Abstract Precise and high efficiency concentration of mid-infrared (mid-IR) light into sub-wavelength volumes is essential for probing low-energy excitations and achieving strong field enhancements, which is hindered by absorption losses and coupling inefficiencies at long wavelengths. Here, we introduce a diamond-based metal-insulator-metal campanile probe that adiabatically compresses free-space mid-infrared light (10 μm) into ≈ 0.7 μm domain. Integrated into a scanning photovoltage microscope, the probe enables sub-wavelength mapping of locally driven photocurrents in graphene, resolving polarization-dependent and contact-sensitive responses at energies down to ≈ 0.1 eV. Experiments reveal a photocurrent signal density enhancement of 10 3 , coupling light power efficiencies above 1%, and an 18-fold increase in near-field light flux density, in agreement with numerical simulations. Operation of the probe with quantum cascade and free-electron lasers demonstrates a robust, spectrally tunable platform for high-resolution exploration of low-energy carrier dynamics in atomically thin materials, opening opportunities for mid-IR optoelectronics and quantum photonics.