Tip-Enhanced Imaging and Control of Infrared Strong Light-Matter Interaction

Optical antenna resonators enable control of light-matter interactions on the nano-scale via electron–photon hybrid states in strong coupling. Specifically, mid-infrared (MIR) nano-antennas coupled to saturable intersubband transitions in multi-quantum-well (MQW) semiconductor heterostructures allow for the coupling strength to be tuned through antenna resonance and field intensity. Here, tip-enhanced nano-scale variation of antenna-MQW coupling across the antenna is demonstrated, with a spatially-dependent coupling strength $g_{\mathrm{aq}}$ varying from 73 (strong coupling) to 24 ${\mathrm{cm}}^{-1}$ (weak coupling). This behavior is modeled based on the spatially dependent local constructive and destructive interference between tip and antenna fields. Using a quantum-mechanical density-matrix model of the MQW system with its designed values of transition dipole moment, doping density, and population decay time, the picosecond IR pulse coupling to intersubband transitions and the associated tip induced strong-field saturation effects are described. These results present a new regime of nonlinear IR light-matter control based on the dynamic manipulation of quantum hybrid states on the nanoscale and in the infrared, with a perspective regarding extension to molecular vibrations.