Probing spatial heterogeneity in exciton and carrier diffusion using transient holographic microscopy

Ultrafast transient microscopic techniques have been used for several years to study exciton and carrier diffusion in materials, down to ultrafast timescales. A common issue remains that all such experiments rely on observing diffusion around a single diffraction-limited spot of the sample, raising the question of how representative the results may be, particularly in samples that are prone to spatial heterogeneity such as polycrystalline thin films, bulk-heterojunctions or 2D materials. The main issue to perform transient techniques in a wide-field configuration is that the slow acquisition speed of cameras limits the modulation frequency of the excitation beam, rendering shot-to-shot signal demodulation impossible even at rather low repetition rates. Here we show how to overcome this issue using a transient holographic microscope that decouples the modulation frequency of the pump beam from the camera acquisition speed, allowing us to obtain ultrafast transient images of the sample covering areas of around 100x100 micrometers. We will demonstrate how to use this to simultaneously acquire diffusion data around over 100 evenly spaced points across the entire field of view, hence obtaining significant statistical information of the sample in a single measurement.