Understanding the Working Principle of a Light-Induced Force Microscope (PiFM)

Let's discuss how a light-induced force microscope works.\nAs shown in the diagram above, the main components of the PiFM microscope include an AFM microscope with an additional off-axis parabolic mirror and a tunable laser as the light source. Typically, the AFM cantilever tip is made of silicon, and to enhance its polarizability, a layer of gold is plated on its surface. The sample is placed on a glass substrate, and the incident light of the PiFM system is focused on the sample laterally using the off-axis parabolic mirror. The total interaction forces between the probe and the sample can be divided into two types: photoinduced force and non-photoinduced force. In the PiFM system, a multifrequency lock-in mechanism is used to separate the photoinduced force from the non-photoinduced force. The non-photoinduced force is independent of the modulation frequency of the incident beam. By controlling the opening and closing of the incident light, the presence or absence of the photoinduced force can be determined. Then, by using an appropriate detection frequency in the locking mechanism, the photoinduced force signal can be isolated.\nBased on this principle, the incident light is modulated at a frequency of fm. It is then focused on the sample surface from the side using the off-axis parabolic mirror. Simultaneously, the tip vibrates close to the sample surface at the first mechanical oscillation frequency, f0. By using a wave mixer to combine the modulation frequency, fm, of the incident light with the first mechanical oscillation frequency, f0, at the probe, sidebands f0+fm and f0-fm are generated. Finally, the photoinduced force signal is effectively extracted through the locking mechanism between the reference force and AFM readout. It is worth mentioning that selecting a specific value of fm that matches the second mechanical resonant oscillation frequency of the cantilever beam and utilizing its high Q factor can improve the signal-to-noise ratio.