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

在这个项目中，我们计划实现和研究一个光学频率转换器，将激光的频率移动几个10太赫兹（> 50 nm， 1微米波长）。在这里，100%的泵浦光子将在纳秒时间尺度上转换，并在一个系统中转换从可见光到中红外边缘的所有波长。频移的幅度跟随施加的电压，使得几乎任意时间变化的频移可以通过电压调制实现，例如线性频率啁啾。这将通过绝热变频实现。它的声学模拟是众所周知的：如果一个人插入吉他弦，并在响铃时间改变它的长度，音高也会相应改变。这个概念可以转移到光学：在这里，光被耦合到一个谐振器中，后者的光学尺寸在衰荡期间发生变化。循环光的频率严格遵循变化的谐振频率。这种光频率转换方案与传统方法相比具有显著的优势。所有腔内光子都被转换，即内部效率为100%，与光强无关。这一过程无需考虑相位匹配，即可以在谐振器中循环的所有波长的光。到目前为止，只实现了100 GHz量级的相对较小的频移（1 μ m波长1 nm）。我们计划将这个值提高两个数量级。这将通过使用由钽酸钾铌酸盐晶体（KTN）制成的低语廊谐振器来实现。它们在铁电相和准电相之间具有温度可控的转变。接近这种相变，KTN晶体具有非常大的电光系数。它们可能为在仅几千伏/毫米的电场下折射率变化10%铺平道路。电光驱动的绝热变频几乎是未知的。为了实现我们雄心勃勃的目标——实现上述变频器——首先，必须回答一些基本的科学问题：通过向电极施加电压而形成的腔内电场的空间分布是怎样的？铁电相和亲电相在内部电场方面有什么不同吗？系统能在多接近相变的情况下运行？外部电场是否有一个极限，超过这个极限，电荷就会注入晶体？后者可能会影响内部电场或减少停机时间。在哪个波长和功率范围内可以实现绝热变频？预计该项目将对新型变频器的科学研究、技术开发和应用产生强烈的启发作用。它们可用于距离测量和快速光谱学。

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.