Dyadic Green's function formalism for photoinduced forces in tip-sample nanojunctions

Abstract

A comprehensive theoretical analysis of photo-induced forces in an illuminated nanojunction, formed between an atomic force microscopy tip and a sample, is presented. The formalism is valid within the dipolar approximation and includes multiple scattering effects between the tip, sample and a planar substrate through a dyadic Green's function approach. This physically intuitive description allows a detailed look at the quantitative contribution of multiple scattering effects to the measured photo-induced force, effects that are typically unaccounted for in simpler analytical models. Our findings show that the presence of the planar substrate and anisotropy of the tip have a substantial effect on the magnitude and the spectral response of the photo-induced force exerted on the tip. Unlike previous models, our calculations predict photo-induced forces that are within range of experimentally measured values in photo-induced force microscopy (PiFM) experiments.PACS numbers: May be entered using the \pacs{#1} command.

Nanoscale spectroscopic origins of photoinduced tip–sample force in the midinfrared

“…The spectral variation of the resonance is one order of magnitude smaller than the nonresonant background value of 0.42. Consequently, the spectral contrast near the molecular resonance is limited, and thus difficult to measure in PiFM measurements, even if the total magnitude of F dip is sufficiently above the noise level (5, 19, 24). The induced dipole force for a 60-nm PS film on a Si substrate is analytically calculated by implementing the finite dipole model (25, 26), indicated in Fig.…”

“…Contrary to the strong oscillator, the magnitude of the polarizability change related to the weak oscillator near molecular vibrational resonance is one order of magnitude smaller than the nonresonant polarizability value. Because the magnitude of F dip depends on both the polarizabilities of the metal tip and the sample, the force on the weak oscillator near a molecular resonance can be large in the range between a few piconewtons to a few tens of piconewtons (19, 37), while its spectral variation due to vibrational resonances is sub-piconewton. Because our system noise level is around 0.1 pN, the corresponding spectral variation for the dielectric materials is hardly detectable.…”

“…Compared to the PTIR and PFIR, the recently developed PiFM operates in the noncontact/tapping mode while monitoring the photoinduced force between the tip and the sample. Theoretically, when the light illuminates the tip–sample junction, the induced dipoles in the tip and in the sample generate a coulombic force whose spectral response follows a dispersive line shape (5, 18, 19). Meanwhile, near molecular resonances, there is a temperature rise of the sample due to light absorption, which results in the thermal expansion of the sample.…”

Photo-Induced Force Microscopy by Using Quartz Tuning-Fork Sensor

“…In classical nearfield scattering theory, αs can be successfully explained with the help of an image dipole model, which yields α s = β α t , where β is the complex electrostatic reflection coefficient, given as β = ε − 1 / ε + 1, where ε is the dielectric constant of the sample. This short-range-induced dipole force typically shows the dispersive spectral line shape and strongly increases in the metal/plasmonic material where ε is negative, whereas the force is typically small under a few pN range in the organic and inorganic sample where ε is positive, even at its molecular resonance [14,15,16,17].…”

Observation of nanoscale opto-mechanical molecular damping as the origin of spectroscopic contrast in photo induced force microscopy

“…While the dipole-dipole force model provides excellent agreement with the electromagnetic near-field measurements in the visible 14 and with mid-IR plasmonic resonance spectra 21 , extending this model to IR vibrational resonances causes discrepancies between experiment and theory 20,22,23 . In particular, the dipole-dipole force model predicts a dispersive spectral response, while the experimental results show a purely dissipative response.…”