Photonic crystal cavity definition by electron beam bleaching of chromophore doped polymer cladding

We demonstrate a novel concept for the fabrication of high Q photonic crystal heterostructure cavities. First, photonic crystal waveguides without cavities are fabricated. The cavities are defined in a later fabrication step by spatially resolved bleaching of a chromophore doped polymer cladding. Bleaching of polymer films either by UV light or by electron beam illumination is well known to reduce the refractive index of the film. The reduction of the cladding refractive index leads to a reduction of the effective lattice constant of the photonic crystal waveguide. The maximum refractive index change was found to be 6·10 ^-2 which corresponds to the effective lattice constant change of 12.2 nm. With this approach it is also possible to achieve very small effective lattice constant shifts of 0.02 nm which is not possible with state of the art lithography. Being able to precisely define the effective lattice constant at every point of the photonic crystal waveguide we are able to impose cavity mode profiles which closely resemble a Gaussian envelope. This leads to a dramatic increase of the Q-factor. In simulations we have obtained Q-factors as high as 3.0·106 for a vertically symmetric polymer cladding. First results for non-vertically symmetric structures are presented. © 2012 SPIE.