In conventional semiconductor lasers, the photons come from the recombination of electrons and holes. But in a new semiconductor laser recently unveiled by a team at AT&T Bell Labs, the photons are emitted as electrons cascade from one energy level down to another in a staircase‐like arrangement of quantum wells. The new laser is thus unipolar, relying only on electrons rather than on both positive and negative charges. While the physics of this “quantum cascade laser” is very exciting, its commercial potential is not. The threshold current is still very high and might limit its practicality. However, the laser could be used as a research tool and should certainly stimulate still more innovations in design.

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