To achieve more compact integrated photonic devices, reducing size of elements is crucial. A factor that limits sizereduction is electro-optic components that require large optical interaction length. In this work, we designed and fabricated an optical modulator where a photonic crystal structure is used to create large phase difference in short distance. Our design is a 2x2 Mach-Zehnder interferometer on the platform of silicon-on-insulator. A left-handed photonic crystal structure that is designed to operate at 1.55 um is placed on one arm of the interferometer to add phase to light. The phase difference between two arms yields amplitude modulation at the output of the interferometer. The photonic crystal is hexagonal air hole lattice and used to switch between negative and positive effective refractive indices. This change is triggered by applying voltage which decreases the refractive index of silicon from 3.480 to 3.477 due to plasma dispersion effect, and causes photonic band-to-band transition. By this way, effective refractive index of the structure jumps from negative to positive values. To be able to realize this, photonic crystal region is sandwiched between n-doped and p-doped materials, which creates a p-i-n diode. By taking the advantage of band-to-band transition at left-handed photonic crystal, we experimentally demonstrated that interaction length is reduced from 255 um to 4.4 um. This reduction leads to low optical insertion loss as well as more compact devices.
The real-time polymerase chain reaction (RT-PCR) analysis using nasal swab samples is the gold standard approach for COVID-19 diagnosis. However, due to the high false-negative rate at lower viral loads and complex test procedure, PCR is not suitable for fast mass screening. Therefore, the need for a highly sensitive and rapid detection system based on easily collected fluids such as saliva during the pandemic has emerged. In this study, we present a surface-enhanced Raman spectroscopy (SERS) metasurface optimized with genetic algorithm (GA) to detect SARS-CoV-2 directly using unprocessed saliva samples. During the GA optimization, the electromagnetic field profiles were used to calculate the field enhancement of each structure and the fitness values to determine the performance of the generated substrates. The obtained design was fabricated using electron beam lithography, and the simulation results were compared with the test results using methylene blue fluorescence dye. After the performance of the system was validated, the SERS substrate was tested with inactivated SARS-CoV-2 virus for virus detection, viral load analysis, cross-reactivity, and variant detection using machine learning models. After the inactivated virus tests are completed, with 36 PCR positive and 33 negative clinical samples, we were able to detect the SARS-CoV-2 positive samples from Raman spectra with 95.2% sensitivity and specificity.
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