Ultrafast Widefield Mid-Infrared Photothermal Heterodyne Imaging

Abstract

Mid-infrared photothermal (MIP) microscopy is a valuable tool for sensitive and fast chemical imaging with high spatial resolution beyond the mid-infrared diffraction limit. The highest sensitivity is usually achieved with heterodyne MIP employing photodetector point-scans and lock-in detection, while the fastest systems use camera-based widefield MIP with pulsed probe light. One challenge is to simultaneously achieve high sensitivity, spatial resolution, and speed in a large field of view. Here, we present widefield mid-infrared photothermal heterodyne (WIPH) imaging, where a digital frequency-domain lock-in (DFdLi) filter is used for simultaneous multiharmonic demodulation of MIP signals recorded by individual camera pixels at frame rates up to 200 kHz. The DFdLi filter enables the use of continuous-wave probe light, which, in turn, eliminates the need for synchronization schemes and allows measuring MIP decay curves. The WIPH approach is characterized by imaging potassium ferricyanide microparticles and applied to detect lipid droplets (alkyne-palmitic acid) in 3T3-L1 fibroblast cells, both in the cell-silent spectral region around 2100 cm–1 using an external-cavity quantum cascade laser. The system achieved up to 4000 WIPH images per second at a signal-to-noise ratio of 5.52 and 1 μm spatial resolution in a 128 × 128 μm field of view. The technique opens up for real-time chemical imaging of fast processes in biology, medicine, and material science.

Video-rate mid-infrared photothermal imaging by single-pulse photothermal detection per pixel

“…Optical phase-detected photothermal imaging was subsequently developed by adding a reference optical field to enhance the image contrast (9)(10)(11)(12)36). Recently, frame rates more than hundreds of kilohertz have been reached with the use of an ultrafast CMOS camera (37). The wide-field approach, which uses spatial multiplexed detection, provides a higher maximum frame rate than scanning approaches, making it a useful tool for submillisecond photography.…”

Widefield mid-infrared photothermal heterodyne imaging in the cell-silent window

“…The WIPH is thus independent of the phase between raw data and reference wave and of fluctuations in the dc background. Further details on the experimental setup, the filter and the data evaluation can be found in the work by Paiva and Schmidt 8 .…”

“…In this work, we use a previously developed widefield mid-infrared photothermal heterodyne (WIPH) system 8 to investigate three promising vibrational probes (alkynes, azides and deuterium) attached to a fatty acid (palmitic acid).…”

Widefield infrared photothermal imaging and spectroscopy

“…More recently, a widefield modality has been incorporated into IR-PHI measurements. [12,13] In the present study, we continue this push towards data parallelization by introducing an ultrafast, widefield modality to IR-PHI, called wIR-PHI, that features high temporal and spatial resolution via use of a nanosecond MIR pump laser in conjunction with an ultrahigh-speed complementary metal-oxide-semiconductor (CMOS) camera.…”