Photoinduced Tip-Sample Forces for Chemical Nanoimaging and Spectroscopy

Area of Science:

• Physics
• Materials Science
• Nanotechnology

Background:

• Scanning probe microscopy (SPM) enables local detection of photoinduced effects for chemical analysis and physical studies.
• A novel SPM technique claimed local chemical sensitivity via optical gradient forces, but signal characteristics contradicted theoretical predictions.

Purpose of the Study:

• To resolve the controversy surrounding photoinduced forces in SPM by analyzing interactions between probe tips and infrared resonant materials.
• To differentiate optical gradient forces from other photoinduced effects in nano-optical measurements.

Main Methods:

• Spectral and spatial force measurements were performed on polymer thin films with infrared-active vibrational modes.
• Analysis focused on force spectra, line shapes, and sample thickness dependence.

Main Results:

• Observed force spectra were symmetric, matching material absorption, unlike the predicted dispersive shape for optical gradient forces.
• Force signals increased with sample thickness beyond expected saturation points for optical dipole forces.
• Results indicate thermal expansion and photoacoustic effects dominate over optical gradient forces.

Conclusions:

• Photoinduced force interactions in this SPM setup are primarily driven by thermal phenomena.
• Provides a method to distinguish optical gradient forces from thermal effects, guiding the development of new SPM modalities.