Optically induced tip-sample forces

Control of optical forces allows nanoparticle manipulation, atom trapping, and fundamental studies of light-matter interactions. Recent studies have claimed localization and detection of these weak forces between a tip and sample surface via infrared vibrational resonances providing for a potentially novel nano-imaging technique. However, the magnitude and spectral lineshape of the optically induced force disagree with recent theory. Through spectral and spatial force measurements, we show that the tip-sample force interaction is dominated by thermal expansion instead of optical gradient forces. Force spectra obtained on PMMA are symmetric, and match the absorption spectrum in contrast to dispersive s-SNOM and optical gradient force behavior. Thickness dependence shows increase in force signal beyond the thickness where the optical dipole force would saturate. Approach curves show that the force signal is confined to regions of physical contact between tip and sample. This strongly suggests that the force is dominated by thermal expansion.