Figure 2. Apparatus of er-SRS microscope.

Synchronized tunable pump and fixed Stokes laser (fixed at 1031.2 nm, 2-ps pulse width, intensity modulated with EOM at 20 MHz) were focused onto a solution sample with a high NA objective. The stimulated Raman loss (SRL$\equiv \frac{\mathrm{\Delta }{I}_{\mathrm{p}\mathrm{u}\mathrm{m}\mathrm{p}}}{I_{\mathrm{p}\mathrm{u}\mathrm{m}\mathrm{p}}}$) is detected as signal. A lock-in amplifier with (amplified) photodiode detection scheme is used for sensitive measurement of SRL. A thin solution sample (~0.13 mm) is used for spectra measurements. Stage-scan is achieved with a XY piezo stage.