Resonant optical gradient force interaction for nano-imaging and -spectroscopy

Abstract

The optical gradient force provides optomechanical interactions, for particle trapping and manipulation, as well as for near-field optical imaging in scanning probe microscopy. Based on recent spectroscopic experiments, its extension and use for a novel form of chemical scanning probe nanoimaging was proposed. Here, we provide the theoretical basis in terms of spectral behavior, resonant enhancement, and distance dependence of the optical gradient force from numerical simulations in a coupled nanoparticle model geometry. We predict an asymmetric line shape of the optical gradient force for molecular electronic or vibrational resonances, corresponding to the real part of the dielectric function of the sample materials. Yet the line shape can become symmetric and absorptive for collective polaritonic excitations. The corresponding magnitudes of the force range from fN to pN, respectively. The distance dependence scales considerably less steeply than simple point dipole model predictions due to multipole effects. The combination of these characteristics of the optical gradient force offers the chance to experimentally distinguish it from competing processes such as thermal expansion induced forces. In addition we provide a perspective for further resonant enhancement and control of optical forces.

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

“…Because modeling of the full three-dimensional problem near the tip-sample junction is costly, simplifications are required to retrieve the essential physics at play. In a recent study, the photo-induced force was evaluated through the MST for an idealized spherical tip apex in a full wave simulation [19]. This approach intrinsically includes multiple scattering mechanism between the tip and its image dipole or between the tip and an isolated particle.…”

Optical force microscopy: combining light with atomic force microscopy for nanomaterial identification

“…Many mechanisms have been involved in developing this new type of scanning probe technique for material identifications. As we introduced in the theoretical section, optical-induced forces can arise from gradient optical forces, scattering forces (radiation pressure), and/or absorption-induced forces [48]. A series of optical force-based microscopy techniques have recently been developed, including photo-induced force microscopy (PiFM -that mainly detects gradient force and radiation pressure), chiral optical force microscopy (COFM -that detects the gradient forces in conjunction with electromagnetic polarizabilities of the sample), photo-expansion microscopy or photo-thermal induced resonance (PTIR and AFM-IR -that detect absorption-induced forces), etc.…”

“…When gradient force and radiation pressure dominate, as shown in Figure 1A, optical forces acting on an AFM tip consist of two parts: an attractive force given by the photoinduced dipole and a repulsive force given by radiation pressure [26,48,49,51],…”

“…While both PiFM and photo-thermal IR microscopy are based on forces originated from light, it remains challenging to differentiate their fundamental mechanisms. Yang and Raschke [48] suggested looking into the lineshapes of the measured forces, because noncontact optical forces and contact thermal forces are dominated by the real and imaginary part of the polarizability, respectively, suggesting an asymmetric lineshape for the optical forces whereas symmetric lineshape for the thermal forces. In two separate studies [57,58], noncontact piconewton range optical forces are directly measured with nonoscillating cantilevers.…”

Photoinduced Atomic Force Spectroscopy and Imaging of Two-Dimensional Materials

“…38,40 The spectral profile of the optical force is also expected to track the absorption spectrum in plasmonic structures. 41,44 Here, we measure photoinduced forces in atomically thin layers of MoS 2 and WS 2 flakes on glass substrates using a modified commercial AFM. MoS 2 and WS 2 are both TMDs with a direct-to-indirect bandgap transition and characteristic excitonic peaks in the visible region of the optical spectrum.…”