Quaternary and secondary structural imaging of a human hair by a VSFG-detected IR super-resolution microscope

► IR super-resolution image of cross section of a human black hair were measured. ► For the amide III band, human hair gave strong VSFG signals at the cortex area. ► Distribution of α-helix based quaternary structure of keratin proteins was observed. ► The VSFG signal disappeared completely when the amide I band was monitored. ► The α-helix of keratin proteins is well aligned along the axial direction in hair.