Transient absorption microscopy setup with multi-ten-kilohertz shot-to-shot subtraction and discrete Fourier analysis

TL;DR

This paper introduces a flexible, cost-effective transient absorption microscopy detection scheme using a photodiode and Fourier analysis, enabling high sensitivity, multi-frequency monitoring, and parallel detection with a 40 kHz laser system.

Contribution

The authors develop a novel detection method that replaces lock-in detection with Fourier-based analysis, allowing for simultaneous multi-frequency and multi-channel transient absorption imaging.

Findings

Achieved benchmark sensitivity comparable to lock-in detection.

Demonstrated 2-D imaging of micro-crystalline molecular thin films.

Enabled parallel detection of multiple pulse sequences.

Abstract

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.