Collinear optical two-dimensional electronic spectroscopy with fluorescence detection at 5 kHz repetition rate

Optical two-dimensional electronic spectroscopy (2DES) is a powerful ultrafast spectroscopic technique that can greatly benefit from the unique features of a femtosecond laser operating at a kHz repetition rate. Lock-in detection of phase-modulated 2DES fluorescent signals requires reference frequencies well below the Nyquist limit of the laser repetition rate. In kHz pulse laser systems, this presents a challenge as the reference frequencies can extend down to the Hz regime, approaching the high intrinsic noise floor of the laboratory environment. We present an experimental study of the signal-to-noise ratio for pump-probe and 2DES phase-modulated fluorescence detection of a rubidium vapor cell with lock-in frequencies down to ∼300 Hz. The study highlights the importance of the lock-in reference frequency in overcoming the limitations imposed by low repetition rates.