Its determination requires all-optical methods, such as the two-pulse coincidence (2PC) technique, which is detailed in Box 2. Briefly, a pulsed pump laser excites the sample and after a short delay, a second laser, the probe laser, examines how many charge carriers can be re-excited. The temporal resolution of this experiment is given by the pulse width of the pump laser and/or the delay time between the two lasers, such that measurements with dt < 100 fs are possible.