Collinear Optical Two-Dimensional Coherent Spectroscopy with Fluorescence Detection at 5 kHz Repetition Rate

Optical two-dimensional coherent spectroscopy (2DCS) is a powerful ultrafast spectroscopic technique that can greatly benefit from the unique features of a femtosecond laser operating at a kHz repetition rate. However, isolating specific nonlinear signal in the collinear geometry, especially with a kHz laser, presents challenges. We present an experimental implementation of optical 2DCS in a collinear geometry using a femtosecond laser operating at a 5 KHz repetition rate. The desired nonlinear signal is selectively extracted in the frequency domain by lock-in detection. Both pump-probe and optical 2DCS experiments were conducted on a rubidium vapor. The signal-to-noise ratio of pump-probe and 2DCS spectra was characterised under various experimental parameters. The study highlights the importance of the lock-in reference frequency in overcoming the limitations imposed by low repetition rates, thereby paving the way for the application of collinear optical 2DCS in material systems requiring kHz femtosecond lasers.