Additional data for the publication "Cryogenic fiber-coupled electro-optic characterization platform for high-speed photodiodes"

We have developed a cryogenic characterization platform for ultrafast photodiodes, whose time domain responses are extracted by electro-optic sampling using femtosecond laser pulses in a pump-probe configuration. The excitation of the photodiodes with the pump beam and the electro-optic sampling crystals with the probe beam are realized in a fully fiber-coupled manner. This allows us to use the characterization platform at different temperatures, ranging from cryogenic to room temperature. As an application example, we characterize the time-domain response of commercial p-i-n photodiodes with a nominal bandwidth of 20 GHz and 60 GHz at temperatures of 4 K and 300 K and in a large parameter range of photocurrent and reverse bias. For these photodiodes, we detect frequency components up to approximately 250 GHz, while the theoretical bandwidth of our sampling method exceeds 1 THz. Our measurements demonstrate a significant excitation power and temperature dependence of the photodiodes’ ultrafast time responses, reflecting, most likely, changes in carrier mobilities and electric field screening. Since our system is an ideal tool to characterize and optimize the response of fast photodiodes at cryogenic temperatures, it has a direct impact on applications in superconducting quantum technology such as the enhancement of optical links to superconducting qubits and quantum-accurate waveform generators.我们研发了一款面向超快光电二极管的低温表征平台，该平台通过采用飞秒激光脉冲的电光采样（Electro-optic Sampling）技术，以泵浦-探测构型（pump-probe configuration）提取其时域响应。泵浦光束对光电二极管的激发，以及探测光束对电光采样晶体的激发，均采用全光纤耦合方式实现。这使得该表征平台可在低温至室温的宽温度区间内开展测试工作。作为应用示例，我们针对标称带宽分别为20 GHz与60 GHz的商用p-i-n光电二极管，在4 K与300 K的温度条件下，以及光电流、反向偏置的宽参数范围内，对其时域响应进行了表征。针对此类光电二极管，我们检测到了最高约250 GHz的频率分量，而本采样方法的理论带宽超过1 THz。我们的测量结果表明，光电二极管的超快时间响应与激发功率和温度存在显著相关性，这大概率反映了载流子迁移率与电场屏蔽效应的变化。由于本系统是在低温环境下表征与优化高速光电二极管响应的理想工具，因此其对超导量子技术领域的相关应用具有直接推动作用，例如优化面向超导量子比特（Superconducting Qubits）的光链路，以及量子精准波形发生器的研发。