  Recording of transient absorption microscopy images requires fast detection
of minute optical density changes, which is typically achieved with
high-repetition-rate laser sources and lock-in detection. Here, we present a
highly flexible and cost-efficient detection scheme based on a conventional
photodiode and an USB oscilloscope with MHz bandwidth, that deviates from the
commonly used lock-in scheme and achieves benchmark sensitivity. Our scheme
combines shot-to-shot evaluation of pump-probe and probe-only measurements, a
home-built photodetector circuit optimized for low pulse energies applying
low-pass amplification, and a custom evaluation algorithm based on Fourier
transformation. Advantages of this approach include abilities to simultaneously
monitor multiple frequencies, parallelization of multiple detector channels,
and detection of different pulse sequences (e.g., include pump-only). With a 40
kHz repetition-rate laser system powering two non-collinear optical parametric
amplifiers for wide tuneability, we demonstrate the 2-D imaging performance of
our transient absorption microscope with studies on micro-crystalline molecular
thin films.
