“…Because the perimeter is circular, the grazing angle of light at the outer interface remains small the entire way along the optical path. This allows for the light to experience total internal reflection, and remain contained within the cavity, as shown in On-chip geometries such as microrings [27,124], microdisks [125][126][127][128][129] (a), double-disks [130][131][132][133][134][135][136][137] (b) and microtoroids [1,26,97,[138][139][140][141] (c) are made through lithography techniques, while microspheres [142][143][144] (d) are typically made by melting the end of an optical fibre (though they can also be fabricated on-chip [145]). Microbottle [146,147] or microbubble [148,149] resonators (e) are made through a more complicated melting and pulling process, and are often used in sensing applications.…”