To investigate the intrinsic response time of a photodetector material, being the physical limit of the response, the two-pulse coincidence technique is used. Here, the device is illuminated by two ultrashort (fs) laser pulses, separated by a delay time Δt. The corresponding photoresponse signal is detected as a function of the delay time Δt between the two pulses, revealing a dip of the signal at coinciding pulses (Δt = 0), due to a non-linear photoresponse. From the exponential increase of the signal with increasing Δt, the intrinsic response time can be determined. Thus, the first pulse induces a change of the photoactive material, which is probed by the second pulse.
Instead of using a typical mechanical delay stage to generate the delay time between the laser pulses, we use asynchronous optical sampling (ASOPS). Two individual coupled lasers with a slight offset in repetition rate (100 MHz ± Δf) are used. This creates a continuously increasing and thus accumulating delay time Δt, creating sweeps of the entire delay window of 10 ns within milliseconds. Accordingly, the combination of the two-pulse coincidence technique and ASOPS allows response time measurements over a huge delay window of 10 ns with unprecedented measurement speed.
