In a pump-probe experiment, a pump pulse excites a sample and induces changes that are measured using a subsequent probe pulse. By varying the time between pump and probe pulses, one can retrieve the recovery time scales of the sample. Pump-probe techniques provide direct, time domain, measurement of gain and refractive index nonlinearities in optical waveguides with sub-picosecond resolution. Knowledge of the relaxation of the complete optical gain and index after a sub-picosecond light pulse is essential to assess the potential of current quantum dot materials for photonic systems applications. Pump-probe spectroscopy is necessary for this since the timescales of carrier processes in materials (typically tens of pico-seconds) are usually much faster than the bandwidth of conventional detectors. The setup at CIT is capable of two colour (heterodyne) pump-probe spectroscopy, in which pump and probe pulses are distinguished by inducing a small frequency shift between them. The technique allows separate extraction of the gain and refractive index dynamics in the waveguide and works for orthogonally as well as parallel polarised pump and probe pulses.
• Coherent Chameleon Ultra Ti:Sapphire laser
• APE GmbH Optical Parametric Oscillator (OPO)
• Acousto-Optic Modulators
• OSA, autocorrelator, lock-in amplifier, etc.
The pump-probe setup is part of the PDD's Femtosecond Physics Laboratory. Click here for a PDF brochure.
At CIT, researchers have used the heterodyne pump-probe method to determine the dependence of the capture time of InAs/GaAs quantum dots on bias current. The measured power law relationship between these two quantities was in good agreement with a model which assumed Auger dominated capture and recombination processes. This finding is important for future optical information processing applications since it implies that the recovery time of the ground state gain should be extremely fast for sufficiently high current levels.
By examining the absorption recovery dynamics, we demonstrated that the hole redistribution processes are extremely fast (1 ps) due to the effective mass asymmetry in InAs QDs. In addition, we have analyzed the gain dynamics far above transparency and found that the ES-GS relaxation is also a fast process, while Auger mediated electron capture to the QD constitutes the main limiting time scale in these devices. We have developed a rate equation model which is in good agreement with experiment. Such results are extremely relevant for the engineering of the next generation of high speed optical components such as regenerators and logic gates as QDs may offer opportunities due to their unique carrier dynamics.