Cover story: Minin IV, Minin OV, Cao YH, Yan B, Wang ZB et al. Photonic lenses with whispering gallery waves at Janus particles. Opto-Electron Sci 1, 210008 (2022)

Since ancient times, gardens have not been watered during the day, as drops of water that collected on the leaves caused sunburn. Seneca (4 BC - 65 AD) mentioned the magnifying effect of spherical particles when he looked through them at small letters. Since the early 2000s, dielectric spheres with a size on the order of a wavelength have attracted the attention of scientists. Such particles are too large to use the Rayleigh approximation and too small to use geometrical optics. Many interesting and unusual optical effects have been discovered for such spheres whose Mie size parameter q is 2π~20π. However, the control of the resonant and focusing properties of such mesoscale dielectric spheres was significantly limited by only two parameters: the size of the sphere (in wavelengths) and the contrast of the refractive index of the sphere material.

An additional degree of freedom is provided by a small truncation of the shadow surface of the sphere, called Janus particle. This particle simple configuration leads to several new effects. Particle truncation allows for a simple and elegant way to generate whispering gallery waves. Truncating a small part of a sphere allows the near field enhancement of local electric and magnetic fields. Moreover, the amplification of the intensity of the magnetic field is several hundred times, while the amplification of the electric field is several times less. On the flat surface of the truncated part of the sphere, a system of hot spots with a deep subwavelength size is generated. These effects can find applications in many areas, including nanolithography.