Small-period titanium-diffused periodically poled lithium niobate waveguides for strongly nondegenerate quantum frequency conversion

Abstract

A titanium-diffused periodically poled lithium niobite (Ti:PPLN) waveguide device with ultra-short poling periods is developed for strongly nondegenerate quantum frequency conversion (QFC) from 369.5nm to 1550nm with a 485nm pump. A practical quantum information network requires a quantum state transfer link between disparate frequencies corresponding to atomic quantum memories and the telecommunication fiber network spectral window. QFC occurs in a nonlinear parametric amplifier. A Ti:PPLN diffused waveguide implementation allows for concentration of mode fields along the full interaction length of the nonlinear crystal. It also allows for engineering of an arbitrary desired nonlinear interaction based on poling parameters through quasi phase matching (QPM). Despite the relative maturity of PPLN technology, the target interaction is quasi phase matched at a period of 2.053μm which represents a significant technical challenge. The periodic poling process requires localization of strong electric fields on the order of 20kV/mm onto stripes which are on the order of 1μm. Thus, nickel- chromium (NiCr) electrodes are used to localize domain nucleation and enhance domain inversion quality by a factor of 200 compared to traditional periodic poling techniques. Devices are fabricated by diffusing Ti stripes into the crystal substrate, then sputtering and wet etching a NiCr layer to form the electrodes. High voltage is applied in an electrolyte bath using a Trek 20/20C high voltage amplifier with a custom high-speed controller utilizing an ARM processor core, FPGA, and A/D converter. Poled devices are then stripped of electrodes and their end faces are polished for optical measurement.