A study of relevant fabrication parameters is presented for a process that uses chemical etching of sacrificial cores to produce long, hollow microchannels. Two different sacrificial materials are investigated, SU8 and reflowed photoresist. These two materials produce channel cross sections with rectangular and arch-shaped cores, respectively. Fabrication times based on etch removal rates of sacrificial materials are reported for SU8 core microchannels and for a hybrid core consisting of reflowed photoresist and aluminum layers. The hybrid design takes advantage of the fast etch times possible for aluminum, but also produces smooth, arched sidewalls. Structural integrity is also investigated for different microchannels, specifically the wall thickness required to produce an intact channel of a given width. Empirical design rules indicate that SU8-based core channels require a wall thickness-to-width ratio of greater than 1:10, and reflowed photoresist based structures require a ratio greater than 1:50.
We discuss the development of novel integrated optical sensors with single molecule detection sensitivity. These sensors are based on liquid-core antiresonant reflecting optical waveguides (ARROWs) that allow for simultaneously guiding light and molecules in liquid solution through micron-sized channels on a chip. Using liquid-core ARROWs as the main building block, two-dimensional planar sensor arrays with sensitivity down to the single molecule level can be fabricated. We present the basic design principle for ARROW waveguides and methods to improve waveguide loss. The influence of surface roughness on the waveguide loss is described. We discuss highly efficient fluorescence detection in both one and two dimensional planar waveguide geometries. Avenues towards subsequent integration with microfluidic systems are presented.
We discuss a new integrated approach to realizing optical quantum interference effects such as electromagnetically induced transparency (EIT), slow light, and highly efficient nonlinear processes on a semiconductor chip. An ensemble of alkali atoms represents one of the canonical systems that exhibit slow light and related phenomena. At the same time, it would be desirable to build slow-light and related devices on a semiconductor platform in order to move to practical applications. We review progress towards combining the large magnitude of quantum interference effects in alkali vapors with the convenience of integrated optics in the form of hollow-core antiresonant reflecting optical waveguides (ARROWs). We discuss the benefits and challenges of this integrated approach with special emphasis on nonlinear optics. We present strategies to optimize the optical waveguides and discuss the current status of building rubidium-filled optical waveguides on a chip. Recent results on optimization of waveguide loss and transfer of rubidium atoms through hollow microchannels on a chip are presented.
We have developed a fabrication method for hollow anti-resonant reflecting waveguides (ARROW) on planar silicon substrates. Our fabrication technique is a bottom-up process making use of a sacrificial core material which is removed in an acid etch, leaving a hollow channel. This method is compatible with standard silicon processing steps, enabling the production of integrated devices. Using different core materials, we have build hollow ARROW waveguides with different core geometries, and have also demonstrated the fabrication of solid-core waveguides to interface with the hollow ARROWs. By optimizing the layer structure and fabrication process, we can reduce the optical loss of these waveguides to below 0.33/cm for liquid-filled waveguides and 2.4/cm for air-core waveguides.
Here we present an analysis of a fully planar optical sensor based on ARROW waveguides. We consider a device geometry that consists of two intersecting ARROW waveguides, one with solid and one with liquid core. The pump wavelength is input from the solid core waveguide and penetrates through the wall into the liquid core of the other waveguide where the fluorescence is excited in the sample material. Then it is captured by the ARROW waveguide and guided to a detector at the end. Pump and signal wavelength can be guided with low loss through the solid and hollow core respectively. At the same time, high loss discrimination inside the core can be obtained by tailoring the thickness of ARROW layers, leading to efficient filtering. The pump wave can be transmitted efficiently in and out of the core, allowing for multiple intersections. In the following experimental part, we investigate the waveguide loss of the liquid core as a function of core width, we measure values as low as 1.7cm<sup>-1</sup> that can be further reduced by improving the thickness control of ARROW layers in the fabrication process. Finally, we report fluorescence experiments on Alexa dye molecules. We demonstrate fluorescence detection at concentrations as low as 10<sup>-8</sup> mol/l from a detection volume of 21pl. Photo-bleaching is observed and discussed as a function of input power intensity.
We present integrated antiresonant reflecting optical (ARROW) structures with hollow cores as a new paradigm for optical sensing of gases and liquids. ARROW waveguides with micron-sized hollow cores allow for single-mode propagation in low-index non-solid core materials where conventional index guiding is impossible. We review design, fabrication and optical characterization of these devices for possible applications in chemical sensing, single molecule fluorescence and Raman spectroscopy, flow cytometry, and pollution monitoring of picoliter to nanoliter volumes. We describe how to determine and control the waveguide loss and dispersion of the ARROW waveguides and design optimization for realistic structures that are compatible with the fabrication constraints. The technology to realize hollow-core waveguides using conventional silicon microfabrication and sacrificial core layers is discussed. We present the first demonstration of waveguiding in integrated ARROW waveguides with both hollow and liquid cores. Single-mode propagation with mode areas as small as 6mm2 and volumes down to 15 picoliters is observed and the loss characteristics of the waveguides are determined. The observation of fluorescence from dye molecules with concentrations of 10 nmol/l is described. Higher-level integration towards compact, planar, and massively parallel sensors on a chip is discussed.