High-sensitivity visualization of ultrafast carrier diffusion by wide-field holographic microscopy

Sketch of the imaging and holographic part of the transient holographic microscope, including the trains of pulses to illustrate the signal modulation approach. The array of diffraction limited excitation spots is created by imaging a pinhole array at the sample position, enabling simultaneous acquisition of transient data around 100 excitation spots.  

Femtosecond transient microscopy is an important tool to study ultrafast transport properties of excited states in solid state samples. Most implementations are limited to photoexciting a single diffraction limited spot at the sample and tracking the temporal evolution of the ensuing carrier distribution, hence covering a very small sample area. Recently, we have demonstrated how to vastly increase the field-of-view of ultrafast microscopes by using off-axis holography to build an all-optical lock-in camera, which decouples the signal demodulation speed from the maximum detector frame rate. 

Whereas in the original work we demonstrated the simultaneous transient imaging of dozens of individual nanoobjects, where photoexcitation of the entire field of view was desirable, in the context of solid state samples where diffraction limited excitation is needed it was not clear how the new holographic technique could be applied. Ideally, an array of diffraction limited excitation spots covering the entire field of view would be generated, so that multiple spots across a large sample area can be simultaneously probed.

In High-Sensitivity Visualization of Ultrafast Carrier Diffusion by Wide-Field Holographic Microscopy we demonstrate how to accomplish this feature by imaging a pinhole array at the sample position. We show that this is not only useful to obtain statistical information on the photophysics of the sample, but also that for homogeneous samples the signal of all spots can be averaged to vastly boost the signal-to-noise ratio.