Imaging by tera-hertz photon counting

Photon counting method is indispensable in visible/near-infrared optical measurements for detecting extremely weak radiation. The method, however, has been inaccessible in tera-hertz region, where the photon energies are more than 100 times smaller and catching individual photons is difficult. Here we review photon counting measurements of tera-hertz waves, by incorporating a semiconductor quantum-dot tera-hertz-photon detector into a scanning tera-hertz microscope. By using a quantum Hall effect detector as well, measurements cover the intensity dynamic range more than six orders of magnitude. Applying the measurement system to the study of semiconductor quantum Hall effect devices, we image extremely weak cyclotron radiation emitted by nonequilibrium electrons. Owing to the unprecedented sensitivity, a variety of new features of electron kinetics are unveiled. Besides semiconductor electric devices studied here, the experimental method will find application in diverse areas of molecular dynamics, micro-thermography, and cell activities. (authors)