A cryogen-free terahertz near-field scanning noise microscope for nanoscale hot-electron imaging in devices at 70-300...

Direct nanoscale imaging of hot electrons is critical for advancing electronic, optoelectronic, and quantum devices, where nonequilibrium carrier dynamics govern energy dissipation, noise, and device performance. Terahertz scanning noise microscopy (SNoiM) has emerged as a powerful tool for this purpose, enabling real-space imaging of hot electrons via current-fluctuation-induced terahertz evanescent fields, a capability well established at room temperature. Extending such imaging to low temperatures, however, has been limited: prior cryogenic SNoiM implementations relied on liquid nitrogen cooling and were restricted to only two discrete lattice temperatures (300 and ∼110 K), preventing systematic studies of temperature-dependent nonequilibrium transport. Such studies are essential, as lattice temperature strongly influences energy relaxation, transport regimes, and the spatial distribution of hot electrons; without continuous temperature control, key transitions and deviations from local equilibrium remain inaccessible. Here, we present a fully cryogen-free, variable-temperature SNoiM enabling continuous nanoscale imaging of hot electrons from 300 K down to ∼70 K. The system employs a two-stage pulse-tube cryocooler: the first stage cools the atomic force microscope and sample with precise temperature tuning, while the second stage maintains the terahertz detector at ∼5 K for high sensitivity. The cryogen-free design further enables cryogenically cooled optics, significantly suppressing background radiation and supporting long-term, low-noise operation. This platform provides a unique experimental capability for real-space investigation of temperature-dependent nonequilibrium electron transport in nanoscale and cryogenic electronic systems.