  Coherent Raman microscopy provide label-free imaging by interrogating the
intrinsic vibration of biomolecules. Nevertheless, trade-off between high
chemical-specificity and high imaging-speed currently exists in transition from
spectroscopy to spectroscopic imaging when capturing dynamics in complex living
systems. Here, we present a novel concept in dual-comb scheme to substantially
beat this trade-off and facilitate high-resolution broadband coherent
anti-Stokes Raman spectroscopic imaging based on down-converted, automatically
varying delay-time in spectral focusing excitation. A rapid measurement of
vibrational microspectroscopy on sub-microsecond scale over a spectral span
~700 cm-1 with solid signal-to-noise ratio provides access to well-resolved
molecular signatures within the fingerprint region. We demonstrate this
high-performance spectroscopic imaging of spatially inhomogeneous distributions
of chemical substances as well as the carotenoids accumulation in plant cells.
This technique offers an unprecedented route for broadband CARS imaging with
hundreds of kHz frame rate using available 1-GHz oscillators.
