Bond-selective transient phase imaging via sensing of the infrared photothermal effect

Phase-contrast microscopy converts the phase shift of light passing through a transparent specimen, e.g., a biological cell, into brightness variations in an image. This ability to observe structures without destructive fixation or staining has been widely utilized for applications in materials and life sciences. Despite these advantages, phase-contrast microscopy lacks the ability to reveal molecular information. To address this gap, we developed a bond-selective transient phase (BSTP) imaging technique that excites molecular vibrations by infrared light, resulting in a transient change in phase shift that can be detected by a diffraction phase microscope. By developing a time-gated pump–probe camera system, we demonstrate BSTP imaging of live cells at a 50 Hz frame rate with high spectral fidelity, sub-microsecond temporal resolution, and sub-micron spatial resolution. Our approach paves a new way for spectroscopic imaging investigation in biology and materials science. A phase imaging technique that provides spectroscopic chemical information could bring new opportunities for studying biology and materials science. Delong Zhang, Lu Lan, and coworkers from Boston, Illinois and Shanghai have developed a scheme called bond-selective transient phase (BSTP) imaging. The approach uses nanosecond pulses of mid-infrared pump laser light to excite molecular vibrations in the sample. The mid-infrared light absorption causes a small temperature change in the sample which results in a change of refractive index and thus a transient phase shift, which is read out by a burst of visible probe pulses and a CMOS camera. Tests with samples including a thin oil film, polyurethane beads, living cells, and the interface between two liquids (olive oil and dimethyl sulfoxide) indicate that BSTP imaging can operate with sub-microsecond temporal resolution and sub-micrometer spatial resolution.