Figure 3.
Comparison between CARS and SRS imaging. (a) The theoretical CARS spectrum resulted from interference between non-resonant background and real part of vibrational resonant contribution. (b)-(d) are forward CARS images of 3T3-L1 cells tuned across the C-H resonance. (b) cell imaged at C-H off resonant condition (2086 cm−1); (c) cell imaged at C-H resonant condition (2845 cm−1); (d) cell imaged at the blue dip of the C-H band at 2950 cm−1. Resonant features appear dark against the non-resonant background; (b)-(d) adapted from Reference (31). Simultaneous (e) epi-CARS and (f) SRS images of a live worm, C. elegans, with the Raman shift being set to the lipid band 2845 cm−1. While SRS specifically probes the lipid contribution, CARS contrast is evidently complicated by non-resonant background from non-lipid structures. Simultaneous (g) epi-CARS and (h) SRS images of a layer of 2 μm polystyrene beads spin-coated on a glass coverslip, with Raman shift being at 2845 cm−1. While SRS shows well-behaved round disks for single beads, the corresponding CARS images show a bright ring due to the interference effect occurring at the edge and a bright spot at the center due to the forward going CARS signal being reflected back by the bead/air interface.