Heterodyne Frequency Modulation in Photoinduced Force Microscopy

Photo induced force microscopy: chemical spectroscopy beyond the diffraction limit

“…Yamanishi et al have considered another aspect of the sideband heterodyne implementation, 79,81 which is that in order to prevent the tip jumping to contact, the cantilever is usually driven at its first mechanical resonance frequency, ν 1 , and the photoinduced force therefore measured near to the second at ν 2 . However, the higher spring constant of the cantilever at ν 2 leads to a lower sensitivity.…”

“…A second approach, called the ''sequential'' heterodyne configuration, which can be combined with the harmonic mode, introduces another modulation to the infrared light source at a frequency n m , is demonstrated with a polarization switch and introduces additional possible modulation channels. Yamanishi et al have considered another aspect of the sideband heterodyne implementation, 79,81 which is that in order to prevent the tip jumping to contact, the cantilever is usually driven at its first mechanical resonance frequency, n 1 , and the photoinduced force therefore measured near to the second at n 2 . However, the higher spring constant of the cantilever at n 2 leads to a lower sensitivity.…”

“…This is particularly relevant to PiFM measurements in vacuum where the higher cantilever Q factor results in a greater degree of signal degradation. Yamanishi et al have proposed 79,81 an alternative ''heterodyne frequency modulation'' arrangement that uses frequency rather than amplitude modulation to create the heterodyne signal. The cantilever is driven at n 1 but, unlike the sideband configuration, the intensity of the illuminating laser is modulated at 2n 1 + n m .…”

Photo-induced force microscopy (PiFM) – principles and implementations

“…[3][4][5] In addition, PiFM has also been used to probe nonlinear polarizations in the sample, resulting in images based on stimulated Raman transitions 6 and pump-probe transitions. 7,8 Along with the development of improved detection strategies, [9][10][11] a great deal of progress has been made on the theoretical description of the PiFM signal, [12][13][14][15] putting the PiFM technique on a more solid footing. Whereas the first applications of PiFM involved inorganic materials 1,16 and metals, 17 the technique has since been used to study organic compounds (polymers 18 and organic semiconductors 19 ), biological samples (collagen fibres 20 and protein structures 21 ), as well as samples of geological significance (biominerals).…”

Theoretical Analysis of Optically Selective Imaging in Photoinduced Force Microscopy

“…Furthermore, it was demonstrated that the combination of mechanical eigenmodes and laser modulation frequency provides images with clearer contrast 17 . Recently, the heterodyne frequency modulation (heterodyne FM) method 18,19 was used for PiFM measurements in ultrahigh vacuum to remove photothermal oscillations and achieve high-resolution imaging of less than 1 nm 20 . Additionally, PiFM can acquire information on samples through their specific linear and nonlinear optical responses.…”