Two-Photon-Excited Photoinduced Force Microscopy

Sensing photoexcitation-induced forces via atomic force microscopy (AFM) has attracted much attention as a promising nanoscale molecular imaging modality. However, most such nanospectroscopies suffer from linear background and noise under single-photon excitation, including scattering into a quadrant detector or photothermal response from the cantilever or substrate. Here, we present two-photon-excited photoinduced force microscopy (TP-PiFM) to suppress these unwanted linear effects. A theoretical analysis shows that signal demodulation at ω_tip ± 2ω_opt can efficiently isolate the photothermal signal under intensity modulation on the pulse train at ω_opt and tip driving at ω_tip. As a proof-of-concept demonstration, we obtained TP-PiFM images of 40 nm gold nanoparticles with a higher resolution than that of AFM topography. Backgrounds in TP-PiFM images were suppressed through the demodulation technique as well as a fitting analysis to extract signals quadratically scaling on the illumination power. These results establish TP-PiFM as a robust, background-free nanoimaging method.