Coherent anti-Stokes
Raman scattering (CARS) microscopy
is an emerging
nonlinear vibrational imaging technique that delivers label-free chemical
maps of cells and tissues. In narrowband CARS, two spatiotemporally
superimposed picosecond pulses, pump and Stokes, illuminate the sample
to interrogate a single vibrational mode. Broadband CARS (BCARS) combines
narrowband pump pulses with broadband Stokes pulses to record broad
vibrational spectra. Despite recent technological advancements, BCARS
microscopes still struggle to image biological samples over the entire
Raman-active region (400–3100 cm–1). Here,
we demonstrate a robust BCARS platform that answers this need. Our
system is based on a femtosecond ytterbium laser at a 1035 nm wavelength
and a 2 MHz repetition rate, which delivers high-energy pulses used
to produce broadband Stokes pulses by white-light continuum generation
in a bulk YAG crystal. Combining such pulses, pre-compressed to sub-20
fs duration, with narrowband pump pulses, we generate a CARS signal
with a high (<9 cm–1) spectral resolution in
the whole Raman-active window, exploiting both the two-color and three-color
excitation mechanisms. Aided by an innovative post-processing pipeline,
our microscope allows us to perform high-speed (≈1 ms pixel
dwell time) imaging over a large field of view, identifying the main
chemical compounds in cancer cells and discriminating tumorous from
healthy regions in liver slices of mouse models, paving the way for
applications in histopathological settings.