Time-Resolved Pump Probe Reflectivity

Undergraduate #287
Discipline: Physics
Subcategory: Materials Science

Elangeni Yabba - University of the Virgin Islands
Co-Author(s): Victor Torres, University of Maryland College Park, Baltimore, MD

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A pump-probe experiment is often used to study ultrafast carrier dynamics in semiconductors. Before conducting our experiment, we hypothesized that the pulse duration would be approximately 7 picoseconds and the carrier lifetime would be approximately 4 nanoseconds. In order to carry out these highspeed measurements, it is important to know the duration of the excitation laser pulse. Autocorrelation measurements are used to estimate the laser pulse duration and involve a series of steps. First, the beam is split into two separate paths. Mirrors are then arranged so that the beams are parallel and within a few millimeters of each other and the path lengths identical. Then, the beams are focused onto a beta barium borate nonlinear crystal. A stepper motor is used to adjust the position of a mirror, which results in the appearance of a third beam, the desired autocorrelation signal between the two beams. The third beam is directed towards a photodetector which is connected to a lock-in amplifier and a computer program (LabVIEW). The program records the data and plots a graph of the amplitude of the pulse versus the time delay. The pulse duration was estimated to be 6.27 picoseconds, in excellent agreement with the manufacturer of the Nd:Vanadate laser. In a time-resolved pump-probe reflectivity experiment of a semiconductor (InGaAs), an ultrashort laser pulse is split into two portions; a stronger beam (pump) is used to excite photocarriers in the semiconductor and a weaker beam (probe) is used to monitor the time dependent change of the reflectivity of the sample. Measuring the changes in the reflectivity as a function of time delay between the arrival of pump and probe pulses yields information about the photocarrier lifetime of the sample. Time-resolved pump-probe experiments permit the measurement of the photocarrier lifetimes of different materials to be evaluated and helps one decide which material is best for different optoelectronic applications.

Funder Acknowledgement(s): This research was funded by the UMBC College of Natural Mathematics and Science and Graduate School and University of the Virgin Islands MARC grant 5T34GM008422.

Faculty Advisor: Anthony Johnson,