of the SPIE)

   Miller, Benjamin L.; Weiss, Sharon M.; Danielli, Amos (Ed.)

   Evanescent field silicon photonics in a silicon-on-insulator or silicon-nitride-on-insulator platforms have been effectively utilized to demonstrate chemical and biosensors over the past decade with applications in the detection of nucleic acids and protein biomarkers for cancers, viruses and infectious diseases, and environmental toxins. By balancing the requirements for efficient low-loss transmission through the waveguide and enhancing light-matter interaction such as with molecules binding on the high index material surfaces in resonant microcavities, slow light and interferometer geometries, various high sensitivity biosensors have been experimentally demonstrated down to few femtograms/ml. various slotted microcavities and waveguides have been experimentally demonstrated. In recent years, subwavelength waveguides have demonstrated high bulk spectral sensitivities approaching ~500nm/RIU (RIU=refractive index unit) in periodic structures with lattice constant (Λ) <<(λ/2n eff ) where n eff is the effective index at wavelength λ. While most experimental demonstrations have been in subwavelength ring resonator geometries, in this research, in addition to experimental demonstration of bulk spectral sensitivity ~775nm/RIU in subwavelength waveguides in interferometer configurations, we investigate optimized geometries that can reach sensitivities ~70,000nm/RIU in compact dimensions. In contrast to Mach-Zehnder interferometer (MZI) sensors of the same geometric interferometer arm lengths, the reflected path in Michelson interferometers (MI) doubles the optical path length, and thus effectively doubles the phase shift in the presence of an analyte. The interference fringe linewidths are narrowed compared to the equivalent MZI and would thus enable smaller changes in analyte concentration to be discerned from the fringe spectra. 

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3. Integrated photonic slow light Michelson interferometer bio sensor

   https://doi.org/10.1117/12.2650514  

   Shen, Jianhao; Donnelly, Daniel; Chakravarty, Swapnajit (March 2023, Proceedings of SPIE)

   García-Blanco, Sonia M.; Cheben, Pavel (Ed.)

   Diverse chip-based sensors utilizing integrated silicon photonics have been demonstrated in resonator and phase shifter/interferometer configurations. Till date, interferometric techniques with the Mach-Zehnder Interferometer (MZI) and Young’s interferometer have shown the lowest mass detection limits (in pg/mm2). Slow light in photonic crystal waveguides integrated with MZIs enables compact geometries due to enhanced optical path lengths as light propagates with high group index. In a typical MZI, light propagating in the signal arm overlaps with analytes and undergo a relative phase change with respect to the light in the reference arm which leads to measured output intensity changes. In this paper, using integrated photonic methods, we demonstrate a slow light enhanced Michelson interferometer (MI) biosensor, wherein the reference and signal arms are traversed twice by the propagating optical mode. As a result, the analyte interaction length is effectively doubled since the propagating optical mode undergoes twice the phase shift as would be observed in a MZI. In an asymmetric MI configuration, the resultant doubling of the phase shift is observed as a doubling of the resonance wavelength shift for a fixed change in the analyte concentration. The device sensitivity is thus doubled with respect to a conventional MZI while also effectively halving the geometric length compared to the MZI sensor 

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4. Experimental Comparison of Slow Light and Subwavelength Waveguide Interferometer Sensors

   https://doi.org/10.1109/JSEN.2024.3450186  

   Shen, Jianhao; Perera, Asela; Rai, Arushi; Kango-Singh, Madhuri; Chakravarty, Swapnajit (October 2024, IEEE Sensors Journal)

   We experimentally demonstrate slow light photonic crystal waveguide (PCW) and subwavelength waveguide (SWWG) loop terminated Mach-Zehnder interferometer (LT-MZI) sensors in a foundry-fabricated silicon-on-insulator (SOI) platform. We compare the experimental results on sensitivity and limit of detection (LOD) on the interferometer sensors with microcavity-type sensors. We show experimentally that 2-D PCW interferometers have higher phase sensitivities than SWWGs of the same length. Based on experimental results, 20- μ m-long 2-D PCW LT-MZI sensors and 200- μ m-long SWWG LT-MZI sensors achieve an LOD of 3.4×10−4 and 2.3×10−4 RIU, respectively, with nearly the same insertion losses in foundry-fabricated devices. We show that by considering the various sources of loss in our benchtop fiber-to-fiber photonic integrated circuit measurement system, it will be possible to reach 10−7 LOD in both slow light PCW and SWWG-based LT-MZI sensors with on-chip integrated light sources and detectors. We show via simulations and experiment that the LOD of a 20- μ m-long slow light PCW LT-MZI is equivalent to that of a 100- μ m-long SWWG LT-MZI, thus enabling more compact LT_MZI sensors when using slow light PCWs versus SWWGs 

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5. Ultrasensitive tapered optical fiber refractive index glucose sensor

   https://doi.org/10.1038/s41598-023-31127-4  

   Ujah, Erem; Lai, Meimei; Slaughter, Gymama (March 2023, Scientific Reports)

   Abstract Refractive index (RI) sensors are of great interest for label-free optical biosensing. A tapered optical fiber (TOF) RI sensor with micron-sized waist diameters can dramatically enhance sensor sensitivity by reducing the mode volume over a long distance. Here, a simple and fast method is used to fabricate highly sensitive refractive index sensors based on localized surface plasmon resonance (LSPR). Two TOFs (l = 5 mm) with waist diameters of 5 µm and 12 µm demonstrated sensitivity enhancement at λ = 1559 nm for glucose sensing (5–45 wt%) at room temperature. The optical power transmission decreased with increasing glucose concentration due to the interaction of the propagating light in the evanescent field with glucose molecules. The coating of the TOF with gold nanoparticles (AuNPs) as an active layer for glucose sensing generated LSPR through the interaction of the evanescent wave with AuNPs deposited at the tapered waist. The results indicated that the TOF (Ø = 5 µm) exhibited improved sensing performance with a sensitivity of 1265%/RIU compared to the TOF (Ø = 12 µm) at 560%/RIU towards glucose. The AuNPs were characterized using scanning electron microscopy and ultraviolent-visible spectroscopy. The AuNPs-decorated TOF (Ø = 12 µm) demonstrated a high sensitivity of 2032%/RIU toward glucose. The AuNPs-decorated TOF sensor showed a sensitivity enhancement of nearly 4 times over TOF (Ø = 12 µm) with RI ranging from 1.328 to 1.393. The fabricated TOF enabled ultrasensitive glucose detection with good stability and fast response that may lead to next-generation ultrasensitive biosensors for real-world applications, such as disease diagnosis. 

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