image credit: Fatima Toor; image source; larger image
Quantum Cascade Lasers
The image above shows a quantum cascade laser captured by a camera that images infrared light. The laser light is the small dot in the middle of the round window. The laser itself--behind the window--is kept at a temperature of -193° C, just above the temperature of liquid nitrogen.
Quantum cascade lasers work like this:
- In a thin slice of a semiconductor, an electron can occupy discrete energy levels.
- When an electron moves from a higher level to a lower level, a photon is emitted.
- If many thin slices of semiconductor are stacked together, the electron can cascade from one to another, emitting a photon of the same frequency in each slice. These photons form the laser beam.
- To learn more, see Quantum Cascade Lasers.
First Supernova Discovered with Laser Guide Star Adaptive Optics
Lasers can help astronomers bring stars into focus. A laser, tuned to a sodium spectral line, shines backward through the telescope, and this laser light excites sodium atoms located in a thin layer of the atmosphere, producing in effect an artificial star. Astronomers can then see how the atmosphere distorts the laser star and then compensate to clarify their images. This relatively new technique means ground-based telescopes can be just as useful as those in Earth orbit. See First Supernova Discovered with Laser Guide Star Adaptive Optics and Rubber Mirrors and Artificial Stars for a fuller explanation.