Integrated MEMS Cavity Optomechanical Oscillators for Wireless and Optical Communications

Recent advancements in micro-optical and micro-mechanical resonator technologies have allowed researchers to exploit coupling between the optical field and mechanical motion in an optical cavity to affect cooling or amplification of mechanical motion. Cooling the mechanical motion of microscale objects has been of high scientific interest, since it facilitates observation and exploration of certain quantum phenomena, e.g., the standard quantum limit of detection. On the other hand, amplification of the mechanical motion allows realization of microscale devices for practical applications, such as light driven low phase noise signal generation by radiation pressure driven Opto-Mechanical Oscillators (OMO’s).The ability to achieve self-sustained oscillation with no need for feedback electronics makes an OMO compelling for on-chip applications where directed light energy, e.g., from a laser, is available to fuel the oscillation, such as Chip Scale Atomic Clocks (CSAC’s). Indeed, an OMO can substantially reduce power consumption of a CSAC by replacing its power-hungry conventional quartz-based microwave synthesizer but this requires that the OMO output is sufficiently stable, as gauged over short time spans by its phase noise.Pursuant to identify phase noise mechanisms, this thesis presents a new phase noise model for OMO’s by deriving an OMO oscillator model with intuitive engineering understanding of its operation consistent with the established OMO theory. Phase noise theory suggests that attaining high mechanical-Q (Qm) is crucial to lower the phase noise while high enough optical-Q (Qo) is required for reasonably low-power operation. This motivates a focus on achieving a high-Qm OMO to have low phase noise while maintaining a high enough Qo for low power operation–a challenge in previous OMO’s that had to trade-off Qm and Qo mainly because they use a single material that sets both.The work in this thesis demonstrates integrated MEMS-cavity optomechanical oscillators that combine the best properties of optical and MEMS resonators in single composite multi- material OMO structures to simultaneously optimize mechanical and optical Q’s. The multi- material coplanar ring OMO structure using a high-Qo silicon nitride optical ring and a high- Qm polysilicon ring simultaneously achieves high Qm > 22, 000, which is more than 2× higher than that of previous-best silicon nitride OMO, and high Qo > 280, 000 on par with single silicon nitride ring demonstrations. With its high Qm, the coplanar ring OMO exhibits a best-to-date phase noise of -114 dBc/Hz at 1 kHz offset and -142 dBc/Hz at 1 MHz offset from its 52-MHz carrier–a 12 dB improvement from the previous best by an OMO constructed of silicon nitride alone. The doped polysilicon structure and electrodes additionally allow tuning of the OMO’s oscillation frequency via voltage control and harmonic locking to an external source, enabling future deployment of the multi-material OMO as a locked oscillator in a target low-power CSAC application. A second integrated OMO structure, dubbed stacked- ring OMO, is also demonstrated using similar silicon nitride and polysilicon ring resonators but this time coupled in a vertical fashion, allowing easy integration with sidewall sacrificial layer defined gap MEMS process technology to achieve high electromechanical coupling in the composite OMO.Enabled by the MEMS integration that allows electrically coupled input-outputs, a new optical communications application based on an OMO is introduced. A super-regenerative optical receiver detecting on-off key (OOK) modulated light inputs has been demonstrated that harnesses the radiation-pressure gain of the electrically-sustained integrated OMO to render its oscillation amplitude as a function of the intensity of light coupled into the oscillator. Unlike previous electronic super-regenerative receivers, this rendition removes the need to periodically quench the oscillation signal, which then simplifies the receiver architecture and increases the attainable receive bit rate. A fully functional receiver with a compact ∼ 90 μm OMO comprised only of silicon-compatible materials demonstrates successful recovery of a 2 kbps bit stream from an OOK modulated 1550 nm laser input. By removing the need for the expensive III-V compound semiconductor materials often used in conventional optical receivers, this OMO-based receiver offers a lower cost alternative for sensor network applications

A high-throughput label-free cell-based biosensor (CBB) system

Cell-based biosensors (CBBs) have important applications in biosecurity and rapid diagnostics. Current CBB technologies have challenges including cell immobilization on the sensors, high throughput fabrication and portability, and rapid detection of responses to environmental changes. We address these challenges by developing an integrated CBB platform that merges cell printing technology, a lensless charge-coupled device (CCD) imaging system, and custom-developed cell image processing software. Cell printing was used to immobilize cells within hydrogel droplets and pattern these droplets on a microfluidic chip. The CCD was used to detect the morphological response of the immobilized cells to external stimuli (e.g., environmental temperature change) using lensless shadow images. The morphological information can be also detected by sensing a small disturbance in cell alignment, i.e., minor alignment changes of smooth muscles cells on the biosensors. The automatic cell alignment quantification software was used to process the cell images (microscopic image was used as an example) and calculate the cell orientation in seconds. The same images were also manually processed as a control to validate and characterize the integrated platform functionality. The results showed software can measure the cell morphology (i.e., orientation) in an automated way without the need for labeling (e.g., florescent staining). Such an integrated CBB system will allow fabrication of CBBs at high throughput as well as rapidly monitor and measure morphological cellular responses

Automated and Adaptable Quantification of Cellular Alignment from Microscopic Images for Tissue Engineering Applications

Cellular alignment plays a critical role in functional, physical, and biological characteristics of many tissue types, such as muscle, tendon, nerve, and cornea. Current efforts toward regeneration of these tissues include replicating the cellular microenvironment by developing biomaterials that facilitate cellular alignment. To assess the functional effectiveness of the engineered microenvironments, one essential criterion is quantification of cellular alignment. Therefore, there is a need for rapid, accurate, and adaptable methodologies to quantify cellular alignment for tissue engineering applications. To address this need, we developed an automated method, binarization-based extraction of alignment score (BEAS), to determine cell orientation distribution in a wide variety of microscopic images. This method combines a sequenced application of median and band-pass filters, locally adaptive thresholding approaches and image processing techniques. Cellular alignment score is obtained by applying a robust scoring algorithm to the orientation distribution. We validated the BEAS method by comparing the results with the existing approaches reported in literature (i.e., manual, radial fast Fourier transform-radial sum, and gradient based approaches). Validation results indicated that the BEAS method resulted in statistically comparable alignment scores with the manual method (coefficient of determination R2=0.92 [R superscript 2 = 0.92]). Therefore, the BEAS method introduced in this study could enable accurate, convenient, and adaptable evaluation of engineered tissue constructs and biomaterials in terms of cellular alignment and organization.National Institutes of Health (U.S.) (NIH R21 (AI087107))National Institutes of Health (U.S.) (NIH R01 (AI081534))Wallace H. Coulter FoundationCenter for Integration of Medicine and Innovative TechnologyUnited States. Army Medical Research and Materiel CommandUnited States. Army. Telemedicine & Advanced Technology Research Cente