Electromagnetic properties of scanning tunneling microscope tip-sample gap in the terahertz frequency range

We have investigated electromagnetic properties of the tip-sample gap of a scanning tunneling microscope (STM) in the terahertz (THz) spectral range. Light in the STM is emitted from the current source excited by tunneling electrons through dipole radiation processes. The antenna factor that measures the efficiency of the dipole radiation contains a factor proportional to the square of the angular frequency ω of the STM light. Hence, it might occur that STM light emissions in the THz spectral range become undetectably weak. The antenna factors for samples of Au and TiO₂, whose plasma frequencies are in the UV and THz spectral ranges, respectively, were investigated as a function of the radius a of curvarure of the W tip. We have found that the effect of ω² in the antenna factor can be compensated for using a tip having an a of approximately 1500 nm.