28 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 1, JANUARY 1, 2006 Integrated Hollow Waveguides With Arch-Shaped Cores John P. Barber, Student Member, IEEE, Evan J. Lunt, Zachary A. George, Dongliang Yin, Student Member, IEEE, Holger Schmidt, Member, IEEE, and Aaron R. Hawkins, Senior Member, IEEE Abstract—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 core’s 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. Index Terms—Antiresonant reflecting optical waveguide (ARROW), fabrication, hollow waveguide, liquid waveguide, micromachining. I. INTRODUCTION H OLLOW waveguides are an important photonic tech- nology because they allow for light guiding in very low refractive index materials, enabling guiding in gas or liquid media that fill the hollow cores. Optical sensors are a key application since guided light can readily interact with the gas or liquid media. The implementation of hollow waveguides has taken several forms including photonic crystals [1] and capillary tubes coated with a low refractive index Teflon layer [2]. A new class of hollow waveguides [3], [4] for picoliter volumes was recently introduced based on the antiresonant reflecting optical waveguide (ARROW) principle. These waveguides were fab- ricated on silicon substrates making them attractive for optical sensor platforms. Hollow-core ARROWs demonstrated in the past had rectangular or trapezoidal cross sections. This letter reports a new type of structure that has a curved or arch shape. We present a discussion of the advantages of an arch-shaped waveguide, fabrication parameters that determine the final arch shape, and a demonstration of optical waveguiding in a representative liquid-filled structure. II. ADVANTAGES OF ARCH-SHAPED CORE The fabrication process used to produce integrated ARROWs has been described in detail elsewhere [5] and involves the Manuscript received July 8, 2005; revised August 29, 2005. This work was supported by the National Science Foundation (NSF) under Grant ECS-0131945 and Grant ECS-0500602, and by the Air Force Office of Scientific Research (AFOSR) under Grant FA-9550-05-1-0432. J. P. Barber, E. J. Lunt, Z. A. George, and A. R. Hawkins are with the Electrical and Computer Engineering Department, Brigham Young University, Provo, UT 84602 USA (e-mail: hawkins@ee.byu.edu). D. Yin and H. Schmidt are with the School of Engineering, University of California, Santa Cruz, CA 95064 USA. Digital Object Identifier 10.1109/LPT.2005.859990 Fig. 1. SEM cross section of hollow ARROWs made using (a) reflowed photoresist sacrificial core and (b) SU8 sacrificial core. Silicon dioxide and silicon nitride are indicated by alternating light and dark layers. coating of a thin line of sacrificial material with layers of silicon nitride and silicon dioxide. The layers are produced using conformal plasma-enhanced chemical–vapor deposition (PECVD). Afterwards, the sacrificial material is removed by chemical etching producing the hollow waveguide core. Light guiding in these structures depends on the thicknesses of the nitride and oxide layers satisfying the antiresonant condition outlined by Duguay [6]. The cross-sectional shape of the sacrificial material before PECVD deposition determines the resulting shape of the hollow core. The rectangular and trapezoidal cores previously produced used SU8 and aluminum, respectively, as sacrificial materials. Arch-shaped cores were produced using reflowed photoresist. The reflow process has been observed for many years and has been used in the past to make a variety of integrated optical el- ements, especially microlenses [7]. The round shape produced by this process results when islands of photoresist are heated above their melting temperature and reshaped into spherical or cylindrical structures. Fig. 1(a) shows an arch-shaped ARROW created by patterning a long line of positive photoresist approx- imately 10 m thick on silicon. The resist was reflowed by placing the silicon wafer on a hotplate at 250 C for 10 min, 1041-1135/$20.00 © 2005 IEEE