Designing whispering gallery modes via transformation optics

Transformation optics makes it possible to design whispering-gallery-mode resonators with high Q factors and unidirectional emission. In dielectric cavities with a rotational symmetry, whispering gallery modes (WGMs) with an extremely long lifetime (that is, a very high Q factor) can be formed by total internal reflection of light around the rim of the cavities. The ultrahigh Q factor of WGMs has enabled a variety of impressive photonic systems, such as ultralow threshold microlasers 1 , 2 , 3 , bio-sensors with unprecedented sensitivity 4 , 5 and cavity optomechanical devices 6 . However, the isotropic emission of WGMs, which is due to the rotational symmetry, is a serious drawback in applications that require directional light sources. Considerable efforts have thus been devoted to achieving directional emission by intentionally breaking the rotational symmetry 7 , 8 , 9 . However, all of the methods proposed so far have suffered from substantial Q -spoiling. Here, we show how the mode properties of dielectric whispering gallery cavities, such as the Q factor and emission directionality, can be tailored at will using transformation optics. The proposed scheme will open a new horizon of applications beyond the conventional WGMs.