Brian S. Phillips

Loss-based optical trap for on-chip particle analysis.

Optical traps have become widespread tools for studying biological objects on the micro and nanoscale. However, conventional laser tweezers and traps rely on bulk optics and are not compatible with current trends in optofluidic miniaturization. Here, we report a new type of particle trap that relies on propagation loss in confined modes in liquid-core optical waveguides to trap particles. Using silica beads and E. coli bacteria, we demonstrate unique key capabilities of this trap. These include single particle trapping with micron-scale accuracy at arbitrary positions over waveguide lengths of several millimeters, definition of multiple independent particle traps in a single waveguide, and combination of optical trapping with single particle fluorescence analysis. The exclusive use of a two-dimensional network of planar waveguides strongly reduces experimental complexity and defines a new paradigm for on-chip particle control and analysis.

Hollow-core waveguide characterization by optically induced particle transport

We introduce a method for optical characterization of hollow-core optical waveguides. Radiation pressure exerted by the waveguide modes on dielectric microspheres is used to analyze salient properties such as propagation loss and waveguide mode profiles. These quantities were measured for quasi-single-mode and multimode propagation in on-chip liquid-filled hollow-core antiresonant reflecting optical waveguides. Excellent agreement with analytical and numerical models is found, demonstrating that optically induced particle transport provides a simple, inexpensive, and nondestructive alternative to other characterization methods.

Tailorable integrated optofluidic filters for biomolecular detection

Spectral filtering is an essential component of biophotonic methods such as fluorescence and Raman spectroscopy. Predominantly utilized in bulk microscopy, filters require efficient and selective transmission or removal of signals at one or more wavelength bands. However, towards highly sensitive and fully self-contained lab-on-chip systems, the integration of spectral filters is an essential step. In this work, a novel optofluidic solution is presented in which a liquid-core optical waveguide both transports sample analytes and acts as an efficient filter for advanced spectroscopy. To this end, the wavelength dependent nature of interference-based antiresonant reflecting optical waveguide technology is exploited. An extinction of 37 dB, a narrow rejection band of only 2.5 nm and a free spectral range of 76 nm using three specifically designed dielectric layers are demonstrated. These parameters result in an 18.4-fold increase in the signal-to-noise ratio for on-chip fluorescence detection. In addition, liquid-core waveguide filters with three operating wavelengths were designed for Förster resonance energy transfer detection and demonstrated using doubly labeled oligonucleotides. Incorporation of high-performance spectral processing illustrates the power of the optofluidic concept where fluidic channels also perform optical functions to create innovative and highly integrated lab-on-chip devices.

Improving solid to hollow core transmission for integrated ARROW waveguides

Optical sensing platforms based on anti-resonant reflecting optical waveguides (ARROWs) with hollow cores have been used for bioanalysis and atomic spectroscopy. These integrated platforms require that hollow waveguides interface with standard solid waveguides on the substrate to couple light into and out of test media. Previous designs required light at these interfaces to pass through the anti-resonant layers.We present a new ARROW design which coats the top and sides of the hollow core with only SiO2, allowing for high interface transmission between solid and hollow waveguides. The improvement in interface transmission with this design is demonstrated experimentally and increases from 35% to 79%. Given these parameters, higher optical throughputs are possible using single SiO2 coatings when hollow waveguides are shorter than 5.8 mm.

Optofluidic notch filter integration by lift-off of thin films

Optofluidic platforms used for biomolecular detection require spectral filtering for distinguishing analyte signals from unwanted background. Towards a fully integrated platform, an on-chip filter is required. Selective deposition of dielectric thin films on an optofluidic sensor based on antiresonant reflecting optical waveguide (ARROW) technology provides the means for localized, on-chip optical filtering. We present a lift-off technique, compatible with thin-film processing including plasma-enhanced chemical vapor and sputtering deposition. The resulting optofluidic notch filters exhibited a 20 dB rejection with linewidths as low as 20 nm for ~1 cm long chips consisting of liquid-core and solid-core waveguides.

Multi-mode mitigation in an optofluidic chip for particle manipulation and sensing

A new waveguide design for an optofluidic chip is presented. It mitigates multi-mode behavior in solid and liquid-core waveguides by increasing fundamental mode coupling to 82% and 95%, respectively. Additionally, we demonstrate a six-fold improvement in lateral confinement of optically guided dielectric microparticles and double the detection efficiency of fluorescent particles.

Optical particle sorting on an optofluidic chip

We report size-based sorting of micro- and sub-micron particles using optical forces on a planar optofluidic chip. Two different combinations of fluid flow and optical beam directions in liquid-core waveguides are demonstrated. These methods allow for tunability of size selection and sorting with efficiencies as high as 100%. Very good agreement between experimental results and calculated particle trajectories in the presence of flow and optical forces is found.

Hollow ARROW waveguides on self-aligned pedestals for high-sensitivity optical sensing

Antiresonant reflecting optical waveguides (ARROWs) provide a promising approach to realizing high-sensitivity sensing platforms on planar substrates. We have previously developed ARROW platforms that guide light in hollow cores filled with liquid and gas media. These platforms include integrated traditional solid waveguides to direct light into and out of sensing media. To improve the sensitivity of these platforms for optical sensing, hollow waveguide loss must be reduced. We are working towards this by using anisotropic plasma etching to create near-ideal hollow ARROW geometries. These structures rely on an etching mask that also serves as the sacrificial core for the waveguide. This self-aligned process creates a hollow waveguide on a pedestal which is surrounded by a terminal layer of air in three directions. We previously produced ARROWs by pre-etching the silicon substrate and aligning the sacrificial core to the pedestal. However, this necessitates using a pedestal which is wider than the core, leading to higher loss and poor reproducibility. We have also increased the hollow to solid waveguide transmission efficiency by using a design that coats the sides and top of the hollow core with a single layer of silicon dioxide. Using this design, we have demonstrated an interface transmission improvement of more than two times. A much improved optical sensor platform will incorporate both of these features, using the self-aligned pedestal process for most of the length of the hollow waveguides to decrease loss, and employing the single layer design only at the interfaces to improve hollow-solid waveguide coupling.

Tailoring the spectral response of liquid waveguide diagnostic platforms

Liquid filled waveguides that form the basis for on-chip biophotonics diagnostic platforms have primarily found application in fluorescence and Raman spectroscopy experiments that require sensitive discrimination between weak analyte signals and a variety of background signals. Primary sources of background signal can include light from excitation sources (strong, narrow frequency band) and photoluminescence generated in waveguide cladding layers (weak, wide frequency band). Here we review both solid and liquid core filtering structures which are based on anti-resonant reflection that can be integrated with waveguides for attenuating undesirable optical bands. Important criteria to consider for an optimized biosensor include cladding layer materials that minimize broad-spectrum photoluminescence and optimize layer thicknesses for creating a desired spectral response in both solid and liquid guiding layers, and a microfabrication process capable of producing regions with variable spectral response. New results describing how spurious fluorescence can be minimized by optimized thermal growth conditions and how liquid-core filter discrimination can be tuned with liquid core waveguide length are presented.

Selective Thin-Film Deposition for Optofluidic Platforms With Optimized Transmission

Selective deposition of dielectric thin films on an optofluidic biosensor provides the means for low loss in hollow-core waveguides and increased solid- to hollow-core coupling. Polymethylglutarimide (PMGI) is used over nonplanar topographies to provide liftoff patterning of dielectric films deposited at temperatures up to 250°C . This fabrication method provides throughput improvement in devices with hollow-cores longer than 2 mm.