Research ArticleAPPLIED SCIENCES AND ENGINEERING

Ultrafast chemical imaging by widefield photothermal sensing of infrared absorption

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Science Advances  19 Jul 2019:
Vol. 5, no. 7, eaav7127
DOI: 10.1126/sciadv.aav7127
  • Fig. 1 Schematic of WPS microscope.

    A nanosecond mid-IR laser (bottom right) was sent through an optical chopper and weakly focused on the sample. The IR beam was partially sampled with a CaF2 plate (P) and sent to an MCT detector. The probe was provided by a 450-nm LED, which was imaged to the back aperture of an imaging objective by a 4f lens system and a 50/50 beam splitter (BS). The sample-reflected light was collected by the same objective and sent to an image sensor with a tube lens. GM, gold mirror; OAPM, off-axis parabolic mirror; CMOS, complementary metal-oxide semiconductor.

  • Fig. 2 Camera-based photothermal imaging.

    (A) Block diagram. The MCT detector was used to capture the IR laser pulses to generate the master clock f at the repetition rate of the IR laser to trigger the function generator, which sent square wave triggers to the camera, chopper, and LED. The internal frequency divider of the camera was set to expose at f/8 frames/s. The chopper divided the trigger pulses by 16 to modulate the pump. A computer was used to control the camera and store the data. (B) Measured pulses of the IR (red), visible (blue), and camera exposure monitor (purple) with an oscilloscope. For the 20-kHz laser repetition rate and 2500-Hz camera frame rate, each frame contained eight probe pulses. (C) Zoom-in view of individual pump and probe pulses. The time scale for each grid is 100 μs in (B) and 500 ns in (C). (D) Image processing procedure to generate a photothermal image. Contrast was created by subtraction between hot and cold frames. Scale bars, 40 μm.

  • Fig. 3 Time-resolved WPS imaging of PMMA film on silicon at the 1728 cm−1 C=O band.

    (A) WPS images of a 486-nm-thick PMMA film at different pump-probe delays. Scale bar, 40 μm. The probe width was 914 ns. Imaging speed: 2 Hz. Imaging contrast: cold-hot. Power at the sample: pump, 5.1 mW; probe, 1.6 mW. (B) Temporal profile of WPS signal (squares) and the exponential decay fitting result (curve). The decay constant was 1.1 μs. (C) Spectral profile of WPS signal (squares) and the reference FTIR spectrum of PMMA (curve).

  • Fig. 4 WPS imaging of etched pattern in PMMA film and microparticles.

    (A) Reflection image of the pattern, where the etched-off parts showed higher reflectivity. (B) WPS image of the same area. (C) First derivative of the intensity profile along the line shown in (B) as squares. a.u., arbitrary units. Gaussian fitting (red line) showed an FWHM of 0.51 μm. (D) Reflection image of 1 μm of PMMA particles. (E) WPS image of the same area with the pump at 1728 cm−1. (F) Off-resonance image showed no contrast. Scale bars, 10 μm.

  • Fig. 5 Ultrafast chemical mapping of nanoscale PMMA film by WPS microscope.

    (A to E) WPS images of 486-nm-thick PMMA film at speeds equivalent to 1250, 250, 50, 10, and 2.4 frames/s. (F) Measured SNR and power function fitting result (solid curve). Scale bar, 40 μm.

  • Fig. 6 WPS imaging of different chemical components in living cells.

    (A) Reflection image of a living SKOV3 human ovarian cancer cell cultured on a silicon wafer. (B to D) WPS images of the same field of view at 1744 cm−1 (lipid), 1656 cm−1 (protein), and 1808 cm−1 (off-resonance), respectively. Scale bars, 10 μm.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/7/eaav7127/DC1

    Section S1. Simulation and experimental results for film samples

    Section S2. Estimation of maximum probe power saturating the camera sensor

    Fig. S1. Simulation and experimental results for film samples.

    Fig. S2. Widefield photothermal imaging of living cells at different depths.

    Movie S1. Movie clip showing the time-resolved imaging of the transient thermal process of a 486-nm-thick polymer film.

    Movie S2. Movie clip showing an ultrafast imaging speed of 1250 frames/s of a 486-nm-thick polymer film.

  • Supplementary Materials

    The PDF file includes:

    • Section S1. Simulation and experimental results for film samples
    • Section S2. Estimation of maximum probe power saturating the camera sensor
    • Fig. S1. Simulation and experimental results for film samples.
    • Fig. S2. Widefield photothermal imaging of living cells at different depths.

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.avi format). Movie clip showing the time-resolved imaging of the transient thermal process of a 486-nm-thick polymer film.
    • Movie S2 (.avi format). Movie clip showing an ultrafast imaging speed of 1250 frames/s of a 486-nm-thick polymer film.

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