THz near-field spatial mapping of dipole–dipole interactions in free space and near interfaces

Abstract

Dipole–dipole interactions are central to energy-transfer processes. Understanding the interaction dynamics is essential across diverse fields, including solar energy harvesting, organic light-emitting diodes, long-range energy transport, and molecular biosensing. Such interactions are fully described by the photonic Green function, which defines the electromagnetic response of a point dipole source. However, direct experimental access to the individual components of Green’s function at the single-dipole level remains challenging. Here, we employ a double-probe terahertz (THz) near-field microscope to directly map the dipole–dipole interactions in free space and near a resonant dielectric interface formed by a pellet of α-lactose and air. In free space, we observe highly anisotropic energy-transfer dynamics arising from the non-radiative near-field contribution of Green’s function, including the pronounced suppression associated with the magic-angle condition. Near the air/α–lactose interface, we reveal strong modifications of Green’s function along both the in-plane and out-of-plane directions: a reduction in the energy-transfer rate near the vibrational resonance of the medium and pronounced oscillations in intensity and phase due to interference between the direct dipole–dipole interaction and the surface-mediated contribution. Our results provide direct access to the non-radiative components of Green’s function and establish a powerful framework for probing and engineering dipole–dipole interactions in complex and resonant photonic media.