Fig. 3

Time-resolved photocurrent spectroscopy of 2D Bi₂O₂Se photodetector. a Photocurrent as a function of delay time between two ultrafast pulses (~100 fs) at zero bias voltage. The red circles and black crosses are data of Bi₂O₂Se and graphene photodetector, while solid lines are exponential fitting. The blue and green curves in the top of the figure represent pump and probe laser pulses, while the black arrow shows the delay time between these two pulses. When delay time is too short, the excitation of the first pulse does not have enough time to turn into current before it meets the excitation of the second pulse, which induces sublinearity in photocurrent and reaches a dip at zero delay. When delay time is long enough, excitations from the two pulses perform independently and result in largest voltage, which is used for normalization value in this chart. b Response time (τ) of Bi₂O₂Se photodetector measured at different incident pump power. The response time is defined as time constant τ in exponential fitting of the photocurrent–delay curve by equation 3. Error bars here describe standard errors of the fitting parameter