Fig.3

The FWM signals of MMF cover from ~ 2700 cm⁻¹ to 2900 cm⁻¹ and interfere with the detection of C–H bond vibrations. The FWM signals of FMF cover from ~ 3400 cm⁻¹ to 3800 cm⁻¹ and avoid interference to the detection of C–H bond vibrations