Fig. 3. Spectral modulation scheme.

While spectrally tailored excitation allows to target which vibrational resonance is excited, it does not directly improve chemical specificity compared to narrowband excitation because there is no mechanism to suppress signal from overlapping bands of interfering species. In STE-SRS, we therefore shape the broadband pump spectrum twice: The first spectrum e⁺(ω_p) mainly contain spectral components resonant with target species (Ω₁ and Ω₃). The second spectrum e^–(ω_p) mainly contain spectral components resonant with target species (Ω₂ and Ω₄) and is weighted to cancel out the interfering signal generated by e⁺(ω_p). The difference SRS signal from e⁺(ω_p) and e^–(ω_p) can thus be freed from all interfering contribution. For simplicity the two spectra can be combined into one effective excitation mask e(ω_p) = e⁺(ω_p) – e^–(ω_p). Such subtraction can be implemented for real-time imaging by modulating between the two excitation spectra at high frequency and extracting the resulting modulation transfer with lock-in amplifier.