The ability to directly measure the electric field of light enables novel observations of light-matter interactions, proving a powerful tool in modern ultrafast science. In this framework, the attoworld team applied an ultrashort visible-UV pulse to a common semiconductor (gallium phosphide). The pulse caused an appreciable change in the carrier density within the semiconductor, lasting for a few femtoseconds. This change in carrier density was harnessed as a gating event, which enabled the team to detect the electric field of a Near infrared test pulse spanning one octave from 110 to 220 THz.
The technique, dubbed linear photoconductive sampling (LPS), is then compared to the performance of electro-optic sampling (EOS). Although both approaches are capable of retrieving the field of the NIR pulse, the scientists observed systematic differences in their response functions. They found out that the deviation between the two techniques grows as they stretched the temporal profile of the VIS-UV gating pulse. As it turns out, that this difference between LPS and EOS was not caused by phase matching in the EOS crystal but rather by how the two techniques respond to the phase of the VIS-UV gating pulse. More accurately, the temporal response function of EOS depends on the complex envelope of the VIS-UV gate pulse, whereas the response of LPS is mostly concerned with the squared modulus of the same quantity.
Since this approach is based on a single-photon absorption process, it does not require high power or high pulse energy to sample the electric field of light. It offers undistorted detection, by injecting a single electron with a well-defined wavepacket in a single, well-defined band. It can sample a large bandwidth without phase-matching limitations in contrast to EOS. And last but not least technique supports the possibility of simultaneously measuring the electric field in two spatial dimensions.

Cover picture by: RMT.Bergues.

Original publication:
N. Altwaijry, M. Qasim, M. Mamaikin, J. Schötz, K. Golyari, M. Heynck, E. Ridente, V. Yakovlev, N. Karpowicz, M. Kling
Broadband Photoconductive Sampling in Gallium Phosphide
Advanced Optical Materials 11, 202202994 (2023)