Evan J. Lunt

Planar optofluidic chip for single particle detection, manipulation, and analysis.

We present a fully planar integrated optofluidic platform that permits single particle detection, manipulation and analysis on a chip. Liquid-core optical waveguides guide both light and fluids in the same volume. They are integrated with fluidic reservoirs and solid-core optical waveguides to define sub-picoliter excitation volumes and collect the optical signal, resulting in fully planar beam geometries. Single fluorescently labeled liposomes are used to demonstrate the capabilities of the optofluidic chip. Liposome motion is controlled electrically, and fluorescence correlation spectroscopy (FCS) is used to determine concentration and dynamic properties such as diffusion coefficient and velocity. This demonstration of fully planar particle analysis on a semiconductor chip may lead to a new class of planar optofluidics-based instruments.

On-chip surface-enhanced Raman scattering detection using integrated liquid-core waveguides

The authors demonstrate surface-enhanced Raman scattering (SERS) detection on an optofluidic chip. Interconnected solid- and liquid-core antiresonant reflecting optical waveguides (ARROWs) form a planar beam geometry that allows for high mode intensities along microfluidic channels containing molecules optimized for SERS. The excitation power and concentration dependence of SERS from rhodamine 6G (R6G) molecules adsorbed to silver nanoparticles were systematically studied. The data can be described by a model that takes into account the microphotonic structure. Detection sensitivity to a minimum concentration of 30nM is found, demonstrating the suitability of ARROW-based optofluidic chips for high sensitivity detection with molecular specificity.

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.

Integrated hollow waveguides with arch-shaped cores

An optical waveguide is described that has a hollow arch-shaped core. Optical confinement for this structure is based on the antiresonant reflecting optical waveguide principle. The waveguides are built on a silicon substrate using a sacrificial etch technique with reflowed photoresist serving as the sacrificial material and producing the cores arch shape. Investigations of fabrication parameters are reported that allow for predicting a final arch-shaped geometry based on initial photoresist width and thickness. Optical mode guiding is demonstrated in an arch-shaped waveguide with a liquid core.

Ultrasensitive Qβ phage analysis using fluorescence correlation spectroscopy on an optofluidic chip

We demonstrate detection and analysis of the Qbeta bacteriophage on the single virus level using an integrated optofluidic biosensor. Individual Qbeta phages with masses on the order of attograms were sensed and analyzed on a silicon chip in their natural liquid environment without the need for virus immobilization. The diffusion coefficient of the viruses was extracted from the fluorescence signal by means of fluorescence correlation spectroscopy (FCS) and found to be 15.90+/-1.50 microm(2)/s in excellent agreement with previously published values. The aggregation and disintegration of the phage were also observed. Virus flow velocities determined by FCS were in the 60-300 microm/s range. This study suggests considerable potential for an inexpensive and portable sensor capable of discrimination between viruses of different sizes.

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.

Optical characterization of arch-shaped ARROW waveguides with liquid cores.

We present the characterization of the optical properties of integrated antiresonant reflecting optical (ARROW) waveguides with arch-shaped liquid cores. Optical mode shapes and coupling, waveguide loss, and polarization dependence are investigated. Waveguide loss as low as 0.26/cm with near-single-mode coupling and mode areas as small as 4.5microm2 are demonstrated. A detailed comparison to ARROW waveguides with rectangular cores is presented, and shows that arch-shaped cores are superior for many applications.

Hollow ARROW Waveguides on Self-Aligned Pedestals for Improved Geometry and Transmission

Micrometer-sized hollow antiresonant reflecting optical waveguides on silicon substrates have been previously demonstrated with liquid and gas-filled cores. Previous designs have nonideal geometries, with nonuniform lateral layers around the hollow core, resulting in higher loss than could potentially be achieved. A new design and fabrication process has been developed involving hollow waveguide fabrication on a self-aligned pedestal (SAP) using anisotropic plasma etching. With the SAP structure, the hollow core is surrounded by uniform layers and a terminal layer of air on three sides, resulting in air-core waveguide loss of 1.54 cm-1 at 785 nm and high fabrication yield.

Microphotonic control of single molecule fluorescence correlation spectroscopy using planar optofluidics.

We demonstrate the implementation of fluorescence correlation spectroscopy (FCS) on a chip. Full planar integration is achieved by lithographic definition of sub-picoliter excitation volumes using intersecting solid and liquid-core optical waveguides. Concentration dependent measurements on dye molecules with single molecule resolution are demonstrated. Theoretical modeling of the FCS autocorrelation function in microstructured geometries shows that the FCS behavior can be controlled over a wide range by tailoring the micro-photonic environment. The ability to perform correlation spectroscopy using silicon photonics without the need for free-space microscopy permits implementation of numerous diagnostic applications on compact planar optofluidic 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.