Photo-induced force microscopy (PiFM) detects photo-induced molecular polarizability of feature sizes down to the molecular level by mechanical detection of the force gradient of the interaction between the optically driven molecular dipole and its mirror image in a metal coated AFM tip. Thus PiFM not only excites the sample with near-field but also detects the response in near-field, a feature truly unique among tip-enhanced optical microscopy techniques. The near-field detection allows PiFM to be significantly easier and more robust to operate than the techniques relying on far-field detection. Another major advantage is the absence of far-field background signal, leading to excellent signal-to-noise ratio even with low excitation power and from extremely thin samples (as thin as ~ 1 nm thick). Yet another advantage is that PiFM relies on non-contact or light tapping AFM mode, which (1) prevents even the softest samples from damage and (2) achieves higher spatial resolution than AFM topography due to the steeper functional dependence of dipole-dipole force on the tip-sample distance.
When an external laser, modulated fm, is focused at the tip/sample area of a tapping mode (TM) AFM (operating typically on the first mechanical resonance of cantilever f0), interaction of the dipole in the sample and the mirror image dipole in the metallic tip creates dipole-dipole force at fm and also modulates the gradient force measured at f0. As such, the Fourier spectrum of the tip movement will look schematically as shown below (a):
The amplitudes of the signal at fm and f0 + fm (see (a) above) correspond to the dipole-dipole force and the gradient of the dipole-dipole force respectively. In order to obtain the highest spatial resolution, we use the gradient force measurement. Also, in order to take advantage of the quality factor Q of the cantilever to enhance the sensitivity, we choose the value of fm such that f0 + fm equals f1, the second mechanical resonance of the cantilever (as shown schematically in (b) above). A schematic diagram of the Photo-induced force microscope in a reflection mode is shown below:
When the frequency of the external laser, ω, matches an excitation energy of the sample, the induced sample dipole and the image dipole on the tip will be the strongest and exhibit a strong attractive force that can be measured by the AFM detection at f1. The resonance can be electronic, plasmonic, or phonon in nature, ranging in frequency from UV to infrared.
When coupled with a tunable infrared laser source to excite the vibrational modes of molecules, Vista-IR produces stunning nanoscale images with chemical specificity. Check out the videos and images at our Gallery to learn more about PiFM and exciting hyPIR imaging capability.
This recent publication gives more detailed theoretical basis for PiFM imaging.