  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 place the characterization platform in almost any temperature
environment. As 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 direct impact on
applications in superconducting quantum technology such as the enhancement of
optical links to superconducting qubits and quantum-accurate waveform
generators.
