ARROW waveguide

The optical mode is leaky, but relatively low-loss propagation can be achieved by making the Fabry–Pérot reflector of sufficiently high quality or small size.

[1] A Fabry-Perot etalon is in resonance when the light in the layer constructively interferes with itself, resulting in high transmission.

Anti-resonance occurs when the light in the layer destructively interferes with itself, resulting in no transmission through the etalon.

The refractive indexes of the guiding core (nc) and the cladding layers (nj, ni) are important and are carefully chosen.

In a typical system of a solid core ARROW, as shown in the figure, the waveguide consists of a low refractive index guiding core bounded on the upper surface by air and on the lower surface by higher refractive index antiresonant reflecting cladding layers.

The confinement of light on the upper surface of the guiding core is provided by the total internal reflection with air, while the confinement on the lower surface is provided by interference created by the antiresonant cladding layers.

The thickness of the antiresonant cladding layer (tj) of an ARROW also needs to be carefully chosen in order to achieve anti-resonance.

Note that the refractive indices of these ARROWs are reversed, when comparing to usual waveguides.

This strong overlap can be made plausible in a simplified picture imagining "rays", as in geometrical optics.

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.

ARROW are often used for guiding light in liquids, particularly in photonic lab-on-a-chip analytical systems (PhLoCs).

As a result, a conventional hollow-core waveguide no longer works once it's filled with water solution, making the PhLoCs useless.

Though ARROWs carry big advantage over conventional waveguide for building PhLoCs, they are not perfect.