Counting terahertz photons

Scientists at the University of Tokyo and the Japan Science and Technology Corporation combined a quantum-dot photon detector (shown here) with a scanning confocal optical system to demonstrate exquisitely sensitive THz microscopy. Previously, such far-IR photons—with energies of about 4 meV—could only be seen in large numbers. The researchers scanned the quantum-dot probe across the face of a quantum Hall device and obtained images with a spatial resolution of 50 µm. (The radiation itself has a wavelength of 132 µm.) Furthermore, the power emitted from the surface was as low as 10⁻¹⁹ watts (about 100 photons per second), arising from a few electrons oscillating at THz frequencies. According to researcher Kenji Ikushima, the technique’s high sensitivity will soon facilitate the study of a single molecule shaking, rattling, and rolling at THz rates. (K. Ikushima et al., Appl. Phys. Lett. 88 , 152110, 2006 http://dx.doi.org/10.1063/1.2194473 .)