Flat dielectric grating reflectors with focusing abilities

Sub-wavelength dielectric gratings have emerged recently as a promising alternative to distributed Bragg reflection dielectric stacks for broadband, high-reflectivity filtering applications. Such a grating structure composed of a single dielectric layer with the appropriate patterning can sometimes perform as well as 30 or 40 dielectric distributed Bragg reflection layers, while providing new functionalities such as polarization control and near-field amplification. In this Letter, we introduce an interesting property of grating mirrors that cannot be realized by their distributed Bragg reflection counterpart: we show that a non-periodic patterning of the grating surface can give full control over the phase front of reflected light while maintaining a high reflectivity. This new feature of dielectric gratings allows the creation of miniature planar focusing elements that could have a substantial impact on a number of applications that depend on low-cost, compact optical components, from laser cavities to CD/DVD read/write heads.