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

Robert Schwarzl, Pascal Heim+5 more|arXiv (Cornell University)|Jun 6, 2022

Advanced Fluorescence Microscopy Techniques21 references3 citations

TL;DR

This paper presents a cost-effective, high-sensitivity transient absorption microscopy setup using shot-to-shot pump-probe and probe-only pulse subtraction with discrete Fourier analysis on a USB oscilloscope, achieving a sensitivity of ∆Ipr/Ipr = 4.7 × 10⁻⁵ (20 µOD) at 40 kHz repetition rate. The method enables simultaneous multi-frequency monitoring, parallelized detection, and flexible pulse sequence control without lock-in hardware, demonstrated on photodamage-sensitive organic microcrystalline films with sub-micron spatial resolution and 80 fs temporal resolution.

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.

Motivation & Objective

• To develop a cost-efficient, flexible detection scheme for transient absorption microscopy that avoids expensive lock-in amplifiers.
• To achieve high sensitivity (ΔIpr/Ipr < 5 × 10⁻⁵) using a conventional photodiode and USB oscilloscope with moderate bandwidth.
• To enable simultaneous monitoring of multiple frequency components and parallelization of detector channels without hardware complexity.
• To allow flexible pulse sequence design, including pump-only and pump-dump-probe configurations, for enhanced dynamic characterization.
• To enable damage-free imaging of photolabile materials such as micro-crystalline organic thin films by minimizing pulse fluence.

Proposed method

• A 40 kHz femtosecond laser system powers two non-collinear optical parametric amplifiers (NOPAs) for wavelength-tuneable pump and probe pulses.
• Shot-to-shot acquisition alternates between pump-probe and probe-only pulses to enable direct subtraction of background signals.
• A home-built photodetector circuit with low-pass amplification is optimized for low pulse energies (down to 5 pJ).
• Digitized signals from a USB oscilloscope (10 MHz bandwidth) are analyzed via discrete Fourier transformation to extract transient absorbance.
• The Fourier-based algorithm enables simultaneous detection of multiple frequency components and supports arbitrary pulse train configurations.
• Spatial resolution is achieved via tight focusing (FWHM ~4 µm), and polarization control enables polarization-resolved microscopy.

Experimental results

Research questions

• RQ1 Can a high-sensitivity transient absorption microscope be built using only a standard photodiode and USB oscilloscope, avoiding dedicated lock-in hardware?
• RQ2 What is the achievable sensitivity (ΔIpr/Ipr) when using shot-to-shot subtraction and Fourier analysis instead of lock-in detection?
• RQ3 Can the method simultaneously resolve multiple frequency components and support flexible pulse sequences such as pump-only or pump-dump-probe?
• RQ4 To what extent is the signal-to-noise ratio limited by laser noise rather than detector or electronic noise?
• RQ5 Can this setup image ultrafast dynamics in photodamage-sensitive materials like micro-crystalline organic thin films with minimal fluence?

Key findings

• The setup achieves a sensitivity of ΔIpr/Ipr = 4.7 × 10⁻⁵ (equivalent to 20 µOD) for a 200,000-pulse measurement in 5 seconds, with a beam diameter of 6 µm.
• The dominant source of noise is the femtosecond laser system, indicating that further improvements in signal-to-noise ratio are possible with more stable lasers.
• The method enables simultaneous monitoring of multiple frequency components and supports parallelization of multiple detector channels without additional cost or complexity.
• The system successfully resolves ultrafast exciton dynamics in individual micro-crystalline organic platelets, showing immediate ground-state bleach followed by non-radiative decay within ~10 ps.
• Polarization-resolved imaging is demonstrated by aligning pump and probe polarizations to maximize contrast with the transition dipole moments of the platelets.
• The technique enables damage-free imaging of photolabile materials, as evidenced by measurements on squaraine-based organic thin films with pulse energies as low as 6.3 pJ for the probe beam.