We have investigated the light emission mechanisms from a scanning tunneling microscope in which the sample surface is a thin film of Au (22.5 nm thick) evaporated on a microscope cover glass. This sample was attached to the flat surface of a hemicylindrical coupler prism. The tunneling electrons from a Pt-Ir tip were directed to the Au film from the air side, and the angle dependence of light emission intensity was measured through the coupler prism using a photon counting system. We found a sharp emission peak centered at 43° from the film surface normal. This is the angle at which the fast mode of surface plasmon polaritons (SPP) mostly localized at the Au/air interface can radiate by conserving the wave vector parallel to the interface. Thus we conclude that most of the light on the prism side is emitted by the delocalized fast mode of SPP of the Au/air interface.
We have investigated the relation between the scanning-tunneling-microscope (STM) tip shape and the efficiency of STM light emission. The light intensity radiated by a current source placed in the tip-sample gap of the STM was calculated for various shapes of the tip using the finite differential time domain method. We found that the highest emission efficiency is obtained for the pyramidal tip with the flat top of 4 nm width and the apex angle of 90 °C.
The optical sampling measurement of a high-speed electrical wave form using a scanning tunneling microscope (STM) is demonstrated. An optical pulse train from a laser diode (LD) is used to turn on and off a photoconductive switch on a STM probe to measure a signal from a Schottky barrier diode (SBD) in a sampling procedure. A time resolution less than 300 ps has been achieved. This novel method has the potential to create a breakthrough in ultra-high-speed wave form measurement through this unique combination of optical sampling and STM technology.
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