Creating electromagnetic cavities using transformation optics
  We investigate the potential of transformation optics for the design of novel electromagnetic cavities. First, we determine the dispersion relation of bound modes in a device performing an arbitrary radial coordinate transformation and we discuss a number of such cavity structures. Subsequently, we generalize our study to media that implement azimuthal transformations and we show that such transformations can manipulate the azimuthal mode number. Finally, we discuss how the combination of radial and azimuthal coordinate transformations allows for perfect confinement of subwavelength modes inside a cavity consisting of right-handed materials only. 
Vincent Ginis, Philippe Tassin, Jan Danckaert, Costas M. Soukoulis,   Irina Veretennicoff
physics.optics cond-mat.mtrl-sci