On-Chip Optical Sensing Platforms

Abstract

Sensing has become an important field of research because of its wide variety of applications, such as; chemical sensing, biomedical diagnostics, environmental gas monitoring and oil quality monitoring. This has spurred researcher across various fields for innovative development of new sensing techniques. These new techniques need to be both sensitive and selective to give direct evidence of the presence of the target analyte/molecule. Compared to the traditional electronic and mechanical based sensing techniques, optical sensing offers superior sensitivity and selectivity. However, to sense low concentrations, meter-long interaction path between the target molecules and light is required, thus making it bulky and unsuitable for portable applications. On-chip photonic sensor based on waveguide could address the challenges in the conventional optical sensing system by taking advantages of reduction in the system footprint by orders of magnitude. Additionally, the compatibility with micro- and nano-fabrication technology reducing the cost. For small-scale production, electron beam lithography (EBL) based fixed-beam-moving-stage (FBMS) technique is widely used for writing stich-error-free structures. However, the writing is limited to primitive patterns such as circles, squares and rectangles. A combination of FBMS and area mode is used to write a smoothly varying polygon, which often results in a misalignment between the two-writing process. To mitigate this, we propose and demonstrate a method that offers smooth and alignment-error-free tapering. Using the proposed method, we experimentally demonstrated a stitch-error and misalignment free patterning of different photonic circuit components such as power splitter, interferometer and resonator. In waveguide-based sensing, sensitivity is a combination of the strength of interaction and the underlying waveguide/resonator’s sensitivity to the change in refractive index or absorbance. In this thesis, we present a slot waveguide based high field interaction waveguide system, that can confine light in a low-index medium for increased sensitivity. We have addressed the problem of coupling from a wire to a slot through a novel coupling scheme. We experimentally demonstrated in silicon on insulator platform, slot mode excitation with the proposed coupling design with high efficiency (99%) and showed athermal behaviour of a PMMA filled slotted ring resonator. A detailed simulation is performed to optimize the slot-waveguide geometry for maximum vii sensitivity. We report a 37% improvement in the sensitivity compared to existing slot waveguide-based ring resonators. Using the sensor, the dynamic refractive index on-chip measurement of different liquids and evaluated in comparison with commercial Abbe refractometer. To further exploit waveguide-based absorption sensing, we propose an integrated photonics circuit-based sensing solution operating in the mid-IR wavelength range. The proposed waveguide configuration reduces the path length from 10’s of cm to less than 1 cm with the use of high field interaction waveguide systems. As an alternative to crystalline germanium, we present deposited amorphous germanium and silicon nitride using low-temperature-plasma-enhanced chemical vapor deposition for high contrast mid-IR waveguide platform. Finally, we demonstrate a grating-based mid-IR filter using the deposited amorphous germanium film on a calcium fluoride substrate. Furthermore, we also study the feasibility of cross-modal sensing to improve sensitivity and selectivity based using on-chip photo-thermal optical readout scheme. We present a highly compact (16x smaller) interrogation grating design that enables probing of smaller vibrating structures. We experimentally demonstrate a resonant displacement sensitivity of 10 μW/nm, which is 100 times better than the static displacement sensitivity. Based on this, we demonstrate resonant sensing of temperature via joules heating and photo-thermal excitation. Experimentally, we measure a thermal sensitivity of 3.1 nm/K and estimate a minimum detectable temperature difference of 129 mK. Furthermore, we experimentally demonstrated a bonding approach for fixing the vibrating structure on top of the interrogation grating.