Abstract | Coherent Anti-Stokes Raman Scattering (CARS) Microscopy has emerged as an important tool for label-free yet chemical-specific imaging of live cells and tissue. The contrast mechanism in CARS derives from the natural vibrational Raman spectrum of the molecule of interest. Since Raman linewidths in the condensed phase are typically 10 cm-1 or so, for the past decade it was assumed that picosecond lasers, having bandwidths in the 10cm-1 range, will be required for CARS microscopy. Femtosecond pulses, being >100x broader, were thought to give poor contrast. In fact, this is not the case if one takes advantage of the control over optical phase that is implicit in femtosecond pulses. Recently, a much simplified approach was demonstrated wherein a single femtosecond oscillator, combined with a photonic crystal fibre, is used for high performance CARS microscopy. In this approach, control over the linear 'chirp' of the optical phase is used as a flexible tool to optimize performance. The method is simple, robust and flexible enough that a major microscope manufacturer recently used it to develop the first commercially available CARS microscope. An overview of the optimally chirped approach is presented, including recent developments and new opportunities, and illustrate these with applications to selected problems in biomedical imaging. |
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