Fig. 1. Spectroscopy principle and probe design of electronic pre-resonance SRS microscopy.

a Schematic illustration of electronic pre-resonance SRS. Two synchronized laser beams were tightly focused on molecules of interest. When the energy difference between two photons matches vibrational transition, one pump photon can be converted to stokes photon and the intensity fluctuation will be detected. As pump energy approaches electronic transition (pre-resonance), such process will be greatly enhanced. ω₀, ω_pump, and ω_vib represents for frequencies of electronic transition, pump beam photon, and vibrational transition, respectively. Γ is the homogeneous linewidth, typically around 700 cm⁻¹. b Design and engineering principles of 9-cyanopyronin library. Three key structural features were rationally tuned to generate new MARS dye library. One sidechain was installed on the amino group to facilitate facile functionalization.