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Optical pump-probe spectroscopy is a powerful tool to directly probe the carrier dynamics in materials down to sub-femtosecond resolution. To perform such measurements, while keeping the pump induced perturbation to the sample as small as possible, it is essential to have a detection scheme with a high signal to noise ratio. Achieving such a high signal to noise ratio is easy with phase sensitive detection based on a lock-in-amplifier when a high repetition rate laser is used as the optical pulse source. However, such a lock-in-amplifier based method does not work well when a low repetition rate laser is used for the measurement. In this article, a sensitive detection scheme, which combines the advantages of a boxcar that rejects noise in time domain and a lock-in-amplifier that isolates the signal in the frequency domain for performing pump-probe measurements using a low-repetition rate laser system, is proposed and experimentally demonstrated. A theoretical model to explain the process of signal detection and a method to reduce the pulse to pulse energy fluctuation in probe pulses is presented. By performing pump-probe measurements at various detection conditions, the optimum condition required for obtaining the transient absorption signal with low noise is presented. The reported technique is not limited to pump-probe measurements and can be easily modified to suit for other sensitive measurements at low repetition rates.

Filtering noise in time and frequency domain for ultrafast pump–probe performed using low repetition rate lasers