Thomas A. Wall

Signal-to-Noise Enhancement in Optical Detection of Single Viruses With Multispot Excitation

We present fluorescence detection of single H1N1 viruses with enhanced signal to noise ratio (SNR) achieved by multispot excitation in liquid-core antiresonant reflecting optical waveguides (ARROWs). Solid-core Y-splitting ARROW waveguides are fabricated orthogonal to the liquid-core section of the chip, creating multiple excitation spots for the analyte. We derive expressions for the SNR increase after signal processing, and analyze its dependence on signal levels and spot number. Very good agreement between theoretical calculations and experimental results is found. SNR enhancements up to 5×104 are demonstrated.

Enhancement of ARROW Photonic Device Performance via Thermal Annealing of PECVD-Based SiO 2 Waveguides

Silicon-based optofluidic devices are very attractive for applications in biophotonics and chemical sensing. Understanding and controlling the properties of their dielectric waveguides is critical for the performance of these chips. We report that thermal annealing of PECVD-grown silicon dioxide (SiO2) ridge waveguides results in considerable improvements to optical transmission and particle detection. There are two fundamental changes that yield higher optical transmission: 1) propagation loss in solid-core waveguides is reduced by over 70%, and 2) coupling efficiencies between solid- and liquid-core waveguides are optimized. The combined effects result in improved optical chip transmission by a factor of 100-1000 times. These improvements are shown to arise from the elimination of a high-index layer at the surface of the SiO2 caused by water absorption into the porous oxide. The effects of this layer on optical transmission and mode confinement are shown to be reversible by alternating subjection of waveguides to water and subsequent low temperature annealing. Finally, we show that annealing improves the detection of fluorescent analytes in optofluidic chips with a signal-to-noise ratio improvement of 166x and a particle detection efficiency improvement of 94%.

Improved environmental stability for plasma enhanced chemical vapor deposition SiO2 waveguides using buried channel designs

Ridge and buried channel waveguides (BCWs) made using plasma-enhanced chemical vapor deposition SiO2 were fabricated and tested after being subjected to long 85°C water baths. The water bath was used to investigate the effects of any water absorption in the ridge and BCWs. Optical mode spreading and power throughput were measured over a period of three weeks. The ridge waveguides quickly absorbed water within the critical guiding portion of the waveguide. This caused a nonuniformity in the refractive index profile, leading to poor modal confinement after only seven days. The BCWs possessed a low index top cladding layer of SiO2, which caused an increase in the longevity of the waveguides, and after 21 days, the BCW samples still maintained ~20% throughput, much higher than the ridge waveguides, which had a throughput under 5%.

Mitigating Water Absorption in Waveguides Made From Unannealed PECVD SiO 2

Water absorption was studied in two types of waveguides made from unannealed plasma enhanced chemical vapor deposition (PECVD) SiO2. Standard rib anti-resonant reflecting optical waveguides (ARROWs) were fabricated with thin films of different intrinsic stress and indices of refraction. Buried ARROWs (bARROWs) with low and high refractive index differences between the core and cladding regions were also fabricated from the same types of PECVD films. All waveguides were subjected to a heated, high humidity environment and their optical throughput was tested over time. Due to water absorption in the SiO2 films, the optical throughput of all of the ARROWs decreased with time spent in the wet environment. The ARROWs with the lowest stress SiO2 had the slowest rate of throughput change. High index difference bARROWs showed no decrease in optical throughput after 40 days in the wet environment and are presented as a solution for environmentally stable waveguides made from unannealed PECVD SiO2.

Optofluidic Lab-on-a-Chip Fluorescence Sensor Using Integrated Buried ARROW (bARROW) Waveguides

Optofluidic, lab-on-a-chip fluorescence sensors were fabricated using buried anti-resonant reflecting optical waveguides (bARROWs). The bARROWs are impervious to the negative water absorption effects that typically occur in waveguides made using hygroscopic, plasma-enhanced chemical vapor deposition (PECVD) oxides. These sensors were used to detect fluorescent microbeads and had an average signal-to-noise ratio (SNR) that was 81.3% higher than that of single-oxide ARROW fluorescence sensors. While the single-oxide ARROW sensors were annealed at 300 °C to drive moisture out of the waveguides, the bARROW sensors required no annealing process to obtain a high SNR.

Silicate overcoat layers for the improvement of PECVD SiO 2 optofluidic waveguides

Silicate spin-on-glass can be used as a protective barrier to coat PECVD SiO2 optofluidic waveguides in order to smooth out surface topology, and lessen moisture absorption on top of the waveguide. The measured optical throughput and mode confinement improved when compared to uncoated waveguides.

Materials and microfabrication processes for ARROW-based optofluidic biosensors

This paper outlines the microfabrication processes and materials used to make an optofluidic lab-on-a-chip biosensor that detects individual biological particles. The biosensor uses a hollow-core ARROW waveguide with a low refractive index liquid core and is fabricated on a silicon wafer using a combination of PECVD deposition, RIE etching, and standard photolithographic processes. As a sensing example, detection of fluorescence signals emitted by labeled oligonucleotides inside the liquid core was used to illustrate the chips potential to identify protein-coding regions of the Zaire Ebola virus genome.