Minimalist laser in a photonic crystal

In the world of the very small, quantum dots (QDs) can behave like artificial atoms, able to emit and absorb photons at discrete frequencies based on an individual dot’s energy levels. Like water drops on a waxed car, QDs can spontaneously form as tiny islands on a semiconductor substrate during epitaxial growth. Such dots interact strongly with lattice vibrations and can be greatly influenced by surfaces and the adjacent wetting layer. Researchers at the University of California, Santa Barbara, found a surprising benefit of those interactions when they buried some indium arsenide QDs in a photonic-crystal membrane—a thin sheet of gallium arsenide having a regular array of drilled air holes—that was missing a few holes, as shown in the electron micrograph. Only a few dots are in the defect area and yet they act as a gain medium to generate coherent laser light. How do they do it? The defect area is essentially an optical nanocavity within which the QDs interact, in effect tuning themselves to efficiently generate the stimulated emission of a laser. What’s more, the laser has an exceedingly low lasing threshold. According to the researchers, the nanolasers are promising for future applications in integrated photonic circuits. For more on quantum dots, see articles in Physics Today, May 2001, page 46 , and October 2002, page 36 . (S. Strauf et al., Phys. Rev. Lett. 96 , 127404, 2006 http://dx.doi.org/10.1103/PhysRevLett.96.127404 .)