Chemically Selective Imaging with Broadband CARS Microscopy

Recently, there has been great interest in a variety of novel ultrafast coherent anti-Stokes Raman scattering (CARS) methods that utilize broadband lasers (1–3), often involving narrowband probe pulses (4–6), chirped pulses (7,8), or fiber-broadened pulses (9–11) to obtain broadband vibrational information from molecules. With the variation of single-pulse, broadband, multiplex CARS microscopy discussed here, broad bandwidth spectra are acquired rapidly during the microscope scan and the spatially variant compositions of samples composed of multiple components are readily determined.

The essential feature of this method is the use of a single, ultra-broadband femtosecond (fs) pulsed-laser oscillator, which simultaneously provides all of the photons for the CARS scattering. By a combination of phase and polarization control of the 10-fs laser pulses, complete broadband spectral images are acquired in the vibrational fingerprint regime. An important advantage of this method is that full chemical composition analysis is obtained in a single microscope scan, rather than the multiple scans that are necessary with narrowband CARS microscopy (12–17). There are, however, tradeoffs necessary to gain this spectral width. With narrowband CARS, the signal sensitivity and signal acquisition rate can be much greater, but with broadband CARS, the spatial locations of multiple species can be obtained in a single scan. In the discussion that follows, the techniques of broadband CARS are discussed and illustrated with examples of multicomponent chemical imaging.

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