Coherent Background Subtraction with an Angle Compensated Temporal Shearing Interferometer for Enhanced Contrast in Widefield Pump-Probe Microscopy

Pump-probe microscopy allows for label-free imaging of structural and chemical features of samples. However, signals in pump-probe microscopy are typically small and often must be measured in the presence of large backgrounds. As a result, achieving measurements with a high signal-to-noise ratio is challenging, particularly when using sensors which are easily saturated by a relatively small number of collected photons, such as CMOS cameras. We present a method for enhancing signal-to-noise ratio when limited by such sensors. In this approach, temporally sheared reference and probe pulses transmit through a sample before and after excitation by a pump pulse. The probe and reference pulses are then recombined in time with a 180 degree phase shift and nearly matched amplitudes to allow for interferometric background reduction. This recombining operation is performed by a novel common-path interferometer. Unlike previous techniques for shearing fields in time, this interferometer demonstrates negligible phase and group delay dispersion with angle of incidence, allowing convenient widefield imaging. To our knowledge, this is the first common-path interferometer with this property. We demonstrate the technique by measuring transient absorption signals in gold nanorod films with a signal-to-background ratio enhanced by over 100% and a signal-to-noise ratio enhanced by about 70%.