shallow angle, when entering the low index inner layer. Thus, one can use the metaphor that these rays "stay very long inside" the low index inner layer. Note this is just a metaphor and the explanatory power of ray optics is very limited for the micrometer scales, at which these ARROWs are typically made. Applications ARROW are often used for guiding light in liquids, particularly in photonic lab-on-a-chip analytical systems (PhLoCs). Conventional waveguides rely on the principle of total internal reflection, which can only occur if the refractive index of the guiding core material is greater than the refractive indexes of its surroundings. However, the materials used to make the guiding core are typically polymer and silicon-based materials, which have higher refractive indexes (n=1.4-3.5) than that of water (n = 1.33). As a result, a conventional hollow-core waveguide no longer works once it's filled with water solution, making the PhLoCs useless. An ARROW, on the other hand, can be liquid-filled since it confines light completely by interference, which requires that the refractive index of the guiding core to be lower than the refractive index of the surrounding materials. Thus, ARROWs become the ideal building blocks for PhLoCs. Though ARROWs carry big advantage over conventional waveguide for building PhLoCs, they are not perfect. The main problem of ARROW is its undesirable light loss. Light loss of ARROWs decreases the signal to noise ratio of the PhLoCs. Different versions of ARROWs have been designed and tested in order to overcome this problem. See also Waveguide (optics) Photonic-crystal fiber External links ARROW Waveguide Layer Thickness Calculator References Optical devices