Adiabatic frequency conversion driven by the electro-optic response of potassium tantalate niobate mixed crystals

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In this project, we plan to realize and investigate an optical frequency converter that shifts the frequency of laser light by several 10 THz (> 50 nm at 1 micrometer wavelength). Here, 100 % of the pump photons shall be converted on the nanosecond time scale and in a single system for all wavelengths from the visible to the edge of the mid infrared. The magnitude of the frequency shift follows an applied electric voltage such that almost arbitrary temporally varying shifts can be realized by voltage modulation, e.g. linear frequency chirps.This shall be enabled by adiabatic frequency conversion. Its acoustic analog is well known: If one plugs a guitar string and varies its length during the ring-down time, the pitch of the tone changes accordingly. This concept can be transferred to optics: Here, light is coupled into a resonator and the optical size of the latter is changed during the ring-down time. The frequency of the circulating light strictly follows the one of the varying resonance frequency. This scheme for optical frequency conversion has significant advantages over conventional methods. All intracavity photons are converted, i.e. the internal efficiency is 100 %, independent of the light intensity. This process works without taking care of phase matching, i.e. for light at all wavelengths that can circulate in the resonator. So far, only relatively small frequency shifts of the order of 100 GHz have been realized (1 nm at 1 µm wavelength). We plan to increase this value by two orders of magnitude. This shall be achieved by employing whispering gallery resonators made of potassium tantalate niobate crystals (KTN). They possess a temperature controllable transition between a ferroelectric and a paraelectric phase. Close to this phase transition, KTN crystals have extraordinarily large electro-optic coefficients. They might pave the way for a 10 % refractive-index change at electric fields of only a few kV/mm.Electro-optically driven adiabatic frequency conversion is almost unexplored. In order to achieve our ambitious goal – the realization of the abovementioned frequency converter – first, fundamental scientific questions have to be answered: What is the spatial distribution of the intracavity electric field built up by applying a voltage to the electrodes? Is there a difference between the ferroelectric and the paraelectric phases regarding the internal electric field? How close to the phase transition the system can be operated? Is there a limit for the external electric field beyond which charges are injected into the crystal? The latter might influence the internal electric field or reduce the rind-down time. In which wavelength and power range adiabatic frequency conversion can be achieved?We expect that this project will strongly inspire the scientific investigation, the technical development and the application of novel frequency converters. They might be employed for distance measurement and fast spectroscopy.