Abstract
Photon counting method is indispensable in visible/near-infrared optical measurements for detecting extremely weak radiation. The method, however, has been inaccessible in terahertz region, where the photon energies are more than 100 times smaller and catching individual photons is difficult. Here we review recent development of single terahertz photon detectors and passive sensing systems to locally pick up spontaneous emission in an object. Applying a passive terahertz microscope with quantum-dot single photon detectors, we image extremely weak cyclotron radiation emitted from quantum Hall effect devices. 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, microthermography, and cell calorimetry.
