Photoinduced Tip–Sample Forces for Chemical Nanoimaging and Spectroscopy

Abstract

Control of photoinduced forces allows nanoparticle manipulation, atom trapping, and fundamental studies of light-matter interactions. Scanning probe microscopy enables the local detection of photoinduced effects with nano-optical imaging and spectroscopy modalities being used for chemical analysis and the study of physical effects. Recently, the development of a novel scanning probe technique has been reported with local chemical sensitivity attributed to the localization and detection of the optical gradient force between a probe tip and sample surface via infrared vibrationally resonant coupling. However, the magnitude and spectral line shape of the observed signals disagree with theoretical predictions of optical gradient forces. Here, we clarify this controversy by resolving and analyzing the interplay of several photoinduced effects between scanning probe tips and infrared resonant materials through spectral and spatial force measurements. Force spectra obtained on IR-active vibrational modes of polymer thin films are symmetric and match the material absorption spectra in contrast to the dispersive spectral line shape expected for the optical gradient force response. Sample thickness dependence shows continuous increase in force signal beyond the thickness where the optical dipole force would saturate. Our results illustrate that photoinduced force interactions between scanning probe tips and infrared-resonant materials are dominated by short-range thermal expansion and possibly long-range thermally induced photoacoustic effects. At the same time, we provide a guideline to detect and discriminate optical gradient forces from other photoinduced effects, which opens a new perspective for the development of new scanning probe modalities exploiting ultrastrong opto-mechanical coupling effects in tip-sample cavities.

Synchrotron infrared nano-spectroscopy and -imaging

“…[16] However, the mechanism underlying the observed contrast of albeit high quality was recently suggested to also be dominated by thermal expansion rather than an optical force at least at IR wavelengths. [17][18][19] These new techniques have extended the spatial resolution of infrared spectroscopy and imaging to the nanoscale, which is 100-1000 times smaller than the diffraction limit. The high spatial resolution implies a signal generated by only a nanoscopic sample volume.…”

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

“…Note that the hard contact region starts near a tip–sample distance of −1 nm where the heterodyne PiFM signal disappears because the tapping amplitude goes to zero. In the heterodyne measurement, the tip-enhanced thermal expansion is dominated by the noncontact interatomic tip–sample force rather than the contact force or the photoacoustic mechanism, as was suggested in the previous work (21), because the signal vanishes in the contact region as well as in the region far away from the surface. SI Appendix , section 4 contains a deeper discussion of the noncontact nature of the heterodyne PiFM measurement.…”

“…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. The spectral response of the thermal expansion follows a dissipative (absorptive) Lorentzian line shape (20, 21), as observed in PTIR and PFIR, which measure the heat exchange between light and matter.…”

Nanoscale force sensing of an ultrafast nonlinear optical response

“…Force probing of the nonlinear optical response at the nanometer scale is insensitive to phase matching conditions, making nonlinear optical experiments less challenging. The presented tr-AFM detection of nonlinear optical responses complements emerging scanning probe techniques, such as THz scanning tunneling microscopy ( 12 – 15 ), photoinduced force microscopy ( 16 – 18 ), scanning near-field optical microscopy ( 14 , 19 – 21 ), or AFM for surface voltage measurements ( 22 – 24 ), to investigate light–matter interactions on the nanoscale with high temporal resolution.…”