A theoretical and experimental analysis of the dominant forces measured in photoinduced force microscopy is presented. It is shown that when operated in the noncontact and soft-contact modes, the microscope is sensitive to the optically induced gradient force (F g ) and the scattering force (F sc ). The reconstructed force-distance curve reveals a tip-dependent scattering force in the 30-60 pN range. Whereas the scattering force is virtually insensitive to the nanoscopic tip-sample distance, the gradient force shows a z −4 dependence and is manifest only for tip-sample distances of a few nm. Measurements on glass, gold nanowires, and molecular clusters of silicon naphtalocyanine confirm that the gradient force is strongly dependent on the polarizability of the sample, enabling spectroscopic imaging through force detection. The nearly constant F sc and the spatially dependent F g give rise to a complex force-distance curve, which varies from point to point in the specimen and dictates the image contrast observed for a given set point of the cantilevered tip.

We perform time-resolved pump-probe microscopy measurements by recording the local force between a sharp tip and the photo-excited sample as a readout mechanism for the material's nonlinear polarization. We show that the photo-induced force is sensitive to the same excited state dynamics as measured in an optical pump-probe experiment. Ultrafast pump-probe force microscopy constitutes a non-optical detection technique with nanoscale resolution that pushes pump-probe sensitivities close to the realm of single molecule studies.

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We review an extensive study of the factors that influence the intensity of coherent, nonlinear four wave mixing (FWM) in carbon nanotubes, with particular attention to the variability inherent to single-walled carbon nanotubes (SWNTs). Through a combination of spatial imaging and spectroscopy applied to hundreds of individual SWNTs in optoelectronic devices, the FWM response is shown to vary systematically with free carrier concentration. This dependence is manifested both in the intrinsic SWNT bandstructure and also by extrinsic and environmental effects. We demonstrate the sensitivity of the SWNT FWM signal by investigating SWNTs transferred from one substrate to another, before and after the introduction of chemical damage, and with chemical and electrostatic doping. The results demonstrate FWM as a sensitive technique for interrogating SWNT optoelectronic properties. I. INTRODUCTION Carbon nanotubes (CNTs) display a rich set of optical and optoelectrical properties 1-3 that render them promising candidates for nanoscopic optoelectronic building blocks in circuits. This prospect motivates the need to develop sensitive tools for characterizing the optical properties of CNT devices. Since CNT fabrication methods generally produce heterogeneous mixtures of nanotubes, correlating the optical response to the electronic structure of CNTs is best carried out at the single nanotube level. In this regard, the sensitivity of optical microscopy techniques, employing either near-field or far-field detection, has proven sufficient for detailed examinations of the optical properties of individual nanotubes. 4 Whereas the linear optical properties of CNTs have been the topic of numerous studies, the nonlinear optical properties of nanotubes have received comparatively little attention. CNTs are known to exhibit high third-order nonlinear susceptibilities, 5, 6 a property that has propelled their use as saturable absorbers in laser cavities. 7, 8 The high optical nonlinearity also enables nonlinear spectroscopic investigations, which offer a closer look at the ultrafast carrier dynamics in nanotubes. Nonlinear optical measurements, and four-wave mixing (FWM) experiments in particular, can be optimized to reveal detailed information on the ultrafast evolution of optical excitations; information that remains hidden in linear spectroscopic measurements. The sensitivity of FWM techniques to both electronic and vibrational excitations makes them attractive probes for dissecting the nonlinear optical response of CNTs. Photon echo 9 and coherent Raman FWM 10 experiments, for instance, have enabled direct recordings of electronic and phonon dephasing, which hold important clues toward the extent of exciton-exciton and exciton-phonon interactions in CNTs. We have recently shown that the FWM technique can be extended to the level of individual nanotubes. 11 By making use of a dual-color picosecond excitation scheme, we collected coherent anti-Stokes Raman scattering (CARS) signals from both metallic and semi-conducting single-walled c...

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