Photonic Crystal Edge Couplers for Sensing Applications
Abstract
Microelectromechanical systems (MEMS) have not only enabled the development of inexpensive sensors but have also improved their performance by lowering their mass thus enabling faster sensor response. However, there are limitations regarding the mass-scaling of conventional MEMS sensors that prevent further miniaturization. This makes the measurement of distributed forces with high spatiotemporal resolution challenging. Optical-based sensors provide low-volume confinement of electromagnetic energy and enable further mass-scaling. This thesis investigates the application of an optical deflection sensing mechanism that relies on the position dependent coupling between dielectric-like edge states on nearby photonic crystal slabs for the purposes of acoustic pressure, wall shear stress, mass and magnetic field sensing.
The performance of these sensors for these different applications developed using this same deflection sensing mechanism are presented in this thesis. The primary element in all these sensors is a suspended photonic crystal (PC) membrane with edge defect waveguides which was fabricated on silicon-on-insulator using a combination of optical chip foundry surface micromachining and post-foundry etching process to selectively remove the buried oxide.
The different dynamic mechanical modes of the suspended membrane, namely in-plane and vertical modes, have been exploited in different sensors. The vertical mode of the PC membrane with a vertical offset was utilized for acoustic pressure sensing while the horizontal in-plane mode was used to develop a wall shear sensor. Thermal fluctuations in the vibrational modes of the membrane position were used to measure the noise floor and calibrate the sensors. The detection limit for the ultrasonic pressure sensor was measured to be 12.5~mPa/$\sqrt{Hz}$. A wall shear stress sensor was demonstrated using a novel experimental setup consisting of a thermoacoustic emitter at the end of an acoustic waveguide. The sensor exhibited a noise floor of 80 \textmu Pa/$\sqrt{Hz}$, resonance at 627 kHz, and 0.12\% full-scale/Pa across 1497--1531 nm. By introducing microbeam arrays, vertical misalignment of PC edges could be reduced leading to lower noise and crosstalk with dynamic pressure. The sensor area is over three-orders of magnitude smaller than sensors with similar sensitivity and can find application in fluid dynamics research where such high resolutions are required.
In the final study, PC directional coupler based suspended membrane sensors as small as 30 \textmu m x 20 \textmu m were metallized to form magnetic sensors relying on the Lorentz force to excite the vertical vibrational mode. The magnetic sensors were tested in ambient conditions and exhibited a noise floor of 130 nT/$\sqrt{Hz}$ with flat mechanical response up to 1.64 MHz resonance. To the best of our knowledge, this is one of the first integrated photonic magnetic sensors to be demonstrated on the SOI platform. Overall, PCDC-based sensors enable low-mass, broad optical bandwidth, high spatiotemporal resolution measurements and compatibility with standardized silicon photonic foundry process.