In photo-induced force microscopy (PiFM), a sharp atomic tip is illuminated by a tightly focused laser beam and the photo-induced force is measured through the changes in the oscillatory motion of the cantilever.

In illuminated tip–sample junctions, the absorption of light by the sample is accompanied by local heating and subsequent thermal expansion of the material. In photoinduced force microscopy (PiFM) experiments, thermal expansion is expected to affect the measured photoinduced force through the thermally modulated van der Waals force. Evidence for such thermal contributions in PiFM measurements has been demonstrated in the mid-infrared range, where the primary excitations are molecular vibrational modes. For PiFM measurements in the vis/NIR, where light-matter energy transfer is mediated through electronic excitations, clear experimental evidence of thermal contributions remains elusive. By developing a frequency domain version of PiFM, we retrieve variations in the photoinduced force on the sub-μs time-scales, allowing a direct registration of the thermal relaxation dynamics of the sample after photoexcitation. Our measurements confirm the presence of the thermal contribution to the PiFM signal in the mid-infrared range and provide strong experimental evidence that thermal components also play a role in the forces measured in PiFM in the vis/NIR range of the spectrum.

We demonstrate experimentally the detection of magnetic force at optical frequencies, defined as the dipolar Lorentz force exerted on a photoinduced magnetic dipole excited by the magnetic component of light. Historically, this magnetic force has been considered elusive since, at optical frequencies, magnetic effects are usually overshadowed by the interaction of the electric component of light, making it difficult to recognize the direct magnetic force from the dominant electric forces. To overcome this challenge, we develop a photoinduced magnetic force characterization method that exploits a magnetic nanoprobe under structured light illumination. This approach enables the direct detection of the magnetic force, revealing the magnetic nearfield distribution at the nanoscale, while maximally suppressing its electric counterpart. The proposed method opens up new avenues for nanoscopy based on optical magnetic contrast, offering a research tool for all-optical spin control and optomagnetic manipulation of matter at the nanoscale.

We theoretically show that the optical chiral properties of tightly focused laser beams can be characterized by means of force detection. To measure the chiral properties of a beam of given handedness in the microscopic focal volume, we determine the photoinduced force exerted on a sharp tip, which is illuminated first by the beam of interest and second by an auxiliary beam of opposite handedness, in a sequential manner. We show that the difference between the force measurements is directly proportional to the chiral properties of the beam of interest. In particular, the gradient force difference Δ⟨F grad ,z ⟩ is found to have exclusive correspondence to the time-averaged helicity density of the incident light, whereas the differential scattering force provides information about the spin angular momentum density of light. We further characterize and quantify the helicity-dependent Δ⟨F grad ,z ⟩ using a Mie scattering formalism complemented with full wave simulations, underlining that the magnitude of the difference force is within an experimentally detectable range.

The process of tip-enhanced Raman scattering (TERS) depends critically on the morphology near the apex of the tip used in the experiment. Many tip designs have focused on optimization of electromagnetic enhancement in the near-field, which is controlled to a large extent by subtle details at the nanoscale that remain difficult to reproduce in the tip fabrication process. The use of focused ion beams (FIB) permit modification of larger features on the tip in a reproducible manner, yet this approach cannot produce sub-20-nm structures important for optimum near-field enhancement. Nonetheless, FIB milling offers excellent opportunities for improving the far-field radiation properties of the tip-antenna, a feature that has received relatively little attention in the TERS research community. In this work, we use finite-difference time-domain (FDTD) simulations to study both the near-field and far-field radiation efficiency of several tip-antenna systems that can be constructed with FIB techniques in a feasible manner. Starting from blunt etched tips, we find that excellent overall enhancement of the TERS signal can be obtained with pillar-type tips. Furthermore, by applying vertical grooves on the tip's shaft, the overall efficiency can be improved even more, producing TERS signals that are up to 10-fold stronger than signals obtained from an ideal (unmodified) sharp tip of 10-nm radius. The proposed designs constitute a feasible route toward a tip fabrication process that not only yields more reproducible tips but also promises much stronger TERS signals. K E Y W O R D S finite-difference time-domain simulations, tip-enhanced Raman scattering