Adiabatic frequency conversion (AFC) in microresonators provides a few advantages over frequency conversion via optical nonlinearity. There are no restrictions due to phasematching. Furthermore, the conversion efficiency does not depend on light intensity, i.e. it can reach 100 % even at the single-photon level. AFC has the following acoustic analogue: A guitar string is picked. Subsequently, the length of the string is decreased during the ring-down time of the sound. Consequently, the tone pitch, i.e. the frequency of the sound, increases. In the optical domain, it works as follows: Light is coupled into a resonator. When the optical length of the cavity is varied on a time scale smaller than its decay time, the frequency of the intracavity light changes.
We demonstrate an electro-optically driven adiabatic frequency converter based on a millimeter-sized whispering gallery resonator made of a lithium niobate crystal. By switching the electric field, we change the resonator’s refractive index and consequently the frequency of the circulating light. The frequency shift is proportional to the applied electric field. Using this simple approach, we achieve frequency shifts exceeding 5 GHz at electric fields as low as 300 V/mm.
Furthermore, we discuss the benefits of employing potassium tantalate niobate crystals. They provide a 33-times larger linear electro-optic response when operated close to their ferroelectric-paraelectric phase transition. Such a strong electro-optic response might pave the way for frequency shifts in the THz range.