High-speed mid-infrared photothermal microscope for dynamic and spectroscopic imaging

Abstract

Mid-infrared spectroscopic imaging, which leverages the inherent vibrational contrast of chemical bonds, has been a powerful analytical tool for sample characterization. However, its use in studying living systems is limited by low spatial resolution and significant water absorption. Recently developed mid-infrared photothermal (MIP) microscopy addresses these limitations by probing the absorption-induced photothermal effect using visible light. MIP microscopy achieves sub-micrometer spatial resolution and reduces water background interference. Yet, the imaging speed of current MIP microscopy is constrained by the challenge of measuring a small modulation over the probe laser background. This low imaging throughput hinders the visualization of living dynamics, and the rich molecular information in the spectroscopic domain is obscured due to the slow acquisition process. This dissertation explores solutions for enhancing imaging speed and spectral throughput and extending MIP imaging into visualizing chemical dynamics in living systems.

In the first part of the dissertation, the mid-infrared photothermal process is studied and modeled in the time, frequency, and spatial domains using heat transfer analysis. Photothermal dynamics imaging (PDI) is introduced with the ability to visualize nanosecond-scale thermodynamics in samples upon laser excitation. By capturing all higher-order harmonics, PDI achieves more than a four-fold improvement in signal-to-noise ratio compared to the lock-in method for detecting low-duty cycle photothermal signals. An imaging speed nearly two orders of magnitude faster than the lock-in counterpart has been reached. In addition, PDI captures the transient thermal field evolution, providing a tool to gauge the target’s physical properties and microenvironment.

In the second part, a video-rate MIP microscope is introduced based on the PDI detection method. In the system, a synchronized IR and visible beam scanning scheme is developed, enabling photothermal detection with a single IR pulse at each pixel. Moreover, synchronized laser scanning allows uniform MIP imaging in a field of view over hundreds of micrometers while maintaining a high spatial resolution. This capability enabled the visualization of fast chemical dynamics inside living fungal cells, cancer cells, and living worms, providing an imaging platform for biology research.

Having reached the speed limitation of single-pulse imaging, we further advanced the speed of spectroscopic imaging by moving beyond the conventional measurement of absorption contrast in the photothermal process. In the final part of this dissertation, we revisited the photothermal process from the perspective of energy deposition, discovering that the absorption coefficient is reflected in the slope of the heating process rather than its overall amplitude. We demonstrated mid-infrared energy deposition (MIRED) spectroscopy using a 32-channel quantum cascade laser array that emits a broadband pulse train in microseconds. With MIRED, we achieved hyperspectral mid-infrared imaging on a microsecond scale.