  Phase-contrast microscopy converts the optical phase introduced by
transparent, unlabeled specimens into modulation in the intensity image. Modern
phase imaging techniques are capable of quantifying phase shift at each point
in the field of view, enabling non-destructive applications in materials and
life sciences. However, these attractive features come with the lack of
molecular information. To fulfill this gap, we developed a bond-selective
transient phase (BSTP) microscope using infrared absorption to excite molecular
vibration, resulting in an optical phase change detected through a customized
phase microscope. By using pulsed pump and probe lasers, we realized BSTP
imaging with high spectral fidelity, sub-microsecond temporal resolution,
submicron spatial resolution at 50 frames per second, limited only by the
camera sensor. Our approach links the missing molecular bond information to the
quantitative phase, which paves a new avenue for spectroscopic imaging in
biology and materials science.
