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.
We present an optomechanical impact sensor, designed by the utilization of a 2D rod-type photonic crystal (PhC) cavity. The PhC cavity is sandwiched by perfect electrical conductor (PEC) boundaries with an air slot between the top of the PhC rods and the bottom of the top PEC layer. Strong light localization in the air slot region makes the PhC cavity characteristics highly sensitive to the air slot width, leading to optomechanical applications such as impact sensing. A suspended mechanical gold membrane, as a replacement of PEC layers for practical realizations, is designed to sense impact acceleration. In the presence of an impact, the mechanical structure deflects resulting in a change in the air slot height, which in turn, tunes the resonant wavelength of the PhC cavity. Calculations show that 16.6 μs response time, much faster than the commercially available ones (around 200 ms), is possible.
We propose a novel way of mechanical perturbation of photonic crystal cavities for on-chip applications. We utilize the equivalence of the 2D photonic crystals with perfect electric conductor (PEC) boundary conditions to the infinite height 3D counterparts for rod type photonic crystals. Designed structures are sandwiched with PEC boundaries above and below and the perturbation of the cavity structures is demonstrated by changing the height of PEC boundary. Once a defect filled with air is introduced, the metallic boundary conditions is disturbed and the effective mode permittivity changes leading to a tuned optical properties of the structures. Devices utilizing this perturbation are designed for telecom wavelengths and PEC boundaries are replaced by gold plates during implementation. For 10 nm gold plate displacement, two different cavity structures showed a 21.5 nm and 26 nm shift in the resonant wavelength. Optical modulation with a 1.3 MHz maximum modulation frequency with a maximum power consumption of 36.81 nW and impact sensing with 20 μs response time (much faster compared to the commercially available ones) are shown to be possible.
Amorphous photonic materials offer an alternative to photonic crystals as a building block for photonic integrated circuits due to their shared short-range order. By using the inherent disorder of amorphous photonic materials, it is possible to design flexible-shaped waveguides that are free from restrictions of photonic crystals at various symmetry axes. Effects of disorder on photonic crystal waveguide boundaries have examined before, and it is shown that flexible waveguides with high transmission are possible by forming a wall of equidistant scatterers around the defect created inside amorphous material configuration. Based on this principle, waveguides with various flexible shapes are designed and fabricated for planar circuit applications. A silicon-on-insulator (SOI) slab with random configuration of air hole scatterers is used. The amorphous configuration is generated through realistic Monte Carlo simulations mimicking crystalline-to-amorphous transition of semiconductor crystals via an assigned Yukawa potential to individual particles. The design parameters such as average hole distance, slab thickness and hole radius are adjusted so that the waveguide is utilizable around 1550 nm telecommunications wavelength. Such waveguides on slab structures are characterized here and the level of randomness and band gap properties of amorphous configurations are analyzed in detail. These efforts have the potential to lead easier design of a wide range of components including but not limited to on-chip Mach-Zehnder interferometers, splitters, and Y-branches.
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