Functionalizing nonlinear crystals

The most efficient nonlinear interaction in quasi phase-matched crystals occurs in periodic crystals when the interacting beams are Gaussian beams. The generated beams can be then routed, filtered or shaped by various elements - mirrors, lenses, pinholes, gratings, spatial light modulators, etc. In order to save space and cost, it is desired to combine the nonlinear process and the above mentioned functions into a single element. One option is to spatially modulate the periodic structure used for phase-matching, for example Ref. [1,2]. However, this technique may reduce the conversion efficiency and requires complex two-dimensional poling capability. We suggest a new approach in which the nonlinear crystal itself is perfectly periodic and the modulation is imposed by patterning a nano-layer of gold sputtered at the exit facet of the crystal. This patterning is achieved with focused ion beam (FIB) milling. The gold layer serves as an amplitude mask that can be patterned to serve as a computer generated hologram (CGH) [3] for beam shaping or as a spatial pinhole for beam quality improvement. In the nonlinear process of second harmonic generation (SHG), both frequencies are passing through the same holographic mask but since the central angle of the diffraction order is proportional to the wavelength, the beams are spatially separated.